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The Tennant Creek Au, Cu, Fe-oxide deposits, NT, Australia
Introduction
These deposits occur within a province of proterozoic sediments, which
include sedimentary iron formations. Mining was originally for copper,
but the high bismuth levels were a serious problem. Gold became a
primary target later in the life of the field and the main gold ores
were not coincident with the high grade Cu ores. The mines are all
hosted in magnetite-chlorite or haematite rich host rocks, with quartz
as a lesser accessory mineral. Some mineralisation models proposed
entirely syn-sedimentary deposition in the iron formation units.
However, these lack widely dispersed Cu or Au and there must have been
another mechanism to focus and concentrate the ores into discrete
deposits. Hydrothermal events are implicated in order to form the ore
deposits. Because quartz is absent, or is a very late, non-ore stage in
most cases, the only record of the hydrothermal fluids is within the
opaque minerals. In this study, magnetite and haematite were analysed
from numerous ore and background locations within the district. The
baro-acoustic decrepitation of these opaque minerals was often intense,
confirming the presence of fluid inclusions as well as the
applicability of the baro-acoustic decrepitation method to study fluid
inclusions in opaque minerals and quartz deficient deposits.
This map shows the regional geology of the Tennant Creek field with
major mine locations. The locations of some of the mines in this study
are highlighted in red. This map is after Zaw
et. al., Microthermometry and geochemistry of fluid inclusions from the
Tennant Creek gold-copper deposits....., Mineralium
Deposita, 1994, V29, pp 288-300. Additional geological information is published by Ross
Large, Zonation of hydrothermal minerals at the Juno mine, Tennant
Creek Goldfield, Central Australia, Economic Geology 1975, V70 pp
1387-1413.
Summary
The economic ore deposits at Tennant Creek are associated with haematite and magnetite with little or no
quartz. This limits the application of conventional fluid inclusion
methods which require a transparent host mineral. However, the
baro-acoustic decrepitation method can provide some information from
fluid inclusions in opaque minerals. At Tennant Creek, both the
haematite and magnetite can contain abundant fluid inclusions. Although
there is not a direct correlation between the decrepitation pattern and
economic value at the regional scale, the method clearly shows the
great hydrothermal complexity of these ores and that they are not
merely synsedimentary stratigraphic deposits as sometimes proposed. The
baro-acoustic decrepitation method has potential application on a mine
scale for discriminating between individual hydrothermal events to
assist in mapping out variations within the ore which are undetectable
in hand samples.
Discussion
The Warrego mine was the largest and best studied deposit in the field.
Here, the copper and gold occurred as close, but distinctly separate pods
within the magnetite, chlorite, quartz host rock. Magnetite comprises
more than 50% of the host rock. Samples of magnetite show intense and
very variable decrepitation patterns, indicating a complex hydrothermal
history during ore deposition. Note the intense decrepitation of
analysis f216 (blue) which contained
some 100g/t of Au. Analyses f209 and f210 are of differing magnetic
separations of the same rock sample, collected at the surface near the
mine headframe. Analyses f209 and f796 are samples from underground.
Based on the lack of decrepitation below 400 C it is suspected that
these fluid systems lacked a high density CO2 rich fluid component.
Magnetite samples from the Juno and Gecko mines also show very variable
decrepitation. Note that the very rich Au sample at Gecko, f218 has
quite low decrepitation in contrast to analysis f216 at Warrego and
f220 at Juno. The decrepitation response alone does not correlate with
Au content across the field and each deposit has to be studied
individually.
Surface samples from various small diggings show the range of
decrepitation from areas lacking economic mineralisation. Only analysis
f148 (red), from the Ace prospect is of magnetite. Analysis f168
(green) is of quartz and lacks decrepitation above 600 C as expected
for quartz. The other 3 samples were of non-magnetic iron oxides,
probably haematite. Note that haematite can give intense decrepitation
and must be a primary hydrothermal phase rather than a supergene
alteration of prior magnetite, as such supergene alteration would have
destroyed any fluid inclusions and given weak or no decrepitation
response.
The Nobles Nob deposit was almost entirely within haematite host rocks
rather than magnetite, but was a very significant gold producing mine.
It is proof that in this field, magnetite alone is not a guide to the
ore deposits and that hydrothermal systems are the real target. Because the pit was inaccesible, all of
these analyses are of non-magnetic surface materials from near the pit. The peak at 600 C in analysis f1202 (red) might be
due to a component of quartz in the sample.
Although we cannot actually see fluid inclusions in these opaque
minerals, the decrepitation counts have been shown to come from an
irreversible event during heating by running the same sample aliquot
twice, as shown and discussed here.
Because the repeated analysis gives no decrepitation it is certain that
the counts are not merely due to crystallographic changes during
heating and come from the explosive decompression of fluids within
opened inclusions.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
