||Characteristics of epithermal Au and Porphyry Cu-Mo mineralizations of the Chatree deposit, Central Thailand
ラダー, タンワッタナヌクンLADDA, Tangwattananukul
The Chatree Au-Ag deposit in Petchabun Province, in the central Thailand is an epithermal vein-type deposit in Triassic age. The size and vertical extension of area of gold mineralization is almost 3 km long, 1.5 km wide and 200 m deep. The total production from 2001 to 2013 was 40 tons of gold and 180 tons of silver. Two mineralization styles that are epithermal Au-Ag and porphyry Cu-Mo mineralization are present in the Chatree mining area. Epithermal Au-Ag deposit, such as the Q prospect and A, C, H, D, Kw and Ke pits of the Chatree deposit, is distributed from north to central part. On the other hand, Cu-Mo mineralization is recognized in the N and V prospects in the southern part of the Chatree deposit.Geology around the Chatree epithermal gold deposit is composed of volcanic and sedimentary rocks ranging from Carboniferous through Permian to Triassic. The sedimentary strata of Carboniferous and Permian ages are thought to be present in deeper part of the Chatree deposit. The geology of the Chatree deposit mainly consists of andesite and rhyolite volcanics. The andesitic volcanic rocks are dominant at the lower part of volcanic succession, which includes monomictic andesitic breccia, polymictic andesitic breccia and volcanic sedimentary breccia in ascending order. The rhyolitic tuff unconformably overlies the andesitic volcanic succession. The Au-bearing quartz veins of the Chatree deposit are mainly hosted in polymictic andesitic breccia and volcanic sedimentary breccia.The Au-bearing quartz vein is characterized by veins, veinlets and network-type veins consisting of large amounts of quartz and calcite with small amounts of illite, adularia, chlorite and sulfide minerals. Based on crosscutting relationships of the Au-bearing quartz veins of the A pit and the Q and B prospects, the Au-bearing quartz veins are classified into five stages. Quartz veins of Stage 1 and network-type veins of Stage 4 at A pit and the Q and B prospects are characterized by intense gold mineralization. The quartz veins containing electrum are characterized by microcrystalline and flamboyant textures that suggest rapid crystal growth from supersaturated hydrothermal solution. Coexistence of laumontite and illite in the Au-bearing quartz vein of Stage Q4 at the Q prospect suggests that the Au-bearing quartz vein of Stage Q4 was formed about 200⁰C. Occurrence of boiling is suggested in the Au-bearing quartz veins of Stage Q4 based on the presence of flamboyant texture and higher oxygen isotopic ratio of quartz (15.0‰). The total pressure of formation of the Au-bearing quartz veins of Stage Q4 at the Q prospect is estimated to be approximately 20 bars. The Au-bearing quartz vein of the Q prospect was formed about 200 m below the paleosurface.Cu-Mo mineralization is present in granodiorite porphyry and andesitic volcanic rocks of the N and V prospects at the southern part of the Chatree deposit. The Cu-Mo mineralization took place about 5 Ma (244±1 Ma) later than epithermal gold deposit (250±1 Ma). The quartz veins of Cu-Mo mineralization consist of a large amount of quartz with small amounts of chlorite, pyrite, chalcopyrite, molybdenum and pyrrhotite. Fluid inclusions of the Cu-Mo mineralization can be divided into liquid-rich two phases fluid inclusion (1 wt% NaCl eq.) and multiphase solid-bearing fluid inclusions (30-44 wt% NaCl eq.). The assemblages of the fluid inclusions are consistent with the assemblage of vapor and fluid after phase separation of magmatic fluid. These conditions the halite-bearing fluid inclusion at the N prospect is thought to be 420 bars at the temperature 450⁰C. Cu-Mo-bearing quartz veins took place at 1 km below the paleosurface.The 87Sr/86Sr ratios of least altered andesite lava, strongly altered andesite lava and quartz veins were examined in the Au-mineralization. The strongly altered andesite lavas (0.707070-0.708765) near the Au-bearing quartz veins have higher initial 87Sr/86Sr ratios than least altered andesite lava (0.705118-0.705309). The initial 87Sr/86Sr ratios of the strongly altered andesite suggest that the hydrothermal solution forming the altered andesite lavas has higher 87Sr/86Sr ratios.The Au-bearing quartz veins and barren calcite veins have different initial 87Sr/86Sr ratios. The initial 87Sr/86Sr ratios of the Au-bearing quartz veins of Stage 1 (0.706949) and network-type veins of Stage 4 (0.708254-0.717607) are higher than those of calcite veins of Stage 5 (0.704843-0.705146). There is a possibility that the hydrothermal system forming the Au-bearing quartz veins is different from the hydrothermal system forming barren calcite vein in later stage in the Chatree deposit. The initial 87Sr/86Sr ratios of the Au-bearing network-type veins of Stage 4 and the strongly altered andesite lava at the A pit and Q prospect are similar to initial 87Sr/86Sr ratios sedimentary and metamorphic rocks. The fact suggests that the hydrothermal solution association with Au-mineralization reacted with sedimentary strata in deeper part in the Chatree area.The Au-bearing quartz veins and altered andesite volcanics formed immediately after formation of new fracture tend to have higher 87Sr/86Sr ratios. This tendency suggests that Au-bearing hydrothermal solution stored in deeper section of the Chatree deposit, under high pressure condition and also reacted with sedimentary strata of Carboniferous and/or Permian. Then the hydrothermal solution in the deeper section was intermittently transported to the shallow section as a result of development of fissures. Structural control such as the development of fissure from deeper to shallower sections is one of important factor for the formation of Au mineralization in the Chatree deposit in addition to presence of source of gold.