DESCRIPTIVE MODEL OF RHYOLITE-HOSTED Sn

MODEL 25h

By Bruce L. Reed, Wendell Duffield, Stephen D. Ludington, Charles H. Maxwell,

and Donald H. Richter

APPROXIMATE SYNONYM Mexican-type.

DESCRIPTION Cassiterite and wood tin in discontinuous veinlets in rhyolite flow-dome complexes and derivative placers (see fig. 128).

Figure 128. Cartoon cross section of rhyolite-hosted Sn deposit showing relationship of cassiterite concentrations to rhyolite dome.

GENERAL REFERENCES Lee-Moreno (1980), Huspeni and others (1984).

GEOLOGICAL ENVIRONMENT

Rock Types Alkali-feldspar rhyolite with SiO2 >75 percent; includes topaz rhyolites of Burt and others (1982) and Christiansen and others (1984). Distinctive accessory minerals may include topaz, fluorite, bixbyite, pseudobrookite, and beryl. Petrochemical signature similar to Climax Mo.

Textures Crystal-poor (5 percent) to crystal-rich (50 percent) rhyolite with quartz and sanidine phenocrysts; rare fayalite, biotite, or hornblende may be present.

Age Range Tertiary; chiefly Oligocene and Miocene.

Depositional Environment Rhyolite flow-dome complexes and related pyroclastic and epiclastic rocks.

Tectonic Setting(s) Silicic volcanic fields, generally in areas of thick continental crust.

Associated Deposit Types None are known, but based on geochemical similarity of associated magmas, these may be a surface expression of Climax Mo.

DEPOSIT DESCRIPTION

Mineralogy Cassiterite (including wood tin) plus hematite (characteristically specularite) ± cristobalite, fluorite, tridymite, opal, chalcedony, beudantite, mimetite, adularia, durangite, and zeolite minerals.

Texture/Structure Most commonly as 0.1- to 10-cm-wide discontinuous veins and veinlets whose other dimensions seldom exceed 75 m. These veins and veinlets may be clustered in zones of somewhat greater dimension. Cassiterite also occurs as disseminations in the matrix of rhyolite flows or fault breccias. These two types of deposits are part of a continuum.

Alteration May be absent; tin may or may not occur in large areas of vapor-phase alteration (tridymite, sanidine, hematite, ± pseudobrookite); alteration directly associated with mineralization may include cristobalite, fluorite, smectite, kaolinite, and other clay minerals.

Ore Controls Deposits are generally in the fractured and brecciated outer parts of flow-dome complexes where permeability is high.

Weathering Weathering is generally minor, but a translucent red-orange clay mineral (smectite) is present in most deposits.

Geochemical Signature Dispersion of associated elements (Sn, Fe, Be, Li, F, As, Sb, Pb, Zn, Bi, REE) in rock is minimal. Best exploration guide is presence of high concentrations of tin (>1,000 ppm) in pan concentrate samples. Cassiterite in stream sediments is usually restricted to within 2-3 km of tin deposits.

EXAMPLES

Black Range, USNM (Fries, 1940; Lufkin, 1972)

Mexico deposits (Foshag and Fries, 1942; Smith and others,1950; Ypma and Simons, 1969; Pan, 1974; Lee-Moreno, 1980)