Compositional Variations of Dolomite from a Chain of Ephemeral Lakes in Coorong Region, South Australia

Michael R. Rosen, Donald E. Miser, John K. Warren

A chain of four shallow ephemeral lakes (Halite, Dolomite, Pellet, and North Stromatolite Lakes) in the Coorong region (Australia) shows three mineralogically, chemically, and isotopically distinct dolomite units in various stratigraphic positions. Type 1, a basal dolomite which forms small centimeter-size concretions and lightly cements a siliciclastic sand, is present in all four lakes in the chain and is chemically similar to the dolomite found in the upper 20 cm of Dolomite Lake. Type 2, a dolomite layer associated with hydromagnesite, is found in the upper 20 to 60 cm of Pellet Lake. Type 3, dolomite found along the margin of Pellet Lake, forms in association with aragonite and magnesium calcite.

Detailed x-ray diffraction (XRD) analyses of samples containing more than 80% dolomite demonstrate that all dolomite types have ordering reflections; however, the degree of ordering between individual samples is highly variable. XRD patterns from Type 1 dolomite indicate that the dolomites has a_o and c_o unit cell dimensions close to ideal dolomite, but slightly expanded in the c_o direction. Type 2 dolomite is expanded in the c_o direction and contracted in the a_o direction (relative to ideal dolomite) and is magnesium rich. Type 3 dolomite is greatly expanded in both a_o and c_o directions and is calcium rich. Transmission electron microscopy reveals that Type 3 dolomite has a relatively homogeneous microstructu e, Type 2 dolomite has a heterogeneous microstructure, and Type 1 dolomite is intermediate.

Type 1 dolomite is characterized by a mean ^dgr¹⁸O of +6.3 ^pmil PDB (stand. dev. ±0.3 ^pmil), and mean ^dgr₁₃C of - 0.5 ^pmil PDB (stand. dev. ±0.8 ^pmil). Type 2 dolomite has a mean ^dgr¹⁸O of + 7.2 ^pmil PDB (stand. dev. ±0.6 ^pmil) and mean ^dgr¹³C of + 3.52 ^pmil PDB (stand. dev. ±0.8 ^pmil). No data are available on Type 3 isotopes.

The mineral associations of each dolomite type, combined with the above data, suggest that variations in the composition of dolomite in these lakes is strongly dependent on the magnesium-calcium ratio of the precipitating fluid.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.