Formation of authigenic Fe2+‐bearing smectite‐vermiculite during terrestrial diagenesis, southern New Zealand

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Abstract
Late Cretaceous to Quaternary terrestrial sediments derived from fault scarps and actively growing folds in schist and greywacke basement are locally preserved in Otago and South Canterbury, New Zealand. Sediments are immature, sparsely carbonaceous, and generally poorly sorted and have undergone little oxidation during rapid erosion and deposition cycles. Mild oxidative alteration of sediment clasts in some horizons has resulted in pseudomorphous replacement of detrital biotite and metamorphic chlorite in greywacke clasts, and chlorite in schistose clasts, by a green, interlayered, ferrous iron‐bearing smectite‐vermiculite mineral of variable composition. Smectite‐vermiculite compositions form a regular trend from low Si and Al, high Fe and Mg to moderate Si and Al, to low Fe and Mg. The Fe2+ smectite‐vermiculite also occurs as pools in the matrix and small veinlets, and as a cement intimately intergrown with fine‐grained clastic matrix. The smectite‐vermiculite is a diagenetic product formed preferentially in sedimentary horizons which are reduced and moderately acidic (pH 3.5–4.5), neutral, or possibly alkaline. There is a close association between the Fe2+ smectite‐vermiculite mineral and authigenic gold deposition; the smectite‐vermiculite is occasionally coated and impregnated with fine‐grained (micrometre scale) gold. Gold precipitation probably occurs by electroless deposition from groundwaters that are undersaturated with respect to gold.