Meso- to epithermal W-bearing Sb vein-type deposits in calcareous rocks in western Thailand; with special reference to their metallogenetic position in SE Asia

Abstract

Calcareous sediments host Sb ore shoots in many Sb mining districts, e.g., in China, Malaysia, Spain, Austria, Italy, Germany, former Yugoslavia and Thailand. Vein-type Sb deposits hosted by or found adjacent to Late Paleozoic calcareous rocks are widespread in the Lampang-Phrae Province in northern Thailand and in the Surat Thani Province in Peninsular Thailand. The limestones are made up of different generations of calcite and dolomite with subordinate quartz, chalcedony, fluorite, pyrite, goethite, hematite and a sheet silicate assemblage dominated by kaolinite. The host carbonates were deposited in a shelf environment and underwent marine and freshwater phreatic diagenesis and dolomitization prior to emplacement of the vein-type Sb–W deposits, as well as intensive late diagenetic/hydrothermal alteration. Their oxygen isotopes indicate formation temperatures from 200 °C to below 45 °C. A complex pattern of lithogenesis suggests that mineralization occurred in the following stages: (1) Deposition and diagenesis of calcareous rocks, (2) dolomitization I, (3) mesothermal W mineralization with wolframite and scheelite, (4) epithermal Sb-(As) mineralization with arsenopyrite, stibnite, fluorite and marcasite, (5) epithermal dolomitization II, (6) hot spring (Sb) deposition with pyrite and Fe–W–Sb oxides, and (7) hypogene and supergene alteration with Fe–Pb–Sb oxyhydroxides. Tungsten mineralization is likely to have formed at temperatures around 300 °C. The mineralizing fluids changed from acidic/ reducing to alkaline/ oxidizing. Unlike high-sulfidation-type Sb mineralization in siliceous host rocks (e.g., dacites) the buffering effect of the carbonate minerals on the low Eh metal-bearing fluids prevented the formation of alunite group minerals in the Thai deposits. Iron was present in suitable amounts to form various Fe minerals but decalcification, common to Sb–Au-bearing SRHDG/Carlin-type (Sedimentary Rock-Hosted Disseminated Gold deposits) and skarn deposits elsewhere, does not occur in the Thai deposits. Gold is present in small amounts only. Ore minerals and carbonate minerals were both emplaced post-kinematically relative to tectonic deformation in this region. Acidic to intermediate igneous rocks changing in age from Triassic in northern Thailand through Cretaceous in Peninsular Thailand have created a geothermal regime which was favorable for the formation of these (meso)- to epithermal (W)-Sb vein-type deposits. Hot spring antimony deposition in the calcareous rocks and hypogene-supergene alteration during the Cenozoic are distinct from the granite-related Sb mineralization of Mesozoic age. From the NW (mainland SE Asia), towards the SE (western Pacific), Sb mineralizations tend to show decreasing age and increasing Au content. The Sb deposits formed on a rather thick lithospheric crust in a more stable terrane behind a zone of late Cenozoic arc-related Cu–Au porphyry and epithermal deposits. It is a mirror image of the metallogenetic setting in the Sb districts across the Pacific Ocean in the Central Andes, Bolivia.