ISOTOPIC AND GEOCHEMICAL CONSTRAINTS FOR WITHERITE (BACO3) FROM THE SEDIMENT-HOSTED ZN-PB-AG MASSIVE SULFIDE DEPOSITS, RED DOG DISTRICT, ALASKA

Massive sulfide deposits in the Red Dog district contain the largest zinc reserves in the world, and include the Main deposit at Red Dog (M) as well as the Anarraaq deposit (AQ) 10 km north. Barite overlies and spatially overlaps the sulfide orebody at the M deposit; barite at AQ overlies but is separated from the sulfide body by 100 meters of organic-rich black shale. At both localities, witherite (BaCO₃) occurs late in the paragenetic sequence. It also occurs as a vein in barite (with no known sulfides) in stratigraphically similar units in the Howards Pass (HP) quadrangle 170 km east of Red Dog. Stable isotopes and chemistry can be used to constrain conditions during formation of the witherite. Samples have light C: -16‰ (M), -21‰ (AQ), and -25‰ (HP). Corresponding O isotope values (VSMOW) are 3‰ (M), 1‰ (AQ), and 18‰ (HP). The C isotopes suggest that carbonate-carbon was primarily derived from oxidized organic matter (OM) with HP representing 100% organic carbon, AQ about 80%, and M about 60%. Oxidized OM could be a product of syngenetic / diagenetic bacterial sulfate reduction (BSR), later higher temperature thermochemical sulfate reduction (TSR), or very late oxidation of OM by oxygen in ground water. The lightest C (HP) is the most closed system with respect to inorganic carbonate contribution. Light O in the M and AQ samples suggest a meteoric water (low temperature) source, whereas HP oxygen in witherite can be from water that equilibrated with silicate oxygen, possibly at higher temperatures, or directly from barite oxygen. Further chemical constraints can be placed on witherite: The SO₄/CO₃ value must have been low (<0.05) with a Ca/Ba ratio of <0.1. Low sulfate conditions can be obtained at the end of BSR or TSR, which also provides high carbonate activity and alkalinity. Ba can be released from barite under alkaline pH or high temperature reduction of sulfate by organic C. In summary the witherite at HP was a result of closed system TSR during burial with later dispersal of H₂S or HS^-. At M and AQ, witherite was post-ore, probably post-burial, formed during uplift-near surface conditions where oxidation of OM and late BSR gave the light C and meteoric O values. Witherite should be sought as an accessory mineral because it can be used to constrain environmental conditions, such as source of fluids, temperature, and bacterial processes.