Most of the principal mines in the Monarch district are situated on Monarch Ridge, about half a mile south of Monarch, and in an area north of Garfield extending from Taylor Gulch west to Columbus Gulch. Outlying groups of mines are situated high up in Middle Fork and North Fork valleys, in Hoffman Park, and near the source of Cree Creek.
The district has produced silver, lead, zinc, gold, and copper.
The ore deposits of the Monarch district consist of replacement deposits in limestone and dolomite, and of veins. The replacement deposits have been overwhelmingly the most productive. They occur in bedded and irregular forms and along faults. The bedded and irregular replacement deposits occur locally in all of the formations that contain limy beds, but they are particularly characteristic of the basal 50 feet of the Manitou dolomite, and in many places rest on granite.
Most of the ore obtained from the replacement deposits has been oxidized, although primary sulfide minerals were reached at varying depths in all the deeper mines. The oxidized ore is typically brown, soft, porous limonite containing variable amounts of cerussite, calamine, smithsonite, and occasional patches or grains of galena. The common primary sulfides are pyrite, galena, sphalerite, and chalcopyrite. Pyrrhotite is present in part of the ore in the Garfield mine.
The unoxidized parts of the veins consist characteristically of variable proportions of galena, sphalerite, and pyrite, and generally lesser amounts of chalcopyrite, in a gangue of white vuggy quartz. Silver is present in nearly all the sulfide ores, and gold is present locally and in variable quantities. Oxidized vein matter is typically brown, somewhat porous limonite or limonitic quartz, accompanied locally by cerussite, smithsonite, calamine, secondary copper stains, and patches or grains of galena. Oxidation is not nearly so pronounced or deep in the veins as in the replacement deposits. The veins in the sedimentary rocks are more deeply altered than those in the crystalline rocks; many are extensively oxidized to depths of several hundred feet, whereas most of the veins in the crystalline rocks contain fresh sulfides at or a few feet below the surface.
Several ore deposits in the Monarch district are associated with minerals characteristically developed by contact metomorphism. All these deposits are near the head of Taylor Gulch or in Cree Creek valley, near a large body of Mount Princeton quartz monzonite which has irregularly metamorphosed the bordering sedimentary rocks to the southeast for as much as half a mile.
Silver ore is reported to be associated with magnetite in the Mountain Chief mine, sphalerite is associated with diopside and andradite near the New York mine, and silver, copper, lead, and gold are associated with garnet in the Clinton mine.
U.S. Geological Survey Professional Paper 289
References
U.S. Geological Survey Professional Paper 289
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