Detrital zircon U-Pb SHRIMP geochronology as an aid to mapping in the Tanami Region

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.