New Mexico Mineral Symposium — Abstracts

Recent geologic studies of the Black Range tin district have indicated that there are several different types of cassiterite occurrences that are associated with the Taylor Creek peraluminous rhyolite. Four types have been distinguished, from early to late: I. high-temperature vapor dominated; II. high-temperature hydrothermal fluid dominated; III. low-temperature hydrothermal fluid dominated; and IV. supergene(?). Type I deposits consist of sparse amounts of cassiterite occurring in lithophysae, miarolytic cavities, and tiny gash veins. Type II deposits consist of well-defined gash veins and anastomosing systems of veinlets that vary widely in orientation and size (1¬10 cm thick and 1-8 m long). Type III deposits consist of veins and stockwork-like areas of reticulate veinlets in the distal edges of rhyolite flow domes and in the underlying tuffs and ash-flow tuffs. Some veins are as much as 30-40 cm wide and 100+ m long. The exposed surface area of the veins may encompass several thousand square meters. Type IV deposits consist of supergene accumulations of wood-tin and stannic acid, Sn02•xH20, occurring in porous zones in sandstone, conglomerate, and tuff. They also occur in subsurface pockets of intensely altered or weathered rock, most commonly in pockets filled with red clay. Thus, deposition of cassiterite and associated minerals occurred over a wide range of conditions. Some of the tin mineralization (Type I) is derived from the host Taylor Creek Rhyolite, but most of the tin mineralization (Type III) is characterized by colloform (wood-tin) cassiterite and is derived from a source outside of the rhyolite flow domes. Most of the tin was deposited in vein systems after the host rocks had solidified, cooled, and fractured.

A list of the minerals identified from each of the four types of deposits is tabulated on the next page.


Electron microprobe and emission spectrographic analyses of cassiterite from all four types of deposits indicate that there is substantial variation in the chemical composition. The earliest formed (Types I and II) cassiterite is characterized by the presence of minor to major amounts of Sb, Fe, ± Ti.

Essentially pure cassiterite is a paragenetically intermediate product, followed by the most abundant type of cassiterite, wood-tin, in Type III deposits. Cassiterite from Type III deposits generally contains as much a 1 wt. % or more of all or some of the following metals: In, As, Si, Pb, Fe, Zn, and Sb.

Mineralogical investigations have shown that a monoclinic(?) derivative of cassiterite, amber to caramel colored, most likely related to the trirutile structure, and containing major amounts of Sb, Sn, Fe, and Ti, exists in rhyolite at Squaw Creek. Some of the material has epitaxial overgrowths of cassiterite. Crystals are all small, usually less than 100 microns long and 10-20 microns wide. Detailed microprobe studies of this material have been done by Paul Hlava of Sandia National Laboratories.

Hematite, particularly high-temperature varieties, may contain as much as 1% Ti, 1.5% Mn, 0.07% Nb, 0.15% Zn, and 0.7% Sn (based on emission spectrographic analyses).

To date, three fluoroarsenates (and/or arsenates) have been found at multiple localities. Durangite, NaAlAsO4F, has been found at two different localities (the Clearing and 74-draw) and has been described in detail in the Canadian Mineralogist (1985, vol. 23, pp. 241-246). A new red to red-orange Na-Ca-Fe¬-Mn-Ti-Al-Mg fluoroarsenate occurs at Squaw Creek and Willow Creek. At Squaw Creek, very small amounts of a new bright yellow Ce-group fluoroarsenate or arsenate, with minor amounts of Ca and Th, coexist with the red fluoroarsenate. Maximum dimensions of crystals of the red fluoroarsenate are as much as 0.5 mm, and those of the yellow REE fluoroarsenate (or arsenate) are about 0.25 mm.

Clinopyroxene, ranging in color from orange to red brown to amber, occurs at several localities (Willow Creek, Squaw Creek, 74-draw, and Lookout Mountain Road). Material from the Lookout Mountain Road locality shows the alexandrite effect, being violet-purplish red under tungsten light and green under natural light. Crystals are typically 0.5-1 mm in mean dimension.

Some of the titanite from Willow Creek, east of the Black Range, has a very unusual composition. It may contain as much as several percent or more of REE, Pb, Nb, F, and other elements. The color is very similar to that of the clinopyroxene from the same locality. Mean dimension is approximately 0.5 mm.

Single-phase high sanidine adularia with the unusual
composition (K0.51Na0.45Rb0.004) ∑ 0.964 (Al.98 Fe.01) ∑0.99 Si3.0108 occurs at Alexander along with Sb-Fe-bearing cassiterite. The adularia coats fracture and/or vein surfaces, and individual euhedral colorless to white crystals are as much as several mm across.

A continuous progression from high- to low-temperature minerals is present, and the occurrence of typically hydrothermal elements, such as Zn, Sb, Pb, and As, along with the geologic observations points toward a principally hydrothermal origin for the deposits.