-
Session 1 Tectonic evolution and metallogenic potential
throughoutearth history
Although it is possible to identify various tectonicprocesses
conducive to mineralisation, the problemremains to identify the
critical factors that control theformation of a large ore deposit
at a particular time andlocation within an evolving orogenic
system. To do this,it is best to examine mineralisation in modern,
activesubduction complexes where it is possible to determinethe
physical properties and dynamics of thelithosphereasthenosphere
system, image plate archi-tecture and measure the rates at which
tectonic andmineralising processes occur. Tectonic processes
thatgenerate magmatism and deformation at convergentmargins
influence the timing and location of ore depositsby inducing
melting, by providing pathways for magmasand by varying the rate of
supply of volatile components.Magmatism provides heat to generate
hydrothermalsystems and also releases fluids that redistribute
metalswithin the crust of the overlying lithosphere. Asorogenesis
progresses from subduction of oceaniclithosphere and generation of
oceanic island arcs, toarccontinent, then continent continent
collisionthrough to post-collisional extension and
orogeniccollapse, so various transient effects due to changes
inplate configurations and subduction architecture cancreate
various conditions conducive to magmatism andmineralisation. King
et al.6 demonstrated that plate re-organisations driven by
instabilities in mantle conventioncan occur on time scales of <
4 Ma. However, transientchanges in plate configurations are
insufficient alone togenerate giant ore deposits. The same types of
porphyrycopper and epithermal gold deposits occur in
differenttectonic scenarios, so that although magmatism may
begenerated by various subduction and mantle wedgeprocesses, the
concentration of mineralisation in largeore deposits has more to do
with the structural evolutionand the state of stress in the upper
plate.
Subduction of oceanic lithosphereIn the SE AsiaSW Pacific
region, for example, seismictomography not only images the present
configuration ofsubduction slab anomalies in the upper mantle but
canbe interpreted in terms of the past history of subductionto test
tectonic plate reconstructions based on surfacegeology and
palaeomagnetic data.4,5 Cenozoic magmaticarcs are richly endowed
with magmatic hydrothermalmineral deposits such as high- and
low-sulphidationepithermal Au, porphyry CuAu and skarn
CuAudeposits.7 Because the magmas are associated with
CuAu mineralisation in both oceanic and continentalarcs, it
seems that the metals were derived from themantle source of the
magmas, rather than a local crustalsource. Most of the magmatism
and ore deposits formedduring three short intervals of plate
re-organisation ratherthan during periods of steady-state
subduction. Theseplate re-organisations were initiated by the
collision of theAustralian continent with the Philippine Sea plate
at 25Ma, rotation or extrusion of Indochina and the cessationof
spreading of the South China Sea at 17 Ma and animportant period of
tectonic reorganisation at 5 Ma.1,7
Arc-related magmatism in transient tectonic settingsproduced the
most abundant and largest ore deposits,most of which have formed
since 5 Ma. For the periodsince 25 Ma, most subduction zones in the
SE AsiaSWPacific region experienced significant arc volcanismduring
hinge retreat (rollback), which in many cases wasaccompanied by
marginal basin formation.7 Mostvolcanic episodes were completed
within 35 Ma. Incontrast, reduction or cessation of volcanic
activityoccurred during hinge advance. Hinge movement appearsto be
a first-order factor in the control of both volcanismand ore
formation. Increased volcanic activity duringperiods of hinge
retreat can be explained by corner flow inthe mantle wedge that is
induced to conserve massbeneath the arc as the slab descends.
Magmatism mayhave resulted from slab melting, melting in the
wedgeabove the subduction zone caused by slab dehydration,or
melting in sub-arc lithosphere caused by inflow of hotmantle during
slab rollback. Mantle melting is mostlikely around 80 km depth,
with 50 km lateral transportof water from the slab.3
Changes in slab architectureSubduction of lithosphere can jump
from one locationto another, usually because there is some
impedimentto subduction, such as the presence of continental
crustand/or thickened lithosphere entering the subductionzone, for
example the collision of Australian con-tinental shelf with the
Banda Arc, where subductionsouth of Timor has recently jumped north
across theBanda Arc to Wetar.9 Subduction can reverse polarityas a
consequence of changes in plate configurations.Northward subduction
south of Timor has switchedto southward subduction of the Banda Sea
plate atWetar. Stalling (and subsequent melting) of
subductedlithosphere, slab steepening and subduction
polarityreversal all appear to have been common regimes for the
Applied Earth Science (Trans. Inst. Min. Metall. B) August 2003
Vol. 112 B107
Tectonic processes conducive to magmatic-hydrothermal
mineralisation
Derek Blundell
Geology Department, Royal Holloway, University of London, Egham,
Surrey TW20 0EX, UK ([email protected])
DOI 10.1179/037174503225001604
-
formation of magmatic-hydrothermal deposits. The ratesof
subduction and rollback may control the volume ofmelt and the
amount of magmatic activity.
Slab detachmentThere is good evidence from earthquake
hypocentrelocations and from seismic tomographic images thatsinking
segments of subducted lithosphere becomedetached, thus relieving
the remaining lithosphere of aload and allowing asthenosphere to
flow through the gap.The resulting changes in temperature and
pressure areconducive to melting of both asthenosphere
andlithosphere. Wortel and Spakman13 have modelled amechanism of
slab detachment through lateral propag-ation of a tear in the
lithosphere. Where torn, thelithosphere is relieved of the load
from the deeper part ofthe slab and rises, whilst the intact part
of the lithospherecarries the full load and is depressed.
Propagation of thetear can be followed by tracking the null point
betweenuplift and downwarping as it progresses around the arc.For
example, migration of the null point started in thewestern
Carpathians at 16 Ma and migrated clockwisearound the arc to the
Vrancea region at its south-easternend, where the slab is still
attached. The tear is initiatedwhere the weight of the slab exceeds
its internal strength,most probably when weak continental crust in
thelithosphere arrives at the trench and starts to be sub-ducted.
Temperatures in the continental lithosphereappear to be the main
factor in determining the depth atwhich the tear initiates, which
can be as little as 35 km. Asthe tear develops, hot asthenosphere
can well up throughthe widening gap, producing a transient heat
pulsethrough the lithosphere of the overriding plate. Modellingthe
thermal consequences, van de Zedde and Wortel12
demonstrate the possibilities, under various
transientconditions, of partial melting of asthenosphere
associatedwith shallow slab detachment, partial melting of
themantle lithosphere of the subducting plate or that of
theover-riding plate, and anatexis of the over-riding crust.Slab
detachment is the natural last stage of the subductionprocess of
continentcontinent collision, when relativeplate motion ceases. It
can thus be the underlying cause ofpost-collisional extension and
orogenic collapse.
The magmatism and mineralisation of the Carpathiansbetween 15 Ma
and 10 Ma2,8 is a well-documentedexample of slab tear and back-arc
extension. Slab tearexplains a diachronous trend in magmatism
clockwisearound the arc. Mineralisation is synchronous but owesmuch
to complexity in the upper plate.
Localised extension within a transpressional regimeLocalised
extensional settings in the over-riding platewithin an overall
transpressional tectonic regime appearto be favourable for the
location of igneous intrusionsand ore deposits. Tosdal and
Richards11 explained thatthe locations of porphyry copper systems
are oftenclosely related to major strike-slip fault systems,
invol-ving both transpression and transtension, which aresteeply
dipping or near-vertical, offering good pathwaysfor upward movement
of magmas. They argued that
periods of normal compression or large-scale extensionare not
conducive to the formation of porphyry copperdeposits but, instead,
many porphyry copper depositsdeveloped under short-lived, transient
conditions duringperiods of plate re-organisation.
Discussing the origins of Variscan mineralisation,Tornos et
al.10 presented an account of oblique northwardsubduction of
oceanic lithosphere beneath SW Iberia andsubsequent
collision-induced thrusting and left-lateraltranscurrent motion of
crustal blocks. Most of themineralisation in the area was
associated withmagmatic/hydrothermal activity controlled by
strike-slipfaulting related to large-scale transpressional
deformationbut located within areas of localised extension.
Variousstyles of ore deposits were formed, involving abnormallyhigh
heat flow and large volume fluid flow along openfractures driven by
the interplay of fault and magmaticactivity to shallow depths. In
the Iberian Pyrite Belt, themineralisation is restricted to (sub-)
surficial levels.Oblique collision and associated lateral escape
promotedthe opening of crustal-scale fractures with crustal
meltingat relatively low pressures, which required
extensionaltectonics and high heat flow conditions, possibly in a
fore-arc extensional setting. Second- and third-order
pull-apartsedimentary basins were also formed, the locations of
thelargest massive sulphide deposits. This tectonic setting
isdistinct from the subduction-related arc and back-arcgeodynamic
environments postulated for most otherequivalent metallogenic
provinces, which may be the keyto understanding what made this part
of the world sofavourable for hosting about 5 Ma of intensely
productivehydrothermal activity around the end of Devonian
times.One possibility is that the mineralisation is associated
withslab break-off and the consequent major heat pulse,providing
conditions for magma generation and theappropriate deformation in
the over-riding plate.
References1. M. E. BARLEY et al.: Geol. Soc. Spec. Publ., 2002,
204,
3947.2. H. DE BOORDER et al.: Earth Planet. Sci. Lett., 1998,
164,
569575.3. J. W. DAVIES and D. J. STEVENSON: J. Geophys.
Res.,
1992, 97B, 20372070.4. R. HALL: J. Asian Earth Sci., 2002, 20,
353434.5. R. HALL and W. SPAKMAN: Earth Planet. Sci. Lett.,
2002, 201, 321336.6. S. D. KING et al.: Earth Planet. Sci.
Lett., 2002, 203,
8391.7. C. G. MACPHERSON and R. HALL: Geol. Soc. Spec.
Publ.,
2002, 204, 4967.8. F. NEUBAUER: Geol. Soc. Spec. Publ., 2002,
204, 81102.9. A. N. RICHARDSON and D. J. BLUNDELL: Geol. Soc.
Spec. Publ., 1996, 106, 4760.10. F. TORNOS et al.: Geol. Soc.
Spec. Publ., 2002, 204,
179198.11. R. M. TOSDAL and J. P. RICHARDS: Rev. Econ.
Geol.,
2001, 14, 157181.12. D. M. A. VAN DE ZEDDE and M. J. R. WORTEL:
Tectonics,
2001, 20, 868882.13. M. J. R. WORTEL and W. SPAKMAN: Science,
2001, 290,
19101917.
B108 Applied Earth Science (Trans. Inst. Min. Metall. B) August
2003 Vol. 112
Session 1 Tectonic evolution
-
This presentation focuses on improving the under-standing of
processes controlling the tectonic genesis ofore systems by
analysing the tectonic mechanisms andassociated stress regimes. The
timing and unique tectoniccharacter coincides with the mineralising
stages. Apply-ing tectogenetic analysis, tectonic mechanisms and
theirnature has successfully identified and explained anumber of
ore systems of various commodities, includ-ing Au, Cu, Ni, ZnPb and
diamond.2,3
Tectonic mechanismsTectonic mechanisms and the corresponding
tectonicand mineralising processes developing in orogenic
andcratonic regimes have been investigated. The TelferAuCu
(Proterozoic Paterson Orogen) and Golden MileAu (Archaean Yilgarn
Craton) world-class deposits ofWestern Australia are used as
examples to explain thetectonic mechanism also commonly observed
elsewherein various modifications.
Tectonic mechanisms at Telfer (orogenic environment)The Telfer
ore system developed within a domal structure(Fig. 1A,B). Most
explanations employ regional com-pression related tectonic
processes, particularly strike-slip
shearing, folding and flexural-slip folding, and thrust
typedeformation mechanisms to explain the formation of thedeposit
domal shape and controls on mineralisation.The results of a study
on the tectonic mechanisms atTelfer indicate that its domal
geometry is not a regionalcompression related feature, but rather a
result of localshear-extensional processes propagated from
thebasement.3 This gives rise to an asymmetric extensionalflexural
bend of the sequence with normal dip-slipkinematics along the
bedding surfaces during themineralising processes (Fig. 1A). This
flexural-bend isa deposit-scale tectonic feature closely linked
with theactivity of the NWSE oriented and basement-rootedmajor
shear/fault structural system, or lineament thatparallels the
Paterson Orogen.
The forces controlling the Telfer-scale extensionaldeformation
siting of mineralisation propagate fromthe basement upwards, and
are mostly related to steepSSWNNE oriented structures mostly with
reverse-slip kinematics. In a number of cases, these
structuresdisplay a specific convex geometry and a tendency tofade
out upwards. In this model, Main Dome is a firstorder (NWSE
elongation), while West Dome is asecond order (NNESSW elongation)
mineralisedfeature. This interpretation explains the lack
ofcorrelation between the reefs forming these twotectonic domains.4
Although formed during thedevelopment of the extensional flexural
bend, the twodomes evolve via differing tectonic forming
processes,tectonic genesis and structural geometry: as a result,
theycannot be considered as en echelon structures in theclassical
sense.
Tectonic mechanisms at Golden Mile (cratonicenvironment)Most
considerations on the Golden Mile (Kalgoorliecamp) tectonic
deformation are concentrated on variousaspects of tectonic
evolution. These include a recent sug-gestion that Golden Mile is
an early orogenic-stagerelating ore system with significant
rotation of thesequence and mineralisation in the later tectonic
events.1
The most prominent fault of the Kalgoorlie region,the
Boulder-Lefroy Fault Zone, is a near-surfaceexpression, generally
with oblique-slip tectonic move-ment, of a major basement fracture
striking NNWSSE.The active role of this basement-rooted feature
duringmineralisation was to propagate the extensional struc-tural
environment upwards, favouring prominent multi-lode Au mineralised
system at Golden Mile and othermajor deposits in the region.
The moderate-to-steep westward dip of the GoldenMile Dolerite
follows the orientation of the contactzone between the Dolerite and
the underlying ParingaBasalt. This contact, together with steep to
vertical
Applied Earth Science (Trans. Inst. Min. Metall. B) August 2003
Vol. 112 B109
Session 1 Tectonic evolution
Tectonic mechanisms and their role in forming the major ore
systems of Western Australia
W. V. Bogacz
Archon Resource Technologies Pty Ltd and BFP Consultants Pty
Ltd, Level 2 Eastpoint Plaza, 233 Adelaide
Terrace, Perth, WA 6000, Australia ([email protected])
1 (A,B) Tectonic mechanism forming the Telfer oresystem
A
B
DOI 10.1179/037174503225001613
-
lithologies for the Western Lodes, determines aspecific
triangular-like tectonic feature (Fig. 2A,B).
The formation of steep reverse-slip faults withconvex-type
geometry and consistent E up/W downmovement, is believed to be the
principal drivingmechanism for propagation of basement forces
upwardsand the creation of the Golden Mile extensional regimeand
accompanying mineralisation. This mechanismpermits the explanation
of the tectonic genesis of theGolden Mile Dolerite hosted
mineralisation (Fig. 2B).This also explains the rich Oroya shoot
(23 Moz), whichis developed within Paringa Basalt, as a local lower
orderfeature controlled by the formation of basement-propagated
reverse-slip Kalgurli and Kalgurli Northfaults (Fig. 3).
ConclusionsThe understanding of the tectonic
mechanismscontrolling extensional deformation and, therefore,the
space preparation processes and correspondingmineralisation
pattern, the nature of structures active
during the mineralising processes, and the overalltectonic
deformation settings in which mineralisationis confined, are
critical for the explanation of thetectonic genesis of ore systems
and their geometry. Iftectonic mechanisms are understood,
prediction ofmineralised zones becomes possible.
Comparing tectonic deformation and mechanismsof mineralisation
emplacement, many similaritiesexist between Proterozoic (orogenic)
and Archaean(cratonic) deposits. Processes related to the
formationof ore systems are strongly influenced by thebasement
activity. Basement-propagated structuralfeatures are dominated by
steep reverse-slip andnormal dip-slip structures that predominantly
controla deposit and local scale extensional deformation,associated
mineralising processes, and determinegeometry and the internal
pattern for many deposits.
More brittle type geomechanical environments,competency and
other geomechanical contrast andboundary zones such as faults and
flexures are moreprone to the propagation of the extensional
deformationand mineralisation in their surroundings. As a
rule,these are basement-propagated zones, which developextensional
openings (controlling mineralisation) alongpre-existing foliation
and other tectonic anisotropysurfaces within the host rock, thus
making them lowerorder secondary structures. Later stage processes
(morebrittle) propagating tectonic deformation along pre-existing
structures are unique when compared to thehost rock (more ductile)
earlier stage tectonic evolution.
References1. BATEMAN et al.: Appl. Struct. Geol. for Miner.
Expl.
Min., AIG Internal Symp., 2325 September 2002,Kalgoorlie, WA,
2002, 68.
2. BOGACZ: Mineral deposits at the beginning of the 21stcentury,
Balkema, 2001, 713.
3. BOGACZ: Appl. Struct. Geol. for Miner. Expl. Min.,AIG
Internal Symp., 2325 September 2002,Kalgoorlie, WA, 2002, 2225.
4. ROWINS et al.: (1997) Econ. Geol., 1997, 92, 133160.5. TRAVIS
et al.: Spec. Publ., Geol. Soc. Aust., 1971, 3.
B110 Applied Earth Science (Trans. Inst. Min. Metall. B) August
2003 Vol. 112
Session 1 Tectonic evolution
B
A
2 (A,B) Tectonic mechanism forming the Golden Mileore system
3 Tectonic interpretation of the Oroya shoot
-
Most passive margins in earth history have experiencedthe same
chain of events: (i) they form by rifting; (ii)subside as the
adjacent ocean basin opens, then closes;and (iii) finally end up
colliding with an arc. Thelifespan of passive margins is of
interest as an indicatorof the earths tectonic regime, and as a
framework forunderstanding their contained ore deposits.
Heatproduction on earth was about 4 times the presentvalue at the
end of the Archaean. Two of the commonlyinvoked mechanisms for loss
of this extra heat aregreater ridge length (implying more, smaller
plates) andfaster plate motions. Either mechanism, or both
incombination, would have resulted in comparativelyshort-lived
ocean basins and, by implication, short-lived passive margins. As a
geologic test of thishypothesis, we examine the lifespan of the
passivemargins within orogenic belts. The lifespan of a
passivemargin is the time from the rift-drift transition to
thedrowning of the platform at the onset of collision, i.e.when the
passive margin evolves into a foredeep.
Ancient passive marginsData from a preliminary compilation of
Proterozoic andPhanerozoic passive margins are plotted in Figs. 1
and 2.Age control is adequate for all the Phanerozoic marginsand
for a few of the Proterozoic ones, but is poor forArchaean margins.
The margins in our dataset had amean duration of 145 million years
(Fig. 1). Surprisingly,the longest-lasting passive margins yet to
be documentedare among the oldest (i.e. the Palaeoproterozoic
southernand eastern margins of the Superior Craton, at ~320 and~285
million years). Presuming that the geochronology isreliable and the
tectonic interpretations are sound, this
result invites an explanation, though the only ones thatoccur to
us are ad hoc. The lack of a secular trend (Fig.2) is unexpected;
the available data certainly do notrequire some combination of
greater ridge length andfaster plates in the Early Proterozoic.
Modern passive marginsThe worlds extant passive margins range in
age fromabout 5 million years (Red Sea) to about 180 millionyears
(Central Atlantic). These margins are only partway through their
life cycles and thus have a youngerage distribution than the
ancient margins. Theirregularity of the age distribution (Fig. 2)
is mainly aresult of the staggered breakup of Pangea. Large partsof
these margins are likely to endure for many millionyears longer,
before finally colliding with something.The worlds population of
extant passive margins willthereby increase in both mean age and
maximum agefor the foreseeable future. If one of the Central
Atlanticmargins can survive unscathed for another 140 millionyears,
it will have matched the lifespan of the southernmargin of the
Superior Craton.
Ore deposits of passive marginsMineral resources associated with
the various phases ofpassive margin evolution include Mississippi
Valley-type (MVT) PbZn, banded iron formation (BIF), andsedimentary
barite. Many of the worlds largest MVT
Applied Earth Science (Trans. Inst. Min. Metall. B) August 2003
Vol. 112 B111
Session 1 Tectonic evolution
Lifespan of passive margins through earth history
Dwight Bradley1 and David B. Rowley2
1US Geological Survey, 4200 University Dr., Anchorage, AK 99508,
USA ([email protected])2Department of the Geophysical Sciences,
University of Chicago, 5734 S. Ellis Ave, Chicago, IL 60637,
USA([email protected])
1 Duration of ancient passive margins through earthhistory
2 Histograms of the lifespans of extant and ancientpassive
margins. The extant margins have yet to liveout their lives and
hence are younger on average. Forextant margins, each age sector
along a diachronousmargin is counted as one, and conjugate pairs
countas two; other protocols yield similar histograms
DOI 10.1179/037174503225001622
-
Mesothermal or, as more recently termed, orogenic lodegold
deposits are the predominant gold deposit type inArchaean
greenstone belts.8 Their defining characteristicsand spatial and
temporal distributions are well-documented.7,9 However, a discrete
sub-set of gold depositswith atypical metal associations have been
identified as acontentious group. They are most abundant in
Late-Archaean terrains, and include several world-classexamples.
The Hemlo Deposit, Canada, is characterised byan anomalous
enrichment in Ba, Mo, and Hg, amongstother elements, and appears
unique. The remainingatypical deposits fall into two groups: those
enriched in Cu Mo (e.g. McIntyre-Timmins, Canada;
Boddington,Australia), and those enriched in CuZn Pb Ag
and/orabundant pyrite (e.g. Bousquet, Canada; Mount
Gibson,Australia). Owing to the abundance of volcanic-hostedpyrite
and ore-stage Cu-enrichment, Bulyanhulu has beenassociated with the
latter category of atypical deposits.8,9
Models for these deposits invoke a consistent spatial ordirect
genetic association with magmatic intrusions, arelationship that
has been long debated in the literature.
Geological setting of BulyanhuluBulyanhulu is situated within
the Sukumalandgreenstone belt, one of a series of Nyanzian
(Late-Archaean, 2825 Ga) terrains developed in theTanzania
Craton.3,4 The terrain, regionally metamor-phosed to greenschist
facies, consists of two oval, sub-concentric belts. They comprise
an inner (LowerNyanzian) belt characterised by basaltic and
andesiticlavas and tuffs, and an outer (Upper Nyanzian)
arcconsisting of banded iron formation with volcanic-lastics. The
terrain has been considered to youngoutwards; however, recent
geochronology contradictsthis, and suggests significant structural
complexity.2,3,10
Bulyanhulu is situated within the inner arc. Syn-
andpost-orogenic granitoids, as well as several generationsof dykes
of lamprophyric and basaltic composition,have been identified. In
the Bulyanhulu area, a series ofNWSE trending shear zones occur and
the main zoneof mineralisation, Reef 1 (105 Moz Au resource),1
ishosted in one such shear structure, with furthermineralisation in
parallel structures termed Reef 2.
deposits are hosted by carbonate sequences that formedalong
passive margins and were later mineralised in
collisional forelands;1 MVT deposits are known as farback as
2300 Ma but peaked around 300 Ma (Fig. 3).Many Proterozoic
Superior-type BIFs formed aspassive margins evolved into
collisional foredeep outerramps of foredeeps.2 Some Phanerozoic
bedded bariteslikewise formed during onset of collision, along
theforedeep axis.4 Secular trends in the existence andabundance of
these deposit types (Fig. 3) are mostlikely due to plate
interactions, superimposed onchanges in climate, ocean chemistry
and atmosphericcomposition.
References1. D. C. BRADLEY and D. L. LEACH: Mineral.
Deposita,
2003, 38, In press.2. P. F. HOFFMAN: Am. Geophys. Union
Geodynamics
Ser., 1987, 17, 8598.3. P. W. JEWELL: SEPM Spec. Publ., 2000,
66, 147161.4. J. B. MAYNARD and P. M. OKITA: Econ. Geol., 1991,
86,
364376.5. C. W. MEYER: Annu. Rev. Earth Planet. Sci., 1988,
16,
147171.
B112 Applied Earth Science (Trans. Inst. Min. Metall. B) August
2003 Vol. 112
Session 1 Tectonic evolution
3 Histograms showing the age distribution of threeore-deposit
types sometimes associated with passivemargins
The Bulyanhulu enigma: an atypical Archaean lode gold deposit
evidence for a pre-oremagmatic input?
C. M. Chamberlain1, J. J. Wilkinson1, R. J. Herrington2 and A.
J. Boyce3
1Department of Earth Science and Engineering, Imperial College,
Prince Consort Road, London SW7 2BP, UK2Department of Mineralogy,
The Natural History Museum, Cromwell Road, London SW7 5BD,
UK3Scottish Universities Research and Reactor Centre, Rankine
Avenue, East Kilbride, Glasgow G75 0QF, UK
DOI 10.1179/037174503225001631
-
Characteristics of mineralisationObservations of the Bulyanhulu
host rocks and min-eralisation show that its evolution includes
thefollowing.
Syn-volcanic activity and carbonaceous
clasticsedimentationTholeiitic volcanic rocks dominate the
stratigraphy, andare intercalated with calc-alkaline rhyodacites
andsubordinate interflow sedimentary units, including theKisii
Shale Unit, which largely hosts the Reef 1 gold-orebody at
Bulyanhulu. The felsic rocks host pyriteclasts similar to those
present in volcanic-hostedmassive sulphide (VHMS) systems. These
volcanicsinclude hyaloclastite flows and polymict sediment-bearing
lava breccias, and show evidence of extensivespilitisation.
Intrusions and related vein fluidsShallow-level porphyries,
geochemically associatedwith calc-alkaline extrusive facies,
intrude thevolcanics and sediments. These show variable degreesof
alteration and mineralisation. The earliest stage ofquartz veining
recognised (stage I) is characterised byfluid inclusions that
contain 1248 wt%equiv. NaCl,with additional CaCl2, and have a
maximum homo-genisation temperature of 420C. The most likelysource
for such a saline fluid is magmatic. This issupported by the
occurrence of directly measuredfluid dD and inferred d18O
compositions that lie in thefield of magmatic water. Taken
together, the fluidcharacteristics are inconsistent with
VHMS-related orepithermal origins, or basinal brines.
Regional metamorphismPeak regional Kahaman metamorphism at
around27002650 Ma was associated with an approximatelynorthsouth
oriented, compressive event. Regionalmetamorphism to greenschist
facies is characterised bythe pervasive development of
chlorite-calcite alteration,with a generally weak fabric
development. Barren shearstructures are enriched in LOI, CaO and
Fe2O3 con-sistent with syn-metamorphic carbonatisation. Thesezones
are distinct from mineralised shears in that theylack
quartz-carbonate veining and gold-bearingsulphides.
Shearing related to shorteningThe principal deformation event
observed atBulyanhulu is characterised by reverse
high-angleshearing and associated quartz vein emplacement.The
strong rheological contrast that the Kisii ShaleUnit provided
resulted in intense focusing of strainallowing the Bulyanhulu Shear
Zone, hosting Reef 1,to develop.
Structurally controlled mineralised veinsSecond-stage quartz
veins (stage II quartz) cut acrossthe shear foliation which, in
turn, overprints theregional metamorphic fabric indicating that
veinformation postdated peak metamorphism. The main
stage of quartzcoppergold-sulphide veining atBulyanhulu is
characterised by a series of steeplydipping lenses, varying in
thickness and continuityalong strike, which overprint the
pre-existing stage Iquartz. Other vein sets cut the shear zone
foliationdisplaying variable degree of progressive
deformationduring shearing events.
Pyrite, the dominant sulphide in the Reef 1 oreassemblage,
occurs at the margins of stage II quartzveins. This pyrite is
accompanied by chalcopyrite,pyrrhotite, microscopic gold and
accessory monazite, intextural equilibrium with arsenopyrite.
Parageneticallylater quartz, associated with boudinage textures,
occurswith coarser gold and chalcopyrite, and accessorysphalerite
and bismuthotellurides. It is not clear whethergold and
chalcopyrite were introduced at this time, or ifthey were simply
remobilised. Sulphur isotopecompositions of stage II sulphides show
a relativelynarrow range, with the dominant population
occurringbetween +20 and +45 supporting a homogeneoussulphur
reservoir.
Aqueo-carbonic fluid inclusions enriched involatiles in addition
to CO2 characterise the stage IIquartz veins, with homogenisation
temperatures of~300450C, and show evidence of phase
separation.Directly determined dD and inferred d18O values
liewithin the overlapping fields of magmatic andmetamorphic
waters.
The results of d13C analysis of vein carbonates arecomparable to
those documented in Archaean golddeposits in the Yilgarn and
Superior Cratons. Modellingof the carbonate alteration halo
demonstrates a two-stage influx of hydrothermal fluids causing the
resultingd13C profile.5
Statistical analysis (specifically principal componentanalysis)
of lithogeochemical data is consistent with twogold associations
AuCu and AuAg.6
DiscussionBulyanhulu has many characteristics consistent withthe
classic model of mesothermal lode gold depositsbut significantly
has a number of atypical features.5
Typical features(i) Like other gold deposits in the
Sukumaland
Greenstone Belt (e.g. Geita, Buck Reef), mineral-isation at
Bulyanhulu is shear-hosted.
(ii) Sub-parallel structures in contrasting lithologicalunits
host gold mineralisation.
(iii) Fluid inclusions indicate low-to-moderate
salinityCO2CH4-bearing fluids homogenising at 300450Cwith evidence
of phase separation during veinformation.
(iv) The narrow zone of largely symmetrical alterationaround the
vein system is consistent with fluidintroduction into wallrocks
from the main shearzones.
(v) Gold is distributed over an extensive strike and diprange,
similar to other major gold camps (e.g.Kolar, Timmins).
Applied Earth Science (Trans. Inst. Min. Metall. B) August 2003
Vol. 112 B113
Session 1 Tectonic evolution
-
The duty of the geologist and the prospector is in fact
todeliver the goods
Sir Lewis L. Fermor, 1951, 6th geological President of
theInstitution of Mining and Metallurgy
Archaean cratons are underlain by relatively thin crust(~3040
km) and thick mantle lithosphere (up to ~250km) that is invariably
enriched in diamonds ranging inage from Phanerozoic to
Mesoarchaean.1,6,8,9 The distri-bution of mineral deposits across
at least seven globallydistributed Archaean cratons also indicates
that many ofthese fragments of early continents each have
uniquepolymetallic fingerprints of their own.2 These differ-ences
appear to reflect regional geochemical hetero-geneities of early
earth. Some of these cratons have Au,Cu, Pb and Zn signatures that
fit with simple collisionalmodels involving accretion of island
arcs with contin-ental and oceanic fragments (e.g. Superior
Province,Yilgarn and Zimbabwe Cratons). Others, however,
areremarkably enriched in siderophile elements such as Ni,Cr, PGE,
both in their crustal and mantle sections (e.g.Pilbara and Kaapvaal
Cratons), whilst others still arerelatively enriched in Sn, W and
U/Th (e.g. Amazonian,Leo-Man, Ntem and South China Cratons). How
theseold continental fragments inherited their
metallogeniccharacteristics is unresolved. In the case of the
oldestArchaean cratons, their dominant metallogenic
fingerprints were formed near the time of their separationfrom
the mantle; thereafter, their inherited metals weremostly
remobilised and redistributed during subsequenttectono-metamorphic
and erosion-sedimentary processes(e.g. Sn in South America; PGE in
Southern Africa).Because different cratons are only small remnants
of oncemuch larger and varied continents, their initial
metalinventories were also in parts recycled. In this
contri-bution, we compare quantitatively (using our extensive
in-house GIS database of mineral deposits2,10) the
mineralinventories of 12 Archaean cratons against mineral
distri-bution across larger Phanerozoic continents (Gondwana,600200
Ma), Africa and South America (2000 Ma) tocompare and contrast the
changing metallogenic finger-prints of earths continental
lithosphere. This workattempts inter alia to chart evolving
partition coefficientsof metalliferous elements between mantle and
lithosphereduring growth and recycling of the continents over
35billion years of earths history.
MethodsA simple linear relationship exists between
litho-diversity and mineral-resource diversity.5,7 This can beused
to predict mineral potential of a specified region,although it
ignores factors such as the infrastructureand exploration history
of the study region. We use
Atypical features(i) Early syn-genetic sulphides in felsic
volcanic shale
package, providing a focus for paragenetically laterhigh-grade
gold mineralisation.
(ii) Evidence for an early saline fluid generation inthe vein
system with a likely magmatic source.
(iii) Statistical lithogeochemical evidence for a two-stage
introduction of metal into the Bulyanhulusystem.6
(iv) Mineralised and altered porphyry stocks inter-sected in
drilling marginal to the ore zone.
(v) High tenor of copper in the gold assemblagealong with highly
elevated Bi and Te.
SummaryWhilst it is clear that structure is the key to
generation ofthe Bulyanhulu deposit, a number of the
atypicalfeatures may be crucial components of the genetichistory of
the deposit. Early syn-genetic pyrite associatedwith black shales
was essentially barren of economicallyimportant elements. However,
it may have acted as ageochemical trap for the precipitation of
gold mineral-isation later on. Early saline fluids are recorded in
quartz
in the gold-bearing vein mineralisation. They suggest anearly
magmatic event which, based on lithogeochemicaldata, probably
involved pre-ore metal enrichment. TheBulyanhulu deposit is,
therefore, interpreted as a two-stagehydrothermal process with the
main lode gold eventoverprinting an earlier CuAu enriched magmatic
system.
References1. BARRICK: www.barrick.com (accessed 2002).2. G. BORG
and T. KROGH: J. Afr. Earth Sci., 1999, 29,
301312.3. G. BORG and R. M. SHACKLETON: Oxford Monogr.
Geol. Geophys., 1997, 35, 608619.4. G. BORG et al.: Geol.
Rundschau, 1990, 79, 355371.5. C. M. CHAMBERLAIN: (2003)
Unpublished PhD thesis,
Imperial College, 2003, 1401.6. C. M. CHAMBERLAIN et al.: Appl.
Earth Sci.(Trans.
Inst. Min. Metall. B), 2002, 111, 137138.7. D. I. GROVES et al.:
Ore Geol. Rev., 1998, 13, 727.8. D. I. GROVES et al.: Econ. Geol.,
2003, 98, 129.9. S. G. HAGEMANN and P. E. BROWN: Rev. Econ.
Geol.,
2000, 13, 1559.10. S. MANYA and M. A. H. MABOKO: Precambrian
Res.,
2003, 121, 3545.
B114 Applied Earth Science (Trans. Inst. Min. Metall. B) August
2003 Vol. 112
Session 1 Tectonic evolution
Metallogenic scents of Archaean cratons: changing patterns of
mineralisation duringearth evolution
M. J. de Wit and C. Thiart
CIGCES and AEON, Departments of Geological and Statistical
Sciences, University of Cape Town, Rondebosch7701, South Africa
([email protected] and [email protected])
DOI 10.1179/037174503225001640
-
these methods to construct normalised fingerprintsof Archaean
cratons (Fig. 1) to facilitate visualisationof relationships
between cratons and specific mineralgroups (see for example 2-D
contingency table andbar-chart of mineral groups and cratons of
Africa;
Table 1, Fig. 2). Resulting profiles of cratons can beeasily
compared against the total enrichment ofminerals across all
cratons. Data in Table 1 are used asan input for correspondence
analysis,4 a technique toconvert rows and columns of contingency
tables into
Applied Earth Science (Trans. Inst. Min. Metall. B) August 2003
Vol. 112 B115
Session 1 Tectonic evolution
1 Selected cratons used in this study (numbers 17 refer to
craton names listed in Table 1; other cratons for whichanalyses is
still to be completed are: 8, Superior; 9, Amazonian; 10, Sao
Fransisco; 11, Pilbara; and 12, Yilgarn
Table 1 Cross-tabulated counts of mineral groups by cratons (6
cratons only)
Map Mineral groups/number of deposits Craton
Id Name Au CrNiPGETi CuZnPbBa MoSnSb W UThREE Other total
1 Kaapvaal 894 241 152 96 5 77 356 18212 Limpopo 0 40 33 0 8 9
15 1053 Zimbabwe 625 114 106 81 305 15 31 12774 Congo 53 2 24 1 0
17 63 1605 Tanzania 7 0 3 1 0 5 4 206 Leo-Man 47 22 11 10 2 9 53
1547 Requibath 0 5 21 0 3 4 10 43
Total deposits 1626 424 350 189 323 136 532 3580
2 Bar-chart of row-profiles (% across rows) for selected
cratons. Total = all 7 cratons listed in Table 1
-
Mineral deposits have a heterogeneous temporal distri-bution,
with characteristic peaks in the abundance ofparticular
mineralisation styles at specific times in earthhistory. This
uneven distribution can be explained by: (i)temporal changes in the
processes that produce mineraldeposits; and (ii) preservational
potential of the environ-ments in which the deposits form. Temporal
changes inmineral-deposit forming processes can, in turn,
beascribed to: (i) the evolution of atmosphere hydrospherebiosphere
systems; (ii) a secular decrease in global heatflow; and (iii)
long-term changes in tectonic processes (e.g.Barley and Groves1).
As shown below, (iii) may be a directconsequence of (ii), and these
factors also affect the long-term preservational potential of
terrains. Althoughindividual lines of evidence for change in the
oxidationstate of the atmospherehydrosphere system are
hotlydebated, the nature of metal deposits for which transportand
deposition are highly affected by redox state (e.g. Fe,Mn, U), and
which formed in sedimentary environments,show marked evolutionary
changes over time. A directcontrol on mineral deposit distribution
by the seculardecrease in global heat flow is the restriction of
NiCudeposits in high-Mg komatiite volcanic rocks to the
LateArchaean and Palaeoproterozoic. In contrast, Palaeo-proterozoic
to Tertiary NiCuPGE deposits areassociated with large intrusions
and giant layered com-plexes that are less magnesian.
Gold deposits as potential tracers of tectonic trendsGold
deposits are particularly useful for testing of secularchanges in
tectonic processes because most such deposits
formed below the influence of surficial processes. There-fore,
most are potentially unaffected by redox changes inan evolving
atmospherehydrosphere biosphere system.Porphyry-and epithermal-type
deposits form at highcrustal levels (< 3 km to surface) in arc
and back-arcenvironments in convergent margins with high
upliftrates and hence they are rarely older than Mesozoic
andTertiary, respectively. Orogenic gold deposits, in contrast,form
over a wide range of crustal environments (320km depth) in the same
convergent margin settings, butduring the main stage of
compressional to trans-pressional deformation that stabilised their
host orogens.These deposits formed over 3 billion years and hence
arepotentially sensitive tracers of temporal changes intectonic
processes.
Orogenic gold deposits through time a preservational
patternMesozoic to Tertiary gold deposits coincide withexternal
ocean margins where accretion of juvenilecrust took place in
environments in which largethermal anomalies were related to
crustal thickeningor upwelling of asthenosphere due to ridge
sub-duction, subduction rollback or lithospheric delamin-ation.
Older deposits appear to have formed in similartectonic settings in
which there were anomalousinputs of thermal energy during gold
mineralisation.Orogenic gold deposits of all ages thus record
orogen-wide fluxes of deeply-sourced auriferous fluid as apart of
the orogenic process in convergent marginsettings. The formation
ages of the orogenic gold
multi-D plots. This exploratory data analysis techniquemakes no
distributional assumptions and is merely auseful preliminary step
towards more structured andtraditional multivariate modelling of
categorical data. Thecorrespondence maps show how the cratons and
theirmineral groups cluster in n-dimensional space. Anormalised
density value (index) for each craton andeach mineral group can
then be calculated. A majoradditional challenge is to normalise the
data further sothat each craton index is weighted per unit area of
craton,as well as for its infrastructure (roads, mining activity).
Forexample, cratons in well-developed mining countries(Kaapvaal)
have been more thoroughly explored thanothers (Congo). We
incorporate this in our finalmetallogenic fingerprints of cratons,
before we comparethem to larger and younger continental
fragments.
References1. M. J. DE WIT: Precambrian Res., 1998, 91, 181226.2.
M. J. DE WIT et al.: J. Afr. Earth Sci., 1999, 28, 3551.3. L. L.
FERMOR: Trans. Inst. Min. Met., 1951, 60, 421465.4. M. J.
GREENACRE: Correspondence analysis in practice,
1984.5. J. C. GRIFFITHS and C. M. SMITH: Comput. Geosci.,
1992,
18, 447486.6. D. E. JAMES et al.: Geophys. Res. Lett., 2001,
28,
24852488.7. M. J. MIHALASKY and G. F. BONHAM-CARTER: Nat.
Resources Res., 2001, 10, 209226.8. S. B. SHIREZ et al.:
Science, 2002, 297, 16831686.9. J. STANKIEWICZ et al.: Phys. Earth
.Planet. Interiors,
2002, 130, 235251.10. C. THIART and M. J. DE WIT: S. Afr. J.
Geol., 2000, 103,
215-230.
B116 Applied Earth Science (Trans. Inst. Min. Metall. B) August
2003 Vol. 112
Session 1 Tectonic evolution
Gold deposits as sensitive indicators of tectonic environments
and their preservationpotential throughout geological history
D. I. Groves1, R. J. Goldfarb2 and R. M. Vielreicher1
1Centre for Global Metallogeny, School of Earth and Geographical
Sciences, University of Western Australia,Crawley, Western
Australia, 6009, Australia ([email protected] and
[email protected])2US Geological Survey, Box 25046, M.S.
964, Denver Federal Center, Denver, CO 80225,
USA([email protected])
DOI 10.1179/037174503225001659
-
deposits define two major Precambrian peaks at27502550 Ma and
21001750 Ma, a marked lack ofdeposits at 1750600 Ma, and a
more-or-less continuous,but cyclic, formation from ~60050 Ma (Fig.
1a). Thepost-600 Ma deposits clearly equate to major
orogenicevents, and are defined by goldfields distributed
alongelongate fold belts normally along the margins ofPrecambrian
cratons or older Phanerozoic fold belts (e.g.Goldfarb et al.3). The
pre-1750 Ma deposits also definegoldfields along elongate belts,
commonly greenstonebelts, but also fold belts of probable
accretionary wedges.These belts are distributed within roughly
equidimen-sional cratons, which contrast in location with
theyounger elongate orogenic belts, which are commonlydeveloped
along the craton margins. Importantly, thepeaks in Precambrian
orogenic gold-deposit formationcorrespond to two major periods
(super-events) ofgrowth of continental crust (Fig. 1b). These
periods arecommonly interpreted to be due to overturn of a
layeredmantle followed by transient whole-mantle convection,with
resulting mantle plumes generating huge volumes ofnew crust by
decompression melting of the lithosphere.These super-events led to
the generation of distinctivebuoyant Archaean sub-continental
mantle lithosphere(hereafter termed lithosphere) and buoyant to
neutralPalaeoproterozoic lithosphere, which contrast with
thenegatively buoyant Phanerozoic lithosphere (e.g. Griffinet
al.4).
Archaean and Palaeoproterozoic orogenic golddeposits were
generated in settings similar to those ofmodern orogens, but were
incorporated into distinctive,relatively equidimensional, buoyant
continental cratonsdue to penecontemporaneous plume activity.
Thiseffectively preserved them from subsequent orogenesisand
erosion. Thus, despite their antiquity, suchArchaean and
Proterozoic gold provinces are still
equivalent in size to much younger gold provinces. Incontrast to
the relatively equidimensional Archaean andPalaeoproterozoic
cratons, juvenile crust created fromabout 1000 Ma to recent is
preserved in microcontinentsor in accretionary collages. These
elongate orogenic beltsare interpreted to have evolved subsequent
to a shiftfrom a strongly episodic, plume-influenced plate
tectonicstyle to a style of less episodic plate-tectonics in a
coolingearth. Orogenic gold deposits formed in juvenile crust
insuch belts would have been susceptible to erosion duringuplift on
the reworked margins of the cratons. The lackof orogenic gold
deposits between ~1750 Ma and ~600Ma could then be explained by
progressive erosion downto the gold-poor high metamorphic-grade
roots of thesethin orogens. Post-600 Ma, segments of the belts
havepresumably not yet been so deeply eroded with,therefore, an
abundance of large orogenic gold provinces,particularly post-~450
Ma.
Age of giant palaeoplacer gold deposits confirmatory
evidenceMost modern placer gold deposits are related to theerosion
of younger Phanerozoic orogenic gold concen-trations, mostly in
MesozoicRecent convergent marginssurrounding the Pacific Rim (e.g.
New Zealand, EasternRussia, California, Alaska), although some
formed due todeep tropical weathering of older orogenic gold
deposits(e.g. Ashanti Belt, Ghana; Tapajtrations, mostly
inMesozoicRecent convergent margins surrounding thePacific Rim
(e.g. New Zealand, Eastern Russia,California, Alaska), although
some formed due to deeptropical weathering of older orogenic gold
deposits (e.g.Ashanti Belt, Ghana; Tapajs, Amazon).
Palaeoplacersderived from older Palaeozoic orogenic gold deposits
(e.g.Ballarat, Victoria) are commonly preserved by
overlyingvolcanic flows. The most remarkable palaeoplacerdeposits
are the giant Witwatersrand deposits, which weredeposited in a
retro-arc foreland basin in essentially thesame time period as the
giant Late-Archaean orogenicgold provinces formed (Fig. 1a). One
reason for theirremarkable preservation is the regional extension
leadingto outpouring of Ventersdorp lavas that followed
theirdeposition. The Witwatersrand Basin also lies above theoldest
known Archaean lithosphere, on which there is themost complete
Archaean to early Palaeoproterozoicsedimentary record on earth.
Smaller, but significant, goldpalaeoplacers at Tarkwa, Ghana (Fig.
1a), in the giantAshanti gold province, were preserved in
Palaeo-proterozoic lithosphere.
Giant iron-oxide CuAu deposits relationship toArchaean
lithosphereFinally, the temporal distribution of giant
iron-oxideCuAu deposits may also be controlled by the distri-bution
of Archaean lithosphere. The earliest knowndeposits formed at ~2550
Ma in the Carajas Province,Brazil,5 whereas others are of
Mesoproterozoic age (e.g.Olympic Dam). They formed close to the
margins ofArchaean lithosphere, and are probably related toalkaline
igneous activity derived from the margins of this
Applied Earth Science (Trans. Inst. Min. Metall. B) August 2003
Vol. 112 B117
Session 1 Tectonic evolution
1 Timing of orogenic gold deposits versus periods ofcrustal
growth. (a) Distribution of major orogenicgold provinces with time:
from Goldfarb et al.3 (b)Temporal evolution of continental crustal
growth:from Condie2
-
The Uralide orogen is one of the major metallogenicprovinces of
the world containing world-class Cr,VMS CuZn, orogenic Au and
Fe-oxide skarndeposits. The VMS deposits alone contain
aconservative 55 Mt of contained metal and the Crreserves at
Kempirsai exceed 4000 Mt. The Uralideorogen forms the western part
of the Altaid collage, atthe boundary between the East European
craton andVendian to Palaeozoic magmatic arcs exposed in theKazakh
uplands and Tien Shan.5 Numerous studies of
Uralide tectonics and metallogeny identify major NSfaults
including the Main Urals Fault (Fig. 1),proposing that the latter
is the main suture separatingthe western and eastern slopes.1,4 The
western slope isuniversally recognised as a deformed
continentalpassive margin, whereas its eastern, oceanic, slope isa
combination of collided and welded magmatic arcs,sutured oceanic
basins and microcontinents. Mineraldeposits help better define the
tectonic evolution ofthe orogen and point to better regional
correlations ofmetallogenic terrains.
Broad domains of the Uralide orogen
Western oceanic complexesThe full section of the oceanic portion
of the orogen isexposed only in the Southern Urals. The
western-mostSakmara zone consists of Ordovician to
Silurianophiolites and immature arc rocks thrust onto
deformedpassive margin of the East European craton. These rockshost
major Cr and medium-sized VMS deposits. TheMagnitogorsk zone,
consisting of Devonian magmaticarc rocks that host major VMS
deposits,2 is also thrustwestward along the Main Urals Fault, but
in the east it istruncated by a west-dipping structure, which
defines itslens-shaped outline in plan view. To the east of this
faultis the NS-trending East Uralian linear megazone whichcontains
fragments of Precambrian, such as theMugodzhar microcontinent,
ophiolite sutures andmagmatic arc fragments.
Syn-collisional axial granitesThe orogen is stitched together
along its central axis byMiddle to Late Palaeozoic granites. The
pre-granitic
lithosphere, which were metasomatised during post-formational
tectonic events. They almost certainly owetheir preservation to
their siting in buoyant Archaeanlithosphere.
ConclusionsMineral deposits, because they reflect an
anomalousconjunction of processes, are important indicators
oftemporal changes in environments and tectonicprocesses. The
distribution of orogenic, palaeoplacerand iron-oxide CuAu deposits
reflects both theirselective formation and preservation at specific
timesin earth history. Of particular importance to
theirpreservation is the evolution from equidimensionalcratons of
relatively buoyant early Precambrian
lithosphere to elongate belts of negatively buoyantPhanerozoic
lithosphere. This, in turn, appears toreflect evolution from
episodic, strongly plume-influenced plate tectonics in the early
Precambrian tomore continuous, modern-style plate-tectonics
post-1750 Ma in response to a cooling earth.
References1. M. E. BARLEY and D. I. GROVES: Geology, 1992,
20,
291294.2. CONDIE: Tectonophysics, 2000, 322, 153162.3. R. J.
GOLDFARB et al.: Ore Geol. Rev., 2001, 18, 175.4. W. L. GRIFFIN et
al.: Proc. 7th Annu. V. M.
Goldschmidt Conf., 1997, 8283.5. F. H. B. TALLARICO et al.:
Econ. Geol., 2003, In press.
B118 Applied Earth Science (Trans. Inst. Min. Metall. B) August
2003 Vol. 112
Session 1 Tectonic evolution
Uralide orogenic evolution through the Palaeozoic and the link
to metallogeny: anupdated model
R. Herrington1, A. Yakubchuk1 and V. Puchkov2
1CERCAMS Group, Dept of Mineralogy, The Natural History Museum,
Cromwell Rd, London SW7 5BD, UK2Ufimian Science Centre, RAS Urals
Branch, Ufa, Russia
1. Schematic map: structure and key deposit types,Urals
DOI 10.1179/037174503225001668
-
rocks in this zone form NE-trending rhombic-shapedstructures,
truncated in the east by the west-dippingKartala fault. Magmatic
arc rocks of the East Uralianmegazone can be tentatively correlated
with similarrocks in the Magnitogorsk zone and possibly Tagilzone,
which would imply a sinistral offset for up to 300km northward
along the Serov-Mauk fault, whichwould, therefore, form a
west-dipping strike-slip faultrather than a thrust fault as it is
traditionallyinterpreted.
Eastern ocean-arc complexesThe eastern-most zone in the Southern
Urals is the Trans-Uralian zone followed to the east by the
Valerianov zone.The latter hosts currently unexploited porphyry
depositsand world-class Fe-oxide Kiruna-type deposits. Alongstrike,
the former consists mostly of Ordovician toDevonian accretionary
complexes and some magmaticarc rocks, also sinistrally offset along
the Kartala faultwith respect to the East Uralian megazone.
Major structuresThe major faults, such as the Kartala and
sub-parallelnetworks, control the distribution of both
granitoidsand orogenic Au deposit clusters. Recognition of
thesestrike-slip structures in the Urals suggests that pre-strike
slip positioning of the Tagil zone in the northernUrals would
reconstruct it as an isolated structureframed by Palaeozoic
ophiolites and Precambriancrustal fragments at the eastern and
western flanks,e.g. it is possible that like the Magnitogorsk zone,
theTagil zone may be completely allochthonous, whichmay be
supported by recent seismic data. It follows,therefore, that only
the western-most zones of theoceanic Urals preserve definite
collisional thruststructures, such features controlling
distribution of theallocthonous Sakmara and Tagil zones. Some of
theeastern zones are bound by inclined sinistral strike-slip faults
which may be late-, even post-collisional.
Broader correlation of the Urals with the AltaidsThe
relationship of the Uralides to the tectonicframework of the Altaid
orogenic collage continues to bethe subject of much debate, mostly
due to naturalisolation of this orogen by
MesozoicCenozoicsedimentary basins and the lack of competent
com-parative studies. Airborne magnetic data help toelucidate the
likely continuity of its magmatic arcs andaccretionary complexes
under these basins. Based onsuch data, the magmatic arcs of the
Urals start underMesozoicCenozoic sediments of the UstYurt
plateauto the south of the exposed Urals, continue northwardinto
exposed parts of the orogen, towards the PolarUrals, and then must
be traced south-eastward underMesozoicCenozoic sediments of the
West Siberianbasin towards Rudny Altai. There are problems with
thissince there are no analogues of the Tagil zone VMS beltin the
Altai-Sayan except for some Cambrian VMSdeposits far to the east in
the Tuva republic. The RudnyAltai hosts numerous Devonian VMS
deposits; but,since subduction polarity in the Rudny Altai and
the
comparative Magnitogorsk zone of the Urals is identical,there
are some geometrical problems. There are VMSdeposits in Kazakhstan
to the south of the Rudny Altaiwhich may be the time equivalents to
the Magnitogorskzone VMS deposits and the Rudny Altai may
correlatebetter with deposits in the Sakmara zone. LatePalaeozoic
oroclinal deformation events are common tothe Uralides and
Altaides, leading to the close structuralconnection and correlation
around the arc of theoroclinal closure apexing in the north Urals.
Theaccretionary complexes of the Trans-Uralian zone can
beconfidently traced into the IrtyshZaissan zone in Altai,defining
an extensive MugodzharRudny Altai arcdraped around an internal
magmatic arc with arcs of theKazakh uplands and the Tien Shan. Such
a suggestiondefines two arc systems in the western part of the
Altaids,accreted to the adjacent East European and Siberiancratons
which, in turn, assembled with the two distinctLate Proterozoic
orogens of the Pre-Uralides (Timanides)and Baikalides.
Implications of the new interpretationsIt follows that the
proposed strike-slip faults of theSouth Urals may also represent
relatively shortfragments of trans-regional fault systems that
mayextend from the Tien Shan to the Polar Urals and Arcticoffshore.
Such faults formed during the collision of theUrals and Kazakh
coupled with oroclinal bending of
Applied Earth Science (Trans. Inst. Min. Metall. B) August 2003
Vol. 112 B119
Session 1 Tectonic evolution
2. Evolution of South Urals during the Palaeozoic links to major
metallogenic events. (A) Primitive arc-related mineralisation
preserved in allochthons ofOrdo-Silurian oceanic complexes (CuZn
VMSdeposits). (B) Devonian oceanic arc complexes(CuZn and
polymetallic VMS deposits). (C) LateDevonian continental margin
magmatism (porphyryCu). (D) Early-Mid Carboniferous volcanism
atrifted continental margin (Fe-oxide, porphyry Cudeposits). (E)
Mid Carboniferous collision andgranitoid intrusion (rare metal,
orogenic Au deposits)
-
Phanerozoic orogenic gold deposits are invariablyassociated with
translithospheric compressional totranspressional-transtensional
shear zones. The depositsare primarily hosted by marine sedimentary
rocksaccumulated on continental margins or in arc-trenchsettings.
Mineralisation occurred post peak meta-morphic and is associated
with the exhumation of theterrain and generally predates the
emplacement ofgranitoids.2 The geodynamic settings of these
golddeposits corresponds to major accretionary processes,which are
typified by transpressive accretion of alloch-thonous terrains to
one another or a continentalmargin identifying Cordilleran style
tectonics.6
The large (> 300 t Au) orogenic Kochkar golddeposit is hosted
by a granite-gneiss complex, that isintrusive into supracrustal
rocks of the East UralianZone (EUZ). Although the deposit shares
manysimilarities with other orogenic gold deposits world-wide, it
is not associated with a major shear zone.7
In this contribution, we use geochemical, petrologicaland
isotope data to describe the magmatic andtectonometamorphic
evolution of the EUZ. The mainaim is to position gold
mineralisation in Kochkar into amodel for the orogenic history of
the Urals.
Regional geologyThe Urals are a bivergent, linear, NS trending
orogenformed during convergence and final EW collisionduring the
late-Palaeozoic.3,8,10,11
Geology of the East Uralian Zone (EUZ)The EUZ comprises
supracrustal rocks and intrusivegranitegneiss complexes. The
granitegneiss complexesare attributed to a long-term orogenic
magmatism,4 whichbegan pre-Middle Devonian by the intrusion of
TTGseries, coeval with Late Palaeozoic regional meta-morphism, and
lasted until the Middle Carboniferous.Younger calc-alkaline and
biotite granites intrudedbetween 290250 Ma into shallow crustal
levels post-dating regional metamorphism. This suite is considered
tobe the product of a large scale melting event in theEUZ.1,4,5
Geology of the Kochkar districtA number of deposits, that are
collectively referred to asthe Kochkar district, are hosted by the
Plastgranitegneiss massif, which is located to the south
ofChelyabinsk. The Plast lithologies are intrusive
intoupper-greenschist to mid-amphibolite facies supracrustalrocks.
The Borisov massif is formed by granitoid rocks tothe west of the
Plast massif. Numerous, steeply dippingmafic dykes describe an
overall radial pattern in the Plastmassif. The width of dykes
ranges from a few centimetresto > 20 m and strike lengths vary
from several tens ofmetres to > 15 km.7
Economic-grade gold mineralisation is hosted bysteeply-inclined
gold-quartz veins developed along aNS trending, 15 km long and 5 km
wide corridor. Theyare spatially associated with the mafic dykes
and occurmostly parallel and directly adjacent to these dykes.
The
the arcs during clockwise rotation of the adjacentcratons. Such
an architecture was likely to havedeveloped during Late Palaeozoic
times when theAltaid/Uralide collage finally assembled and when
theexternal arcs of the Kazakh uplands and Tien Shanbecame rotated
and pushed against the East Europeancraton.
Tectonic evolution and metallogenyLinkage of metallogeny to
tectonics is best explainedin the South Urals where the most
complete exposurefrom the western oceanic complexes, oceanic
andcontinental arcs and complex collision collage isassembled. The
simplified evolution after removingperceived strike-slip
complications resulting from theoroclinal bending in the Late
Palaeozoic can besummarised as in Fig. 2:
AcknowledgementsPart of this work has been funded by the
EuropeanCommunity, Cordis-RTD projects, 5th FrameworkINCO-2,
project number ICA2-2000-10011. This is aGEODE publication.
References1. G. S. GUSEV, A. V. GUSCHIN, V. V. ZAYKOV et
al.:
213295; 2000, Geodynamics and metallogeny: theoryand
implications for applied geology, Moscow, Ministryof Natural
Resources of the RF and GEOKART Ltd.
2. R. J. HERRINGTON, R. N. ARMSTRONG, V .V. ZAYKOV etal.: AGU
Monograph, 2002, 132, 155182.
3. V. A. KOROTEEV, H. DE BOORDER, V. M. NETCHEUKHINand V. N.
SAZONOV: Tectonophysics, 1997, 276, 391300.
4. V. N. PUCHKOV: Spec. Publ. Geol. Soc. Lond., 1997,
121,201236.
5. A. YAKUBCHUK, R. SELTMANN, V. SHATOV and A.COLE: SEG
Newslett., 2001, 46, 114.
B120 Applied Earth Science (Trans. Inst. Min. Metall. B) August
2003 Vol. 112
Session 1 Tectonic evolution
Orogenic evolution of the East Uralian granitegneiss terrain and
timing of goldmineralisation at Kochkar, Russia
J. Kolb, S. Sindern and F. M. Meyer
Institute of Mineralogy and Economic Geology, Aachen University,
Wuellnerstrasse 2, D-52056 Aachen, Germany([email protected])
DOI 10.1179/037174503225001677
-
massive and/or laminated, greyish to milky quartz veinsare, on
average, 021 m wide and show distinctlytabular geometries. They
contain, on average, 46 g/tAu. In contrast, associated alteration
zones show onlysubeconomic gold grades of < 1 g/t Au. Prominent
gold-quartz veins can be traced along strike for over 500 mwith
similar subvertical down-dip extents. Fluidinfiltration during
hydrothermal gold mineralisationwas promoted particularly along the
interface betweenmafic dykes and granitoids. Repeated dilation
normal todyke walls readily explains the dyke-parallel laminatedor
ribbon textures of gold-quartz veins. Mafic dykesacted as weak
layers during regional-scale EW directedhorizontal shortening.7
Wall-rock and alteration petrologySchollen- and raft migmatites
together with undeformedgranitoids of the Plast massif comprise
perthitic feldspar,plagioclase, quartz, microcline, biotite, and
muscovite.Accessories are titanite, apatite, zircon, rutile,
monazite,and ilmenite. Lithologies of the Borisov massif show
agranitic to gneissic texture and comprise microcline,plagioclase,
quartz, biotite, and subordinate muscovite,apatite, and zircon. The
gneissic fabric and recrystal-lisation textures of feldspars and
quartz in both massifsindicate a metamorphic overprint. Feldspars
are locallyaffected by saussuritisation and sericitisation, and
maficminerals are locally replaced by chlorite and rutile.
Mafic dykes display highly variable textures rangingfrom
fine-grained, massive, to porphyritic or stronglyschistose dykes.
The peak metamorphic hornblende-plagioclase assemblage was used to
calculate temper-atures to 635 40C. This metamorphic assemblage
isalmost invariably replaced by alteration mineralscomprising
biotite, actinolite, albite, K-feldspar, quartz,epidote,
tourmaline, and sericite. Gold-quartz veinscomprise quartz with
minor amounts of calcite, sericite,scheelite, biotite, tourmaline,
actinolite as well as pyrite,arsenopyrite, subordinate
chalcopyrite, sphalerite,fahlores, galena, and bismuthinite. Gold
is fine grained(< 20 mm) and mainly occurs as specks of free
gold inquartz.7 Biotite and tourmaline of the alterationparagenesis
were used to calculate the temperature ofgold mineralisation to 500
20C. A second retrogradeoverprint is indicated by the replacement
of formermineral assemblages by chlorite, green biotite,
andsericite, which reveals lower-to-mid greenschist
faciesconditions.
Wall-rock geochemistryThe modal variation of the Plast gneisses
are alsoreflected by their major element composition. SiO2ranges
between 6072 wt% and peralominosity between1114. Na2O/K2O ratios
(wt%) are highly variablebetween 03 and 42. As a consequence, some
samplesare trondhjemitic whereas the other Plast samples
aregranitic to granodioritic.
The Plast gneisses have low HREE-contents (Yb03416 ppm),
relatively low Y-concentrations (4719ppm) which, together with the
relatively high Al2O3,
gives them an adakitic character. However, Sr-concentrations are
variable (259508 ppm) and Sr/Y-ratios vary from 285 to 1081. This
deviates fromadakitic signatures which might be due to
hydrothermaleffects as well as the variation of Na2O/K2O
ratios.
The mafic dykes are heterogeneous with respect totheir major
element composition. Trace elementsignatures are most consistent
with a subductionrelated origin.
GeochronologyIn order to obtain information on the timing
ofmagmatism and the tectonometamorphic evolution,four samples were
chosen for geochronology. Zircons ofa Borisov massif sample were
dated at 362 24 Ma,which is interpreted as the intrusion age of the
granitoids.RbSr analyses of a least altered and an altered
Plastmassif sample indicate isotopic disequilibria for thevarious
mineral separates. This can be interpreted byopen system behaviour
during the retrograde meta-morphic evolution. RbSr analysis of
separates from aquartzsericite veinlet, which formed during
theretrograde greenschist facies event, yields an isochrondefining
an age of 265 3 Ma.
Model for the orogenic evolutionDating of the Borisov (362 24
Ma) and Plast massifs(341 20 Ma)9 indicate that these arc-related
granitoidsintruded the supracrustal rocks of the EUZ in the
EarlyCarboniferous. Field observations, however, show thatthe
intrusion of the Borisov massif caused radialfractures in the Plast
massif, suggesting that intrusionof the Borisov postdated that of
the Plast massif. Theradial fractures were intruded by mafic,
arc-relatedmelts in the Carboniferous.7 Amphibolite faciesmineral
parageneses and fabric development in thegranite-gneiss complexes
as well as in the mafic dykesdeveloped during doming and sinistral
shearing alongNS trending shear zones resulted from regional
EWcompression. Continuous compression caused shearingand gold
mineralisation along the mafic dykePlastgranitoid contact in the
lower amphibolite facies. Thetectonometamorphic evolution is
finalised by greenschistfacies veining at 265 3 Ma. Disequilibria
in the RbSrisotopic signatures of altered Plast gneisses at
Kochkarsuggest that the retrograde metamorphic evolutionoccurred
between 320265 Ma.
ConclusionsThe EUZ represents a mid-crustal section of an
islandarc and shows a complex orogenic evolution in a longlasting
EW compressional environment. Peakmetamorphic doming of the
granitegneiss complexeswas followed by a retrograde exhumation and
goldmineralisation at Kochkar, which coincides with theintrusion of
Permian granites. This and the fact thatthe hypozonal Kochkar
deposit is not hosted by amajor shear zone contradicts a
Cordilleran tectonicsetting. Instead, a post-collisional slab
rollback or
Applied Earth Science (Trans. Inst. Min. Metall. B) August 2003
Vol. 112 B121
Session 1 Tectonic evolution
-
The Caledonian hinterland of northern Britain com-prises mainly
Late Proterozoic metasediments andmetavolcanics (locally termed
Moinian and DalradianSupergroups), and Lower Palaeozoic (Cambrian
toLower Devonian) sediments and volcanics which havebeen
amalgamated during the Caledonian Orogeny. TheOrogeny resulted from
the complex closure of theIapetus Ocean mostly over the period
470390 Ma, theclosure lineament, the Iapetus Suture Zone extending
toNewfoundland, mainland Canada, and easternGreenland. On the basis
mainly of key faunal,structural, radiometric and geophysical
parametersthese Caledonian rocks have been divided up into anumber
of discrete terrains. Our work relates to fourterrains in
particular which are best known as (fromNS) the North Highland,
Grampian, SouthernUpland an Lakesman terrains.
A wide range of sulphur isotope studies have beenpublished for
the Palaeozoic and older terrains ofNorthern Britain over the last
20 years. A large datasetof d34S (in excess of 600 analyses) now
exists, fromsulphide disseminations in Caledonian intrusions
andtheir related mineralisations,3,9,12,13 base-metal
vein,stratiform and stratabound deposits,1,11,14,16,18 and pyritein
metasediments (e.g. Hall et al.8,9 and referencestherein). Our
analysis of this dataset highlightsdistinctive isotopic variations
which not only providefurther evidence for the delineation of the
putativeterrain boundaries, but also hints at deep crustal
andpossibly subcrustal variations across the region.
Here we discuss the data for four terrains from NSand assess the
environment of formation of thesulphides and sulphates and the role
of sedimentarysulphur in subsequent mineral deposit formation.
Northern Highland Terrain and ForelandThe foreland sequence
consists of cratonic basementoverlain by unmetamorphosed
Neoproterozoic andPalaeozoic sediments. Pyrite segregations in the
LateArchaean Lewisian gneiss basement of the forelandhave values
from 1 to +5. Lower Proterozoicmetasedimentary units of the Loch
Maree Supergroupin the cratonic sequence of the foreland are host
toboth disseminated and stratiform sulphides with d34Sbetween 3 and
1. A value of 3 has also beenrecorded for sulphide in the
terrestrial Torridoniansediments overlying the cratonic
sequence.
The Northern Highland cratonic margin terraincontains inliers of
cratonic basement directly beneath theMoinian succession, and this
shallow basement providesmuch of the sulphur seen within
mineralisation of theterrain. The Moinian sequence of generally
coastalsediments contains only minor primary sulphides withvalues
around +3 to +5.10 The Hebridean forelandlithologies and their
extension beneath the NorthernHighland Terrain are thus a source of
sulphur in therange 3 to +5.
Grampian TerrainThe Dalradian Supergroup, with its wide range
oflithologies representing changing environments ofdeposition has
the widest range of sulphide sulphurd34S of these terrains ranging
from 15 to +43.
The shallow shelf sequence of the lower AppinGroup is
characterised by d34S values ranging from+12 to +16.4,10
The middle Dalradian Argyll Group is notable forthe onset of
strong tectonic controls on sedimentation
delamination of the lithosphere possibly provides theheat source
for the large scale Carboniferous toPermian melting event in the
EUZ and the fluidplumbing system for gold mineralisation at
Kochkar.
References1. F. BEA et al.: Tectonophysics, 1997, 276, 103117.2.
F. P. BIERLEIN and D. E. CROWE: Rev. Econ. Geol., 2000, 13,
103139.3. H. P. ECHTLER et al.: Science, 1996, 274, 224226.
4. G. B. FERSHTATER et al.: Tectonophysics, 1997, 276,87103.
5. A. GERDES et al.: Int. J. Earth Sci., 2002, 91, 319.6. R.
KERRICH et al.: Rev. Econ. Geol., 2000, 13, 501551.7. A. F. M.
KISTERS et al.: Mineral. Deposita, 2000, 53,
157168.8. V. N. PUCHKOV: Geol. Soc. Lond. Spec. Publ., 1997,
121,
201237.9. V. N. SAZONOV et al.: Econ. Geol., 2001, 96,
685703.10. L. P. ZONENSHAIN et al.: Tectonophysics, 1984, 109,
95135.11. L. P. ZONENSHAIN et al.: Am. Geophys. Union
Geodynamics
Ser., 1990, 21, 2754.
B122 Applied Earth Science (Trans. Inst. Min. Metall. B) August
2003 Vol. 112
Session 1 Tectonic evolution
Sulphur isotope signatures of Neoproterozoic and Palaeozoic
terrains of NorthernBritain environments of formation
D. Lowry1, A. J. Boyce2, A. E. Fallick2, W. E. Stephens3, A. J.
Hall4 and N. V. Grassineau1
1Geology Department, Royal Holloway, University of London,
Egham, Surrey TW20 0EX, UK ([email protected])2Isotope
Geoscience Unit, Scottish Universities Environmental Research
Centre, East Kilbride G75 0QF, UK3School of Geography and
Geosciences, St Andrews University, St Andrews, Fife KY16 9AL,
UK4Archaeology Department, Glasgow University, Glasgow G12 8QQ,
UK
DOI 10.1179/037174503225001686
-
and it shows the widest d34S variations and generallyrepresents
the widening, deepening and eventuallyfailed rifting of the
Dalradian marginal basin. Thebase of the sequence is marked by the
Port Askaigglacial tillite. This was followed by a shift to a
muchwarmer climate for the overlying Bonahaven dolomitewhich has
exceptional pyrite values of up to +43.8
Globally, the seawater sulphate reached its highestd34S value
during this Snowball Earth period ataround +35 and the associated
fall in sea level mayhave marginalised the basins in this region
cutting offthe recharge of seawater sulphate. The following onsetof
rapid subsidence and localised basin developmentresulted in
contemporaneous deep and shallow watersedimentation, such as the
Easdale Slates of the westcoast5 and the Ben Eagach Schist which is
at a similarstratigraphic level further east15,16 with d34S of +12
to+22. The d34S values of seawater sulphate remainedheavy as seen
in the schist-hosted syn-sedimentaryAberfeldy barite and the
smaller BaPbZn Loch Lyonhorizon with sulphide d34S values range
from +16 to+26 and barite values from +27 to +40.16,18
After a period of basin filling, the major period ofbasin
opening and failed rifting commenced with shelfmuds, now
represented by the Ardrishaig phylliteswhich have d34S of 15 to
1.7,18 This is followed byan enrichment in 34S again, through the
Ben LawersSchist (4 to +4),15 the Crinan Grit (+1 to+8)10,18 and
Ben Challum Quartzite units (+8 to+15).15 The Southern Highland
Group of theDalradian includes pillow lavas, limestones and
gritswhich have d34S values between 2 and +4.18
Within the Dalradian sequence as a whole, only
theArdrishaig/Craignish phyllite units have values typical ofopen
system bacterial reduction (fractionation of 3550from seawater
sulphate values) suggesting that there wasrecharge of seawater
sulphate and possibly an opening toIapetus. The other units are
dominated by fractionationsrelative to seawater of 1030,
representing closed- orsemi-closed-system bacterial reduction.
Southern Upland TerrainThe main sulphur isotopic characteristic
of this terrain isa depletion in 34S. Ordovician diagenetic pyrite
in theMoffat Shale Unit has d34S of 17 to 0, representingopen
system reduction of Ordovician seawater sulphate(+28)2 to H2S and
then pyrite.
1
Lower Carboniferous PbZn-rich vein systems hostedby the Lower
Palaeozoic sediments at Wanlockhead andLeadhills (and additionally
the Salterstown mineralisationin NE Ireland) have d34S of 10 to
3.1,14 TheSouthern Uplands Terrain and its continuation
intoLongford Down, Ireland, therefore represents a 34S-depleted
reservoir.
Lakesman TerrainThe main sulphur isotopic characteristic of this
terrain11
is an enrichment in 34S. The dominantly argillaceaousOrdovician
Skiddaw Group formed in an arc basinwhich was closed to, or only
periodically recharged with,
Ordovician seawater sulphate resulting in diageneticpyrite d34S
values of +11 to +28 formed by closedsystem bacterial reduction of
H2S.
Devonian and Carboniferous veins hosted by theOrdovician
volcano-sedimentary sequence with d34S of+13 to +23 represent
partially homogenised sulphurfrom a Skiddaw Group (or similar)
source.
SummaryThe d34S data in these terrains highlight manyimportant
features, but two are particularly relevantto this discussion.
First is the distinct variation intenor across the Iapetus Suture
Zone, in lithologies ofthe same age, implying a radical difference
in thehistory of diagenetic sulphide on either side of
thepalaeo-ocean. The lower Palaeozoic Lakesman andSouthern Uplands
terrains represent sources of 34S-enriched and 34S-depleted
sulphur, respectively, whichhave been related to different modes of
bacterialseawater sulphur reduction either side of the
IapetusSuture.11
Second is the large within-terrain variation across theDalradian
Supergroup which reflects basin developmentand climate controls.
Where estimates of seawatersulphate d34S are possible in the Argyll
Group, they areconsistent with the global record for this period of
theNeoproterozoic reaching all time highest values by
thePrecambrian-Cambrian boundary.17 Proponents ofSnowball Earth
hypotheses surmise that globalglaciation would cause massive
fluctuations in seawaterd34S. This was followed by
tectonically-controlled basins,rifting and basic magmatism with
large fluctuations inwater depth and rates of sulphate
recharge.
The features of the d34S data in these terrains can all
berelated to the upper crustal sulphur, with the exception
ofmineralisation hosted-by sub-crustally sourced intrusives.The
disseminated diagenetic and metamorphic sulphidescan be related to
their environment of formation,controlled by the d34S of seawater
sulphate (possiblyinfluenced by global glaciation) and basin
tectonicscontrolling the sedimentation depths and the continued
orrestricted supply of sulphate. These changes over theperiod from
750450 Ma have given the terrains their ownpersonal d34S tenor,
often traceable laterally for hundredsof kilometres. This tenor is
the primary control on d34S ofsubsequent SilurianCarboniferous vein
mineralisation.The dominant upper crustal units in each terrain
providethe sulphur for local copper, lead and zinc
veinmineralisation, being partly homogenised during theprocesses of
scavenging and transport.
References1. I. K. ANDERSON et al.: J. Geol. Soc. Lond., 1989,
146,
715720.2. G. E. CLAYPOOL et al.: Chem. Geol., 1980, 28,
199260.3. T. A. FLETCHER et al.: J. Geol. Soc. Lond., 1989,
146,
675684.4. A. J. HALL et al.: Chem. Geol., 1987, 65, 305310.5. A.
J. HALL et al.: Mineral. Mag., 1988, 52, 483490.6. A. J. HALL et
al.: Chem. Geol., 1991, 87, 99114.7. A. J. HALL et al.: Scott. J.
Geol., 1994, 30, 6371.
Applied Earth Science (Trans. Inst. Min. Metall. B) August 2003
Vol. 112 B123
Session 1 Tectonic evolution
-
Recent advances in understanding the processes thatform
Mississippi Valley-type (MVT) PbZn-(FBa)deposits clearly point to
specific environments andperiods of mineralisation. These new
advances, inconjunction with knowledge of the regional geology,may
help to identify new areas for the exploration ofmineral resources.
The primary objective of thisabstract is to present the
explorationist with a shortsynopsis of the key factors necessary to
form aMississippi Valley-type PbZn deposit/district.
Geochronology
Host rocksIt has long been recognised that MVT deposits occur
inplatformal carbonate successions. While the age ofmineralisation
is difficult to determine, the age of thehost rocks can be
established with a fair degree ofcertainty. Fig. 1 illustrates that
MVT district are hostedby carbonate rocks ranging in age from the
Neoarchaeanto Tertiary. A histogram of host rock ages reveals
that~60% of MVT deposits are hosted by Palaeozoic rocks,while 25%
are hosted by Precambrian rocks, and 15% byMesozoic and Cenozoic
rocks (Fig. 1). As illustrated inFig. 1, the Palaeozoic is marked
by maxima in theCambro-Ordovician and Devono-Carboniferous,
yetdeposits are less common in rocks of Silurian andPermian age.4
When compared with palaeogeographicreconstructions of the earth,7
it becomes clear that duringthe Cambro-Ordovician and
Devono-Carboniferous thesupercontinent Laurentia was located in
equatorialregions, thereby allowing for the formation of
thickcarbonate sequences.
MineralisationRecent advances in age-dating techniques have had
aprofound impact on our understanding of MVT deposits.However, only
a short summary will be provided. For amore detailed analysis, the
reader is referred to Leach et
al.5 MVT deposits world-wide appear to have formedduring four
periods Palaeo- and Mesoproterozoic,Devonian to Permian, and
Cretaceous-Tertiary (Fig. 2).The most important periods of MVT
genesis appear to bein the Palaeozoic Era.5 They note that this is
exemplifiedby more than 75% of the combined metal produced arefrom
deposits that have dates that correspondtoDevonian through Permian
time. Indeed, if the ageof mineralisation is compared with
palaeogeographicreconstructions, it becomes quite clear that
thesemineralising events closely correspond in time withconvergent
orogenic events, thereby indicating that theformation MVTs is not
simply a passive process, butrather a dynamic one.
B124 Applied Earth Science (Trans. Inst. Min. Metall. B) August
2003 Vol. 112
Session 1 Tectonic evolution
New insights into the exploration for Mississippi Valley-type
leadzinc deposits
C. R. McClung1, D. L. Leach2, J. Gutzmer1, D. Bradley3 and S.
Gardoll4
1Paleoproterozoic Mineralization Research Group, Department of
Geology, Rand Afrikaans University,
PO Box 524, Auckland Park 2006, South Africa
([email protected])2US Geological Survey, PO Box 25046, DFC MS
973, Denver, CO 80225, USA3US Geological Survey, 4200 University
Drive, Anchorage, AK 99508, USA4Centre for Global Mineralogy,
School of Earth and Geographical Sciences, University of Western
Australia,
35 Stirling Highway, Crawley, WA 6009, Australia
1 Age distribution of MVT host rocks. Scale at leftrefers to
number of districts. Here we describe adistrict as a group of
deposits that share a closegeographic association. Therefore a
single deposit(e.g. Pering) is classified as a district, as well as
agroup of geographically close deposits (e.g. SEMissouri). Modified
from Sangster6
8. A. J. HALL et al.: Mineral. Mag., 1994, 58, 486490.9. R.
LAOUAR et al.: Geol. J., 1990, 25, 359369.10. D. LOWRY: Unpublished
PhD thesis, St Andrews
University, 1991, 1625.11. D. LOWRY et al.: J. Geol. Soc. Lond.,
1991, 148, 9931004.12. D. LOWRY et al.: Trans. R. Soc. Edinb.,
1995, 87, 221237.13. D. LOWRY et al.: Appl. Earth Sci. (Trans.
Inst. Min. Metall.
B), 1997, 106, 157168.
14. R. A. D. PATTRICK and M. J. RUSSELL: Mineral. Deposita,1989,
24, 148153.
15. R. A. SCOTT et al.: BGS Stable Isotope Report, 1987,
130,140.
16. R. A. SCOTT et al.: Trans. R. Soc. Edinb., 1991, 82,
9198.17. G. A. SHIELDS: Geophys. Res. Abs., 2003, 5, 10859.18. R.
C. R. WILLAN and M. L. COLEMAN: Econ. Geol., 1983,
78, 16191656.
DOI 10.1179/037174503225001695
-
Tectonic environmentAs recently noted by Bradley and Leach,1 it
has onlybeen within the last 20 years that the connectionbetween
MVT mineralisation and plate tectonics wasrealised. Leach and
Rowan3 and Bradley and Leach1
illustrated that most, but not all, MVT depositsappear to form
in foreland basins or in rocks thatformed in a foreland
environment. Despite the factthat MVTs may form in foreland basins
or in forelandbasin rocks, one thing is clear, locally within
thedeposit or district, most MVTs are controlled by deep-seated
structures. Bradley and Leach1 describe somecharacteristics of
these structures. Regardless of howthese structures form, it is
important to note that theydo localise mineralisation (i.e. Irish
Midlands, USMid-continent, Metaline, Cracow-Silesian,
UpperMississippi Valley district, and many others).
Fluid flowAlthough several fluid flow mechanisms have
beenproposed, most of these can be grouped into either: (i)sediment
diagenesis and compaction; or (ii) tectonicforce.4 Sediment
diagenesis and compaction have beenproposed as a viable mechanism
to generate the fluidflow required to move MVT ore fluids. However,
(asargued in Leach and Sangster4) this method is unlikelyto provide
adequate discharge rates and heat neededto form MVT
mineralisation.2 In the alternativemodel, during plate convergence,
uplift along theflank of a basin will cause groundwaters, recharged
inthe highlands to migrate through the deep portions ofthe basin,
collecting heat and metals, and discharge atthe opposite side of
the basin.1,2,4
Contained metalsAs stated above, over half of all known MVT
depositsoccur in the Palaeozoic with maxima during the early
andlate Palaeozoic. Similar to Fig. 1, a plot of host-rock
agesversus contained MVT Zn + Pb metal indicates maximain the
Cambro-Ordovician (~35% of known MVT metal)and Devono-Carboniferous
(~45% of known MVTmetal). In comparison, the Precambrian contains
~5% ofknown MVT metal, while the Mesozoic contains ~15% of
known MVT metal (Fig. 3). All contained metal valueswere
calculated from published grade and tonnage values;however, not all
known deposits/districts have publishedgrade and tonnage values.
Therefore, not all MVTdeposits/districts are included here.
Implications for explorationIt has become clear that MVT
formation is mainlyrestricted to two windows of geological time
Devono-Permian and Cretaceous-Tertiary. In theirevaluation of MVT
deposits world-wide, Leach et al.5
determined that these windows of time correlate withthe
assimilation of Pangea (Devono-Permian) and theNorthern Hemisphere
AlpineLaramide orogenies ofthe Cretaceous-Tertiary. Based on this
and other linesof evidence, Leach et al.5 and Bradley and
Leach1
concluded that MVT mineralisation is directly relatedto major
fluid flow events associated with global-scalecollisional
orogenies.
As illustrated in Figs. 1 and 3, over half of all knowMVT
occurrences and ~80% of the combined metal occursin Palaeozoic
carbonate host rocks. Therefore, whenexploring for major MVT
deposits or districts one shouldconcentrate on Palaeozoic
platformal carbonate succes-sions that have been later affected by
a collisional orogeny(preferably Pangean or Cretaceous-Tertiary in
age).
References1. D. C. BRADLEY and D. L. LEACH: Mineral. Deposita,
2003,
DOI 10.1007/s00126-003-0355-2.2. S. GE and G. GARVEN: J.
Geophys. Res., 1992, 97,
91199114.3. Leach, D. L. and Rowan, E. L: Geology, 1986, 14,
932935.4. D. L. LEACH and D. L. SANGSTER: Geol Soc Can. Spec.
Paper 1993, 40, 289314.5. D. L. LEACH et al.: Mineral. Deposita,
2001, 36, 711740.6. D. L. SANGSTER: Appl. Earth Sci. (Trans. Inst.
Min.
Metall. B), 1990, 2142.7. C. R. SCOTESE: Atlas of Earth history,
vol. 1, Paleo-
geography, PALEOMAP Project, 2000, Arlington, Texas.
Applied Earth Science (Trans. Inst. Min. Metall. B) August 2003
Vol. 112 B125
Session 1 Tectonic evolution
2 Age distribution of MVT mineralisation. Scale atleft refers to
number of districts. Modified fromLeach et al.5
3 Age distribution of MVT host rocks versus theamount of
contained Zn+Pb metal
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The hydrothermal systems responsible for the generationof
orogenic gold deposits5 are associated with sub-duction related
accretion.7 Under these conditions,major thermal anomalies occur
involving fluid liberationand widespread granitoid intrusions.
These granitoidscan, therefore, be correlated with both the fluid
liber-ation and subsequent gold mineralisation.
This paper describes the tectono-magmatic evolutionof the
Hutti-Maski Greenstone Belt (HMGB), DharwarCraton, India. It was
here that the key environment wasobtained for the development of
Indias largest goldmine (Hutti Gold Mine). New UPb zircon
SHRIMPages of the syn-tectonic Kavital granitoid in addition
tofelsic metavolcanic host rocks, add to the
structuralinterpretation of the belt and, therefore, the timing of
thegold mineralisation.
Geological settingThe Dharwar Craton can be subdivided into
easternand western blocks, due to lithological
variations,differences in volcano-sedimentary
environments,magmatism and the grade of metamorphism. Thecontact is
formed by an elongate body of granitetrending NS to NWSE, the
Closepet granite.9
The basement of the western block is characterisedby 2933 Ga10
tonalitetrondhjemitegranodioriteintrusives referred to as the
Peninsular Gneiss.13
Volcanic and sedimentary schistose units in thePeninsular
Gneiss, the Sargur Group,13 have beendated between 29633 Ga.11
Younger, volcanic andsedimentary rocks collectively termed the
DharwarSupergroup13 were deposited between 2926 Ga.13
In c