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

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 smectitevermiculite mineral of variable composition. Smectitevermiculite 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 Fe 2+ smectite-vermiculite also occurs as pools in the matrix and small veinlets, and as a cement intimately intergrown with fine-grained clastic matrix. The smectitevermiculite 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 Fe 2+ smectitevermiculite mineral and authigenic gold deposition; the •imectite-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.

Initial sedimentation and subsequent diagenesis in the Eastern Southland Lignite Basin, southern New Zealand

“…The distinctive green Fe-K bearing vermiculite-smectite clay minerals in the ESLB lithic conglomerates (Figs. 9A, 10A, B) are similar to postdeposition alteration minerals described by Craw et al (1995) in immature lithic conglomerates derived from the Otago Schist.…”

Sulphur mobility and sulphur‐mineral precipitation during early Miocene‐Recent uplift and sedimentation in Central Otago, New Zealand

“…These sulphides show no sign of having undergone any physical transport, in marked contrast to the larger abraded or partially rounded detrital (primary) pyrite cubes that also occur in the fan gravels (described above). These smaller, (Craw et al 1995). Fan gravels along the Dunstan Range (Fig.…”

Lithostratigraphy of gold-bearing Quaternary gravels, middle Manuherikia Valley, Central Otago, New Zealand

“…Gravel beds near the base of the formation are typically white because of their high quartz clast contents, and outcrops of gravels higher in the formation weather brown because of their higher schist component. Tinkers Formation gravels exposed in active mine workings at Matakanui in the 1980s were distinctly green in colour, reflecting the preservation of unoxidised iron phyllosilicates below the water table (Youngson & Craw 1993;Craw et al 1995).…”