Work

Formation of the youngest Ni-Cu sulfide-mineralized intrusion in the Chinese Eastern Tianshan: Constraints from geochronology, PGE geochemistry and Hf-Sr-Nd-C-S isotopes

Year: 2024

Type: article

Abstract: The timing of magmatic sulfide mineralization in orogenic belts is critical for Ni-Cu mineral exploration. In the Chinese Eastern Tianshan, the Ni-Cu sulfide mineralization is presumed to have occurred within a limited period (∼5 Myrs) in the early Permian (∼285–280 Ma). However, the discovery of the Baijiantan ore-bearing intrusion in this region extends the mineralization period to ∼15 Myrs. This unique intrusion provides an excellent example for investigating diverse metallogenic events across different tectonic stages during the long-lived orogenic mafic magmatism. The Baijiantan sulfide mineralization is hosted in a strongly altered gabbroic intrusion, in contrast to the regional magmatic Ni-Cu sulfide deposits that are predominantly found in ultramafic rocks. New zircon U-Pb dating reveals an emplacement age of 270.9 ± 1.0 Ma for this gabbroic intrusion, making it the youngest sulfide-mineralized intrusion in the Eastern Tianshan to date. Geochemically, the gabbroic intrusion is characterized by light rare earth element (LREE) enrichment, pronounced Nb-Ta negative anomalies, highly positive zircon ε Hf (t) (+12.9 to +15.1) and whole-rock ε Nd (t) (+6.0 to +7.3) values, and low initial 87 Sr/ 86 Sr ratios (0.7032 to 0.7038). These geochemical signatures, along with the widespread occurrence of hornblendes, suggest that the intrusion was likely originated from a metasomatized depleted mantle source. Sr-Nd isotopic mixing calculations indicate that the Baijiantan parental magma experienced no more than 23 wt% crustal contamination with the juvenile crust during its ascent. The disseminated sulfide ores show high Cu/Pd ratios (up to 7.1 × 10 5 ) and slight depletion in PGE relative to Ni and Cu. Numerical modeling using bulk sulfide compositions reveals that the Baijiantan magma is significantly depleted in platinum group elements (PGEs) due to previous sulfide segregation at deeper feeders or staging chambers, rather than sulfide retention in the mantle. The anomalous negative δ 13 C values of the Baijiantan gabbroic rocks (−28.9 to −27.4 ‰) suggest that a reduction process caused by the assimilation of organic matter may have played a significant role in triggering sulfide saturation in the magma. The sulfide ores show high S/Se ratios (4118–7290), indicating assimilation of sulfur-rich country rocks during magma ascent. The δ 34 S values of sulfide ores range from −1.2 to +4.0 ‰, which slightly differs from typical mantle value (0 ± 2 ‰). The variations in the S/Se ratio and S isotopes in the sulfide ores can be explained by the addition of external sulfur with positive δ 34 S values and variable R-factors of no more than 3500. Therefore, we propose that addition of crustal sulfur at depth plus assimilation of organic matter was critical in triggering sulfide segregation to form the Baijiantan ore-bearing intrusion. These findings support that mafic intrusions produced by small-scale lithospheric delamination at late post-collision stage may represent prospective targets for regional Ni-Cu mineralization in orogenic belts that have previously been overlooked.

Source: Ore Geology Reviews

DOI: 10.1016/j.oregeorev.2024.106392