Fluid inclusions and H-O-S-Pb isotopic characteristics of the Jiagantan gold deposit in Gannan, West Qinling
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摘要:
加甘滩金矿床是西秦岭甘肃省甘南藏族自治州夏河-合作矿集区近年发现的资源量最大的金矿床。金矿体赋存在中上三叠统细碎屑岩中, 矿体的产出受NW向逆冲断裂及其次级构造控制, 呈雁行状、羽状分布, 矿体形态为板状、脉状、分叉状。矿床热液期主成矿阶段矿物组合为石英-黄铁矿-毒砂-辉锑矿-自然金。为进一步查明矿床成矿物质来源, 分析矿床成因, 本次在矿床地质调查的基础上, 开展了系统的流体包裹体、氢-氧-硫-铅稳定同位素测试分析。加甘滩金矿主成矿阶段石英包裹体类型为气液两相包裹体, 其中, 富液相两相包裹体最为常见。成矿流体均一温度均值为248.67℃, 盐度均值为3.78‰ NaCl, 具有中低温、低盐度的特征; 石英δ18OH2O值为10.42‰~13.82‰, δD值为-101.2‰~-93.2‰, 成矿流体组成较复杂, 可能既有岩浆水, 也有变质水的参与。主成矿阶段黄铁矿和毒砂δ34S值为-13.4‰~-7.5‰, 可能来源于岩浆作用或变质沉积地层; 铅同位素组成相似, 主要来源于造山带铅, 有部分上地壳铅和地幔铅加入。加甘滩金矿床成因类型为与岩浆有关的金矿床, 它的形成与印支期洋壳俯冲产生的深部岩浆作用密切相关。
Abstract:Jiagantan gold deposit is the largest gold deposit discovered in Xiahe-Hezuo area of West Qinling in recent years. The gold ore bodies occur in the fine clastic rocks of the Middle and Upper Triassic. The gold ore body occurs in the Middle and Upper Triassic fine clastic rocks. The occurrence of the ore bodies is strictly controlled by the NW thrust faults and its secondary structure, and they are distributed in echelon and pinnate forms, and the ore bodies are in vein, plate and bifurcated forms. Main mineral assemblages in the main ore-forming stage of the hydrothermal stage are quartz-pyrite -arsenite -stibnite -natural gold. In order to identify the source of ore forming materials and analyze the genesis of the deposit, a systematic fluid inclusion and H-O-S-Pb stable isotopic testing analysis was conducted based on the geological survey of the deposit. The quartz inclusion type in the main mineralization stage of the Jiagantan gold deposit is gas-liquid two-phase inclusion, and liquid-rich two-phase inclusions are the most common. The average temperature of ore-forming fluid is 248.67℃, and the average salinity is 3.78% NaCl, which is characterized by low temperature and low salinity. On the δD - δ18OH2O diagram, the samples are close to the primary water range of the primary magma and mantle. The δ34S value of sulfide (-13.4‰~-7.5‰) is characterized by obvious negative value and narrow range of variation. The Pb isotopic compositions of pyrite and arsenopyrite are similar. The lead is mainly from orogenic belt, and some of the lead from upper crust and mantle is added. The genetic type of the Jiagantan gold deposit is a wide-range low-temperature hydrothermal gold deposit far away from the ore-forming geological bodies. Its formation is closely related to the deep magmatism produced by the subduction of the oceanic crust in the Indosinian period.
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图 5 加甘滩金矿流体包裹体均一温度-盐度- 密度(Ahmad，1980)
Figure 5.
图 7 铅同位素构造模式图和构造环境判别图(Zartman et al., 1981；A、B、C、D为各区域中样品相对集中区)
Figure 7.
表 1 加甘滩金矿矿石中石英流体包裹体均一温度、冰点与盐度
Table 1. Homogenization temperature, freezing point and salinity of Jiagantan gold deposit
序号 矿物 包体类型 大小/ μm 气液比 均一温度/℃ 冰点温度/℃ 盐度/%NaCl 1 石英 V+L 6×6 20 329.3 -1.7 2.90 2 石英 V+L 9×6 35 342.5 -2.1 3.54 3 石英 V+L 5×4 10 262.7 -2.9 4.78 4 石英 V+L 8×3 25 361.5 -1 1.74 5 石英 V+L 8×2 12 279.8 -2.1 3.54 6 石英 V+L 6×3 20 340.9 -2.9 4.78 7 石英 V+L 8×5 8 214.7 -0.9 1.57 8 石英 V+L 6×4 28 355.2 -1.8 3.06 9 石英 V+L 8×4 8 224.9 -3.2 5.23 10 石英 V+L 9× 8 8 163.7 -1.3 2.24 11 石英 V+L 16×12 5 173.6 -2.4 4.01 12 石英 V+L 10×6 10 181.7 -2.8 4.63 13 石英 V+L 9×5 8 167.4 -0.3 0.53 14 石英 V+L 6×4 5 176.2 -1.4 2.40 15 石英 V+L 6×5 3 180.3 -2.8 4.63 16 石英 V+L 10×3 12 216.6 -3.5 5.66 17 石英 V+L 7×2 l5 326.3 -1.3 2.24 18 石英 V+L 8×4 10 349.6 -3.7 5.95 19 石英 V+L 5×4 15 278.2 -5.1 7.85 20 石英 V+L 7×2 22 292.3 -2.7 4.47 21 石英 V+L 6×2 28 264.7 -0.6 1.05 22 石英 V+L 8×4 12 266.4 -2.3 3.85 23 石英 V+L 8×6 25 328.3 -0.7 1.22 24 石英 V+L 6×2 32 365.9 -1.4 2.40 25 石英 V+L 9×2 l8 286.7 -0.9 1.57 26 石英 V+L 9×5 17 279.9 -2.6 4.32 27 石英 V+L 16×12 15 259.2 -2 3.38 28 石英 V+L 8×5 9 250.7 -2.9 4.78 29 石英 V+L 8×7 15 344.6 -1.3 2.24 30 石英 V+L 14×7 8 201.3 -2.4 4.01 31 石英 V+L 8×6 8 219.3 -3.6 5.81 32 石英 V+L 10×6 10 201.7 -2 3.38 33 石英 V+L 5×3 1 261.3 -1.8 3.06 34 石英 V+L 9×5 12 273.4 -1.7 2.90 35 石英 V+L 8×3 10 247.4 -2.8 4.63 36 石英 V+L 9×6 15 263.9 -4.1 6.52 37 石英 V+L 8×5 5 132.7 -2.1 3.54 38 石英 V+L 11×8 12 179.4 -2.9 4.78 39 石英 V+L 12×4 7 135.2 -1.9 3.22 40 石英 V+L 10×5 10 149.1 -3.2 5.23 41 石英 V+L 6×4 8 146.2 -3.3 5.37 42 石英 V+L 8×7 8 169.7 -3.7 5.95 注：V—气态；L—液态 
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表 2 加甘滩金矿氢-氧同位素分析结果表
Table 2. H-O isotopic analysis results of Jiagantan gold deposit
样品原号 岩石/ 矿物 δDV-SMOW/‰ δ18OV-SMOW/‰ δ18OH2O/‰ JZK1408-6 石英 -99.3 20.1 11.32 JZK1408-15 石英 -99.9 21.3 12.52 JZK809-5 石英 -98 19.2 10.42 JZK1805-3 石英 -97.5 20.4 11.62 JZK1609-6 石英 -94.1 21.6 12.82 JZK1609-7 石英 -93.7 21.2 12.42 JZK1609-11 石英 -94.5 21.3 12.52 JGT2-2 石英 -101.2 20.3 11.52 JGT2-3 石英 -93.2 22.6 13.82
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表 3 加甘滩金矿硫、铅同位素分析结果
Table 3. S and Pb isotopic analysis results of Jiagantan gold deposit
样品号 样品名称 S Pb δ34SV-CDT/‰ 208Pb/204Pb 207Pb/204Pb 206Pb/204Pb Δβ Δγ JZK1408-20 黄铁矿 -12.5 38.35 15.59 18.17 17.03 23.38 JZK809-7 黄铁矿 -7.5 38.65 15.68 18.55 22.77 31.44 JZK1609-11 毒砂 -13.4 38.50 15.59 18.41 17.16 27.41
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① 甘肃省地质矿产勘查开发局第三地质矿产勘查院. 甘肃省夏河县西科—塔哇一带1: 5万矿产远景调查成果报告[R]. 2022.
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