Evolution of fractures and alteration in the Sierrita-Esperanza hydrothermal system, Pima County, Arizona

Abstract

The Sierrita-Esperanza porphyry copper complex is central to a large fossil hydrothermal system located in the southeastern part of the Sierrita Mountains, Arizona. Abundant, closely spaced fractures exert a profound control of sulfide minerals and alteration in the mineralized center of the system. The extent and limits of the hydrothermal system in peripheral areas have been defined in this study by determination of the distribution and abundance of the altered and mineralized fractures. Mapping of these elements to their limit of occurrence has resulted in recognition of an area of about 60 km 2 where fractures related to porphyry copper mineralization occur and a contiguous area of about 40 km 2 where slightly older altered fractures, related to the first stages of Laramide igneous activity in the region, are present.Within the orebody, distinctive fracture types, as recognized by fracture style and alteration assemblages, occur in a consistent paragenetic succession. This same succession has been recognized in the areas peripheral to the pits but with variations for each alteration type as influenced by wall-rock mineralogies. The establishment of a sequence of vein alteration, commencing with quartz-orthoclase alteration, passing through precipitation of quartz and sulfides, and ending with formation of epidote-orthoclase-chlorite, has allowed separation of times of fracture formation in the orebody periphery.Fracture density (fracture length/sample area) maps have been prepared from data collected at 1,310 stations in the study area. The data were processed and transformed into maps of moving average contours of fracture abundance for different times of fracture formation, as interpreted from the alteration paragenesis. The distinctive differences in alteration types in crosscutting veins reveal the episodic nature of fracturing and the fracture density maps show shrinking of fractured rock volumes toward the central porphyry mass with successive fracture events. Evaluation of the fracture area in a 1-km-thick slab of the 40-km 2 area at the time of precipitation of quartz and sulfide minerals in veins indicates that 4 X 10 5 km 2 of crack area existed. A slightly smaller value of 6 X 10 4 km 2 per km-thick slab existed at the time of the earliest quartz-orthoclase alteration.