No abstract

Platinum-group elements (PGE) and gold are a promising tool to assess the processes of mantle melting beneath the subduction zones. However, fractionation processes in magmas inevitably overwrite the initial metal budgets of magmas, making constraints on the melting processes inconclusive. Moreover, little is still known about the geochemical behavior of a particular metal in a single arc magmatic system, from mantle melting towards magma solidification. Here we compare noble metals in lavas from several eruptions of the Tolbachik volcano (Kamchatka arc) to better understand the effects of magma differentiation, estimate primary melt compositions and make constraints on the mantle melting. We show that Ir, Ru, Rh and, to a lesser extent, Pt are compatible during magmatic differentiation. The pronounced incompatible behavior of Cu and Pd, observed in Tolbachik magmas, rules out the significant influence of sulfide melts on the early magmatic evolution in this particular case. Gold is also incompatible during magmatic differentiation; however, its systematics can be affected by the inferred gold recycling in the plumbing system of Tolbachik. Although the Tolbachik lavas show only slightly higher PGE fractionation than in MORB, a notable negative Ru anomaly (higher Pt/Ru and Ir/Ru) is observed. We attribute this to be a result of greater oxidation in the subarc mantle (by 1–4 log units), which promotes crystallization of Ru-bearing phases such as Fe3+-rich Cr-spinel and laurite. The estimated Pd contents for the parental melt of the Tolbachik lavas approaches 6.5 ppb. This is several times higher than reported MORB values (1.5 ± 0.5 ppb), suggesting the enrichment of Pd in the mantle wedge. Our results highlight the influence of the subduction-related processes and mantle wedge refertilization on the noble metal budgets of arc magmas.

—We present results of petrographic, geochemical, and mineralogical studies of apogranite metasomatites associated with sulfide–quartz gold ore veins. The studies show a predominance of muscovite and quartz–muscovite metasomatites. Formation of muscovite metasomatites was accompanied by the accumulation of W, Sc, Zr, Hf, Ga, REE, U, Th, Ta, and Nb and the genesis of new accessory minerals: monazite-(Ce), apatite, zircon, scheelite, W-containing rutile, uraninite, thorianite, cassiterite, etc. Compared with the primary granites, quartz–muscovite metasomatites are richer in Pb, Bi, As, Sb, Co, Ni, Ba, In, Cd, Mo, Te, Ag, and Au (elements of the gold ore assemblage). The high contents of these trace elements are due to abundant galena, fahlores, chalcopyrite, and pyrite among the accessory minerals. Metasomatism of granites was followed by the removal of SiO2, which was then spent for the formation of quartz veins. We have revealed that the distribution of metasomatites of different types within a dike body affects directly the distribution of sulfide–quartz veins and thus determines the ore content of the dike body fragments.