Summary of results. Laser ablation ICP-MS in situ apatite geochronology of the base metal and copper-gold-bismuth deposits of the Rover field, and the copper-nickel mineralisation of the Bluebush area, Warramunga Province

Abstract

This Record presents isotope and chronologic data collected on apatite at the Australian Research Council Centre of Excellence in Ore Deposits at the University of Tasmania. Apatite U–Pb geochronology analyses were carried out from April to June 2021 in support of Northern Territory Geological Survey’s (NTGS) Resourcing the Territory initiative. Imagery, isotope data and interpretations for apatite geochronology were derived from five drill core samples taken from four mineralised sites in the Warramunga Province, Northern Territory. Apatite was analysed in order to constrain the timing of mineralisation at the copper–gold–bismuth Rover 1 deposit, the copper–silver–lead–zinc Explorer 108 deposit and its satellite lead–zinc–copper Curiosity prospect, and the copper–nickel–chromium mineralisation in the Bluebush area.

Two samples from the Rover 1 deposit (BW20PGF127A and BW20PGF094A) yielded apatite with very low U content, resulting in unresolvable ages. The Rover 1 mineralisation assemblage includes coffinite and uraninite as accessory phases. A possibility is that these accessory minerals fractionated and accumulated most of the available U during mineralisation, resulting in U-depleted apatite.

A sample of chlorite-altered, magnetite-hematite-rich siliciclastic rock (GS19DLH0060A) from the mineralised zone of the Curiosity prospect yielded a lower intercept age of 1740 ± 55 (2s) Ma, interpreted to record the timing of mineralisation of this unit. A sample of talc-chlorite-carbonate-hematite-altered rock (GS20PGF133) from the core of the Explorer 108 orebody yielded a lower intercept age of 1724 ± 77 (2s) Ma, interpreted to record the timing of base metal mineralisation remobilisation. Apatite from the porphyritic actinolite basalt (TC20PGF070) collected in the Bluebush area yielded a lower intercept age of 1758 ± 78 Ma (2s), interpreted to represent the age of the orthomagmatic copper-nickel mineralisation and a proxy for the crystallisation age.

The estimated ages of base metal mineralisation in the Rover field and the orthomagmatic copper-nickel mineralisation in the Bluebush area are younger than the ca 1850 Ma gold-copper-bismuth mineralisation of the Tennant Creek mineral field, irrespective of the large apatite age uncertainties. However, they are closer in age to that of other mineralisation in the region, ie the copper-tungsten mineralisation at the Explorer 27 prospect (1711 ± 8 Ma and 1719 ± 8 Ma Re- Os model ages), and the shear-hosted gold-copper-bismuth metallogenic event at Orlando East and Navigator 6 prospects (SHRIMP U-Pb-Th monazite ages of ca 1659 Ma).

The rare earth element (REE) composition of apatite from the Curiosity prospect (GS19DLH0060A) and Explorer 108 (GS20PGF133) are alike and characterised by light rare earth elements (LREE) enrichment over heavy rare earth elements (HREE). However, differences in Eu/Eu* anomaly values and relative LREE and HREE abundances outlines two different apatite groups: group 1 has a concave REE pattern and negative Eu/Eu* anomaly; and group 2 has positive Eu/Eu* anomaly, higher common lead and higher LREE and HREE. Apatite in group 2 is associated with sphalerite (zinc) and galena (lead), whereas apatite in group 1 is hosted in a chlorite-biotite-rich domain (GS20PGF133). Apatite in the Bluebush sample (TC20PGF070) also yielded two different REE patterns: an apatite group with a flat REE pattern and negative Eu/Eu* anomalies, and a second group associated with mineralisation, characterised by lower REE content and positive- to nearpositive Eu/Eu* anomalies. It is interpreted that the apatite with positive Eu/Eu* anomaly and overall lower REE content formed under hydrothermal influence (Curiosity and Explorer 108) or local mineral assemblages (sulfide-allanite-epidote in Bluebush). The apatite with negative Eu/Eu* anomaly and overall larger REE content represents earlier phases of mineralisation (for Curiosity and Explorer 108; group 1) or magmatic apatite (Bluebush), which are chronologically indistinguishable from the apatite related to mineralisation (group 2).