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贵州水银洞金矿床地质特征及成因研究
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摘要
贵州水银洞金矿床是一个比较典型的大型卡林型隐伏金矿床。行政区划属于贵州省黔西南地区贞丰县小屯乡。大地构造位置处于扬子准地台西南缘与华南褶皱系两个Ⅰ级构造单元的交汇部位,属于华南褶皱系右江褶皱带。区域上地处北东向弥勒~师宗深断裂、北西向紫云~垭都深断裂和近东西向开远~平塘深断裂围成的“三角形”夹块,即滇黔桂“金三角”地带。构造受前期深大断裂的影响和制约,构造线主要呈东西向展布。不同期次、不同方向的构造形迹叠加复合,构造变形的组合和发育特点受岩相、岩性制约,在空间上有明显的差异。矿体产出主要受灰家堡背斜及附近断裂控制。矿区内岩浆岩不发育,为少量的偏碱性超基性岩和峨嵋山玄武岩。矿区内出露及钻探地层有:二叠系中统茅口组,二叠系上统龙潭组、长兴组、大隆组,三叠系下统夜郎组、永宁镇组第一段。
     水银洞金矿床的赋矿层位有长兴组、龙潭组和构造蚀变体(位于茅口组和龙潭组之间的不整合界面)。矿体以受岩性控制的层状矿体为主、断裂型矿体为辅,以及少量的似层状矿体。矿石的岩石类型有:生物灰岩、粉砂岩、断层角砾岩、泥岩等。矿石中矿石矿物有:黄铁矿、毒砂、赤铁矿、辉锑矿(偶见)、辰砂(偶见)、雄黄(偶见)。脉石矿物有:石英、白云石、方解石、水云母、绢云母、高岭石、萤石、海绿石、沸石、有机炭、变质沥青。黄铁矿是金的主要载体,含金黄铁矿粒度主要在0.0005mm~0.05mm即0.5μm~50μm之间,肉眼难以见及,显微镜下呈莓状、球状、皮壳状、环带状等。Au主要呈超显微状态存在。毒砂含量不高,不超过1%,但往往与高品位矿石关系密切。粘土矿物也有一定含金性。矿石中金的品位变化较大,为1.06~48.40×10-6,平均11.72×10-6,变化系数102.03%。Au与As、Sb的相关性较好。碳酸盐岩型矿石品位较高,平均值为21.82×10-6,含量也较稳定,变化系数为59.27%。泥岩型矿石品位较低,平均值为6.73×10-6,变化也较大,变化系数为113.28%。矿石的有用组分Ag、Sb、Cu、Zn、Pb等有益元素不具综合利用价值。有毒有害元素如Tl、Hg等含量很低,不会造成环境问题。As平均含量0.40%,需采用高温预氧化工艺选矿。常量元素分析,矿石中具有较高的硅、镁、三价铁、硫等元素含量。砂岩化学成分的构造背景分析表明,水银洞金矿床砂岩的构造环境可能为活动大陆边缘。矿石中具有较高的Ag、Mn、In等元素,而元素Li、Be、Sc、V、Cr、Co、Ni、Cu、Zn、Ga、Rb、Sr、Y、Cd、Cs、Ba则含量较低。Au与主要成矿元素Cu、Zn、As、Sb、Tl、Bi、Sn均呈负相关,线性相关度均不高,相对而言,与As、Sb、Tl关系较为密切。矿石中的稀土总量、轻稀土含量、重稀土含量和轻重稀土比值均较低,灰岩矿石比砂岩矿石低;Eu负异常较弱,Ce负异常较明显。根据矿床构造、围岩蚀变、矿石结构构造和矿物共生组合及其相互关系等,可将水银洞金矿成矿作用划分为两个成矿期:沉积成岩期和构造-热液期。构造热液期划分为三个阶段:石英-白云石-黄铁矿阶段、自然金-黄铁矿-毒砂-石英阶段、雄黄-辰砂-石英-方解石阶段。
     二叠纪中晚期的海相盆地边缘淡化区沉积,为水银洞金矿床准备了岩性条件,即容矿岩层(生物屑灰岩或粉砂岩)和有利的封闭岩层(粘土岩)。印支晚期~燕山早期的构造运动,伴随着深大断裂和岩浆活动,为深部成矿物质提供了来源和通道。古地温的高异常,为成矿热液提供了热力和热动力条件。矿石黄铁矿Pb同位素组成分析,成矿物质可能主要来源于造山带,以幔源为主,并有上地壳物质混入。成矿流体显示中低温热液特点;包裹体氢氧同位素显示,流体可能主要源于大气降水成因的地下热水。热液相对富集轻稀土元素,由于与深部作用和富含长石的岩区有关,而具有正Eu异常。成矿热液沿深大断裂向上运移,并向两侧有利成矿的岩层渗透,次级断裂和褶皱滑脱面,提供了导矿、容矿构造,让成矿物质得以达到有利的容矿场所富集成矿,形成了沿岩层或断裂或褶皱滑脱面产出的矿体。
The Shuiyindong gold deposit is a typical large Carlin-type gold deposits concealed.Administrative division belongs to Xiaotun town, Zhenfeng county, Guizhou Province.Tectonic position is in site of the intersection between the south-west margin of Yangtze platform and the two tectonic units of gradeⅠof the South China fold line, which belongs to Youjiang fold line of South China fold belt. It locates in a "triangle" block, that is, Dianqiangui "Golden Triangle" area, which are surrounded by the north-east Mile-Shizong deep fault, the north-west Ziyun-Yadu deep fault and the near east-west Kaiyuan-Pingtang deep fault. By the impact and constraints of deep fault, construction lines are mainly east-west distribution, and the principal stress is of the north-south level extrusion. Structural features of different periods and different directions compositely superimposed. The characteristics of tectonic deformation of development and combination, affected by lithofacies and lithology, are obviously different in space. The output of orebody is controlled by huijiabao anticline and the near fault. Magmatic rocks in the mining area is not developed, being a small amount of alkaline ultrabasic rocks and Emeishan basalt.The strata drilled and exposed in Mining area are as follows: in the Permian Maokou EC, the EC Permian Longtan Formation, Changxing Group and Dalong Group, Lower Triassic Yelang EC group, the first paragraph of Yongning Town Group.
     Ore-bearing strata of Shuiyindong gold deposit are of Changxing Group, Longtan Group and the constructure-altered body(located in the unconformity interface between Maokou and Lungtan Group).Ore bodis are mainly lamellar bodies controlled by lithology, fault-type ore bodies as a supplement, as well as a small amount of layered like ore bodies. Ore-bearing rocks are mainly biogenic limestone, siltstone, and fault breccia. Ore minerals are: pyrite, arsenopyrite, hematite, stibnite (occasionally), cinnabar (occasionally), realgar (occasionally).Gangue minerals are: quartz, dolomite, calcite, water, mica, sericite, kaolinite, fluorite, glauconite, zeolite, organic carbon, asphalt deterioration. Pyrite is the main vehicle for gold. Gold-bearing pyrite, mainly in the size of 0.0005mm-0.05mm (0.5μm-50μm), the naked eye difficult to see, under the microscope was strawberry-like, spherical, Shell-like,ring band and so on. Ultrastructure showed mainly a state of disseminated non-uniform distribution in the belt loop of the hydrothermal arsenic pyrite. Arsenopyrite content is not high, not more than 1%, but are often closely linked with the high-grade ore. Clay minerals are also of a certain gold-bearing.Gold grade in ore changes from 1.06×10-6 to 48.40×10-6. Au has better correlation with As, Sb. Carbonate-type high-grade ore, with a mean of 21.82×10-6, the content is also relatively stable, changes in coefficient of 59.27%. Mudstone-type low-grade ore, with a mean of 6.73×10-6, the larger changes, changes in coefficient of 113.28%.The useful components of ore, Ag, Sb, Cu, Zn, Pb,and other useful elements,are of the value of non-utilization. Toxic and harmful elements such as Tl, Hg content is very low and so will not cause environmental problems. The average level of As is 0.40 percent, ore must to be dressed by high-temperature pre-oxidation process. Analysis of constant element shows that ore is with high silicon, magnesium, ferric iron, sulfur. Chemical composition analysis about sandstone shows that the tectonic setting of sandstone of Shuiyindong gold deposits may be active continental margin. Ore with a higher Ag, Mn, In, and elements of Li, Be, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Cd, Cs, Ba content is over low. The main ore-forming elements, Au between Cu, Zn, As, Sb, Tl, Bi, Sn, show a negative correlation,and linear correlation is not high. Relatively speaking, Au has closer relationship with As, Sb, Tl. TheΣREE, LREE, HREE and LREE/HREE of ore are lower than that of rock, that of limestone ore lower than sandstone ore. Eu negative anomaly of ore is weak, and Ce negative anomaly of it more pronounced. Under the structure of deposit, wall-rock alteration, the fabric of ore, the symbiotic composition and relationship of minerals, and so on, mineralization can be divided into two periods: period of sediment diagenesis and construction-hydrothermal period. construction-hydrothermal period can be divided into three phases: calcite-pyrite stage, quartz-dolomite-pyrite stage and realgar-cinnabar-quartz-calcite stage.
     In the late Permian, marine sedimentary basin margin prepared the lithology conditions for the gold deposits, that are ore-hosting rock (limestone or biological siltstone) and the favorable closure of rock (clay rocks). The tectonic movement from late Indosinian to early Yanshanian, accompanied by a deep fault and magmatic activity, provided channels for the ore-forming minerals from the deep.High paleotemperature anomaly provided the heat and thermal power conditions for the ore-forming hydrothermal. Ore-forming materials mainly come from the deep , shallow source material mixed. Ore-forming fluids show the epithermal characteristics; inclusion hydrogen and oxygen isotopes show that fluid may mainly derive from underground hot water coming from precipitation.Hydrothermal has LREE enrichment, and has a positive Eu anomaly relating to the deep feldspar-rich rock. Hydrothermal migrates upward along the deep fault, and penetrate the both sides of rock benefiting ore-forming. Secondary faults and the detachment surface of folds provided the mine-leading and ore-hosting structures for ore-forming minerals to reach favorable sites, forming orebodies along rock, fracture or the detachment surface of folds.
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