application of multi-element geochemistry in the weathered ... · application of multi-element...

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

Application of multi-element geochemistry in the weathered environment: Controls, considerations and implications for exploration Fiona Best Matthew Readford Kristina Walcott South32 South32 Freegold Ventures 108 St Georges Terrace 108 St Georges Terrace PO Box 10351 Perth Perth Suite 888 - 700 West Georgia Street WA 6000 WA 6000 Vancouver, BC V7Y Australia Australia Canada [email protected] [email protected] [email protected]

INTRODUCTION

Geochemical data is collected at every stage of an exploration program, from initial soil sampling through to resource drilling. Datasets, comprising high-quality, multi-element geochemistry, have many uses in exploration including identifying commodity element anomalies at the surface, understanding lithostratigraphy, mapping hypogene alteration patterns and dispersion halos, and producing resource and geometallurgical models. In unweathered bedrock the geochemistry will reflect primary lithology ± hypogene alteration processes. However, in the surface and oxidised environment, the geochemistry will likely be influenced by element mobility during chemical and physical weathering processes. Understanding the mobility of individual elements in the oxidised environment is therefore important for the accurate interpretation of exploration geochemical data collected from the weathered environment. This paper aims, through the use of multiple exploration case studies, to demonstrate how: (1) chalcophile and pathfinder element mobility in the near-surface weathered environment impacts interpretation, and how ignoring this in exploration targeting can lead to missed opportunities; (2) subtle changes in mobile element geochemistry can be used to map the oxide-fresh rock boundary on a deposit scale; and (3) conversely, how immobile elements can be used to map basement geology in residual soils and weathered bedrock. In turn, this paper highlights the importance of collecting high quality, multi-element geochemistry with low detection limits at every stage of exploration.

ELEMENT MOBILTY IN THE WEATHERED ENVIRONMENT

The rate of release of elements into the weathering profile is dependent on the stability of primary minerals. Resistate minerals, e.g., quartz, talc, zircon and chromite, are predominantly stable in the weathering environment and preserve their primary element compositions (Anand and Paine, 2002). However, many primary rock-forming minerals are unstable in this environment, having formed at much higher temperatures and pressures. As a rock weathers chemically, these mineral constituents change to new, more stable assemblages and the contained elements are either partly redistributed into new minerals or taken into solution, in some cases to be incorporated in to other parts of the weathering profiles (Figure 1). Destruction of ferromagnesian minerals and feldspars results in depletion of Mg, Ca, Na, K, plus hosted

SUMMARY Understanding the mobility of individual elements in the oxidised environment is important for the accurate interpretation of geochemical data from regolith and weathered bedrock samples. In the weathered environment, immobile element geochemistry can reflect primary lithological and in situ mineralisation signatures, whereas mobile elements can help establish the degree and extent of weathering. Three exploration case studies are presented here to demonstrate the application of multi-element geochemistry in the weathered environment: (1) An example from the Shorty Creek Project, Alaska (Freegold Ventures), highlights the importance of reviewing and understanding pathfinder elements in soil, weathered bedrock and fresh basement. Here, commodity elements Cu and Zn are highly mobile and relatively depleted in the soils and weathered zone over Cu-Au mineralisation, whereas Au, As, Bi and Sb are less mobile and highly anomalous in the oxidised bedrock and associated soils. (2) The Hermosa Deposit, Arizona (South32), where downhole geochemistry can help map the oxide-fresh rock boundary at the deposit scale. Subtle depletion in mobile elements allows the weathered zone to be identified in altered rhyolites, and Zn:S and Pb:S ratios in the mineralised zone helps distinguish Zn and Pb oxides from Zn and Pb sulphides. (3) A review of residual soils over a Ni-Cu-PGE prospect in Northern Queensland, where immobile elements are reliable discriminants of primary lithologies in highly weathered environments. Zirconium, Y, Th and Nb can be used to distinguish felsic from mafic bedrock, and variations in Cr, V, Al, Fe and Sc in soils confidently identify blind, compositionally distinct mafic-ultramafic bodies. This paper also highlights the importance of collecting high quality, multi-element geochemistry with low detection limits at every stage of exploration. Key words: Multi-element geochemistry, immobile elements, weathering, oxide - fresh boundary, exploration

Application of multi-element geochemistry in the weathered environment Best, Readford and Walcott


trace elements Co, Cr, Cu, Mn, Ni, Ba, Cs, Rb and Sr (McQueen & Scott, 2008). Unsurprisingly, these elements are typically highly mobile in the weathering environment and cannot be used for lithological classification of weathered/altered rocks. Conversely, Al, Ti, Sc, Zr, Hf, Nd, Nb, Th, Y, Hf have been found to be relatively immobile in the weathering profile, being concentrated in resistate phases or secondary minerals (e.g., Middelburg et al. 1988; Barnes et al., 2013). The geochemical behaviour of Mn, Cr, V, Fe and Ce is very dependent on redox conditions.

Figure 1. Mineral stability and secondary mineral pathways in a weathered profile (from Anand and Paine, 2002) There is a strong relationship between pH-Eh and base metal mobility (Mann, 1982; Thornber, 1992). Under humid, oxidising and acid conditions, most sulfides are highly unstable and rapidly breakdown. Consequently, S will be depleted, or present as sulfate either in solution or as precipitates, and elements hosted by the sulphides pass into solution or are incorporated into neo-formed (secondary) minerals (McQueen & Scott, 2008). Cations are commonly highly mobile in this environment, hence commodity-elements Cu, Co, Cd, Ni and Zn rapidly depleted in the highly weathered environment. Conversely, pathfinder elements such as As, Sb, Te, W and Mo, which form anions, may remain with the Fe- and Mn-oxide minerals. This has implications for exploration, with anomalies comprising only pathfinder elements a valid target in the weathered zone. The factors affecting the mobility of ‘free’ elements (i.e., elements released from primary and secondary minerals) in the weathered environment are numerous and complex. They include solution processes (e.g., sorption, complexation, oxidation state), influenced by pH and Eh, as well as gas-vapour, biological and mechanical activity. Although these processes are not discussed in detail here, it is highlighted that cations (commodity elements) and anions (pathfinder elements) commonly behave differently in solution. For example, in acidic environments mineral surfaces are commonly positively charged, leading to adsorption of anions (e.g., Mo, Se, As) and repulsion of cations (Cu, Pb, Zn, Co, Cd; Thornber, 1992).

THE IMPORTANCE OF PATHFINDER ELEMENTS IN SURFACE GEOCHEMISTY

A sulphide body with high proportions of iron sulphide minerals (particularly pyrite), will produce highly acidic weathering conditions, potentially causing depletion in commodity elements but retention of pathfinder elements (Thorneber and Taylor, 1992). The case study presented here, from Freegold Ventures’ Shorty Creek Project (Alaska), demonstrates this weathering process and highlights the

importance for following-up pathfinder-element surface geochemical anomalies, even without commodity element enrichment. Copper-Au-Ag mineralisation, hosted by disseminated and vein-controlled pyrite, pyrrhotite, chalcopyrite and bornite in flysch units, has been intersected at the Hill 1835 prospect, Shorty Creek Project (Freegold Ventures investors presentation, 2019). These sulphides are also commonly enriched in W, As, Sb, Bi, Zn and Co. The oxide zone extends to approximately 80 – 100 m below surface and is obvious in downhole geochemical profiles based on the significant depletion in S, Cu, Zn and Co. Conversely, Au, Ag, W, As, Sb and Bi do not show depletion in the oxide zone, with Sb and Bi slightly enriched in this zone (Figure 2). Soil geochemistry over the Hill 1835 prospect reflects the deep weathering profile observed in the drilling data. Residual soils are highly enriched in Au, As, Bi and Sb, but give little indication that Cu mineralisation is significant at depth (Figure 3).

MAPPING THE OXIDE-FRESH ROCK BOUNDARY USING DOWNHOLE

GEOCHEMISTRY Understanding the depth of weathering and identifying the oxide-sulphide boundary is crucial at the advanced exploration and resource definition stage. Logging weathering intensity is highly subjective and, when this data is collected over multiple drilling programs by numerous different geologists, confidence is this data is often low. Multi-element geochemistry can be used to validate this logging and help map the oxide zone and base of fresh rock. The case study presented here, from South32’s Hermosa Project (Arizona), demonstrates that subtle depletions in mobile elements can identify the weathered zone and Zn:S and Pb:S ratios can help distinguish the oxide/sulphide boundary. The Hermosa Project comprises two major mineral deposits – the Taylor Zn-Pb-Ag sulphide and the Clark Ag-Mn-Zn oxide deposit. Host rocks predominantly comprise Jurassic Rhyolites and Paleozoic Limestone. Within the mineralised zones, Zn:S and Pb:S ratios were able to help distinguish samples dominated by sphalerite and galena, from those dominated by Zn and Pb oxides. In unmineralized to weakly mineralised areas, this approach could not be used to map the base of weathering, and subtle depletion in mobile elements, such as Ca and Na, in volcanic units allowed the depth of weathering to be mapped across the deposit. At Hermosa these techniques largely verified the logging but, in other deposits where the oxide boundary is poorly understood, downhole geochemistry offers a powerful tool to help improve knowledge and model the oxide-fresh boundary.

MAPPING BEDROCK GEOLOGY THROUGH INSITU REGOLITH

Residual regolith forms from the weathering of rock without significant lateral movement of the solid weathering products (Eggleton, 2001). As documented in Figure 1, the mineralogy and major element chemistry of residual soils will be significantly different to that of the parent rock. However, trace elements which behave as immobile components during weathering are reliable discriminants of primary lithology and



allow bedrock geology to be accurately mapped using residual soil geochemistry. This is demonstrated here using soils collected over a Ni-Cu-PGE prospect in Northern Queensland. The Dido Prospect, located close to the SE margin of the Georgetown Region (near Greenvale), is dominated by the Dido tonalite-diorite batholith. Fourteen potential mafic-ultramafic intrusions were identified within the Dido Batholith using airborne magnetic and radiometric data. Soil sampling occurred over two of the larger bodies (Palmer North and Palmer South). Using immobile elements Zr, Y, Th, Nb and P, the extent of these largely blind intrusions was mapped (Figure 4); soils over the mafic-ultramafic intrusions are depleted in these elements compared to those over the more felsic Dido Batholith. Further, the soil geochemistry highlighted that the two mafic-ultramafic intrusion were geochemically distinct. Whereas soils over the Palmer North intrusion are characterised by anomalous levels of Cr, Ni, Mg, Cu and Co relative to background, those over Palmer South are enriched in Fe, V, Sc, Cu and Pt. The interpretation from the soil geochemistry was that the Palmer North intrusion was likely a primitive, ultramafic intrusion, whereas the Palmer South intrusion formed from more evolved, iron-enriched magmas. This interpretation was confirmed by diamond drilling (Figure 5) – Palmer North comprises olivine-rich mafic and ultramafic cumulates, whereas Palmer South comprises magnetite-rich pyroxenites and gabbronorites.

Figure 5. (a) Downhole geochemical profiles from a representative drillhole from the Palmer North intrusion; (b) Downhole logs from a representative drillhole from the Palmer South intrusion; (c) Thin section (cross-polarised light) showing troctolite from the Palmer North intrusion; (d) Thin section (cross-polarised light) showing olivine cumulate with abundance of magnetite, from the Palmer South intrusion.

Identifying the basement geology using soil geochemistry is important for exploration because it allows variations in background element concentrations to be understood and anomaly thresholds to be accurately established. In the case of the Palmer intrusions, both intrusions are mafic-ultramafic and should be prospective for magmatic Ni-Cu-PGE. However, without levelling soil geochemistry by lithotype the Palmer South intrusion, which has lower olivine concentrations hence lower background nickel concentrations in soil, would not have been drill tested.

CONCLUSIONS This paper highlights the importance of collecting high quality, multi-element geochemistry with low detection limits at every stage of exploration. Through the presentation of three exploration case studies, the application and importance of such data in the weathered environment was demonstrated. Key points include: (1) chalcophile and pathfinder elements commonly have different mobilities in the near-surface weathered environment and ignoring pathfinder-element-only anomalies when exploration targeting can lead to missed opportunities; (2) subtle changes in mobile element geochemistry, such as Ca, Na, K and Mg can often be used to map the oxide-fresh rock boundary on a deposit scale; and (3) immobile elements such as Zr, Th, Nb, Al and Nd can be used to map basement geology in residual soils and weathered bedrock.

ACKNOWLEDGMENTS We would like to thank South32 for their support in the publication of this abstract. We would also like to acknowledge all those people at the Hermosa Project, Arizona (South32) and Shorty Creek Project, Alaska (Freegold Ventures) whose hard work provided the geochemical data presented here. The Dido Prospect case study comprised part of a PhD (Best, 2012) which was funded by Anglo American Exploration, Australia.

REFERENCES Anand, R.R., and Paine, M., 2002, Regolith geology of the Yilgarn Craton, Western Australia: Implication for exploration, Australian Journal of Earth Sciences, 49:1, 3 – 162. Barnes, S.J., Fisher, L.A., Anand, R., and Uemoto, T., 2014, Mapping bedrock lithologies through in situ regolith using retained element ratios: a case study from the Agnew-Lawlers area, Western Australia. Australian Journal of Earth Sciences, 61:2. Eggleton, R.A., 2001, The Regolith Glossary, CRC LEME Perth, 144. Mann, A.W., 1982, Mobilities of metal ions. In: Smith, R.E. (ed.), Geochemical exploration in deeply weathered terrain. CSIRO Institute of Energy and Earth Resources, 76 – 106. McQueen. K.G., and Scott, K.M., 2008, Rock weathering and structure of the regolith. In: Scott, K.M., Pain, C.F., (eds) Regolith Science, 103 – 124. Middelburg, J.J., van der Weijden, C.H., Woittiez, J.R.W., 1988, Chemical processes affecting the mobility of major,



minor and trace elements during weathering of granitic rocks. Chemical Geology, 68 (3-4), 253-273. Thornber, M.R., 1992, The geochemical mobility and transport of elements in the weathering environment. In: Butt, C.R.M, and Zeegers, H. (eds), Handbook of Exploration Geochemistry. Vol. 4. Regolith exploration geochemistry in tropical and subtropical terranes, 79 – 96. Thornber, M.R., and Taylor, G.S., 1992, The mechanisms of sulphide oxidation and gossan formation. In: Butt, C.R.M, and Zeegers, H. (eds), Handbook of Exploration Geochemistry. Vol. 4. Regolith exploration geochemistry in tropical and subtropical terranes, 1992 119 - 138

Figure 2. Downhole geochemical profiles from drillhole SC1801, Shorty Creek Project, Alaska (Freegold Ventures).

Figure 3. Shorty Creek Prospect, Alaska (A) Percentile-based element distribution grids of soil geochemistry. (B) Soil grid overlain on airborne magnetic image and showing the location of Hill 1835, 1710, 1890 and Shorty Creek magnetic anomalies.



Figure 4. Percentile-based element distribution grids for Mg, Cr, Ni in soils over the Palmer North and Palmer South mafic-ultramafic intrusions, northern Queensland. The dashed outlines for the individual intrusions are based on airbourne magnetic signatures.

application of multi-element geochemistry in the weathered ... · application of multi-element...

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