Are the Olympic IOCG and Central Gawler Gold provinces merely …energymining.sa.gov.au/__data/assets/pdf_file/0019/... · 2019-10-24 · I present this talk as an independent geologist

Are the Olympic IOCG and Central Gawler Gold provinces merely the more intensively explored parts of a larger Hiltaba-GRV IOCG province

– THAT is the question!

TelluricMagneto

Skirrow et al., G3 2018, Figure 1.

I present this talk as an independent geologist who once spent more than a decade working at and researching the Olympic Dam IOCG deposit (and researching many other IOCG deposits), and more than a decade exploring in and near the Gawler Ranges, eastern central Gawler Craton, South Australia -arguably western central Gawler Craton because remnants of the eastern margin of what was the Gawler cratonic block at ~ 1.59Ga are now likely located in the Curnamona block, and probably further east beneath the Delamerian Orogen based on the presence of inherited 2.5Ga zircons in Palaeozoic granites. This is at odds with some claims that deposits in the Olympic IOCG Province formed at a craton margin.

Blame for the pseudo-Shakespearian title of this talk rests partly on the pseudo-Shakespearean title conjured by Justin Gum, previous speaker in this workshop. To both of us, Shakespeare would no doubt reiterate “I do desire we may be better strangers”. The quotes from James Hutton and Sam Harris are verbatim.

I have deliberately borrowed several of the diagrams used early in this talk from the publications and presentations of others and annotated them (mostly adding a “?” or two) as examples to illustrate my point that for decades there’s been a strong inference that where IOCG deposits are known “to be” is the only place they will be, and everywhere else they’re “not to be”. This thinking (and much research focus) has been clagged in the province trap!

Borrowed diagrams used in the latter part of this talk are from sources more open to the ideas and observations expressed here.

I gratefully acknowledge permission from Terramin Australia Ltd, Freeport McMoRan Ltd and BHP Ltd to speak generally about some locations on their tenements and include photos of rocks from these.

Disclaimer / Combobulation

High potentialfor discovery of near-surface IOCG deposits– by simply hunting stones!

Central GawlerGold Province~ the western limit of the IOCG province ?

GawlerCratonMargin

OLYMPICIOCGProvincezone of world-class IOCG deposits

and some clagged thinking

High potential for discovery of new deeply buried IOCG depositsrequiring bleeding edge technologies & Tier 1 budgets

“Lord pity the arse that's clagged to a head that will hunt stones”

GRV

0 100

km

Cariewerloo Basinprohibitively thick sedimentary cover ?

Gawler Ranges

Multistage hematite-rich heterolithichydrothermal breccia cropping out in Gawler Ranges GRV (note GRV clasts)

James Hutton

Skirrow et al., G3 2018, Figure 1.

Note the band of A-type Hiltaba granites along the eastern, southern and western margins of the GRV.

Additional Hiltabagranites known to occur within and adjacent to the southern GRV margin are not shown here.

Skirrow et al., G3 2018, Figure 11

Gawler

Note:The interpreted zone of metasomatisedSCLM (yellow) extends well to the west of the Olympic IOCG province - into the Central Gawler Craton

Ranges

Skirrow et al., G3 2018, Figure 4a

?

?Potential for additional IOCG deposits on the fringes of the C1 conductor

Skirrow et al. G3 2018 Figure 12

?

?

Potential for IOCG deposits above the southern and western parts of the metasomatised SCLM

huntstones here!

are these epithermal break-outs through upper GRV lavas

capping high-level IOCG hydrothermal systems within or beneath the lower GRV?That is another question!

Interesting ‘stones’ : hematite-quartz veins & brecciasfrom the upper GRV, exposed in the central Gawler Craton ….

Variably siliceous hematite-rich breccias with multi-generation epithermal quartz, quartz-hematite and hematite fragments + variably silica-hematite altered GRV fragments: bulk compositions up to ~60% Fe with variably anomalous LREE, Ba, Cu, Au, Mo, As, Sb, Bi, V, Ag, Pb, ± Cr contents

Olympic Dam DrillingGawler Ranges Outcrop

Olympic Dam Drilling Gawler Ranges Outcrop

Hematite-quartz-GRV breccias & veins:Gawler Ranges outcrops

Macro photograph of polished thin sections Note multiple phases of veining and brecciation

Copper in gossanous quartz-hematite vein, Gawler Ranges

Magnetics

Gravity

Data

Model

Model

Data

Example of a MagGrav-J Model Section- southern Gawler Ranges

Preliminary model data supplied by J Hanneson, Adelaide Mining Geophysics, 2018

*

*

* *

Major early shear zones – low P/T cataclasis – persisted intermittently until Delamerian (major lineaments through GRV, shear zones to west)

GRV magmatism was penecontemporaneous with main-stage hydrothermal brecciation and metal deposition

Felsic GRV fragments in ODBC hematitic breccias - some barely altered

Zones of intense silicification and metal depletion overly ore zones

Epithermal quartz-hematite veins are preserved in peripheral areas of the ODBC – comparable to veins in Gawler Ranges GRV

Epithermal quartz vein fragments occur in ODBC heterolithic hematite-rich breccias

Which ‘stones’ are important at Olympic Dam?This talk focuses on six of the many aspects of Olympic Dam Breccia Complex (ODBC) geology that lead to exploration for analogues in the Gawler Ranges:

Early cataclasis of granite

Shearing of cataclasite+ hem-sericite alteration

Foliation of mafic-ultramafic dykes injected in fault zones (micro wov 5mm)

Multi-stage cataclasite-hematite-sulphide breccia

Granite cataclasite clast in sheared hematite-sulphide-cataclasite breccia

Cataclasite matrix replaced by red-brown hematite + sulphides

Multi-stagemineralisation disrupted by multistage faulting

py-cpy

bn-cc

py-cpy

cpy-bn

bn-cc

bn-cc

hematitic clasts in altered granite-dominated cataclasitematrix

Granitic cataclasiteclasts in Cu-rich hematiticmatrix

Physical mixing of disequilibrium sulphide associations

Delamerian (~500Ma) faulting (reactivation?) with barite-fluorite fault-vein infill

- underground at OD

Ehrig et al, Garry Davidson Symposium, , Hobart, 2018

Increasing recognition of moderate displacement multi-age faulting events within and near the ODBC

Ehrig et al, Garry Davidson Symposium, , Hobart, 2018

TERRAMIN AUSTRALIA LIMITED SLIDE No. 25Multi-stage hematite-rich hydrothermal breccia containing fragments of GRV (v) & epithermal quartz veins (q)

Millions oftonnes@ ~10g/t Au

Hundreds of millions of tonnes @ ~2-3% Cu Billions of

tonnes@ ~1% Cu

low Cu no Au

modified from WMC Resources Ltd, 1985

TERRAMIN AUSTRALIA LIMITED SLIDE No. 26

Hem-qz

XS C-DLocation ODBC Plan

ODBC Section

Ore zone “continuity” and deposit symmetry are gross simplifications.

Ore is multi-stage and multi-style (in detail).

Barren hematite-quartz breccias are widespread (and typical) above sulphide mineralised zones

modified from WMC Resources Ltd, 1985

cropped from Curtis & Wade, RB2015-21

from Ehrig et al., 2018 GDS

lower GRV

upper GRV

Note the widespread distribution of lower GRV, and the occurrences of GRV at (hosting) Acropolis and near Wirrda Well (see inset).

IOCG alteration penecontemporaneouswith GRV magmatismIOCG/IOA veining within GRV – Acropolis Prospect

Ages

OD - GRV

from Jagodzinski 2016

from Ehrig SAEMC 2017

from Hayward and Skirrow (2010)

? intruded its lower GRV extrusive equivalents

lower GRV, including reworked volcaniclastics, possibly capped (~5Ma later) by upper GRV.

GRV mostly removed by Marinoan(± Sturtian) glaciation(s)

Likely formed a local maar depression in a regionally extensive ephemeral lacustrine basin (salar)

Gawler Ranges – so little effective drilling!

MSDP05

Thick Cariewerloo Basin cover has deterred exploration west of the Olympic IOCG Province, and perceptions of thick barren upper GRV has deterred drilling to target lower GRV-hosted IOCGs further west.Fieldwork and MSDP drilling have confirmed that hydrothermal activity was widespread in the upper GRV

Cariewerloo

Basin

Gawler Ranges

Modified from Reid, Minerals 2019

Modified from Curtis & Wade DEM RB2015-21

Modified from Flint 2016

Published GRV Stratigraphy

~1587 Ma

~1592 Ma

Suggested stratigraphy in southern Gawler Ranges

Menninnie IgnimbriteMain stage IOCG

Minor IOCG“breakouts”

?

Welded ignimbrite, tr py, intensely altered locally= Menninnie Ignimbrite?

Ag-Pb-Zn (W),Au-Mo, Mt-Au-Cu, Sn

IOCG Exploration Target Models– Gawler Ranges

Cc

Bn

Cp

Py

Epithermal quartz-hematite veins & breccia outcrops

(high LREE low Au-Cu)

Current SurfaceStructure hidden under cover

High grade gold zone? High

grade Cu-Au zone

Lower grade

Copper zone

Au

upper GRV

lower GRV

Predicted Gravity Anomaly Profile

? 50mto

? 200m

1587 Ma

1593 Ma

~ 6 My u/c

Native gold : high grade

Chalcocite ± bornite (Au)Bornite ± chalcocite (Au)Chalcopyrite (Au)Pyrite

Metal Zones

Cc

Bn

Cp

Py

Epithermalquartz-

hematitevein float

(high LREE, low Au-Cu)

Current Surface

breccias beneathalluvium?

High grade gold zone?

Lower grade

Copper zone

Au

Native gold : high grade

Chalcocite ± bornite (Au)Bornite ± chalcocite (Au)Chalcopyrite (Au)Pyrite

Metal Zones

upper GRV

lower GRV

Predicted Gravity Anomaly Profile

50mto

100m 1587 Ma

1593 Ma~ 6 My u/c

Faults active pre-and post-

upper GRV

Post-1587Ma fault movements produced shear and shatter zones in upper GRV that have been preferentially eroded to form geomorphological depressions and linear valleys throughout the upper GRV.

Some of these alluvium-draped depressions have abundant hematite-rich breccia float and ?subcrop, but gravity stations spacings are broad 2km +

High grade Cu-Au zone

1. upper GRV are locally thin, with inliers of lower GRV exposed- supported by Archimedes’ remodelling of the magnetic and gravity datasets, but not confirmed by drilling yet; lower GRV sintersand geyserite are exposed where upper GRV were thought to be ~ km’s thick

2. The regionally extensive, porous, highly reactive unwelded outflow equivalent of the lower GRV ignimbrite hosting the Viper Pb-Zn deposit is potentially a “trap rock” for additional hydrothermal mineral deposits :- IOCG breccias, Ag-Pb-Zn replacement types and Ag-rich deposits.

3. Porous reactive ignimbrite is extensive at the lower GRV / upper GRV boundary (unconformity)

4. IOCG-style hematitic breccias occur in outcrop and float in the Gawler Ranges

5. Olympic Dam style hematite-quartz epithermal veins are extensive in outcrop & as float

6. Model: IOCG hydrothermal breccia systems formed along major fault zones in lower GRV and buried by upper GRV while still active – producing epithermal vein “breakouts” along faults reactivated in basement and lower GRV, and propagating upwards through the upper GRV flow(s)

7. IOCG epithermal systems are unlike Andean and Western Pacific types – IOCG fluids have extreme excess of Fe over S, low acidity (ie moderate pH) and high salinity. Precipitation of Fe & Cu sulphides leads to sulphur exhaustion which (with or without rapid dilution by groundwater) resulted in precipitation of higher grade Au0 ± Cu0 zones

8. Pervasive silicification (± reddening) is the dominant alteration style above the sulphide and gold zones

9. Fresh and altered (some greisenised) mg-cg Hiltaba granites intruded to high levels within the upper GRV in the Gawler Ranges

10. At least one extensive conventional epithermal system is present in the Gawler Ranges region – possibly an intrusion-related Ag-Pb-Zn system with local enrichments in Ag, and separate local enrichments in Au-Mo or Sn

11. Gravity station spacings in parts of the area is 4-8km = potential for undetected Tier 1 IOCG deposits

12. Throughout much of the Gawler Ranges gravity station spacing is 2km x 2km, which doesn’t preclude presence of Carrapateena-sized/shaped IOCG deposits at shallow depths (with ~ 400m diameter gravity footprints)

Some Key Geological Features

So, we now know that 1.59 Ga hydrothermal systems formed OD-style multistage hematite-rich veins and breccias in GRV in the central Gawler Craton, but can we believe that significant IOCG deposits may have formed west of the sacred Olympic IOCG Province ……..

… well, in the words of Sam Harris(neuroscientist, rationalist, and expert cognitive unclagger)

“wherever we look we find otherwise sane men and women making extraordinary efforts to avoid changing their minds”