Explained by Alan R. Wallace
On the choice of deposit models
Small stratiform sulfide deposits are present in Early Proterozoic metasedimentary rocks in Colorado. These may be sedimentary exhalative deposits related to contemporaneous submarine volcanism. The descriptions of many massive sulfide deposits in Colorado, such as in south-central Colorado and the Front Range, are generally inadequate to definitively assign those deposits to a particular model. Only in the northern Park Range (Klipfel, 1992) and the northern Sawatch Range (Tweto, 1974) have the deposits been studied in sufficient detail to make an assignment to this deposit type.
On the delineation of permissive tracts
The Early Proterozoic basement of Colorado, south of a suture zone with the Archean Wyoming Province near the Colorado-Wyoming border, is composed of metasedimentary and metavolcanic rocks that formed in a complex back-arc setting (Reed and others, 1987). Sediments deposited during submarine volcanism could have included exhalative products from the volcanic systems. Areas underlain by unit Xb (biotite schist) on the State geologic map (Tweto, 1979) in proximity to metavolcanic rocks are designated permissive for sedimentary exhalative deposits; these rocks are in the northern Sawatch Range, the Gore Range, and the central Front Range. Sulfide deposits are known to be associated with some of these rocks. The permissive tract also includes areas in southern Wyoming described by Klipfel (1992).
Important examples of this type of deposit
Occurrences of stratabound sulfides are scattered through the Early Proterozoic metasedimentary and metavolcanic rocks of Colorado (Sheridan and others, 1990; Lovering and Goddard, 1950). Most occurrences, such as the broad belt of disseminated sulfides in the northern Sawatch Range near Homestake Reservoir (Tweto, 1974), are semiconformable with the enclosing gneisses, but some contain minor crosscutting veins. Due to the degree of metamorphism and deformation , these veins may contain sulfides that were remobilized locally from the stratabound sulfides. Lovering and Goddard (1950) and Sheridan and others (1990) describe small sulfide deposits and occurrences in Proterozoic metasedimentary rocks, but the descriptions were either inadequate to confidently assign deposits to specific models or the authors of those reports concluded that such assignments could not be made due to conditions such as extreme metamorphic grade and deformation. Production from those deposits was either trivial or not reported.
On the numerical estimates made
Although Klipfel (1992) concluded that some stratabound sulfide deposits in the northern Park Range were sedimentary exhalative deposits related to volcanism that produced nearby Besshi-type and mixed Besshi-sedex deposits; we do not agree with this view, and include undiscovered deposits from the northern Park Range in the assessment of Besshi-type deposits of southern Wyoming and northernmost Colorado.
Production from known zones of stratabound sulfides has been very small (less than a few thousand tons) to nonexistent (Sheridan and others, 1990), and none of the known occurrences are as large as the deposits in the grade and tonnage model of Menzie and Mosier (1986) for sedimentary exhalative lead-zinc deposits. Considering the relatively high degree of knowledge about the geology of the Proterozoic basement, especially in the permissive area, and the very small sizes of known occurrences, the team judged that there was only a small chance of there being an undiscovered deposit, and, for the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more sedimentary exhalative Zn-Pb deposits consistent with the grade and tonnage model of Menzie and Mosier (1986).
Klipfel, P.D., 1992, Geology and metallogeny of the southern portion of the Encampment district, Colorado and Wyoming: Golden, Colorado School of Mines, unpub. Ph.D. dissertation, 244 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.
Menzie, W.D., and Mosier, D.L., 1986, Grade and tonnage model of sedimentary exhalative Zn-Pb, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 212-215.
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., 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.
Tweto, Ogden, 1974, Geologic map and sections of the Holy Cross quadrangle, Eagle, Lake, and Pitkin Counties, Colorado: U.S. Geological Survey Miscellaneous Investigations Map I-830, scale 1:24,000.
Tweto, Ogden, 1979, Geologic map of Colorado: U.S. Geological Survey Map, scale 1:500,000.