Rapid fluid inclusion data
for exploration (decrepitation)
Reported at NTGS AGES 2002 conference, Alice Springs NT
N.T. Geological Survey record 2002-0003
S Wygralak and Terrence P Mernagh
The Palaeoproterozoic Tanami Region is one of the most rapidly developing gold provinces in Australia. Its steadily growing gold resource currently stands at 12.5 Moz, including past production of 4.1 Moz. The entire Region contains some 60 gold occurrences. Most of these are concentrated in three goldfields - Tanami, The Granites and Dead Bullock Soak (DBS). Mined deposits usually contain reserves of 0.01-0.1 Moz Au. Unique in size and a notable exception is the Callie deposit (DBS goldfield). Prior to 30 June 2001, this open cut and underground mine produced 1.7 Moz Au and there is a remaining underground resource of 4.3 Moz Au. Other significant deposits include Groundrush (0.5 Moz Au), Titania (0.3 Moz Au) and Minotaur (0.1 Moz Au). The Coyote deposit in Western Australia also appears to contain significant mineralisation, but its resource has not been announced as yet.
In 1999, the Northern Territory Geological Survey commenced a major multidisciplinary project in the Tanami Province. This was designed to facilitate mineral exploration in the region by provision of a new generation of geological maps and the development of mineralisation models. Results of this work to date have been published in Hendrickx et al 2000, Dean 2001, Vandenberg et al 2001, Wygralak and Mernagh 2001 and Wygralak et al 2001.
Earlier reported metallogenic work (Wygralak and Mernagh 2001) concentrated on the delineation of physico-chemical characteristics, and the origin and evolution of hydrothermal fluids in the Tanami, The Granites and DBS goldfields. Pilot work has also been performed on the newly discovered Groundrush deposit.
Mineralising fluids in each of the goldfields have a unique physico-chemical signature. Fluids in the Tanami goldfield have a temperature range of 120-220oC and contain almost no gases. Mineralisation occurred at shallow depths of 0.4-1.8 km. Gold was precipitated as a result of decreasing pressure and temperature. In The Granites goldfield, fluid temperature was in the range 260-312oC and the fluid contained significant amounts of CO2 mixed with minor CH4 and N2. Gold precipitated due to reaction of the fluid with host rocks containing magnetite, graphite and, in the case of the Bullakitchie deposit, carbonates. The depth of mineralisation is estimated at 3.8-7.5 km. In the Callie deposit (DBS goldfield), mineralising fluid had a temperature of 310-330oC and contained CO2 and N2, but no CH4. Gold precipitation occurred at a depth of 3.2-5.8 km as a result of the reaction of fluid with carbonaceous sediments. In the Groundrush deposit, fluid temperature was in the range 390-430oC. Fluid was dominated by CH4 and there was a minor amount of CO2. Mineralisation occurred at depths of 5.7-8.3 km and phase separation was the precipitation mechanism.
During the 2001 field season, similar studies were conducted on gold occurrences in the Winnecke area, Falchion prospect and White Range deposit. In the Winnecke area, spotty gold mineralisation in quartz veins hosted by the Winnecke Granophyre was previously reported by Otter Gold. Fluid inclusion and Raman spectrometry work on samples from this locality revealed that gold-related fluids had a temperature range of 200-220oC and contained only minor amounts of CO2 and CH4. Fluid inclusion data indicate that gold was precipitated from a boiling fluid at shallow depths of 0.4-0.5 km.
Auriferous fluids in the Falchion prospect had a temperature range of 320-340oC. Raman analysis indicated the presence of CO2, CH4 and N2. These gases occur in extremely variable proportions. This indicates either the presence of several fluids or one fluid, which has interacted with a variety of rocks so as to generate locally high CH4 during reaction with graphitic rocks and N2 during reaction with sedimentary rocks. Another group of inclusions contains graphite, suggesting the presence of an additional strongly reduced fluid. At this stage, there are insufficient data to estimate the depth of mineralisation.
Fluid inclusion and Raman work performed on Heavitree Quartzite-hosted gold mineralisation in the White Range deposit revealed boiling fluids, with temperatures in the range 320-340oC. Gold precipitated at a depth of 2 km as a result of fluid boiling.
An important finding from our work is that granites played no genetic role in gold mineralisation, despite a close spatial relationship between mineralisation and felsic intrusions. This statement is based upon different Pb/Pb isotopic signatures of granites and auriferous sulfides, and on the age difference between the intrusives and mineralisation. The former appear to be about 100 million years younger. Close spatial relationships between felsic intrusives and some gold deposits most likely indicate that the intrusives provided a favourable structural setting for mineralising fluids.
A 1710 ± 20 Ma 40Ar/39Ar age has been obtained for biotite associated with ore-stage veins in Callie and similar younger ages have been obtained from the Titania and Galifrey prospects (Wygralak et al 2001). These are suggestive of a link with the Strangways Event (1720-1730 Ma), which was responsible for widespread deformation and metamorphism in the Arunta province to the southeast of the Tanami Region. This significantly enhances the gold prospectivity of Strangways terranes in the Arunta province.
Except for the Tanami goldfield, where fluids have a strong meteoric water signature, oxygen and hydrogen isotopes of fluids do not distinguish between magmatic and metamorphic origin. The provenance of gold is therefore unclear, but it is likely that gold was scavenged from country rocks by high-pressure fluids circulating along D5 faults and percolating into surrounding rocks. Subsequent fault failures and a resulting sudden drop in pressure reversed the direction of gold-bearing fluids back into fault systems.
pilot study has been conducted into the suitability of acoustic
as a cheap and rapid exploration tool to detect CO2 in fluid
inclusions hosted by hydrothermal quartz veins. In most cases, CO2
is a favourable gas component of fluids associated with gold
and its detection in quartz veins enhances their gold prospectivity.
decrepitometry was conducted on samples with fluid characteristics that
were already known from previous microthermometric and Raman work.
on the samples studied, decrepitometry has proved to be a credible
for detection of CO2 in inclusion fluids that contain CO2-H2O±NaCl.
However, the method has limitations and is unable
to detect CO2 if it is mixed with significant amounts of
gases such as CH4. (This
statement is incorrect - see explanation below)
Note: Decrepitation DOES detect mixtures of CO2 and CH4. Thermodynamics (the gas law) asserts that gases behave identically as temperature increases. But CO2 and CH4 can react to produce a carbon film and reduce the total internal gas pressure of the inclusion. The second author is unable to actually provide me with gas measurement data, and claims this is because it exists only as rough work-sheets and the calculations have not yet been done. In addition, samples sent to the laboratory were misnumbered and the first author declines to resolve the mis-numbering, which renders 3 of the samples as unreliable and/or incorrect. His assertion that decrepitometry does not produce a recognizable low temperature decrepitation peak on CH4 rich fluids cannot be deduced from this seriously flawed data and in fact is inconsistent with the gas law!
The first author's presentation showed a decrepitation graph on a sample number that was never submitted to the laboratory. The report's first author has declined to resolve the sample numbering error and so 3 (at least) of the sample numbers cannot be accepted as reliable data. (K. Burlinson comment)
Our future work will concentrate on: (i) tracing regional changes in the physico-chemical character of fluids; (ii) establishing the difference between fluids in mineralised and distant areas; and (iii) establishing the chronology of fluid flow. The first stage of this work will be focussed on THE GRANITES, HIGHLAND ROCKS, MOUNT SOLITAIRE and MOUNT THEO and will attempt to establish the differences in fluid characteristics between the Tanami and North Arunta provinces.