Page 95 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

94 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
minerals are silica in boxwork form, anglesite, cerusite, chalco- cite, covellite, malachite, azurite, smithsonite, limonite, free gold, native silver, argentite, cerargyrite, bromyrite and embolite. (See footnote, page 59.)
The maximum production from any one shoot has consisted of a few hundred tons of shipping ore, and most of them have yielded from a few tons up to a few carloads.
Near the surface these runs or shoots of ore are oxidized. Solution of the limestone has occurred along the fracture zone, leaving a series of small open chambers in the limestone along one side of the ore shoot. The ore has then been oxidized and leached, with the removal of most of the silver, copper and zinc, and part of the lead, and with the development of a silica box- work structure and the deposition within it of limonite, anglesite and cerusite. A claylike deposit on the floors of the solution cav- ities consists of the impurities in the limestone left behind when the latter was dissolved, together with limonite, anglesite, ceru- site, fragments of the silica boxwork, and fragments of rock and of the primary vein matter dropped in during the processes of oxidation and leaching. In some places this residue nearly fills the cavity ; in others it is merely a thin coating on the lower walls and on the bottom.
Farther down along the ore shoot and near or at water level, the solution cavities in the limestone have been lined with a crust, often one layer deep, of cubical crystals of galena up to 11/2 inches along an edge. Zinc and copper sulfides are not found associated with these large crystals, and the shoot alongside is filled with the silica boxwork, limonite, anglesite, cerusite and residual galena. Native silver is an accompanying mineral, and cerargyrite and other halides of silver may be present. This mode of occurrence of the galena suggests that it has been formed by secondary processes, which have involved its solution in the zone of oxidation, its transportation in some soluble form, possibly as the chloride, and its precipitation as a sec- ondary sulfide from weak solutions in open cavities just above water level and above the point at which the other secondary minerals begin to concentrate.
Below water level in these shoots are the primary sulfides. Here the galena is in small crystals and often approaches steel galena in its fineness. It is associated with primary chalcopyrite, bornite, and sphalerite, and these have been replaced by second- ary chalcocite, covellite, argentite, and locally native silver. Solution cavities in the limestone are lacking at this depth.
The ore shipped has been extremely high grade, assaying on an average 250 ounces silver a ton and 40 per cent lead, but this ore was obtained by sorting, as a result of which over 50,000 tons of rock, which assays 10 to 20 ounces of silver, is said to have been left on the dumps of the camp, and from which it is said a screened product containing 30 ounces of silver per ton could be made. In July, 1932, Mr. C. F. Ross was working on a new