Major and trace element geochemistry of garnets from the Geumseong Mo skarn deposit, South Korea: Genesis and exploration implications

Abstract

Few studies have focused on analyzing the compositional variations of trace elements in garnets from Mo skarn deposits to provide their genetic implication and inform potential applications for both distinguishing skarn types and mineral exploration. To achieve this, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), in conjunction with electron probe microanalysis and sulfur isotope analysis, was employed to the Geumseong Mo skarn deposit located within the Hwanggangri mineralized district of South Korea. The garnets in the skarn were divided into early-formed isotropic dark red (Grt Ia) and light red (Grt Ib) garnets, and later-formed zoned red brown garnets (Grt II). These garnets were found to be replaced by molybdenite and pyrrhotite, respectively. The analysis revealed that Grt Ia and Grt Ib comprise a narrow compositional range, with Gro_50–75Adr_8–34 and Gro_66–89Adr_5–28, respectively. They exhibit positive slopes for light rare earth element (LREE), and variable Eu anomalies and heavy rare earth element (HREE) patterns. On the other hand, Grt II demonstrates a wider range of Gro_3–71Adr_20–96 with increased LREE/HREE ratios compared to Grt Ia and Ib, along with distinct positive Eu anomalies. Moreover, there was a systematic decrease in U concentrations as well as Eu anomalies from the core to the rim in Grt II. The observation suggests that early garnets likely formed under near-neutral pH conditions, while later garnets precipitated under mildly acidic conditions. Sulfur isotope values of sulfides including molybdenite, pyrite, pyrrhotite, and sphalerite give relatively elevated δ³⁴S values, ranging from 10.7 to 13.4 ‰ (avg. 12.4 ‰, n = 10). The sulfur isotope characteristics suggests a significant contribution of sulfur from carbonate host rocks and an isotope disequilibrium under high fluid–rock interactions. The textural and compositional evidence indicates that Grt Ia and Grt Ib were formed under stable conditions through diffusive metasomatism via a fluid system in equilibrium with the host rocks. Conversely, the formation of Grt II appears to have occurred under variable physicochemical conditions with intermittent fluid infiltration. Discrimination diagrams for trace elements demonstrate that the Mo skarn compositional field largely overlaps with the Cu field but is distinct from the W and W-Mo fields. The finding implies that the composition of trace elements in garnet can be utilized to discriminate Mo skarn deposits from other types of skarn mineralization, and thus could be potentially applied to the exploration of Mo skarn deposits.