Stratabound skarn Pb-Zn mineralization in the Yunkai Domain (South China): The Fozichong case

Abstract

The Fozichong Pb-Zn ore district, located in the Yunkai Domain, southern section of the Qin-Hang Metallogenic Belt (QHMB) in South China, consists of several large-size stratabound ore deposits. The ore bodies are mainly stratiform and stratoid, bounded within the Ordovician and Silurian strata that are intruded by the Yanshanian porphyries. These deposits are debated between syngenetic sedimentary exhalative (SEDEX)- and epigenetic skarn-type. Since the occurrence of ore bodies is accompanied by the green-color stratiform rock (GSR), the GSR origin may play an important role to resolve the controversy. This motivates us to conduct a set of petrographic investigations, whole-rock and mineral chemistry analyses of the GSR coupled with zircon U–Pb geochronology of the intrusions. Detailed petrographic observations reveal that the GSR is mainly composed of a mineral assemblage of garnet, pyroxene, epidote, amphibole and chlorite, in analogy with that of traditional skarns. In addition, the concentrations of ore-related Pb, Zn, Cu and Ba consistently decrease from rim to core of an individual GSR layer, supporting an origin of hydrothermal fluid replacement rather than comparatively homogeneous sedimentation. Therefore, the Fozichong GSR is referred to as the stratiform skarn in literature. The Mn-poor and Ca-rich andradite, diopside-hedenbergite and calcic amphibole in the skarn provide robust evidence for the magmatic hydrothermal origin. The skarn is believed to be formed by the infiltration of magmatic hydrothermal fluids since it is not confined in the intrusion contact zone. Where the hydrothermal fluids derived from is complicated due to the multi-stages magmatic intrusions present in the district. Detailed investigations indicate that the late Yanshanian porphyries (zircon U–Pb dated at ~ 100 Ma, including dacite porphyry and granite porphyry) have a genetic link with the Fozichong skarn mineralization, while the Indosinian Dachong pluton and the early Yanshanian Guangping pluton exhibit insignificant correlation with the mineralization. The redox state of the skarn system changes from oxidation (prograde and retrograde stages) to reduction (late retrograde and quartz-sulfide stages), as recorded by the Fe2+/Fe3+ ratios in skarn minerals (i.e., 0.05 for garnet, 0.01–0.23 for amphibole and 55.6–178.1 for chlorite, respectively). During the redox transition, the mineralization has been developed with the precipitation of chlorite at moderate temperatures (~255 to 278 °C). In combined with the above analyses, the Fozichong is representative of a stratabound skarn Pb-Zn mineralization system in the Yunkai Domain, South China.