Page 50 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

GENERAL FEATURES 49
veins connected with intrusive rocks of early Tertiary age, which contain principally gold, copper and silver ; veins and replace- ment deposits in limestone consisting of silver-lead, silver-lead- zinc, silver-lead-manganese, and fluorspar-lead-silver deposits ; and probably also most of the copper deposits in sandstone.
A few veins in the region are known to have been formed following the extrusion of the rhyolitic rocks, and these consist of tin deposits, gold veins carrying minor amounts of copper and silver, and gold-silver veins with associated tellurium and bis-
muth.Structural control was of paramount importance in the localization of ore bodies. Faulting and fracturing created the channels through which ore-bearing solutions gained access to the host rock, and even in the case of contact-metamorphic and blanket-replacement types of deposits it can be shown that the localization of mineralization was due to a relatively intricate system of fracturing rather than to simple permeability of the beds and the infiltration of solutions for great distances from the major fractures and bedding planes. Differences in chemical and physical character of the host rock were important in the processes of selective replacement. In many places early solu- tions deposited silica, calcite or other gangue minerals, sealing off the wall rock and filling the smaller fractures, so that the later ore-bearing solutions were forced to deposit their load in the openings remaining in the wider portions of the veins. This resulted in the formation of many small tortuous ore shoots con- taining much high-grade material, rather than continuous ore bodies in the veins or beds with possibly a lower metallic con- tent. In places a cover of impervious shale caused the slowing up and stagnation of the mineralizing solutions, with consequent replacement of the underlying favorable beds and precipitation of the ore in the form of irregular blankets.
Beds that dip toward the igneous intrusion were mineralized to a greater extent than those that dip away from these rocks. Anticlinal folds formed traps, which served to concentrate the solutions and precipitate the ore minerals, especially where a bed favorable for replacement was capped by an unfavorable imper- vious bed. In other instances, as at the Lookout mine near Tierra Blanca and at some of the deposits in the Sierra Cuchillo, gentle folding of brittle beds produced tension cracks at the crest of arches, in which ore minerals were concentrated. In the gold- silver-copper veins in the volcanic rocks, open-space filling in fault planes along the contact between dikes and country rock was almost the only mode of ore deposition. At Copper Flat near Hillsboro and at the junctions of some of the more impor- tant veins in the district with cross veins, the pronounced vein characteristics were lost in a general fracturing of the surrounding area, producing stockworks in which the mineralization was