Page 188 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 185
GEOLOGY
The Macho mining district is near the eastern of of the
thick lava flows that cover most of the western half of the county. (See Plate I.) The principal rocks of the district are andesite breccia and andesite flows. The low hills surrounding the dis- trict are composed principally of andesite flows higher in the series, which are slightly more resistant to erosion than the basal breccias. In places, particularly to the west in the foothills of the Black Range, patches of rhyolite overlie the andesite and tend to form steep cliffs of light buff to pink color, in sharp contrast to the dark brown and purple colors of the andesites. Dikes of latite porphyry cut the lower andesites ; these dikes are generally much weathered and decomposed, and the outcrops in many places are characterized by the conspicuous yellow color of the altered rock, rather than by prominently projecting outcrops. Sedimentary rocks do not appear at the surface in the district, and the nearest outcrops are at Lake Valley and in the Cooks Peak region. A drill hole from the 400-foot level of the Anni- versary shaft, which reached a depth of 700 to 800 feet below the surface, may have penetrated Fusselman (Silurian) limestone. No reliable data on this hole appear to be available.
STRUCTURAL RELATIONS
The dikes in this district form a radiating system, the cen- ter of which is believed to be a short distance southwest of the Anniversary mine. The Anniversary shaft is on one of the northeastward-trending dikes, and some exploration has been done on adjacent radiating dikes to the east and west of it. Brec- ciated andesite, suggestive of faulting, occurs generally along one wall of the latite porphyry dikes. Part of this movement is be- lieved to have taken place simultaneously with the intrusion and part later, but before the period of mineralization had ceased. This intrusion of latite porphyry and the fracturing and move- ment which accompanied and followed it served to break up the andesite, thus affording ready access of mineralizing and alter- ing solutions from below and later oxidizing surface waters from above. The fractured and altered rocks have been easily eroded, with the result that the mineralized area is now a basin.
ORE DEPOSITS
The ore deposits of the Macho district are in veins, occurring
between well-defined walls of andesite, in brecciated andesite, and within the latite porphyry dikes. The veins strike northeast ; near the surface they dip steeply or are nearly vertical, but as depth is attained they flatten to the west. In the brecciated andesite the mineralization is principally open-space filling with subordinate replacement of the brecciated material. In the latite the mineralization is almost entirely by replacement, but in places of slight fracturing and brecciation, open-space filling has