Evaporites, brines and base metals: low-temperature ore emplacement controlled by evaporite diagenesis

Salt beds and salt allochthons are transient features in most sedimentary basins, which through their dissolution can carry, focus and fix base metals. The mineralisation can be subsalt, intrasalt or suprasalt, and the salt body or its breccia can be bedded or halokinetic. In all these evaporite-associated low-temperature diagenetic ore deposits there are four common factors that can be used to recognise suitably prepared ground for mineralisation: (i) a dissolving evaporite bed acts either as a supplier of chloride-rich basinal brines capable of leaching metals, or as a supplier of sulfur and organics that can fix metals; (ii) where the dissolving bed is acting as a supplier of chloride-rich brines, there is a suitable nearby source of metals that can be leached by these basinal brines (redbeds, thick shares, volcaniclastics, basalts): (iii) there is a stable redox interface where these metalliferous chloride-rich waters mix with anoxic waters within a pore-fluid environment that is rich in organics and sulfate/sulfide/H2S; and (iv) there is a salt-induced focusing mechanism that allows for a stable, long-term maintenance of the redox front, e.g. the underbelly of the salt bed or allochthon (subsalt deposits), dissolution or halokinetically maintained fault activity in the overburden (suprasalt deposits), or a stratabound intrabed evaporite dissolution front (intrasalt deposits). The diagenetic evaporite-basemetal association includes world-class Cu deposits, such as the Kupferschiefer-style Lubin deposits of Poland and the large accumulations in the Dzhezkazgan region of Kazakhstan. The Lubin deposits are subsalt and occur where long-term dissolution of salt, in conjunction with upwelling metalliferous basin brines, created a stable redox front, now indicated by the facies of the Rote Faule. The Dzhezkazgan deposits las well as smaller scale Lisbon Valley style deposits) are suprasalt halokinetic features and formed where a dissolving halite-dominated salt dome maintained a structural focus to a regional redox interface. Halokinesis and dissolution of the salt bed also drove the subsalt circulation system whereby metalliferous saline brines convectively leached underlying sediments. In both scenarios, the resulting redox-precipitated sulfides are zoned and arranged in the order Cu, Pb, Zn as one moves away from the zone of salt-solution supplied brines. This redox zonation can be used as a regional pointer to both mineralisation and, more academically, to the position of a former salt bed. In the fault-fed suprasalt accumulations the feeder faults were typically created and maintained by the jiggling of brittle overburden blocks atop a moving and dissolving salt unit. A similar mechanism localises many of the caprock replacement haloes seen in the diapiric provinces of the Gulf of Mexico and Northern Africa. Evaporite-associated Pb-Zn deposits, like Cu deposits, are focused by brine flows associated with both bedded and halokinetic salt units or their residues. Stratabound deposits, such as Gays River and Cadjebut, have formed immediately adjacent to or within the bedded salt body, with the bedded sulfate acting as a sulfur source. In allochthon/diapir deposits the Pb-Zn mineralisation can occur both within a caprock or adjacent to the salt structure as replacements of peridiapiric organic-rich pyritic sediments. In the latter case the conditions of bottom anoxia that allowed the preservation of pyrite were created by the presence of brine springs and seeps fed from the dissolution of nearby salt sheets and diapirs. The deposits in the peridiapiric group tend to be widespread, but individual deposits tend to be relatively small and many are subeconomic. However, their occurrence indicates an active metal-cycling mechanism in the basin. Given the right association of salt allochthon, tectonics, source substrate and brine pending, the system can form much less common but world-class deposits where base-metal sulfides replaced pyritic laminites at burial depths ranging from centimetres to kilometres. This set of diagenetic brine-focusing mechanisms are active today beneath the floor of the Atlantis II Deep and are thought to have their ancient counterparts in some Proterozoic sedex deposits. The position of the allochthon, its lateral continuity, and the type of sediment it overlies controls the size of the accumulation and whether it is Cu or Pb-Zn dominated.