The origin and evolution of heavy rare earth element mineralisation in the Browns Range area, Northern Australia

This thesis investigates a regional-scale heavy rare earth element (HREE) mineralisation style that appears as several structurally-controlled orebodies distributed from the Halls Creek Orogen to the Tanami Region, in an area labelled the North Australian HREE+Y (NAHREY) mineral field. The ore minerals consist only of xenotime [(Y,HREE)PO₄] and minor florencite [LREEAl₃(PO₄)₂(OH)₆], and occur mainly near a regional unconformity between the Archean metasedimentary rocks of the Browns Range Metamorphics (BRM) and overlying Paleoproterozoic Birrindudu Group sandstones in northwest of the Tanami Region. The BRM are medium- to coarse-grained arkosic metasandstones that host the bulk of the HREE mineralisation in the NAHREY mineral field. The BRM consists mainly of detrital quartz and feldspars with minor granitic lithic fragments. Isotopic data acquired from detrital zircons from the BRM and intruding felsic igneous rocks yielded a well-defined age of ca. 3.2 to ca. 3.0 Ga, with relatively radiogenic εHf values (εHf = –1.7 to +5.1), indicating derivation from a Mesoarchean granitic basement of juvenile origin, and deposition in a continental rift basin setting. The sedimentation is constrained to between the ca. 3.0 Ga age of the source rocks and ca. 2.5 Ga age of the felsic igneous bodies that cross-cut the BRM. The ca. 2.5 Ga zircons from the felsic igneous rocks have εHf model ages comparable to those of the ca 3.2 to ca. 3.0 Ga detrital and inherited zircons (ca. 3.4 to ca. 3.1 Ga), consistent with formation via partial melting of the BRM, or the Mesoarchean granitic basement. The unconformably-overlying Gardiner Sandstone of the Birrindudu Group contains detrital zircons of ca 2.6 to ca 1.8 Ga age with no trace of Mesoarchean age, which discounts a significant contribution from the underlying BRM. A detailed paragenetic study of the mineralisation revealed; (1) a pre-ore stage displaying mostly a greenschist-facies overprint, with the detrital/metamorphic minerals including quartz (several generations), alkali feldspar, plagioclase, and coarse-grained muscovite aligned in the pre-mineralisation foliation; (2) syn-ore quartz and white mica alteration associated with a complex multi-stage mineralisation of the ore minerals, primarily in breccias and veins; (3) a post-ore stage characterised by several generations of quartz, hematite, barite, anhydrite and pyrite veining and brecciation. Isotopic dating of xenotime ore from across the NAHREY mineral field constrained the main stage of ore formation to between ca. 1.65 Ga and ca. 1.60 Ga, which is significantly younger that the pre-ore muscovite ⁴⁰Ar/³⁹Ar age of ca. 1.72 Ga that corresponds to a regional metamorphism. The ca. 1.65-1.60 Ga timeframe does not correlate to any local magmatism or orogeny but was coincident with the collision of the North Australian Craton with the Arunta Inlier and Laurentia and subsequent initiation of the Isan and Liebig Orogenies. Far field stresses from these craton-scale events potentially acted as drivers of large-scale fluid flow and fault (re)activation that led to the HREE ore formation. Ore petrography indicates multiple stages of xenotime and florencite crystallisation and recrystallisation. Early xenotime (up to 1 mm), coexisting with early florencite, appears in breccias (breccia-hosted) and mineralised quartz veins (vein-type). Late xenotime (<100 μm) occurs largely as pyramid-shaped overgrowths on the pre-existing xenotime and coexists with late florencite that mainly replaces early xenotime and also appears as narrow rims on early florencite. Compared with early xenotime, the late xenotime overgrowths are richer in the HREE and more depleted in P and LREE, owing to crystallisation of late florencite. Moreover, early florencite has a nearly pure florencite composition whereas the late florencite is defined by a broad chemistry including components of svanbergite, goyazite and woodhouseite. Both xenotime and florencite incorporated quantities of trace elements via a number of substitution mechanisms. High U content of xenotime and composition of early florencite potentially support a genetic association between the HREE mineralisation and the coeval U deposits of northern Australia that formed across the same basin. Samples of the BRM are variably depleted in HREE compared to sedimentary protoliths, and also have unradiogenic Nd isotope compositions that are comparable to the orebodies, but quite distinct from the igneous rocks or other sedimentary rocks (Birrindudu Group) from across the North Australian Craton. These observations demonstrated that the ore metals were derived directly from the BRM. Moreover, investigation of a large number (ca. 550) of primary fluid inclusions from both mineralised and barren quartz veins, revealed three types of hydrothermal fluids available only in the mineralised samples including type I low salinity H₂O-NaCl (largely <5 wt.% salinity; consistent with meteoric water), type II medium salinity H₂O-NaCl (12-18 wt.% salinity) and type III high salinity H₂O-CaCl₂-NaCl (up to 25 wt.% salinity). The trapping temperature and pressure during the ore formation was between 100 to 250 °C and between 0.4 and 1.6 Kbar, respectively. Trace element analysis detected Y, Ce and Cl only in the type III fluid inclusions, which indicates that transportation of ore metals was (at least partly) by Cl complexes in the type III fluid. The P required for phosphate ore mineral formation was likely transported by the type I fluid. Moreover, mineralised quartz samples returned δ¹⁸Ofluid values in the range defined by the BRM (δ¹⁸Ofluid = +1.8 to +5.2‰) and the Birrindudu Group sandstones (δ¹⁸Ofluid = +8‰). Combining whole-rock, fluid inclusion and isotopic data, an ore genesis model is developed that suggests mixing of at least two hydrothermal fluids, one (type III) leached HREE+Y from the BRM and moved upward along fault structures in the vicinity of the regional unconformity, and there mixed with another down-flowing P-bearing fluid (represented potentially by the type I fluid inclusions) originated from the Birrindudu Group sandstones. Leaching of ore metals was greatly enhanced by halogen (Cl, F) complexes. Introduction of P during fluid mixing/dilution and an increase in pH as recorded by the syn-ore muscovite alteration, resulted in HREE deposition. Globally, the closest analogue to the NAHREY ore deposits is the Maw Zone, which formed in a very similar geological setting in the Athabasca Basin, Canada. Collectively, this style of REE mineralisation is unlike any other known REE ore style, and is herein labelled "Unconformity-Related REE deposit". There is great potential for further unconformity-related REE deposits to be found in intercontinental basins in close proximity to regional unconformities between Archean basement rocks and overlying Proterozoic sedimentary sequences.

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