EARLY ARCHEAN SULFATES FROM WESTERN AUSTRALIA FIRST FORMED AS HYDROTHERMAL BARITES NOT GYPSUM EVAPORITES

Pioneering work on bedded barites from the North Pole area of Western Australia indicated that they are diagenetic replacements of evaporitic gypsum crystals that had grown on or just below the Archean sea floor. This interpretation was based on interfacial angles measured with a universal stage on relatively few crystals. In order to improve the crystallographic database, X-ray computerized tomography was used to image in situ barite crystals. At the operating energy levels, barite attenuates X-rays significantly more than metallic iron, so the contrast between the barite crystals and the quartz matrix is profound. Analysis of the processed images, casts of natural external molds of barite crystals, and large thin sections using polarized light, conclusively demonstrates that the original mineralogy was barite, not gypsum. A close genetic link between the bedded barites and syndepositional barite dikes and veins is provided by plots of all four stable isotopes of sulfur which form arrays that parallel terrestrial fraction lines but have negative (D³³S) and positive (D³⁶S) y-axis intercepts significantly far from zero. This is excellent evidence that the hydrothermal fluids involved in dike and vein production were cycling sulfates from the same source as the bedded barites. The non-zero intercepts are indicative of sizeable mass-independent fractionations of the sulfate. As these effects are only known to occur in gas-phase reactions, it is likely that the barites were formed from sulfate produced by shortwave UV photoxidation of volcanic SO₂. Although pyrites intimately associated with the barites have a negative D³³S, those in coeval black cherts have large positive D³³S anomalies (Runnegar et al., Topical Session T11). Thus, bacterial sulfate reduction cannot have been responsible for the disseminated pyrite found in Archean black shales, and it is unlikely that sulfate evaporites formed during the Archean.