Carbonation reactions and coupled element and isotope redistribution during shallow crustal gold mineralisation, New Zealand

Abstract

Hydrothermal alteration zones rich in carbonate are excellent vectors for orogenic gold-bearing veins, but the extent of element mobility is difficult to establish. Whole rock and in situ mineral analysis was performed to better understand the element and isotope (Sr, Nd, C) redistribution around Miocene quartz-carbonate-sulphide-gold-bearing veins in meta-turbidite (greyschist) and meta-basite (greenschist) in the Otago Schist in New Zealand. The new data provide insight into the fluid-rock interaction within a lithologically, chemically and isotopically heterogeneous upper few kilometres (< 5 km) of crust. The relative resistance of metamorphic minerals to complete breakdown in mineralising fluids with high CO2 activity was epidote < titanite < actinolite < chlorite and muscovite, with their breakdown promoting simultaneous precipitation of carbonate, white mica, rutile, quartz, pyrite, arsenopyrite, barite, scheelite and gold. At the hand specimen scale, mass balance calculations suggest that major elements and large-ion lithophile elements (K, Rb, Cs, Ba ± Sr) were mobile, whereas high-field strength elements (HFSE; Ti, Nb, Zr, Hf) were immobile. In detail, however, Zr and Hf were mobile at the microscopic scale during the replacement of metamorphic titanite. Similarly, rare earth elements (REE; La to Lu) appeared immobile at the hand specimen scale but were mobile at the microscopic scale because epidote, the main metamorphic REE reservoir, was replaced by REE-free white mica. The limited hand specimen-scale REE mobility was due to the formation of hydrothermal calcite, ankerite and magnesite, which almost immediately bound these elements into their crystal structures. Conventionally obtained δ13C and bulk 87Sr/86Sr isotopes, augmented with in situ laser ablation analysis of 87Sr/86Sr of hydrothermal carbonate and metamorphic epidote, show that the mineralising fluids reacted with greyschists and greenschists, even when these lithologies were not close by. The elemental and isotopic compositions of the hydrothermal minerals in this shallow gold-bearing system were evidently affected by distal and proximal metasomatic reactions.