Page 108 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 107
contacts in such a manner that parallel fractures or faults dip toward the monzonite porphyry masses in depth, and hence they could readily function as channels for ore-bearing solutions con- centrated at the walls of the igneous masses or migrating from greater depths along these contact surfaces.
ORE DEPOSITS
Ores of silver and of silver and lead are important in this district. The silver occurs as native silver and as the chloride and bromide in the oxidized portions of the veins, and as the sul- fide, argentite, in the zone of secondary enrichment and in the primary zone. It is also in combination with the sulfides of lead and copper. The silver and silver-lead ores occur chiefly as pockets and pipes in limestone underlying the upper shaly beds of Lake Valley limestone (Crinoidal limestone of Lake Valley) along narrow zones bordering the porphyry intrusions in the dis- trict. In these zones the ore in the limestone is as much as 150 to 200 feet away from the contact. Fractures in the limestone have been channels along which solutions have traveled upward, and low structural arches under the shale have acted as traps to retain the solutions and permit the precipitation of their min- eral load to form the commercial ores.
Gold is found in the top of the Fusselman (Silurian) lime- stone in a thin bed that has been replaced by silica. This topmost silicified portion of these beds is a persistent feature of the Fusselman limestone, and it is thought to be a brecciated zone caused by differential movement between Fusselman and Percha beds at the time of folding in the region, and prior to'the time of faulting and igneous activity. The silica-bearing solutions are thought to belong to an early stage of the mineralization and to have come up along faults and fractures, replacing the lime- stone breccia and converting the fragments to flint. Pink and white quartz fills the interstices between the fragments with the formation of many crystal-lined vugs, and small amounts of gold were deposited along with it. The ledge has been reported to assay $8 to $9 per ton in gold in places, but no verification of these figures is at hand, and so far as the writer knows, no shipments have ever been made of this material and no systema- tic prospecting and sampling of this ground has ever been attempted. (See footnote, page 59.)
Gold mineralization of a later period also occurs in this region, being associated with the rhyolite dikes and flows which were emplaced after the early gold, silver and silver-lead deposits had been formed. This gold occurs in veins along the contact of rhyolite dikes and sills within the limestone. The gold is native and is associated with hessite (silver telluride). The gold-silver telluride, calaverite, which has been reported, was not seen by the writer, but it may be present in parts of the deposit.