Salt Diapirs and Base Metals: Key to Capricorn Orogen Metallogeny

Halokinetics and halotectonics are some of the most interesting and widely under-regognised metallogenic processes within evaporite basins throughout geological history, present during cratonic sedimentary successions from their appearance in the first palaeoproterozoic basins at c. 2400Ma (with minor Archaean examples).

Without treading on the toes of John Warren's fantastic work, salt when buried in a sedimentary basin becomes both a source of sulphur and halides, a locus for fluid entrapment and a self-propagating structural phenomenon that exploits, and can indeed create, the throughgoing pathways for base metal mineralisation within basin successions.

The most fascinating phenomenon about salt is that once the basin gets a shove, it disappears almost entirely from the stratigraphy, via halotectonic and mass flow processes, and when coupled with hydrothermal fluid flow (of which it can, in fact, be a causative factor) it can be removed almost entirely from the rock record.

Within the Capricorn Orogen of Western Australia there is an unrecognised evidence of significant halotectonic processes operative between the Tooloo Group and MIningarra Group sediments, present as portions of the Yelma Formation and equivalents that occupy the top of the Yerrida Basin stratigraphy. This is a widespread but irregularly distributed interval of conglomeratic and rudaceous dolomite sediment, crystalline quartz, and base metal anomalism and mineralisation.

This interval is arguably the eruptive remnants of salt diapirs, with salt sourced from evaporitic strata in the Finlayson Sandstone and Juderina Formation, that erupted through the basin stratigraphy during the c. 1950Ma metamorphic event and orogeny.

One example of such mineralisation is the Paroo (Magellan) oxide lead deposit, located in the south-east Yerrida Basin, which is hosted within a rudaceous dolomite breccia (including rounded quartz cobbles and 5-30mm quartz with fluid inclusion equilibrated at c. 250°C) which lies in a broadly flat orientation over steeper dipping Maraloou Formation carbonaceous shales and mudstones.

My contention is that this is the breccia remnants of a significant namakier (salt glacier) deposit of Proterozoic age. Similar deposits of quartz crystalline, cobble conglomerate and dolomite rudites are present at and proximal to the Corktree Cu prospect, the Elmos Zn prospect, the Enigma Cu prospect and in between the Juderina and Johnson Cairn Formation around the NE of the Yerrida Basin. In this situation, these rudaceous sediments represent a salt weld breccia or rauwacke, and record halokinetic flow within the sequence. Of course, the salt is now gone.

My observations of halokinetic deposits and their association with base metals in the Yerrida Basin (figure modified from Occhipinti et al 2017)

Throughgoing salt diapirs, I argue, provide and indeed create the space and fluid pathways to localise Mississippi Valley Type (MVT) mineralisation, within both the Yerrida and Earaheedy Basins, and are intimately associated with both porosity creation, halide fluids, oils, and deep tapping structures. Examples of diapir-associated MVT's exist in Morocco and elsewhere.

Association of diapirs with MVT deposits (Bouhlel et al, 2016).

Further evidence of MVT prospectivity in the Yerrida Basin and Eararheedy Basin is the presence of oils. The Thaduna Cu deposit, whilst it postdates basin formation by ~500Ma, occupies a breached oil system in a fault that cuts the Thaduna synform. Here so-called "graphite alteration" as described by Ventnor Resources and Sandfire Resources, is in fact pyrobitumen. Further examples of pyrobitumen are known from air core drilling within the Johnson Cairn Formation. Arguably salt diapirism is a fundamental process in forming oil fairways, and thereafter, providing reductive traps for the Thaduna Cu deposit some half a billion years later.

Similarly, migration and pooling of wet gas and dry gas in conventional peridiapiric positions, intimately familiar to petroleum geologists, provides scope for MVT's within diapiric stems and salt welds, both vertical and horizontal.

Beyond this, rauwacke horizons within basement onlapping sediments are simply highly porous, halide and oil saturated environments to host base metal accumulations.

Recognition of these key elements in the Capricorn Orogen metallogeny is a critical lack in most if not every exploration play book, and arguably the ontological garbage bag called "chert breccia" has been used to incorrectly describe a swathe of clear namakier and rauwacke textures within the Yerrida and Eararheedy basins, and lack of exposure between outcrops hinders recognition of cross-cutting cisional features like salt welds and namakiers.

Exploration therefore, can be improved, and discoveries made, by recognising the missing salts within these basins, and others.

References:

Bouhlel, S, Leach, DL, Johnson, CA et al. (3 more authors) (2016) A salt diapir-related Mississippi Valley-type deposit: the Bou Jaber Pb-Zn-Ba-F deposit, Tunisia: fluid inclusion and isotope study. Mineralium Deposita, 51 (6). pp. 749-780. ISSN 0026-4598?https://dx.doi.org/10.1007/s00126-015-0634-8

Barhoumi et al. (2021) Tectonic controls on the salt diapir-related Mississippi Valley-type lead-zinc mineralization of Fej El Adoum ore deposit (Northern Tunisian Atlas): Constrains from detailed gravity and drill hole data. Journal of African Earth Sciences 181. https://doi.org/10.1016/j.jafrearsci.2021.104237

Garnit H., Bouhlel S, Barca D., Johnson C.A., Chtara C., 2012. Phosphorite-hosted zinc and lead mineralization in the Sekarna deposit (Central Tunisia). Mineralium Deposita 47 pp 545-562.

Occhipinti S, Hocking R, Lindsay M, Aitken A, Copp I, Jones J, Sheppard S, Pirajno F, Metelk V, 2017. Paleoproterozoic basin development on the northern Yilgarn Craton, Western Australia. Precambrian Research 300 pp 121-140. https://dx.doi.org/10.1016/j.precamres.2017.08.003

Pirajno F., Burlow R., Huston D., (2010) The Magellan Pb deposit, Western Australia; a new category within the class of supergene non-sulphide mineral systems. Ore Geology Reviews 37 pp 101-113