Geologic description
The rocks in the prospect area are part of a belt of Mesozoic or Paleozoic sedimentary and volcanic rocks that have been metamorphosed to Tertiary or Cretaceous schist and gneiss. The belt is about 1 1/2 mile wide and strikes northwest (Brew, 1997; George and Wyckoff, 1973). The metamorphic rocks are bounded on the east by a thick, regionally extensive, 60 to 70 Ma tonalite sill and on the west by a 90 Ma granodiorite pluton (Brew, 1997 Still and others, 2002). On the north side of Groundhog Basin, about 2 miles northwest of this prospect, the metamorphic rocks are intruded by a 16.3 Ma biotite 'tin' granite pluton about 1,000 by 2,0000 feet in size. The granite is probably the source of the numerous rhyolite dikes and sills that extend from it and the mineralization in the area (Newberry and Brew, 1989).
The East Marsh Peak prospect was mapped and sampled by El Paso Natural Gas Company in the early 1970s; they dug three, 20- to 60-foot, samples on the property. Several other companies have looked at the property and Still and others (2002) mapped and sampled the prospect as part of a regional mineral assessment for the BLM. The mineralization here is similar in origin to that at the he well known mineralization at the Groundhog Basin deposit (PE040) to the north and the numerous other base-metal prospect sin the area. The East Marsha Peak prospect is associated with a fault zone that strikes about N 25 E and crosscuts the northwest-trending layering in the gneiss and the rhyolite sills in the gneiss The shear zone is 30 to 40 feet wide and extends for about 2,000 feet. The hanging wall is highly silicified with vugs of quartz and fluorite. Sphalerite, galena, chalcopyrite, and pyrrhotite occur in masses and disseminations in brecciated gneiss along the shear zone and in gouge. The ore minerals also occur in a network of fractures that extend out into the gneiss wallrock for up to 40 feet. El Paso Natural Gas Company (George and Wyckoff, 1973) and Still and others (2002) collected numerous samples in the trenches and in surface outcrops. In one trench at an elevation of about 4,075 feet, El Paso sampled a zone up up 30 feet thick that averaged 3.19 percent zinc, 1.67 percent lead, and 1.99 ounces of silver per ton. Their trench at an elevation of about 4,100 feet exposed a zone 13 feet thick that averaged 1.45 percent zinc, 2.75 percent lead, and 1.23 ounce of silver per ton; an additional 18 feet across the mineralization averaged 1.79 percent zinc with little lead and silver. Other samples gave similar values. |
