Geochemistry, Zircon U–Pb Ages, and Hf Isotopes of the Devonian Mafic and Acidic Dykes of the Jinba Gold Deposit, NW China: Petrogenesis and Tectonic Implications

Abstract

The Jinba deposit is an orogenic gold deposit located in the Markakuli shear zone, in the southern part of the Altay orogenic belt, northwestern China. Several granite and diorite dykes are present in the area of the mine, with ore bodies occurring in the diorite dykes. However, the diagenetic age, genesis, and evolution of these magmas, and the tectonic setting of the dyke emplacement process, remain unclear. The present study is based on a detailed geological survey, zircon U–Pb dating, Lu–Hf isotopes, and geochemical analysis of the granitic and diorite dykes. The crystallization ages of the granitic and diorite dykes are 384.5 ± 1.2 Ma and 393.9 ± 3.5 Ma, respectively, which indicates formation in the Early to Middle Devonian. Zircon εHf(t) values of the granitic and diorite dykes are 1.43 to 5.2 and −4.47 to −1.18, respectively, with a corresponding two-stage model of depleted mantle (TDM2) ages of 1046 to 1285 Ma and 1242 to 2623 Ma, respectively. This indicates that the granitic and diorite dykes were formed by the mixing of mantle magma and crustal materials to varying degrees, and diorite dykes are more obviously contaminated by the lower crust. Geochemical analysis shows that the granite dykes have a high SiO2 (72.51%–74.87%) and moderate Al2O3 (12.88%–14.04%) content, a total alkali of (K2O + Na2O = 5.51%–6.44%), and aluminous (A/CNK = 1.0–1.02). Granite dykes are enriched in LREE and Th, U, and Pb, and depleted in P, Sr, and Nb elements, with clear Eu negative anomalies (δEu = 0.62~0.66). The SiO2 content of diorite dykes is 51.48%–53.71%, Al2O3 contents are high (14.70%–15.99%), K2O is 1.94%–2.54%, Na2O is 2.97%–3.96%, MgO contents are high (5.15%–6.46%), and TFe2O3 is (13.42%–15.13%), enriched Sr, U, Pb, deficient Th elements, rare earth fractionation is not obvious, and Eu anomaly is not obvious (δEu = 0.93~1.1). We conclude that the Early to Middle Devonian magmatism in the southern margin of Altay (which corresponds to the Jinba gold deposit) may have formed in an island arc-related subduction environment.