Page 51 - Geologic Investigations in the Lake Valley Area, Sierra County, New Mexico

Sugarlump Tuff; therefore the jasperoid is Eocene or older and predates caldera magmatism. The jasperoid in the Fusselman Dolomite that holds up Quartzite Ridge is aphanitic and white to gray colored, and is in places brecciated and recemented by aph­ anitic silica. Jigsaw puzzle textures are found locally. However, large areas of red to brown to white jasperoid are unbrecciated and aphanitic. Locally, manganese oxide and crystalline quartz veinlets cut the jasperoid, and drusy quartz and iron and manga­ nese oxides fill vugs. Visible pyrite is rare to absent except where the jasperoid is cut by faults, but microscopic finely disseminated pyrite, in many places altered to hematite and goethite, is com­ mon. Lovering and Heyl (1989) reported these jasperoids to be barren except where cut by the Lake Valley fault.
Description of Deposits
Descriptions of the mine workings at Lake Valley are from Clark (1895), Harley (1934), Creasey and Granger (1953), and this study. Major workings, many of which are discussed in the text, are shown in figure 5 and table 5. Following Creasey and Granger (1953), the district is separated into four areas: Grande, Bella-Strieby, Apache, and Stone Cabin areas (fig. 5); most of the ore was extracted from the Grande and Bella-Strieby areas.
The mineral deposits occur in two forms: (1) fault- and fracture-controlled replacements and (2) stratabound replace­ ment bodies. The fracture- and fault-controlled deposits are in irregular, steeply dipping zones locally at the intersection of northeast- and northwest-striking fractures and faults (Creasey and Granger, 1953) and along west-striking faults. Mineralized rock is most common in crosscutting structures in the western part of the mining district and in the Alamogordo Member of the Lake Valley Limestone. Deposits at the Daly shaft (fig. 5) and at the No. 14 surface cut (near the Boiler shaft northwest of the Bridal Chamber) are typical of fault- and fracture-controlled deposits. Locally, the lower 0.3–0.9 m of the Nunn Member is mineralized and, in underground workings at the Daly shaft, the Andrecito Member is mineralized for approximately 0.6–0.9 m from the controlling structure.
Tabular, stratabound replacement deposits are adjacent to the fault- and fracture-controlled deposits. These deposits are typically in the upper Alamogordo Member, near the contact with the overlying Nunn Member. The silver ore bodies were thin, irregular tabular zones 0.9–2 m thick that were underlain and laterally surrounded by larger manganese replacement bodies 1–9 m thick. The high-grade silver zones were mined out in the 1880’s, leaving only manganese replacements, some of which contain low-grade silver. Thinner and smaller manga­ nese-silver deposits, 1–3 m thick, are in the Apache and Stone Cabin area workings.
A high-grade silver pocket was mined from the Bridal Chamber, and smaller silver bonanza pockets from the Empo­ ria incline and Bunkhouse shaft (Clark, 1895). The Bridal Chamber consisted of nearly pure cerargyrite in a pocket about 100 m long and 7 m thick (MacDonald, 1909). A streak of pure cerargyrite or chlorargyrite (AgCl) was 1 m thick, and assays as high as 20,000 oz/short ton were common. Silver pockets in the Emporia incline contained 200–500 oz/short ton
Table 5. Silver production from mines in Lake Valley district, 1878–1893.
[Clark (1895)]
Mine
Bridal Chamber Thirty stope Emporia incline Bunkhouse Bella Twenty-five cut Apache area Total
Production (oz of silver)
2,500,000 1,000,000 200,000 300,000 500,000 200,000 300,000 5,000,000
44 Geologic Investigations in the Lake Valley Area, Sierra County, New Mexico
silver and 50–60 percent lead as galena (Clark, 1895). Ore from the Bunkhouse mine contained 200–500 oz/short ton silver. A 5-ton shipment from the Columbia mine contained 3,600 oz/short ton silver (Silliman, 1882).
Primary silver minerals, since mined out, were stephanite, proustite, pyrargyrite, and argentiferous galena (Harley, 1934). Oxidized minerals (cerargyrite, embolite, native silver, cerus­ site, vanadinite, wulfenite, endlichite, descloizite, iodyrite) were either deposited as primary minerals when the fluid evolved to a more oxidizing state or were formed by later supergene fluids. Other minerals found in the district include pyrolusite, mangan­ ite, psilomelane, limonite, hematite, calcite, anderite, and apa­ tite (Silliman, 1882; Genth and von Rath, 1885; Clark, 1895; Lindgren and others, 1910).
Alteration minerals at Lake Valley are primarily quartz , calcite, and clay. The quartz is mostly jasperoid occurring in aphanitic veins. The calcite-clay assemblage is at and near the contact between the Alamogordo and Nunn Members; in addi­ tion, several faults and fracture zones in the underground work­ ings are filled with calcite and clay minerals. Locally, clay zones, overlying or adjacent to mineralized jasperoids, contain elevated silver, lead, and other metals. (See Appendix, samples Lake 74, 83, 98.) Elsewhere, a zone consisting of fine-grained white calcite and clay overlies the manganese deposits.
At the Bridal Chamber, a 0.3–1 m silicified zone overlies the Ag-Mn deposit at the top of the Alamogordo Member. This silicified zone is low in metals concentration (Appendix, sam­ ples Lake 89, 90) and is unconformably overlain by the pale­ ochannel-filling gravels. A sample of clay from between the Nunn Member and the poorly consolidated gravel at the Bridal Chamber contains elevated silver, lead, and other metals (Appendix, sample Lake 91) and is interpreted as a supergene enrichment zone; the paleochannel may have eroded part of the ore deposits in the Bridal Chamber area.
A crude zonation that exists in the district is predominantly a reflection of the abundance of silver and jasperoid in the west- ern part of the area. The western deposits, known as the Grande area workings (Clark, 1895; Apell and others, 1947; Creasey and Granger, 1953), contain the largest and highest grade depos­ its (Clark, 1895; Creasey and Granger, 1953) and consisted of predominantly siliceous ore. Early ore mined from these work­ ings averaged 65 percent silica, 6 percent iron, 12 percent man­ ganese, and 20 oz/short ton silver (Clark, 1895). High-grade