Page 167 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 165
present. Along the walls of the dikes, stringers of quartz, pyrite and chalcopyrite make off into the monzonite porphyry along shearing planes trending N. 50° E. The trap dikes strike N. 50" E. and average 4 feet in width. In general the dikes are altered with the formation of much chlorite, and they have a dove-gray color. A few specimens obtained from the dump were vesicular and less altered in appearance and looked much like basalt. It appears probable that these dikes are of the same age as the basalt of the district. Because of their porosity they have easily altered, and they have been impregnated with the copper min- erals by circulating ground waters, which have produced sec- ondary enrichment elsewhere in the basin. Along the dike in which the Sternberg shaft and workings have been opened, the west wall is said to be bordered by 4 inches of quartz containing abundant pyrite and molybdenite.
The Sternberg shaft is reported to be 150 feet deep, with a
level at 90 feet consisting of 255 feet of drifting in the trap dike
and 120 feet of crosscutting. At 175 feet from the shaft, a fault
with a strike of N. 50° E. offsets the southern extension of the
dike 10 to 15 feet to the southwest. At the time the property
was visited the shaft contained water to within 75 feet of the
collar, and all the surrounding prospect shafts over 50 feet deep
contained some water. The shear zone carrying enriched copper
deposits is about 200 feet wide where it is cut by Grayback Gulch,
and samples taken in the various open cuts, tunnels and natural
exposures along the gulch contain from a trace of gold and 0.3
per cent copper to 0.15 oz. gold a ton, a trace of silver, and 6.55
per cent copper. Hand-sorted material from these old workings
assayed 1.01 oz. gold, and 26 to 39 per cent copper, with the re-
ject from sorting assaying 0.52 to 1.3 per cent copper.
FREIDBERG MINE
Northwest of the El Oro mine and on the outward northerly slope of the Animas Hills is the old Freidberg mine, also known as the Brauer group. No shipments have been made from this property, but it is of interest for the reason that the workings have prospected a residual patch of rhyolite flows and tuffs. A shaft 60 feet deep bottoms in a dark shaly material, which is considered to be a sedimentary accumulation on the andesite sur- face made during the interim between the last andesite flow and the earliest rocks of the rhyolite sequence. Other workings in- clude a tunnel, several crosscuts and two short winzes. All these workings are said to be in rock averaging $1.50 per ton in gold, with some small shoots and patches of $25 ore. It is reported that the grade of the material encountered at the shale contact in the bottom of the shaft is much higher than elsewhere in the workings. At the portal of the tunnel is an obsidian dike 3 feet wide that strikes due east and is nearly vertical ; this dike was