Epidote geochemistry of the Chating porphyry Cu-Au deposit, eastern China: Metallogenic and exploration implications for porphyry Cu deposits associated with carbonate wall rocks

Abstract

Epidote is one of the main alteration minerals in propylitic alteration, and its mineral chemistry has been shown to provide vectors to assist in exploration for hidden arc-related porphyry copper deposits hosted in volcanic rocks. However, it is unknown whether the same vectors are applicable in exploration for porphyry deposits hosted in carbonates. Chating copper–gold deposit in the Middle Lower Yangtze River Metallogenic Belt of eastern China is a porphyry deposit associated with limestone wall rocks. We systematically analyzed epidote associated with Chating deposit, using EMPA and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), to assess the factors controlling epidote geochemistry and the effectiveness of epidote chemistry as a vector in porphyry deposit which hosts in carbonates.

The petrographic study showed that there are three types of epidote in Chating: Ep1 (vein type), Ep2 (replacing magmatic minerals such as plagioclase and amphibole), and Ep3 (replacing skarn minerals such as garnet). All are hosted in the ore bearing quartz diorite porphyry. The major and trace element analysis results show that epidotes in Chating are enriched in Ca, Mn, Zn, Mg, P, Ti, Sr, As and Sb, with higher Ca and Al in Ep3 than in Ep1 and Ep2, and all epidotes are depleted in Cu, Au, Mo and Ag. Epidote chemistry in Chating varies with distance to the ore deposit center, with As, Sb, Pb, Ca and Al high distal from the deposit, whereas Fe, Sn and Fe/Al ratios are low. However, Mn, Zn, Cu and Au, which have proven to be effective vectors in subduction porphyry deposits, did not show systematic spatial variations. We suggest that epidote chemistry in Chating is controlled by several factors, including fluid composition, temperature, oxygen fugacity and the degree of fluid-rock interaction. The dominant factors that resulted in the restricted scale of propylitic alteration, and differences in the spatial trend of elements in epidote between Chating and subduction related porphyry deposits is the wall rock lithology and the degree of fluid-wall rock interaction. The spatial trends of As, Sb, Pb, Ca, Al, Fe, and Sn, and the content of Cu, Au and Mo in Chating epidote can also provide vectoring and fertility information, with potential to be applied in exploration for carbonate hosted porphyry deposits elsewhere.