Fluid inclusion studies at the Donlin Creek Gold Deposit, Alaska, possible evidence for reduced porphyry-Au to sub-epithermal transition

摘要

Donlin Creek contains a resource of 718 tons gold associated with illite altered rhyodacite dikes and quartz-ankerite-dolomite-arsenopyrite-pyrite veinlets. The district contains 3 styles of mineralization; early high-temperature reduced porphyry-Au veins at Dome, later low-temperature sub-epithermal ACMA-Lewis veins/veinlets that comprises the main gold resource, and minor late quartz-stibnite veins. Veins at Dome contain coexisting high salinity brine and low salinity CO_2-CH_4+-N_2 bearing vapor, and formed between 260 to 410°C at 1500-3000 m depth. Veins at ACMA-Lewis formed from low salinity (2.5 wt.% NaCl equivalent) CO_2+-N_2 bearing (2 to 6.5 wt.% CO_2) aqueous fluid inclusions between 140-220°C at around 1600 m depth. A third fluid, which is possibly transitional between fluids at Dome and ACMA-Lewis, is found mainly around the margins of Dome, and partially overlaps ACMA-Lewis veins. This marginal fluid is characterized by low salinity aqueous fluid inclusions that homogenize between 230 and 300°C and contain significant CO_2-CH_4+-N_2. We propose that as the reduced (pyrrhotite stable) magmatic-hydrothermal fluids at Dome boiled, a low-density H_2O-CO_2-CH_4+-N_2 vapor phase sequestered Cu-Au-As-Sb-Hg as volatiles and transported them into overlying low salinity waters. As these low salinity waters cooled Cu and other elements were deposited at temperatures as low as 230 and 300°C, while Au-As-Sb-Hg were deposited mainly between 140-220°C as a result of boiling. The high CO_2 content of the ore fluid resulted in boiling and Au deposition below the traditional epithermal environment.