Volcanic–plutonic connection and associated Au‐Cu mineralization of the Tulasu ore district, Western Tianshan, NW China: Implications for mineralization potential in Palaeozoic arc terranes

Abstract

Constructing the plutonic–volcanic connections in volcanic arc setting is essential for understanding a variety of magmatic processes from continental crust differentiation to formation of magmatic‐hydrothermal ore deposits. Based on the combination of zircon U–Pb geochronology and Hf isotope and whole‐rock geochemistry and Sr‐Nd isotopes, we evaluated the potential connections between volcanic and plutonic rocks from the Tulasu Basin, the largest epithermal gold ore district in the Chinese Western Tianshan. The peak of volcanism coincides well with the major plutonism, representing an important magma event at ~347–367 Ma. These contemporaneous volcanic and plutonic rocks show broadly similar arc‐related geochemical signatures and zircon in‐situ Hf (εHf[t] = −1.0–14.3) and whole‐rock Nd (εNd[t] = −4.2–4.6) isotopic compositions, indicative of formation from parental magmas derived from a common depleted mantle source metasomatized by slab‐derived melts, despite diverse consequent processes (fractional crystallization, crustal contamination, and magma mixing/mingling) during their differentiation in a subduction‐related setting. Volcanic rocks from the Axi caldera in the centre of the Tulasu district are geochemically less evolved than the coeval porphyritic granite, which indicates that the intrusive rocks might originate from residual melts left behind by extraction of erupted magmas. In contrast, the porphyry Cu mineralized Kexiaxi pluton, which occurs in the margin of the Tulasu district, is compositionally indistinguishable from the coeval volcanic suites. This implies that the Kexiaxi pluton represents the intrusive equivalent of a common magma reservoir feeding the volcanic eruptions, and magma fluxes may have controlled their differentiation. We thus suggest that the depleted mantle derived calc‐alkaline magma, modified during slab‐related melt metasomatism, crustal contamination, and magma mixing may have played an important role in copper enrichment at Kexiaxi. Finally, the relatively water‐rich and fertile nature of arc magma of the Tulasu district indicates high exploration potential for significant porphyry Cu‐Au deposits. The existing alteration‐immobile fertility proxy (V/Sc) for intrusive rocks may also be useful to assess the fertility of cogenetic volcanic rocks, in turn with implications for unexposed porphyry Cu deposits at depths.

Geology, geochemistry and genesis of Tabei: A newly identified intermediate‐sulphidation epithermal Pb–Zn deposit adjacent to low‐sulphidation Au deposit in the Tulasu Basin, Chinese Western Tianshan

“…This is further supported by that the data of sphalerite from Tabei are all plotted within the area between continental crust and the source regions of basalt in the t-87 Sr/ 86 Sr diagram (Figure 12). Note that the 87 Sr/ 86 Sr (332 Ma) ratios of sphalerite are comparable to those of orehosting volcanic rocks and granitoids in the Tulasu Basin and Au-bearing quartz from Axi (Figure 12), further suggesting that the strontium, and plausibly both the Pb-Zn and Au, was sourced from the Dahalajunshan Formation volcanic rocks that were generated by the interaction between mantle wedge peridotites and subducted oceanic sedimentderived melts with minor slab-derived fluids (An et al, 2013;Peng, 2015;Tang et al, 2013;Zhao et al, 2020).…”

“…Some W-E-and WNW-trending , 1993). The Dahalajunshan Formation volcanic eruption is commonly considered to be triggered in response to the southward subduction of the North Tianshan Ocean beneath the YCTP (An et al, 2013;Peng et al, 2017;Tang et al, 2009;Zhao, Xue, Symons, et al, 2014) and are genetically associated with the epithermal Au-Pb-Zn deposits (Figure 2; Peng et al, 2017;Zhai et al, 2006;Zhao et al, 2020). The Lower Carboniferous Aqialehe Formation comprising sandstone and limestone is located in the central part and east of Tulasu Basin, unconformably overlying the Dahalajunshan Formation volcanic rocks.…”

“…Sources for Sr isotope data: Quartz from Axi (H. Q. Li et al, 1998); ore-hosting andesite in the Tulasu Basin (An et al, 2013;Tang et al, 2013;Zhao et al, 2020); Granitoids from Tawuerbieke and Kexiaxi (H. G. Wang, 2014;Zhao et al, 2020) F I G U R E 1 3 δD H2O versus δ 18 O H2O plot of ore-forming fluids from the Cu-Au-Pb-Zn deposits with different styles of mineralization in the Tulasu area. Sources for D-O isotope data: Calcite and quartz (this study); quartz from Kexiaxi (H. G. Wang, 2014); quartz from Jingxi-Yelmand (W. J.…”

Accretionary tectonics, deep structures and metallogeny of southern Altaids

“…Constructing the plutonic–volcanic connections in a volcanic arc setting is essential for understanding a variety of magmatic processes from continental crust differentiation to formation of magmatic‐hydrothermal ore deposits. Based on the combination of zircon U–Pb geochronology, Hf and Sr–Nd isotopes, and whole‐rock geochemistry, Zhao et al (, this issue) evaluate the potential connections between volcanic and plutonic rocks from the Tulasu Basin, the largest epithermal gold ore district in the Chinese Western Tianshan. The data also suggest that the depleted mantle derived calc‐alkaline magma, modified during slab‐related melt metasomatism, crustal contamination, and magma mixing may have played an important role in copper enrichment at Kexiaxi.…”