Geological Characteristics And Origin Discussion Of The Nanlao W-Cu Deposit In Laojunshan,SE Yunnan

The Laojunshan ore concentration area is located in the southwest margin of the South China plate,which is an important part of the tungsten-tin polymetallic metallogenic belt in SE Yunnan.In this paper,the newly discovered Nanlao tungsten-copper deposit in western Laojunshan ore concentration area was selected as the research subject.On the basis of detailed field investigation,the analysis of the metallogenic material source,diagenetic and metallogenic chronology,ore-controlling tectonic have been carried out.Combined with deposit geological characteristics and metallogenic geological background,the main ore-controlling factors,metallogenic epoch,and metallogenic model of the deposit were discussed.The main insights are as follows:1)Electron probe analysis shows that the main chemical components of garnet of the Nanlao tungsten-copper deposit are Si O₂(35.94%?38.93%),Al₂O₃(17%?19.98%),Ti O₂(0.06?0.27%),Fe O(5.22%?7.45%),Mn O(0.13%?0.85%),Mg O(0?1.47),Ca O(33.11%?35.01%),which belong to the andradite-grossular series;The main chemical components of epidote are Al₂O₃(27.61%?28.56%),Si O₂(38.03%?39.01%),Ca O(23.65?24.21%),Fe O(6.53%?7.87%),witch are characterized by low Fe O content and high Al₂O₃content.Both of them were formed in the stage of skarnization,reflecting the environment of relative reduction.Combined with field geological characteristics,it's proposed that they are the products of bedding metasomatism of the Laojunshan magmatic hydrothermal fluids during Yanshanian.2)Zircon U-Pb dating of the Nanlao gneiss yielded a concordant age of 405.4±8 Ma,belonging to Caledonian magmatic rocks.Re-Os isotopic dating of molybdenite in the ore-bearing quartz vein obtained two metallogenic periods of(437.1±6.2)?(428.1±9.9)Ma and(180.6±4.0)?(159.3±2.7)Ma,representing the Caledonian and early Yanshanian metallogenic events,respectively.3)Based on the study of geological characteristics of the deposit and the analysis of diagenetic and metallogenic chronology,it is considered that the Nanlao tungsten-copper deposit experienced three important mineralization periods:Caledonian period(440-400Ma),early Yanshanian period(180-160Ma)and late Yanshanian period,forming altered granite type W ore body,quartz vein type W-Cu ore body and skarn type W-Cu ore body respectively.4)The evidences such as ore-bearing space and structural deformation characteristics show that the F₀detachment fault is the most important ore-controlling structure in the deposit,which plays an important role in multi-stage metallogenic events.In Caledonian,the Nanlao gneiss intruded under the Xinzhai formation,and the schist of Xinzhai formation initially enriched minerals from the Nanlao gneiss.Late indosinian to early Yanshanian,the Nanwenhe metamorphic core complex evolution under the background of regional extension,the Nanlao gneiss and the schist of Xinzhai formation interface as the weak structure surface(magma emplacement contact interface/lithologic interface)developed the F₀detachment fault.The F₀fault further enrich minerals from upper and lower layers,forming a serious of bedding ore-bearing quartz veins.The Laojunshan magmatic activity in late Yanshanian brought a large amount of hydrothermal and metallogenic materials.On the one hand,the F₀detachment fault provided a channel for hydrothermal migration;on the other hand,the schist and gneiss in the upper and lower walls of the fault provided a good shielding effect for mineralization,leading to further mineralization in this area,and superimposed and reformed the early mineralization.