Setting the stage for the genesis of the giant Bendigo ore system

The Ordovician sediments that host the giant Bendigo saddle reef gold deposits consist of a 3 km-thick sequence of turbiditic sandstones and interbedded siltstones and mudstones. Sedimentological studies suggest the succession formed within a major deep marine channel–levee complex similar to those described from contemporary continental margin to basin plain settings outboard of major river systems (e.g. the Amazon, Mississippi and Congo). Black shales, which are commonly the immediate host rocks to the epigenetic gold reefs, are interpreted to be over-bank deposits or abandoned channel fills, developed adjacent to active channels which were sandstone-dominated and had an incised axial thalweg marked by the coarsest-grained sediments present. Organic carbon content of the black shales at Bendigo varies from 0.2 to 2 wt%, compared with the grey shales, siltstones and sandstones, which vary from 0.05 to 0.2 wt%. Trace elements fall into two main groups: (a) elements that have a linear relationship with aluminium, and are controlled by the detrital clay content (Sn, Ba, Rb, Li, Cs, Mn, Cr and Tl); and (b) elements that show relationships with both aluminium and organic carbon (V, U, Ni, Zn, Cu, Bi, Pb, Se, Ag and Au) and are controlled by both the clay and organic matter content in the carbonaceous shales. The elements in the second group are enriched in the black shale facies. The background gold content of the black shales in the drill holes distal from mineralization averages 8.9 ppb, compared with the sandstones with 1.5 ppb. Most of the gold in the shales is present in diagenetic pyrite and marcasite, which laser ablation inductively coupled mass spectrometer (LA-ICPMS) analyses indicate varies from 5 to 3850 ppb and averages 370 ppb Au. The geochemical data suggest that this syngenetic gold was most likely sourced by erosion of the hinterland, and transported attached to detrital clay particles or as colloidal gold, by a high-volume feeder river system. High Rb/K ratios in the shales support a highly weathered source typical of a giant river system. By analogy with modern systems, following transport into deep marine channel–levee complexes via continental margin canyons, gold and other redox sensitive trace elements were ultimately trapped by reduction, adsorption and complexation with organic matter in the sub-oxic to anoxic over-bank deposits. Oxidation of much of the organic matter during diagenesis released the gold and certain trace elements (Ni, Co, Se, Ag, Cu, Bi, Pb), which became incorporated into diagenetic pyrite. Enrichment of gold in diagenetic pyrite of the black shale facies of the Ordovician turbidites at Bendigo was the first stage in a two-stage process that produced the world-class quartz–gold saddle reef deposits.赋存巨型本迪戈鞍状礁金矿床（Bendigo saddle reef gold deposits）的奥陶纪（Ordovician）沉积岩系，为一套厚度达3 km的浊积砂岩（turbiditic sandstones）与互层粉砂岩、泥岩序列。沉积学研究表明，该沉积序列形成于大型深海水道-堤岸复合体（deep marine channel–levee complex）中，类似现代大型河系外侧（如亚马孙河、密西西比河及刚果河）的大陆边缘至盆地平原环境产出的沉积体。通常作为后生金矿床直接赋矿围岩的黑色页岩，被解释为漫溢沉积（over-bank deposits）或废弃水道充填体，发育于以砂岩为主的活动水道附近；活动水道发育有以最粗粒沉积物为标识的下切轴向深槽（axial thalweg）。本迪戈地区黑色页岩的有机碳含量介于0.2 wt%至2 wt%之间，而灰色页岩、粉砂岩及砂岩的有机碳含量仅为0.05 wt%至0.2 wt%。微量元素可分为两大组：（a）与铝呈线性相关、受碎屑黏土含量控制的元素（Sn、Ba、Rb、Li、Cs、Mn、Cr及Tl）；（b）同时与铝及有机碳存在关联的元素（V、U、Ni、Zn、Cu、Bi、Pb、Se、Ag及Au），受碳质页岩（carbonaceous shales）中的黏土与有机质含量共同调控。第二组元素在黑色页岩相中发生富集。远离矿化区的钻孔中，黑色页岩的背景金含量平均为8.9 ppb，而砂岩的背景金含量仅为1.5 ppb。页岩中的大部分金赋存于成岩黄铁矿（diagenetic pyrite）与白铁矿（marcasite）中，激光剥蚀电感耦合等离子体质谱仪（laser ablation inductively coupled mass spectrometer, LA-ICPMS）分析显示，这些矿物中的金含量介于5 ppb至3850 ppb之间，平均含量为370 ppb Au。地球化学数据表明，这类同生金最可能源自内陆区的侵蚀作用，通过大型补给河系以附着于碎屑黏土颗粒或胶体金的形式被搬运。页岩中较高的Rb/K比值佐证了大型河系典型的高度风化物源区特征。类比现代沉积系统，金及其他氧化还原敏感微量元素经大陆边缘峡谷搬运至深海水道-堤岸复合体后，最终在次氧化至缺氧环境的漫溢沉积中，通过还原作用、吸附作用及与有机质的络合作用被捕获。成岩作用过程中大部分有机质的氧化释放了金及部分微量元素（Ni、Co、Se、Ag、Cu、Bi、Pb），并使其赋存于成岩黄铁矿中。本迪戈奥陶纪浊积岩黑色页岩相的成岩黄铁矿中金的富集，是形成世界级石英-金鞍状礁矿床的两阶段成矿过程的第一阶段。