Page 140 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 139
largely replaced the primary sulfides, only residual grains of the latter now being present within the mass of the new min- erals. In other places, as along thin seams and on disseminated grains, it is usual to find only a thin film of secondary minerals coating grains of primary minerals that largely retain their original outline. Copper is the metal most affected by this pro- cess, and valuable copper deposits may result from it. Silver is less soluble than copper and more easily precipitated and hence it is usually deposited through a wider vertical zone, and less often forms highly concentrated bodies of ore at water level. Sphalerite is highly soluble, and zinc may migrate farther than either silver or copper ; in part, however, it may remain in the oxidized ore as the carbonate, smithsonite, or as the hydrated silicate, calamine. Lead only occasionally forms the secondary sulfide, being more often oxidized to the sulfate, anglesite, or to the carbonate, cerusite, immediately adjacent to the original galena. This process has been much less important than the gold enrichment in the oxidized portions of the veins.
In veins that outcrop along ridges, where erosion has been slow, free-milling gold in an oxidized and porous gangue has been found and profitably mined to a depth of 500 feet, but where the veins outcrop in arroyos, or at other low points where erosion has been rapid and has outstripped oxidation, sulfides are gen- erally found at or near the surface, and only small, scattered bodies of free-milling gold ore have been worked in these local- ities. An intermediate condition, giving a better balance between the processes of oxidation and erosion, has been widespread in the district. Under this balance, oxidation has progressed in a normal manner, and the gold has been freed of much of its con- taining gangue and has been converted largely to the free-milling form. As a cyclic process, following the oxidation of the out- crops and due to periodic changes in the elevation of the land surface caused by continued faulting along old lines of weakness, erosion has followed more or less closely and has kept the com- pletely oxidized zone worn down to a minimum thickness. It is possible that future mining operations near water level in these areas will disclose workable secondary enrichment deposits of copper and silver.
HISTORY AND PRODUCTION
Gold is reported to have been first discovered in the Hills- boro district in 1877 by two prospectors who picked up float on the hillside below what is now called the Opportunity mine. Assays were made of the float rock at an old mill on the Mimbres River, and the returns of $160 per ton in gold and silver caused considerable excitement. In a short time ore was discovered at the Rattlesnake vein, just to the west of the Opportunity. In August, 1877, the first house was built on the present site of the town of Hillsboro.