Copper-lead mineralization in the regionally-zoned Agnigundala Sulfide Belt

Copper mineralization at the Nallakonda deposit, copper-lead mineralization at the Dhukonda deposit and lead mineralization at the Bandalamottu deposit constitutes the Agnigundala Sulfide Belt of Paleoproterozoic Cuddapah basin. A sandstone-shale-carbonate association of shallow marine origin hosts the mineralization. Copper sulfide mineralizations in sandstones are stratabound and disseminated type and lead sulfide mineralizations occur as stratabound fracture filling veins and/or replacement veins within dolomite. Systematic mineralogical and sulfur, carbon and oxygen isotope studies of the three deposits indicate that the same ore-fluid that deposited copper at Nallakonda at relatively higher temperature, also deposited copper-lead at Dhukonda and lead at Bandalamottu under progressive cooling during migration through sediments. The ore-fluid was a low temperature (<200°C), oxidized fluid containing enough metal and sulfate to form sulfide mineralization in the Agnigundala Sulfide Belt. Further, the present study indicates thermochemical reduction of basinal water sulfate to account for the sulfur for sulfide ores. It is envisaged that basal red-bed and evaporite bearing rift-related continental to shallow marine sediments might have acted as the source for the metals in low temperature and oxidized ore-fluid. Rift related faults developed during sedimentation in the basin might have punctured the ore-fluid pool in the lower sedimentary succession and also acted as conduits for their upward migration. The ore-fluid, under progressive cooling, deposited the copper sulfide ores at Nallakonda, copper-lead sulfide ores at Dhukonda and lead sulfide ores at Bandalamottu. The ore bearing horizons have participated in deformations during basin inversion without any recognizable change in mineralization patterns.纳拉孔达（Nallakonda）矿床的铜矿化、杜孔达（Dhukonda）矿床的铜铅矿化以及班达拉莫图（Bandalamottu）矿床的铅矿化，共同构成了古元古代卡德帕（Cuddapah）盆地内的阿格尼贡达拉硫化物带（Agnigundala Sulfide Belt）。一套浅海成因的砂岩-页岩-碳酸盐岩组合赋存了该矿化体。砂岩中的硫化铜矿化属层控浸染型，而硫化铅矿化则以层控裂隙充填脉及/或白云岩内交代脉的形式产出。对这三个矿床开展的系统矿物学、硫、碳及氧同位素研究表明，在相对较高温度下于纳拉孔达矿床沉淀铜矿化的同一成矿流体，在运移穿过沉积物的过程中随温度逐步降低，同时在杜孔达矿床形成了铜铅矿化，在班达拉莫图矿床形成了铅矿化。该成矿流体为低温（<200℃）氧化型流体，富含足够的金属与硫酸盐，足以在阿格尼贡达拉硫化物带内形成硫化物矿化。此外，本研究表明，硫化物矿石所需的硫来自盆地水硫酸盐的热化学还原作用。研究推测，与裂谷作用相关的陆相至浅海相沉积序列底部的红色岩层及含蒸发岩沉积物，可能是该低温氧化型成矿流体中金属的来源。盆地沉积期形成的与裂谷作用相关的断层，可能刺穿了下部沉积序列中的成矿流体储集层，同时也作为流体向上运移的通道。成矿流体在逐步降温过程中，分别在纳拉孔达矿床沉淀硫化铜矿石、在杜孔达矿床沉淀铜铅硫化物矿石、在班达拉莫图矿床沉淀硫化铅矿石。含矿地层在盆地反转过程中发生了变形，但矿化类型未出现可识别的变化。