Page 134 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 133
eral places along the fault break already formed. The great weight of these superimposed lava flows probably caused a sink- ing of small blocks of sediments on either side of the main fault. Continued movement relative to the limestone blocks to the south, especially at the point of the projecting angle now occupied by the southern cupola of the monzonite stock, caused a shearing between the two buried blocks and continued the line of original faulting through the newly erupted andesites. This period of activity was followed by a short period of erosion, during which sediments consisting of andesitic detrital material were laid down in a few small areas. Then came the period of intrusion of the monzonite stock and associated monzonite porphyry dikes, porphyritic latite dikes at a higher level, and the local sills and flows of latite porphyry. This period was followed by one during which thick rhyolite flows and tuffs were laid down over the en- tire region. From the end of Oligocene time to the beginning of the Pleistocene was a period of quiet and of continuous erosion, during which the various fault blocks were undergoing gentle but fairly continuous readjustments of position. It is supposed that the central block at Hillsboro was gently rising at this time. The arched up and fractured central part of this area was eroded away in cycles, following each rise of the central mass, and a large alluvial fan was built out to the east of the range. Pleis- tocene time was a period of basaltic flows in the region, followed by the formation of Palomas gravels, and these in part were cov- ered by still later flows of basalt. Faulting continued on at least one major plane of movement almost to the end of Pleistocene time, as indicated by the continuation of the fault scarp in Fus- selman limestone southward through the gravels south of the Rio Percha.
ORE DEPOSITS
CHARACTER
The ore deposits of the Hillsboro or Las Animas mining dis- trict are of four general types as follows : (1) Fissure veins in andesite flows, (2) disseminated deposits in monzonite porphyry, (3) replacement deposits in limestone, and (4) placer gold de- posits. (See footnote, page 59.)
Fissure Veins in Andesite Flows.—Most of the production of this district has come from fissure veins in the andesite, and particularly from those in the southern part of the district. It is believed that the source of the veins, as well as the accompany- ing dikes, is the deep-seated part of the monzonite porphyry stock in Copper Flat. The cupola northeast of Hillsboro and other small masses of monzonite porphyry in the district are con- sidered to be of the same age and to have come from the same deep-seated reservoir.
Within Copper Flat erosion has bared the lower portions of the veins within and adjacent to the stock, and here the impor-