Types of mineralization related to fluorine-rich silicic lava flows and domes

Several types of mineralization appear to be related to the emplacement of fluorine-rich silicic lava flows and domes. An important example is the beryllium deposit at Spor Mountain, west-central Utah, where bertrandite, fluorite, amorphous silica, and Mn-Fe oxides replace dolomite fragments in tuffaceous surge deposits just beneath a topaz-bearing rhyolitic lava flow. The Be-mineralized zone is also highly enriched in F, Sn, W, Nb (and presumably Ta), Zn, Pb, and several other metals (but not in Mo). The uniform lateral character of the mineralization, the restriction of Be mineralization to the upper-most few meters of tuff, and the lack of mineralization in fluorite-bearing breccia pipes (tuffaceous vent breccias, in some cases) in underlying dolomite suggest (Bikun, 1980) that the beryllium mineralization resulted from the devitrification of the overlying lavas (a “steam iron” model). Mass-balance calculations based on comparisons of the chemical compositions of glassy and devitrified rhyolite are consistent with this unconventional interpretation. A second important example is provided by “Mexican-type” fumarolic tin deposits, characterized by cassiterite in carapace breccias of rhyolitic domes. Low-temperature dissolution and reworking of early fumarolic cassiterite may produce the colloform “wood tin” common in this deposit type. Deposits of this type occur in Nevada and New Mexico, as well as in many areas of northern Mexico. Fluorine-rich intrusive domes (better known as plutons) may also host metal mineralization, generally of the porphyry type. Examples include the well-known Climax-type porphyry molybdenum deposits of Colorado, New Mexico, and Utah, and the porphyry tungsten deposit at Mount Pleasant, New Brunswick, Canada. These subvolcanic deposits, in common with those associated with extrusive silicic lava flows and domes, are believed to have been derived by the crystallization-devolatization of highly fractionated magma.