Two basic factors on formation of molybdenum deposits, at Questa mine, New Mexico

Abstract

Molybdenum mineralization associated with hydrothermal alteration occurs at the Questa mine on the western slope of the Taos Range of the Sangre de Cristo Mountains in Taos County, northern New Mexico. Flows, tuffs, and breccias; andesitic, latitic, quartz latitic, and rhyolitic in composition, are extruded in a depressed zone in Precambrian metamorphic rocks. The earliest eruption is of andesitic composition, in succession to this the more salic rocks are erupted. Dikes and plugs of monzonite, quartz monzonite, granite, and quartz porphyry intrude the effusive rocks. Compositional variation through these rocks in different stages follows those of calc-alkalic rock series, i.e., increasing silica and alkali, and decreasing iron from the initia stage with no concentration at middle and later stage (Kuno, 1965). However, each component shows somewhat alkaline character-high in K2O and low in CaO, giving the alkali-lime index 54. Into the effusive rocks intruded granitic stocks of Log Cabin granite, Questa mine aplite, and Red River aplite. These granites are highly differentiated as shown by an example of a parameter, DI of which the granites have 93-95 for the main phase, 96-97 for the latest phase. During the latest magmatic stage, the fractionation is best shown in increase of K2O. The Questa mine aplite, whic accompanies the strongest molybdenum mineralization along the west flank of the sole aplite stock, is the one most fractionated among three granitic stocks. Though little is known about behavior of trace elements in magma, Mo should be concentrated in residual liquids, because in following the “Goldschmidt rule” there are no suitable positions for the element to be settled in major rock-forming minerals. Degree of fractionation may be an importance for concentration of molybdenum in the Questa mine area. Each of the stocks has slightly different characters in mode of the intrusion, The Log Cabin granite may have been intruded into faulted area, and the Red River aplite extruded partly onto surface. They have been in the situation which the molybdenum-bearing ore fluid could have been diffused along faults and vents, instead precipitated. On the contrary the Questa mine aplite was formed in a somewhat closed environment. This could have been the second importance for the formation of the Questa molybdenum deposits.