Variable Source: Pb isotopes reveal basement influence on mineralisation at 1590 Ma Ben Nicolson Geological Survey of South Australia - Discovery Day 2016
Variable Source: Pb isotopes reveal basement influence on mineralisation at 1590 Ma
Ben NicolsonGeological Survey of South Australia - Discovery Day 2016
Background
Pb isotopes in isotope geochemistry
Two principal uses
• First used historically to determine the ages of rocks and
minerals, and still the case (geochronology)
• Secondly, in petrogenetic studies to identify geological
processes and sources (isotope geology)
2
Radiogenic isotopes provide a means to sample the
deep interior of the earth, which otherwise can only
be studied indirectly using geophysics
Background
Isotope geology and Pb-isotope
evolution (growth curves)
Average Growth Curves:
Indicated by studies on large populations from
similar geological settings; e.g. Doe & Zartman,
1979; and Plumbotectonics model (at left);
Zartman & Doe, 1981)
3Plumbotectonics model lead-isotope evolution curves (Zartman & Doe, 1981)
206Pb/204Pb
20
7P
b/2
06P
b
A = mantle
B = orogene
C = upper crust
D = lower crust
D
A
BC
CB
AD
20
8P
b/2
06P
b
206Pb/204Pb
4
Pb isotopes: Variable Source
Mixing of crustal and mantle reservoirs into the orogene source reservoirAfter Doe and Zartman (1981).
In assessing origin, we expect that several trends will become
apparent (no single curve is sufficient in an area of such
complexity) that all represent different lead reservoirs
ab
c
d
5
Regional Geology & Mineral OccurrencesNumerous
mineral
prospects that
contain Pb
~100 Pb-Zn-Ag
occurrences
Clusters along
southern
margin of GRV
and in Cleve
uplands
Au & Cu association??
NuytsDomain
Cleve-Coulta Domain
6
Regional Geology & GeochronologyLarge number
of dated sites
on Eyre Pen.
Selected: ~70 Represent:
•Mesoarchean
granites
•Sleaford
Complex
•Kimban-aged
intrusions
•Hiltaba Suite
•Gawler Range
Volcanics
NuytsDomain
Cleve-Coulta Domain
7
Nd Data (TDM)
Wide range in ages calculated from Nd values
Based on previous studies, variations in Pb ratios tend to follow similar trends
K-spar sample medium: comparing like samples
NuytsDomain
Cleve-Coulta Domain
The source of metals
- previous studies
8Department of State Development
Dominated by Upper Crustal Source
Lower Crust
Mantle
Orogene
Roache (1996)
20
7P
b /
20
4P
b
206Pb / 204Pb
Focussed on signatures of
mineralisation
~250 analyses from Menninnie Dam
~50 analyses from other Pb prospect
Roache 1996:
• Gawler Range Volcanics / Hiltaba Granites
• Hutchison Group
Numerous additional sources possible include:
Kimban-aged intrusions
Sleaford basement (reworked in the Kimban)
Limited work on feldspars regionallyWork was site-specific relating mineralisation to local sources
Gawler Craton Pb isotopes – recognised as not fitting to general curves readilySomething different about the Galwer Craton evolution??
Comprehensive review of geochemical
data on regional geochronological
samples shortlisted 23 granite or felsic
gneiss samples for analysis
Considerations:
Granites with highest Pb contents
Lowest U contents
Presence of K-feldspar*
Minimal alteration**
9
Isotope Geology: determining our sourcesFirst two points are straight forward
• * K-spar isn’t always present, or preserved intact.
Many samples contain significant perthite lamellae
(intergrown Na-spar).
• ** The region has undergone at least two major
orogenies and one major volcano-plutonic event.
Metasomatic alteration, regional isochemical
calcsilicate formation are particularly strong effects of
regional metamorphism. Disturbance to older rocks is
widespread, in some cases causing severe problems
with interpretation of data in this study.
• Despite attempts to sample unaltered rocks, many
samples have disturbed ratios and are unusable,
such that an iterative sampling & analysis approach
to obtain usable samples is required
• This trend towards greater disturbance correlates
with increasing age, however,
there have been a few
surprises!
10
1.1 Ga
1.6 Ga
2.0 Ga
1.6 Ga Note: Age points in 0.1 Ga increments
ResultsPreliminary interpTwo distinct sources:
11
Results
1.1 Ga
1.6 Ga
1.6 Ga Note: Age points in 0.1 Ga increments
Preliminary interpTwo distinct sources:
1) Upper curveDefine a source with a 238U/204Pb (µ) valueestimated to be ~13.7 (very high)Reflecting strongly on the Kimban OrogenyEven completely rehomogenising Pb in some rocks of Neoarchean age
2.0 Ga
12
1.1 Ga
1.6 Ga
2.0 Ga
1.6 Ga Note: Age points in 0.1 Ga increments
ResultsPreliminary interpTwo distinct sources:
2) A more juvenile source, which closely follows the S-K curve, and indicates a source with a 238U/204Pb (µ) value closer to 10 (less clear at this stage), but includes some Hiltaba Granites
13
1.6 Ga
1.1 Ga
2.0 Ga
Mineralisation trends follow the feldspars:Menninnie TS, intersects upper curve at an age of ~1590 Ma!Other mineral zones appear to follow the more juvenile curve (possibly Paris)Others are mixed
Results - Mineralisation
Note: Advanced argillic alteration (alunite) Ar-Ar, dated @ 1580 Ma, spatially related to GRV (Nicolson et al, in review).
14
Still early days, but:Origin of Pb at Menninnie fits with pattern of upper crustal (supracrustal) to Orogene source (Hutchison Group & Kimban-aged granites of Peter Pan Supersuite)Our contention is that in Eyre Peninsula, Hiltaba-GRV contributed mainly heat, but little PbParis – more complex, very limited data mixed source including GRV-Hiltaba, (nearby alteration at Nankivel Hill constrains mineralisation to
Conclusions
No Pb-isotope ratios (galena) to suggest pre-1590 Ma mineralisation was identified.Anecdotally, this may be a function of preservation. Why?Geobarometry indicates basement rocks of the modern-day surface were formed at ~550 MPa. This is equivalent to a depth of ~17 km. This is a ductile environment and exceeds the depth at which hydrothermal deposits form (optimally, they form at 0-15 km depth)•Quartz veining (apart from scattered veins associated with radiogenic occurrences, an
indication of hydrothermal activity) is notably absent on central Eyre Peninsula
No mineralised samples
Mineralisation, tailing towards increasingly radiogenic ratios
~1580 Ma?)
15
Further Work• The Gawler Craton is a large area; what applies in one area may not apply elsewhere• Highly variable degrees of metamorphism are preserved across the craton• Work commenced on an area containing elements of the Archean origin of the Craton
which has seen two very significant orogenic cycles, and generation of a SLIP• All of these events likely produced mineralisation• Other parts of the craton, which clearly demonstrate a different history are most likely
going to be prospective for different styles of mineralisation (i.e. areas with a strong juvenile component are more prospective for magmatic-hydrothermal deposits including VHMS and porphyry-style deposits.
• Continued regional assessment of Pb isotopes will help resolve the origin of lead (and by inference) other metals.
• May be a useful, predictive tool to apply to areas of limited exposure and without known mineralisation as a tool to assess mineral potential.
Thankyou!
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