SEDIMENTARY ENVIRONMENT OF PYRITE ORE DEPOSIT IN PUTOU CHENGBU COUNTY HUNAN PROVINCE

The pyrite ore deposit discovered in Hunan Province in recent years is a medium scale sedimentary deposit.Its ore-bearing strata are the laiziqiao Formations of the Middle Devonian Series.In this region, the Devonian system possesses two petrographical division and marl division，and this mine belongs to the latter.In this division, the Qiziqiao Formation can be divided into four lithological sections.The ore-bearing section is situated at the top(the fourth section)，and is composed of thin beded,dark grey- siliceous rock, siliceous banded limestone and carbonaceous and pelitic limestone，about 20-60m thick.The ore beds are stratiform,stratoid and Tense-like with a thickness of 0.4一31m and conformably deposited the strata in synchronous fold.It extends in NE direction, and the thickness of its northern part is greater than that of the southern one.The ore is composed of pyrite w ith framboids texture.lts struc-ture is massive, closely banded, and sparsely banded.A number of Tentaculite fossila still retained in the ore rock.The rate of Co/Ni in the single mineral of pyrite is 0.01.The composition of sulfur isotope is rich in S³², its variation range is from-17.4 to -28.0‰. and its average is -23.7‰. In the upper and lower ore beds the values of the δS³⁴ are stable.Along the strike of the ore body, the value of the δS³⁴ is also even, but from east to west it decreases from -20 to -27‰ in the dip.In this region, the Devonian system was the first sedimentary covering strata after the Caledonian Movement.In the early stage of Qiziqiao, the whole region was in the open shallow sea environment, and in the middle and late stages, the large rift emerging in NE direction in the Caledonian Movement began to revive.And then,the landform of the sedimentary region presented the relief in alternat-ing high and lov- parallel to the NE direction.The high division formed the half-li-mited and limited regions of carbonate platform，and the low division w-as the region of deep water basin.The rock type.chemical composition.teaiure and structure.organic assemblage and ecological characteristics of the sediments were distinctly different in various sedimentary regions.Particularlt，in the late stage of Qiziqiao.the basin was completely situated in the environment of deep water.low grade energy and lacking of oxygen. The characteristics of the sediments were very similar to those of the Craton basin carbonate zone described by J，L.Wilson(1975)，but these basins appeared within the platform and developed in a short time.Their forms mostly occurred in the narrow and linear structure which related to the specific teotonic setting in southern C hina.Therefore，they are specially called intraplatform linear basins.The palaeogeographical site of Putou pyrite ore deposit appeared at the eastern margin of the basin in the extension of NE direction.The sedimentary environment of Putou pyrite deposit was in the deep water basin lacking of om gen, and the supplies of organic m atter and ferriginous m ateri-als were comparatively- sufficient.T he relief difference caused the existence of bioherm was not compensated it that time. Therefore, the relative strong retarding environ went appeared in the west side of the reef body，which was more favoura-ble for the concentration of the ore forming material and the organic matter.The study of sulf ur isotope of the mine has proved that the sulphates in marine water were continuously replenished to the sedimentary strata in the open condition.In the dia-genesis stage, the ore beds of pyrite, with well一developed horizontal laminated structure of large thickness and rich in S³², were formed。