GLOBAL CHANGE PARAMETERS IN THE EVOLUTION OF EARTH'S ORE DEPOSITS

Mineral deposits have an uneven distribution through geologic time that has been related to long-term changes in element abundances, global heat flow, tectonic patterns, compositions of the atmosphere and ocean, and biologic activity. One factor appears to dominate in some cases, such as the decay of 238U that created enriched Phanerozoic crustal source rocks for Pb-deposits and the Paleoproterozoic Great Oxidation Event (GOE) that increased oxygen in Earth's atmosphere ending formation of U-bearing conglomerates. A concomitant increase in oxygen and sulfate in seawater has been linked to the appearance of sedex Zn-Pb deposits and probably extended to subsurface ore-forming environments in sedimentary basins facilitating formation of unconformity/sandstone-U and MVT Zn-Pb deposits in Mesoproterozoic and younger rocks. At greater depth, GOE-related changes in magmagenesis might have created more oxidized magmas allowing the Phanerozoic increase in porphyry Cu, carbonate-replacement Zn-Pb, and epithermal Au-Ag deposits. Decreasing heat flow through time probably accounts for the restriction of komatiite Ni-Cu deposits to Archean and Paleoproterozoic rocks, and the formation of thick lithosphere in which anorogenic magmatism formed deposits of Sn, Fe-oxide Cu-Au, PGEs and diamonds. These longer-term trends have been modulated by episodic tectonic processes and supercontinent cycles, possibly including “snowball Earth”, that generated apparent exceptions. Examples include plume-related rifting and magmatism that supplied Fe for post-GOE, 1.8 Ga Fe-formations, changes in seafloor spreading and calcite-dolomite balance that increased marine sulfate, allowing MVT Zn-Pb deposits to form in Paleoproterozoic rocks but suppressing them in early Phanerozoic rocks, and increased continental aggregation that provided a platform for deposits related to basin sedimentation and anorogenic magmatism. The wide array of elements contained by ore deposits makes them very sensitive to changes in geochemical and tectonic environment, and an important source of insights in our efforts to link the composition of the lithosphere to that of the atmosphere and hydrosphere or, in other words, to make Earth System Science truly global.