Arumpo bentonite deposits: distinctive indicators of past volcanic events in the Murray Basin, southeastern Australia

Abstract

Sampling in the Arumpo leases in southwest New South Wales over an area of 5 × 10 km has delineated the structure and composition of two large high-purity bentonite deposits, spatially adjacent but deriving from two separate volcanic events. They are overlain by fluvial and other sediments including later bentonite formations. All of the bentonite deposits are considered to be formed from volcanic ash falls deriving from lavas from related magma sources. Recent aeromagnetic surveys have shown the presence of extensive clusters of highly magnetic pipe-like structures, presumably volcanic plugs, lying as close as 15 kilometres from the deposits. The two major deposits are generally flat-lying and 5-8 m thick with Na and Mg as the dominant exchange cations, with minor exchangeable K and Ca. They constitute an Na-Mg variety of bentonite as compared with Na- or Ca-dominant bentonites elsewhere in the world. The deeper deposit lies in a graben near the edge of the Neckarboo Fault, and presumably was deposited in seawater. The more elevated adjacent deposit suggests ash deposition in a large near-ocean lagoon. Both bentonites have considerable exchangeable Na, ranging from 30 to 56 cmol(+)/kg within total exchangeable cation contents of 80-97 cmol(+)/kg. The deeper deposit is more saline and has exchangeable Na to exchangeable Mg (Na:Mgexch) ratios of 1.3-1.6 and pH 8-8.5, while the shallower has a Na:Mgexch ratio of ∼0.7-0.9 and a pH of 4.8. Major- and trace-element compositions are very similar in both deposits, as are the ratios of concentrations of the oxides of Zr, Ti, Ga and Al. Silica and Na2O plus K2O contents are also similar. Together, these data indicate that both deposits have a related volcanic parent of intermediate mafic composition.