Geology and characterization of two hydrothermal nontronites from weathered metamorphic rocks at the Uley Graphite Mine, South Australia

Mining operations during the early 1990s at Uley Graphite Mine near Port Li ncoln on southern Eyre Peninsula, South Australia, uncovered abundant nontr onite veins in deeply weathered granulite facies schist, gneiss, and amphib olite of Palaeoproterozoic age. Two types of nontronite are present: a brig ht yellowish-green clay (NAu-1) distributed as veinlets and diffuse alterat ion zones within kaolinized schist and gneiss, and a massive to earthy, dar k-brown clay (NAu-2) infilling fracture networks mainly in amphibolite or b asic granulite. The nontronites are the product of low-temperature hydrothe rmal alteration of primary minerals, biotite, and amphibole. The principal chemical difference between NAu-1 and NAu-2 is a higher alumina content in NAu-1, which was either inherited during hydrothermal alteration of biotite in the host rock or acquired through recrystallization of nontronite durin g subsequent weathering and associated kaolinization. Sufficient bulk sampl es of both NAu-1 and NAu-2 were collected to supplement reference nontronit e of the Source Clay Repository of The Clay Minerals Society. The clay frac tion of the bulk samples is typically >85%. NAu-1 contains minor kaolin and quartz which are easily removed to give a high purity nontronite of compos ition M-1.05(+)[Si6.98Al1.02] [Al0.29Fe3.68Mg0.04]O-20(OH)(4), similar to t hat of nontronite from Garfield, Washington. NAu-2 contains fewer total imp urities but the presence of trace amounts of submicron carbonate and iron o xyhydroxide requires additional chemical treatment to produce a nontronite of purity comparable to NAu-1. Composition of NAu-2 was calculated as M-0.7 2(+)[Si7.55Al0.45][Fe3.83Mg0.05]O-20(OH)(4), although infrared data indicat e that at least some Fe is in tetrahedral coordination.