Abstract

Quantitative data from the margins of the Sierrita porphyry copper deposit were derived by direct measurements of fractures in the field and fluid temperatures in the laboratory. These data constrain interpretations of the nature and extent of fracture permeability at different stages of cooling of the quartz monzonite porphyry which invaded the Ruby Star Granodiorite.Fracturing coeval with formation of the orebody extends into the granodiorite some 4.5 km from the center of the hydrothermal system. Integrated fracture densities decrease from values of 0.2 to 0.3 cm (super -1) near the system's center to a background value of 0.03 cm (super -1) or less outside of the system. Four mineralogically different vein types with a distinct paragenetic sequence are present. Early veins, characterized by a quartz-orthoclase assemblage, are crosscut by two distinct types of sulfide-bearing quartz veins. The bulk of the copper mineralization is associated with the later of these two events. The final vein set present consists of an orthoclase-epidote assemblage. Distribution of the vein types in the granodiorite indicates that the areal extent of fracturing associated with each type diminishes with time.Vein filling temperatures determined from fluid inclusions at two localities reveal a general decrease in the temperature of deposition of vein quartz in each vein type as the system evolved. Homogenization temperatures in excess of 350 degrees C were observed in the quartz-orthoclase vein set. Temperatures in the quartz-sulfide vein assemblages were considerably lower, never exceeding 320 degrees C, and those in the final orthoclase-epidote vein set were lower still, never exceeding 260 degrees C. Sulfide-bearing quartz veins collected at sites 1.8 and 2.9 km from the system's center showed similar temperatures of formation; however, secondary inclusions representing later events, present at the 1.8-km site, were absent at the 2.9-km site, suggesting that the late, lower temperature event related to their formation was less widespread.

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