| Description |
At the Ontario Mine, Park City District, Utah, the tertiary age mesothermal lead-zinc-silver-copper minerealization is located in the Mississippian Humbug and Deseret Formations and is associated with a diorite intrusive. Hydrothermal alteration products were determined in various sedimentary host rocks within the 1500 and 1600 foot levels of the Ontario Mine. Systematic studies were made from ore outward into the Upper Humbug Limestone and across the Deseret Limestone-Dolomite intrusive contact. Alteration of the sediments consisted of an inner sericite-quartz zone and an outer kaolinite zone. The width of the sericite-quartz zone ranged from 2 or 3 feet in the lime-stone to more than 50 feet in the Doughnut Humbug Shale. The kaolinite zone is more widespread in the limestone and quartzite host rock than in the shale host rock and is greater than 50 feet. The contact zone of the quartz diorite intrusive and the Deseret Limestone-Dolomite has produced an assemblage of high magnesium-iron minerals which are serpentine, brucite, and some magnetite. These minerals are associated with the igneous contact and not with alteration around the ore. Alteration within the intrustive has been varied. The porphyritic quartz diorite sampled at the intersetion of 1590 drift west and 1544 crosscut south is very fresh and contains only a small amount of kaolinite and chlorite. Samples of quartz diorite porphyry from 1649 drift east and 1639 drift east are severely altered to montmorillonite, kaolinite, and some chlorite, and limonite. The large amount of montmorillonite is believed to be due to local weathering and/or deuteric alteration. The maximum temperature of formation of the contact and the altering solution has been estimated by the use of phase diagrams and a chart prepared by stringham (1952). They are about the same and believed to be between 247C and 425C. |
