Abstract

The San Manuel copper deposit of southwest Arizona was discovered by drilling done under the strategic-minerals program of the United States Government. The ore is disseminated throughout a large zone in quartz monzonite (Oracle granite) of Precambrian age and monzonite porphyry of Tertiary (?) age. Oxidation and enrichment have played an important part in development of present conditions within the deposit.The oxidation and enrichment took place during an erosion cycle that preceded the present cycle and probably the last two cycles. A conglomerate, here referred tentatively to the Gila conglomerate, was laid down on the eroded and oxidized ore deposit; in some places the conglomerate reaches a thickness of over 1,000 feet and perhaps much more. The age of the Gila conglomerate is believed to be late Pliocene and possibly Pleistocene, but solution of the problem of the correlation of the conglomerate in the San Manuel area requires detailed work over a large area. Extensive tilting and faulting followed the deposition of the conglomerate. Alluvial slope gravels were laid down across the eroded edges of the conglomerate, and later faulting affected both gravels and conglomerate.The zones of oxidation and enrichment are not related to the present water table. The depth to the bottom of the oxide zone ranges from 285 feet to 1,630 feet. Differences in surface altitudes and probably tilting of the original oxide zone account for the great differences in depth.Chrysocolla is the most important copper mineral in the oxide zone, which contains tens of millions of tons of low-grade material averaging about 0.70 percent copper.Sulphide enrichment was variable. In general the increase in copper content averages only a few tenths of a percent. Some holes in the eastern part of the area show a relatively thin zone that was enriched fairly heavily. An unusual feature is the presence of considerable disseminated native copper and goethite in the sulphide zone.

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