Page 138 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 137
largely confined to the southern part of the district and consist there of small, irregular pockets of lead-vanadium-molybdenum ore replacing the Fusselman limestone along bedding planes. Since the deposition of the ores the overlying Percha shale has been entirely eroded away. Much of the primary galena has been oxidized, and the lead is now in the form of anglesite, ceru- site, wulfenite and vanadinite. Endlichite, a rare variety of vanadinite in which arsenic replaces the chlorine, occurs in these pockets. Residual patches of galena, coated with concentric shells of anglesite and cerusite, are usually present, even in the most highly oxidized bodies of ore. For the most part the ceru- site, vanadinite and wulfenite, together with considerable limon- ite, form a coating on silica boxwork, and this boxwork, in turn, may enclose residual grains of galena. These rich pockets change abruptly to a low-grade galena-calcite-quartz primary ore of no commercial value. Along the bedding planes several of these oxidized bodies of ore may be connected by small, irregular and often not easily followed stringers of oxidized ore, which may be coated with caliche and honeycombed with solution cavities. In this part of the district much manganese and manganiferous iron ore has been mined from pockets in the limestone.
Placer Gold Deposits.—The gold placer deposits of the dis- trict consist for the most part of alluvial fans, the material of which has been derived from the erosion of the Animas Hills, particularly that part which once lay over the area now called Copper Flat. It is estimated that more than 1,000 feet of rock has been eroded from above the present top of the Animas Hills. The upper part of the original range was composed of rhyolite tuffs and flows that probably contained very little gold. The lower parts of the fans consist largely of these rocks, and gold is absent.
The upper parts of the fans are composed of andesite frag- ments, with pieces of latite and monzonite in subordinate amounts, these being the next lower rocks in the sequence shown in the Animas Hills. It is within this material that the particles of gold and of black sand, much of which contains gold, are found lying on layers of "false bedrock." The principal fan in the dis- trict is that formed by the drainage toward the east out of Cop- per Flat, principally through Grayback Gulch but in part through Dutch Gulch. Erosion to the south on the outside slope of the hills, through Wicks Gulch and down the draws from the Rattle- snake and Opportunity mines, although much less important in quantity of detrital material moved, has nevertheless produced some high-grade placer ground, from which several small for- tunes are reported to have been made. The original discovery in the district was made at the placer deposits of Wicks Gulch. Nuggets of gold worth several dollars each have been found near the heads of the placers, but for the most part flattened grains about the length and width of a grain of wheat and about