The genesis of the bellerophon gold deposit, St Ives Camp, yilgarn craton- an example of a magmatic-hydrothermal gold deposit

The Bellerophon gold deposit in the St Ives camp, Yilgarn craton, Western Australia, has been studied to assess the importance of magmatic systems in the formation of ancient gold deposits. The majority of the gold grains occur in the quartz-albite-carbonate-pyrite veins and quartz-pyrite veins. Intense albite and hematite alteration surrounds the mineralized veins, which show enrichment in Na, S, and Sr, and depletion in K, Cs, Rb, Cr, V, and Ni, compared with their unaltered equivalents. During mineralization, Bi, Te, Pb, and Mo were introduced together with Au. Multi sulfur isotopes measurements of pyrites that are genetically related to the gold mineralization from the Bellerophon, Victory, and Wallaby deposits in the Yilgarn craton, and the Beattie deposit from the Abitibi belt, Canada, show that the sulfur in the gold-bearing fluid is dominated by mass dependent fractionation (MDF), which requires the fluid to have a mantle origin or to have been derived from magma of mantle origin. Three types of fluid inclusions were identified: Type I low salinity carbon dioxide inclusions, Type II high salinity aqueous inclusions, and Type III disordered carbon bearing high salinity aqueous inclusions. Type I fluid inclusions can be further divided into Type Ia and Type Ib with high and low ratios of carbon dioxide to water, respectively. The similarity between homogenization temperatures, and the occurrence of the Type Ia and Types II inclusions within the same clusters suggest that they formed by liquid immiscibility at about 300 degree celcius. Type III fluid inclusions, which have high temperature (exceed 550 degree celcius), high salinity (38 weight percent NaCl equiv.), and a redox state that lies close to the carbon dioxide and methane buffer, are interpreted to represent the parental hydrothermal fluid. The exsolution of methane from the hydrothermal fluid, during fluid immiscibility, lead to oxidation of the remaining fluid, which in turn triggered gold precipitation. Two types of hydrothermal scheelite were identified in association with mineralization, Type I, which is enriched in the middle rare earth elements (REE), and Type II, which has a flat REE pattern. Hydrothermal apatite has a similar REE pattern to the Type I scheelite. Metamorphic apatites are distinct from the hydrothermal apatite and scheelite in geochemistry. The Sr isotopes composition of hydrothermal scheelite is similar to that of apatite from the host rocks, which indicates that the Sr isotopes in hydrothermal fluid have been modified by interaction with the wall rocks. The wide range of Sr isotopic compositions suggests that the ore-forming fluid has been transported over a long distance. The age of the Bellerophon gold deposit was determined by Ar-Ar dating muscovite extracted from ore veins. The age obtained was 2614 Ma, which is appreciably younger than the associated monzonite intrusions, which have U-Pb zircon age of 2660 Ma, and cannot be the source of the mineralization. Combining all the published ages for gold mineralization in Yilgarn craton showed that the mineralization form over an extended period from ca. 2700 Ma to ca. 2600Ma