Page 54 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

GENERAL FEATURES 53
centers in the county there are still good chances for locating mineralized extensions of known veins, or for finding veins that lie buried under a mantle of detrital material or of later flow rocks.
All the known silver-lead-manganese deposits in the county are in the Paleozoic sediments, where they have formed largely by replacement of favorable beds in the Silurian limestone just under the Percha (Devonian) shale, in the Lake Valley (Missis- sippian) limestone under its topmost shaly member, or in the Magdalena (Pennsylvanian) limestone underlying one of the contained shaly members. Considerable ore has been found on
that side of an intrusion where the
igneous rock. Valuable ore deposits are located along the axes at the tops of anticlinal folds, in tension cracks at the tops of such arches, and in larger fractures which may or may not be in conjunction with folding. The contact-metamorphic deposits of lead-zinc and of copper in the Cuchillo Negro district occur in the Magdalena (Pennsylvanian) limestone and are quite similar to the silver-lead-manganese deposits in shape and relation to the enclosing beds. In the igneous rocks ore has been found within the intrusive ones of Tertiary age at Hillsboro as veins and stockworks. In the volcanic areas the older flows of ande- site and andesite breccia have been much more productive of gold, copper and silver than have the later rhyolites. Most of the ore in the andesites appears to be of the leptothermal type and is closely associated with the latite and latite-porphyry dikes and sills that penetrated and disturbed the andesites prior to the rhyolite extrusions, and which are considered to be direct off- shoots from the deeper monzonite stocks of the region. A few weak veins of the epithermal type containing gold and silver have been found in or associated with the rhyolites, but these rocks are notably lacking in valuable mineral deposits within Sierra County.
Practically all of the ore so far discovered in Sierra County has shown evidence of its presence on the surface by actual assay. There is always the chance, however, that concentrations of secondary sulfides at water level may be found under prac- tically barren leached outcrops. As far as the writer has any knowledge, there is no reason why other concentrations of metals in favorable beds underlying impervious shale members should not be found at greater depth under known ore bodies, especially where fractures cross the formation. It is certain, however, that ore found at deeper horizons will consist entirely of primary minerals, and hence these bodies probably will be of smaller size and materially lower grade than those exploited in the past. In the bonanza type of ore deposits in the lava flows, a few ore shoots have been found that apparently had no surface indica- tions of their presence, or the evidence was of such a nature as to have passed unnoticed. In general, considering the rather
,
sediments dip toward the