UBC Theses and Dissertations
Structural controls on mineralization and constraints on fluid evolution at the Sacrificio Cu (Zn-Pb-Ag-Au) skarn, Durango, Mexico Patterson, Keith Mark
The Sacrificio deposit, located in Durango State, Mexico, is hosted by mid- Cretaceous limestone, chert, and minor shale of the Cuesta del Cura and Indidura formations. Intrusive rocks include two temporally and compositionally distinct suites, dated at 109.1 ± 0.4 Ma (mid-Cretaceous) and 40.1 ± 0.5 Ma (Middle Eocene) by U-Pb methods. The Middle Eocene granite suite is associated with the development of skarn and related sulphide mineralization that forms the Sacrificio deposit. Three episodes of pre-intrusion regional deformation (D^ to D3 ) and two episodes of syn-intrusion local deformation are recognized at the Sacrificio deposit. Skarn and skarn-related mineralization is localized by these regional and local structures and their intersections. Alteration at the Sacrificio deposit is divided into pre-mineralization contact metamorphism, prograde skarn, and retrograde skarn. Styles of skarn-related sulphide mineralization include: (1) disseminated bornite and lesser chalcopyrite filling porosity in prograde garnet skarn; (2) fracture-controlled, silver-rich, bornite-chalcopyrite + sphalerite ± galena ± arsenopyrite cutting prograde garnet skarn and marble; and (3) semi-massive to massive sulphide bodies (mantos) containing sphalerite, galena, chalcopyrite, arsenopyrite, pyrrhotite, pyrite, and bornite associated with retrograde amphibole skarn. Rare quartz-arsenopyrite veins cut all alteration and mineralization. Skarn and sulphide-forming fluids at the Sacrificio deposit are interpreted to be magmatic in origin from lead isotope data. Compositions of garnet (Ad ₆₅- ₁₀₀Gr ₀.₃₅) and pyroxene (Di ₆₈.₉₄HD ₅.₃₀Jo₁.₃) from prograde skarn classify the Sacrificio deposit as a copper skarn. Fluid inclusion studies indicate that mineralizing fluids were dominantly H20-NaCI mixtures (0.5 - 19.7 wt. % NaCI eq.) containing divalent cations (i.e., Mg²⁺, Fe²⁺, and Ca²⁺) . Pressures during the development of the Sacrificio deposit were approximately 0.5 to 1.0 kbar. Temperatures recorded during skarn formation and sulphide deposition show a gradual cooling of the skarn system. Prograde skarn formed at 460° to 580° C, disseminated Cu-Ag mineralization at 291° to 504° C, and mantos at 247° to 396° C. A final pulse of hotter fluids (318° to 480° C) formed the late quartzarsenopyrite veins. Results of this study have allowed the construction of a detailed structural and fluid evolution model for the Sacrificio deposit. Application of these findings to other skarn deposits or prospects in Mexico, and elsewhere in the world, may lead to the discovery of important new orebodies.
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