Trace element and fluid inclusion geochemistry constraint for genesis of gold-polymetallic mineralization in the Songhung deposit, DPR Korea

Abstract

The Songhung deposit is located in the southwestern part of the Yangdok metamorphic core complex (YMCC) in the northern portion of the Pyongnam Basin, the eastern part of the Sino-Korean Craton. In this paper, we present a detailed study on the magma sources of the granitoids (Yangdok and Hoechang graintic intrusions), genesis of pyrite and fluid inclusion characteristics for sufficient and additional understanding of tectonic events, petrogenesis of the granitoids and mineralization in the study deposit. The Yangdok granite has lower Al2O3/TiO2 ratios (mean value of 60.58) than the Hoechang granite (mean value of 994.46), indicating the higher temperature magma source of the former. On the Rb/Ba vs. Rb/Sr and CaO/(TFeO + MgO + TiO2) vs. (CaO + TFeO + MgO + TiO2) discriminant diagrams, the magma source of the Yangdok granite belongs in the plagioclase-rich and clay-poor source rocks, while those of the Hoechang granite plots either in the plagioclase-rich and clay-poor source rocks (for granite and fine-grained granite) or in the plagioclase-poor and clay-rich source rocks (for granite porphyry and felsite). On the Nb vs. Y, Ta vs. Yb, Rb vs. Y + Nb and Rb vs. Yb + Ta tectonomagmatic discrimination diagrams, these two granites are connected with the volcanic arc-related setting and plot in post-collision setting. Moreover, some data of the Hoechang granite fall in WPG field, suggesting that the volcanic arc setting may be changed to an intraplate setting from Middle Cretaceous.The pyrite trace element geochemistry, showing the high Co/Ni ratio than one with an average of 8.2 and high Co content and low Ni content, suggests that its formation was closely linked with I-type granitic magma-derived hydrothermal fluid mixed with meteoric water. It could also be assumed by the Co and As contents in pyrite, indicating a certain degree of meteoric water in the ore forming fluid. It is consonant to the Pb, S, C and O isotope geochemistry in the previous study, suggesting that ore-forming fluids and metals could be derived from both the Yangdok granite and the Proterozoic and Paleozoic meta-sedimentary rocks exposed in the study area. All of the pyrite grains have not compositional zoning and the similar distribution patterns of Co and Ni are appeared in single pyrite grain (from core to rim), suggesting the stable crystallization environment of pyrites and same ore forming fluid source. In pyrite grains, the positive correlation between gold and arsenic is appeared. The Au exists either as a micro- and nano-size inclusion in pyrite and other minerals or as a solid solution in pyrite lattice. Based on the petrological and microthermometric characteristics of the fluid inclusions in ore-bearing quartz, we suggest that the ore-forming fluid is represented as NaCl + H2O or NaCl + KCl + H2O systems with low salinity. The absence of CO2-enriched phase and vapor-only or liquid-only primary inclusions in quartz indicates the homogenous magma-derived ore forming fluids and a lack of boiling during ore-forming process. The formation temperature and pressure of this deposit approximately vary from 317 to 326 °C and from 105.46 to 111.69 × 105 Pa, respectively, exhibiting the hydrothermal deposit origin. In conclusion, the Songhung deposit is originated from I-type granitic magma source-derived hydrothermal fluid which was mixed with meteoric water.