Page 50 - Geologic Investigations in the Lake Valley Area, Sierra County, New Mexico

The district is in a wedge of rocks bounded on the south- west by the northwest-striking Lake Valley fault and on the north by westerly-striking faults (Chapter A, pl. 1). As explained in Chapter A, the Lake Valley fault in the area of the district is interpreted as having multiple movements, including a component of right-lateral strike-slip in the Laramide and normal movement in the Tertiary. The map pattern shows that in the mining area, near the Lake Valley fault, the beds have a slightly more easterly strike than elsewhere in the map area; this bend in strike may relate to drag along the Lake Valley fault. In the district itself, west-striking faults of small offset and associated fractures are the dominant structures. These faults are about parallel to a larger west-striking fault that approximately bounds the district on the north and cuts Quartz­ ite Ridge (pl. 1). West-striking faults are rare in the map area outside the district.
Mineralized Rock
Small and irregular zones of mineralized rock are localized at and near the contact between the Alamogordo and Nunn Members of the Lake Valley Limestone. Cross sections by Clark (1895) across the district emphasize the stratabound char­ acter of the ore. The Alamogordo Member is thick- to medium- bedded, fine-grained to aphanitic limestone that shows a con­ choidal fracture. In contrast, the overlying Nunn Member is thin bedded, contains abundant clays, and breaks along bedding planes. The contact between the members is exposed in the old workings, where slickensides indicate bedding plane slippage of
the ductile Nunn Member over the rigid and fractured Alamo­ gordo Member. Jasperoid replaces fractured Alamogordo, and is itself brecciated. Harley (1934) reported 0.3–0.6 m of jasper­ oid at the top of the Alamogordo. In the western part of the dis­ trict, the jasperoid is brecciated and hosts ore, whereas in the eastern part it is less disrupted and forms the footwall of miner­ alized rock in the lower part of the Nunn Member.
The richest ore in the Lake Valley mining district is near the Lake Valley fault. Jicha (1954) stated that the ore is in the drag zone of normal movement on the fault. The detailed map of the mining district and underground workings by Creasey and Granger (1953) shows that although ore is concentrated near the Lake Valley fault, it is not along the fault. However, minor faults northeast of the Lake Valley fault contain mineralized rock (Creasey and Granger, 1953).
Ore in the district is spatially associated with the Quater­ nary gravels, as shown on the map by Creasey and Granger (1953). Clark, who referred to the paleochannel gravels as por­ phyrite, noted that all the deposits, with exception of a small one in the Apache group of claims, are near the gravel unit and that the largest, the Bridal Chamber and Bunkhouse, are directly under it. Both the Bridal Chamber and Bunkhouse deposits are in caverns and consist largely of cerargyrite. Clark also stated that the overlying gravel carried up to an ounce of silver per ton.
Jasperoid occurs not only in the Lake Valley Limestone, but also in the upper part of the Fusselman Dolomite and in the Mon­ toya Group north and northeast of the mining area. The best developed and most extensive jasperoid is in the Fusselman Dolomite (fig. 4). In the map area, jasperoid in the Fusselman Dolomite is overlain by unaltered Rubio Peak Formation and
Figure 4. Jasperoid in the Fusselman Dolomite. Photograph from Lovering (1972).
Mineral Deposits of the Lake Valley Mining District 43