The carbon, hydrogen, and oxygen isotopic composition of the hydrothermal fluids responsible for the lead-zinc deposits at Providencia, Zacatecas, Mexico

Abstract

The relative deuterium concentration of water in primary fluid inclusions in calcite, quartz, and sphalerite from several mines at Providencia, Zacatecas, Mexico, was determined to fall in the narrow range --6.8 to --8.3%. Essentially the same range was obtained for each of the three minerals throughout the area and for fluid inclusions having a wide range of filling temperatures and salinities.Detailed studies of the carbon and oxygen isotopic composition of four successive calcite generations from three mines indicate only small variations in the isotopic composition of individual generations. Successive generations of calcite, with one exception, have increasingly heavier carbon and oxygen isotopic compositions. This trend coincided with a 150 degrees C decrease in the temperature of mineral deposition. The results can be interpreted to indicate that the oxygen isotopic composition of the hydrothermal fluid was nearly uniform in time and space during ore deposition, and that the trend toward higher delta O 18 values with time was a result of increasing isotopic fractionation as temperatures of deposition decreased. The results also indicate that the isotopic composition of carbon from the source of the fluids was uniform throughout calcite deposition, and that the trend toward higher delta C 13 values with time for the bulk of calcite deposition was the result of either increasing isotopic fractionation of carbon with decreasing temperature, or the mixing of limestone carbon with deep-seated carbon, or both.The oxygen isotopic composition of the hydrothermal fluids was calculated from the isotopic composition of the calcites, the filling temperatures of the fluid inclusions, and experimental fractionation factors. Results show that the bulk of the calcites precipitated from fluids having an oxygen isotopic composition between 6.4 and 7.9ppm (relative to SMOW). Similar calculations based on the oxygen isotopic composition of quartz fall within the range calculated from the calcite data.The isotopic data are interpreted in terms of the origin and history of the hydrothermal fluids that were responsible for the ore body. The isotopic data strongly point to a magmatic source for the hydrothermal water, and to a deep-seated source for at least 50 percent of the carbon in the calcites. The large salinity variations in the Providencia fluid inclusions, from 5 to 40 equivalent weight percent NaCl, resulted either from events at the source of the fluids, or from intermittent incorporation of sedimentary salt.