Nature and source of mineralizing fluids at the Presidio mine, Shafter district, Presidio County, Texas

Rambaud, Fabienne Michèle
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The object of this research is to determine the nature and source of the mineralizing fluids that formed the Presidio Zn-Pb-Ag deposit, and to determine the relationship of the Presidio deposit to the rest of the Shafter district, including the Red Hills porphyry Cu-Mo deposit, exposed about 7 km to the west. The Presidio deposit is located in the Trans-Pecos mid-Tertiary volcanic field, near the Chinati Mountain caldera margin. Deformation of the Permian and Cretaceous strata has been attributed to the late Cretaceous to early Eocene Laramide Orogeny, with additional effects related to local caldera development. The ore-hosting Permian carbonates are highly fractured below the pre-Cretaceous unconformity, which allowed mineralizing fluids to form manto deposits. Recent studies have established that the Red Hills intrusion is of Laramide age (64 Ma). Thus, mineralization in the Shafter district is unlikely to be related to the 32 Ma Chinati Mountain caldera. A plutonic body extending from the Red Hills to the east could have been the source of mineralizing fluids that flowed eastward (and upward) creating a metal zonation from Cu-Mo to ZnPb-Ag. Extensive cores drilled between Shafter and Red Hills provide a valuable data base to attempt to link the two parts of the system. A fluid inclusion study was performed on calcite and quartz samples collected in the Presidio mine and on calcite, quartz, and sphalerite collected from deep core intercepts west of the mine. Based on eutectic behavior of fluid inclusions two possible mineralizing fluids are suggested. H₂O-NaCl-KCl and H₂O-NaCl-CaCl₂. Both could represent a magmatic hydrothermal system. Isotopic studies of calcite revealed that oxygen isotopes of formation waters are between 17 and 6‰ VSMOW. The spread of δ¹⁸O values overall is interpreted to result from a mixing of high δ¹⁸O values fluids, likely caused by high temperature exchange in the sedimentary rocks, with small amounts of meteoric water or other low δ¹⁸O fluid. Carbon isotope results suggest input of CO₂ and/or CH₄ likely from organic compounds in underlying sedimentary strata. Sulfur isotope analyses show a narrow δ³⁴S range near 0 ‰ CDT and suggest a magmatic source for the sulfur involved in sulfide mineralization. Fractionation indicated by sulfide pairs suggests a mineralization temperature of 170° to 294° C. The metal zonation, fluid inclusion, and stable isotope data suggest a magmatic source, probably from a largely buried Laramide-age magmatic system that produced a hydrothermal system that cooled laterally (Red Hills to Shafter), vertically, and over time, perhaps in response to fluid mixing