Physical and Chemical Evolution of the Dabaoshan Porphyry Mo Deposit, South China: Insights from Fluid Inclusions, Cathodoluminescence, and Trace Elements in Quartz
Physical and Chemical Evolution of the Dabaoshan Porphyry Mo Deposit, South China: Insights from Fluid Inclusions, Cathodoluminescence, and Trace Elements in Quartz
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DOI：
10.2113/econgeo.112.4.889
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发表时间：
2017-06
期刊：
影响因子：
5.8
通讯作者：
Wei Mao;B. Rusk;Fuchun Yang;Mingji Zhang
中科院分区：
文献类型：
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作者：
Wei Mao;B. Rusk;Fuchun Yang;Mingji Zhang
南岭大宝山多金属矿床为斑岩型和夕卡岩型钼矿床，与侏罗系斑岩侵入体和邻近中泥盆统灰岩中的层控型铜铅锌矿化有关。斑岩钼矿化的特征是多代含石英脉的交错叠加，包括：贫石英脉（V1）、含钾长石蚀变晕的石英-辉钼矿脉（V2）、含白云母蚀变的石英-黄铁矿脉（V3）、含泥质蚀变的晚期贱金属矿化（V4），以及含灰岩的层控铜铅锌矿化（VS）。流体包裹体岩相学和显微测温结合阴极发光结构和石英中的微量元素揭示了形成矿床的热液系统的压力和温度变化。V1和V2脉体以低盐度（1 ~ 6 wt % NaCl当量）、含co2 （4 ~ 10 mol %）的两相包裹体为主，约35 vol %的气泡被困在流体溶质上方的单相场中。V1脉以CL-亮粒状石英嵌合体为主，CL强度高于其他脉型。该石英的Ti含量也高于其他脉状石英（24-89 ppm），而Al、Ge和Li含量与其他脉状石英重叠。含钼矿的V2脉也以CL-亮粒状石英嵌合体为主，但V2脉的CL强度略低，Ti浓度较低（10 ~ 65 ppm）。V3脉的空间分布比V1和V2脉更宽，从斑岩内部向邻近灰岩延伸。这些矿脉还以低盐度（2 - 6 wt % NaCl当量）、含co2 （4-7 mol %）的两相包裹体为主，这些包裹体在其溶剂上方含有约45 vol %的气泡。V3脉以CL-暗石英为主，呈自形生长带，CL强度振荡，Ti浓度低于前几代脉（1.5 ~ 12 ppm）。小的V4脉切断了以上所有的脉代，在V3脉中也出现了晚期充填。低矿化度（4-7 wt % NaCl当量）、含co2 （4-5 mol %）的两相包裹体以约20 vol %气泡为主。V4脉的石英CL强度在所有脉型中最低，存在振荡CL强度自形生长带，Ti浓度在所有脉型中最低（0.54 ~ 5.3 ppm）。流体包裹体等等值线与ti -in-石英等等值线的相交表明，热液系统从近岩浆压力和温度2.7±0.2 kbars和650°±40°C （V1脉）演化到1.9±0.2 kbars和530°±40°C （V2脉）演化到0.65±0.2 kbars和400°±40°C （V3脉）。当体系在0.40 ~ 0.65 kbar的最大压力下冷却至250°~ 300°C时，形成了缺乏金红石的V4脉。与几乎所有其他报道的斑岩型矿床不同，大宝山斑岩钼矿的含石英脉在任何脉型中都很少含有含卤石或蒸汽为主的流体包裹体。这类流体包裹体的缺乏，加上各脉型中含两相和三相低盐度流体包裹体的丰富，说明在h2o - nacl - co2体系中，V1 ~ V4脉的形成条件仍停留在V-L表面以上，因此在大宝山热液体系中很少发生流体解混。所有脉型中石英溶蚀结构的罕见证据进一步支持了这一观点，表明压力主要高于石英逆行溶解度带。流体包裹体、氯离子结构和石英微量元素数据表明，大宝山斑岩钼矿床是形成最深的斑岩型矿床之一，形成深度为地表以下6 ~ 7 km。极端的深度和流体解混的缺乏抑制了Cu在斑岩体系中的沉淀，反而使Cu留在溶液中，与周围泥盆系灰岩相互作用，与Pb和Zn沉淀。
The Dabaoshan polymetallic deposits in the Nanling Range, South China, consist of porphyry and skarn-type Mo mineralization genetically related to Jurassic porphyritic intrusions and adjacent strata-bound Cu-Pb-Zn mineralization hosted in mid-Devonian limestone. Porphyry Mo mineralization is characterized by the superposition of multiple generations of crosscutting quartz-bearing veins including: barren quartz veins (V1), quartz-molybdenite veins with K-feldspar alteration halos that host the bulk of the Mo mineralization (V2), quartz-pyrite veins with muscovite alteration (V3), and late base metal mineralization with argillic alteration (V4), as well as limestone-hosted strata-bound Cu-Pb-Zn mineralization (VS). Fluid inclusion petrography and microthermometry combined with cathodoluminescent textures and trace elements in quartz reveal changes in pressure and temperature of the hydrothermal system that formed the deposit. V1 and V2 veins are dominated by low-salinity (1–6 wt % NaCl equiv), CO 2 -bearing (4–10 mol %) two-phase inclusions with about 35 vol % bubble trapped in the one-phase field above the solvus of the fluid. V1 veins are dominated by CL-bright granular quartz mosaics with higher CL intensity than any other vein type. This quartz also contains more Ti than any other vein generation (24–89 ppm), while Al, Ge, and Li concentrations overlap with other vein generations. Molybdenum ore-hosting V2 veins are also dominated by CL-bright granular quartz mosaics, but V2 veins display slightly less CL intensity and correspondingly lower Ti concentrations (10–65 ppm). V3 veins have a broader spatial distribution than V1 and V2 veins, extending from inside the porphyries out into the adjacent limestone. These veins are also dominated by low-salinity (2–6 wt % NaCl equiv), CO 2 -bearing (4–7 mol %) two-phase inclusions, these containing about 45 vol % bubble trapped above their solvus. V3 veins are dominated by CL-dark quartz with euhedral growth zones of oscillating CL intensity and systematically lower Ti concentrations than previous vein generations (1.5–12 ppm). Minor V4 veins cut all the above vein generations and also occur as late infill in V3 veins. Low-salinity (4–7 wt % NaCl equiv), CO 2 -bearing (4–5 mol %) two-phase inclusions with about 20 vol % bubble prevail in V4 veins. V4 veins have the lowest CL-intensity quartz of all vein types, with euhedral growth zones of oscillating CL intensity and the lowest Ti concentration of all vein types (0.54–5.3 ppm). Intersections of fluid inclusion isochores with Ti-in-quartz isopleths indicate that the hydrothermal system evolved from near-magmatic pressures and temperatures of 2.7 ± 0.2 kbars and 650° ± 40°C for V1 veins to 1.9 ± 0.2 kbars and 530° ± 40°C for V2 veins to 0.65 ± 0.2 kbars and 400° ± 40°C for V3 veins. V4 veins, which lack rutile, formed as the system cooled to 250° to 300°C at maximum pressures of 0.40 to 0.65 kbar. Unlike nearly all other reported porphyry-type ore deposits, quartz-bearing veins from the Dabaoshan porphyry Mo deposit contain few halite-bearing or vapor-dominated fluid inclusions in any vein type. The dearth of such fluid inclusions, coupled with the abundance of two- and three-phase CO 2 -bearing low-salinity fluid inclusions in all vein types is evidence that the formation conditions of V1 to V4 veins remained above the V-L surface in the H 2 O-NaCl-CO 2 system, such that fluid unmixing rarely occurred in the Dabaoshan hydrothermal system. This is further supported by only rare evidence for quartz dissolution textures in all vein types, implying that pressures were dominantly higher than zone of retrograde quartz solubility. Taken together, the fluid inclusions, CL textures, and quartz trace element data indicate that the Dabaoshan porphyry Mo deposit is one of the deepest formed porphyry-type ore deposits, having formed at depths of 6 to 7 km below surface. The extreme depth and lack of fluid unmixing inhibited Cu precipitation in the porphyry system, and instead allowed Cu to remain in solution to precipitate with Pb and Zn upon interaction with the surrounding Devonian limestone.