Low-salinity Liquid-rich Or Vapor-like Fluids in a Porphyry-type Mo Deposit, South Korea

Abstract

Small porphyry-type molybdenum (Mo) mineralization, the Geumeum deposit in the Gyeongsang Basin, South Korea, is associated with the crystallization of a Cretaceous granodiorite, exsolution of magmatic hydrothermal fluids, and related hydrofracturing. Quartz and molybdenite occur with minor amounts of uneconomic chalcopyrite, pyrite, sphalerite, and galena that precipitated from exsolved magmatic fluids and formed hydrothermal fissure-filling vein ores. Three distinct fluid inclusion assemblages responsible for the precipitation of molybdenite are present in the vein quartz. The earliest fluid is represented by low-salinity liquid-rich Type I fluid inclusions, which displayed homogenization temperatures ranging from 298 to 352 °C, and salinities from ∼0 to 9 wt.% NaCl equiv. The Type I inclusions were trapped in a liquid-stable, vapor-absent, one-phase field. The intermediate fluid is represented by CO2-bearing vapor-rich Type IV inclusions, generally showing no visible CO2 liquid phase. These CO2-bearing inclusions totally homogenized by vapor disappearance at 327∼340 °C or vapor bubble expansion at 327∼369 °C, exhibiting near-critical behaviors. The textural relationships and phase equilibria constraints indicate that the Type IV inclusions were derived from a single population, which formed as pressure and temperature fluctuated at near-critical conditions. The latest fluid can be represented by vapor-rich Type II fluid inclusions. No microthermometric data were obtained for these latest assemblages as no visible amounts of liquid phases are evident in small inclusions. Type III inclusions, which commonly refer to halite-bearing inclusions, are absent in the quartz veins studied. The oxygen and hydrogen isotopic fluid compositions of the vein quartz (δ18OSMOW = 4.3 to 6.9‰ and δDSMOW = −65 to −84‰ at 400 °C) is consistent with a magmatic origin with a possible slight influence from meteoric water. Molybdenum mineralization at Geumeum is a product of hypogene hydrothermal processes that were strongly fracture-controlled, highlighting the importance of low-salinity liquid-rich to vapor-like supercritical fluids for the mineralization. It seems likely that the magmas responsible for the formation of the deposit at Geumeum were emplaced at greater depths than those reported for economic porphyry copper deposits in the world. The deposit could thus have survived long periods of erosion, and represent the weakly mineralized “base” of porphyry systems in the Gyeongsang Basin, South Korea.

Fluid Inclusions and S–Pb Isotopes of the Reshui Porphyry Mo Deposit in East Kunlun, Qinghai Province, China

“…The main processes that can add carbon to ore-forming fluids are magma genesis, where the carbon is directly derived from deep magma reactions with H 2 O, decomposition of organic matter, oxidation reactions related to graphite, and decarbonization reactions of calcium-silica carbonate rock. No sedimentary strata were exposed in the Reshui Mo deposit, so the presence of CH4 in the inclusions indicates that the ore-forming fluid reacting H 2 O with reduced carbon in the crust and created CO-CH 4 [66,[69][70][71]. The homogenization temperatures of the stage 2 FIs range from 238.7 to 312.6 • C, which correspond to salinities of 4.34-42.64% NaCl eq.…”

“…The ore-forming fluid thus belongs to a high-temperature H 2 O-NaCl-CO 2 ± CH 4 system with a wide range in salinity. This is the main metallogenic stage; similar to the high-temperature ore-forming fluid system of the Dabie-Qingling collision-type porphyry Mo deposit, it exhibits a wide range of salinity characteristics [71][72][73][74][75]. In stage 3, homogenization temperatures range from 198.3 to 228.9 • C, which correspond to salinities of 0.7-6.3% NaCl eq.…”

“…When the ore-forming fluid is at higher temperature and pressure, the molybdenum complex has a relatively high stability. However, the temperature decrease and the decompression boiling of the fluid in stage 2 may have caused a large amount of volatile matter to escape (CO 2 , H 2 , H 2 S, SO 2 ), and when the fluid was exsolved from the crystallizing magma, the movement of CO 2 (H 2 , H 2 S, SO 2 ) out of the system may have induced a sudden increase in pH [71], thereby, raising the pH and causing Mo 6+ to convert Mo 4+ and SO 4 2− to convert to S 2− to form metal sulfides, depositing sulfides, native metals, and other ore minerals [72]. According to Kim et al [71], pressure changes that transformed the system to near critical environments or critical H 2 O conditions have created favorable environments for the fundamental depositional mechanism.…”

“…However, the temperature decrease and the decompression boiling of the fluid in stage 2 may have caused a large amount of volatile matter to escape (CO 2 , H 2 , H 2 S, SO 2 ), and when the fluid was exsolved from the crystallizing magma, the movement of CO 2 (H 2 , H 2 S, SO 2 ) out of the system may have induced a sudden increase in pH [71], thereby, raising the pH and causing Mo 6+ to convert Mo 4+ and SO 4 2− to convert to S 2− to form metal sulfides, depositing sulfides, native metals, and other ore minerals [72]. According to Kim et al [71], pressure changes that transformed the system to near critical environments or critical H 2 O conditions have created favorable environments for the fundamental depositional mechanism. The incorporation of meteoric water in stage 3 may have caused the temperature and pressure of the mineralizing fluid to drop sharply, leading to a decrease in the solubility of metal complexes and the precipitation of molybdenum [65,66,72] in the Reshui Mo deposit (Figure 8).…”