Zircon and Garnet U–Pb Ages of the Longwan Skarn Pb–Zn Deposit in Guangxi Province, China and Their Geological Significance

Abstract

Garnet is the most common alteration mineral in skarn-type deposits, and the geochronological research on it can limit the mineralization age. The Longwan Pb–Zn deposit, situated within the Fozichong Pb–Zn ore field in Guangxi, lacks precise geochronological data, limiting the in-depth comprehension of its genesis and tectonic setting. This study employs LA-ICP-MS U–Pb dating of garnets developed in the skarn orebody and zircons in the associated granitic porphyry to determine the deposit’s mineralization age. Backscatter electron images and electron probe microanalysis reveal common zonation characteristics in garnets from the Longwan Pb–Zn deposit, with dominant end-member compositions of Andradite and Grossular. The values of U concentrations range from 1.8 ppm to 3.7 ppm, and a garnet U–Pb age of 102.6 ± 1.9 Ma was obtained, consistent with the zircon U–Pb age of 102.1 ± 1.2 Ma from the granite porphyry within the deposit. The Longwan Pb–Zn deposit formed during the late Early Cretaceous as a skarn deposit resulting from contact metasomatism between the granite porphyry and the host rock. The deposit likely formed in response to the Neo-Tethys plate subducting beneath the South China continent during the Cretaceous, followed by a retreat during the Late Cretaceous. The Cenxi-Bobai Fault experienced reactivation under the extensional tectonic regime induced by the Neo-Tethys Ocean’s retreat, leading to a series of magmatic activities along the NE-trending direction within the Fault. The Longwan Pb–Zn deposit formed during the processes of magma emplacement and contact metasomatic reactions with the country rock.