The
northeast-southwest trending ~25 km2 area incorporating Bald Hill is
located 18 km northeast of Haast, and 5 - 7 km west of the Alpine Fault in South
Westland, New Zealand. The geology consists of Greenland Group greywacke
overlain by a Cretaceous-Tertiary sedimentary sequence and fluvio-glacial/marine
terrace deposits of Quaternary age. Alluvial gold in Cole Creek, draining west from the summit of
Bald Hill, may be derived either from vein mineralisation in basement rocks or
from fluvial or marine terraces.

This
paper focuses on the potential for in
situ gold and base-metal mineralisation in sporadic, siderite-rich quartz
veins that cut basement greenschist facies metasediments that have been
overprinted by contact metamorphism.

Throughout the area,
vein compositions are dominated by quartz and commonly feature zoned
plagioclase. Alteration adjacent to veining within the contact aureole is
dominated by 1 - 2 mm, coarse grained biotite, with small amounts of sulphide,
and is pervaded by abundant siderite veinlets. Outside the aureole, wall rock
alteration to veins consists of fine grained chlorite. Trace element XRF
analysis has been used to ascertain base metal concentrations within samples of
vein and alteration zones collected from throughout the area. Vein and
alteration zone mineral assemblages containing <5 per cent sulphide, with low>
concentrations of As, Cu, Pb and Zn indicate low potential for gold and
base-metal mineralisation in the Bald Hill region. Contact metamorphism of
Greenland Group basement in the northeast of the region, is likely due to
emplacement of a poorly exposed Cr- and Ni- enriched gabbro. The presence of
biotite and andalusite in hornfelses indicate contact metamorphism occurred at
temperatures >350C and at depths <10 km. widespread vein distribution>
does not appear to be related to contact metamorphism or gabbro emplacement.
Further research is required in order to establish the fluid source from which
the veins were precipitated.