Effects of Sedimentation and Diagenesis on Distribution of Gold of Banna Formation,Middle Triassic in NW Guangxi,Sw China

Banna Formation is the source bedds and host rocks of fine-impregnation gold ore deposits in Northwest Guangxi. The formation is turbidites with 150-5000m thick,and composed of sandstone,siltstone,mudstone and mirl. Through statistical analysis of gold concentration of 240 samples collected from 11 major geological sections and considering sedimentary and petrological characters of those samples,the authors report 7 factors which affect the concentration of gold of Banna Formation; 1:Gold concentration in geological sections is classified into original,barent and enriched type, which include 6 sub-types ;and regionally classified tnto original, barent and enriched area. Among them, part of gold in gold-enriched area is from Jiangnan Uplift and Ermei Basalt Belt. 2: There are little differences among mean concentration of sandstone, siltstone, mud-stone and mirl,but gold concentration in siltstone has comparetively higher variation coefficient,which indicats that gold in siltstone is mobile and easy migrating. 3:Gold concentration is low in the rocks having high percentage of quartz,but the law is only for those samples which have comparetively slight dizginesis. 4:In Bouma Sequence of turbidites,division C has higher variation coefficient of gold. In parts of geological profiles,gold concentration in fine-grain division is higher than that of underlying coarse-grain division. 5: Gold concentration is low in the rocks which had suffered strong chloritization and carbonatization in late diagenetic stage and subsequent stage. 6: Gold distribution in geological sections is classified into stable type and pulsatory type. In pulsatory type,concentration of gold is low in lower parts and higher in upper parts in each petrological member.