New Mexico Mineral Symposium — Abstracts

The Sierra Ladrones are situated within the Rio Grande rift and are a basement block uplift formed as a result of the extensional tectonism prevalent in this region for the last 30 m.y. Alteration and mineralization in the region has spanned at least the last 500 m.y. since the intrusion of Cambrian carbonatites into the Precambrian country rock (McMillan and McLemore, 2004). Recent observation suggests that Precambrian basement rocks in the Ladrones Mountains have experienced a much older history of hydrothermal alteration and brittle deformation.

The earliest documentation of mineralization at the Cascabel mine is based on misinterpreta¬tion of the local geology and therefore necessitates a contemporary genetic model for the tectonic history and mineral paragenesis. Previous observation attributed fluorite, quartz, and hematite formation to a 1-2-m-wide "diabase dike" that intruded the granitic gneiss country rock. Several factors negate this "intrusion" as the cause of mineralization: 1) Precambrian basement rock throughout the region contains linear amphibolite bodies (not diabase) not associated with country rock alteration; 2) Diabase dikes are water poor and silica poor and would not generate a hydro-thermal system capable of severely altering approximately 2 km2 of country rock at the Cascabel mine. We prefer a genetic model that relies on circulation of hot, silica-rich fluids that infiltrated and exploited faults and fractures in the country rock. Oxidized iron responsible for the coloration of amethyst implies interaction of meteoric water during the last stage of mineralization. This con¬strains the depth of the host rock at the time of quartz deposition to several kilometers below the paleosurface. Open voids for quartz mineralization were the result of faulting and/or volumetric loss related to chemical alteration. Goethite pseudomorphs after pyrite show that sulfides were once present in the country rock and were the likely source of acid that caused the locally intense alteration. Following this, silica saturated fluids precipitated large, milky-quartz crystals as much as 6 inches in length. Fluorite crystallized after this. A second episode of mineralization began with deposition of hematite followed by a fluid lightly charged with silica that left a thin coat of dear quartz on the milky crystals. Finally, the amethyst formed as the weak silica solution gained oxidized iron. At least two episodes of mineralization are clearly delineated as nearly all the early quartz crystals were broken, then healed, or in some cases formed "dumbbells" as amethyst grew on both tips.

The age of mineralization has not yet been definitively determined, but several observations suggest a Proterozoic ancestry: 1) host rock for mineralization is Precambrian metamorphic rock; 2) Paleozoic sediments overlying the deposit are unaltered relative to Precambrian rocks. Localized fracturing and dilation within an extensional tectonic regime are conducive to mineralization. Proterozoic extension in the southwestern United States has been documented during two time periods at ca 1000 and 800 Ma (Timmons et al., 2001). ⁴⁰Ar/³⁹Ar K-feldspar thermochronology of the basement rocks in New Mexico indicates that rocks were at approximately 10-km depth at ca 1000 Ma (Heizler, M.T., pers. comm. 2003), and were likely too deep for the style of mineralization observed at the Cascabel mine. However, by ca 800 Ma the basement over much of the region had been exhumed to a few kilometers depth that places the Precambrian host rocks at the Cascabel mine at shallow crustal levels. Based on this, we believe that - 800 Ma is a good representation for the timing of mineralization and alteration.

References:

1. McMillan, N. J., and McLemore, V. T., 2004, Cambrian-Ordovician magmatism and extension in New Mexico and Colorado: New Mexico Bureau of Geology and Mineral Resources, Bulletin 160, in press.
2. Timmons, J. M., Karlstrom, K. E., Dehler, C. M., Geissman, J. W., Heizler, M. T., 2001, Proterozoic multistage (ca 1.1 and 0.8 Ga) extension recorded in the Grand Canyon Supergroup and estab-lishment of northwest- and north-trending tectonic grains in the southwestern United States; Geological Society of America, Bulletin, v. 113, no. 2, pp. 163-180.