Alteration, mineralization, and genesis of the zoned Tongshan skarn-type copper deposit, Anhui, China

摘要

The Tongshan skarn-type copper deposit is located in the Anqing-Guichi ore cluster of the iron-copper metallogenic belt which occurs along the Middle-Lower Yangtze River Valley, China. In the study area, skarnization and mineralization took place along the contact zone between carbonates and granodiorite porphyries. The contact zone shows significant horizontal and vertical variations in alteration and minerali zation. In the horizontal direction, the garnet content is high in the skarns near the intrusive body (proximal skarns), the diopside content is high farther from the intrusive body (distal skarns), and hedenbergite is concentrated in the skarns adjacent to the marble zone. Limestones located far from the marble zone experienced a strong silicification. In the vertical direction (from higher to lower levels), the rocks change from hornfels to calcareous skarn to magnesian skarn. Mineralogical studies show that the skarns near the intrusion are relatively oxidized, and the garnet in the skarns is relatively andradite rich. High concentrations of Cu are found in the porphyries with quartz veins, as well as in the calcic skarns, magnesian skarns, hornfelses, and marbles, which are located at distances of 13, 10, 43 and 25 m from the porphyries, respectively. High concentrations of Zn are found in silicified limestones and skarns located even farther from the porphyries. The present findings suggest that the Tongshan deposit was subjected to prograde alteration and mineralization, followed by retrogression. The alteration can be divided into a sequence of stages: contact metamorphism, prograde metasomatism, early retrogression, and late retrogression. The copper mineralization occurred mainly during the early retrogression, and the copper was further enriched in quartz veins within the porphyries during the late stages of magma evolution.