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Figure 4. Deposit size-distribution plot for oxide zone endowment showing hypothetical Zipf endowment model with a cut-off of 0.03 Moz and assuming the largest undiscovered deposit contains approx. 1 Moz of gold.

Figure 5. Deposit size-distribution plot for total natural endowment showing hypothetical Zipf endowment model with a cut-off of 0.1 Moz and assuming the largest undiscovered deposit contains 4.76 Moz of gold.

An assessment of the potential orogenic gold endowment of the Sandstone Greenstone Belt using a mineral systems framework for comparison with the Agnew Gold Camp

Figure 2. Comparison between Sandstone Greenstone Belt and Agnew Gold Camp showing deposit locations and drilling below a vertical depth of 100m.

Introduction

The Sandstone greenstone belt is an exploration-immature, regolith-covered, approx. 1000 sq. km belt, in the Southern Cross Domain of theYilgarn Craton (Fig. 1). The total residual gold endowment within theoxide zone of the Sandstone greenstone belt is estimated, by applicationof quantitative statistical assessments. This mineralisation is most likelycontained in extensions of known deposits (Table 1) and severalundiscovered deposits. The fresh rock of the Sandstone greenstone beltremains poorly explored. However, a conceptual endowment estimatecan be made, based on a minerals system comparison between theSandstone greenstone belt and the well-explored, geologically-analogousAgnew greenstone belt, 100 km to the east (Fig. 1).

Research Methodology

Judging the undiscovered resource endowment of a search space represents a challenging but vital step in the mineralexploration process, as it is key to defining the potential economic cost-benefit of selected ground. Traditional approaches to theprediction of residual endowment rely on the subjective, albeit expert, judgement of a group of geologists and is open to biasesbased on the experience and knowledge of the geologists, often resulting in over-optimistic predictions of residual mineralendowment (Fallon et al., 2010).

In this study, pre-existing datasets and new critical data for the immature Sandstone Greenstone Belt are compiled tosystematically define regional- to deposit-scale characteristics of its gold mineralisation. The datasets are interrogated using themineral systems framework (McCuaig and Hronsky, 2014; Hagemann et al., 2016: Table 2) for comparison with the geologicallyanalogous Agnew Gold Camp (Fig. 2), providing an integrated conceptual, empirical, and quantitative assessment of goldendowment, providing the framework for the Mineral Systems Resource Assessment (Fig. 3).

Discussion and Conclusion

The Sandstone and Agnew Belts display similar geometry, geology, belt-scale structures, deposit styles, a shared tectonichistory from 2.72 Ga, and similar timing of gold mineralisation, between 2.66 and 2.63 Ga (Fig. 2). The mineralising systemsthat led to their endowment also share several important characteristics outlined in Table 2. Based on this comparison, it isproposed that the total gold endowment of the Sandstone Greenstone Belt may be roughly equivalent to that of the morecomprehensively explored Agnew Gold Camp. It is likely that geological difference between the belts in terms of degree ofstrain (probably a neutral factor), abundance of sedimentary rocks (more positive for the Agnew Gold Belt), and metamorphicgrade (more positive for the Sandstone Greenstone Belt) cancel each other out in terms of potential endowment.

A power law distribution (Lee & Singer, 1994; Guj et al. 2011) normally defines the frequency distributions of mineral depositsizes (e.g. Hronsky & Groves, 2008). The total residual gold endowment within the oxide zone is estimated to be 2.2 Moz (Fig.4), and in comparison with the more mature Agnew Gold Belt, the remaining primary-zone mineralisation of the SandstoneGreenstone Belt is estimated to be 12.1 Moz (Fig. 5).

Future Research

Past studies of mineral endowment have shown that resource assessment methods work at a range of scales (e.g. Guj et al.2011). Further research will use data from the entire Yilgarn Craton, and from areas of comparable geology elsewhere in theworld, to apply the Mineral Systems Resource Assessment to individual terranes, domains, and greenstone belts within theYilgarn Craton (Fig. 6). Such analyses would demonstrate gold endowment and exploration maturity, identifying areas withrelative immaturity, yet high prospectivity.

Rhys S. Davies, David I. Groves, Allan Trench, John Sykes & Jonathan G. Standing

Figure 1. Tectonic setting of the Sandstonegreenstone belt, within the Southern CrossDomain of the Yilgarn Craton (modified fromChen et al. 2005).

Figure 6. Conceptual diagramshowing application of quantitativeassessment analysis to entire of theYilgarn as well as each of theseparate Terranes. Power laws havebeen applied as part of endowmentassessments at various scales, froman entire Craton to a single alluvialgold patch.

References

Chen, S. F., Morris, P. A., and Pirajno, F., 2005,Occurrence of komatiites in the Sandstonegreenstone belt, north-central Yilgarn Craton:Australian Journal of Earth Sciences, v. 52(6), p.959-963.

Fallon, M., Porwal, A., and Guj, P., 2010,Prospectivity analysis of the Plutonic MarymiaGreenstone Belt, Western Australia: Ore GeologyReviews, v. 38, p. 208-218.

Guj, P., Fallon, M., McCuaig, T. C., and Fagan, R.,2011, A timeseries audit of Zipf's law as ameasure of terrane endowment and maturity inmineral exploration: Economic Geology, v. 106, p.241–259.

Hagemann, S. G., Lisitsin, V. A., and Huston, D. L.,2016, Mineral system analysis: Quo vadis: OreGeology Reviews, v. 76, p. 504-522.

Hronsky, J. M. A., and Groves, D. I., 2008, Scienceof targeting: definition, strategies, targeting andperformance measurement: Australian Journal ofEarth Sciences, v. 55(1), p. 3-12.

Lee, P. J., and Singer, D. A., 1994, Using PETRIMESto Estimate Mercury Deposits in California:Alberta, Institute of Sedimentary and PetroleumGeology, Geological Survey of Canada.

McСuaig, T. C., and Hronsky, J. M. A., 2014, TheMineral System Concept: The Key to ExplorationTargeting: SEG 2014: Building ExplorationCapability for the 21st Century, p. 153-175.

Singer, D. A., 1993, Basic Concepts in the Three-Part Quantitative Assessments of UndiscoveredMineral Resources: California, USA, U.S.Geological Survey.

Table 1. Deposits of the SSGB, showing historical productionand resources (measured in ounces). Data pre-1954 wascalculated from historical records (Appendix 1) and post-1954was sourced from Troy Resources internal reports.

Table 2. Comparison of critical mineral system elements between Sandstone Greenstone Belt and Agnew Gold Camp.

Mineral Systems Resource Assessment Framework

Figure 3. The constituent steps of a mineral systems assessment of undiscovered resources. The process of analysistends towards a more qualitative or quantitative approach depending on the maturity of the defined explorationsearch space, along with the availability and quality of underlying data. Source: Adapted from Singer, 1993.