An Approach to the Controls of strata,Sedimentary Facies and Location on the Beishan Silver,Zinc,Cadmium and pyrite Deposit in Guangxi

The Beishan deposit in Guangxi, a large-scale sulfide ore deposit in which silver, zinc, cadmium and pyrite occupy first place : lead comes second, is situated on the outer side of the southwestern margin at the southern end of the Jiangnan old land, i. e., at the northwestern end of the central Guangxi folded fault boundle. The Devonian and Carboniferous strata cropped out around the deposit, and the pre-Devonian strata and the magmatic rocks of various types on the old land to the north and east of it. The development of the study area involved both Pre-Caledonian geosynclinal and post-Caledonian paraplatformal stages. The geosynclinal history in the area ended as a result of Caledonian Movement during which the NNE-dominated and NWW-subordinated tectonic framework was formed. The combination of the ancient NNE-trending fault zones led to the formation of many finger uplifts on the margins of the old land Which stretch southwards into the marine basin and constituted a series of finger bays with rises alternating with depressions. During early and middle Early Devonian, the area was once a denudation area and then received the littoral elastic deposits due to transgression in the late stage, while from Middle to Late Devonian, although a larger-scale transgression resulted in the development of organic reef facies in some parts of the uplifted area, the area was still dominated by shallow-marine carbonate rocks. Since the areca was adjacent to the margins of the old land, many rivers on the old land poured into the marine basin and provided abundant mineral material for the area. In addition, the migration and accumulation of the mineral material gradually from the depression to the uplifted area contributed to the formation of the deposits in the area under the influence of folding, faulting, organic reefs, dolomitization and other geological processes. There are well-developed strata, from old to new, cropping out in the Guangxi area, among which the Devonian strata act as the most important ore-bearing horizons. For example, the Devonian Pb-Zn deposits occupy first place in the area both in the numbers of mineral occurrences and in the reserves. The controls of sedimentary facies and palaeogeographic environments on the ore deposits (orebodies) are included as follows. (1) The NNE-trending zonal distribution of the deposits (mineral occurrences) is controlled by the uplifted areas on the margins of the old land; (2)the distance of the deposits (mineral occurrences) from the margins of the old land vary with different rises and depressions; (3) the deposits (orebodies) are all confined to the transgressive cycles at some distance above unconformity interface; (4)the enrichment of the deposits (orebodies) is governed by lithologic associations, source beds, reservoirs and overlying strata, and (5)the emplacement of the deposits (orebodies)by the anticlines and the faults on the anticlines in the uplifted area at the Devonian sedimentary basement.