Frederick J. Sawkins

Sulfide Ore Deposits in Relation to Plate Tectonics

The dynamics of lithospheric plate motions, as denned by plate tectonic theory, appear to exert a fundamental control on many geologic processes, including sulfide ore deposition. An analysis of basic types of sulfide ore deposits, in relation to various plate tectonic regimes reveals a systematic pattern. The calc-alkaline magmatism apparently generated via the subduction process at convergent plate boundaries gives rise, under favorable circumstances, to Kuroko-type (conformable massive sulfide) deposits in submarine volcanic environments, and Cordilleran-type (postmagmatic) ore deposits in epizonal plutonic environments. Porphyry copper deposits, an important subclass of Cordilleran-type deposits, exhibit a remarkable spatial relationship to present or former convergent plate boundary regimes. Certain copper-nickel deposits occurring in ultramafic extrusive or penecontemporaneous intrusive rocks also appear to be related to plate convergence. Sulfide ore deposits generated at divergent plate boundaries (spreading-center systems) are apparently rare, but this may be a function of their submarine habitat. No examples of sulfide ore deposits formed at transform plate boundaries are known. Stratiform copper deposits and most Mississippi Valley-type lead-zinc deposits are generated in continental intraplate environments. Magmatic deposits in layered mafic complexes also occur in intraplate environments, where spreading-center activity was initiated but failed to develop further. Many gold deposits in volcanic arc sequences of greenstone type can be related to a two-stage concentration process operative at inferred convergent plate boundaries. Plate tectonic theory may provide insights into the generation of many metallogenic provinces, and detailed studies of these provinces and their constituent ore deposits will probably provide insights into the processes operative at some plate boundaries. Finally, the plate tectonic approach to the generation and distribution of sulfide ore deposits has much to offer the exploration geologist.

Metal Deposits Related to Intracontinental Hotspot and Rifting Environments

Most non-ferrous metal deposits are formed at convergent and divergent plate boundaries because these are areas of major magmatic and geologic activity. Within plates magmatism is largely related to hotspot activity. The tectonic, igneous and sedimentary processes that accompany subcontinental hotspot activity provide favorable environments within which ore-generating systems can operate, and a spectrum of resultant ore deposit types can be identified. Certain tin deposits are associated with crustal melting during the early stages of hotspot activity. Many hotspot controlled geologic regimes exhibit copper mineralization, and some contain important hydrothermal and/or stratiform copper deposits. In addition some lead-zinc deposits of Mississippi Valley-type can be related to hotspot activity. Other metal deposits that may fit into this general scheme are those associated with some layered mafic intrusives, and certain stratiform lead-zinc deposits in shales. Analysis of the geologic record suggests widespread hotspot activity in association with the fragmentation of Pangea and an earlier (∼1 b.y.) period of widespread hotspot activity that resulted in the fragmentation of a postulated proto-Pangea. The age of many of the non-ferrous ore deposits that have formed in intracontinental environments can be closely fitted to either the first or second of these major fragmentation events. The relationships, between geologic environments that can be correlated with variable degrees of confidence with hotspot activity and certain types of non-ferrous metal deposits, if valid, should provide important guidelines for the exploration geologist.