This study relates morphology, size, texture, and chemistry of arsenian pyrite in the Twin Creeks Carlin-type gold deposits to the chemical position and amount of gold that it contains. Arsenian pyrite is confirmed as the most important gold-hosting mineral and four types of gold-bearing arsenian pyrite are recognized. Among these four types, the lowest amount of gold and arsenic (<1 wt % As; 17-60 ppm Au) is contained in arsenian pyrite with relatively large grain sizes (10-30 mu m). It formed at relatively high temperatures (250 degrees C) and K (super +) /H (super +) ratios (adularia stable), probably relatively slowly. The largest amount of gold and arsenic (1.05-2.43 wt % As; 595-1,465 ppm Au) is contained in arsenian pyrite with small grain sizes (up to 2 mu m) that formed at low temperatures (120 degrees -200 degrees C) and moderate K (super +) /H (super +) ratios (illite stable), probably by relatively rapid deposition.XANES, EXAFS, and SIMS analyses show that gold in arsenian pyrite is both structurally bound (Au I ) and in submicroscopic inclusions of free gold (Au 0 ). Highest amounts of structurally bound gold were observed in the relatively coarse-grained arsenian pyrite noted above, whereas most inclusions of free gold were observed in fine-grained arsenian pyrite. In general, there is a good correlation between gold and arsenic in arsenian pyrite, although Au/As ratios vary from one pyrite to another. It is suggested that gold in arsenian pyrite was deposited by direct deposition of native gold and by adsorption of Au I from hydrothermal solutions. Adsorption of gold by arsenian pyrite can take place from solutions that are not saturated with respect to native gold. Relative volumes of gold-bearing pyrite from different paragenetic stages at Twin Creeks suggest that as much as 50 percent of the total gold in the deposit could have formed from solutions that were not saturated with respect to native gold.Because both Au 0 and Au I are found together in arsenian pyrite, it is likely that they were deposited by the same process. Consideration of possible depositional reactions suggests that a decrease in the activity of reduced sulfur is most likely to cause gold deposition.

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