The geochronology of the Tennant Creek inlier and its ore deposits, Northern Territory
Creators: name: Compston, David Mark
Publisher: name: The Australian National University
Year: 1994

description: The Early Proterozoic Tennant Creek Inlier, located in northern Australia, has produced over -125 tonnes of gold and -230,000 tonnes of copper from several separate deposits hosted by hydrothermal "ironstones" within the Warramunga Formation. Understanding the origin of these deposits is, in part, dependent on the determination of reliable age constraints on both the mineralisation and its regional environment. Previous studies indicate the gold-copper mineralisation overprints the ironstone host rocks, but had not constrained the time difference between their formation. Isotopic studies of potassic alteration (muscovite) associated with both the gold mineralisation and the ironstone alteration show they are indistinguishable in age, at about 1825 ± 4 Ma. This is contemporaneous with the waning stages of felsic magmatism associated with the Barramundi Orogeny and implies that the magmatism was the source of thermal energy driving the mineralising fluid flow, and supports the interpretation of stable isotope data that there was a magmatic component in that fluid. SHRIMP U-Pb zircon (and baddeleyite) age determinations on crustal components of the Early Proterozoic Tennant Creek Inlier have identified a relatively short (ca 30 my) history for the major magmatic events in the area, including those related to the Barramundi Orogeny, and independently confirms the revisions of the Tennant Creek stratigraphy proposed by the Northern Territory Geological Survey. The oldest known rocks from the area are the volcaniclastic turbidites of the Warramunga Formation, which were deposited in a rapidly subsiding basin at 1862 ± 9 to 1859 ± 13 Ma. Subordinate zircon populations in the greywackes indicate earlier magmatic activity at 1930 ± 12 and 1908 ± 15 Ma. The regional shortening and contemporaneous intrusion of voluminous and chemically homogeneous granites (and associated felsic porphyry dykes), known across northern Australia as the Barramundi Orogeny, occurred here between 1858 ± 12 and 1845 ± 4 Ma. Following uplift, the felsic volcanics and volcaniclastics of the Flynn Subgroup were extruded between 1845 ± 4 and 1827 ± 9 Ma. During this interval, felsic porphyry dykes and minor granite bodies probably related to the Flynn Subgroup volcanism intruded the Warramunga Formation and the lower parts of the Flynn Subgroup. Magma source regions contain components as old as 3.0 Ga, as revealed by zircon inheritance in most of the analysed rocks. The youngest recognised igneous activity in the area are the intrusions of the "Warrego granite" at 1700- 1650 Ma and the Gosse River East granite at 1712 ± 5 Ma. Following the Barramundi Orogeny, deposition of sediments (and volcanics) occurred primarily in the northern and southern parts of the Tennant Creek Inlier. The upper parts of Churchills Head Group, to the north, have a maximum depositional age of 1749 ± 17 Ma, at least 50 Ma younger than the Hatches Creek Group, to the south. The amphibolite-grade metamorphic rocks are not basement to the Tennant Creek area, with U-Pb zircon analyses indicating protoliths ranging in age from 1842 ± 8 to 1827 ± 9 Ma. K-Ar and 40 Ar- 39 Ar of biotite and muscovite ages indicate that metamorphism occurred at about 1700 Ma.