The mineralization age, evolution, genesis of Indosinian magmatic rocks and their related mineralization in the Niukutou ore district, Qinghai Province

The Niukutou skarn type deposit is located in the middle section of the Qimantage area of the East Kunlun orogenic belt, which is one of the largest Pb-Zn polymetallic deposits discovered in this region recently. Previous studies suggested that the mineralization in the Niukutou ore district is dominated by middle to late Devonian Pb-Zn mineralization. However, in recent years, the presence of Indosinian magmatic rocks and related mineralization has been discovered in the southwestern part of the ore district. In order to investigate the relationship between the Indosinian magmatic rocks and mineralization in the ore district, this paper study the chronology, evolutionary characteristics, genesis and source properties of Indosinian magmatic rocks through petrographic analysis, LA-ICP-MS chronology of zircons and garnets, whole rock main and trace elements, and Lu-Hf isotope analysis of zircons. The results indicate that the Indosinian rock mass is composed of gabbro diorite, diorite, granodiorite and porphyritic granite formed at 225.4±1.4Ma~224.0±1.2Ma, while its associated garnet skarns were formed at 228.9±2.4Ma~219.5±0.9Ma. The coupling of the two during the Indosinian period further confirm the existence of Indosinian mineralization in the Niukutou ore district. The geochemical characteristics indicate that the Indosinian rock mass belong to the high potassium calcium alkaline series, with rare earth elements showing enrichment in light rare earth elements and depletion in heavy rare earth elements, with enrichment in large ion elements and depletion in high field strength elements. Diorite (gabbro diorite), granodiorite and porphyritic granite, quasi aluminum, belong to Ⅰ-type granite. Compared to the former, porphyritic granite has a more pronounced differentiation of light and heavy rare earth elements and negative δEu anomaly, and higher differentiation and evolution of major and trace elements. The εHf(t) values of the porphyritic granite is between -4.51 and 5.55, and the tDM2 is between 897Ma and 1454Ma (with an average of 1192Ma). Based on comprehensive analysis, it is believed that the Indosinian magmatic rocks are of crust-mantle mixed origin. The Indosinian porphyritic granite is formed by the crystallization differentiation of diorite (gabbro diorite) and granodiorite. The Indosinian porphyritic granite has a high degree of evolution and is a directly ore-forming rock mass. Based on the comparative study of Indosinian magmatic rocks in this region, this article suggests that the Niukutou Indosinian rock mass may have been formed by mixing of crust derived magma from partial melting of thickened lower crust (ancient crust) and mantle derived magma formed under a post-collisional tectonic setting during 230Ma to 220Ma. The study of the Indosinian mineralization age, magmatic rock evolution and genesis can provide scientific basis for the study of mineralization laws and predictions, and thus open up new ideas for mineral exploration in the Niukutou ore district.