AN EVOLVING UNDERSTANDING BETWEEN MINERAL DEPOSITS AND GLOBAL TECTONICS

Pre-1970’s, descriptive metallogenic studies related the mineral deposits of major ore producing regions mainly to their distinctive host rocks. The rapid evolution in the understanding of global tectonics during the last two to three decades subsequently demonstrated the importance of tectonic environments and processes in controlling both the nature of igneous, sedimentary and metamorphic host rock sequences and their many important types of contained mineral deposits. During the last 1 b.y. of Earth history, plate tectonic processes have differed significantly in continental and oceanic crust. Rifting, with or without separation, affects both these environments and sea-floor spreading causes collisions between both types of crust. The entire spectrum of differing types of ore deposits and their geologic settings are products of resulting intra- or inter-plate processes.

Because both marine and continental successions and their contained deposits are co-products of these varied generative tectonic environments, both serve as indicators of specific geodynamic processes. A simple plot of the time-space distribution of important types of mineral deposits shows that these become increasingly numerous and diverse through time. Only four types--small banded iron formations, komatiitic Ni ores, Pb-poor VMS deposits and gold lodes, which are everywhere similar--occur in Archean greenstone belts. Paleoplacers, magmatic Ni-Cu-PGE ores and huge Fe-Mn formations appear in Early Proterozoic sequences and sedex deposits, Fe oxide-Cu-Au and unconformity-type uranium ores in Middle Proterozoic ones. Sedimentary copper, carbonate-hosted Zn-Pb, and porphyry type ores appear in Late Proterozoic-Phanerozoic strata. As in faunal evolution, these changes include extinctions, disappearances / re-appearances, sudden proliferations and increasing diversity of types through the geologic record. Collectively, as products of an evolving Earth, these indicate major changes from globally similar to complex and varied geotectonic environments through approximately 3 b.y. of geologic time. They suggest that Archean global tectonic environments were akin to those of modern oceanic crust, and that continental crust and its geodynamic settings were generated by subsequent, episodic orogenesis.