Abstract

The Silver City stock, one of several resurgent intrusions along the margin of the Tintic caldera, has been historically accepted as the source for ore fluids at Tintic, or at least responsible for the heat to drive the hydrothermal system. Fluid inclusions in quartz from fissure veins, however, reveal a thermal gradient that traverses exposures of the Silver City stock, reaching a thermal maximum 3 km south of the exposed stock. Sulfur isotope values from sulfide minerals in the vein and replacement deposits are typical of igneous values (about 1ppm) near the thermal high, but are distinctly lower (as low as -6.8ppm) to the north, though still located in the Silver City stock.Main-stage mineralization at Tintic may have been the product of fluids emanating from a very late resurgent granitoid intrusion in the Tintic district, one that is still largely unexposed. An apophysis of this intrusion may approach the surface near the thermal high delineated by fluid inclusion homogenization temperatures. Additional evidence for the existence of such an intrusion includes widespread propylitic alteration of the Silver City stock and surrounding volcanic rocks, a ladder-style pattern of fissure veins traversing the district, and rare exposures of granitic aplite dikes crosscutting the Silver City stock.

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