Bainiuchang Super-Large Silver-Polymetallic Ore Deposit Related To Granitic Magmatism In Mengzi, Yunnan, China

Bainiuchang silver-polymetallic deposit, one of the super-large oredeposits discovered in 1980s, lies in Mengzi County, Yunnan. It occurs inthe southeast Yunnan tin-polymetallic metallogenic belt. In this paper,choosing the Bainiuchang ore deposit as the study area, being guided byregional metallogeny and prospecting systematic engineering, selectingthe relationship of granite and ore-forming as the starting point, and themagmas-tectonic-fluid-ore forming as the main line, the author hasstudied on the ore-forming geological conditions, ore geologicalcharacteristics, formational mechanism and mineralization regularit, thenestablished a metallogenic model and prospecting model, and put fowardmetallogenic prognosis for the deposit at the last. The main Researchprogress, some new viewpoints and results are summarized as follows.(1) Summed up the characteristics of metallogenic series of thesoutheast Yunnan tin-polymetallic deposits. There are three Yanshanianperiod tin-bearinr granitic intrusions named Gejiu, Bozhushan andLaojunshan in SE-Yunnan. The granitic intrusions are all accompaniedsuper-large tin (silver) Polymetallic ore deposits which are respectivelyGejiu, bainiuchang and Dulong. It is suggested that the metallogenicseries (from granites to country rock) is tin (tungsten, beryllium)→copper→zinc→lead→silver→antimony.(2) The main strata in the Bainiuchang ore area are Tianpeng Group((?)₂t) and Longha Group ((?)₂l) of the middle Cambrian. The TianpengGroup belongs to shore-tidal flat sediment which is grey-oxford grey,thin-middle thick (silt) dolomite-(calc or carbonaceous)siltstone-(bioclastic or oolitic) limestone, and the Longha Group belongsto shoreline upper-tidal zone environment sediment which is thin-middlethick(silt-crystal or silt or bioclastic) dolomite. The tin, silver, lead andzinc are mainly enriched in Tianpeng Group and Longha Group and the strata below of them which are considered as parts of the source ofore-forming components. There are approximately 200 ore bodies. Mostof them are located in Tianpeng Group ((?)₂t), and a few in are locatedLongha Group ((?)₂l) of the middle Cambrian system.(3) The main structures are NW-NWW direction and disharmonicwith NE direction in the ore area, as result of the reasons granitic magmasintrude into strata along NW direction and produce a series of NWdirection structures which overprinted at NE direction structures ofIndo-China period.(4) It is verified by drilling projects that the large concealed granitlicrock body exists in the deep of the Bainiuchang ore area. The rock bodycontrolled by drilling is about 2100m long, 1000m wide. It intrude intothe Tianpeng Group of middle Cambrian system with NW-SE trend, andits top altitude is between 1220m and 1455m. The Rb-Sr isotopic modelage of the granitlic rock is 68.80±2.60Ma, so it is formed in lateYanshanian period. The Bainiuchang granite has the main characteristicsof the tin-bearing granites in the world. The type of the granitic rock isbiotite monzonite granite. Of the granitic rock, alkali, tin and otherore-forming elements is rich, K₂O＞Na₂O, A/NKC≥1, high REE andLREE, with obvious depleted Eu anomalies, (⁸⁷Sr/⁸⁶Sr)_i＞0.708. Itbelongs to S-type granite.(5) Bainiuchang granite rock being rich in tin, zinc and so onprovides the main ore-forming materials for the ore deposit. Thebainiuchang ore is very similar to the granite rock on the characteristicsof the trace element compositions and the REE distribution pattern. Thesulfur isotopic composition characteristics show its deep magma source.The granitic magma intrusion is the dynamics mechanism of theformation of the main contain ore body fault (F3) in Bainiuchang oredeposit. With the intruding of granitic magma, there occur a series ofextension fractures and interlayer fracture zones which may be the waysof the ore-forming fluid transport and the advantageous spaces for thesettlement of the mineral materials.The space distribution of the deposit is controlled by magmaticactivities. From the concealed granitic body to the country rock, there exists tin-copper-zinc-lead-silver-antimony mineralization belt. TinOre-bodies occur in the contact metamorphic belt above the the concealedgranitic body. Copper Ore-bodies usually occur in 50—300m,Independent Zinc Ore-bodies 200—300m, and the V₁ (Ag-Sn-Pb-Znore-body) more than 400m, far from the concealed granitic body. On thetop margin of the V₁ lies the independent silver or-bodies. Theindependent Sb ore-bodies lie in the exocontact of the the V₁. As far asthe V₁ is concerned, the value of w (Zn)/w (Zn+Pb) in the upper sectionis lower than 0.60 (ω(Zn)/ω(Zn+Pb)＜0.6), on the other hand, that ofω(Zn)/ω(Zn+Pb) in the lying section is higher than 0.60.(6)The genesis model of Bainiuchang ore-deposit is described asfollows: In late Yanshanian period, there happens intrusion of Graniticbecause of area spreading movement. Because the magma bearingore-forming fluids contacts with country rock, a series of contactmetasomatism happened. When the granitic magma contacts withcarbonatites, the country rock is of skarnization, meanwhile theskarn-type tin and copper ore-bodies are formed. Driven by thehigh-temperature granitic magma, the fluids bearing the ore-formingmaterials rise into the crannies and faults produced by granitic magmaintruding into country rock, and absorb silver, lead and so on elementsfrom country rock, and mix with atmospheric precipitation in clasticrock-carbonate rock stratum. With the distance increasing, temperatureand pressure become lower and lower, and other physicochemicalconditions change, so the mixed fluid deposit successively copper-(zinc)ore-bodis, zinc ore-bodies, zinc-lead-silver-(tin) ore bodies, silver orebodies and antimony ore bodies and so on in places such as advantageouslithology, interlayer fracture zone, fractured zone and fault crushed zones.(7) The zinc-lead-silver-(tin) ore-bodies proved previously are onlypart of Bainiuchang metallogenic series. Prospecting potential for tin,copper, zinc ore-bodies may be favorable in future. The deep ofChuanxindong—Yutang (â… ) and the deep of Awei ore-block and its southand north sides (â…¡) are circled as two metallogenic prediction areas inthis paper. Through drilling tests after the EH-4 survey and analyzingore-prospecting information are carried out in the two areas, great development for ore prospecting is made. A tin ore-body (22.85m thickgrading 1.331% Sn, accompanying with 0.15% WO₃) and two copperore-bodies(one 12m thick with 0.60% Cu, another 11m thick with 0.63%Cu) are found in the area (â… ). In the area (â…¡), a independent tinore-body (9.90m thick with 0.718% Sn) and a biggish zinc tin ore-body(11.4m thick with 6.23% Zn) are discovered.