GSA Connects 2021 in Portland, Oregon

Paper No. 62-8
Presentation Time: 2:30 PM-6:30 PM


KEPEZHINSKAS, Pavel1, BERDNIKOV, Nikolai1, KEPEZHINSKAS, Nikita2 and KONOVALOVA, Natalya1, (1)Kosygin Institute of Tectonics and Geophysics, Russian Academy of Sciences, Khabarovsk, 680000, Russian Federation, (2)Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, University of Alberta, Edmonton, AB T6G 2E3, Canada

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.
  • GSA 2021 - Avachinsky xenoliths poster (2).pdf (11.3 MB)