Element behaviour during interaction of magma and fluid: A case study of Chamuhan Granite, and implications on the genesis of W – Mo mineralisation

Abstract

The Chamuhan Granite is located in the eastern part of the Central Asian Orogen where it crops out as an elongated, NE-trending composite monzogranite. The pluton consists of two phases with a fine – grained monzogranite (G2) at depth in sharp contact with a medium – grained monzogranite (G1) above. The medium-grained monzogranite can be subdivided into G1a and G1b based on the degree of metasomatism affecting it, with the G1b being more altered. Tungsten–molybdenum mineralisation is hosted by quartz (−fluorite) veins or forms disseminated sulfides in G1a, indicating a genetic association between the monzogranite and ore. Magmatic zircons from G1a yield a weighted mean age of 144 ± 2 Ma, and hydrothermal zircons from G1a yield a weighted mean age of 143 ± 1 Ma, indicating that the hydrothermal zircons are very close in age to the medium-grained monzogranite. Monazite grains yield U – Pb isotope ages of 138 ± 3 Ma for G1a, 137 ± 1 Ma for G1b, and 139 ± 2 Ma for the fine-grained monzogranite (G2) phase. These dates are the same within analytical error, and are coeval with the molybdenite Re – Os weighted mean age of 140 ± 2 Ma, indicating that the monazite and molybdenite were deposited during hydrothermal alteration. The Chamuhan Granite is geochemically a metaluminous to slightly peraluminous high – K calc-alkaline granite, and is especially characterised by its REE tetrad pattern and a huge Eu depletion that are accompanied by a non – “Charge-and Radius-Controlled” (CHARAC) trace element behaviour. This and the O-isotopic data from the granite phases are indicative of a highly differentiated magma that interacted strongly with a co–existing fluid emanating from the magma during the late stage of its crystallisation. The interaction between the magma and related hydrothermal fluid resulted in the enrichment of W–Mo(–Nb–Ta–Th–U–Hf–Rb–Y). All the studied granitic rocks have homogeneous initial isotope values with positive εNd(t) values of 0.1–3, and TDM2 dates between 924 and 691 Ma. This is consistent with granites derived from a pre-existing juvenile crust derived from a depleted mantle during the Neoproterozoic. The molybdenite in the Chamuhan W – Mo deposit has a narrow δ34SVCDT range of 0.1–1.6‰ characteristic of a magmatic origin, and the Pb isotopic compositions with 206Pb/204Pb ratios between 18.298 and 18.882, 207Pb/204Pb ratios between 15.494 and 15.538 and 208Pb/204Pb ratios between 38.046 and 38.206, are similar to the calculated initial Pb isotopic ratios of the Chamuhan Granite. We thus suggest that there is an intimate relationship between the Chamuhan Granite and mineralisation, and that the mineralising process is related to a post-collisional extensional event following the formation of an over-thickened continental crust and during closure of the Mongol-Okhotsk and Paleo-Pacific oceanic plates.