Detrital zircon U-Pb SHRIMP geochronology as an aid
to mapping in the Tanami Region
Andrew
Cross
Introduction
The Tanami Region is located approximately 600 km
northwest of Alice Springs and is a major Australian gold province. Straddling
the Northern Territory-Western Australian border, the region is host to over 50
known gold occurrences in established gold fields at: Dead Bullock Soak; The
Granites; and; Tanami and also significant prospects at: Groundrush; Crusade;
Coyote and; Kookaburra. The Tanami region is characterised by an extensive
regolith cover that is punctuated by sparse and variably weathered outcrop,
rarely are the contacts between the various stratigraphic packages observed in
the field. Because of this, the relationships between, and relative timing of
the various packages have been constrained using a combination of: the known
age of intrusive contacts; geophysical interpretations; structural and;
metamorphic criteria. During 2001-2002 Geoscience Australia and the Northern
Territory Geological Survey commenced a joint study to determine whether
detrital U-Pb SHRIMP geochronology can be used as an aid to further
understanding Tanami stratigraphy. The revised stratigraphy is presented in
Figure 1.
Geology
Basement rocks of the Tanami Region are represented by two
known late Archaean inliers, the Browns Range Metamorphics in the northwestern
Tanami and the Billabong Complex southeast of The Granites mine. A banded
granitic gneiss from the Billabong Complex has been assigned a magmatic age of
2514 ± 3 Ma (Page, 1995). The Palaeoproterozic MacFarlane Peak Group is
interpreted as the oldest sequence in the Tanami region and generally consists
of mafic volcanics and volcaniclastic sediments. Overlying the MacFarlane Peak
Group is the Tanami Group which is a thick sequence of clastic sediments
comprising the lowermost Dead Bullock Formation and the upper Killi Killi
Formation. Vandenberg et al. (2001) have suggested that deposition of the Killi
Killi Formation was halted by the onset of the Tanami Orogenic Event (TOE)
between 1825 Ma and 1835 Ma. Following the TOE, Hendrickx et al. (2000) have
suggested a period of extension resulting in felsic volcanism of the Winnecke
Group and also the extrusion of basalt and deposition of volcaniclastic turbidites
of the Mount Charles Formation. Intrusion of granites throughout the Tanami
characterise the period between 1815-1790 Ma. Unconformably overlying the Killi
Killi Formation are the shallow marine sediments of the Pargee Sandstone. This
unit in turn underwent significant peneplanation before deposition of the
overlying siliclastic sediments of the Birrindudu Group.
The
reader is directed to Hendickx et al. (2000) for a more comprehensive review of
Tanami geology.
Figure 1. Time-event diagram for the Tanami
region. Modified from (Crispe, 2002).
Results
of recent Tanami detrital zircon U-Pb geochronology
Sediments
from the MacFarlane Peak Group, Killi Killi Formation and Pargee Sandstone were
sampled for detrital zircon SHRIMP geochronology. In order to maximise zircon
yield, 25-35 kg of rock was sampled at each site. This approach was taken in an
effort to compensate for the generally weathered condition of outcrop. Most
samples contained high percentages of zircons with discordant compositions, presumably
the result of an extended history of weathering.
MacFarlane Peak
Group
Preliminary
SHRIMP data suggest that this group was deposited after about 1865 Ma, slightly
younger than the interpretation by Hendrickx et al. (2000), which suggests
sedimentation of this group was between 1910-1880 Ma. The maximum depositional
age of ~1865 Ma for this group supports the suggestion by Hendrickx et al.
(2000) that it is the lowermost sequence in the Tanami, and also indicates that
it may correlate with the ~1860 Ma Warramunga Formation in the Tennant Creek
Inlier.
Killi Killi
Formation
Zircons
from six samples of Killi Killi Formation confirm this unitÕs widespread
distribution, consistent provenance signature and maximum deposition age. Killi
Killi Formation provenance is dominated by Barramundi-aged detritus (1880-1840
Ma), always contains a subordinate input of ~2500 Ma detritus and has a
remarkably consistent maximum deposition age of about 1840 Ma. Also identified
was a previously unrecognised phase of sedimentation in the Tanami with a
maximum deposition age of about 1815 Ma. This currently unnamed unit, is
non-magnetic in outcrop but overlies highly magnetic rocks, and therefore
contrasts with the non-magnetic Killi Killi Formation (Crispe, 2002).
Pargee Sandstone
This unit was previously interpreted as a syn-orogenic
molasse deposited during the TOE (1825-1835 Ma). SHRIMP U-Pb detrital zircon
results for this sample prove the previous interpretation to be incorrect,
because the youngest zircons define a maximum depositional age of about 1750
Ma. This result has significantly changed the interpretation for the Pargee
Sandstone, and also strongly indicates that the unconformably overlying
Birrindudu Group was deposited sometime after ~1750 Ma.
Conclusions
Detrital zircon U-Pb SHRIMP geochronology is a useful aid
to mapping in regions typified by well-developed regolith cover and poor
outcrop. Individual zircon ages potentially provide maximum depositional ages
for units, and also a provenance signature that can be used to correlate
sedimentary packages both within, and between different geological regions.
References
Crispe, A.J. (2001) Geology
of the Tanami region: in ÔAnnual Geoscience Exploration Seminar (AGES 2002).
Record of AbstractsÕ. Northern Territory Geological Survey, Record.
Hendrickx, M.A., Slater,
K.R., Crispe, A.J., Dean, A.A., Vandenberg, L.C. and Smith, J.B. (2000)
Palaeoproterozoic stratigraphy of the Tanami Region: regional correlations and
relation to mineralisation -preliminary results. Northern Territory
Geological Survey, Record 2000-0013
(electronic pre-release).
Page, R.W. (1995)
Geochronology of an exposed late Archaean basement terrane in The
Granites-Tanami region. Australian Geological Survey Organisation, Research Newsletter 22, 19-20.
Vandenberg, L.C., Crispe,
A.J., Hendrickx, M.A., Dean, A.A. and Slater, K.R. (2001) Geology and
mineralisation of the Tanami gold region: in Annual Geoscience Exploration
Seminar (AGES 2001). Record of Abstracts, Northern Territory Geological Survey, Record, 2001-0006.