Analysis of Microfacies,Microfacies Associations and Sedimentary Environments of Carbonate Rocks of the Middle Devonian in Xicheng Pb-Zn Orefield

The Xicheng Pb-Zn orefield locates in Gansu Province, is one of the most famous ore-fields in China. Its controlling horizon are Anjiacha and Xihanshui formations of the middle Devonian,and its ore-bearing rocks belong to marine clastic and carbonate rocks. Carbonate rocks characterized wide spreding, multi-rock types, rapid facies changment and good ore-bearing bedds. On the basis of textural origin classification and consulting the types of grains and sedimentary structures,the carbonate rocks can be divided into four types and twenty-six kinds of microfacies: include seven kinds of grainstone(MG₁-MG₇),six kinds of wackestone (GM₁-GM₆),nine kinds of mudstone(M₁ -M₉) and four kinds of hermatobiolith(R₁ -R₄). The microfacies represent respectively different sedimentary conditions and environments. Ten kinds of microfacies associations have been estabilished according to neighbouring and paragenetic relationship of micro-facies. Each association corresponds a designated sedimentary environment and its evolution: such as (Fig. 1) open platform, uppertidal zone-swamp in restricted platform, sag basin of semi-open platform, reef-fore-reef-reef-back, biogenic bank(or stratigraphic reef)-shallow basin behind bank-shelf and shelf point reef shelf. On the basis of microfacies associations controlling Pb-Zn ore,this paper shows that sag basin of semi-open platform and shollow basin behind the bank are a favourable environments to form Pb-Zn ore.