GOLD AND ARSENIC IN IRON SULFIDES FROM SEDIMENT-HOSTED DISSEMINATED GOLD DEPOSITS - IMPLICATIONS FOR DEPOSITIONAL PROCESSES

Secondary ion microscopic elemental distribution maps and quantitative measurements of Au and As concentrations in samples from five sediment-hosted disseminated gold deposits in Nevada present a consistent picture of the location and nature of gold in these samples. Gold is present in arsenian pyrite that forms overgrowth rims on, and narrow veinlets in, gold-free pyrite (and possibly other minerals, but only to a minor extent). Auriferous overgrowth rims that may exhibit compositional zoning, either simple or oscillatory, range from approximately 25 mum in thickness to the limits of microscopic resolution. Because all of the observed gold-bearing pyrite contains sufficient As to be metastable (i.e., in excess of approximately 0.5%), it is suggested that small native gold grains occasionally observed in unweathered sediment-hosted disseminated gold ores resulted from exsolution of gold from the arsenian pyrite lattice. Available evidence suggests that gold in arsenian pyrite is most likely present as a charged species (Au+3) and that gold probably was deposited with arsenic as coupled solid solution substitutions in pyrite. The codeposition of gold and arsenic in pyrite in these and many other deposits may be a function of a redox reaction involving oxidation of gold and concurrent reduction of arsenic. Commonly proposed reactions involving gold deposition in sediment-hosted disseminated gold deposits usually involve gold precipitation as a neutral species from a bisulfide complex. It is proposed that such reactions are inappropriate for gold deposition in arsenian pyrite but that reactions involving oxidation of gold and concurrent reduction of arsenic are probably more appropriate. Mechanisms effecting gold deposition as native metal also will effect deposition as Au+3 if it is deposited along with arsenic (in either arsenopyrite or arsenian pyrite).