The Ranger uranium mineral system, and temporal-spatial controls on the formation of unconformity-related uranium deposits in the Pine Creek Orogen, Northern Territory, Australia  

Authors / CoAuthors

Skirrow, R.G. | Mercadier, J. | Armstrong, R. | Kuske, T.J.

Abstract

The Ranger 1 unconformity-related uranium deposit in the Northern Territory of Australia is one of the world's largest uranium deposits and is currently Australia's second largest producer of uranium. Mineralisation at the Ranger, Jabiluka and other major unconformity-related deposits in the Alligator Rivers Uranium Field (ARUF) occurs in Paleoproterozoic metamorphic basement rocks immediately beneath the unconformity with the overlying and partly preserved Paleo- to Mesoproterozoic McArthur Basin. New geochronological data and observations of the relative timing of tectonothermal and hydrothermal events at the Ranger 1 number 3 orebody provide fresh insights on the spatial-temporal controls on ore formation in this deposit and more broadly in the ARUF. Uranium mineralisation and associated alteration at Ranger were fundamentally controlled by reactivated D1 shear zones that were initially localised by rheological contrasts and graphite-bearing units within the metasedimentary Cahill Formation. A new model of the Ranger 1 ore-forming mineral system attempts to reconcile the key features of the Ranger deposit and those of other major deposits and regional alteration in the ARUF. The results of the study demonstrate the importance of pre-ore structural and chemical architecture of the basement in controlling the location of uranium mineralisation. As in most recent models, McArthur Basin-derived, oxidised, diagenetic brines are envisaged as crucial in transporting uranium downwards into reduced basement rocks where uranium was deposited either by fluid-rock reaction and/or by fluid mixing. However, we propose a different architecture of fluid flow in which these basinal fluids penetrated extensive volumes of basement rocks beneath the unconformity to leach uranium, particularly from Archean felsic orthogneisses across basement paleo-highs. Possibly driven by convection and/or extensional tectonism during the period ~1725 Ma to ~1705 Ma the uranium was transported laterally through microfractures in this regional sub-unconformity alteration zone, along the unconformity, and via fault networks until chemical gradients were encountered in D1 shear zones in the Cahill Formation. Uraninite deposition at Ranger most likely resulted from reduction of ore fluids via reaction with pre-existing Fe2+-bearing minerals and/or via fluid mixing, although the evidence for mixing remains cryptic. Simultaneously, intense Mg metasomatism generated very acidic and silica-rich fluids that migrated laterally and downwards within the D1/D2 shear zones to dissolve and silicify adjacent carbonate rock units, explaining the observed spatial association of chloritic ore breccia adjacent to, and in places above, thinned and silicified carbonate rock units, both at the Ranger and Jabiluka deposits. The possible role of fluid mixing above and below the unconformity requires further study to better understand why no significant uranium deposits have yet been discovered within the McArthur Basin, a characteristic that appears to differentiate the ARUF from the Athabasca Basin in Canada.

Product Type

nonGeographicDataset

eCat Id

82512

Contact for the resource

Keywords

• External PublicationScientific Journal Paper

( Theme )

• economic geology

( Theme )

• geochronology

( Theme )

• metallogenesis

( Theme )

• uranium

Australian and New Zealand Standard Research Classification (ANZSRC)

• Earth Sciences

• Published_Internal

Publication Date

2014-01-01T00:00:00

Creation Date

Security Constraints

Australian Government Security ClassificationSystem    

Classification - unclassified

Legal Constraints

Creative Commons Attribution 4.0 International Licence    

Access - license

Use - license

Status

Purpose

Maintenance Information

notPlanned

Topic Category

geoscientificInformation

Series Information

Lineage

Sampling at the Ranger 1 deposit was undertaken in 2009 and 2010, and analyses of samples undertaken at Geoscience Australia, the Australian National University, and the University of Lorraine (Nancy, France).

Parent Information

Extents

Reference System

Spatial Resolution

Service Information

Associations

Downloads and Links

Source Information

New analytical data by Geoscience Australia, stored iin the whole-rock geochemical database and in the geochronology database (SHRIMP zircon results).