“煌斑岩"与金矿的实际观察与理论评述

LAMPROPHYRE AND GOLD MINERALIZATION AN ASSESSMENT OF OBSERVATIONS AND THEORIES

新的分析资料和研究表明,Rock等人提出的煌斑岩中温热液金矿成因模式在理论上和实际上都存在明显的问题。根据金和铂族元素、铜等元素的地球化学相关性,可以鉴别出绝大多数金异常高的煌斑岩类岩石受到次生蚀变和金富集作用。一些钾玄岩类岩石与中温热液金矿时空上的密切共生,是由于两者形成的构造环境相同。扩大“煌斑岩”这个术语的运用及其所包含的岩石类型,容易造成岩石学分类的混乱,并给岩石学和经济地质学研究带来困扰。因此,我们建议将其限制用作野外工作术语。

The proposal of Rock (1987, 1991) to broaden the term 'lamprophyre' from an essentially field-oriented usage to a term covering various rock types other than tholeiites has added confusion to petrogenesis and economic geology. This paper presents a review of various issues related to the alleged genetic connection between mesothermal gold mineralization and lamprophyres. This model is evaluated in terms of the gold contents of fresh and altered lamprophyres and gold mineralization resulting from hydrothermal alteration as well as some theoretical analyses. Altered 'lamprophyres', especially those proximal to gold mineralization affected by carbonation, have significantly higher Au contents. Secondary Au enrichment can be identified by a mantle-normalized Cu-PGE-Au diagram, on which Au shows a peak relative to Cu and neighbouring platinum group elements. Thus, those samples with anomalously high gold contents are most likely to be the product of hydrothermal alteration. S-undersaturation is a fundamental condition for concentration of Au, Pd and Pt as incompatible elements in magma during partial melting of the mantle.The relationship between 'lamprophyres' and mesothermal gold mineralization varies. In most cases, e. g. in the case of Archean 'lamprophyres' associated with gold deposits in the Superior Province, Canada, and Yilgarn region, Western Australia, 'lamprophyres' appear unlikely to have contributed significant Au or other components to the mineralization. In contrast, 'lamprophyric rocks' (shoshonitic suite) of the Lachlan Fold Belt in SE Australia have produced porphyry and skarn Cu-Au mineralization through magmatic differentiation and release of mag-matic fluids. They also acted as gold sources for later mineralization in shear zones and fault zones in company with deformation and metamorphism.