E.J. Mikucki

3.26 Ga black smoker-type mineralization in the Strelley Belt, Pilbara Craton, Western Australia

Early Archaean (3.26 Ga) volcanogenic massive sulphide (VMS) deposits located within the low strain and very-low metamorphic grade Strelley greenstone belt, East Pilbara, Western Australia, contain exceptionally well-preserved sulphide textures which are directly analogous to those formed in black smoker chimneys at present-day submarine hydrothermal vents. The VMS deposits are associated with seafloor alteration zones in and below silicacarbonate laminites, at the top of a pile of tholeiitic to calc-alkaline intermediate and felsic volcanic rocks with trace element abundances indicative of modem subduction-related processes. VMS mineralization comprises Zn-rich sulphide lenses with significant Cu and barite, and variable Pb and Ag, above Cu-rich stringer zones, all of which are similar to Phanerozoic VMS deposits. Sulphide-sulphate mineralization implicates hydrothermal discharge into sulphate-bearing ocean waters.

Archean lode-gold deposits: fluid flow and chemical evolution in vertically extensive hydrothermal systems

Abstract The range of conditions of formation of lode-gold deposits from the sub-greenschist to the lower-granulite facies in Archean greenstone belts, and the generally steeply plunging, vertically continuous pipe-like or tabular geometries of individual deposits, indicate long-distance hydrothermal fluid advection along well-defined channelways in the upper and middle crust. From presently available gold solubility data, destabilisation of gold-bisulphide complexes through H 2 S loss from the fluid to the wallrock was the dominant gold precipitation mechanism within these hydrothermal systems as a whole. This inference is supported by the S:Au ratios of ores. Sulphur and Au precipitation in the hydrothermal system is estimated to be relatively inefficient, with only 10–50% of S or Au contained in the fluid precipitated over any kilometre length of fluid channelway. The relative inefficiency of gold precipitation allowed mineralisation over a significant depth range in a crustal profile.

Archaean Au−Ag mineralisation at Racetrack, near Kalgoorlie, Western Australia: a high crustal-level expression of the Archaean composite lode-gold system

The Racetrack Au−Ag deposit, in the Archaean Yilgarn Block, Western Australia, is hosted by a porphyritic basalt in a low greenschist facies setting and is associated with a brittle strike-slip fault system. Three distinct and successive stages of hydrothermal activity and late quartz-carbonate veining resulted in multiple veining and/or brecciation: Stages I and II are Au-bearing, whereas Stage III and late veins are barren. The ore shows features of both classic epithermal and mesothermal deposits. Alteration assemblages, typified by sericitization, carbonization, silicification and chloritization, are similar to those of mesothermal gold deposits, wheras the quartz vein-textures including comb, rosette, plumose and banded, ore mineralogyof arsenopyrite, pyrite, chalcopyrite, sphalerite, galena, freibergite, tetrahedrite, tennantite, fahlore, electrum and gold, and metal associations (Cu, As, Ag, Sn, Sb, W, Au and Pb) are more characteristics of epithermal deposits. Fluid inclusions related to Stage II are two phase and aqueous with 1–8 (average 4) wt. % NaCl equiv. and CO2 content of <0.85 molal. Pressure-corrected homogenisation temperatures range from 190°C to 260°C. Mineral assemblages indicate that ore fluid pH ranged between 4.2 and 5.3, fO2 between 10−38.8 and 10−39.6 bars, and mΣs between 10−3.2 and 10−3.6. Calculated chemical and stable isotope compositions require a component of surface water in the ore fluid depositing the mineralisation, but evidence for deep crustal Pb indicates that deeply sourced fluids were also involved. The deposit is interpreted to have formed in a shallow environment via mixing of deeply sourced fluids, from at least as deep as the base of the greenstone belt, with surface waters. It therefore represents the upper crustal end-member of the crustal depth spectrum of Archaean lode-gold mineralisation.

Alteration and primary geochemical dispersion associated with the Bulletin lode-gold deposit, Wiluna, Western Australia

Abstract The Bulletin lode-gold deposit is within the northernmost part of the Norseman–Wiluna greenstone belt in the Archaean Yilgarn Block, Western Australia. It is located within a brittle–ductile shear zone and hosted by tholeiitic metavolcanic rocks. Syn-metamorphic wallrock alteration envelops the gold mineralisation and is pervasive throughout the entire shear zone and extends up to 150 m into the undeformed wallrocks. Alteration is characterised by the sequence of distal chlorite–calcite, intermediate calcite–dolomite, outer proximal sericite and inner proximal dolomite–sericite zones. The thickness of the alteration envelope, and the occurrence of dolomite in the alteration sequence, can be used as a rough guide to the width, extent and grade of gold mineralisation, because a positive correlation exists between these variables. Mass transfer evaluations indicate that chemical changes related to the wallrock alteration are similar in all host rocks: in general, Ag, As, Au, Ba, CO 2 , K, Rb, S, Sb, Te and W are enriched, Na and Y are depleted, and Al, Cr, Cu, Fe, Mg, Mn, Nb, Ni, P, Se, V, Zn and Zr are immobile, while Ca, Si and Sr show only minor or negligible relative changes. The degree of mobility of each component increases with proximity to gold mineralisation. The largest potential exploration targets are possibly defined by regional As (>6 ppm) and Sb (>0.6 ppm) anomalies. These anomalies, if real, extend laterally for >150 m from the mineralised shear zone into areas of apparently unaltered rocks. Anomalies defined by Te (>10 ppb), W (>0.6 ppm), carbonation indices, local enrichment of Sb (>2.0 ppm) and As (>28 ppm), and potassic alteration indices also form significant exploration targets extending beyond the HJB shear zone and the Au anomaly (>6 ppb) and, locally, into apparently unaltered rock. Gold, itself, has a restricted dispersion, with an anomaly extending for 1–35 m from ore, and being restricted to within the shear zone itself. Amongst individual geochemical parameters, only As and Sb define significant, consistent and smooth trends (vectors) when laterally approaching the ore. However, the respective dimensions of individual geochemical anomalies can be used as an extensive, though stepwise, vector towards ore.