Sandstone gold endowment - Davies et al - Apr 2017 - Centre for Exploration Targeting

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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 the
Yilgarn Craton (Fig. 1). The total residual gold endowment within the
oxide zone of the Sandstone greenstone belt is estimated, by application
of quantitative statistical assessments. This mineralisation is most likely
contained in extensions of known deposits (Table 1) and several
undiscovered deposits. The fresh rock of the Sandstone greenstone belt
remains poorly explored. However, a conceptual endowment estimate
can be made, based on a minerals system comparison between the
Sandstone greenstone belt and the well-explored, geologically-analogous
Agnew 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 mineral
exploration process, as it is key to defining the potential economic cost-benefit of selected ground. Traditional approaches to the
prediction of residual endowment rely on the subjective, albeit expert, judgement of a group of geologists and is open to biases
based on the experience and knowledge of the geologists, often resulting in over-optimistic predictions of residual mineral
endowment (Fallon et al., 2010).
In this study, pre-existing datasets and new critical data for the immature Sandstone Greenstone Belt are compiled to
systematically define regional- to deposit-scale characteristics of its gold mineralisation. The datasets are interrogated using the
mineral systems framework (McCuaig and Hronsky, 2014; Hagemann et al., 2016: Table 2) for comparison with the geologically
analogous Agnew Gold Camp (Fig. 2), providing an integrated conceptual, empirical, and quantitative assessment of gold
endowment, 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 tectonic
history from 2.72 Ga, and similar timing of gold mineralisation, between 2.66 and 2.63 Ga (Fig. 2). The mineralising systems
that led to their endowment also share several important characteristics outlined in Table 2. Based on this comparison, it is
proposed that the total gold endowment of the Sandstone Greenstone Belt may be roughly equivalent to that of the more
comprehensively explored Agnew Gold Camp. It is likely that geological difference between the belts in terms of degree of
strain (probably a neutral factor), abundance of sedimentary rocks (more positive for the Agnew Gold Belt), and metamorphic
grade (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 deposit
sizes (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 Sandstone
Greenstone 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 the
world, to apply the Mineral Systems Resource Assessment to individual terranes, domains, and greenstone belts within the
Yilgarn Craton (Fig. 6). Such analyses would demonstrate gold endowment and exploration maturity, identifying areas with
relative immaturity, yet high prospectivity.
Rhys S. Davies, David I. Groves, Allan Trench, John Sykes & Jonathan G. Standing
Figure 1. Tectonic setting of the Sandstone
greenstone belt, within the Southern Cross
Domain of the Yilgarn Craton (modified from
Chen et al. 2005).
Figure 6. Conceptual diagram
showing application of quantitative
assessment analysis to entire of the
Yilgarn as well as each of the
separate Terranes. Power laws have
been applied as part of endowment
assessments at various scales, from
an entire Craton to a single alluvial
gold patch.
References
Chen, S. F., Morris, P. A., and Pirajno, F., 2005,
Occurrence of komatiites in the Sandstone
greenstone 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 Marymia
Greenstone Belt, Western Australia: Ore Geology
Reviews, v. 38, p. 208-218.
Guj, P., Fallon, M., McCuaig, T. C., and Fagan, R.,
2011, A timeseries audit of Zipf's law as a
measure of terrane endowment and maturity in
mineral exploration: Economic Geology, v. 106, p.
241–259.
Hagemann, S. G., Lisitsin, V. A., and Huston, D. L.,
2016, Mineral system analysis: Quo vadis: Ore
Geology Reviews, v. 76, p. 504-522.
Hronsky, J. M. A., and Groves, D. I., 2008, Science
of targeting: definition, strategies, targeting and
performance measurement: Australian Journal of
Earth Sciences, v. 55(1), p. 3-12.
Lee, P. J., and Singer, D. A., 1994, Using PETRIMES
to Estimate Mercury Deposits in California:
Alberta, Institute of Sedimentary and Petroleum
Geology, Geological Survey of Canada.
McСuaig, T. C., and Hronsky, J. M. A., 2014, The
Mineral System Concept: The Key to Exploration
Targeting: SEG 2014: Building Exploration
Capability for the 21st Century, p. 153-175.
Singer, D. A., 1993, Basic Concepts in the Three-
Part Quantitative Assessments of Undiscovered
Mineral Resources: California, USA, U.S.
Geological Survey.
Table 1. Deposits of the SSGB, showing historical production
and resources (measured in ounces). Data pre-1954 was
calculated from historical records (Appendix 1) and post-1954
was 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 analysis
tends towards a more qualitative or quantitative approach depending on the maturity of the defined exploration
search space, along with the availability and quality of underlying data. Source: Adapted from Singer, 1993.