Explained by William F. Cannon, Suzanne W. Nicholson, and Laurel G. Woodruff
On the choice of deposit models
In northern Michigan, two major sediment-hosted copper deposits, White Pine and Presque Isle, occur in basal beds of a Middle Proterozoic black shale interbedded with continental red beds. The strata are about 1.1 billion years old and were deposited during the closing stages of development of the Midcontinent rift. The White Pine deposit has been in production since the 1950's and has a very large reserve that could support production for several more decades. Copper is mostly in the form of chalcocite, but native copper is also an important ore mineral; silver is an important by-product.
The ore formed mostly by late diagenetic replacement of early diagenetic pyrite by chalcocite as copper was introduced into the rocks by slightly heated groundwater (White, 1971; Brown, 1971). A second stage of mineralization introduced native copper and additional chalcocite, probably during regional native-copper mineralization seen mostly in underlying basalts (Mauk, 1993).
The Presque Isle deposit is well known as a result of extensive drilling that has outlined a significant undeveloped resource. The deposit appears to be virtually identical to White Pine.
The known mineralization is most like the sediment-hosted copper model (30b) of Mosier and others (1986). But for this quantitative assessment we have modified model 30b in two ways. First we have removed red-bed deposits which tend to be small compared to deposits hosted in reduced facies. Second we have excluded deposits from the Copper Belt of Zambia and Zaire. The Copper Belt accounts for roughly half of the known reduced-facies deposits of the world. The distribution of tonnages and grades for Copper Belt deposits is very different from those of deposits elsewhere. For instance, median tonnage about five times higher and median copper grades are about double the medians of the tonnage and grade distributions of all other reduced facies deposits. Moreover, the overall geology of the Copper Belt is markedly different from the midcontinent of the U.S. These differences suggest to us that Copper Belt mineralization is a special case in which, for reasons not yet fully understood, exceptionally numerous, large, and rich deposits were formed. We believe that a global model that does not include the Copper Belt (Mark3 index 102) is a better analogue for undiscovered deposits in the Midcontinent rift, where all known mineralization is of low grade.
On the delineation of permissive tracts
The permissive tract consists of red clastic rocks with interbedded reduced-facies rocks approximately correlative with the mineralized rocks in northern Michigan. The tract includes areas where these rocks are at the surface in parts of northern Michigan and Wisconsin and direct extensions beneath Lake Superior, as well as areas where they are covered by less than 1 km of younger rocks in parts of Wisconsin, Minnesota, Iowa, Nebraska, and Kansas. The location of the covered part of the tract is determined by geophysical methods and drill holes (Sims, 1990). In the covered areas the character of the red-bed sequence and the amount and location of interbedded reduced-facies rocks are very imperfectly known. Several deep petroleum test wells have intersected reduced-facies rocks at widely scattered locations (Newell and others, 1993), so the occurrence of favorable host rocks is judged to be possible throughout the tract. There is no evidence that mineralizing processes have occurred anywhere other than near the known mineralized areas in northern Michigan, but data is very fragmentary and substantial mineralization could be concealed. The two known deposits are around the sediment-covered flanks of a stratavolcano where stratigraphic thinning of paleoaquifers onto the volcanic edifice may have focused the flow of heated groundwater to produce the copper enrichment (White, 1971). If such focusing of flow was required to produce mineralization, only a small part of the permissive tract is likely to be mineralized.
On the numerical estimates made
At 90th, 50th, and 10th percentiles, the estimated numbers of undiscovered deposits in the tract are, respectively, 2, 5, and 10 or more deposits consistent with the grade and tonnage model (Mark3 index 102). The permissive tract is large and poorly explored except in part of the exposed belt in northern Michigan and Wisconsin closest to the known deposits. On the other hand, known deposits and their subeconomic fringes cover very large areas so that similar deposits could be detected with widely spaced drilling. Thus, a somewhat limited area exists in which very large deposits might still be found in the permissive tract. The two known deposits in the tract are very large compared to the global tonnage distribution of deposits of this type suggesting that even if no additional large deposits exist, there is likely to be a substantial number of moderate- to small-sized deposits.
Brown, A.C., 1971, Zoning in the White Pine copper deposit, Ontonagon County, Michigan: Economic Geology, v. 66, no. 4, p. 543-573.
Mauk, J.L., 1993, Geologic and geochemical investigations of the White Pine sediment-hosted stratiform copper deposit, Ontonagon County Michigan: Ann Arbor, University of Michigan, Ph.D. dissertation, 194 p.
Mosier, D.L., Singer, D.A., and Cox, D.P., 1986, Grade and tonnage model of sediment-hosted copper, in Cox, D.P., and Singer, D.A., eds. 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 206-208.
Newell, K.D., Burruss, R.C., and Palacas, J.G., 1993, Thermal maturation and organic richness of potential petroleum source rocks in Proterozoic Rice Formation, North American Midcontinent rift system, northeastern Kansas: American Association of Petroleum Geologists, v. 77, p. 1922-1941.
Sims, P.K., 1990, Precambrian basement map of the northern midcontinent, USA: U.S. Geological Survey Miscellaneous Investigations Map I-1853-A, scale 1:1,000,000.
White, W.S., 1971, A paleohydrologic model for the mineralization of the White Pine copper deposit, northern Michigan: Economic Geology, v. 66, no. 1, p. 1-13.