Page 217 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 213
The gravel in place has been estimated to contain one-sixth by volume of coarse boulders, one-sixth of cobbles from 1 to 3 inches in diameter, and two-thirds of gold-bearing gravel and sand. In the treatment processes used the sand and gravel are screened out and the gold is separated from them.
Several sets of samples of these gravels have been taken. No details of the methods used were obtained, but it is reported that they represent gravels in place. One set of samples from 684 holes is reported to have varied in gold content from 29c to $18.80 per yard in place. Another set is reported to average $1.00 per yard throughout, with a reported maximum sample of $108.00 per yard, and another sample assaying $28.64 per yard. The lower grade samples on the hills are reported as averaging 8c per yard. Mr. Burke, who conducted the writer over the field, was inclined to accept the $1.00 a yard average valuation for aft gold-bearing gravels on the property, and from measurements of the various pits he thought that they would average 3 feet in thickness on the hills and 6 feet in the gulches.
ORIGIN OF THE GOLD
The gold of the Shandon placers unquestionably has been derived from quartz veins in the pre-Cambrian granites and schists in the lower part of the escarpment of the Sierra Caballos east of Shandon. In this part of the escarpment, quartz and peg- matite veins are much more abundant than elsewhere in the range, and they vary in width from knife-blade seams to several feet. Some of these veins contain gold in quantities varying from a trace to $4.00 per ton, but they are probably too low grade for profitable mining. Supporting this origin for the gold of the placers are the following: (1) The gold varies from coarse to fine, but the particles are uniformly unworn in appearance, indi- cating that they have traveled only short distances ; and (2) gold occurs in some of the veins in the pre-Cambrian rocks from which the gold-bearing gravels were derived.
At the eastern end of the placer ground next to the granite face of the escarpment, there is little or no gold in the gravels. It is believed that the eroded upper parts of the veins were richer in gold than the remaining lower parts, and that these veins were rapidly eroded and carried down to form a thin veneer of detrital matter over the Tertiary lava flows and sediments. As the veins were eroded down, the average gold content of the detrital mate- rial decreased. The recent erosional products from the scarp face have formed a thin, nearly barren covering over the older productive gravels.
POSSIBILITIES OF THE DISTRICT
Although numerous sets of samples have been taken in the
district, they do not appear to the writer to have been entirely representative of the area. Most of them have been obtained