Abstract

The porphyritic to seriate-textured quartz monzonite host-rock at Bagdad was changed by hypogene alteration to a granular rock in which the plagioclase became albitic, orthoclase and quartz increased in amount, and hornblende and book-biotite were recrystallized to pale brown leafy biotite. Hypogene sulfides, pyrite and chalcopyrite, were added during the process of alteration, accompanied in part by sericite. Molybdenite is younger than the chalcopyrite. Locally, the host rock was altered to a quartz-orthoclase-sericite facies containing more quartz and orthoclase than is found in the main alteration facies. Chemical analyses indicate that alteration and mineralization of the host rock resulted in loss of iron, lime, and soda, and addition of sulfur, copper, and potash.Supergene changes include the formation of a chalcocite blanket in late Pliocene or early Pleistocene time. Chalcocite replaced chalcopyrite in preference to pyrite. The chalcocite blanket is truncated by the present erosion surface and its distribution can be recognized from the character of the iron oxide left in the leached cavities. Along some of the fractures and faults, supergene clay is present, consisting of kaolinite and a member of the saponite-montmorillonite group.

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