Petrogenesis and Ni-Cu-Co Sulfide Formation of Mafic Enclaves in Tulaergen Mafic-Ultramafic Intrusive Rocks, Eastern Tianshan, Northwest China: Implications for Liquid Immiscibility and Hydrothermal Remobilization of Platinum-Group Elements

The Tulaergen Ni-Cu-Co sulfide deposit is associated with a small differentiated mafic-ultramafic body (ca. 300 Ma) in the eastern Tianshan orogenic belt, northwestern China. The mafic-ultramafic complex comprises amphibole lherzolite, amphibole olivine websterite, amphibole websterite, and amphibole gabbro. Amphibole lherzolite is characterized by the occurrence of mafic enclaves (gabbro-granodiorite), some of which show coarse blebby to semimassive sulfide mineralization. No chilled or baked margins exist along the margins of mafic enclaves. In contrast, inward-pointing cusps/embayments of amphibole lherzolite around the margins of mafic enclaves are observed. Some mafic enclaves form accumulations (tens of cm) comprising discrete patches (several mm–5 cm). These features indicate that mafic enclaves formed as a molten state within the hosting magma. Mafic enclaves and ultramafic host rocks exhibit similar trace element signatures (e.g., similar incompatible trace element patterns and comparable Th/Nb values, avg. 0.51 relative to avg. 0.52) and overlapping initial Sr and Nd isotope ratios (⁸⁷Sr/⁸⁶Sr_i(t) = 0.7028–0.7036 relative to 0.7021–0.7047 and ¹⁴³Nd/¹⁴⁴Nd_i(t) = 0.5126 relative to 0.5125–0.5127). The uniform geochemical features among mafic enclaves and ultramafic host rocks rule out the possibility that the mafic enclaves were products of contamination of crustal xenomelts or insufficient mixing with a foreign intermediate magma. Mafic enclaves contain unique hydrous alteration assemblages; hence, they were originally enriched in aqueous fluids. The coexistence of aqueous mafic enclaves and hydrous Tulaergen silicate melt (primary amphibole and phlogopite) is consistent with the fluid immiscibility of magma, which could be induced by partial melting of metasomatized mantle occurring above the second critical endpoint at about 3.8 GPa and 1,000°C (~110 km).

The mineralized mafic enclaves contain a pyrrhotite-pentlandite-chalcopyrite assemblage and platinum group element (PGE) patterns similar to those of the other sulfide ores in the Tulaergen Cu-Ni-Co sulfide deposit, indicating the same source of sulfide segregated from the Tulaergen primary magma. However, they are distinguished from the other sulfide ores by strong enrichment in Pt-Pd-Cu (Pd/Ir = 396 and 834 in mineralized mafic enclaves relative to avg. 80, and Ni/Cu = 0.2 and 1.0 in mineralized mafic enclaves relative to avg. 5.8). Lack of negative correlations between Ir and Pt-Pd tenors contradicts the effect of sulfide fractionation on the Pt-Pd-Cu enrichment in the residual sulfide liquid. Together with the aqueous nature of mafic enclaves and evidence of hydrothermal remobilization of sulfide in mineralized mafic enclaves, Pt-Pd-Cu enrichment in mineralized mafic enclaves is attributed to their high mobility in aqueous fluids relative to Ir-Ru-Rh-Ni.

The Tulaergen mafic-ultramafic intrusive rocks were derived from a magma that originated from depleted mantle (εNd(t) ~4.9–8.5), which was metasomatized by slab-derived fluids during the closure of the ancient Tianshan Ocean. Partial melting of the metasomatized mantle above approximately 110 km generated immiscible aqueous fluid and hydrous silicate melt, which eventually gave rise to mafic enclaves and the Tulaergen mafic-ultramafic complex, respectively. The general PGE depletion in sulfide deposits within the eastern Tianshan orogenic belt could be attributed to a low PGE budget in the depleted mantle source(s) and/or low degree(s) of partial melting.