The Georgia, USA, kaolins contain three major occurrences of Fe-sulphide minerals, namely (1) in burrow structures, (2) as nodules associated with lignite and (3) as finely disseminated material. We have used sulphur isotope analysis and X-ray powder diffraction to understand these three occurrences in order to gain insight into the diagenetic conditions and microbial influences affecting the kaolin mineralogy. Burrows and nodules are dominated by corroded marcasite with thin pyrite overgrowths. The δ34S composition of each sulphide sample has internal fractionations (δ34Shigh – δ34Slow) ranging from 22‰ to 67‰. The secondary pyrite phase tends to be the most 34S depleted, with a low δ34S value of –47‰, whereas marcasite displays a δ34S range from 30‰ to +33‰. Early acidic environments from organic acids could have facilitated the initial marcasite crystallization, with a pH increase during later diagenesis producing pyrite overgrowths. Sulphide formation would have continued until burial restricted the diffusion of sea-water sulphate. At this point, the metabolism of sulphate-reducing bacteria would have been retarded, producing δ34S values ranging from –30‰ (open sulphate supply) to +40‰ (highly restricted sulphate supply). Under the nutrient-starved conditions of later-diagenesis, disproportionate sulphur-reducing bacteria would likely have become more dominant, yielding additional δ34Ssulphide fractionation and producing the observed fractionation (δ34Ssulphate – δ34Ssulphide) values of +67‰.

The kaolin deposit at the Adelita mine, located in the province of Rí;o Negro (Patagonia, Argentina), was studied. Petrographic studies on thin sections and chemical analyses of major, minor and trace elements on bulk samples were carried out. The kaolin content of the rock ranges from 31% to 65%. The mineralogy of the working front, which is about 45 m thick, varies from the upper zone, where the rock is strongly silicified, stained with iron oxides and carbonated, grading downwards to a kaolinized zone, with a quartz-kaolinite-dickite assemblage with relict lithic particles and a smaller amount of iron oxides, and ending in the deepest zone where dickite, alunite, diaspore, quartz and scarce associated kaolinite occur.

The kaolin mineralogy was determined by SEM, XRD, DTA-TG, IR and δ18O and δD isotope analyses. The S, Ba and Sr contents are enriched during hydrothermal alteration, whereas Cr, Nb, Ti and lanthanide elements are concentrated mainly during weathering. The (Ba+Sr) concentrations in the samples studied vary between 600 and 6000 ppm and (Ce+Y+La) between 2 and 150 ppm; (Cr+Nb) remains constant for all the samples, whereas (TiO2+Fe2O3) is below 0.3%. In the hypogene deposits P2O5 is also more abundant and increases with the degree of alteration. Chondritenormalized rare earth element diagram shows a marked enrichment in LREE relative to HREE, with negative europium anomalies and D18O values range between 3.8‰ and 7.7‰ and δD between –123‰ and –103‰, suggesting that kaolin formed from the hydrothermal alteration of rhyolitic tuffs.