Alunite alteration of tuffaceous layers and zircon dating, Upper Permian Kennedy Group, Carnarvon Basin, Western Australia

Abstract

Alunite layers were found in two locations in the Binthalya Formation, Upper Kennedy Group, of Late Permian age, in the Merlinleigh Sub-basin, onshore Carnarvon Basin. Alunite has not previously been reported from the Kennedy Group, or from a similar setting in Western Australia. The alunite occurs in discrete layers that have different chemical signatures to surrounding, unaltered sedimentary rocks. The lack of alteration in surrounding sedimentary rocks suggests that alteration was localised by initial differences in lithology, rather than by fortuitous localisation of alteration in a single layer. Alunite is thought to be the product of alteration of K-feldspars, clay minerals and pyrite under acidic, specifically sulfuric, conditions. It is found in hydrothermal systems and weathering profiles. Because the precursor minerals of alunite are believed to be feldspars, clays and sulfur-rich minerals, tuffaceous layers are the probable precursors of the alunite layers. Sulfur for the reaction was derived from within the tuffaceous deposits or from the oxidation of pyrite, which was probably present as cementation of trace fossils in other beds. The alteration is thought to have occurred during extensive oxidation, weathering and laterite formation during the Cenozoic, because the low density and powdery nature of the alunite indicates that it developed at a shallow depth. U-Pb dates obtained from zircons extracted from the alunite layers returned ages ranging from ca 2690 Ma to ca 270 Ma. The majority of zircons are inherited from the same source as the Kennedy Group sediments, indicating mixing of the initial tuffaceous layers with background sedimentation, either during the sedimentation process or afterwards by reworking or biological disturbance.