Research For Ore-Forming Material Source Of Layer-Controled Lead-Zinc Deposits In Western Hubei

In recent years, great progress has been achieved in the exploration of layer-controlled lead-zinc deposits in western Hubei, however, the relational mineralization theory about these deposits is yet not completely clear.The western Hubei lies in the area involved in Qinling orogenic belt and Yangtze platform which are divided according to the Qingfeng-Xiangguang fault zone. The former lies in the south to this fault zone, however, the later in the north. Magmatite which are mostly intrusive rock distributes in Huangling faulted dome, Shennongjia faulted dome and part of Qinling orogenic belt. Stratum was exposed completely form Archaeozoic to Quaternary sediments. According to their rock composition, Doushantuo formation can be divided into four paragraphs and Dengying formation can be divided into three paragraphs both of which contains many black rock series.More than 150 lead-zinc deposits or ore points was found in western Hubei and they mainly lies in the south to Qingfeng fault zone, around Shennongjia faulted dome and in the north of Huangling faulted dome. Mostly, ore-bearing rocks of these deposits, which mainly contains brecciaous dolostones of Dengying formation in Yichang district and near the Qingfeng fault zone and brecciaous dolostones of Doushantuo formation in Shennongjia district, are stable and mineralization was controlled by layers obviously.Bingdongshan, Muyuhe, Aozigang and Tanbao lead-zinc deposits are four typical layer-controlled lead-zinc deposits and 30 rock geochemistry samples, 20 lead and sulfur isotopic samples and 3 fluid inclusions samples from these deposits were studied in this paper.Bingdongshan and Muyuhe deposits are located around the Shennongjia faulted dome. Ore bodies were born in the brecciaous dolostones, nipped between two black rock series, of the 4^th paragraph of Doushantuo formation. The sedimentary circumstance of ore-bearing rocks belonged to limited mesa and lagoon. Minerals of ore are mainly sphalerite, galenite, pyrite, dolomite, quartz and barite or so and their formation had several stages. Tantalums have high contents in wall rocks. Silicalite in wall rock was biogenesis sediment at continent margin but not submarine hydatogenic sediment. The facts that rare-earth elements of silicalite, carbonaceous shale and dolostone show obvious Ce negative anomaly and faint Eu positive or negative anomaly in different samples indicate that thermal water participated weakly in the process of sedimentationAozigang zinc deposit is located in the north-east of Huangling faulted dome, Ore bodies were born in the upper dolostones of Hamajing paragraph of Dengying formation. The sedimentary circumstance of ore-bearing rocks belonged to tidal flat of carbonatite mesa. Minerals of ore are mainly sphalerite, smithsonite, pyrite and minerals of gangue are dolomite, calcite and quartz. Tantalums have high contents in wall rocks. Rare-earth elements display obvious Ce negative anomaly and MREE enrichment. It indicates that many particle-reactive sensitive elements could be adsorbed greatly in seawater of that time and Pb and Zn could be enriched to a certain degree.Tanbao lead-zinc deposit is located in Xihaoping near to the south of Qingfeng fault zone. Ore bodies were born in dolostones of upper part of Dengying formation. The sedimentary circumstance of ore-bearing rocks belonged to wide mesa syetem. Minerals are mostly sphalerite, galenite, pyrite, dolomite, quartz and fluorite or so. Dolostones, the host rock near to ore body, have obvious negative Ce and positive Eu and show HREE depletion distribution pattern. Positive Eu also appears in farside wall rocks, such as packsand. These show that the mixing of seawater and hotwater took place obviously in the process of sedimentation.Lead isotope samples of all ores and most of host rocks fell in the supracrust field of the diagrams of plumbotectonics model. Only one sample of host rock from Tanbao fell in the orogenic belt field. These lead isotopic charaters display that lead element in host rocks and ores of Bingdongshan, Muyuhe, Aozigang Pb-Zn deposits, also in ores of Tanbao Pb-Zn deposit, were from supracrust, however, lead element in host rocks of Tanbao deposit had mixing sources of supracrust and orogenic belt.Ores of the former three deposits have higher positive value ofδ³⁴S from 17.57‰to 33.90‰, but ore of the last has lower slightly positive value ofδ³⁴S about 12.35‰. It indicates that sulfur mainly came from marine sulphate, in other words, from stratum, however, hydrotherm sulfur from orogenic belt was added into the wall rocks of Tanbao deposit.Gas phase component of fluid inclusion from Muyuhe deposit are mainly CH₄ and few CO₂ and fluid phase is basically water. In Aozigang deposit, ice-point temperature vary from -14.4℃to -5.1℃which corresponds to a salinity from 9.79% to 18.27% and homogeneous temperature vary from 100.3℃to 171.6℃and displays one peak value in the statistical column diagram which indicates that ore mineral was formed in one stage. In Muyuhe deposit, ice-point temperature vary from -24.1℃to -0.5℃which corresponds to a salinity from 0.87% to 25.42% and homogeneous temperature vary from 80.6℃to 167.3℃and displays three peak value in the statistical column diagram which indicates that ore mineral was formed in three stages. The ore-forming fluids of these two deposits were hypogeal hot brine with low temperature and low pressure.Study about metal contents in black rock series and in dolostones shows that metal elements from supracrust source, including Pb and Zn or so, could be enriched initially in sedimentary process of the both and formed ore-source layers or mineralized layers, even ore layers.In conclusion, we can suggest the following four viewpoints about the layer-controlled lead-zinc deposits in western Hubei.1.Ore-forming metals, including Pb and Zn or so, mainly came from supracrust and were greatly absorbed initially in sedimentary process of black rock series and carbonatites of Doushantuo and Dengying formations and were enriched sequentially in the lithification process to form ore-source layers or mineralized layers, even ore layers, moreover, these layers could be changed by hydrotherm because of orogenetic movement which mixed orogenetic belt lead into partial wall rocks.2.Sulfur mainly came from marine sulphate, in other words, from stratum, however, hydrotherm sulfur from orogenic belt was added into some wall rocks nearby the Qingfeng fault belt.3. Metallogenic fluid mainly was hypogeal hot brine or sedimentary constraction water. In the early sediment- lithification stage, the basinal hot brine could filter and extract metals and removed them into dolostone breccias to form ores. In the terminal orogenetic movement, the infiltrating-cycling hot brine could carry mineral into layer-inside structure belts or fault crannies to form ores and changed early ore layers in certain degree.4.The genesis of layer-controlled lead-zinc deposits in western Hubei vary slightly in different structure positions and ore-bearing layers. Three types of deposit genesis can be divided as sedimentary deposit, sedimentary-reconstructive deposit and hydrotherm-reforming deposit. Their typical deposits are respectively Aozigang zinc deposit, Bingdongshan lead-zinc deposit and Tanbao lead-zinc deposit.