贵州老万场金矿床红土化作用及对金赋存状态的制约

Laterization and its control on gold occurrence in Laowanchang gold deposit,Guizhou Province

红土的矿物成分、硅酸盐成分和化学蚀变指数(CIA)特征表明,贵州老万场红土化过程经历了粘土化、铝土矿化、铁化三种化学风化作用;地球化学上属早期红土化作用阶段。风化过程中K2O、Na2O、CaO相对于Al2O3的淋失量很高,显示较强的化学蚀变作用。红土剖面中成矿元素Au、As、Sb含量变化很大。Au通常为40～4551ng/g;Sb普遍小于0.1%;As通常为0.n%,最高1.8%。它们普遍高于大厂层硅质岩中相应成矿元素的含量。红土化过程中,Au、As的富集与Fe的富集作用有明显的关系,并与红土剖面中相对还原的部位有关,而Sb则在相对氧化的环境中聚集。红土剖面中稀土元素含量普遍较高,轻重稀土元素之比通常小于10,稀土元素分布模式非常相似,与大厂层稀土元素特征有明显差别。微量元素图解显示红土剖面中的大离子亲石元素、不相容元素与大厂层也有明显的不同。成矿元素间的差异性也很明显,表明大厂层岩石可能并不为老万场红土层的发育提供直接的物质来源,红土层的发育似乎与后期第四系的冲积物、洪积物有关。成矿物质也可能不与大厂层直接有关。带能谱的透射电镜(TEM-EDX)观察表明金的存在形式多样,虽然金主要以浑圆状的自然金颗粒产出,但也可以被针铁矿(含砷针铁矿)、石英硅质吸附,或被伊利石等吸附。显示红土化过程中铁还原作用?

Mineral compositions, chemical components and chemical indexes of a lteration of laterites indicate three chemical weathering processes including ar gillaceous, bauxitic and ferric process can be observed in Laowanchang laterizat ion; geochemically, the laterization can be attributed to early laterization sta ge. K2O, Na2O and CaO components highly leached during weathering showing strong chemical alteration. Contents of ore-forming elements of Au, As and Sb vary g reatly along the lateritic profiles. Au content is usually in the range of 40～ 4 551 ng/g, Sb content usually less than 0.1%, while As normally in the grade of point numeral percent with a highest value of 1.8%. Au, As and Sb contents in laterites are obviously higher than those in Dachanceng sillcalites. During l aterization, enrichment of Au and As was obviously related with the enrichment o f iron, and usually concentrated in the relatively reductive positions, while Sb in the relatively oxidized circumstances. REE contents are normally high in the lateritic profiles, and the LREE to HREE ratios are usually less than 10, and t heir REE distribution patterns of lateritic samples are very similar, but signif icantly different from those of Dachanceng samples. Trace element diagrams also reveal great differences for large ionic lithophile elements and imcompatible el ements between lateritic samples and Dachanceng samples. Difference for ore-fo rming elements in laterites and Dachanceng samples are also obvious. All these g ave supports that material sources for laterites would not directly be from Dach anceng rocks but mainly related with Quaternary sediments. Ore-forming substan ces for Laowanchang gold deposit also less directly connected with Dachanceng. T EM-EDX observations testify various occurrences of gold in laterites. Gold was observed existing mainly as granular natural gold, but it could be adsorbed by goethite, siliceous or quartz, also by illites and arsenic bearing goethite, whi ch showed the importance of reduction of iron during laterization.