PRELIMINARY FLUID INCLUSION EVIDENCE OF ACID BRINES IN HALITE OF THE NEOPROTEROZOIC BROWNE FORMATION, CENTRAL AUSTRALIA

The Neoproterozoic Browne Formation from central Australia has a lithological assemblage similar to known modern and Permo-Triassic acid saline lake systems. It is composed of red bed siliciclastics, bedded and displacive halite and gypsum/anhydrite, and stromatolitic carbonates. There has been no comprehensive depositional study. Marine origin has been suggested by the presence of evaporites, the presence of stromatolites, and a seawater-like major ion composition of fluid inclusions in halite. However, none of these is diagnostic for a marine origin. Other characteristics of the fluid inclusions may provide more definitive data about depositional waters. The goal of this study is to investigate primary fluid inclusions from bedded halite in the Browne Formation to determine if it is acidic in origin.

Samples of the Browne Formation from the Empress 1A core were examined by petrography, microthermometry, and laser Raman spectroscopy. Primary fluid inclusions from nine intervals of bedded halite consisting of chevrons and cumulates were used. Primary fluid inclusions have negative crystal shape, are all liquid or liquid-solid, and are on growth bands. Most primary fluid inclusions are ~5 to 50 microns in diameter. Rare, large (>100 microns diameter) fluid inclusions contain gas. Some solids in fluid inclusions have crystal shapes, and others have spherical and rod shapes. Preliminary freezing-melting runs show that samples cooled to -196 ˚C did not freeze. Laser Raman spectroscopy of inclusion liquid has identified peaks at ~420, ~1022, and ~1054 cm^-1.

Petrography indicates that the Browne Formation bedded halite was precipitated by shallow, ephemeral saline surface waters and that its primary fluid inclusions are direct remnants of that Proterozoic water. Failure of inclusions to freeze suggests that waters were acidic and/or highly saline. Crystals within fluid inclusions indicate that, in addition to halite, waters were supersaturated with respect to other minerals. Spherical and rod-shaped solids in inclusions are consistent with microorganisms. Raman peaks are characteristic of bisulfate, sulfuric acid, and hydronium. This assemblage of characteristics has been documented in other acid-precipitated halites. We propose that bedded halite of the Browne Formation was deposited from acid brines.