HOW ZN-PB-RICH ARE SEDIMENTARY BRINES THAT FORM CARBONATE-HOSTED ZN-PB ORE DEPOSITS? RECENT PROGRESS AND UNRESOLVED QUESTIONS

Knowledge of the concentrations of Zn and Pb in sedimentary ore-forming brines is essential to understanding the origin of carbonate-hosted Zn-Pb ore deposits, including mechanisms of precipitation, the mode of metal transport, and the duration of ore formation. However, determining the concentration of Zn and Pb in sedimentary ore-forming brines has been challenging. The Zn and Pb concentrations reported in analogous modern sedimentary brines range over several orders of magnitude, to maximum values of nearly 1000 ppm and about 100 ppm, respectively. Attempts to calculate Zn and Pb concentrations theoretically from solubility relationships are hampered by large uncertainties in the pH, redox potential, and sulfur concentrations of the ore fluids. LA-ICP-MS analyses of fluid inclusions have yielded important insights but remain equivocal in other respects. For example, fluid inclusions hosted by dolomite gangue in MVT deposits are found to have low Zn and Pb concentrations up to 1’s of ppm, but this dolomite at best only partially overlaps periods of sulfide mineral precipitation in the paragenetic sequences. LA-ICP-MS analyses of sphalerite-hosted fluid inclusions from various types of carbonate-hosted Zn-Pb deposits ranging in scale from trace occurrences to large ore bodies show wide-ranging apparent Pb concentrations over several orders of magnitude, up to 1000’s of ppm. If these concentrations genuinely represent aqueous solute, then they strongly imply that mixing between metal-rich and sulfide-rich fluids is a common mechanism for sulfide mineral precipitation. However, fluid inclusion Pb concentrations do not correlate well with other elemental concentrations and are not highly consistent within fluid inclusion assemblages, raising the possibility that measured Pb signals actually represent Pb mineral accidentals randomly entrained within fluid inclusions. Ore fluid Zn concentrations predicted from disequilibrium distribution coefficient theory using solid solution Zn concentrations in limited occurrences of ore-stage calcite are in the range of 1’s to 10’s of ppm. However, this theory has not worked well for predicting concentrations of Pb and other elements that preferentially form carbonate minerals with structures other than calcite.