Abstract

The Dunmore lode is localized along a persistent fissure zone over two miles long and averaging nearly 100 feet in width along the length of the Dunmore claim. The fault in which the lode is located offset the pre-Cambrian quartzite and slate walls about 4,500 feet prior to deposition of the overlying thick San Juan tuff. Late Tertiary faulting extended the fissure into the San Juan tuff, dropping the south side about 80 feet. Although most of the lode crops out in the pre-Cambrian rocks, it extends upward across the profound unconformity into the tuff. The lode is complex, containing sharply marked fissure veins, sheared slate dragged into the fault zone, a sheared dike, breccia chimneys, and pebble dikes. An assemblage consisting of quartz, sericite, kaolin, and pyrite appears to be of early Tertiary age. The more productive part of the lode, however, is late Tertiary and consists of a hematite chimney inclosing a copper shoot, a tungsten chimney, and compound base-metal fissure veins. These chimneys and veins are mostly localized along early Tertiary structures, but their mineral sequences correlate them with the second stage of late Tertiary deposition recognized by Burbank in the adjoining Red Mountain and other districts. The Dunmore lode, however, is believed to have been deposited at greater depth and higher temperature and nearer to a source of supply than the lodes of these districts. In structure as well as mineral assemblages the Dunmore lode shows a connection between the chimneys and veins of the region.

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