Abstract

Disseminated pyrite-chalcopyrite mineralization of the porphyry copper type occurs in a porphyritic dacite (Dacite I)--one of three major phases of the igneous complex at Copper Hill. This dacite intrudes a quartz diorite and is intruded by a second porphyritic dacite (Dacite II).The mineralized dacite was a hornblende-plagioclase porphyry, texturally similar to other dacites found within the area. However, chlorite + sericite + pyrite + calcite + or - epidote alteration almost obliterates the porphyritic texture in some cases. Changes in the relative abundances of sericite, chlorite, and epidote reflect several distinct zones within the altered Dacite I as follows: an intensely sericitized core, a chlorite + calcite-rich zone, an epidote + calcite zone with much fresher plagioclase, and a peripheral zone with epidote, remnant magnetite, and occasional hornblende phenocrysts.Chlorite and sericite become increasingly Fe rich and plagioclase more calcic away from the core.The major chemical changes are decreases in Si, Al, and S, with increases in Fe, Mn, Mg, Ca, and water from the central sericitic zone through the chlorite to the epidote-calcite zone. K, Ba, and Rb are highest in the central zone, while Na, Sr, and Cu are highest in the chloride zone.Zonation also occurs in the sulfide mineral distributions. The central sericitic zone is mainly pyritic. The highest chalcopyrite contents occur in the surrounding chloritic envelope, the chalcopyrite/pyrite ratio then decreases outward. High Cu contents also occur in the margins of the intruded volcanics. Mo and Ag are associated with the two innermost zones. Pb, Zn, and some barite occur toward the boundaries of Dacite I.All these changes are envisaged as resulting from saline fluids which introduced K, S, CO 2 , Cu, Pb, Zn, and some Si, Fe, and Ca to the system and removed Na and Sr.

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