Most epithermal Au-Ag deposits in the Hauraki Goldfield are hosted in late
Miocene andesites. Broken Hills is an exception, being one of the few economic
deposits hosted in rhyolite. Investigation of Broken Hills provides the
opportunity to collate structural data that may be compared with that collected
from andesite-hosted deposits in order to evaluate the importance of rock type
on vein development. The rhyolite host rock at Broken Hills is strongly
flow-banded, but this anisotropy does not appear to affect the orientation of
mineralised structures. There is a range of structures at Broken Hills: veins,
faults, fractures, and large breccia pipes. Veins generally strike north-south,
with localised changes in strike and thickness. Slip vectors determined from
striations indicate near pure dipslip on the main lode, indicating an
extensional tectonic setting. The orientations of vein deflections are parallel
to slip vectors. This correlation is common in modern extensional fault arrays
where fault growth and linkage produces subvertical deflections in fault
orientation (eg TVZ). There are three main types of breccias:
1. Primary breccias related to rhyolite emplacement.
2. Jigsaw type secondary breccias that are that developed during the life of
the hydrothermal system.
3. Breccia pipes. Broken Hills was a dynamic epithermal mineralising
system.