Page 131 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

130 GEOLOGY AND ORE DEPOSITS OF SIERRA CO.. N. M.
granular to fine granular, and in it numerous grains of feldspar can be seen. In color the rock varies from gray to light buff or cream color where alteration and staining with iron solutions has progressed very far. In the exposure near Hillsboro the monzonite generally is of finer grain and equigranular, with a slightly greater proportion of the darker minerals present, giv- ing it a darker gray to greenish-gray color ; stringers of aplite cut through it in all directions.
Under the microscope the rock of the central mass is coarsely porphyritic, containing large phenocrysts of orthoclase with perfect crystal outlines, smaller crystals of plagioclase near andesine, and hornblende and augite crystals in subordinate amounts. The groundmass is coarsely to finely equigranular and is composed principally of grains of orthoclase, subordinate plag- ioclase, and some biotite, magnetite and quartz. Sericite, chlorite, calcite and epidote are the common secondary minerals. In gen- eral the phenocrysts and smaller grains of orthoclase are fairly fresh, but the plagioclases are altered to sericite. The dark min- erals and the groundmass generally are much altered. The rock of the darker colored cupola is similar to that in Copper Flat ex- cept in having a greater proportion of the darker minerals in the groundmass, which in turn have produced a larger amount of chlorite in the outcrops. The eastern part of this exposure is still more basic in appearance, and in places where it is in contact with the Fusselman limestone it contains segregations of almost pure biotite. The eastern differentiate of this cupola is distinctly dioritic in composition.
Basic Dikes.—Near the Sternberg shaft in Copper Flat six dikes, which appear at the surface and in the underground work- ings, are locally called trap. The rock is dark gray and very fine grained and has a few minute crystals of a dark chloritized min- eral and some small altered feldspar. This dike rock is some- what vesicular, and the vesicles are filled with the hairlike form of cuprite known as chalcotrichite.
Near the west border of the monzonite porphyry cupola in Copper Flat, a typical basalt dike cuts through it in a northeast direction for a short distance and then disappears under surface detritus.
Rhyolite and Rhyolite Tuff.—Residual patches of rhyolite flows and tuffs are all that remain of the immense outpourings of these rocks that once covered this region. One of these is in the northern part of the district and caps a low hill north of Dutch Gulch. It is connected with a rhyolite dike that strikes northeast along the northern border of the monzonite porphyry stock. This dike varies between 8 and 12 feet in width, and it is composed of glass along its walls for a width of 12 to 18 inches. In the southeast part of the district two small patches of this material project through the wash south of the highway, and appear to be directly in contact with Fusselman limestone.