THE CHEMICAL EVOLUTION OF MELTS AND FLUIDS ASSOCIATED WITH THE PORPHYRY-TYPE COPPER-GOLD-PGE DEPOSIT AT ELATSITE, BULGARIA

The Elatsite Cu-Au-Pd-Pt deposit; Bulgaria, represents one of the few porphyry-type ore deposits with distinct enrichments of PGE's, namely Pd and Pt (e.g., Tarkian and Stribrny, 1999). Here we explore how such a PGE enrichment may have formed in relation to the porphyry-type Cu-Au mineralization, using LA-ICPMS analysis of sulfide and silicate melt inclusions in magmatic amphibole phenocrysts and fluid inclusions in hydrothermal quartz of porphyry-stage ore veins. Sulfide melt inclusions show significant signals for Cu, Au and Pd after correction of the signals on 105Pd and 108Pd for the isobaric interferences of 65Cu40Ar and 108Cd, respectively. Preliminary data reveal concentrations of 1.3 to 2.3 wt-% Cu, a few mg/g Pd and about 1 mg/g Au, resulting in Cu/Au ~15000 and Pd/Au ~5. Early polyphase brine inclusions in porphyry-type quartz-sulfide veins have salinities of up to 51 wt-% NaClequiv, and preliminary metal contents are a few wt-% Cu and a few mg/g Au, translating into a Cu/Au ratio of ~10000. Ratios of Pd/Au in the fluid are variable between 0.2 and 1.2, and Pt was generally below the limit of detection of ~0.2 mg/g. These element ratios compare well with those of the immiscible sulfide melt present in the source magma and suggest complete resorption of immiscible sulfide melt by the aqueous fluid that exsolved from the magma during crystallization (Halter et al., 2002) unless it was preserved as inclusions in phenocrysts. Ratios of Cu/Au and Au/Pd in flotation concentrates are between 10000 - 25000 and 0.05 - 0.1 (Tarkian and Stribrny, 1999). If directly comparable, it would indicate that the precipitation efficiency decreased from Cu to Au to Pd, or that much of the PGE's was lost from the fluid before the onset of Cu-Au precipitation. Further work on both the sulfide melt inclusions and the early porphyry-stage brines is required to confirm the tentative Pd and Pt analyses. The aim is to resolve the importance of source control versus that of precipitation mechanisms for Cu-Au and PGE enrichments observed in the Elatsite ore deposit.

References: Tarkian M. and Stribrny B. (1999) Mineralogy and Petrology 65, 161-183. Halter W. E., Pettke T., and Heinrich C. A. (2002) Science 296, 1844-1846.