Abstract

During the last years several small- to medium-sized Pb-Zn deposits have been explored in the western mountain chains of the Province of Kanchanaburi, Thailand. Sulfide mineralization is stratabound and closely related to the development of reeflike algal crinoidal buildups which are incorporated in a thick Ordovician limestone sequence. Fine-grained galena-sphalerite-pyrite ores prevail with accessory fahlore and various Sb-Ag sulfosalts in a gangue of barite, silica, and carbonate. Sulfides are usually intimately intergrown, grain boundaries of the impingement type. Sphalerite has a low Fe content whereas galena is notably argentiferous. Sulfide precipitation was mainly controlled by primary porosity and permeability of the host rocks; however, in some places cavern fillings and collapse breccias of a paleokarst are also mineralized. Relationships between carbonate cementation and ore structures indicate an Ordovician ore emplacement at shallow depth. Homogenization temperatures of fluid inclusions range from 107 degrees to 174 degrees C. The lack of major replacement structures and the fact that carbonate gangue and sulfides have often coprecipitated show that the ore-bearing solutions were almost neutral in pH. The freezing temperatures of fluid inclusions near 0 degrees C, the high contents of Ag, Sb, and Hg, and a zonal pattern of these elements are unusual for most carbonate-hosted lead-zinc deposits. The origin of the metal-bearing solutions is uncertain, but chemical characteristics and mineralized tuff breccias near the deposits possibly indicate a relationship to an igneous source.

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