The Old Man
Range, south of Alexandra, in central Otago consists of a package of highly
folded greenschist facies schist of the Wanaka lithologic association. The range
is crosscut by several east to ENE-striking high angle gold-bearing quartz
veins, (eg Whites Reef and Grays Reef), which were extensively worked in the
late 1800s. The range is bounded on its east side by the Old Man Fault, which
separates these rocks from generally flat lying and planar Caples Terrane
schist. To the west of the Old Man Fault, about halfway up the range, a large
subparallel, 100 m wide fault zone strikes NNW and appears to cut off the
historic veins. The fault zone consists of highly sheared and folded rocks and
numerous high angle cataclasites and centimetre-scale clay gouge zones.
Hydrothermal alteration is not apparent and the individual faults and shears are
clay altered, possibly due to weathering, without visible quartz veins. Gold,
limonite pseudomorphs after pyrite and rutile are concentrated in some of the
narrow cataclasites and gouge zones. These mineralised structures are texturally
similar to the gold-bearing reefs and may be structurally related. It is
postulated that the NNW-striking shear zone and ENE-striking veins may be a
conjugate array formed during a period of Cretaceous normal
faulting.