Page 139 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

138 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
half as thick represent the upper limit of size. From this size the particles grade down to thin flattened specks of gold so small as to readily escape detection by the unaccustomed eye. Most of the gold near the heads of the fans is coarse and easily recov- ered, and the exceedingly fine gold becomes relatively more abundant at distances of over a mile from the heads. When re- covered by dry methods the particles'of gold may have a gray or a rusty appearance due to a film of caliche or of limonite, but when wet methods have been used they have been cleansed of these films and are a beautiful reddish-golden color. The black sand that is associated with the gold consists in part of magnetite and the other dark and heavy accessory minerals that have formed in the igneous rocks. One of the most important consti- tuents, however, is limonite, derived from the pyrite of the veins by oxidation. The pyrite in the veins carries most of the gold in the district,' and its oxidation and disintegration have liberated the gold. In part, the crystals have been oxidized into pseudo- morphs of limonite, which retain the gold. These pseudomorphs are sufficiently abundant in some parts of the placer ground to give the black-sand concentrate, after all free gold particles have been removed, commercial value as a product to be shipped to the smelter for treatment.
SECONDARY ENRICHMENT
Secondary enrichment of the veins in the Hillsboro district is of two types. One of these has resulted in a residual concen- tration in the oxidized ore of gold and subordinately of other metals, due to the dissolving and removal of the more soluble minerals. The sulfides of iron, copper, silver and zinc have been oxidized with the formation of various soluble sulfates of these metals and of free sulfuric acid, and these products have been carried downward by meteoric waters. Acid-reacting waters have altered the various vein and wallrock minerals, principally to convert the feldspar and sericite to kaolin, and the ferromag- nesian minerals to chlorite. These soft, earthy secondary products have been partly removed by the circulating ground water, and as a result, the massive, compact primary ore has been altered to a porous and much lighter ore containing practi- cally all of the original gold. The upper oxidized parts of the veins that were profitably mined in the early history of the camp were the end product of this type of enrichment.
The other type of secondary enrichment has been due to the removal in solution of the copper and silver from the oxidized portions of the veins and their deposition as the secondary sul- fides, chalcocite and argentite, which replace the chalcopyrite and pyrite of the primary ore in the zone of precipitating conditions at and near water level. The secondary sulfide minerals form films coating grains of primary sulfide minerals, and fillings in cracks and seams within the grains of these minerals. In places, as along the larger fractures, the secondary sulfides have