The Mount Julia-Henty gold deposits are a series of small tonnage (< 500,00
0 t) high-grade (10-30 g/t Au) sheet-like lenses hosted in an extensive pac
kage (> 20 Mt) of quartz-sericite altered volcanics. The alteration system
is hosted in Cambrian submarine volcaniclastic and hyaloclastic dacites ass
ociated with interbedded carbonates and calcareous volcaniclastic samdstone
s located at the Mount Read Volcanics Tyndall Group-Central Volcanic Comple
x boundary.
The alteration zone forms a subvertically dipping tabular sheet over 3 km i
n length and between 10 and 100 m in width. Although this zone is broadly s
trata bound in the north, it crosscuts stratigraphy and is hosted 50 to 300
in deeper to the south, well below the Central Volcanic Complex-Tyndall Gr
oup contact.
The alteration zone is bound updip by the South Henty fault and downdip by
abruptly decreasing alteration intensity. Mineralization is confined to len
ses within the subvertically dipping alteration zone. The alteration zone i
s strongly, deformed due to reactivation of the South Henty fault. The seri
citic alteration assemblage is strongly foliated and mylonitized and the or
ebody is now located on the steeply west dipping, overturned limb of a broa
d, shallowly south plunging syncline.
Alteration is distinctly asymmetric and can be subdivided into three catego
ries: (1) footwall alteration, (2) alteration associated with mineralizatio
n (A zone), and (3) hanging-wall alteration. Alteration in the southern are
a (Mt, Julia) is similar to the north (Zone 96) but has a much lower sulfid
e content. Footwall alteration consists of intensely sericite +/- pyrite +/
- carbonate altered schistose rhyolitic and dacitic volcanics. The main min
eralized zone (A zone) is also zoned from intensely leached, massive quartz
alteration, to quartz-sericite alteration, to an outer quartz-sericite-pyr
ite-chlorite alteration. Minor massive pyrite and massive sulfide lenses ar
e located at the top of the A zone. Hanging-wall alteration consists of chl
orite-albite-quartz alteration in andesitic volcaniclastic rocks and albite
-quartz alteration of rhyolitic volcaniclastic rocks and lavas.
Bedded carbonates and calcareous volcaniclastic rocks are mainly associated
with the upper parts of the A zone alteration zone but are also found towa
rd the foot-wall and well up into the overlying sequences.
The foot-wall and A zone alteration zones are intensely Na2O depleted and K
2O enriched due to feldspar destruction and sericitization. The hanging-wal
l alteration is strongly Na2O enriched due to intense albitization. Aluminu
m and K2O have been strongly depleted from the massive quartz alteration wi
thin the A zone, suggesting leaching by highly acidic fluids.
Mineralization consists of pyrite and chalcopyrite with lesser galena and s
phalerite and with minor gold, electrum, galenobismuth, and native bismuth.
Gold, copper, and bismuth are mainly confined to the massive quartz and qu
artz-sericite alteration zones, whereas the outer quartz-sericite-chlorite
alteration halo is dominated by pyrite. Metal zonation extends from a gold-
silver-rich core associated with copper, lead, and bismuth, to a proximal h
alo of copper, lead, and bismuth, and then to a distal halo of zinc.
Modeling of oxygen and carbon stable isotopes suggests that the carbonates
formed from an early phase of basin-wide magmatic CO2 devolatilization that
commenced early in the hydrothermal event and continued for a long time pe
riod. The carbonates precipitated due to mixing of small amounts of magmati
c CO3-rich fluid with seawater at and below the seawater interface. An alte
rnative model suggests that the observed carbon and oxygen stable isotope s
ignature results from fluid-rock interaction between a preexisting carbonat
e and a later hydrothermal fluid.
The Au-Cu-Bi-Ag association and alteration zonation suggests that the Henty
-Mount Julia system formed from an evolving submarine Cambrian hydrothermal
system, with some low-sulfidation epithermal characteristics. Phase separa
tion of magmatic volatiles from metalliferous magmatic brines formed the al
teration halo first, followed by the mineralizing event. Deposition occurre
d through mixing of the magmatic fluid with bicarbonate/H2S-rich seawater c
irculating through the unconsolidated Lynchford Member and Central Volcanic
Complex volcanics. Mineral deposition was less efficient at Mount Julia wh
ere the magmatic volatile phase was dominant and the late, reduced seawater
influx: less focused, resulting in discontinuous, lower grade mineralizati
on and extensive areas of barren massive quartz and quartz-sericite alterat
ion. The metal zonation is a result of both sulfur availability and decreas
ing temperature.
The Henty-Mount Julia alteration system possibly represents a gold-rich end
member of a large, regional, submarine magmatic copper-gold hydrothermal e
vent. This mineralizing event is probably responsible for most of the depos
its found on the southeastern side of the Henty fault, including the Mount
Lyell deposits.