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Description
The Silver mining district consists of thirty square miles of the southern end of the Trigo Mountains, Yuma County, Arizona. From 1879 to 1900, 2,329,059 pounds of lead and 1,559,201 ounces of silver were recovered largely from argentiferous galena and silver halides from oxidized fault-fissure veins of the Red Cloud and Clip mines. The rocks of the district include Pre-Tertiary schist, gneiss, phyllite, hornfels, and amphibolite intruded by a late Cretaceous or early Tertiary granodiorite stock, overlain by middle Tertiary volcanic flows, flow breccias, and pyroclastics of andesitic to rhyolitic composition. All of these rocks are unconformably overlain by basalt of Pliocene or Pleistocene age. Schistose and gneissic rocks display a prevailing parallel trend of lineation, foliation, and fold axes to the northeast with steep dips to the southeast or northwest. Persistent north to northwest-trending normal faults are occasionally offset by later strike-slip faults with east-west trends. Mineralization occurs in north to northwest-trending fault fissures associated with granodiorite intrusion. More favorable zones of ore deposition are at intersections of these faults with transverse faults, where ore shoots have formed. Typical ore deposits are limited to a depth of approximately 200 feet of limonitic, quartz-barite-calcite fissure-fillings with irregular zones containing argentiferous galena, lead oxides, argentite, silver halides, cerussite, smithsonite, and anglesite. Pyrite and sphalerite are thought to have been present in the primary ore, but oxidation and supergene action has removed all traces of these sulphides. Ore shoots usually are limited to lengths of less than 100 feet and widths of less than 10 feet. Average assay values are 6 per cent lead, 4 per cent zinc, and 10 ounces of silver, and 0.02 ounces of gold per ton of ore. Future exploration in the district can be supplemented with trace analyses of carbonate and silicate rocks associated with normal faulting and hydrothermal activity to define promising ore targets. Improved ore recovery methods and an analysis of ore reserves in the district are desirable.
