Geology and gold paragenesis in Torlesse greywacke, Wellington, and in Caples schist, Marlborough

Abstract

Mesothermal gold-bearing vein deposits have been mined at Terawhiti Hill, Wellington, and at Waikakaho, Marlborough. Geological, geochemical, structural and geothermometry data describe and interpret emplacement history of the gold deposits and their host rocks. Basement rocks at Terawhiti Hill consist of Late Triassic Torlesse terrane greywacke whereas those at Waikakaho comprise predominantly Caples terrane metamorphosed volcanogenic semi-pelitic lithotypes. The quartzofeldspathic Torlesse terrane was derived from a continental-margin source while Caples terrane rocks were derived from a volcanic island arc. Mapping in both areas shows that basement rocks have been subject to complex deformation and low grade metamorphism. The dominant structural feature at Terawhiti Hill is a north-south striking, steeply dipping shear zone, newly described and named here as the Terawhiti shear zone. Fabric orientation and kinematic indicators reveal high-angle reverse faulting. Neometamorphic minerals developed along cleavage planes and a brittle-ductile style of deformation indicate the shear zone developed at temperatures and pressures of 250-300°C and 3-4kb (10-14km depth), near the base of the seismogenic part of the crust. Deformation events recorded in the Caples terrane rocks at Waikakaho include greenschist facies metamorphism and late stage strike-slip faulting. Early Cretaceous quartz veins (V3) that post-date folding are found over most of Terawhiti Hill and are both subhorizontal (some V3a) and subvertical (some V3a and V3b) with variably striking orientations. Gold-bearing veins (V3b) are steeply dipping and occur within the Terawhiti shear zone. V3a veins are typically less than 5m in length and average 1-2cm thick. V3b gold-bearing veins have similar lengths but may lense up to 0.5m thick. Geothermometry indicates emplacement of V3 veins occurred from large quantities of fluids at temperatures of 230-300°C. Chemistry of gold grains and vein neometamorphic mineral assemblages indicate that the Terawhiti Hill gold deposits formed deeper in the sedimentary pile than the Never-Despair Phoenix gold deposit 3km to the east. The extensively fractured Terawhiti shear zone acted as a conduit for rising fluids. In the Waikakaho No. 3 and Southern Cross mines at Waikakaho, two episodes of gold-bearing quartz veins were deposited following metamorphism in the Cretaceous. The early V2 veins are predominantly parallel to S2 schistosity, 30cm in length and 1-2cm thick. They formed at temperatures between 230-300°C. V4 late-stage gold-bearing quartz veins were deposited in strike-slip shear zones. These veins, 0.01m to 2m thick, strike predominantly north. Emplacement of V4 veins from large volumes of fluid occurred in ductile conditions at temperatures between 310-450°C. The presence of graphite in the schist (not observed elsewhere in Marlborough) created a reducing environment preferentially precipitating gold. The mineral assemblage within veins from both Terawhiti Hill and Waikakaho includes quartz + calcite ± pumpellyite ± prehnite ± epidote ± haematite ± pyrite ± arsenopyrite ± pyrite ± galena ± native gold. The veins have a predominantly blocky internal texture with precipitation of new minerals along the median line indicating antitaxial vein growth. Recrystallisation of quartz and calcite is observed indicating post-depositional deformation. Wall rock and fluid interaction has led to arsenic enrichment in the Terawhiti shear zone. At Waikakaho enrichment of muscovite, pyrite and arsenopyrite and depletion of albite, calcite and epidote occur within one metre of the gold-bearing veins. A sedimentary source for the gold is proposed for both Terawhiti Hill and Waikakaho. Deposition of the sedimentary sequence followed by prograde metamorphism and an associated rise of geotherms led to the creation of metal enriched fluids. Fluids and incompatible elements such as K, Rb, Sr and Ba were released from reacting minerals (e.g. epidote and chlorite) and transported together with residual fossil water retained during burial of sediments. Vein systems cross-cutting earlier metamorphic fabrics provide the simplest evidence for mobile fluid phases. Sulphides and native elements found within the veins indicate that the fluid contained SiO2, CO2, Au, As, Fe, S and possibly Pb. Hydraulic fracturing induced by thermal expansion of water may explain fluid loss from the metamorphic pile. More likely, the saline fluids migrated into fractures and pore spaces which progressively dilated during long-term stress accumulation. Seismic events allowed sudden drainage of the overpressured reservoir. This cycle may be repeated many times. The aggregate effect of these processes is to bring about vein formation, gold precipitation and wallrock alteration. The above genetic model provides an improved understanding of the environment of gold deposition at Terawhiti Hill and Waikakaho. The model for the formation of the vein systems may also provide a useful framework for mineral exploration.