Page 133 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

132 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
known of the relative positions of the rock masses here, except that in the El Oro workings andesite is the country rock on the bottom level at a depth of 500 feet.
Within the central area a wide zone of shearing is evident along the courses of the dikes and veins, from the Bonanza vein on the west through the Rattlesnake and Opportunity group to the Ready Pay vein on the east. This zone of shearing trends northeast through Copper Flat, where shearing and fracturing are visible in the monzonite floor. North of Dutch Gulch two minor faults and an intervening small patch of monzonite have a linear arrangement in a north-south direction and seem to extend the line of shearing in a general way. It is thought that the horizontal component of the total shearing movement has been greater than the vertical component and that the rocks on the west side have been shifted to the north. Along with this general shearing there may have been a slight hinging action, which has dropped the north end of the east block and raised the south end. The monzonite porphyry intrusion and the many dikes and veins in this area have formed principally in the zone of faulting and shearing.
Another type of movement has been confined to the area of andesite in the central part of the district. When the stock of monzonite porphyry was intruded into the andesites and under- lying rocks, it exerted a doming effect on these rocks, and the flanks of this dome dip gently in all directions from the center. The intrusion came in along the zone of northeast shearing, but it added to these fractures by causing additional radiating ten- sion cracks, along which dikes and veins formed just as they did along the shear planes.
GEOLOGICAL SEQUENCE
The geological sequence of the district may be summarized
as follows : In late Cretaceous time, when igneous activity had started in the southwest corner of the State and elsewhere in the Colorado Plateau region, gentle warping accompanied by local faulting was taking place in Sierra County, and the present ranges were beginning to take on their general form and outline. At Hillsboro a long fault line supposed to be continuous with simi- lar breaks in the Sierra Cuchillo and in.the Lake Valley Hills had developed, and a large block of sediments had begun to rise along the line now indicated by the fault scarp in the south end of the district, the sheared zone in the central portion, and the minor north-south faulting at the northern end. Erosion accompanied this movement, and the sediments were removed until only patches of Lake Valley limestone and Percha shale were left on the surface of partly eroded Fusselman limestone. In early Ter- tiary (Oligocene) time, widespread outpourings of lava occurred. These lavas issued from many vents, and in the Hillsboro area the andesites probably found an easy path of emergence at sev-