POST-CONVERGENT PORPHYRY COPPER DEPOSITS—WHAT, WHERE, WHY

Although most porphyry copper deposits worldwide are the products of arc magmatism related to subduction of oceanic lithosphere, an important subset are not related directly to subduction. These porphyry copper deposits are similar to those found in magmatic arcs. The magmas that sourced the deposits are generally hydrous, oxidized, and calc-alkaline to alkaline; they show evidence for partial melting of protoliths that contained hornblende, as exemplified by relatively elevated (Sr/Y). However, the tectonic setting for the origin of these magmas is quite different than for those associated with subduction. Origins postulated for these magmas include 1) melting of thickened mafic lower crust or 2) delaminated lower crust, modified by the inclusion of varying amounts of mantle component or 3) melting of previously metasomatized mantle. Suggested triggers for these various styles of melting include 1) lithospheric thickening, 2) extension, 3) delamination, 4) aesthenospheric upwelling, or 5) translithospheric strike-slip and/or normal faulting.

The 60 post-convergent deposits that we have identified appear to have significantly higher Mo, Au, and Ag grades, although the Cu grade and tonnage distributions for these deposits are not significantly different than those for subduction-related deposits. The differences in Au and Ag grades in particular, are based on a few rich deposits, and we do not yet propose a separate grade and tonnage model.

These deposits are found throughout much of the world, and have ages ranging from Miocene to as old as Jurassic. The most important localities are the Jurassic and Early Cretaceous of Eastern China, the Tertiary of the Tibetan Plateau (Gangdese, Dali, and Yulong belts), the Tertiary of the western United States (Great Basin and Rocky Mountains), and the Tertiary of Eastern Europe (Northern Carpathians, Apuseni Mountains, and Aegean region). These and other areas may be the site of new discoveries of this recently recognized variety of porphyry deposit. We will explore the settings and characteristics of these deposits by comparing their characteristics to those of deposits formed in more conventional settings using the large databases compiled by the Global Mineral Resource Assessment Project of the USGS.