Abstract

Metatholeiites and interflow sediments of Cambrian age are exposed in the Lake Cooper quarry located in the northern segment of the Heathcote greenstone belt, central Victoria, Australia. The exposed sequence is 281 m thick and comprises nine massive to pillowed lava flows and seven interflow sequences. The metabasalts from the Lake Cooper quarry have strong petrological and geochemical similarities to normal midocean ridge basalts (N-MORB) and to metabasalts from greenstone suites elsewhere in Victoria, suggesting that they have not been exposed to extensive hydrothermal alteration. The interflow sediments in the quarry are subdivided into units comprising finely bedded sulfides, layers of sulfidic carbonaceous argillites, chert, and sedimentary carbonate rocks. Unusual mineral assemblages in these sediments, as well as their trace element concentrations and rare earth element signatures, suggest an exhalative volcanogenic origin. Whereas gold values in the tholeiites from the Lake Cooper quarry generally remain below 2 to 5 ppb, gold contents in the interflow sediments, which are also characterized by elevated As, Cu, and Zn values, range from <5 to 243 ppb and average 67 ppb. On the basis of sedimentological features and sulfide petrology, accumulation of the ore metals in these sediments is inferred to have occurred during deposition of the exhalative precipitates, possibly as a result of extraction and transfer of ore metals from the thick boninite sequence known to occur below the tholeiites in Victoria. Mass balance calculations show that remobilization of gold from the interflow sediments by metamorphic fluids and subsequent redeposition within dilatation zones and structural traps at higher crustal levels during the early to middle Paleozoic could account for a significant component of the central Victorian gold province. Thus, these exhalative metasediments might have played a vital role in accumulating, preconcentrating, and providing the gold required to form the world-class turbidite-hosted lode deposits in the Lachlan fold belt of Victoria.

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