First evidence of magmatic Ni-Cu sulfides and hydrothermal Zn mineralization in Jumun Island, central-western Korean peninsula

Jumun Island is tectonostratigraphically situated on the marginal zone of the Gyeonggi Massif. The Massif is in contact with the southwestern margin of the Imjingang Belt and adjacent to Boreum Island, where ultramafic rock with magmatic Fe-Ti oxide deposits occurs. The northwest of Jumun Island, facing the Boreum ultramafic rock with Fe-Ti oxide ores, is composed of Precambrian Boreumdo schists containing a few magmatic intrusives, the exact ages of which are unknown. In Jumun, the ultramafic intrusion (Mg# = 75), which is confined to a narrow zone along the seaside, mainly consists of olivine (Fo = 81–82), amphibole (magnesio-horn-blende to tremolite), and phlogopite. The olivine is strongly serpentinized and encompassed by amphibole and phlogopite. The Ni-Cu sulfide mineralization found in the ultramafic rock is weak but has a typical assemblage of pyrrhotite-pentlandite-chalcopyrite with a small amount of magnetite. Notably, the Ni-Cu sulfides are closely associated with amphibole and phlogopite and are found in the fractures and interstitials of the olivine grains. The pyrrhotite (n = 2) and chalcopyrite (n = 1) are compositionally close to pure samples, whereas the pentlandite (n = 2) is characterized by enrichment with Co (up to 6.9 wt%). The sphalerite-bearing quartz vein cuts across the Precambrian gneissic rock and strikes N70 °W with an 80 °NE dip. This vein, which is traceable to a limited extent and approximately 40 cm wide, shows mineralogical zonation in the inward direction from pyrite to sphalerite-dominant. Consisting of sphalerite, pyrite, quartz, and chlorite with minor amounts of chalcopyrite, pyrrhotite, and pentlandite, it is composed of 9.56 wt% Zn with < 1.0 wt% As, Co, Cu, In, Mn, Ni, and Pb and below-detection limits (0.001 ppm) amounts of Bi, Ge, Mo, Se, Sb, Te, and W. Sphalerite, a principal ore mineral, is coarse-grained and reddish-brown and is composed of 57.3–58.8 wt% ZnS, 8.0–9.2 wt% FeS, and 32.0–32.4 wt% S with small amounts of Cu, Mn, As, and Cd. The recently discovered Ni-Cu sulfide mineralization and quartz vein with sphalerite, along with the linear array of magmatic Fe-Ti oxide deposits, provide conclusive evidence that the marginal zone of the Gyeonggi Massif may be a geologically favorable area for the formation of magmatic and magmatic-hydrothermal deposits. For exploration purposes, it is necessary to contextualize the source, tectonic setting, and magmatic evolution.