Broken Hills is an adularia-sericite epithermal Au-Ag
deposit, hosted in flow-banded rhyolites and overlying rhyolitic pyroclastic
rocks in the eastern Hauraki Goldfield. The 7.1 Ma deposit was the largest
producer of gold in the eastern Hauraki Goldfield, and was one of very few
economic deposits hosted in Pliocene rhyolites. This paper presents the results
of a recent study in the upper levels of the deposit. Maps and cross-sections
constrain structural and lithological relationships, and the lateral and
vertical extent of a breccia pipe - the major feature of the upper levels.

Previous studies in the
upper levels of Broken Hills have shown that gold and silver values are
concentrated around and within a large breccia pipe, mined by a series of cave'
stopes and in veins. Contour diagrams of Au-Ag data from channel samples
indicate that the breccia pipe was a major fluid channel-way for mineralising
solutions.

The
breccia pipe deposits are part of the dynamic geothermal system that formed the
Broken Hills ore bodies; they developed as a result of hydrothermal brecciation
in areas of structural weakness within a rhyolite pyroclastic unit that overlies
a massive rhyolite lava-flow unit. The main breccia pipe begins near the
rhyolite/pyroclastic contact, having the shape of an inverted, inclined cone,
and consists of silicified breccia, stringers of quartz, and irregular patches
of clay-altered country rock. Smaller zones or pockets of hydrothermal eruption
breccia are silicified and contain low grade gold in the matrix and in the
adjacent silicified wall rocks.

Quartz has been
classified according to its physical characteristics as seen in situ and
hand specimens indicate that vein definition decreases upwards, perhaps as a
result of changing lithological conditions and/or increasing proximity to the
paleosurface. Hydrothermal quartz is widespread as veins and as a replacement
mineral, or lining fractures and cavities and overgrowing rock fragments.
Breccias contain vein quartz, but some quartz veins cross-cut breccias,
indicating that vein and breccia formation was relatively synchronous and
overlapped in time.

Disseminated
gold-silver mineralisation is associated with vein quartz and silicic alteration
of the wall rock within the breccia pipe. The auriferous zones are highly
irregular and erratic, and sampling results show a wide range of gold-silver
ratios, suggesting that highly volatile physiochemical conditions prevailed
during mineralisation.