The geochemistry and geochronology of garnet from the Jinchanghe Fe-Cu-Pb-Zn deposit, Baoshan Block, western Yunnan

Abstract

The Jinchanghe polymetallic Fe-Cu-Pb-Zn deposit is located in the northern region of the Baoshan Block in western Yunnan. Using electron probe micro-analysis and laser ablation inductively coupled plasma mass spectrometry, we analyzed the major, trace, and rare earth elemental contents of garnets from the Jinchanghe deposit. Moreover, U-Pb dating was performed using the laser ablation sector field inductively coupled plasma mass spectrometry (LA-SF-ICP-MS) method to determine the direct mineralization age of the Jinchanghe deposit. Petrographic analyses revealed three garnet generations: early (GrtI), middle (GrtII), and late (GrtIII). GrtI (And55.69-64.47Gro30.28-41.21Ura + Pyr + Spe + Alm2.35-5.49) was characterized by irregular shapes and weak optical properties, and primarily comprised Fe-rich grossular. GrtII (And53.69-96.17Gro2.78-45.07Ura + Pyr + Spe + Alm0.59-2.12) was characterized by andradite, in which GrtIIa was a homogeneous body, whereas GrtIIb shows exhibited oscillatory zoning with abnormal interference colors. Fine-veined andradite aggregates were predominant in GrtIII (And70.51-90.33Gro6.34-27.80Ura + Pyr + Spe + Alm1.03-3.32). The GrtI, GrtIIa, and the rims of GrtIIb were depleted in heavy rare earth elements and enriched in light rare earth elements, with positive Eu anomalies, indicating that these garnets crystallized from mildly acidic magmatic hydrothermal fluids. However, the cores of GrtIIb had negative Eu anomalies and may have crystallized from a relatively neutral Cl-rich fluid. GrtIII was severely depleted in light rare earth elements with positive Eu anomalies, indicating the presence of a weakly acidic environment. Accordingly, owing to a decrease in temperature and an increase in oxygen fugacity, the pH of the Jinchanghe deposit may have transitioned from an early weakly acidic to neutral, then to a late weakly acidic environment. Furthermore, the ages of GrtII and GrtIII ranged from 520 to 496 Ma, which places the formation of the Jinchanghe deposit during the Late Cambrian. The deposit is closely related to the arc magmatic rocks produced by the subduction of the Proto-Tethys Ocean.