Explained by Alan R. Wallace
On the choice of deposit models
Small mines and prospects of this type occur in Early Proterozoic bimodal metavolcanic rocks in an east-trending belt in south-central Colorado, in northern Colorado and southern Wyoming, and in several small areas in the Front Range. Although these deposits have been studied to some degree, many of the descriptions are inadequate to permit a definitive assignment to a particular model. The association with metavolcanic rocks, coupled with descriptions of the Sedalia deposit near Salida (Sheridan and Raymond, 1984), indicate that it is most likely a kuroko-type deposit, and are the key criteria for choosing this model. The criteria used for this assessment followed those outlined by Eckstrand (1984) and Singer (1986) for regional evaluations of Precambrian terranes similar to those of Colorado and southern Wyoming.
On the delineation of permissive tracts
The assessment criteria include: (1) submarine volcanic host rocks of felsic composition; (2) an island-arc tectonic setting, with local extensional deformation at the time of mineralization; and, (3) exhalative horizons within the supracrustal sequence associated with the volcanic rocks (for example, iron-formations or cherty horizons) that indicate the evidence for submarine hydrothermal hot-spring activity during volcanism. The Early Proterozoic basement of Colorado and southern Wyoming is composed of metasedimentary and bimodal metavolcanic rocks that formed in a complex back-arc setting south of a suture zone with the Archean Wyoming Province near the Colorado-Wyoming border (Reed and others, 1987). The kuroko-type massive sulfide deposits in Colorado and southern Wyoming are associated with the Early Proterozoic bimodal metavolcanic rocks. These largely are exposed in (1) an east-west elongate belt in the south-central part of the State, (2) an area along the Wyoming-Colorado border in the Park Range and Sierra Madre Mountains, and (3) several small tracts in the central Front Range (Lovering and Goddard, 1950). Those in Colorado are all within unit Xfh (felsic and hornblende gneisses) on the State map of Colorado (Tweto, 1979). The overall proportion of volcanic rocks in these areas is small, although they may be dominant locally. These rocks are predominantly mafic flows with associated felsic intrusive rocks and tuffs (Sheridan and others, 1990), typical of bimodal volcanic rocks associated with kuroko-type massive sulfide deposits. The massive sulfide deposits associated with metavolcanic rocks in southern Wyoming were classified by Klipfel (1992) as Besshi-type and sedimentary exhalative massive sulfide deposits, but the area is nevertheless permissive for kuroko massive sulfide deposits.
Important examples of this type of deposit
The east-trending belt in south-central Colorado contains scattered small prospects and mines, including the Sedalia deposit near Salida from which nearly 100,000 metric tons of Cu-Zn ore was produced (Sheridan and others, 1990). Very minor amounts of ore came from the Front Range deposits. Lovering and Goddard (1950) and Sheridan and others (1990) describe small sulfide deposits and occurrences in Proterozoic metamorphic rocks of a variety of compositions, but the descriptions are inadequate to confidently assign deposits to specific models. Production from those deposits either was trivial or not reported. The Encampment district in the Sierra Madre Mountains of southern Wyoming has produced nearly 1,000 metric tons of copper, with some zinc, lead, silver, and gold, from deposits in a metamorphosed sequence of Proterozoic volcanic and sedimentary rocks (Hausel, 1989), although how much of this came from kuroko, as opposed to Besshi, deposits is unknown.
On the numerical estimates made
The permissive tract has a few small deposits, including that at the Sedalia mine near Salida. However, the rest of the occurrences are very small, and production of zinc, lead, copper, silver, and gold at the Sedalia deposit, the largest in Colorado, was less than for the smallest deposit on the grade-tonnage curves for kuroko-type deposits (Sheridan and others, 1990; Singer and Mosier, 1986). The area thought to have highest probability was the Encampment district in Wyoming, with an estimated 25 percent chance of hosting such a deposit. All of the assessed areas have received scrutiny by industry and USGS mappers, so the geology is fairly well known. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 4, and 5 or more deposits consistent with the grade and tonnage model of Singer and Mosier (1986).
Eckstrand, O.R., Canadian mineral deposit types—A geological synopsis: Geological Survey of Canada Economic Geology Report 36, 86 p.
Hausel, W.D., 1989, The geology of Wyoming's precious metal lode and placer deposits: Geological Survey of Wyoming Bulletin 68, 248 p.
Lovering, T.S., and Goddard, E.N., 1950, Geology and ore deposits of the Front Range, Colorado: U.S. Geological Survey Professional Paper 223, 319 p.
Reed, J.C., Jr., Bickford, M.E., Premo, W.R., Aleinikoff, J.N., and Pallister, J.S., 1987, Evolution of the Early Proterozoic Colorado province—Constraints from U-Pb geochronology: Geology, v. 15, p. 861-865.
Sheridan, D.M., and Raymond, W.H., 1984, Precambrian deposits of zinc-copper-lead sulfides and zinc spinel (gahnite) in Colorado: U.S. Geological Survey Bulletin 1550, 31 p.
Sheridan, D.M., Raymond, W.H., Taylor, R.B., and Hasler, J.W., 1990, Metallogenic map of stratabound exhalative and related occurrences in Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-1971, scale 1:1,000,000.
Singer, D.A., 1986, Descriptive model of kuroko massive sulfide deposits, in Cox and Singer, eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 189-190.
Singer, D.A., and Mosier, D.L., 1986, Grade and tonnage model of kuroko massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 190-197.
Tweto, Ogden, 1979, Geologic map of Colorado: U.S. Geological Survey Map, scale 1:500,000.