Pre-1.8 Ga Crustal Evolution of the

Mount Isa Fault Region – Insights

from SHRIMP U-Pb and in-situ Lu-Hf

Analysis of Zircons

Frank P. Bierlein, Lance Black, Janet Hergt, Geordie Mark

predictive mineral discovery

Cooperative Research Centre

SHRIMP U-Pb age dating and laser ablation Hf-isotope analysis of zircons from pre-1.8 Ga

basement rocks on either side of the metallogenically important Mount Isa Fault indicate that

the entire region was part of the same lithospheric block, and that the Mount Isa Fault cannot

represent the surface expression of a Barramundi-aged transcrustal suture as has previously

been proposed (e.g., MacDonald et al., 1997; Hobbs et al., 2000). Magmatic ages of ca. 1.85 Ga

in samples from the Yaringa Metamorphics and from enclaves in the Kalkadoon Granite

constrain the thermo-tectonic history of the proto-Mount Isa Fault region, with ca. 2.4 – 3.2

Ga inherited zircons suggesting that Archaean crust contributed to the source of the

Palaeoproterozoic sequence.

The Lu-Hf composition of zircons analysed by SHRIMP, and the absence of evidence for

large-scale tectonism support the notion that there is no lithospheric break across the Mount

Isa Fault (e.g., Bierlein and Betts, 2004). Furthermore, the 176Hf/177Hf isotope data confirm

that Archaean – Palaeoproterozoic magmatic zircons on both sides of the Mount Isa Fault

were sourced from the same parental lithospheric reservoir which evolved over time from

more primitive mantle to more supracrustal compositions. The results of this study impact on

crustal-scale models for the development of shale-hosted massive sulphide Pb-Zn-Cu

mineralisation and whether trans-lithospheric faults are an important ingredient for the

development of this deposit type.

Frank P Bierlein

Centre for Exploration Targeting

University of Western Australia

35 Stirling Highway, Crawley WA 6009

Phone: (08) 6488 7846

Fax: (08) 64881178

Email: Your fbierlein@tsrc.uwa.edu.au

www.pmdcrc.com.au

1860 MaMcDonald et al. (1997)

MI-829; 1858 Ma

MI-8211; 1853 Ma

MI-827; 1847 Ma

1890 MaPage & Williams (1988)

‘magmatic’population

(= compositionof lithosphereat ca. 1.9 Ga)

inheritedzircons

(= compositionof lithosphere

between 3.4and 1.9 Ga)

(n = 169)

176/177 versus age

2.804E-01

2.806E-01

2.808E-01

2.810E-01

2.812E-01

2.814E-01

2.816E-01

2.818E-01

0 500 1000 1500 2000 2500 3000 3500 4000

Lu-Hf isotopic evolutionary trendof pre-1.9 Ga basement in

western succession east & westof Mt Isa Fault

Yaringa Creek Metamorphics

Kalkadoon Granite mafic

enclave

Kurbaya Migmatites

0.5

mm

Plum Mountain Gneiss

Yaringa Metamorphics (west of Mt Isa Fault);1858 ± 4 Ma (n = 17)

Yaringa Metamorphics (west of Mt Isa Fault);1853 ± 5 Ma (n = 17)

FBMI-829 FBMI-827FBMI-8211

Enclave in Kalkadoon Granite, NE of Mt Isa;1847 ± 3 Ma (n = 20)

References

Bierlein, F.P., Betts, P.G. (2004). The Proterozoic Mt Isa Fault Zone, northeastern Australia – is it really a ca. 1.9 Ga terrane-bounding suture? Earth and

Planetary Sciences Letters, 225, 279-294.

Hobbs, B.E., Ord, A., Archibald, N.J., Walshe, J.L., Zhang, Y., Brown, M., Zhao, C. (2000). Geodynamic modelling as an exploration tool, in: After 2000 – The

Future of Mining Conference.

McDonald, G.D., Collerson, K.D., Kinny, P.D. (1997). Late Archean and Early Proterozoic crustal evolution of the Mount Isa block, northwest Queensland,

Australia. Geology, 25, 1095-1098.

Mt Isa