Post-metamorphic structural evolution of Otago Schist
resulted in a pattern of northwest and northeast striking faults and fractures
cutting the basement rocks. These faults have been reactivated several times
since Mesozoic metamorphism. Gold-bearing quartz veins which have been exploited
sporadically over the past 130 years are almost invariably hosted by northwest
striking structures, and most exploration has been focussed on these structures.
There is evidence for hydrothermal activity and mineralisation along some of the
northeast striking structures, although there has been very little work done on
these structures. Our study is attempting to characterise the hydrothermal
processes in the northeast structures, in comparison to the northwest
structures, in order to see if further detailed exploration is warranted. There
is evidence for both normal and reverse senses of movement on northeast striking
faults, and some of the faults have evidence for early ductile deformation
overprinted by later brittle structures. Northeast striking structures host
stibnite mineralisation in places, and faults are locally silicified or
extensively impregnated with ankeritic carbonate. Sulphides are rare in veins,
but do occur in some hydrothermally altered host rocks. Hydrothermal haematite
accompanies extensive silicification along the Manuherikia fault zone near
Alexandra. Anomalous gold
concentrations occur in some hydrothennal zones, and in downslope sediments in
some localities.