2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 4
Presentation Time: 1:30 PM-5:30 PM


ANTHONY, Michael W.1, EMSBO, Poul2 and HOFSTRA, Albert H.1, (1)U.S. Geol Survey, P.O. Box 25046, M.S. 973, Denver, CO 80225, (2)U.S. Geol Survey, MS 973, Box 25046, Federal Center, Denver, CO 80225, manthony@usgs.gov

The dual Ion Chromatography (IC) system is capable of analyzing 24 cations and anions in fluid inclusions in hydrothermal minerals and can aid in understanding the source of ore-fluid components and geochemical processes that form ore deposits. This one-of-a-kind dual IC system simultaneously analyzes (F-, Cl-, Br-, I-, NO3-, CO3-2, PO4-3, HS-, SO3-2, SO4-2, S2O3-2, SeO3-2, SeO4-2, and C1-C4 organic acids; Li+, Na+, NH4+, K+, Rb+, Cs+, Fe+2, Mg+2, Ca+2, Sr+2, and Ba+2) at sub-nanogram concentrations in inclusion fluids extracted from very small mineral grains. Main minerals that have been analyzed include: sphalerite, quartz, dolomite, galena, fluorite, calcite, siderite, stibnite, chalcopyrite, and barite. Multiple runs of sphalerite standards from the Lucky Dog mine in Canada by the IC method give consistent results with minimal standard deviation of all concentration levels.

Fluid inclusion compositional data is being used to understand fluid rock reactions and the migrational history of deep crustal fluids, and to develop models for the genesis of a variety of deposit types and assess the regional impact of hydrothermal systems that are geologically and geochemically realistic. Recent results have opened new avenues of study and demonstrate this new method to be an important tool in ore genesis studies. Mississippi Valley-Type (MVT) Zn-Pb deposits have long been known to form from hot brines, yet the source(s) of these brines has remained highly contentious. Implementing the dual IC fluid inclusion method, we were able to measure the chemical composition of inclusion fluids in sphalerite from 17 MVT districts in North America and Europe; when used in combination with new techniques for the display and interpretation of the brine data, this method constrains the origin of the brines, maps brine provinces, and identifies subsequent compositional modifications resulting from mineral precipitation, fluid rock reactions, and fluid mixing. For example, data from this method indicates the commonly invoked mechanism of halite dissolution for brine generation is not indicated in any of the districts studied. Rather, the ore brine are compositionally consistent with residual bitter brines that result from subaerially evaporated seawater. These results place crucial constraints on genetic models for MVT deposits.