Page 130 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 129
stony to extremely fine granular in appearance, and in some places it gives the suggestion of a glassy texture, although for the most part the freshness of the groundmass is clouded by the presence of sericite and chlorite.
The third phase is that of the dikes found at a lower geologic level within the basin of Copper Flat. These dikes, although much altered to sericite and bleached and iron stained, are clearly porphyritic in character and in general have a coarser ground- mass than have the dikes at higher levels. They show consider- able shearing, and quartz has been deposited within the shear planes, often accompanied by pyrite and chalcopyrite. Through- out the dikes, minute cavities formed by removal of pyrite crys- tals are visible, and the rock is crossed by bands of yellow, purple, brown and black limonite staining. These dikes are 4 to 40 feet wide; in general they are wider than the first-described type, which with few exceptions have a width of 4 to 8 feet.
Under the microscope these rocks show a decided similarity in composition. The phenocrysts consist of orthoclase, plagio- clase close to andesine, and hornblende, with biotite locally prom- inent, particularly where the rock has taken the form of sills or flows. Many of the orthoclase crystals have perfect outlines and are twinned according to the Carlsbad law. The plagioclase phenocrysts for the most part are corroded and are almond shaped. The groundmass is trachytic with some glass present, and often a flow structure is apparent. Laths of orthoclase pre- dominate in the groundmass, and plagioclase is subordinate. Reds and small books of biotite are numerous. The rock contains some magnetite and in places a small amount of quartz. The feldspars are sericitized more or less completely, and the ferro- magnesian minerals are chloritized. Secondary epidoje, calcite, quartz and pyrite are present, especially in areas where these dikes occur in proximity with propylitized andesites.
Monzonite.—Two intrustions of monzonite outcrop in the district. One of these forms a prominent hill about a mile northeast of Hillsboro and near the road leading into Hillsboro from Hot Springs. It is irregular in shape and has a maximum length of about three-quarters of a mile. In the basin of Copper Flat in the central part of the district, another mass of irregular outline occurs, having a length from north to south of a little over a mile and an extreme width of nearly three-quarters of a mile. The top of this mass has been eroded, and it now forms the floor of Copper Flat, as shown on Plate VI. Radiating from this central mass of monzonite are the dikes described in the previous section under "Latite and Latite Porphyry." These dikes in general extend in a southeast direction from the stock.
In hand specimens the monzonite of the central cupola or stock is coarsely porphyritic, with large phenocrysts of ortho- clase and plagioclase, and occasionally coarsely megascopic grains of hornblende and augite. The groundmass is medium