Alteration-mineralization zoning, Red Mountain, Arizona

Abstract

The Red Mountain porphyry copper deposit is a hypogene deposit, occurring at depths of more than 3,500 feet beneath surface exposures of high-pyrite, phyllic alteration. The deposit was discovered as the result of deep drilling, predicated on the pattern of vertical zoning in alteration and mineralization noted during initial exploration of supergene chalcocite mineralization.The alteration-mineralization system is believed to be centered on a caldera subsidence structure which was associated with explosive volcanism and subvolcanic intrusive activity. Surface exposures reflect a zoned pattern of alteration and mineralization, centered on an area of phyllic alteration and copper-molybdenum mineralization that is surrounded successively by zones of pyritic-argillic and propylitic alteration, with effects of hydrothermal alteration evident over an area seven to eight miles in diameter. Vertical zoning in alteration mineralogy appears to be related to a gradual decrease in sulfur content with increasing depth and shows a gradational change from near-surface, sulfur-rich phyllic alteration, through weak-potassic alteration, to lowsulfur potassic alteration at depth. The lateral and vertical zoning pattern is also reflected by the distribution of lead, zinc, molybdenum, and copper minerals, both in the zones of pervasive disseminated sulfides and within the exterior veins of the propylitic alteration zone, which are considered to be an integral part of the alteration-mineralization system.The Red Mountain alteration-mineralization system exhibits two different types of hypogene copper mineralization. Enargite is associated with the near-surface, highpyrite, phyllic and pyritic-argillic alteration. Chalcopyrite occurs with weak potassic and potassic alteration at depth. Although not ore-grade, the enargite mineralization did provide a protore source for the copper that was later concentrated in a high-level chalcocite enrichment blanket. Ore-grade chalcopyrite mineralization occurs at depths of 3,500 feet or more beneath the surface. The zoning pattern is characterized by a gradual increase in the grade of copper mineralization with increasing depth, within the zone of weak potassic alteration and the upper part of the potassic alteration zone.