Uranium and gold deposits in the Pine Creek Orogen (North Australian Craton): A link at 1.8 Ga?

Abstract

The Pine Creek Orogen (PCO) in the North Australian Craton (NAC) hosts world-class unconformity-related U deposits and numerous orogenic or intrusion-related Au deposits, whose timing and genesis are the subject of ongoing controversy. This study reports new geochronology results for an unconformity-related U deposit, Ranger 1 Number 3 orebody, to constrain the timing of processes leading to alteration development and U mineralisation within a regional geological and metallogenic context. It also examines why some U deposits in the eastern PCO contain elevated Au concentrations, unlike their unconformity-related U counterparts in Canada and elsewhere. Key results are as follows. (i) The Ranger 1 deposit and the PCO Au deposits share several characteristics of hydrothermal mineralogy, including chlorite—white-mica—monazite—(±xenotime)-bearing alteration assemblages, some sulfides (pyrite, chalcopyrite), silica (quartz veining or silicification), and Au. (ii) The primary host structure at the Ranger 1 deposit is a shear zone which was reactivated at low-grade metamorphic conditions, as during formation of many of the syn-tectonic Au deposits in the PCO. (iii) Ion probe U–Pb dating of low-Th monazite in an assemblage of hydrothermal white-mica and Fe-rich chlorite from an outer alteration zone at the Ranger 1 No. 3 orebody yielded an age of 1800±9Ma. This age is ∼60 million years earlier than the oldest previously reported age of unconformity-related U mineralisation in the PCO and is within error of several U–Pb ages reported for monazite and xenotime associated with Au mineralisation in the PCO. (iv) Iron-rich chlorite intimately associated with the dated monazite is similar compositionally and texturally to chlorites in alteration zones at other U deposits in the PCO.Two scenarios for the formation of the Ranger 1 deposit are consistent with the available evidence. We prefer a two-stage model where an Fe-chlorite—white-mica—monazite hydrothermal alteration assemblage, with probable gold was introduced replacing medium-grade metamorphic assemblages within a reactivated shear zone at ca. 1800Ma, contemporaneous with the formation of some Au deposits within the PCO. Subsequently, U ore was formed within this environment at ∼1740Ma and/or later, via the reaction of oxidised U- and Mg-bearing basin-derived fluids with chemical reductants in the basement.The new U–Pb geochronology result for monazite points to an important hydrothermal event at the site of the Ranger 1 deposit during regional tectonothermal activity of the Shoobridge Event at ∼1.8Ga in the PCO, and suggests that this pre-ore alteration and tectonism was a key factor in localising later unconformity-related U mineralisation. The pre-ore alteration at ∼1800Ma at the Ranger 1 deposit therefore provides a link between the genesis of the unconformity-related U deposits and the orogenic or intrusion-related Au deposits that formed in the PCO during the Shoobridge Event. At the broader scale, the newly identified ∼1800Ma hydrothermal activity at the Ranger 1 deposit may be part of a craton-wide metallogenic and tectonothermal event at ∼1.8Ga that includes Au mineralisation in the PCO and in The Granites-Tanami Orogen, as proposed in other studies.