Turbidity development and dissipation in paleoplacer gold deposits, southern New Zealand

Abstract

Placer gold mining inevitably produces highly turbid processing waters. Five historic paleoplacer mining sites in Central Otago, New Zealand, provided 24 samples which were used in laboratory-based settling experiments. Turbidity was examined in the context of stratigraphy of paleoplacer deposits, their environments of formation, and the groundwater processes that affected the deposits as well as the underlying bedrock. Settling rates were characterised by measuring turbidity levels over time (up to 40 days) using turbidimeter and Coulter counter methods. High clay mineral (mostly kaolinite) contents of the materials were confirmed by X-ray diffraction. Grain size distributions of suspended materials were very comparable across all samples with majority of particles falling between <1.2 and 2.5 μm. The levels of turbidity produced by the auriferous sediments were partly controlled by the level of sorting and winnowing that the sediments were subjected to during transport and deposition. Debris flow material generated high turbidity [initial levels of 120–420 nephelometric turbidity units (NTU)] which settled slowly, as did eolian siltstone (1,600 NTU). Fluvial sediments generally generated lower turbidity which settled more rapidly, on the scale of hours to days. It was found that cementation of the sediment can reduce turbidity generation by limiting disaggregation of the clay minerals. On the other hand, the presence of altered lithic clasts within the sediments contributes to higher turbidity production. There was poor correlation between the level of bedrock alteration, as indicated by Chemical Index of Alteration, and the resulting turbidity. Settling rates were more rapid in experiments conducted in saline solution, as opposed to stream water, due to floc formation.