摘要
一六钨矿大地构造位置位于南岭成矿带中段南缘,粤北曲仁盆地西南缘,是粤北地区近年来重要的找矿勘查成果之一。矿床为典型的矽卡岩矿床,矿体赋存于上泥盆统帽子峰组矽卡岩以及NWW向钾长石-石英-白钨矿脉和云母石英脉中。通过野外观察和镜下研究,本文将成矿过程分为矽卡岩期(A)和热液期(B),矽卡岩期可以分为早期矽卡岩阶段(A1)、晚期矽卡岩阶段(A2)、钾长石英白钨矿阶段(A3),热液期可以分为云母石英脉阶段(B1)和石英碳酸盐阶段(B2)。矿区包含4种类型的包裹体:含子矿物三相包裹体(Ⅰ型)、气液两相水溶液包裹体(Ⅱ型)、CO_2水溶液三相包裹体(Ⅲ型)、纯CO_2包裹体(Ⅳ型),Ⅰ型包裹体仅见于A3阶段;Ⅱ型、Ⅲ型以及Ⅳ型包裹体在A3和B1阶段石英中均有发育,在A3和B1阶段白钨矿中还发育Ⅱ型包裹体。A3阶段Ⅰ型包裹体完全均一温度为162~381℃,盐度为30.1%~45.4%(wt%NaClequiv,下同省略),Ⅱ型包裹体完全均一温度为154~363℃,盐度为1.49%~11.0%,Ⅲ型包裹体完全均一温度为290~390℃,盐度为2.20%~6.88%;B1阶段Ⅱ型包裹体完全均一温度为152~381℃,盐度为1.65%~9.32%,Ⅲ型包裹体完全均一温度为281~378℃,盐度为2.00%~8.82%。激光拉曼探针分析表明,A3阶段和B1阶段流体中存在H_2O、CO_2、CH_4和少量CO_3~(2-),指示流体处于还原的环境。包裹体完全均一温度—盐度关系图表明,数据点主要集中于三个区域:a区对应早期出溶成因的高盐度流体,b区反映流体发生了不混溶作用,c区反映早期高盐度流体与低盐度地下水混合特征。各区包裹体代表了岩浆期后残余原始流体不同阶段的演化产物。通过Ⅰ型包裹体计算得出的成矿压力范围为86.0~415.8MPa,用Ⅱ、Ⅳ型包裹体对成矿压力进行校正得出,A3阶段成矿压力范围为86~115MPa,成矿温度为176~279℃;B1阶段成矿压力范围为55~93 MPa,成矿温度为160~228℃,估算成矿深度范围为3.62~4.26km。研究认为,流体在演化早期存在局部高压,流体不混溶作用要比外来流体混入更早发生,而流体混入促进了流体的不混溶作用。流体物理化学条件的改变、外来流体混入以及流体不混溶作用是引起钨矿沉淀的主要原因。
The Yiliu tungsten deposit,tectonically located in the southern margin of the central Nanling metallogenic belt and in the southwestern margin of the Quren basin in northern Guangdong,is one of the most important prospecting achievements of North Guangdong in recent years.It belongs to typical skarntype deposit,with ore bodies occurring in both skarn of the Upper Devonian Maozifeng Formation and NWW-trending K-feldspar-quartz-tungsten veins and mica-quartz veins.Based on field and microscope observations,this study divides the ore-forming process into skarn period(A)and hydrothermal period(B).The former is composed of early skarn stage(A1),late skarn stage(A2)and K-feldspar-quartztungsten stage(A3),whereas the latter is composed of mica-quartz vein stage(B1)and quartz-carbonate stage(B2).Four types of fluid inclusions have been identified:daughter mineral-bearing inclusions(typeⅠ),gas-aqueous two-phase inclusions(typeⅡ),three-phase CO_2-aqueous inclusions(type Ⅲ)and pure CO_2 inclusions(typeⅣ).Type I occurs only in A3 stage quartz.Type Ⅱ,type Ⅲ and type Ⅳ can be found in A3 and B1 stage quartz,whereas two-phase aqueous inclusions can be found in A3 and B1 stage tungsten.The homogenization temperatures of the type Ⅰ,type Ⅱ and type Ⅲ fluid inclusions in stage A3 vary from 162℃to 381℃,154℃ to 363℃ and 290℃ to 390℃,respectively.The salinities of type I inclusions range from 30.1% to 45.4%(wt%NaClequiv),while those of type Ⅱ and type Ⅲ range from1.49%to 11.0% and 2.20% to 6.88%.The homogenization temperatures of type Ⅱ and type Ⅲ fluid inclusions in B1 stage vary from 152℃ to 381℃ and 281℃ to 378℃,and the salinities of each type of inclusion ranges from 1.65% to 9.32% and 2.00% to 8.82%.Raman spectroscopy shows that the fluid compositions are mainly H2 O,CO_2,CH_4 with a little CO_3~(2-),indicating a reducing environment.Temperature-Salinity diagram of fluid inclusion data show three groupings.Zone "a"represents the originally high salinity exsolved fluid,zone "b"shows fluid immiscibility,and zone "c"shows the originally high salinity fluid mixed with lower salinity underground water.The fluid inclusions of each zone represent different evolutionary stages of primitive fluids which eventually formed the magmatic intrusion.The ore-forming pressures calculated using fluid inclusions of typeⅠ range from 86.0 MPa to415.8 MPa,and the pressures of A3 stage corrected using data from fluid inclusions of typeⅡand typeⅣrange from 86 MPa to 115 MPa,while those in B1 stage range from 55 MPa to 93 MPa.The ore-forming temperatures in A3 and B1 stages vary from 176℃ to 279℃ and 162℃ to 228℃,the ore-forming depth varies from 3.62 km to 4.26 km.This study shows that high pressure occurred locally at the early period of fluid evolution,and fluid immiscibility may occur earlier than fluid mixing and further promoted fluid immiscibility.Changes of the fluid's physical-chemical conditions,fluid immiscibility and fluid mixing may be the leading factors of tungsten precipitation.
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