YUNSHUEN WANG1,3, MUNETAKE SASAKI2, MASAKATSU SASADA2, and CHENG-HONG CHEN3
1Central Geological Survey, P.O. Box 968, Taipei, Taiwan
2Geological Survey of Japan, Tsukuba, Ibaraki, 305 Japan
3Department of Geology, National Taiwan University, Taipei, Taiwan
The Chinkuashih deposits associated with Pleistocene arc volcanic rocks are the largest gold-copper deposits in Taiwan. The deposits include high-sulfidation types of the Penshan and Tsushihshan-Changjen groups, and low- sulfidation types of the Chiufen-Wutanshan group. Fluid inclusion data are available for late vug-filling quartz from the Penshan group. Two-phase fluid inclusions occur as primary and secondary types with homogenization temperatures (Th) ranging from 162o to 298o C. The ice melting temperatures (Tm) correspond to fluid salinities ranging from 0 to 8.14 wt% NaCl eq. These fluid inclusions were trapped at different stages over the life-span of the hydrothermal fluids. Microthermometric data indicated that variable amounts of boiling and mixing had occurred in the system. Based on the Th of inclusions trapped from the boiling fluid and the CO2 contents of the inclusions, the hydrostatic pressure is estimated to have been 68 atm. A possible pre-boiling theoretically parent fluid was identified by the chloride-enthalpy diagram at about 297o C. This parent fluid ascended to about 850m below paleo-water table and began to boil at a temperature about 271o C. As vapor was lost slightly during boiling, the cooling fluid gradually increased in salinity. The gas-rich vapor rose and condensed at shallow levels, heating local groundwater to 75o C. This steam-heated water subsequently acted as a diluent and mixed with the ascending chloride fluid.
Late vug-filling quartz from the Tsushihshan-Changjen group contains two-phase inclusions, with Th from 196o to 267o and salinities from 0.88 to 4.96 wt% NaCl eq. However, the salinities at constant Th show a bimodal distribution. This phenomenon can be explained by the intermittant incursion of brine at depth. In one sample which occurs as quartz crystals cement, the porous residual silica might be deposited from hypersaline magmatic fluids. Its fluid inclusion types include two-phase, liquid-rich and vapor-rich, as well as poly-phase saline. The poly-phase saline inclusions contain liquid + vapor +/- halite +/- additional daughter minerals. These inclusions homogenize between 307o and 484o C and have a prominent mode between 300o and 370o C while salinities vary from 30 to 59 wt% NaCl eq. Two-phase liquid-rich inclusions homogenize at a temperature range from 310o to 387o C and salinities between 0.71 and 12.39 wt% NaCl eq.
In the Chiufen-Wutanshan group, small amounts of samples from restricted areas are available. Quartz veins from the Wutanshan orebody were deposited from dilute fluids (0.71-3.55wt% NaCl eq.), with Th measurements from 231o to 280o C. On the other hand, quartz deposition for the Chiufen orebody occurred between 197o and 302o C from fluids of 0.71 to 6.88 wt% NaCl eq salinity.
Gas compositions of fluid inclusions were analyzed for all three groups by a quadrupole mass spectrometer. The results of gas bulk composition analysis indicate that inclusions contain H2O ranging from 99.50 to 99.72 mol%, while CO2 ranges from 0.27 to 0.52 mol%. N2 is about one order of magnitude less abundant than CO2. CH4 is highly variable but consistently less than N2. Small amounts of SO2 were also detected in three samples from the high sulfidation types of ore deposits. Gas chemistries reflected different fluid sources, and were also modified by deep rock interaction and shallow mixing.
The results of individual inclusion analysis indicates that the inclusion fluids comprise mainly H2O and a small and variable amount of CO2. The heterogeneity of gas content among individual inclusions within a single sample may represent different stages of progressive cooling fluids with varying gas content, or be due to heterogeneous trapping of two-phase boiling fluid. SO2 may have been produced by the decomposition of metastable sulfur species that were formed by the oxidation of H2S during mixing of the hydrothermal fluids with low pH groundwater.