Page 216 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

212 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
1905 the production was 2,316 ounces, most of this coming from the Shandon placers. Since 1905 production has been unim- portant. For several years a lawsuit involving rights of owner- ship and other matters interfered with any operations in the district, but this controversy was finally settled, and in the autumn of 1931 the property was under a 20-year lease to Callo- way and Burke. At the time of the writer's visit to these workings, they were being sampled by means of pits. Measured quantities of gravel from these pits were being hauled to Trujillo Gulch, and were being treated in a plant consisting of an elevator, trommel, and sluice box 60 feet long by 3 feet wide. When these placers were first worked, water from wells sunk on the bank of the river was forced to the workings by powerful pumps. This equipment had been rehabilitated, and by means of Diesel power, water was being pumped through a 6-inch pipe line to a reservoir on the hill above the placers, from which it was being delivered to the sluices.
The placer gravels of the Shandon district occupy approxi- mately 1,270 acres. One-third of this area is in the gulches that dissect the sloping plain between the mountain escarpment and the river bluffs, and two-thirds is on the terrane between the draws. The terraces are covered to an average depth of 3 to /1 feet with coarse sand, gravel and boulders derived from the decomposition of the granite of the fault scarp. In the, arroyos near the river the gravel is reported to have a depth of 60 to 70 feet, while at the main workings in Trujillo Gulch bedrock is about 7 feet from surface. Near the east end of the area the placer gravels rest on a bedrock of rhyolite and rhyolite tuff, beneath whicn andesite is exposed in places. The gravels imme- diately above bedrock are impregnated and partially replaced by a mixture of caliche, crystallized calcite, and manganese oxides for a thickness of a fraction of an inch to 3 feet. In places massive slabs of psilomelane have formed, largely by replacement of the gravels ; in other places only interstitial spaces are filled by psilomelane. In places in the gravels caliche with scant mangan- ese is the cementing material, while in others calcite has formed finely crystalline crusts on open channels overlying similar crusts of psilomelane. Near the river the gravel on the hills is low grade to barren, but farther to the east the gold increases. In the gulches near the river the placers- are wide and deep, and a few samples taken near bed rock are reported to be high grade. Farther east in these gulches the gravels are narrow and are not over 9 feet thick. One area worked in Trujillo Gulch near the east side of Sec. 20 was 150 feet long by 75 feet wide and aver- aged 8 feet deep. The gold-bearing gravels of the arroyo evidently have been reworked and concentrated from the under- lying sands and gravels of the alluvial plain, which have been 'eroded away during the dissection of the plains by the arroyos.