贵州锦屏新元古界青白口系下江群稀土、微量元素分布特征——探讨金的来源问题

Geochemical Characteristics of REE and Trace Element:Implication for Gold Source in the Xiajiang Group of Neoproterozoic Qingbaikouan System,Jinping County,Guizhou Province,China

贵州东南地区的新元古界下江群产大量的石英脉型金矿,有关金的矿源一直是争议的问题。通过对下江群逐层采样,进行微量、稀土元素测试,结果表明金主要在变余砂岩中富集,特别是在变余砂岩透镜体或板岩封闭的变余砂岩顶部是含金石英脉形成的部位。Au与Mn、Sr、Cu等成正相关,特别是Au与Mn呈显著的正相关,可能是Mn胶体对金具有显著的吸附作用,Mn含量高的层位金含量高。而Au与Hg、Ba、W、As呈弱的负相关性,Ti、Co与Au呈负相关性,说明Au与陆源物质没有关系。下江群中稀土配分非常类似,表明地层被后期热液充分混合作用,金往往在稀土含量较低的层位富集,说明流体运移到砂岩内,而砂岩中的石英对稀土有稀释作用。富含金的pq-15样品具有与下江群完全不同的稀土配分,其表明金的成矿物质可能除来源于下江群外,还有深部流体参与成矿。

Numerous quartz vein-type gold deposits occur in the Neoproterozoic Xiajiang Group, Jinping county, southwestern Guizhou Province, China, but the source of gold is an endless debate. A great number of the samples were collected from the Jinping-Pingqiu section of Xiajiang Group for analysis of trace elements and rare earth elements by ICP-MS and X-ray spectrum in the Yichang Institute of Geology and Mineral. The results show that gold is mainly enriched in low metamorphic-sand rock, especially in lensoid low metamorphic-sand rock and the top of low metamorphic-sand rock overlaid by slate. Gold shows a positive relation especially with Mn, Sr and Cu, which is probably attributed to Mn＇s absorption to Au. Therefore, horizons with high Mn content are also enrichment in Au. Au show weak negative relation with Hg, W, Ba and As, and negative relation with Ti, Co and Au, indicating no relation between gold and terrestrial sediment. The distribution of rare earth element of 32 samples （except No. pq-15） sampled from the Xiajiang Group is very similar, showing that mixing of low-temperature fluid took place in the strata. Au is always enriched in the strata containing less amount of rare earth element, and this indicates that the fluid flows into sandstone because quartz in the sandstone dilutes the REEs. No. pq-15 sample with a high content of REE and Au is of completely different pattern of REEs from that in the Xiajiang Group, implying that gold not only originated from the Xiajiang Group but the deep fluid containing ore-forming materials.