MASS-INDEPENDENT FRACTIONATIONS OF FOUR SULFUR ISOTOPES AS TRACERS OF ARCHEAN BASE METAL AND GOLD ORE-FORMING FLUIDS

The early Archean (3.24 Ga) Panorama volcanic-hosted massive sulfide deposits of the Pilbara Craton, Western Australia, were formed by deep water fumarolic activity above the subvolcanic Strelley Granite (Vearncombe et al., 1995; Huston et al., 2001; Buick et al., 2002). It has recently been argued that the metals were leached from large volumes of andesites/basalts at the base of the volcanic pile (Kangaroo Caves Formation) and redeposited at fluid conduits near its top to form the evenly-spaced ore bodies (Huston et al., 2001). This interpretation challenges current enthusiasm for a direct magmatic source for the metal-rich ore fluids.

Measurements of the ratios of the four stable isotopes of sulfur (³²S, ³³S, ³⁴S, ³⁶S) in pyrite from the ore bodies and the underlying volcanics using a Cameca ims 1270 ion microprobe provide some support for the hydrothermal leaching model. The sulfur in the ore body sulfides has a small component (5%), derived from seawater sulfate that was anomalously depleted in ³³S (D³³S < 0), mixed with a major component (95%) of essentially mantle composition (d³⁴S, D³³S = 0). In the underlying volcanics, pyrite (and sphalerite) is anomalously enriched in ³³S (and anomalously depleted in ³⁶S) indicating that about 12% of the sulfur had experienced gas-phase processing in the atmosphere (Farquhar et al., 2001). However, this sulfur left the atmosphere as insoluble S⁰ (possibly S₈) and was reduced to sulfide during subsurface hydrothermal circulation. There is no evidence that the reduction of sea water sulfate contributed to the sulfur in these widely disseminated sulfides. Most of the sulfur in both the ore bodies and the immediately underlying volcanics is probably magmatic host rock sulfur that, together with the base metals, was leached from lower down in the volcanic pile as suggested by a recent regional geochemical survey (Huston et al., 2001). In contrast, the absence of any mass-independent signature in pyrite from the White Reef, West Rand Consolidated Mine, RSA provides additional evidence for a placer, rather than a hydrothermal, origin for the Wiwatersrand gold.