PRELIMINARY APPROACH TO THE CONTROL-ORE FACIES MODEL OF XICHENG LEAD-ZINC ORE-FIELD

Control-ore facies is the sedimentary facies of controlling ore deposit formation,The fades may be a single or a composition of facies. The control-ore facies in the Yicheng Lead-Zinc Ore-Field is a complicated composition,The Yicheng Lead-Zinc Ore-field lies in the southeast of Gansu Province, It is well known that the ore-field is a famou strata-bound lead-zinc deposit. There exists a close relationship between the deposit and sedimentary facies (especially carbonatite sedimentary facies) of the Mfiddle Lower Devonian.The relationship shows that the control-ore facies is characterized by a polyfacies in the Xicheng Lead-Zinc Ore-field.This paper preliminarily presents a synthetical model of control-ore facies for discussion and reference.On the basis of the facies analyses of carbonatite and their adjoining rocks related to the ore, the control-ore facies in the Xicheng Lead-Zinc Ore-field can be divided into two control-ore facies areas and again subdivided into siy control-ore.facies groups and some control-ore fades.1. The platform fades area lies in the east of the Xicheng Lead-Zinc Ore-field.It is made up of the widespread and very thick carbonatite of the Lower Devonian.It is subdivided into three sedim entary facies groups related to the ore.(1)The finger hill-beach facies group of platform edge is a“finger" of the carbonatite platform. There are two "fingers'(Changba and Bi jiashan)in the Xicheng Lead-Zinc Ore-field. The Changba finger hill-beach facies group is by far the most typical cxample. The facies group may be subdivided into four control-ore facies、biological hill(reef)facies(controlling ore-body No. I of Changba),back hill depression fades(controlling ore bodies No. I I and VIII of Changba), front hill depression facies(controlling.No. III of Lijiagou)and biological beach or sand facies(controlling No. I andII of Lijiagou).(2)The platform facies group is made up of tvco control-ore facies:intraplatform ridge facies (controlling Xiaowanli ore deposit).andplatform cvaporation facies(controlling Guanzigou ore doposit).(3)The intraplatform depression facies group consists of two control-ore facies:stagnant depression facies(controlling Heitangou copper deposit) and depre-ssion ridge fades (controlling Laodongshan manifestation etc.).2. The basin facies area is a widely distributed in the Xicheng Ore-field besides platform facics areas. It is:,set of basin facies sediments of shallow sea of the Rliddle Devonian, and can be subdivided into three sedimentary facies groups of control-ore.(1)The biological hill(reef)facies group is an isolated hill(reef)in the basin fades area, for example, Luoba ore deposit, It is made up of two control-ore facies:the biological hill(reef) facies and back hill fades in close connection with the ore.(2)The biostrome reef-beach facies is a thick carbonatite bed in argillaceous sediments of the basin facies area, hor example.Dengjiashan and Yeshuihe ore deposits occur in the thinning out part of the reef-beach facies or in the part near the contact zone with back beach facies.(3)The spot reef facies(group)is made up of some little coral reef bodies in the basin facies area, and these little reef bodies can be completely replaced by galenite and sphalerite,From what we have mentioned above, some regularities are suggested as follows.Both facies areas:the carbonatite platform facies of the Lower Devonian and the basin facies area of the Middle Devonian control all the Xicheng Lead-Zinc ore-feild.The facies group controls the ore zone. For eiample, the finger hill-beach facies group of the platform edge controls all the Changba ore-zone. However, the depression, the hill and the beach control individual ore deposits or ore-bodies.All the characteristics the synthetical model of of the control-ore facies are synthesized and simulated as control-ore facies as shown in Fig. 2.