Page 34 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

GENERAL FEATURES 33
lect in large numbers on the surface. They are reported to con- tain about $1.25 per ton in gold, and due to their hardness, the suggestion has been made that they would afford a cheap local source of grinding pebbles. In the fresh rock the epidote segre- gations are often surrounded by a thin bleached zone, while the center of the segregation may be open textured due to a partial leaching out of some of the earlier constituents, and these open spaces are often lined with minute coatings of iron-stained cal- cite or silica. It is possible that these segregations have been formed by gases or solutions trapped in the rock while it was solidifying, which later spread from these centers of accumula- tion to react with the original rock minerals in various ways, with the formation of deuteric minerals, and the partial bleaching and solution of others. Another suggestion has been offered, which states that these segregations were originally fragments of sediments trapped in the molten rock and later altered by propylitizing solutions. The writer leans toward the former view, as he was not able to find evidence of channels along which the solution might have traveled through the rock mass to the segregations. Further, it seems improbable that so many small fragments of.a sediment could be trapped in a rock without some larger partly altered xenoliths having been preserved, and it would also appear that such small fragments would have been completely assimilated in the magma long before the period of deuteric mineralization became effective.
The latites occur as dikes, small sills, and as flows,laid down on the old andesite surface. The original flows were largely if not entirely removed before the rhyolite of the region was poured out, and in many places erosion has again cut so deeply that what were once sills of this rock within the andesite are now found at the present land surface. The latites when fresh are gray rocks somewhat lighter in shade and without the greenish cast that characterizes the fresh andesite. Within the dikes the texture is felsitic to fine grained, although in their deeper portions and close to the intrusive monzonite stocks from which the latites are supposed to be direct offshoots, the texture is coarser and may approach that of a fine to medium grained monzonite. Within the sills the texture is porphyritic, and the rock contains abun- dant medium-sized to large tabular crystals of feldspar in a fel- sitic to fine-grained groundmass. The phenocrysts are andesine and are oriented so that the long axes and tabular faces lie parallel to the surfaces of the sills. Most of the phenocrysts are corroded and have rounded outlines, instead of the usual sharply defined angular intersection of faces, giving to the rock a characteristic appearance, and the local name "birds-eye porphyry." The composition of these rocks is similar to that of the monzonite porphyry of the intrusive group.
The following paragraphs describing the rhyolites of the Black Range but applicable to the whole of Sierra County where