DOLOMITE, MICROBIALITES AND SALT LAKES: CELEBRATING 75 YEARS OF LACUSTRINE DOLOMITE RESEARCH

An historic event of considerable significance passed without fanfare last year: 2004 marked the 75th anniversary of the discovery of modern dolomite. As we move into the next quarter century of research on this important and still enigmatic mineral, it is fitting to return to the Australian continent where this watershed discovery was made. Sir Douglas Mawson's 1929 report of the presence of modern, non-detrital CaMg(CO₃)₂ associated with algal biscuits in ephemeral saline lakes at the south end of the Coorong Lagoon was the first of over 40 occurrences of lacustrine dolomite that have been described from Australia and nearly one hundred occurrences world-wide. Despite this, it is fair to conclude that after 75 years of study, there is still considerable difference of opinion regarding the genesis of modern dolomite. Although the precise mechanism(s) by which dolomite is forming in many of these lacustrine settings remains elusive, there is nearly universal agreement that fine-grained, unconsolidated and poorly consolidated Mg-Ca carbonate material (protodolomite, neodolomite) can form both as a true primary (often biogenically-mediated) precipitate and as a penecontemporaneous diagenetic mineral.

In addition to marginal marine/lagoon localities that are hydrological and geochemically similar to the distal lakes of the Coorong, modern dolomite occurs in many inland lacustrine settings. Foremost among these true continental settings are the lakes of central and western Victoria. The Western District Volcanic Plains contain the world's largest number of extant dolomite-precipitating lakes. There are two basic types of lakes in which neodolomite is forming: (i) playas and shallow lakes and (ii) deep, perennial volcanic maars. Dolomite formation within each type is, like that of the marginal marine settings, complex. There is now abundant geochemical, petrographic and sediment trap data to confirm that true primary precipitation of dolomite occurs in the offshore areas of both perennial lakes as well as ephemeral playas. In contrast, the petrography and geochemistry of modern microbialites, indurated crusts and hardgrounds in the nearshore areas of these lakes indicate a complex interaction of both primary bio-induced precipitation and penecontemporaneous dolomitization.