川西甲基卡区域伟晶岩分异演化及锂成矿过程：来自白云母矿物学的约束

甲基卡位于松潘-甘孜造山带内, 为我国超大型伟晶岩型锂矿床之一, 具有较大的经济价值。甲基卡伟晶岩在空间上具有良好的分带, 以二云母花岗岩为中心, 向外依次为微斜长石伟晶岩带(Ⅰ带)→微斜长石-钠长石伟晶岩带(Ⅱ带)→钠长石伟晶岩带(Ⅲ带)→锂辉石伟晶岩带(Ⅳ带)→白云母伟晶岩带(Ⅴ带)。为了研究甲基卡区域伟晶岩脉空间演化和稀有金属富集规律, 本文对各分带伟晶岩的白云母进行了主量、微量元素研究。根据矿物内部结构和化学成分, 区域伟晶岩存在两阶段演化: 早阶段在Ⅰ带至Ⅳ带形成均一结构的原生白云母; 晚阶段Ⅴ带形成具有成分分带的白云母, 二者在成分上Li、Rb、Cs含量和K/Rb、K/Cs比值呈现明显差异, 表明演化程度明显加大, 流体组分比例升高, 表明体系由以熔体为主的阶段进入以熔流体为主相对不稳定的阶段。从Ⅰ带至Ⅳ带, 原生白云母的K/Rb、Kb/Cs比值降低有限, 微量元素Li、Rb、Cs、Ta含量总体略微升高, 表明甲基卡区域伟晶岩脉经历了中等程度的结晶分异演化。V带云母的主微量成分呈振荡变化, 该现象主要受熔体不混溶过程的控制。总体上, 原生白云母均有具有高Li、Cs、B含量的特征, 表明初始熔体极具成矿潜力。白云母中K/Rb比值小于等于20或Cs含量大于等于400×10^-6可以作为评价Li-Cs-Ta(LCT)伟晶岩发生锂辉石矿化的指标。

     

Neoproterozoic crustal growth: The solid Earth system during a critical episode of Earth history

The behavior of the solid Earth system is often overlooked when the causes of major Neoproteozoic (1000–542 Ma) climate and biosphere events are discussed although  20% of the present continental crust formed or was remobilized during this time. Processes responsible for forming and deforming the continental crust during Neoproterozoic time were similar to those of the modern Earth and took place mostly but not entirely at convergent margin settings. Crustal growth and reworking occurred within the context of a supercontinent cycle, from breakup of Rodinia beginning  830 Ma to formation of a new supercontinent Greater Gondwana or Pannotia,  600 Ma. Neoproterozoic crust formation and deformation was heterogeneous in space and time, and was concentrated in Africa, Eurasia, and South America during the last 300 million years of Neoproterozoic time. In contrast, the solid Earth system was relatively quiescent during the Tonian period (1000–850 Ma). The vigor of Cryogenian and Ediacaran tectonic and magmatic processes and the similar timing of these events and development of Neoproterozoic glaciations and metazoa suggest that climate change and perhaps increasing biological complexity was strongly affected by the solid Earth system.     

The physical and chemical evolution of low-salinity magmatic fluids at the porphyry to epithermal transition: a thermodynamic study

Fluid-phase relationships and thermodynamic reaction modelling based on published mineral solubility data are used to re-assess the Cu–Au-mineralising fluid processes related to calc-alkaline magmatism. Fluid inclusion microanalyses of porphyry ore samples have shown that vapour-like fluids of low to intermediate salinity and density (~2–10 wt% NaCl eq.; ~0.1–0.3 g cm^–3) can carry percentage-level concentrations of copper and several ppm gold at high temperature and pressure. In epithermal deposits, aqueous fluids of similar low to intermediate salinity but liquid-like density are ubiquitous and commonly show a magmatic isotope signature. This paper explores the physical evolution of low-salinity to medium-salinity magmatic fluids of variable density, en route from their magmatic source through the porphyry regime to the near-surface epithermal environment, and investigates the chemical conditions required for effective transport of gold and other components from the magmatic to the epithermal domain. Multicomponent reaction modelling guided by observations of alteration zonation and vein overprinting relationships predicts that epithermal gold deposits are formed most efficiently by a specific succession of processes during the evolution of a gradually cooling magmatic–hydrothermal system. (1) The low-salinity to medium-salinity fluid, after separating from the magma and possibly condensing out some hypersaline liquid in the high-temperature porphyry environment, must physically separate from the denser and more viscous liquid, and then cool within the single-phase fluid stability field. By cooling under adequate confining pressure, such a vapour will evolve above the critical curve and contract, without any heterogeneous phase change, to an aqueous liquid of the same salinity. (2) High concentrations of gold, transported as stable Au bisulphide complexes supporting >1 ppm Au even at 200°C, can be maintained throughout cooling, provided that the fluid initially carries an excess of H₂S over Cu+Fe on a molal scale. This condition is favoured by an initially high sulphide content in a particularly low-salinity magmatic fluid, or by preferential partitioning of sulphur into a low-salinity vapour and partial removal of Fe into a hypersaline liquid at high temperature. (3) Acid neutralisation further optimises gold transport by maximising the concentration of the HS^– ligand. This may occur by feldspar destructive alteration along pyrite±chalcopyrite±sulphate veins, in the transition zone between the porphyry and epithermal environments. An alternative acid/base control is the dissolution of calcite in sediments, which may enable long-distance gold transport to Carlin-type deposits, because of the positive feedback between acid neutralisation and permeability generation. The three physical and chemical transport requirements for high-grade epithermal gold mineralisation are suggested to be the common link of epithermal gold deposits to underlying magmatic–hydrothermal systems, including porphyry-Cu–Au deposits. Both mineralisation types are the result of gradual retraction of isotherms around cooling hydrous plutons in similar tectonic and hydrologic environments. As magmatic fluid is generated at increasing depths below the surface the importance of vapour contraction increases, leading to the typical overprinting of potassic, phyllic and advanced argillic alteration and their related ore styles.Editorial handling: B. Lehmann     

改造型盆地含油气系统分析 ——以雪峰山西侧盆山过渡带为例

南方是中国海相碳酸盐岩的主要分布地区之一,发育厚8000~10000m的震旦系—下三叠统海相沉积,形成了以震旦系、寒武系、奥陶系、志留系为主体的海相含油气系统下组合和以泥盆系、石炭系、二叠系、中下三叠统为主体的海相含油气系统上组合。上述含油气层系和油气组合在雪峰山西侧地区也有不同程度的发育,并具有复杂的圈闭条件、保存条件和成藏演化过程,蕴藏着丰富的油气资源。笔者着重讨论了改造型含油气盆地的研究思路、方法及研究区油气系统的地质作用过程,并在此基础上对4个重点区的含油气系统进行了分析,以期为深入开展雪峰山西侧地区油气地质调查和勘探目标区优选提供依据。