Abstract

The Iron Mountain volcanogenic massive sulfide deposit is contained in a Devonian bimodal suite of island-arc volcanic rocks in the southern end of the West Shasta mining district. It is the largest of the West Shasta ore deposits and is composed of a series of structurally related orebodies that have been separated by a series of high-angle normal faults. The ore deposits are predominantly pyritic with subordinate chalcopyrite and sphalerite. The major gangue mineral of the massive sulfide is quartz which occurs in veins and lenses as well as interstitial to the sulfides in the orebody. The deposit shows metal zonation with copper and zinc grade increasing in the central and upper portions of the orebody.Whole-rock oxygen and hydrogen isotope analyses of the rocks underlying the orebodies show the effects of hydration and silicification, from a hydrothermal fluid composed of seawater. Pyrite from the massive ore has a narrow range in delta 34 S values from 3.2 to 5.3 per mil and shows no variation with texture, stratigraphic position, or grade of ore. The narrow range can be explained by a single hydrothermal event of relatively constant isotopic composition, whose principal source of sulfur was leached magmatic sulfur from the underlying rocks. Calculated temperatures from coexisting chalcopyrite-pyrite mineral pairs yield temperatures predominantly in the 400 degrees to 700 degrees C range. The unrealistically high temperatures indicate disequilibrium conditions although clustering of temperatures suggests some regularity governing the isotopic relations.The isotopic values of interstitial quartz in the orebodies show a trend of decreasing delta 18 O values with depth. This isotopic trend along with the similarity in isotopic values between the lower portion of the orebody and the quartz veins in the underlying rock suggests that the quartz was deposited from fluids moving through a temperature gradient of approximately 150 degrees C.From the isotopic and metal zonation data, Iron Mountain is interpreted to have formed from continued zone refining in which successive replacement of sulfides has accounted for both the isotopic relationship in the coexisting sulfide mineral pairs, the isotopic trends in interstitial quartz, and the metal zonation.

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