US Geological Survey
Date of this Version
1987
Abstract
Scheelite-molybdenite stockwork mineralization constitutes one component of the Lake George polymetallic (Sb-W-Mo-Au-base metal) deposit, a complex hydrothermal center of Late Silurian (ca. 412 m.y.) age in the Fredericton trough of the northern Appalachians. The stockwork, hosted by Silurian graywackes, in part calcareous, is spatially and temporally related to a postkinematic cupola of biotite monzogranite, and its formation overlapped in time with the emplacement of monzogranitic porphyry dikes. Mineralogical and textural evidence indicates that contact metamorphism associated with the cupola had ceased before the initiation of W-Mo mineralization and that it occurred, at pressures of less than 1.75 kb, in two stages: a peak stage (T > 600°C), evident only in rocks of pelitic composition; and a lower temperature reequilibration (T < 500°C), recorded in rocks of both pelitic and marly compositions.
The W-Mo deposit comprises three different scheelite- and/or molybdenite-bearing veinlet types. Type 1 bodies, the earliest formed, are calc-silicate (granditic garnet, wollastonite, clinopyroxene, and calcic amphibole) quartz veinlets, with ubiquitous Ca and H metasomatic alteration envelopes. Mineralogical and fluid inclusion relationships indicate that the fluids ranged in temperature from 550° to 22B°C and that temperature decreased away from the cupola. The succeeding type 2 veinlets comprise quartz and lesser amounts of perthitic alkali feldspar, muscovite, calcite, scheelite, molybdenite, and pyrite. Fluid inclusion evidence shows that mineralization dominantly occurred from 400' to 175OC, under a confining pressure of 1.3 kb. Higher grade scheelite and molybdenite deposition was focused in a lower temperature zone, to the north of the cupola, in which CO, effervescence occurred. Type 3 veinlets, the last to form, consist of prehnite, molybdenite, and quartz and represent a volumetrically minor mineralization type.
In both type 1 and 2 systems, scheelite and molybdenite deposition appears to have been controlled by decreasing temperature and increasing pH. Temperature was a function of distance from the cupola for both veinlet types, but the controls on pH were specific to each. Thus, the pH of type 1 fluids was controlled by wall-rock interaction (H metasomatism), whereas that of type 2 fluids was controlled by CO, effervescence.
The economic stibnite-quartz veins (Scratch et al., 1984) occupy fractures which transect, and therefore, postdate all stages of W-Mo mineralization.
Comments
Published in Economic Geology Vol. 82, 1987, pp. 1259-1282.