Geochronology, geochemistry and petrogenesis of Late Triassic dolerites associated with the Nibao gold deposit, Youjiang Basin, southwestern China: Implications for post-collisional magmatism and its relationships with Carlin-like gold mineralization

摘要

The relationship between gold mineralization and magmatism within the Youjiang Basin of SW China remains poorly constrained as a result of the lack of outcropping igneous units in this region. This study presents new geological, geochronological, and geochemical data for recently discovered dolerite dykes associated with the Nibao gold deposit, which is located within the NW Youjiang Basin of SW China. Zircon laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of these dykes yielded an age of 213.6 +/- 2.5 Ma, indicating that these dykes were emplaced during the Late Triassic and providing new evidence for Late Triassic magmatism within the study area. The dykes also represent an opportunity to investigate the petrogenesis of mafic rocks and the tectonic setting of the Late Triassic magmatism in the Youjiang Basin. These Nibao dolerite dykes contain relatively low concentrations of SiO2 (46.03-47.66 wt%), have moderate total alkali contents (K2O + Na2O = 2.71-3.15 wt%) and high total Fe as Fe2O3(TFe2O3) contents (13.49-14.96 wt%). Their trace element compositions are indicative of an OIB-like geochemical affinity barring depletion in the high field strength elements (HFSE) such as Nb, Ta and Th. The Nibao dolerites have homogeneous Lu-Hf and Sr-Nd isotopic compositions with (Sr-87/Sr-86)(i) and (Nd-143/Nd-144)(i) values of 0.70562-0.70677 and 0.51233-0.51239, respectively. These dykes have a narrow range in associated epsilon(Nd)(t) and epsilon(Hf)(t) values from -0.70 to + 0.51 and from + 0.04 to + 3.24, respectively. These geochemical data indicate that the dolerites associated with the Nibao gold deposit were derived from an enriched region of the asthenopheric mantle ( > 90km depth) and the magmas that formed these dykes were generated by 5-10% partial melting of similar to 75% garnet peridotite and similar to 25% spinel peridotite material. The ascending magmas underwent both crustal contamination and the fractional crystallization of olivine, clinopyroxene and Fe oxides (e.g., magnetite or ilmenite) prior to emplacement. The Nibao dolerites are geochemically distinct from mafic rocks related to the Emeishan large igneous province, suggesting that they are not linked to this mantle plume event. Instead, they formed in an extensional setting generated by the subduction of the Song Ma oceanic plate beneath South China Block and the subsequent collision between the Indochina and South China blocks. The breakoff of the Song Ma oceanic slab caused the partial melting of the mantle source region for the Nibao dolerite dykes. These relationships suggest that the post-collision extensional environment associated with this breakoff contributed to the deeply sourced, mantle-derived Late Triassic magmatism in this area and, perhaps as well, the gold mineralization in the Youjiang Basin. Most importantly, combining the new geochronological data presented in this study with the timing of extension in this region and the results of previous research suggest that the Carlin-like gold deposits in the Youjiang Basin formed at around 213 Ma (i.e., the Late Triassic).