Groundwater interactions control dolomite and magnesite precipitation in saline playas in the Western District Volcanic Plains of Victoria, Australia

In this paper, a review is offered on lacustrine sites in Australia where both dolomite and magnesite are found to precipitate today (but not through diagenesis) and have been doing so during the Holocene. Information thus far obtained in the Coorong region of SE South Australia is provided and this follows with a discussion of the published literature from the volcanic plains of Western Victoria where modern dolomite and magnesite are also precipitating in saline playa lakes today. The work presented here clearly identifies that the volcanic rises (and topographic highs) in Western Victoria, such as Warrion Hill, play a vital role in groundwater recharge first, with subsequent flow paths towards the low plains in the region where saline playas are located. In the topographically-high recharge areas, the waters are predominantly rich in Ca and, as water progressively flows towards the playas, Ca is lost through calcite precipitation in scorias and soils, and is also presumably exchanged for K, with a considerable enrichment of the groundwaters with respect to Mg/Ca and K/Ca. This cationic exchange is thought to occur on clays in the subsurface, which are the by-product of weathering of the local Quaternary basalts. Eventually, the lake waters become supersaturated with respect to Mg-rich minerals and both dolomite and magnesite precipitate naturally in the lake waters to finally settle on the lake floors. It is more than likely that a brine pool occurs below each lake and it is from this that precipitation occurs once that pool waters reaches the lake surface. High levels of bicarbonate are found in the groundwaters principally as a result of carbonic acid supplied through rain and also from hydrolysis of silicate minerals that originally occurred in the basalts. The high concentrations of bicarbonate contribute to the precipitation of the carbonate minerals. Chemical analyses of the groundwaters and the lakes are tabled and help understand the chemical changes along the groundwater flow paths. In addition, examination of the δ¹⁸O and δ¹³C of the Western Victorian lake sediments, by analogy with those of Coorong lakes, indicates that although the carbonates in the Coorong lakes are precipitated as a result of sulphate bacterial reduction, this appears not to be the case for the Western Victorian lakes. Due to the high turbidity of the Western Victorian dolomite-precipitating lakes as well because of their very high salinities (often ≫50TDS), there are no macrophytes, and little if any algal growth.