Prediction of Metal Mobility from Sulfidic Waste Rocks Using Micro-analytical Tools, Baal Gammon, Northern Australia

Predictions on the behavior of environmentally significant elements at mine sites requires the use of advanced laboratory techniques. The aim of this contribution is to demonstrate the use of electron microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to gain an understanding of likely element behaviour. Sulfidic boulders sampled from an acid rock drainage (ARD) impacted ephemeral stream adjacent to the historical Baal Gammon workings are dominated by chalcopyrite, arsenopyrite, pyrrhotite and lesser pyrite. Micro-analytical investigations using EMPA and LA-ICPMS reveal that chalcopyrite contains significant quantities of Ag, Cd, Sn, In and Zn either substituted directly into the crystal lattice or occurring as discrete sphalerite and stannite inclusions. Arsenopyrite, comprising more than 50 % of some boulders, is most notably rich in Co, Ni, Sb and Se, but it also contains inclusions of sphalerite, chalcopyrite and stannite. By contrast, pyrrhotite contains relatively few trace elements, but it may be a significant contributor to ARD development. The trace element composition of Fe-oxides in the oxidized rinds of these boulders is likely directly influenced by the mineralogy of the sulfidic boulders on which they precipitate. Although significant quantities of As, Bi, Cu, In, Pb and Zn occur in Fe-oxides at Baal Gammon, these elements may be liberated during acid flushing of the ephemeral stream. Consequently, EMPA and LA-ICPMS represent valuable tools for evaluating the source and potential mobility of environmentally significant elements at mine sites.