SULPHUR ISOTOPIC RECORDS OF PRECAMBRIAN-ORDOVICIAN PB-ZN-AG DEPOSITS OF MYANMAR

This study examines the sulphur isotope geochemistry of total (28) sulphide and sulphate ores from Yadanatheingi, Bawdwin and Bawsaing Pb-Zn-Ag producing mines in Myanmar. The Yadanatheingi deposit formed as fracture, vug and pore space fillings, which are discordant to the structure of country rocks within tectonically and structurally disturbed Precambrian metasediments. The Bawdwin deposit, on the other hand, consists of massive lenses of course and disseminated ore body occurring within a volcanic complex of Cambrian age. This host rick made up of Bawdwin Tuffs and Agglomerates that are laterally interstratified with quartzites and siltstones. The major ore body is offset by the cross faults and formed the three divisions known as Shan, Chinaman and Meingtha Lodes. In the Bawsing mine, the ore body is interpreted to have formed by erratic strata bound syn-sedimentary deposition in an Ordovician limestone. d ³⁴S values of galena from Yadanatheingi mine vary from 0.2 to 8.6 while a pyrite mineral separate yielded a value of 11.6. These values and above mentioned geological features suggest that mineralization possibly occurred in fissure veins with some input from metasomatic processes during Precambrian time. In the Bawdwin Mine, d ³⁴S values of galena, sphalerite, chalcopyrite and rhyolite porphyry from Shan Lode are (-1.5) - (6.5), 3.7 and 0.3-2.2 and 0.6 respectively. In addition, galena, chalcopyrite and pyrite from Chinaman Lode in the same mine show d ³⁴S values of (-0.6),(0.69), 3.7 and 0.1 - 9.3 accordingly. Galena and malachite from Meingtha Lode, yielded d ³⁴S values of 0.8 and 0.3 while a barite sample indicates a value of 17.9. These d ³⁴S values indicate that hydrothermal process rather than seawater could have significantly affected formation of ore body at Bawdwin Mine during the Precambrian to Upper Cambrian period. d ³⁴S values from Bawsaing mine show 15.3 to 16.9 for galena, 0.6 for malachite and 30.0 -30.2 for barite. The galena was formed by high reduction of seawater sulfate with little or no fractionation. Those values of galena and barite suggest that the ore was contemporaneously deposition of limestone in a restricted and major evaporate basin input of seawater during the period of Middle to Upper Ordovician.