| 其他摘要 | 黔西南地区是中国最重要的金资源地之一，该区金矿主要赋存于右江裂谷盆地的沉积岩地层中。这些金矿床与美国内华达州卡林型金矿具有很多相似的特征，包括：金矿床在区域内成群成带分布，矿体均赋存于细碎屑岩-碳酸盐岩建造的沉积岩中，具有Au、As、Hg、Sb、Tl等元素组合特征，以及金主要以不可见金的形式赋存于含砷黄铁矿中。泥堡金矿是黔西南地区最大的金矿床之一，是一个受断裂和地层双重控制的特大型金矿。本次研究首次通过激光剥蚀等离子质谱（LA-ICPMS）和激光剥蚀多接收等离子质谱（LA-MC-ICPMS）等分析手段对泥堡金矿的矿物共生组合、矿物结构、黄铁矿和其它硫化物的原位微区微量元素、黄铁矿的原位微区硫同位素、石英包裹体的氢-氧同位素和方解石的碳-氧同位素进行详细研究，来解决泥堡金矿的矿物生成顺序、矿床形成环境、成矿物质来源和流体演化问题，进而为泥堡金矿的成矿模式提供依据。此外，本次研究还通过围岩和矿石的对比来探讨了金等成矿元素的沉淀机制问题。具体的研究结果和获得的主要认识如下： （1）本次研究根据矿物结构和微量元素组成确定了的四种黄铁矿类型和两种毒砂类型，分别为：草莓状黄铁矿（Py1），自形-半自形且包围草莓状Py1的更粗大黄铁矿（Py2），富硅质、碳质包裹体的“海绵状”黄铁矿（Py3），以及包围其它黄铁矿类型的狭窄边缘黄铁矿（Py4）；第一类毒砂Aspy1与Py1和Py2关系密切，第二类毒砂Aspy2和Py4密切相关。此外，我们将硫化物的生成顺序划分为两个主要阶段：沉积成岩阶段和热液阶段（早期、中期、晚期）。其中，沉积成岩阶段的硫化物包括Py1、Py2、闪锌矿和白铁矿，早期热液阶段的硫化物包括Py3和Aspy1，中期热液阶段的硫化物为Py4，晚期热液阶段的硫化物包括Aspy2、黄铜矿、黝砷铜矿、雄黄、雌黄和辉锑矿。 （2）四种黄铁矿类型的微量元素组成由于盆地系统的演化而有一些变化。但是总的来说，Py1和Py2富As（Py1中值8029ppm，Py2中值6976ppm）且含有一定含量的Au(Py1中值0.13ppm，Py2中值0.17ppm)，高于Py3的金含量（Py3中值0.08ppm）。Py4也富集与Au（中值31.1ppm）和As（中值28096ppm）等微量元素，包括Cu、Se、Ag、Sb、Hg、Tl、Pb、Bi等。通过Au和其它元素的二元图解分析，发现Py3和Py4的微量元素组成大多落在Py1和Py2的范围之内，而且Py3和Py4的Co/Ni比值也小于1，和Py1、Py2的一样，是在沉积黄铁矿的范围内。此外，Py3和Py4的Zn/Ni比值也是在沉积黄铁矿的范围内。这些说明Py3、Py4具有与Py1、Py2相同的微量元素来源，即都是沉积来源。结合Py1和Py2都富As且含有一定量的Au，且地层中富含有机质，所以，我们认为Py3和Py4中的Au、As和其它微量元素是来自于沉积盆地本身。 （3）泥堡金矿Py1和Py2的Co/Ni比值<1、Mo含量<50ppm说明了早期黄铁矿是在富有机质、缺氧的静海环境中形成的。当对泥堡金矿各种黄铁矿类型的Cu/Ni比值进行分析时，发现Py1的Cu/Ni比值（1.32）低于世界范围沉积黄铁矿（从太古代到现今）的Cu/Ni比值（0.01-2），而Py2的Cu/Ni比值（2.45）高于世界范围沉积黄铁矿的Cu/Ni比值，说明盆地中可能比较富集Cu，而且Cu在成岩阶段在黄铁矿中的富集程度比在沉积阶段的强烈。当对泥堡金矿各种黄铁矿类型的As/Ni比值进行分析时，发现Py1和Py2的As/Ni比值（Py1为35.46，Py2为29.53）都高于沉积黄铁矿的As/Ni比值（1~10），说明沉积盆地中非常富As，才能导致Py1和Py2中的As/Ni比值远高于沉积黄铁矿的As/Ni比值。 （4）从泥堡金矿的Aspy1、Py3、Py4、Aspy2、黄铜矿和黝砷铜矿的微量元素组成可以看出，泥堡金矿的热液流体富含微量元素，包括那些在卡林型金矿中通常出现的元素组合，如Au、As、Hg、Sb、Tl、Cu等。其中与Cu与Au呈强的正相关(R=0.87)，而其它元素与Au相关性不强。 （5）通过对泥堡金矿黄铁矿的硫同位素进行研究，发现硫同位素组成分布非常集中，在-2~-6‰之间。其中Py2硫同位素的平均值为-3.44‰，而Py4硫同位素的平均值为-3.42‰，非常接近。所以泥堡金矿的硫很可能是地层来源，支持了成矿物质来自于盆地本身的观点。石英流体包裹体的氢同位素组成变化范围在-75.4~-88.1‰之间，氧同位素组成为8.2~10.9‰，说明成矿流体有可能是大气降水与变质水的混合流体。方解石的碳、氧同位素研究说明了矿区的方解石脉主要是由该区灰岩发生溶解作用形成的。 （6）泥堡金矿围岩与矿石的对比分析结果表明，黄铁矿与石英呈负相关，与伊利石呈正相关；矿石发生了去碳酸盐化作用，矿石中的黄铁矿大多具有环带结构，部分围岩也发生了去碳酸盐化作用，而围岩中的黄铁矿一般不具有环带结构；Au明显加入到构造蚀变体的矿石中，而CaO、MgO、S、Ba、Be等从其围岩中带出；Au、Sc、As和Fe2O3明显加入到逆冲断层破碎的矿石中，SiO2、CaO、Sr、W、Be等则显示从其围岩中带出。综合分析认为，去碳酸盐化和硫化作用是泥堡金矿的主要成矿机制。在矿化前，去碳酸盐化作用为成矿提供了有利的环境；在成矿过程中，矿化通过硫化作用形成了黄铁矿的载金含砷边缘。 （7）不同黄铁矿类型的微量元素含量和其它们之间的关系，以及和世界范围内从太古代到现今沉积黄铁矿的对比说明了炭质沉积岩源区成矿模式在泥堡金矿是成立的。在这种模式下，金和砷及其它微量元素如Co、Ni、Cu、Se、Sb、Tl、Hg和Pb，在早期阶段就被带入右江盆地，并且在沉积成岩过程中部分的在Py1和Py2中预富集。到了主成矿热液阶段，盆地深处受到了构造变形和热液作用，金、砷等微量元素从Py1和Py2中释放出来，然后被热液流体运移到构造有利部位富集沉淀形成Py4，从而形成具有经济意义的金矿床。; The southwestern Guizhou Province, China, contains many sediment-hosted gold deposits, and is one of the most important sources of gold production in China. The gold deposits in the region mostly host in the stratas of the Youjiang Rift Basin. These gold doposits show many characteristics similar to the namesake deposits in Nevada, USA. The similarities include the tectonic setting, structural and stratigraphic control, hydrothermal alteration, mineral assemblage of the deposits and the occurrence of invisible gold ionically bound in arsenian pyrite. The Nibao deposit is one of the largest gold deposits in the region. The majority of the orebodies at the deposit are controlled by a reverse fault and an unconformity between the Middle Permian and Upper Permian stratigraphic units. In this study, for the first time, we took detailed study on the mineral assemblage, mineral structure, in situ trace element composition of pyrite and other sulfide, in situ sulfur isotopic composition of pyrite, hydrogen-oxygen isotopic composition of fluid inclusion of quartz, and carbon-oxygen isotopic composition of calcite by a series of analytical methods such as LA-ICPMS and LA-MC-ICPMS, to answer the questions of mineral paragenesis, the deposited environment conditions, the revolution of hydrothermal fluid, and the source of ore forming. The results of these studies will further provide evidence for the ore-forming model at Nibao. Furthermore, a comparison between wallrock and ore is used to explore the ore forming mechanism in this study. Our results and conclusions are as follows. (1) Four pyrite types and two arsenopyrite types were identified, based on texture and trace element content: framboidal pyrite (Py1), euhedral to subhedral coarser pyrite overgrowing framboidal pyrite (Py2), ‘spongy’ pyrite containing abundant inclusions of silicate and carbonate minerals (Py3), and narrow pyrite rims overgrowing the sponge-textured Py3 (Py4). Aspy1 commonly relates to Py1 and Py2. Aspy2 usually relates to Py4. We further established the mineral paragenesis. The sulfide minerals were divided into two main stages: the sedimentation/diagenetic stage and the hydrothermal stage. According to the trace element geochemistry, the hydrothermal stage were further divided into the early hydrothermal, the middle hydrothermal and the late hydrothermal stage. Sulfides formed during the sedimentation/diagenetic stage include Py1, Py2, sphalerite and marcasite. Py3 and Aspy1 are considered as sulfides formed during the early hydrothermal stage. Py4 is the only sulfide formed during the middle hydrothermal stage. Sulfides formed during the late hydrothermal stage include Aspy2, chalcopyrite, tennantite, realgar, orpiment, and stibnite. (2) The concentration of trace elements in four pyrite types varied due to the changing physicochemical conditions as the basin system evolved. In particular, Py1 and Py2 are rich in As (median 8,029 ppm for Py1 and 6,976 ppm for Py2) and contain relatively higher Au contents (median 0.13 ppm for Py1 and 0.17 ppm for Py2) than that of Py3 (median 0.08ppm for Py3). Py4 contains most diverse trace element suite of all pyrite types, including Cu, Se, Ag, Sb, Hg, Tl, Pb, and Bi along with As (median 28,096ppm) and Au (median 31.1ppm). The bivariate plots of Au versus other trace elements show that the trace element compositions of Py3 and Py4 are predominantly within the range of Py1 and Py2. The Co/Ni ratios of Py3 and Py4 are lower than 1, similar to that of Py1 and Py2, and within the range of worldwide sedimentary pyrite (ranging from the Archean to present). Moreover, the Zn/Ni ratios of Py3 and Py4 are within the range of worldwide sedimentary pyrite, too. All these suggest that the trace elements in Py3 and Py4 have the same source with that of Py1 and Py2. (3) Both the Co/Ni<1 and Mo content<50ppm in Py1 and Py2 indicate these early-stage pyrites were deposited in an organic matter-rich and anoxic to euxinic environment. When examined the Cu/Ni ratios of all pyrite types in Nibao deposit, Py1 show Cu/Ni ratio (1.32) lower than that of worldwide sedimentary pyrite (0.01~2) while Py2 show Cu/Ni ratio (2.45) higher than that of worldwide sedimentary pyrite, suggesting that the sedimentary basin is rich in Cu and Cu is more likely to concentrate during diagenetic stage. When examined the As/Ni ratios of all pyrite types in Nibao deposit, both Py1 and Py2 show As/Ni ratios (35.46 for Py1, 29.53 for Py2) higher than that of worldwide sedimentary pyrite (1~10), suggesting the sedimentary basin is very rich in As. Only the sedimentary contains abundant As can caused the As/Ni ratios of Py1 and Py2 much higher than that of worldwide sedimentary pyrite. (4) The trace element compositions of Aspy1, Py3, Py4, Aspy2, chalcopyrite and tennantite suggest the hydrothermal fluid is apparently rich in trace elements, including elements commonly found in pyrite from Carlin-type deposits in Nevada, USA (e.g., Au, As, Sb, Hg, Tl). Among these trace elements, Au is most correlated with Cu (R=0.87), and shows no apparent correlation with other trace element. (5) The results of sulfur isotopic composition of pyrite show that the sulfur distribution is very narrow, ranging from -2~-6‰. The average sulfur isotopic composition of Py2 and Py4 is -3.44‰ and -3.42‰, respectively. It is very close to each other. We concluded that it is more likely that the sulfur is sourced from the basin, which supports the view that the sedimentary basin is the source for the deposit. The hydrogen-oxygen isotopic compositions of quartz fluid inclusions show that the hydrogen isotopic composition is ranging from -75.4~88.1‰, while the oxygen isotopic composition is ranging from 8.2~10.9‰, suggesting that the ore-forming fluids are more likely to be the mixture of meteoric fluids and metamorphic fluids. The results of carbon-oxygen isotopic compositions of calcite indicate that the dissolution of the limestone causes the forming of calcite vein. (6) The results of comparative study of the wallrock and ore from the Nibao deposit indicate that pyrite shows a negative correlation with quartz and a positive correlation with illite. Ore samples have undergone decarbonation, and pyrite in the ore samples generally shows core-rim texture. A part of wallrock samples also have undergone decarbonation, but pyrite in the wallrock samples does not show core-rim texture. Au was added to the ore, while CaO, MgO, S, Ba and Be were removed from the wallrock in the uncomformity. In the reverse fault, Au, Sc, As and Fe2O3 were added to the ore while SiO2, CaO, Sr, W and Be show the characteristics of being removed from the wallrock. The integrated results suggest that decarbonation and sulfidation are the main ore forming mechanisms at the Nibao gold deposit. Before mineralization, decarbonation provides favor environment for ore forming. During the ore forming process, minerlization through sulfidation to form gold bearing arsenian rims on pyrite. (7) The trace element contents of different pyrite types and their relationships in this study, and a comparison with worldwide sedimentary pyrite ranging from the Archean to the present suggest the carbonaceous sedimentary source-rock model of Large et al. (2011) is valid for the Nibao deposit. In this scenario, Au and As would have been sourced from the broader Youjiang basin and locally pre-concentrated in Py1 and Py2, along with a suite of other trace elements, including Co, Ni, Cu, Se, Sb, Tl, Hg, and Pb. During the main ore-forming hydrothermal event, Au and As would have been remobilized from Py1 and Py2 by hydrothermal fluids in the deep basin, and then transported by these fluids to favourable trap sites where they were precipitated in Py3 and Py4. |
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