Geological setting, mineralogy, alteration, and nature of ore fluid of the H zone, the Chatree deposit, Thailand

The Chatree gold deposit occurs within the Permo-Triassic volcanic rocks of the Loei-Phetchabun Volcanic Belt and is located at the boundary of Phetchabun and Phlchit Provinces, central Thailand. The Chatree deposit (geological resource of 33 Mt @ 1. 7 git Au and 11 git Ag) is classified as a low-sulphidation epithermal deposit and has been subdivided into several distinct zones. This study concentrates on the H zone. The stratigraphy of the H zone consists of plagioclase pyroxene phyric andesite, andesitic lithic breccias, crystal-rich andesitic pumice breccias, crystal-rich quartz feldspar lithic breccia, quartz-rich volcanic sandstone/mudstone and limestone. Andesitic-dacitic and basaltic dykes have intruded the host rocks and mineralised veins. Whole rock geochemistry indicates that the host rock composition ranges from sub-alkaline and calc-alkaline basalt to andesite. These volcanic rocks formed in a volcanic arc, with most rocks of continental volcanic arc affinity. The environments of host rock deposition are mainly subaerial to shallow submarine. Five stages of vein mineralisation have been identified by petrography, PIMA and electron microprobe. These include Stage 1: hydrothermal breccia; Stage 2: gold - bearing quartz - calcite vein including quartz - calcite - chlorite - illite - illite/smectite - sericite - ankerite - dolomite - epidote - adularia - pyrite - hematite - rhodochrosite - chalcedony - sphalerite - galena - chalcopyrite - electrum; Stage 3: quartz - K-feldspar - carbonate - epidote - pyrite vein; Stage 4: calcite veinlet; and Stage 5: laumontite coating. Stage 2 mineralised veins occur in a NE trending dilational jog, dipping 45°NW in the southern part of the H zone and slightly flatter (to 20°NW) at the northern end of the H zone. Mineralised vein textures include colloform-crustiform bands, comb, breccia and vuggy textures. Gold occurs as electrum in the forms of ( 1) inclusions within pyrite, (2) disseminated and (3) in contact with pyrite, chalcopyrite, galena and sphalerite within finegrained quartz and calcite gangue. The deposit has a gold/silver ratio of approximately I :4 and gold fineness ranging from 544 to 690 with an average of 609. Gold-bearing veins are associated with four separate hydrothermal alteration zones. The mineralogy of these zones has been identified by PIMA, electron microprobe and petrography. The zones include: Zone I (Kaolinite - montmorillonite - illite - pyrite); Zone II (Quartz - carbonates - illite - chlorite - pyrite - sericite - adularia); Zone III (Illite - chlorite - adularia - quartz - carbonate - epidote - sericite - albite ); Zone IV (Epidote - chlorite - illite) and supergene enrichment. The alteration assemblages suggest that ore fluids were neutral to alkaline, reduced chloride waters with high concentrations of dissolved CO2 and temperatures ranging from 100 to 300°C. 834S values from pyrite grains within the mineralised veins range from -1. 7 to +5.2%o, suggesting that the sulphur has been derived from a predominantly magmatic source. 818C values from calcite in various paragenetic stages and altered limestone range from -0.8 to -7.3%0. Corresponding 8180 values range from +7.5 to + 14.9%0. The initial composition of fluids has 813C and 8180 values of -3%o and +2%o, respectively. The carbon and oxygen isotope data suggests that carbonate minerals precipitated from the mixture of two fluids (magmatic and meteoric waters), accompanied by cooling and fluid-wallrock interaction