This was a joint project between Geoscience Australia and the Northern
Territory Geological Survey from 2000 to 2004 to try and determine the
extent of orogenic gold mineral systems in the Tanami and Arunta
regions of central Australia.
Outcrop in the area is sparse (~5%)
and most of gold mineralisation is covered by a blanket of regolith
consisting predominantly of transported sand and laterite. The regolith
is usually 10-20 m deep, but at places may reach more than 100 m.
Our strategy has been to carefully characterise the composition of the
ore-bearing fluids at the major gold deposits in this region and then
to expand our studies further a field. To do this we have collected
over 100 samples of quartz or quartz/carbonate veins that outcrop over
a wide region of the Tanami and Arunta blocks.
Timing of vein
formation and fluid flow are other important facto0rs that need to be
considered and so, where possible, we have focused on veins with
similar orientation to the D5 veins associated with mineralisation or
we have used Ar/Ar dating of biotite or mica within the veins to
determine their age.
The largest gold mine in the area is the Callie mine, which is a
world-class deposit with over 4.5 Moz Au.
It started as an
open-pit but mining now continues underground. Mineralisation is entirely sediment hosted and consists mostly of free
gold within steeply dipping sheeted quartz veins that are typically 2 –
20 cm thick.
Cross section of the Callie mine, Wilson shoot.
2 types of fluid inclusions are observed in the quartz veins. The first
are aqueous inclusions (Type D) with a small vapour phase and salinities up to
20 wt.% NaCl.
The 2nd type (Type A) are vapour-rich inclusions containing CO2 and N2 as well
as a small amount of water. They typically homogenise at temperatures
between 250 and 326 C and have salinities below 8 wt.% NaCl
At The Granites goldfield there is stratabound mineralisation in
quartz-carbonate veins hosted by amphibolite facies BIF that wraps
around the edge of the Inningarra Granite.
Once again we observe 2 and 3 phase CO2 inclusions in the quartz veins.
They homogenise between 240 and 312 C.
We also observe 2-phase aqueous Type D inclusions and Type E inclusions
which contain a small halite cube as well.
At the Tanami goldfield, mineralisation occurs along a lineament known
as the Tanami trend.
Mineralisation occurs in quartz veins in both basalt and sedimentary
units of the Mount Charles Formation.
The Tanami quartz veins contain wispy trails of secondary inclusions.
The veins are dominated by aqueous Type D fluid inclusions with a small
vapour bubble.
Occasional vapour-rich inclusions are due to necking-down processes.
A few rare inclusions also contain an unidentified solid as well.
In a few of the pits we were able to observe primary fluid inclusions
in growth bands in vuggy quartz.
Some of these inclusions consisted of 3-phase CO2 inclusions.
The growth bands also contained aqueous inclusions, some showing signs
of necking down.
Coexisting CO2 and aqueous inclusions suggest that phase separation was
occurring at the time of trapping.
This diagram summarises the fluid inclusion data from the Tanami gold
deposits.
The deposits formed over a range of
depths.
Salinities varied up to 21 wt.% NaCl but CO2-rich
inclusons typically had salinities below 8 wt.%
NaCl.
Homogenisation temps varied from about 240 to 450 C but
there is general overlap of most deposits between 250 and 350
C.
THE MOST IMPORTANT POINT IS THAT ALL DEPOSITS CONTAIN
CO2-BEARING FLUIDS
We also sampled a series of E-W striking veins to the NE of The
Granites mine.
The fluid inclusion data indicate the presence of 2 fluids
the 1st population of inclusions homogenise at temperatures of 320-360 ºC, low
salinity, CO2 > CH4 (graphite, N2).
The 2nd population homogenise at temperatures of 120-230 ºC, have high salinity and no
detectable gases.
The yellow zones in the diagram below show regions where we also observed veins with
crustiform or bladed pseudomorphs which might be interpreted as
epithermal or epizonal textures.
There are 2 populations of inclusions, both homogenising between 120 and 180 C. The first population has very low salinities
while the second population has moderate to high salinity.
No gases could be detected by Raman microprobe.
Ar/Ar dating of mica in veins in this region produced older ages in the
west 1810-1750 Ma (which overlap with the age of granites in this
region)
and slightly younger ages in the east 1740 – 1700 Ma
There appear to be two main sets of outcropping veins in the Birrindudu
region. The first, and often dominant set of veins strike approximately
N – S whereas the other set of veins strike approximately ESE.
As well as these mesothermal veins we also observed some veins with
epithermal textures such as the bladed pseudomorphs and crustiform
textures.
Mesothermal veins at temps 123 – 370 ºC, moderate salinity, CO2
>> CH4.
Epithermal veins at temperatures of 110 – 260 ºC, moderate to high salinity and no
detectable gases.
This region in the Arunta block contained veins with relatively low-temperature inclusions.
The veins themselves sometime had chalcedonic and other texture
suggestive of high crustal level emplacement.
There appears to be both a very low salinity and a high salinity
population of inclusions both with similar homogenisation temps.
There was no detectable CO2 or other gases.
40Ar/39Ar dating of muscovite associated with these veins gives ages
ranging from 1590 to 1430 Ma. Perhaps related to the Chewings Orogeny.
In the Mt Thomas quartzite, we again observe 2 types of fluid inclusions in the veins, both
homogenising over a similar temperature range.
The first population has moderate to high salinities and appears
similar to the higher temperature inclusions in the last diagram.
The second population has low salinities and contains liquid CO2 and
varying proportions of CH4.
This latter population is similar to the ore-bearing fluids in the
Tanami region suggesting that there is potential for similar styles of
gold mineralisation in this area.
40Ar/39Ar dating of muscovite in the veins has returned unusually young
ages of 947 and 1005 Ma. These younger ages may be due to isotopic
resetting caused by the thermal event related to intrusion of the
Stuart Dyke Swarm.
This area along the boundary of the Ngalia Basin contains veins with a
population of CO2-rich inclusions, with a remarkably constant N2/CO2
ratio of 0.06 in the vapor phase.
Clathrate melting temperatures (below zero) indicate high salinities.
Hydrothermal muscovite from a quartz vein in this locality yielded a
40Ar/39Ar age of 1478 +/- 17 Ma. Possibly related to the Anmatjira Uplift
Phase (1500 – 1400Ma).
Quartz veins from these base metal and tungsten mines contain only low temp,
highly saline aqueous inclusions.
Ar/Ar dataing of veins in this region gives ages ranging from
1562 – 1538 Ma. Possibly related to the Chewing Orogeny.
In the region just to the West of the Ngalia basin there are veins with highly saline fluid
inclusions which range in temperature from 230 – 450 C.
The inclusions are gas-rich with the vapour phase consisting mainly of
CH4 with minor CO2 and N2.
The higher temperature of these fluids relative to those in previous
slides, and the more reduced (i.e. CH4-rich) nature of the fluids
suggest that these may be metamorphic fluids.
40Ar/39Ar dating of muscovite from a quartz vein in this locality
returned an age of 1636 +/- 4 Ma, which is synchronous with the Liebig
Orogeny of the Warumpi Province.
Limited work on fluid inclusions from quartz veins from the Warumpi
Province indicate the presence of at least two populations of fluid
inclusions.
The first population of inclusions are highly saline but have
relatively low temperature (170 – 190 C).
The second population of fluid inclusions homogenise at much higher
temperatures (330 – 540 C) and are CO2-rich.
Limited 40Ar/39Ar dating yield ages ranging from 1100 to 918 Ma. Once
again, these younger ages are thought to be due to isotopic resetting
related to the intrusion of the Stuart Dyke Swarm in the southern
Arunta.
Summary
So in relation to mineralisation, the northern area contains fluid
inclusion evidence for the wide distribution of typical orogenic gold
style fluids of similar age to the known gold deposits and with low –
moderate salinities and moderate concentrations of CO2.
The southern region contains evidence for several phases of fluid flow
typically with ages corresponding to the Chewings Orogeny or younger.
There is still a fairly wide distribution of CO2-bearing fluids but the
salinities and homogenisation temperatures seem to be higher than those
in the north. These southern fluids have characteristics similar to
those reported from intrusion-related gold deposits (Thompson &
Newberry, 2000) but further work is required to confirm this.
And these 2 regions appear to be separated by a region of gas-poor
fluids which have greater potential for base-metal or Sn-W styles of
mineralisation.
Clearly, there is a relationship between the fluid types mapped out and
the potential for the occurrence of different mineral systems. The
presence of a high temperature, low salinity, CO2 rich fluid is a good,
but not necessarily unique, indicator for orogenic gold systems. Higher
salinity, CO2-rich fluids may indicate potential for intrusion-related
gold systems while high salinity fluids lacking in CO2 and other gases
may indicate potential for Sn-W and/or base metal mineral systems.
Tabulations of fluids in other mineralised areas of Northern Australia.