The Paragenesis of the Orogenic Gold Deposits of Te Oneroa Bay, Preservation Inlet, Southwest Fiordland

Abstract

The Te Oneroa Bay area in Preservation Inlet, southwest Fiordland, has been a well-known source of hard rock and alluvial gold since the late 1800’s. Such Au-deposits are found within the Buller Terrane mid-Ordovician Preservation Formation, composed of mid-greenschist facies meta-turbidites deformed into hundred-metre scale waveforms. Large faults and shear zones cut the Preservation Formation on approximate NNW-SSE strikes along the limbs of the large waveforms. Higher permeability in the shears/faults enabled the concentration of hydrothermal fluid flow subsequently causing the precipitation of mineralised quartz vein controlled dominantly by the sericitisation and chloritisation of the host rock, decreasing pressure-temperature, and boiling of the fluid. Mineralisation is thought to have occurred via a two-step model, whereby an early stage of pyrite and arsenopyrite, and lesser galena and chalcopyrite, were introduced along with shear zone quartz pods. A secondary stage of mineralisation later occurred closely associated with ductile deformation-controlled quartz vein recrystallisation, the remobilisation of the already formed sulphide minerals, and the introduction of native gold, tetrahedrite, sphalerite, bournonite, and boulangerite along stylolites. Mineralisation likely occurred just below the brittle-ductile transition at between 1-3.5 kbar of pressure and 180-350 °C, from a fluid with near-neutral pH, low-moderate volatile content, and low salinity. A main structurally controlled branch of mineralisation is inferred to exist approximately between the historic Morning Star and Alpha Mines, as well as a smaller side branch at Golden Site Mine. Gold ranges from ~1-20 wt% silver and is thought to be hypogene; no evidence for supergene concentration exists. Due to the similarities of general geology, metamorphic grade, structure, and mineralisation of the Te Oneroa Bay area to other Buller Terrane Au-deposits, mineralisation is thought to be orogenic in origin. Fluids were predominantly produced via the dehydration of chlorite during metamorphism at ~375-330 Ma. Subsequently, Au-mineralisation is likened to have occurred at a similar time period.