Super-enrichment mechanisms of precious metals by low-melting point copper-philic element (LMCE) melts

Abstract

低熔点亲铜元素（LMCE）As、Sb、Bi、Hg、Pb、Se、Te、Tl、Sn等，均具有亲铜性、低熔点、半金属的特性，在成矿过程中可以形成LMCE熔体，并对Au、Ag、PGE等贵金属的高效富集沉淀起到一种重要的桥梁作用。作者对前人研究资料与LMCE热力学相图进行了分析，并结合浅成低温热液型、造山型、卡林-类卡林型、碱性-偏碱性侵入岩型金矿床的研究成果，探讨了LMCE熔体形成、类型及其对Au、Ag、PGE等贵金属富集成矿的机理，并提出了LMCE熔体参与成矿的矿物组合与结构特征标志。LMCE熔体可以在岩浆过程、（岩浆）热液过程及变质过程中形成，是贵金属矿床重要的成矿机制之一。LMCE熔体中存在大量原子团簇，团簇间的聚集生长会使熔体难以达到相平衡，形成许多非平衡矿物组合，如包含LMCE的自然元素、金属互化物及含LMCE的多相矿物。Au在LMCE熔体中也可以团簇存在，金团簇聚集形成球状或片状，并形成巨富的金矿体。LMCE熔体形成的矿物常以浑圆状、近浑圆状、不规则状的单个或群体组合的乳滴、珠滴、气泡的微粒包体产在硫化物、硒化物、碲化物、氧化物和硅酸盐矿物内或沿矿物裂隙线形排列，这些LMCE微粒包体是熔体扰动导致熔-熔或熔-液间发生乳化所致，流体沸腾是引起熔体扰动的主要机制。LMCE熔体不能快速淬火结晶，通常在低温下缓慢冷却达到相平衡，形成复杂的矿物组合，该特点即使在微米到纳米级的矿物微粒中也显著存在。熔体-流体包裹体是LMCE熔体参与成矿作用最为直接的证据。固溶体分解结构、熔体退火结构、矿物-熔体二面角结构、溶解-再沉淀结构等也是LMCE熔体参与成矿的标志性结构。;The low-melting point chalcophile elements (LMCE), including As, Sb, Bi, Hg, Pb, Se, Te, Tl, Sn and so on, have characteristics of chalcophile behavior, low melting point and semi-metallic properties, which can form LMCE melt during mineralization process and play an important role for the efficient enrichment and precipitation of Au, Ag, PGE and other precious metals. In this paper, the previous research data and the LMCE thermodynamic phase diagrams were analyzed. Combining research results of epithermal, orogenic, Carlin to Carlin-like and alkaline to meta-alkaline intrusion-related gold deposits, the authors discussed the formation and type of LMCE melts and their mechanisms for the enrichment of Au, Ag, PGE and other precious metals, and summarized mineral compositions and characteristic textures of mineralization that benefited by LMCE melts. The LMCE melts can be formed during magmatic, (magmatic-) hydrothermal and metamorphic processes, and belong to one of the important metallogenic mechanisms for precious metal deposits. There are many ion clusters in the LMCE melts, and aggregation between the clusters precludes the melt to reach phase equilibrium, which results in many non-equilibrium mineral combinations, including the coexistence of native LMCE, intermetallic compounds and multiphase minerals containing LMCE. Gold could also exist as ion clusters in the LMCE melt that gather to form spherical or flakes native gold, and form the super-rich ore bodies. The minerals formed by LMCE melt often exist as single or group combinations of emulsion droplets, beads, and bubbles in round, nearly round, and irregular particle inclusions in sulfides, selenides, tellurides, oxides and silicates, or distribute along fractures of minerals. These LMCE micro-inclusions are derived from melt disturbance that result in melt-melt or melt-liquid emulsification. Fluid boiling should be the main mechanism that causes the melt disturbance since the LMCE melt cannot be quenched and crystallized quickly at this process. It is usually cooled slowly at low temperature to achieve phase equilibrium and form complex mineral compositions. This feature is significant even in micro-to nano-scale mineral particles. Melt-fluid inclusions are the most direct evidence for the involvement of LMCE melts during mineralization. Solid solution decomposition texture, melt annealing texture, mineral-melt dihedral texture, dissolution-reprecipitation texture are also characteristic textures of LMCE melts involved in mineralization.