Evaporites & lower temperature ore deposits

There is an association of evaporite settings with the larger of the known MVT, Sedex, Stratiform Sedimentary Copper and some IOCG associations (A; see Warren 2016, Chapter 15 and 16 for detailed case histories and models).

 

The role of evaporites in focusing metalliferous ore accumulations is three-fold:

1) In solution (halite-dominant precursor) they can act as chloride-rich metal carriers and,

2) Locally, as beds or masses (especially of CaSO4), their dissolution products, especially if trapped, can supply sulphur (mostly as bacteriogenic or thermogenic H2S)

3) Dissolving evaporites set up chemical interfaces that act as foci in the setup of brine mixing conditions suitable for precipitation of metal sulphides or native elements.

 

Hence, most evaporite-associated ore systems tend to be epigenetic, rather than syngenetic. Subsurface salt beds and masses are merely the solid part of a large ionic recycling system; dissolved metals are another part, and zones of mixing between the two are typically sites where ores tend to accumulate. Halokinesis steadies the position of a redox interface, tied to a salt dissolution brine halo, and enables an extended phase of focus to a metal precipitative (redox) interface at a stable location in subsurface earth-space (Warren, 2016).

At the world-scale, evaporite-associated metalliferous systems are driven by plate tectonics. Halite-dominated sequences, deposited in the drawdown basin centres, tend to dissolve in burial, and so supply chloride ions to the brine system. Salt beds that are thick enough tend to flow and so focus the upward, and centripetal passage of basinal and hydro-thermal fluid flows. Dissolving gypsum or anhydrite beds, typically deposited higher on the basin platform or diagenetically accumulated along salt dissolution edges and touchdowns can supply sulphur, via bacterial or thermochemical sulphate reduction, while simultaneously focusing metalliferous brine flows into the precipitation interface.

 

The figure plots a selection of the many metalliferous ore deposit provinces where hypersaline brines and/or evaporites beds or there dissolved counterparts played a significant role in the forming of the deposit. See Warren 2016 for a full discussion of these and many more similar evaporite associated deposits. Various web pages in this section discusses some of detail in a few of these world-scale deposits.

 

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