THE FORMATION OF GIANT STRATIFORM SEDIMENT-HOSTED ZN-PB-AG DEPOSITS

Sedimentary basins that host large stratiform Zn-Pb-Ag deposits have the following characteristics: (1) cycles of rift-phase clastics and volcanics overlain by sag-phase shales and/or carbonates, (2) long-lived, steeply-dipping syn-sedimentary faults that penetrate basin fill, and in some cases are rooted in the basement, (3) evaporitic facies in both the rift and sag packages that supply salinity to the hydrothermal system, and (4) organic-rich shale and siltstone facies within the sag cycle that host the mineralisation. Geological and sulphide paragenetic studies suggest that stratiform sulphides are deposited either on the seafloor by hydrothermal exhalative processes, and/or below the basin floor by selective sediment replacement and pore space in-fill processes within the organic-rich shale facies. Numerical modelling of fluid flow and heat transport in sedimentary basins suggests that free convection, driven by density changes in sedimentary brines, may be the most important process in forming giant SEDEX deposits. In response to periods of active tectonism during the sag phase of sedimentation, extensional faults are reactivated such that they penetrate the shale and/or carbonate cap rocks to depths of several kilometres and access fluids contained in the rift phase clastic reservoir. The deep metalliferous brines ascend the faults and discharge on, or close to, the basin floor. Metal precipitation occurs in the anoxic, organic-rich sub-basins developing adjacent to the syn-sedimentary feeder faults. After the initial release of connate brines, usually over a period on the order of 30,000 years, marine waters recharge the basin via second- and third-order faults, travel through the clastic aquifer sequence below the sag package seal, and drive continued fluid convection and metal release from the rift package. This leads to an extended period of fluid discharge and ore deposition adjacent to the master discharge faults. Understanding the fault nature and history, and the aquifer controlled recharge-discharge patterns in sedimentary basins, is the key to predicting the location of giant stratiform Zn-Pb-Ag ore deposits.