Explained by William F. Cannon, Suzanne W. Nicholson, and Laurel G. Woodruff
On the choice of deposit models
Copper in metallic form occurs in altered volcanic rocks deposited in the Midcontinent rift about 1.1 billion years ago (White, 1968). Production was from twelve principal deposits, and numerous smaller deposits, all in the Keweenaw Peninsula. No deposits have been in production since the 1960's but substantial known resources are left in the ground at marginally economic concentrations. The native copper deposits have few known analogues elsewhere in the world so the occurrence model and tonnage-grade model are developed solely on features in the Lake Superior region. The tonnage grade model (Mark3 index 99) was developed using the 12 deposits and is believed to represent the undiscovered deposits in the permissive tract.
On the delineation of permissive tracts
The permissive tract lies along the trend of the Midcontinent rift where rift-related basalts are within one kilometer of the surface. The tract includes the principal mining district in northern Michigan and extensions of the basalts along the rift trend as far south as northern Kansas. In northern Michigan, Wisconsin, and parts of Minnesota, these basalts form the bedrock surface but farther south are covered by Paleozoic strata (Sims, 1990). The tract also lies partly under Lake Superior. Even where buried, the distribution of the basalts is quite well constrained by geophysical data and drilling.
On the numerical estimates made
At 90th, 50th, and 10th percentiles, the estimated numbers of undiscovered deposits in the tract are 10, 20, and 40 or more, respectively consistent with the grade and tonnage model (Mark3 index 99). This assessment is based on the large size of the tract and very incomplete exploration. Except for the heart of the mining district, little or no modern exploration has been done. Even within the mining district, a large percentage of the ground is not adequately tested, and a major new deposit was discovered there only a few years before the district was closed. Occurences of native copper are widespread and distinctive rock alteration related to native copper mineralization is common throughout the outcrop belts of the basalts. Within the outcrop belts, much favorable rock is covered by glacial deposits, so important mineralization could be concealed at the bedrock surface. Where the basalts are covered by Paleozoic strata their character is not well known, but there is no geologic reason to conclude that they are not significantly mineralized.
Sims, P.K., 1990, Precambrian basement map of the northern midcontinent, USA: U.S. Geological Survey Miscellaneous Investigations Map I-1853-A.
White, W.S., 1968, The native-copper deposits of northern Michigan, in Ridge, J.D., ed., Ore deposits of the United States, 1933-1967: American Institute of Mining, Metallurgical and Petroleum Engineers, p. 303-325.