Genetic concepts versus observational data in governing ore exploration

CIM Bulletin, Vol. 76, No. 852, 1983

JOHN DREW RIDGE, Department of Geology, University of Florida Gainesville, Florida

Over the years, ore geologists have made innumerable observations as to the relationships of certain kinds of ore deposits to particular types of rocks and to the types of primary and secondary structures that these rocks contain. On the basis of these, the places in which to look for new ore deposits are more clearly defined than they would be in an exploration program based largely or solely on concepts of ore genesis. This paper discusses the affinity of certain rock types and structures for such ore deposits as: (1) tin-tungsten associated with granitoid rocks; (2) lead-zinc of simple mineralogy in carbonate rocks; (3) massive base-metal sulphides in sedimentary or volcanic rocks; (4) massive base-metal sulphides and oxides at, or near, contacts with igneous rocks; (5) gold or gold-uranium in, or adjacent to, conglomerates; (6) shale- or sandstone-hosted copper; and (7) porphyry coppers. Additional varieties could be considered, but it appears that these examples are sufficient to demonstrate the validity of the suggestion that the use of observational characteristics is far more useful in mineral exploration than that of theories of ore genesis. As by-products of the theory of plate tectonics, many concepts have been put forward to explain how ore deposits of different kinds have been formed. Most, if not all, of these theories seem to be as valid as present-day informatin permits. However, do these ideas add to the ability of the ore geologist to find ore deposits? I doubt it, and offer reasons as to why I think so. To produce maximum results in any exploration program, the one that makes the greatest use of information produced about the geologic environment of the area being investigated is the one that will provide the greatest return in deposits found for the sponsors of the program. Such exploration is best designated as total exploration, and it certainly is the most effective way of uncovering such new deposits as may exist in the area being studied in particular and of adding to the sum total of non-fuel minerals deposits available to the industrial world in general. It is reasonably estimated that, in searching for deposits of non-fuel minerals, the contribution provided by the totality of tools available for exploration (excluding geochemical, geophysical and remote sensing data) derives 75 per cent from an understanding of the stratigraphy of the area, including the relationships of all rocks to each other—sedimentary, igneous and metamorphic. Of the remaining 25 per cent, three-quarters of that is made up of a comprehension of the structures—internal and external—of the rocks involved, with both mega- and microstructures being determined and used. The final quarter of that 25 per cent is provided from all other geologic knowledge obtained by the team engaged in the exploration process. Such knowledge includes the identities of the minerals present—ore, gangue and wall-rock; their paragenetic relationships (mega- and microscopic) to each other (in the narrow sense); the temperatures at which they were introduced (or metamorphosed), as determined by fluid inclusion, isotope and other studies; and concepts as to how the deposits sought may have been formed.

Keywords: Mineral exploration, Economic geology, Exploration geology, Plate tectonics, Tin-tungsten deposits, Lead-zinc deposits, Sulphide deposits, Gold deposits, Uranium deposits, Porphyry copper deposits.