Page 179 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

176 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
indicate that the monzonite or latite porphyry was an offshoot from a deep-seated source of supply, probably cutting across the sedimentary beds and the andesite flows as a dike, following some plane weakness. At the contact of the andesite and the eroded surface of the limestone, or at the contact of the Blue limestone with overlying Crinoidal beds, this intrusion formed a sill, which extends for a distance of over a quarter of a mile parallel to the main trend of the ore bodies of the district. About 300 feet north of the northeast end of this intrusion is another area of altered monzonite about 200 feet wide and 600 feet long in a northerly direction, which appears to follow in general the trend of the Columbia fault. Two other patches of igneous rock, classed by Clark and Gordon as andesite, appear in the sedimen- tary area as residual patches of the surface flows of this material.
Several breaks striking N. 45° E., parallel to the escarpment face of the sedimentary block, and having a throw of a few inches to 5 feet, had an important localizing influence on the ore deposits of the district ; and several minor faults and fractures striking northwesterly, roughly parallel to the Columbia fault, have modi- fied the pattern of the ore bodies as determined by the strike- fault system. In the southwestern part of the district, where the sedimentary rocks have been elevated in relation to the rhyolite of Porphyry Hill, they have been dragged along the fault plane to such an extent that they dip to the south and southeast at angles of 30° or more. They have undergone considerable frac- turing and minor folding in this zone. On the dip slope of the sedimentary block the beds are slightly undulating and have been warped and distorted over areas of a few square feet, with only a few inches to a few feet of vertical elevation, but these small dome-like areas have acted as traps for the retention of mineral- bearing solutions and the precipitation of their mineral content.
ORE DEPOSITS
The ore deposits of the Lake Valley district are all of the bedded type. They are confined to the top of the Blue limestone and to the base of the Crinoidal limestone, and are from 3 to 15 feet thick. Where the limestone beds are not contorted, as in the northeastern part of the district, the ore has replaced the base of the Crinoidal limestone, and the hard cherty layer 1 to 2 feet thick at the top of the Blue limestone forms the footwall of the ore deposits. Near the limestone-rhyolite fault contact, however, where the beds have been dragged down by the movement, the brittle chert layer has been much fractured, and the silver-bear- ing manganese-iron solutions have completely recemented the chert breccia and have formed a siliceous ore. Varying thick- nesses of the Crinoidal limestone overlying this chert zone have also been converted to ore by the replacing process. Along this dragged zone, especially in the Bridal Chamber and in the