THERMAL WATER SEDIMENTATION AND PB-ZN-BARITE DEPOSITS

The centeral Guangxi Province is located in a platform which was consolidated-in Caledonian orogeny. Stratiform barite deposit (SB) and vein Pb-Zn-barite deposit (VB) occur in a rift basin in Devonian system. The Devonian lithologic units are listed here: (1) red clastic rocks of littoral and facies, (2) lutite-carbonate of neritic facies, (3) chert layer and banded limestone. The SB occurs in the chert layer in Upper Devonian system and the VB occurs in the neritic strata in Middle and Lower Devonian system. In space the VB is located in both sides of the basin, but the SB occurs in the middle of the chert layer which is 150m in thickness in the western side of the basin, and ore layer is more stable in thickness and extends up to several km. The apicall, floor and band plates of the ore layer are the chert layers. Grey and grey-black SB with banded and striped structures is composed of fine, grey barite. The section of the barite layer shows symmetricl eye shape. Composition of the SB is very simple. The radiospherical barite is located in the outside of eye structure. Fluid inclusions are very rare and very tiny, whose homogenous temperature is about 140℃, and the enclosure system is SO4-Cl-Na-K. The δ₍₃₄₎S values of the barite is about +29‰. SB is associated with stratiform Mn, U, Mo mineralization. However, VB occurs in clusters containing basemetal veins (PV), basemetal-barite veins (VB) and single barite veins (MVB). The ore veins show zonation in horizontal and vertical directions. The horizontal zoning of the barite vein cluster is PV, VB, MVB outside around a deep fault controlling the rift basin, and the vertical zoning displays its feature in extended depth of the oreveins, mineralization temperature, mineral composition and country rock alteration etc. The δ_(34)S‰ values of VB are 19-30 (average 26.5), of MVB are about 23-26 (average 25.1). The enclosure system in MVB is SO₄-Cl-Xa-K. VB and SB are different produces of same mineralization, the envidences are: (1) VB and SB occur in the same rift basin, and are closely associated with each other, VB is located under the SB layer, (2) ore composition, structure, texture, feature and temperature of mineralizing solution, ore-bearing country rocks (from silication rock to chert layer) and country rock alterations are similar and display obvious and systematic evolution from VB to SB,(3) the δ₍₅₄₎S values of VB and SB are similar and approximated to the average value of sulfate in Later Devonian marine water, but are not similar to average value of sulfate in Early Devonian water, so that we propose a thermal water mineralization model. The study indicates the SB and country rock-chert layer are typically deposited by thermal water, so it is necessary to propose a stratiform-vein type deposit which is typical representation of thermal water mineralization.