Metallogenic Provinces of Northeast Pacific: ABSTRACT

Abstract

Metal provinces of contrasting sizes and shapes in western North America include deposits of various ages and appear to be largely unrelated to recognized major elements of crustal tectonics, as pointed out by J. A. Noble. When considered in terms of respective structural and petrologic associations, apparent ages, and implied genesis, however, the known deposits can be assigned to metallogenic provinces with a geologically systematic pattern. Five principal kinds of metal concentrations are especially useful in this connection: (1) relatively massive sulfide deposits associated with thick sections of subaqueous volcanic rocks; (2) stratiform deposits in marine sedimentary rocks; (3) stratiform deposits in terrestrial sedimentary rocks; (4) deposits in host rocks of conti ental orogens; and (5) deposits associated with major volcanic accumulations of continental affinities. The volcanogenic sulfide concentrations, which provide a long-term clue to crustal concentration processes, include Fe-Cu-Zn-Au-Ag deposits of Precambrian age that may well reflect contributions from a primitive mantle, Fe-Cu-Pb-Zn-Ag deposits of younger Precambrian and Mesozoic ages in less mafic volcanic rocks and associated eugeosynclinal strata, and post-Paleozoic Fe-Cu-Au deposits of the ophiolitic type that evidently represent mantle exhalations along zones of sea-floor spreading. Such exhalations also appear to have been responsible for accumulation of Fe, Cu, Mn, and other metals in pelagic sediments of deep ocean basins during Cenozoic time. In marked contrast are other deposits that bespeak early separation into the earth's sialic crust of metals such as Mo, W, Sn, U, and V, and continuing differentiation in this direction for Pb, Ag, and Zn. Unlike those of more direct mantle derivation, these deposits evidently have required recycling of metals through various combinations of sedimentation, crustal melting, vapor transport, and new mantle contributions to explain their levels of concentration. Thus current models of metallization along zones of continental rifting, sea-floor spreading, and subduction of oceanic crust can account directly for the development of some important deposits, but they must include at least partly related processes of concentration and reconcentration within the continental crust to explain all of the recognized metallogenic provinces. The copper province of Arizona is perhaps the best example of such complicated interplay over a very long period of geologic time. End_of_Article - Last_Page 1442------------