Abstract

The Scuddles volcanogenic massive sulfide prospect together with the nearby Gossan Hill (Golden Grove) prospect constitute the largest known potentially economic Cu-Zn sulfide deposits in the Archean Yilgarn block of Western Australia. Enclosed in a submarine sequence of felsic to intermediate volcanics, epiclastics, and exhalative chemical sediments, the Scuddles deposit lies in a regionally extensive mineralized horizon. This horizon is up to 80 m thick and is dominated by sulfidic and cherty exhalites, and epiclastics. A sequence of felsic to intermediate lavas, pyroclastics, and epiclastics lies above the mineralized horizon, with felsic pyroclastics and epiclastics beneath. Pyroclastic rocks may have been resedimented by mass flow processes. The entire sequence has sustained the effects of hydrothermal alteration and regional metamorphism and dips steeply to the west, lying on the east limb of a major synclinal structure. Due to alteration, major element values of the rocks are not an accurate guide to their primary composition. However, abundances of Ti, Zr, Y, Nb, and V are commonly unaffected and confirm an andesitic to rhyolitic spectrum. Zoning of ore metals in the Scuddles mineralized horizon is typical of that in many Precambrian felsic volcanic-associated exhalative sulfide deposits. The top is defined by a laminated magnetite-bearing iron-formation (enriched in Fe, Mn, and base metals) which conformably overlies several lenses of massive Fe-Zn sulfides (in places containing significant Pb, Ag, Au, and Cu) grading downward into pyritic sulfides. In turn, the massive sulfides overlie a zone of epigenetic stockwork, stringer, and disseminated Cu-Fe sulfides in the middle and lower portions of the mineralized horizon. Massive to disseminated exhalative sulfides are in places accompanied by quartz, chlorite, talc and other silicates, carbonates, and magnetite, also interpreted to be at least partly exhalative in origin. The lower, Cu-Fe sulfide zone is considered to have formed immediately below the sea floor, perhaps occupying an extensive feeder system for hot springs. The upper, Zn-Fe sulfide zone may have formed by precipitation out of hydrothermal plumes as they vented into and mixed with seawater.Ore mineral deposition was attended by the development of a volumetrically large (> 10 km long, several hundred meters thick) strata-bound zone of hydrothermal alteration, well-represented in the footwall and mineralized horizon. Upper hanging-wall rocks are little altered, but elsewhere the sequence displays moderate to intense feldspar-destructive development of quartz, chlorite, sericite, and locally abundant carbonate, talc, and sulfides. These mineralogical changes are accompanied by pervasive depletion of Na, Ca, and Sr (and locally, K). The mineralized horizon and much of the footwall is enriched in Fe, Mg, Mn, base and precious metals, As, S, CO 2 , and H 2 O, interpreted to be the result of addition from exhalative sources. The strata-bound alteration zone represents a volume of hydrothermal flux in which major sites of sulfide deposition at Scuddles and Gossan Hill were focused zones of Fe-Mg-base metal enrichment below, and adjacent to, positions of submarine metal-bearing exhalations. Precipitation of exhalites occurred under relatively quiescent conditions in the mineralized horizon; however, the stratigraphic sequence indicates that, at other times, active mass flow deposition was taking place.

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