Geology and origin of mineralized breccia pipes in Copper Basin, Arizona

Abstract

The Copper Basin district, Yavapai County, contains a Cu type Cu-Mo deposit in which the bulk of ore mineralization appears to have been controlled by pipe structures. Mineralization is associated with a composite stock of presumed Laramide age which has intruded older Precambrian metamorphosed sediments and intrusive rocks. The Laramide stock is thought to have an elongated beet shape with a restricted orifice at depth. Its intrusive units range in composition from diorite to aplite, but mineralization appears to be most closely associated with quartz monzonite and quartz monzonite porphyry units. Mineral deposits of the district have a rough zonal arrangement with Cu-Mo mineralization in the center surrounded by an aureole of Pb-Zn-Ag occurrences. Mineralized breccia pipes are roughly cylindrical, near vertical structures ranging in diameter from 50 to 600 ft. A composite pile is composed of a central core of heterogeneous, rotated, angular to rounded rock fragments surrounded by a zone of nonrotated crackle breccia. The fragments are cemented by quartz and may be mineralized with pyrite, chalcopyrite, and molybdenite. The pipes are thought to have been conduits through which late magmatic fluids, collecting near the restricted root of the stock, passed upward. An individual pipe was initiated by passage of fluid upward along a vertical line such as an intersection of faults or fractures. From this channel, fluid worked into the adjacent fractured rocks and corroded fragments that were eventually loosened and moved in the conduit, gradually enlarging the pipe. Quartz later precipitated, filling all of the open space and choking the conduit. Successive flexures of the stock or recurrent movement on regional faults fractured this quartz in some of the pipes, and later hydrothermal solutions tended to follow these conduits, both because of their location on deep-seated structures and because the fractured quartz provided relatively high permeability. Hydrothermal solutions, preceding and accompanying ore deposition, spread outward from the pipes and formed overlapping aureoles of alteration. Pyrite, chalcopyrite, and molybdenite were deposited along fracture surfaces throughout a large area in Copper Basin, but higher grade mineralization was generally confined to fractured pipe structures.