Relatively little attention has been given to variations in ore‐forming metal sources of Pb–Zn polymetallic deposits hosted in or near the Linzizong volcanic rocks from the eastern to western Gangdese belt. Based on the S and Pb isotopic data of typical Pb–Zn polymetallic deposits from the eastern to western Gangdese, the ore‐forming sources are discerned. The δ 34 S V‐CDT values of the typical Pb–Zn polymetallic deposits in the Gangdese metallogenic belt have a relatively large range of −11.6‰ to 6.0‰, different from those of the porphyry Cu ± Mo ± Au deposits (mainly vary from −2.0‰ to 1.0‰) in the Gangdese metallogenic belt, indicating that the sulphur in the ore‐forming fluid of Pb–Zn polymetallic deposits was dominantly of magmatic origin, but also influenced by strata nearby. The Nuocang and Beina deposits in the western Gangdese have a predominantly upper crustal source of Pb (average values: 207 Pb/ 204 Pb = 15.705, 208 Pb/ 204 Pb = 39.102), while the Narusongduo and Dexin deposits in the central Gangdese and the Leqingla and Xingaguo deposits in the eastern Gangdese are characterized by a mixed source in which Pb was derived from both the subducted slab and the ancient Lhasa basement. From the eastern to western Gangdese, Pb isotopic data show a gradually increasing scale of crustal materials from the Lhasa terrane basement. Combined with the geochemical data of the ore‐related intrusions and Linzizong volcanic rocks from the six deposits, or nearby, we proposed that the heterogeneity of Lhasa terrane crust resulted in more ancient Lhasa basement components contributing to ore‐forming sources from the eastern to western Gangdese. Moreover, relatively higher contents of Au, Ag elements could be identified in the Linzizong volcanic rocks of the western Gangdese than those of in the central and eastern Gangdese. Therefore, in addition to finding the cryptoexplosive breccia, epithermal, skarn, hydrothermal vein‐type Pb–Zn polymetallic deposits, there is exploration potential for epithermal Au–Ag deposits associated with the Linzizong volcanic rocks, western Gangdese.
Abstract To investigate the effect of melt‐rock reaction on Zn isotope fractionation and mantle Zn isotopic heterogeneity, we analyzed Zn isotopic compositions of peridotites, pyroxenites, and mineral separates from the Bohemian Massif, Central Europe. The Mg‐lherzolites (Mg# = 90.9 to 89.1, FeO T = 7.9 to 9.0 wt %) are melting residues with only moderate metasomatism and have δ 66 Zn from 0.11 to 0.20‰. In contrast, the Fe‐rich peridotites (Mg# = 88.2 to 80.3, FeO T = 10.0 to 14.5 wt %) and pyroxenites have larger ranges of δ 66 Zn from 0.11 to 0.31‰ and −0.33 to 0.42‰, respectively. Large disequilibrium intermineral Zn isotope fractionation occurs in the Fe‐rich peridotites and pyroxenites with Δ 66 Zn Opx‐Ol = −0.50‰, Δ 66 Zn Grt‐Ol = −0.55 to −0.39‰, Δ 66 Zn Grt‐Opx = −0.28 to −0.05‰, and Δ 66 Zn Grt‐Cpx = −0.50 to 0.12‰. Combined with their low SiO 2 contents and radiogenic Sr‐Nd‐Os isotopic compositions, the high δ 66 Zn of the Fe‐rich peridotites is attributed to reaction between Mg‐lherzolites and percolating SiO 2 ‐undersaturated basaltic melts that incorporated isotopically heavy crustal components. Crystallization of the isotopically heavy percolating melts migrating through the lithospheric mantle yield the high‐δ 66 Zn pyroxenites. The low δ 66 Zn of the pyroxenites and large intermineral Zn isotopic disequilibrium may result from kinetic Zn isotope fractionation during melt‐rock reaction. Collectively, these observations indicate that melt‐rock reaction can cause intermineral Zn isotopic disequilibrium and significant Zn isotopic heterogeneity in the mantle. This study thus highlights the potential use of Zn isotopes to trace melt‐rock reaction events in the mantle.
Abstract We determined Zn isotopic compositions of 21 orogenic peridotites from the Baldissero and Balmuccia peridotite massifs in Ivrea‐Verbano Zone, Italian Alps, to investigate Zn isotope behaviors during partial melting and melt percolation in the mantle. The samples include lherzolites, harzburgites, and dunites. Lherzolites are strongly depleted in light rare earth element relative to middle and heavy rare earth element with (La/Sm) PM from 0.009 to 0.265 and (La/Yb) PM from 0.003 to 0.125, which can be explained by 5–15% fractional melting of a primitive mantle source. Harzburgites and dunites with nearly identical Mg# (molar 100 * Mg/(Mg + Fe) = 90.2–91.0) have (La/Sm) PM and (La/Yb) PM higher than but Zn contents similar to or lower than those of the parental lherzolites, suggesting that they were influenced by Zn‐depleted silicate melt percolation. Lherzolites have δ 66 Zn from 0.13 to 0.27‰ showing no correlations with indicators of melt extraction (e.g., Al 2 O 3 , Mg#, and La/Yb) and Zn contents. Three sulfide melt‐affected lherzolites show similar δ 66 Zn to the other normal ones. These observations indicate that 5–15% partial melting and sulfide melt percolation cause limited Zn isotope variations in the mantle. The metasomatic harzburgites and dunites display high δ 66 Zn (up to 0.46‰) negatively correlated with Zn contents. Such correlations are attributed to kinetic effect during silicate melt percolation, whereby 64 Zn preferentially diffuses out from mantle minerals (e.g., olivine) to the percolating silicate melts. A diffusion model suggests that the negative correlation between δ 66 Zn and Zn contents in dunites can be explained by an empirical β Zn (i.e., β Zn ‐exponent in D 66Zn / D 64Zn = (m 64Zn /m 66Zn ) βZn ) of 0.05–0.06 in olivine.
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