The Golden Cross, low-sulfidation, epithermal deposit
shows a number of features that are directly or indirectly related to boiling
hydrothermal fluids. Occurrences of lattice calcite and their quartz pseudomorph
equivalents in veins, and occurrences of adularia in veins and in the
surrounding altered rocks in the vicinity of ore, are both direct evidence of
deposition in the presence of boiling hydrothermal fluids. Loss of carbon
dioxide causes calcite deposition (platy variety) near the level of first
boiling, while adularia deposits due to the attendant pH increase along the
cooling flow path.
Indirect evidence of boiling include
crustiform-colloform quartz banding, late massive calcite veins, clay-carbonate
alteration in the shallow and peripheral parts of the ore zone, and the
restricted vertical interval of precious metal mineralisation. The colloform
quartz banding strongly resembles the banding in amorphous silica deposits found
in geothermal surface pipes. This implies that fluids ascending the Empire Vein
structure were saturated in amorphous silica. If so, then they must have
undergone phase separation, which initiated at considerable depth (e.g. 1000 m)
and very hot temperatures (e.g. 300C).
On the basis of stable isotope data, late massive
calcite veins appear to have deposited from CO2-rich steam-heated
waters. Calcite deposited along heating paths as these waters descended into the
upflow zone during late stage collapse of the hydrothermal plume. In active
systems, such steam heated waters form by deep boiling. The high CO2
contents of these waters promote hydrolytic alteration and the formation of
clay-carbonate alteration.
Reaction path modelling using CHILLER and a deep,
Broadlands-Ohaaki water as an analogue, shows that minerals deposit along
boiling flow paths over discrete spatial intervals. Calcite and adularia deposit
near the site of first boiling, followed by gold and then by amorphous silica.
For deep boiling starting at 300C, these minerals deposit over a path length
that exceeds 1000 m. The distribution of banded quartz and gold at Golden Cross
are consistent with such a boiling model. However, the elevation of first
boiling must have fluctuated over 500 m to account for the coincidence of
lattice quartz (pseudomorphs of platy calcite) and adularia,
crustiform-colloform silica and gold over distances of a few centimeters in
veins. We deduce that a downward propagating boiling front and expansion of
two-phase flashing in a flowing water column occurred during episodes of high
vertical permeability and fluid flux in the Empire vein
structure.