Explained by Michael L. Zientek, Stephen E. Box, and Arthur A. Bookstrom
On the choice of deposit models
These stratabound, massive Zn-Pb sulfide deposits form by the precipitation of sulfide and sulfate minerals from metalliferous brines that were exhaled along active submarine faults during deposition of the enclosing sedimentary sequence. The mineral deposits are dominantly iron sulfide accumulations, which enclose economic ore deposits as layers and lenses rich in Zn and Pb sulfides. These syngenetic deposits are hosted in euxinic marine sedimentary rocks in epicratonic and intracratonic basins, often associated with synsedimentary faults related to rifting and subsidence of the sedimentary basin (Briskey, 1986). Proterozoic sedimentary strata deposited in intracratonic basins that formed along the rifted continental margin of North America (Belt Supergroup and Yellowjacket Formation) have Pb-Zn deposits and occurrences as well as evidence for sydepositional faulting and fluid movement. We were examined the general applicability of the grade and tonnage model of Menzie and Mosier (1986) to this Proterozoic basin, in particular, the question of whether the grade and tonnages of Proterozoic and Phanerozoic deposits are different. No differences are apparent, and we decided that no modification of the grade and tonnage models are warranted.
On the delineation of permissive tracts
Traditionally, exploration for sedimentary exhalative Zn-Pb deposits in the Belt basin has focused on a particular horizon (the host horizon of the Sullivan deposit) in the Prichard Formation, the lowest unit of the Belt Supergroup. More recent exploration strategies have expanded the search to all black shale horizons throughout the Belt basin. Two other black shale horizons are now known from the lower Belt. In the central and western parts of the basin, one black shale horizon in the Ravalli Group, one in the middle Belt carbonate rocks, and up to four in the upper Belt are considered viable targets. Thus, the permissive area for undiscovered deposits was defined by the entire outcrop area of the Middle Proterozoic Belt Supergroup in northern Idaho and western Montana (including highly metamorphosed areas marginal to the Idaho batholith) as well as rocks mapped as Yellowjacket Formation and Lemhi Group in Idaho. The Uinta Mountains Supergroup may contain similar rocks, but that area was not included in the tract because we lacked information.
Important examples of this type of deposit
Only one large sedimentary exhalative Zn-Pb deposit is known in the Belt basin: the Sullivan deposit in southeastern British Columbia (Hamilton and others, 1982). That deposit is much larger than the median or mean deposit size of the grade and tonnage distributions of Menzie and Mosier (1986).
On the numerical estimates made
Exhalative massive sulfide deposits are known only from the lower Belt, which includes the Prichard, Aldridge, and Newland Formations, and from the Yellowjacket Formation. Exhalative mineralization includes Pb-Zn deposits such as the Sullivan mine in Canada but also Cu-Co deposits at Sheep Creek, Montana and Blackbird, Idaho. In addition to these 3 deposits with reported grades and tonnages, exploration for sedimentary-exhalative deposits in the lower Belt basin has identified 11 prospects or districts that have been drilled or mined that have ore grades and 55 exploration targets that have unknown grade but meet 2 or more of the following criteria: (1) appropriate depositional environment, (2) evidence of syndepositional extensional tectonics, (3) evidence for the circulation of hydrothermal fluids, and (4) evidence of synsedimentary sulfide mineralization (J.W. Whipple, oral commun., 1993). Seven of the exploration targets are in British Columbia, 12 are in Idaho, and 36 are in Montana. Information on the location of these targets was not available. These are exhalative targets and may contain either Pb-Zn or Cu-Co mineralization.
Although we lack specific information about the location of all the deposits, prospects, and exploration targets summarized above, some favorable areas can be identified. Zn-Pb mineralization has been reported at Sheep Creek, Montana and in the Highland Mountains (Soap Gulch prospect?), Montana (Thorson, 1984; Thorson, 1993; Zieg and Leitch, 1993). Areas favorable for Sullivan-type Pb-Zn mineralization were identified in both the Wallace (the Perma-Plains area) and Dillon (Highland Mountains) 1° x 2° sheet mineral resource assessments (Pearson and others, 1990; Harrison and others, 1986). Tourmalinization is the alteration type most closely related to mineralization at Sullivan (Turner and others, 1993); Slack (1993) identified at least 30 locations in Proterozoic sedimentary rocks in the United States that may have some relation to sedimentary-exhalative mineralization. At least two of these localities, Trestle Creek (Beaty and others, 1988) and Morning Glory (J.F. Slack, oral comm., 1994), have been drilled. Synsedimentary massive sulfide mineralization has also been described in the Whitehall area (Foster and others, 1993).
Although extensive exploration efforts have focused on the Prichard Formation (and stratigraphic equivalents), only one large Pb-Zn deposit, Sullivan, has been found. One industry source indicated that over $15 million has been expended for exploration in the Prichard in the U.S. over a 15 year period. Mineralized Pb-Zn occurrences that have less than a million metric tons of rock have been identified but no mineable deposits have been found.
Despite this apparent lack of success, there is still optimism about the discovery of new deposits. Exploration efforts have focused on the lower Belt; reports of sedex-type mineralization in the middle Belt suggest that some of the other argillaceous units in the Belt Supergoup may be prospective and are underexplored. The Wallace breccias appear to be most favorable units. One industry source offered the following conservative estimates for the discovery of a mineable Pb-Zn deposit in various parts of the Proterozoic basin: 50 percent chance in the Prichard Formation, 50 percent chance in the Helena embayment, 25 percent chance in Wallace breccias within the middle Belt carbonate rocks, <10 percent chance in the Yellowjacket Formation, and <10 percent chance in the argillaceous units in the Ravalli and Missoula Group.
The team was cautiously optimistic that the large permissive area, the presence of one very large known deposit, and many prospective areas indicates a reasonable chance that undiscovered deposits exist in the upper kilometer of the crust in the permissive tract. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 2, 4, 6, and 8 or more deposits consistent with the grade and tonnage model of Menzie and Mosier (1986).
Beaty, D.W., Hahn, G.A., and Threlkeld, W.E., 1988, Field, isotopic, and chemical studies of tourmaline-bearing rocks in the Belt-Purcell Supergroup—Genetic constraints and exploration significance for Sullivan-type ore deposits: Canadian Journal of Earth Sciences, v. 25, p. 392-402.
Briskey, J.A., 1986, Descriptive 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. 211.
Foster, F., Chadwick, T., and Nilsen, T.H., 1993, Paleodepositional setting and synsedimentary mineralization in Belt Supergroup rocks of the Whitehall area: Belt Symposium III, Program and Abstracts, Belt Association, Inc., unpaginated.
Hamilton, J.M., Bishop, D.T., Morris, H.C., and Owens, O.E., 1982, Geology of the Sullivan orebody, Canada, in Hutchinson, C.D.S. and Franklin, J.M, eds., Precambrian sulphide deposits: Geological Association of Canada Special Paper 25, p. 597-665.
Harrison, J.E., Cressman, E.R., Long, C.L., Leach, D.L., and Domenico, J.A., 1986, Resource appraisal map for Sullivan-type stratabound lead-zinc-silver deposits in the Wallace 1° x 2° quadrangle, Montana and Idaho: U.S. Geological Survey Miscellaneous Investigation Series Map I-1509-G.
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.
Pearson, R.C., Hanna, W.F., James, H.L., Loen, J.S., Moll, S.H., Ruppel, E.T., and Trautwein, C.M., 1990, Map showing mineral resource assessment for silver, cobalt, and base metals in Proterozoic sedimentary rocks and for iron, chromium, nickel, talc, chlorite, gold, and graphite in Archean crystalline rocks, Dillon 1° x 2° quadrangle, Idaho and Montana: U.S. Geological Survey Miscellaneous Investigation Series Map I-1803-D.
Slack, J.F., Models for tourmaline formation in the Middle Proterozoic Belt and Purcell supergroups (Rocky Mountains) and their exploration significance: Current Research, Part E, Geological Survey of Canada, Paper 93-1E, p. 33-40.
Thorson, J.P., 1984, Suggested revisions of the lower Belt Supergroup stratigraphy of the Highland Mountains, southwestern Montana, in Hobbs, ed., The Belt, abstracts with summaries, Belt Symposium II, 1983: Montana Bureau of Mines and Geology Special Publication 90, p. 10-12.
Thorson, J.P., 1993, Controls of massive sulfide mineralization in the Belt Supergroup: Belt Symposium III, Program and Abstracts, Belt Association, Inc., unpaginated.
Turner, R.J.W., Hagen, Arthur, Hoy, Trygve, and Leitch, C.H.G., 1993, The Sullivan-North Star corridor—Regional-scale alteration, fragmental rocks and mineralization associated with the Sullivan stratiform lead-zinc deposit, southeastern British Columbia: Belt Symposium III, Program and Abstracts, Belt Association, Inc., unpaginated.
Zieg, G.A. and Leitch, C.H.B., 1993, The geology of the Sheep Creek copper deposits, Meagher County, Montana: Belt Symposium III, Program and Abstracts, Belt Association, Inc., unpaginated.