Microfacies analysis of some ordovician carbonate sediments with associated Lead-Zinc ores at Song Toh Mine, Amphoe Thong Pha Phum, Changwat Kanchanaburi

Abstract

The Song Toh Lead-zine Mine area is located between KhwaeYai and KhwaeNoi rivers approximately 160 kilometres in the northwestern part of Kanchanaburi. The study area is rectangular of 1.1x2.5 square kilometres. The central part of the area, where the lead-zinc ores are exploited, is elongated in the valley with north-south trend. The average ground surface elevation is between 610 to 670 metres above the mean sea level. The present study aims at defining the microfacies of both physical and geochemical aspects of the Ordovician carbonate sediments in the area. Additional attempt will be focusing upon the associated lead-zinc ores and the possible relationships between the carbonate-host rock and the mineralization. Approximately, 450 rock and ore samples have been collected from three measured sections of about 850 metres thick. In addition, about 90 samples have been collected from the surface exposures covering the area of approximately 2 square kilometres. Detailed laboratory studies include petrography, X-ray diffractometry, geochemistry, and ore-microscopy have been undertaken. The microfacies profiles of the Ordovician carbonate sediments can be categorized into two aspects, namely, physical and geochemical. The physical parameters are allochems, orthochems, carbonate minerals (calcite and dolomite), quartz, acid insoluble residue, ores and gangue minerals. For geochemical aspect, the parameters concerned are Ca, Mg, Fe, Ba, Mh, Sr, Pb, Zn, Cd, Ag, Cu, Ag, Cu, Ni, As, and Hg. The carbonate sequences are consisting of dismicrite, fossiliferous micrite, biomicrite, biosparite, cosparite, colitic pelsparite, and pelmicrite which have been partially and/or wholly dolamitized. Upon comparison of the microfacies in the study area with carbonate depositional facies models, it is concluded that the Ordovician carbonate sediments of Song Toh Mine area were deposited in the intertidal-subtidal zone of carbonate shelf environment. It is also noted that the Ordovician carbonate sediments in this area show strongly diagenetic changes and have been structurally deformed by folding, faulting, and fracturing. The sulphide ores associated with the Ordovician carbonate sediments are mainly galena, sphalerite whereas, pyrite, barite; quartz, dolomite, calcite, and clay are gangue minerals. Besides, secondary ares of cerussite, smithsonite, hemimorphite and hydrozincite are also present. The are bodies are oriented approximately in the north-south trend with moderate dipping to the east parallel to the regional attitude of the Ordovician carbonate-host rock. Despite the fact that ares of the Song Toh deposit have been post-depositional deformed and mobilized, some microscopic characteristics of primary or early-formed are textures can still be observed, i.e., framboidal pyrite, and primitive or early-formed sphalerite. With respect to the genetic model of the lead-zinc mineralization of the Song Toh Mine area, many lines of evidence indicate that it is of sedimentary origin. The ore-bearing fluid is believed to be either squeezed out of the underlying Cambro-Ordovician fine-grained clastic rocks or the metal-rich brine which might be derived from exhalative processes to be precipitated penecontemporaneously with the Ordovician carbonate-sediments under favourable conditions. Lastly, subsequent tectonic deformation have further complicated the mineralized zone.