Insights on landscape geochemistry and mineral exploration in … · 2019-09-18 · Insights on...

AEGC 2019: From Data to Discovery – Perth, Australia 1

Insights on landscape geochemistry and mineral exploration in the Fraser Range, Albany-Fraser Orogen, Western Australia I. González–Álvarez W. Salama, T. Ibrahimi M. leGras CSIRO CSIRO CSIRO CSIRO Perth, WA 6151 Perth, WA 6151 Perth, WA 6151 Perth, WA 6151 [email protected] [email protected] [email protected] [email protected]

INTRODUCTION

The Albany–Fraser Region (AFR) is a regolith/cover-dominated terrain, where exotic sediments with distinctive mineralogical and geochemical features underlie the regolith profile. The cover results in increased depths to basement rock, of up to ~150 m.

Several discoveries of major mineral deposits highlight the potential of this region as an emerging metallogenic province: the Tropicana-Havana Au system at ~6 Moz (discovered in 2005), Nova-Bollinger Ni-Cu (discovered in 2015; Fig. 1) and the polymetallic Trilogy deposit (Cu–Au–Ag–Pb–Zn; discovered in 1997). These three deposits were discovered by detecting their anomalous geochemical fingerprints within the overlying cover.

Figure 1. Landscape in the study area. Matsa’s tenement, Symons Hill, is located less than 10 km to the SW of the Nova–Bollinger Ni-Cu system (Figs. 1 and 2). In the last 5 years Matsa Resources Ltd. has collected a high quality and diverse datasets including: geophysics (airborne and ground electromagnetics, gravity, magnetics), and a large geochemical and drilling dataset within the tenement. Building upon previous work carried out on the basement rocks (Spaggiari et al., 2015 and references therein; Kirkland et al., 2015 and references therein), on the characterisation of the cover in the AFR (González-Álvarez et al., 2014; 2016a, 2016b; Salama et al., 2016) and extensive studies in the Yilgarn (e.g., Anand and Butt, 2010 and references therein), the Symons Hill tenement was used to develop a regolith-landform model, which will form the basis for the interpretation of surface geochemical data to address specific exploration problems at the tenement scale in this region.

METHOD

This study applied the following workflow: (1) selection of available drill holes (RC: reverse circulation drilling and AC: aircore drilling) from Matsa Resources for regolith logging; (2) collection of samples representing different residual and transported regolith units and underlying bedrock for geochemistry and petrology; (3) geochemical analyses of ~80 regolith and basement rock samples collected from selected RC and AC drill holes; (4) generation of a high–quality reference mineralogical (Fig. 3) data using optical microscope, XRD and SEM and geochemical data set for understanding the geochemical dispersion of metals through the regolith (Fig. 4) using XRF and ICP–MS/OES for major element, trace element and mineral analyses, and spectral mapping using CSIRO’s HyChipsTM; (5) integration of all resulting data to characterise the regolith and basement rocks, including drilling data, stratigraphy from drill holes, mineralogy and geochemistry; and (6) based on drilling data availability, company data on the

SUMMARY The Albany-Fraser orogen is an emerging recently opened mineral exploration province in Western Australia due to the recent discoveries of the Tropicana-Havana Au system in 2005, and the Nova-Bollinger Ni-Cu deposit in 2015. Simons Hill is located less than 10 km to the SW of the Nova–Bollinger system in the Albany –Fraser Orogen in Western Australia. This study analyzed the nature and distribution of the regolith, and its stratigraphy, weathering and depositional history that have led to a variety of residual and transported regolith types. It aims to expand the understanding of the landscape geochemistry associated with Symons Hill in order to provide insights into mineral exploration techniques in this region. Symons Hill is located on a palaeo-topographical high that did not experience the transgression–regression cycles that affected the Albany–Fraser Region for the last 60My. Hence the Simons Hill stratigraphy lacks the marine geochemical influence which is widespread in the Norseman area. The landscape at Symons Hill evolved in a lacustrine/swamp environment. Poor drainage resulted in an extensive, thin (<30m) and homogeneous fine–grained transported cover that displays the same geochemical footprint as the underlying unweathered basement rocks. This cover includes the thicker palaeochannel sequences at the south of the tenement. Most of the transported cover is <5m thick. Since this cover is the result of local recycling of weathered outcrops, it is an appropriate sampling medium to assess the geochemical composition of the rock at depth. Even if metal geochemical anomalies were displaced laterally, the source of the anomaly at depth is localized, and is most often located within a few 100s of metres from its source. This is a key element to keep in mind when undertaking mineral exploration in the Albany-Fraser Orogen. Key words: Landscape evolution, regolith, geochemical dispersion, Fraser Range, Albany-Fraser, Mineral exploration

Landscape geochemistry in the Albany-Fraser I. González–Álvarez, W. Salama, T. Ibrahimi and M. LeGras


geochemistry of the drill holes and soil samples, and on landscape features, selected AC and RC drill holes were described and logged. Out of 90 drill holes, 83 samples were selected for whole–rock geochemistry, spanning the lateral and vertical stratigraphic extent of all of the regolith units at Symons Hill.

Figure 2. Maps of the regional regolith context of the study area: (A) Digital Elevation Model (DEM; Geosciences Australia, 2009); (B) DEM data after algorithm filtration (MrVBF, Gallant and Dowling, 2003); (C) interpreted regolith map from satellite image (GSWA, 500K, 2003); and (D) interpreted vegetation map (CSIRO, 2007).

RESULTS AND DISCUSION

Geochemical relation between the basement, saprolite and the transported cover

Basement rock suites in the Symons Hill tenement are mainly mafic, with scattered intervals of quartzo–feldspathic gneiss, granite and sedimentary rock suites (e.g., black shale). The mafic basement is a source of MgO, CaO, MnO and Fe2O3, and transition metals such as Sc, V, Cr, Co, Ni and Cu. The felsic units are a source of Zr, Hf, Nb, Ta, REE, Th and U. The black shales and basement sulfides are a source of sulfur. Sample 81161 from drill hole 13SHRC06 (40–44m) has the lowest Th/U ratio (1.5) and the highest Zr/Hf ratio (50), with a concentration of Ni at 6,100ppm. This may indicate a change in the geochemical conditions of the original magma due to fluctuating redox conditions (Th/U variability) by crustal contamination, evidenced by unusually high values for Zr/Hf. Sample 80487 from drill hole SHAC065 (50–51m) displays Cr and Ni >3,000ppm, and has an unusually high Total REE content at ~1,700ppm. These features are rare for the geochemical background in the area and may be indicative of prospective basement at depth. The presence of black shales in the stratigraphy, containing SO2 concentrations up to 4wt%, makes it difficult to assess the importance of sulfur concentration in the regolith as a proxy for NiS in the unweathered basement. Sulfides are easily oxidised along the weathering front and are redistributed throughout the regolith profile. As sulfur is also present in the black shales, caution must be taken in employing sulfur as a proxy to detect mineralisation at depth at Symons Hill. In addition, company data shows that Fe and S are gradually enriched upward in the

weathering profile (in the saprolite) in drill hole SHAC065, for which Ni, Co, Cr, Cu and Zn are enriched in the mafic bedrock/saprock. In other drill holes, S is associated with As, K, Al and Pb, potentially in the form of secondary sulfates.

Figure 3. Microphotographs of indurated mottled saprolite above mafic basement rocks. The mafic basement rocks display strong relations between Sc and REE with r2>~0.80, as well as Co, Cu and Zn at r2>~0.75, ~0.95 and r2>~0.90, respectively. Nickel, MnO and Fe2O3 do not correlate with transition metals, nor REE. However, the mafic saprock displays MnO with REE at r2>~0.65, preserves Co with REE at r2>~0.75 and Ni correlates with MnO at r2>~0.60. Even though most of the basement rocks are rich in plagioclase, muscovite and orthoclase, illite and smectite are not abundant in the weathered profile. Kaolinite is the dominant clay mineral. This is interpreted as the result of weathering in a tropical wet-dry climate. The presence of kaolinite indicates pH conditions between 4 and 9, which is typical for weathered areas containing mafic magmatic suite rocks. The presence of goethite throughout the ferruginous saprolite, but not in the transported cover, is due to climatic control. Dry climate favours the formation of hematite in the transported cover, coupled with an increase in Fe2O3 and a decrease in MnO, which is interpreted as resulting from a redox shift between oxidising and reducing conditions, between the saprolite and the transported cover. During weathering (supergene enrichment), a change in Eh–pH due to downward movement of Fe–Mn–rich fluids causes the precipitation of Fe in preference to Mn. Manganese is stable in solution over a wider Eh–pH range, particularly under moderately reducing conditions. A limited increase in pH may lead to the selective elimination of Fe from Fe–Mn bearing solutions, and a further increase in pH or Eh may cause the formation of stable Mn oxides or Mn carbonates (Maynard, 1983). This pH/Eh shift



segregates the way the transition metals associate with different mineral assemblages in the stratigraphic sequence.

Figure 4. Vertical variability of transition metal concentrations for drill hole SHAC026 normalised to UCC for each of the stratigraphic units present. In the ferruginous saprolite, Ni and Co display a positive correlation with MnO (r2>~0.90 and r2>~0.95, respectively). Conversely, Fe2O3 correlates with Cr and Cu at r2>~0.85 for each, and with Sc at r2>~0.70. This was likely caused by the formation of goethite, which absorbs Cr, Cu and Sc, and Mn oxides which absorb Ni and Co. Goethite is a common mineral in weathering environments, and may contain a variety of transition metals such as Cr, Cu and Sc in its crystalline lattice. Selective uptake of Ni and Co by Mn oxides results from oxidation of soluble Co2+ to insoluble Co3+ by Mn3+/Mn4+. This mineral–surface redox reaction accounts for the geochemical relation between Co and Mn under weathering conditions. In saprolite, the Light REE decreases their relationship with the Middle and Heavy REE, and P2O5 becomes linked with total REE with an r2>~0.60. In the unweathered mafic rock and mafic saprock P2O5 correlates with REE at r2<~0.10. This is due to redistribution of phosphates and absorption of REEs in the formation of new mineral phases in the saprolite. The presence of phosphate minerals in the saprolite is a common feature.

Phosphate minerals in the bedrock are in the form of apatite, xenotime and monazite. When these minerals are weathered under acidic conditions (generated from the weathering of sulfides, primarily pyrite) new secondary minerals of Al phosphates and sulfate (APS) minerals are formed within the saprolite. These minerals are high in Light REE concentrations. Available data shows that Al, S, As, Pb, K and P are common in secondary sulfates (alunite, jarosite, crandallite and plumbogumite). In the Ni-rich ferruginous saprolite, elemental relationships are unique as compared with the rest of Ni–poor saprolite. Fe2O3 becomes independent of the transition metals, except for a relation with V and Cr at r2 >~0.85 for each, and with REE. Co is linked to MnO, MgO, CaO, K2O and Na2O with r2>~0.85-0.95, and Ni is linked to MnO, MgO, Na2O with r2>~0.75, and to LREE with r2>~0.80. MnO, MgO, K2O and Na2O are strongly linked with Light and Middle REE with r2>~0.85. Major oxides, transition metals, HFSE (Zr, Hf, Nb, Ta) and REE patterns resemble Primitive Mantle patterns. This suggests that the ferruginous Ni-rich saprolite may be the result n of supergene enrichment alongside residual weathering of mafic/ultramafic cumulates. The Ni+Co transition metal budget is detached from Fe2O3, MnO and CaO within the basement rocks, whereas in the saprolite it covaries with Mn oxides, but not in the transported cover. In the mafic basement rocks Cu+Cr+V correlates slightly with MnO, though not with CaO. However, Cu+Cr+V displays a significant correlation with Fe2O3 throughout all of the stratigraphic units in the basement, the saprolite and the transported cover. This indicates that the best medium to estimate the Ni+Co budget is the saprolite and saprock that is rich in MnO, and to estimate the Cu+Cr+V budget is in the transported cover. Metal anomalies with linear profiles may trace deeper structures that may have been conduits for fluid flow and geochemical dispersion due to their higher porosity and high fluid pressures. Available data shows that the Ni–rich saprolite/saprock at the base of the weathering profile is coincident with the trend of the Ni anomalies in the overlying transported cover. This is exemplified by Ni, Co, Cr, Cu and Mn enrichments along the drilling trend. This drill hole trend coincides with the southern part of the north–south–trending Ni anomaly found in the soil. The high concentration of Ni, Co, Cr, Cu and Mn in narrow zone trends may suggest prospective mafic-ultramafic lithologies, a contact between different lithological rock units, or possibly a deep structure (fault) along which Ni, Co, Cu and Mn were vertically dispersed. Arsenic is linked to Fe2O3 which explains why all of the transported cover is enriched in As (due to the presence of ferruginous granules), which correlates to the As content of the unweathered basement rock at depth. Calcrete lenses are geochemically enriched in total REE (~140ppm), Ag, Cr and Ni relative to UCC average values. Calcium and Mg in the calcrete lenses at the top of the transported cover is likely derived from dissolution of mafic lithologies at depth, and form via precipitation of carbonates due to evaporation under alkaline condition. This calcrete transition metal enrichment (V, Cr, Co and Ni) is not a reliable proxy of metal concentrations at depth, since metal mobility may have been lateral as well as vertical. The transported cover is locally recycled from a local residual weathering profile. This interpretation is supported by the unusually low total REE budget in the red, green and grey clay,



which displays much lower values than UCC (~150ppm), varying from ~10 to 50ppm, and even lower values than saprolitic total REE values, varying from ~25 to 100ppm. Clay minerals tend to have high total REE budgets, which are increased during transport and mixing prior to deposition. The transported clay cover at Symons Hill is locally sourced, mixed and deposited at the tenement scale. This is further supported by the elemental values of Ni and Cr throughout the transported cover, which are up to ~10 times UCC values. Vertical geochemical dispersion from the unweathered mafic basement to the surface displays a coherent pattern for transition metals, REE, HFSE and transition metals. Original mafic units are weathered to ferruginous saprolite with metal anomalies of Ni, Co, etc enhanced by supergene enrichment and concentrated due to the formation of Mn oxides. However, the major trace element relationships are maintained throughout the profiles. This indicates that the geochemical signature of the basement rocks is preserved in the saprolite and is also expressed in the transported cover. However, Ni signatures are better preserved in the saprolitic Mn-rich stratigraphy.

IMPLICATIONS FOR EXPLORATION

(1) The geochemical signature of the unweathered basement rocks is preserved throughout the overlying stratigraphy and is detectable in the transported cover. Since the transported cover is the result of local recycling of the weathered units, this is an appropriate sampling medium to assess the geochemical composition of the rocks at depth. (2) Geochemical Ni/metal anomalies that are displayed as narrow bands may be related to structural features at depth, through which Ni/metal dispersion may have occurred. Those anomalies that appear as halos do not necessarily to reflect structural features at depth, but rather locally prospective mafic-ultramafic lithologies. (4) Sample 81161 from drill hole 13SHRC06 (40–44m), and sample 80487 from drill hole SHAC065 (50–51m) display geochemical features that are anomalous in the area, and are indicative of prospective basement rocks at depth. (5) Ni and Co haloes are more easily identified in the ferruginous saprolite and soil, whereas Cr, Cu, V and As haloes reside in the transported cover. (6) Calcrete may contain transition metal anomalies. However, due to lateral and vertical mobility of metals, these anomalies are not necessarily linked to a source at depth. (7) Sulfur abundance and distribution in the regolith is sourced from black shales and mafic units at depth. The presence of black shales in the stratigraphy, even if in minor proportions, lowers the efficiency of sulfur concentration as an indicator of NiS mineralisation.

CONCLUSIONS FOR LANDSCAPE EVOLUTION The landscape at the Symons Hill tenement evolved in a lacustrine/palustrine environment. Poor drainage resulted in an extensive, thin (<30m) and homogeneous fine-grained transported cover that displays the same geochemical footprint as the unweathered basement rocks at depth. Even if metal geochemical anomalies were displaced laterally, the source of

the anomaly at depth is local and most likely within a few 100s of metres distance. Palaeochannels are a minor feature of the tenement and display small scale (~20m thickness) and low energy (clay, silt, fine–medium sand) fill (Fig. 5). The transported cover displays the geochemical footprint of the unweathered basement rocks. Transition metals, REE and HFSE (Zr, Hf, Nb and Ta) elements are conservative throughout the saprolite and transported cover. Ni and Co abundances are mainly preserved in the saprolite due to their affinity for MnO and goethite, whereas in the transported cover, with its low goethite content, Cr, Cu and V are linked to Fe2O3 (Fig. 5).

ACKNOWLEDGMENTS We would like to thank the traditional owners of the land. Matsa Resources Ltd. is specially thanked for allowing access and providing data for this project. Derek Winchester is thanked for preparing thin sections and Carmen Krapf for her critical review. This project received funding from Innovation Connect. We also like to thank K. Green for his support.

REFERENCES Anand, R.R., Butt, C.R.M., 2010. A guide for mineral exploration through the regolith in the Yilgarn Craton, Western Australia, Australian Journal of Earth Sciences 57, 1015-1114. Commonwealth Scientific and Industrial Research Organisation, 2007. Interpreted Vegetation cover of Australia. Gallant, J.C., Dowling, T.I., 2003. A multi-resolution index of valley bottom flatness for mapping depositional areas, Water Resources Research 39, 1347, 4-1/4-13. Geological Survey of Western Australia, 2003. 1:5,00,000 regolith map (500 meter grid) of Western Australia. Department of Mines and Petroleum, ID: ANZWA1220000494 http://mapserver.doir.wa.gov.au/datacentre Geosciences Australia, 2009. Digital Elevation Model of Australia. SRTM-derived 1 Second Digital Elevation Models Version 1.0. http://www.ga.gov.au/scientific-topics/national-location-information/digital-elevation-data. González-Álvarez, I., Anand, R.R., Hough, R., Salama, W., Laukamp, C., Swetapple, M., Ley-Cooper, Y., Sonntag, I., Lintern, M., Abdat, T., leGras, M., Walshe, J., 2014. Greenfields geochemical exploration in a regolith-dominated terrain: the Albany-Fraser Orogen/Yilgarn Craton margin. GSWA Report 144, MRIWA Report 305, CSIRO Report EP1312804, 212 pp. González-Álvarez, I., Salama, W., Anand, R.R., 2016a. Sea-level changes and buried islands in a complex coastal palaeolandscape in the South of Western Australia: implications for greenfields mineral exploration. Ore Geology Reviews Special Issue 73, 475-499. González-Álvarez, I., Ley-Cooper, Y., Salama, W., 2016b. A geological assessment of airborne electromagnetics for mineral exploration through deeply weathered profiles: the southeast Yilgarn cratonic margin, Western Australia. Ore Geology Reviews Special Issue 73, 522-539.



Kirkland, C.L., Smithies, R. H., Spaggiari, C., 2015. Foreign contemporaries – Unraveliling disparate isotopic signatures from Mesoproterozoic Central and Western Australia. Precambrian Research 265, 218–231. Maynard, J.B., 1983. Geochemistry of Sedimentary Ore Deposits. Chapter 5. Manganese, 121–145. Berlin, Heidelberg, New York, Tokyo: Springer–Verlag, 305pp. Rudnick, R.L., Gao, S., 2003. The composition of the continental crust. In: Rudnick, R.L., Holland, H.D., Turekian, K.K. (Eds.), The Crust. Treatise on Geochemistry, vol. 3. Elsevier, Oxford, New York, pp. 1–64. Salama, W., González-Álvarez, I., Anand, R.R., 2016. Significance of weathering and regolith/landscape evolution for

mineral exploration in the NE Albany-Fraser Orogen, Western Australia. Ore Geology Reviews Special Issue 73, 500-521. Spaggiari, C.V., Kirkland, C. L., Smithies, R. H., Wingate, M. T. D., Belousova, E.A., 2015. Transformation of an Archean craton margin during Proterozoic basin formation and magmatism: The Albany–Fraser Orogen, Western Australia. Precambrian Research 266, 440–466.

Figure 5. Simplified geochemical model for the Symons Hill area.

Insights on landscape geochemistry and mineral exploration in … · 2019-09-18 · Insights on...

Documents

Transcript of Insights on landscape geochemistry and mineral exploration in … · 2019-09-18 · Insights on...