Page 93 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

92 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
cuts through the mass just north of the hotel. North of the stream the greater hardness of the andesite causes it to stand out as a ridge, which de- creases in height toward the north and finally disappears under the covering of andesite tuff. Between this ridge of andesite and the limestone to the east is a depression underlain by the Palomas gravel, which here evidently con- stitutes a filling in a previously eroded valley. The mines of the Antelope group are located in the limestone near the contact with the andesite.
STRUCTURAL RELATIONS
Aside from the regional faults which have sliced the range into long, narrow blocks over such long distances, three major faults having important relations to the ore deposits occur in the limestone section of the Hermosa district. These are named, from east to west, the Palomas Chief, Pelican and Bullfrog. The Palomas Chief and Bullfrog faults are parallel and strike N. 13° E. The Pelican fault, which strikes N. 40° W., cuts the Palomas Chief fault on the south side of the canyon and the Bullfrog fault about half a mile north of the canyon and about five-eighths of a mile to the west of the first intersection. As seen on the sections, figure 6, numerous other faults trend in the same general direc- tons as these three principal breaks. The heavy lines A-B indi- cate in a general manner the amount of folding and its location in the sedimentary beds prior to the occurrence of faulting. In the lower section of figure 6 a series of eastward-trending breaks, which have also displaced the beds relatively to one another, parallels the course of Palomas Creek.
ORE DEPOSITS
The ore deposits occur principally in the lower part of the
Magdalena limestone (Sandia formation) under the bed of dark carbonaceous shale described by Gordon in the above section on the geology of the region. In places, as along the Pelican fault and in the Antelope mine at the western end of the camp, ore occurs in very small quantities above this shale horizon. Below the shale the ore may be in contact with it, but often it is as much as 50 feet below the contact. The three main fractures have been the principal channels along which the solutions have migrated upward, and the ore is in shoots within these fault planes, or in beds and pockets following the bedding planes of the limestone for short distances away from the faults. As shown by figure 6, the principal concentrations of ore have been at places where the limestones were originally arched into low folds, and presumably it has been the trapping of the solution at these points that has caused the abundant precipitation of the metals.
The ores of the district are principally silver-bearing sul- phides of lead, copper and iron. The primary ore minerals are galena, chalcopyrite, bornite, sphalerite, and associated silver and gold. Antimony minerals are said to occur locally, but none were seen by the writer. In the primary ore the silver, which has been the principal metal of value in the district, is associated with the galena, chalcopyrite and bornite. Among the secondary