Paper No. 62-8
Presentation Time: 2:30 PM-6:30 PM
METALS IN KAMCHATKA MANTLE WEDGE LINKED TO CU-AU-AG DEPOSITS IN MAGMATIC ARCS
Ultramafic xenoliths in Avachinsky and Bakening volcanoes in Kamchatka record diverse metasomatic interactions between depleted mantle wedge and slab fluids and melts. Least-depleted Avachinsky harzburgites (bulk Al2O3=0.23-1.66 wt.% and CaO=0.26-1.72 wt.%) are composed of Mg-olivine (Fo90-92) and Al-poor orthopyroxene and contain Ni-Cu-Fe sulfides (pentlandite, pyrrhotite, chalcopyrite). Cr-spinel carries droplet-shaped Na-Al-Si-rich inclusions with major element chemistry resembling adakite. Other metasomatic features in Avachinsky xenoliths include LREE, Sr and Ti enrichment of secondary-textured clinopyroxenes coupled with presence of amphibole, rutile, (La, Ce)-bearing barite, (Ce, Nd)- and (Y, Th)-bearing armalcolite, (La, Ce)-bearing Ti-silicate and Cu-Ag-Sb-Pb-Bi sulfosalt (similar to vikingite-eskimoite homological series). Avachinsky harzburgites contain Cu-Ag-Au (Cu31.4-46.0Ag4.8-8.7Au49.2-59.8, Cu5.9Ag3.9Au90.2, Ag90Cu10) alloys along with native Pt, Cu, Bi, Pb, Zn metals, W1Fe0.9Mn0.2 alloy and iron silicide. Metasomatic pyroxenites from Bakening are composed of Al-rich (3.24-8.55 wt.% Al2O3) Ti-augite (Ti=792-5530 ppm; Sr=29-110 ppm; LaN/YbN=0.15-5.19), olivine (Fo86-88), Al-rich (38-59 wt. Al2O3) spinel, Ti-magnetite (6-24 wt.% TiO2) and Al- and Ti-rich amphibole associated with blebs and patches of plagioclase-bearing (An48-83) adakitic glass (Sr/Y=84-106, La/Yb=5-41). Metal assemblages in Bakening xenoliths include Cu-Ag alloy and cerargyrite (AgC)l associated with galena, Ni-pyrrhotite, Ni-pyrite and apatite. Data from Kamchatka xenoliths suggest fluid- and melt-related fluxes of ore metals (Cu, Au, Ag, Pb, Zn, Sb, Bi and, to a lesser extent, Pt) from subducted slab into the overlying mantle wedge under low fO2 conditions (below the fayalite-magnetite-quartz buffer). Metals were sourced from altered basaltic crust and metalliferous sediment and transported via siliceous melt (adakitic glass and Ti-minerals) and H2O-S-Cl fluid (barite, cerargyrite and LREE-phases). Later-stage generation of hydrous mafic magmas from hybrid mantle wedge resulted in the efficient transfer of ore elements into the overlying arc crust. Differentiation of metal-rich magmas in crustal conduits and exsolution of ore fluids lead to the formation of Cu-Au-Ag mineralization in porphyry and epithermal environments in magmatic arcs.