We've Moved!
Visit SDSU’s new digital collections website at https://digitalcollections.sdsu.edu
Description
From 1897 to 1911 the King of Arizona and North Star Mines produced over 220, 000 ounces of gold and 90, 000 ounces of silver from vein gold deposits. The North Star Mine lies at the base of a 900-meter-thick Tertiary sequence of thick rhyolite flows, ignimbrites, and andesite flows. The underlying metasedimentary rock attains amphibolite facies metamorphism and is intruded by northeast-trending pegmatite dikes and plugs. The King of Arizona Mine is found in an exotic volcanic breccia that lies in or near a volcanic vent. A pervasive northwest-trending dike swarm intrudes the district. The Kofa Mountains are divided into numerous fault blocks that dip to the northeast. Ignimbrites and rhyolite flows onlap against steep slopes composed of Tertiary rhyolite flows and Mesozoic metasedimentary rock. Most faults in the district have a west-northwest strike. This fault set is offset by a fault set which strikes north. Manganese, lead, and minor gold mineralization is found on this fault set. Virtually all the precious metal mineralization is found on the west-northwest fault set, but the mineralization often occurs where the host vein is offset by faults that strike north. The geology of this district is very similar to the geology found at other precious metal deposits of the southwestern United States. The North Star Vein and King of Arizona Vein possess a mineralogy that is typical of vein gold deposits of manganese oxide association. The North Star Vein contains abundant chalcedony, vuggy quartz, chlorite, manganese, and iron oxides, calcite, adularia, pyrite, manganiferous calcite, boulangerite, epidote, fluorite, and clay. The King of Arizona Vein contains quartz, vuggy quartz, calcite, manganiferous calcite, manganese oxide, pyrite, adularia, tourmaline, polybasite, and argentite. Both of these veins have had episodes of intramineralization brecciation. Numerous mineralized veins in the district are filled with manganese oxide. Unfortunately there is no spatial relationship between specific manganese minerals and gold mineralization. Wall rock alteration on the North Star Vein is typical of that found on previous metal vein deposits. The rock near the vein is silicified and adularized; the rock around the ore deposits has phyllic alteration, and propylitic alteration extends a great distance from the vein. Alteration at the King of Arizona Mine is complicated by multiple episodes of alteration that are a result of volcanic venting and diking. Metasedimentary rock within and surrounding the volcanic agglomerate are propylitized. The King of Arizona Vein is adularized, pyritized, and enveloped by zones of pyrite and silica alteration. An exploration model for the district is based on the assumption that undiscovered ore deposits are located in the same geological environments as the North Star Mine and King of Arizona Mine. These deposits are hosted by veins which strike west-northwest and are offset by faults which strike north. Rhyolite porphyry dikes are found in both deposits. Alteration mapping reveals that different exploration strategies should be used for the two main veins. At the North Star Vein, phyllic alteration is found around two ore sites and is a useful guide for ore. At the King of Arizona Vein, silica and pyrite alteration serve as exploration guides.
