Abstract

Prior to mining, the Hannan South deposit contained 0.26 Mt at 4.98 g t−1 Au confined within a weathered, highly saline water-saturated regolith beneath a salt lake. There was no surface expression, although the host rock sub-cropped at 3–4 m depth. The water table seasonally fluctuates between 0 and 10 m depth. The rock is a highly altered and structurally disrupted basalt, now a sulphidic epidote-rich unit. This unit is truncated at 80 m depth within sub-vertically interlayered Archaean felsic sedimentary and intermediate volcanic rocks. No fresh host rock is preserved. There is a sharp sub-horizontal iron redox front at 25–30 m depth between upper and lower saprolite. At this front, Eh conditions are sufficient to oxidize dissolved Fe2+ from weathered Fe-bearing minerals in the lower saprolite, to Fe3+ with the production of strong acid solutions (pH 0.5–2). Acid-enhanced weathering forms the upper saprolite as the front descends through the profile. As the pH increases, Fe3+ precipitates as Fe oxides, mainly goethite, above the front.

Primary Au containing 3–5 wt% Ag is hosted within pyrite and accessory sulphide minerals of As, Cu, Co, Pb, Bi, W and Zn in the saprock. The progressive oxidation of the sulphides has released soluble ore-associated elements, including Au as a thiosulphate complex (Au–Ag(S2O3)23−). This complex is metastable and has precipitated fine-grained secondary Au containing 3 wt% Ag around pseudomorphs of the host sulphides. The other elements are widely dispersed but are largely confined to the host rock in the lower saprolite due to limited groundwater flux. Poorly defined acid sulphide and subtle Mn redox fronts within the lower saprolite have little influence on trace element dispersion. Silver is the most strongly depleted. In the upper saprolite, descent of the redox front over time has led to leaching of Au under acid, oxidizing conditions. Gold has probably dissolved as organic or chloride complexes at the front, dispersed laterally down-gradient and precipitated as pure crystals (99.9 wt% Au) in a sub-horizontal supergene enrichment blanket just above the front. Copper, Ag, Co, Pb and Zn are also strongly leached from the acidic saprolite above the iron redox front and are dispersed and depleted with increasing groundwater flux in the upper profile. In comparison, As, Sb, Bi, Mo and W, which form oxyanions, are relatively immobile and are preferentially retained in goethitic zones. These buried multi-element halos hosted by goethite provide near-surface geochemical exploration targets within otherwise barren upper saprolite.

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