US EPA

1439998 

Journal Article 

Geology of the Ranger Uranium Mine, Northern Territory, Australia: structural constraints on the timing of uranium emplacement 

Hein, KAA 

2002 

1 

Ore Geology Reviews
ISSN: 0169-1368
EISSN: 1872-7360 

20 

3-4 

83-108 

WOS:000177978400001 

The geology of the No I and 3 pits at the Ranger Mine in the
Pine Creek Inlier (PCI) of Australia is dominated by Palaeoproterozoic volcanic, carbonate and
sedimentary sequences that unconformably overlie Archaean granitic gneiss of the Nanambu Complex
(2470 +/- 50 Ma). These sequences are folded, faulted and sheared, and crosscut by east-trending
granite (sensu stricto) dykes and pegmatite veins, and gently dipping N-NE trending mafic dykes
of the Oenpelli Dolerite ( similar to 1690 Ma). Regional metamorphism is to greenschist facies
and contact metamorphism is to homblende-homfels facies. The rocks of the Ranger Mine have been
subjected to at least two phases of ductile-brittle deformation (D2-D3) and one phase of brittle
deformation (D4). These events were preceded by regional diastathermal or extension-related
metamorphism (D1) and the development of an ubiquitous bedding-parallel cleavage (SI). D2
resulted in the development of NNE-NNW trending mesoscopic folds (F2) and a network of thrusts
and dextral reverse shears. The modelled palaeo-stress directions for the emplacement of
pegmatite veins suggests that they formed early in D2. D3 resulted in the development of WNW-NW
trending mesoscopic folds (173), a weakly defined axial planar cleavage (S3) and sinistral
reactivation of D2 shears. D2-D3 are correlated with deformation during the Maud Creek Event of
the Top End Orogeny (1870-1780 Ma), while the emplacement of granite dykes and pegmatite veins is
correlated with emplacement of regional granites at 1870-1860 Ma. D4 is associated with brittle
deformation and resulted in the development of normal faults and fault breccias during a period
of east-west extension. This event is correlated with regional east-west extension during
deposition of Palaeo- to Mesoproterozoic platform sequences. The sequence of tectonic events
established in this study indicates that uranium-bearing ore shoots in the Ranger No I and 3 pits
formed during extension in D4, and after emplacement of the Oenpelli Dolerite at 1690 Ma.
However, the currently accepted 1737 +/- 20 U-Pb Ma age places the mineralising event at time of
regional post-orogenic erosion, after the Top End Orogeny and before emplacement of the Oenpelli
Dolerite and extension in D4. The U-Pb age is not consistent with Sm-Nd ages for primary uranium
mineralisation at Nabarlek and Jabiluka at 1650 Ma [Econ. Geol. 84 (1989) 64] and does not concur
with currently accepted regional tectonic data of Johnston [Johnston, J.D., 1984. Structural
evolution of the Pine Creek Inlier and mineralisation therein, Northern Territory, Australia.
Unpublished PhD Thesis, Monash University, Australia], Needham et al. 

Uranium; Ranger Mine; Palaeoproterozoic; structure controls; tectonics