Abstract

Although circumstantial evidence from ore deposit mineralogy and geochemistry can imply potential sources for fluids and metals, rarely is direct evidence for metal leaching from source rocks seen in the vicinity of deposits. Here, we investigate the source of metals for a series of fault zone– and shear zone–hosted uranium occurrences in the Mount Isa inlier, Australia. As well as containing uranium, these deposits are enriched in Zr and rare earth elements (REEs), requiring that unusual fluids were responsible for addition of these typically immobile elements. During the Isan orogeny, highly saline metamorphic fluids infiltrated the sheared margin of a highly evolved granite intrusion, which contains elevated U, Th, F, Zr, and REEs. Synchrotron X-ray fluorescence spectroscopy and high-resolution electron microprobe mineral mapping show that this radioactive element–rich characteristic caused zircon crystals to become highly metamict, allowing the elements therein to become mobile. Thus, when orogenic fluids pervasively infiltrated along shear zones ∼100 m.y. after granite intrusion, their unusually saline character allowed enhanced dissolution of regional carbonates and fluorite from the granite, providing the ligands needed for transport of uranium and the normally immobile elements from the metamict zircons.

INTRODUCTION

Determining the origin of metals that form metamorphogenic ore deposits is often difficult and controversial. Although circumstantial evidence can be drawn from deposit mineralogy, geochemistry, and age dating, rarely is direct evidence for metal leaching from potential metasedimentary or igneous source rocks obtained. This is an important issue because understanding source processes helps determine which regions have greater ore potential. Source region metallogeny is consequently one of the hot research topics in economic geology (see Muntean et al., 2011; Lee et al., 2012; Tomkins, 2013; Tomkins and Evans, 2015). In the Mesoproterozoic Mount Isa inlier (Queensland, Australia), there is a series of unusual uranium occurrences with elevated Zr and rare earth elements (REEs) that are spatially but not temporally associated with the granitic Sybella batholith (Fig. 1). The batholith formed over 1671–1655 Ma (Page and Bell, 1986; Hoadley, 2003; Gordon, 2004), whereas the largest U deposit likely formed during regional metamorphism at 1590–1565 Ma (Polito et al., 2009; Wilde et al., 2013). A lack of equivalent-aged batholithic intrusions implies that regional metamorphic fluids mobilized U during the Isan orogeny, thus forming orogenic U deposits.

Given the spatial association, and that the Sybella batholith is anomalously enriched in U, REEs, and fluorine (Budd et al., 2001), we investigated whether regional fluids could have scavenged U from the intrusions. Once crystallized, felsic intrusions are generally not considered to play an active role in ore deposit genesis. However, some felsic intrusions can be highly anomalous in incompatible U, Th, and rare metals relative to other rocks because these elements become enriched through protracted fractionation. This enrichment makes granites a plausible passive source of U, provided that orogenic fluids can somehow interact with and extract metals from these typically impermeable rocks. Indeed, Oliver et al. (1999) suggested that leaching of the U-rich Burstall Granite (and/or adjacent skarns) may have facilitated formation of the Mary Kathleen uranium-bearing REE (U-REE) deposit, 50 km to the east of Mount Isa.

Fine-grained accessory minerals and grain boundaries are considered the major reservoirs for U, Th, REEs, and Zr in granites (Bea, 1996; Hiraga et al., 2004). However, the accessory minerals that contain these elements, particularly zircon, tend to be relatively stable in the presence of ordinary metamorphic fluids; the neutral-pH, weakly oxidized, low-salinity fluids associated with orogenic gold deposits typically do not extensively mobilize the elements in these accessory minerals. Zircon is used as a geochronometer specifically because it is stable for billions of years when it contains low U concentrations. Valhalla, the largest uranium deposit in the Mount Isa region, is anomalously enriched in Zr and REEs, and contains throughout, an unusual U-Zr–rich silicate mineral with up to ∼20% UO2. These deposits are also free of quartz veins associated with main-stage mineralization, but contain abundant fine calcite and dolomite veins and are fluorine enriched; ore-forming fluids were thus distinct from orogenic gold-related fluids found globally.

We used a combination of synchrotron X-ray fluorescence and high-resolution electron microprobe element mapping to compare the mineralogical and textural setting of U, Th, and Zr in primary Sybella batholith granite with samples from the margin of the intrusion, which was subject to ductile deformation and metasomatism during regional metamorphism. The results provide an explanation for the liberation of U via dissolution of zircons, which occurs through a combination of early metamictization and subsequent interaction with unusually saline fluids. We show that U loss and remobilization of Zr and Th occurred during orogenesis-driven fluid flow through the sheared granite margin. The results have important implications for understanding the source, transport path, and types of regional fluids needed to form orogenic uranium deposits.

GEOLOGICAL SETTING

The anorogenic Sybella batholith is one of several granitic intrusions anomalously enriched in radiogenic heat-producing elements within the Mount Isa inlier (McLaren et al., 1999). The Sybella batholith was emplaced ∼100 m.y. before regional greenschist to mid-amphibolite facies metamorphism during the Isan orogeny (Page and Bell, 1986; Hoadley, 2003; Gordon, 2004). Peak metamorphism during the D2 event occurred at ca. 1600–1570 Ma (Rubenach, 1992; Giles and Nutman, 2002; Hand and Rubatto, 2002; Rubenach, 2008).

We studied the most radiometrically anomalous Kitty Plain Microgranite (KPM) phase of the Sybella batholith, which is closest to several U occurrences hosted in interbedded metabasalt and metasediments of the Eastern Creek Volcanics (Fig. 1A). Here, the sheared eastern KPM margin links to the regional network of major first-order fault and shear zones and related second- and third-order albitized faults that host the U occurrences. These deposits are unusual for their high Zr, REE, and F content (Gregory et al., 2005; Polito et al., 2009) and uranium-zirconium silicate mineralogy (Wilde et al., 2013). A near-peak metamorphic timing for mineralization has been interpreted based on the observation that metasomatic riebeckite, resulting from sodic alteration of mafic rock, is oriented parallel to the peak-metamorphic D2 foliation (Wilde et al., 2013). This riebeckite has an oldest 40Ar/39Ar step age of 1566 ± 11 Ma and is intimately associated with brannerite that produced a U-Pb discordia age of 1564 ± 27 Ma (Polito et al., 2009).

METHODS

Samples were collected from undeformed KPM and its sheared eastern margin. Fusion disks were analyzed for trace elements by laser ablation–inductively coupled plasma–mass spectrometry at ALS Geochemistry Brisbane (Australia) and Acme Analytical Labs Ltd. (Vancouver, Canada). Polished thin sections and thin wafers were prepared using kerosene to avoid water-induced alteration of sensitive minerals. Airborne radiometric data were obtained from Geoscience Australia (Canberra), processed using Leapfrog software (http://www.leapfrog3d.com) by normalizing U and Th concentrations in granite rocks against average country rock, and a Th/U image generated.

At the Australian Synchrotron (Clayton, Victoria; XFM beamline), we used a Maia 384 detector array to perform X-ray fluorescence mapping (SXRFM). The intensity of the synchrotron X-ray beam allows information to be collected on element distribution throughout the thickness of the section being analyzed. Eight rock wafers and thin sections 100 μm thick were examined. X-rays were focused to an ∼1.5 μm beam spot using a Kirkpatrick-Baez mirror–based lens system, and the sample was rastered through the beam using a dwell time of ∼1–2 ms per pixel (at 16.5 KeV). Platinum, Mn, Ni, and Fe foils were used to calibrate the beam. This system enables rapid element mapping across large areas at high spatial resolution (e.g., ∼2 cm2 in 4.5 hr at ∼5 μm resolution). First-pass low-resolution element maps were rapidly compiled before collecting higher-resolution data at lower detection limits over small areas of interest. Qualitative element maps of the distribution of Ca, Fe, Ti, Zn, Cu, Mn, Sr, Rb, U, Th, La, Ce, Y, Hf, Pb, and Zr were generated following deconvolution of the fluorescence emission spectra using GeoPIXE software (http://nmp.csiro.au/Geopixe.html; following Ryan et al., 2014).

High-resolution element maps were then collected using a JEOL 8500 CL-FEG-EPMA (field emission gun electron probe microanalyzer) HyperProbe and converted to mineral maps using a cluster analysis approach. Because EPMA mapping collects data from the top few microns of a polished section, the images have a sharper resolution than the synchrotron element maps. Also, the synchrotron system was limited to elements with atomic number >20, so although a single EPMA map can take >12 h to collect, the results complement the synchrotron work with more detailed mineralogical maps. Operating conditions were 12 kV accelerating voltage, 60 nA beam current, 400 nm step size, and 40 ms per step dwell time.

RESULTS

Primary KPM is distinctly pink colored and contains moderately abundant 1–3 cm fluorite-biotite clusters. Within the marginal shear zone, the granite has been bleached white through albitization and the biotite-fluorite clots have been destroyed. Chloritization indicates greenschist facies fluid through-flow. Whole-rock geochemistry indicates that this marginal zone is U, F, and K2O depleted and has elevated Th/U ratios (Fig. 2; Table DR1 in the GSA Data Repository1). Significant Zr loss from the marginal zone was not detected, but variability in whole-rock Zr concentration in granite shear-zone samples suggests local Zr mobility. Airborne radiometrics (Fig. 1B) also indicate that the marginal KPM has elevated Th/U and thus was subject to significant U loss. Similar high-Th/U lineaments within the KPM, varying in orientation from north-northeast to northwest, are connected to the marginal shear zone and thus to the major regional north- to northwest-trending fault network and U occurrences (Fig. 1B).

SXRFM shows that uranium has been mobilized along grain boundaries in the vicinity of metamict zircon (Fig. 3). Subsequent high-resolution EPMA mapping of key areas identified by SXRFM reveals that mobilized U and Th are associated with minerals produced by zircon metamictization (Fig. 4). Zirconium silicate minerals with varying proportions of U and Th occur in microveinlets that penetrate grain boundaries between common silicates, fluorite, and REE fluorocarbonate. In samples from the sheared granite margin, there are numerous veinlets of Th-Zr silicates with low U content on grain boundaries and within pervasive fractures associated with chloritized biotite. Monazite veinlets are also evident and rutile is a common accessory phase, whereas fluorite was not detected.

DISCUSSION

The similarity between the granite trace element geochemistry and that of rare uranium-zirconium silicate minerals in nearby U deposits suggests a genetic link. The enrichment of Zr, F, P, and REEs in these deposits is highly unusual because Zr and REEs are immobile in typical orogenic fluids found in other settings (e.g., Rubin et al., 1993); for example, orogenic gold deposits are not enriched in these elements. A satisfactory explanation for U deposit formation here must adequately explain this unusual element suite; U can be transported by a range of fluid types, but Zr mobilization requires special fluids.

The SXRFM and EPMA results indicate that zircon stability controlled the mobility of U, Th, and Zr in the KPM, as in many granitoid complexes globally. The U-Th-Zr silicate–filled microveinlets indicate localized mobilization of these elements in the granite, and the intimate association of these microveinlets with metamict zircon suggests that the elements were mobilized from that source. If zircon forms with high U and Th it can quickly become metamict through radiation damage (Meldrum et al., 1998), releasing these elements and allowing their subsequent mobilization during deformation and hydrothermal fluid infiltration if the fluid chemistry is appropriate for mobilization (cf. Nasdala et al., 2010). Given the well-known stability of non-metamict zircon, we suggest that this process would only be possible in granites with unusually high proportions of radioactive elements.

The U loss from the sheared granite margin, indicated by the U-depletion signature in the whole-rock geochemistry data and the airborne radiometrics, implies that deformation during the Isan orogeny was responsible for uranium mobilization. Because the delicate U-bearing microveinlets are focused on grain boundaries in the primary granite (Figs. 4B and 4C), they would have been highly susceptible to fluid attack in the marginal shear zones given that deformation and fluid flow is focused at grain boundaries during shearing.

The contrast between low Th/U in the Valhalla deposit (<<0.5; data from Polito et al., 2009) and high Th/U in the granite margin (>5) can be investigated by considering experimental data. In experiments at 2 kbar and 750 °C, Keppler and Wyllie (1991) showed that U and Th are partitioned effectively into fluids from melts by fluoride complexes. However at the same conditions, addition of CO2 inhibits Th mobility while permitting CO2 complexation with U. Zirconium is also able to complex with F [as ZrF(OH)3 and ZrF2(OH)2] under similar hydrothermal fluid conditions (Migdisov et al., 2011). Similarly, Ayers et al. (2012) and Wilke et al. (2012) showed that highly sodic hydrothermal fluids are able to transport Zr. The albitization of both the sheared granite and the U deposit host rocks is consistent with prior work (Oliver et al., 2004) showing that large areas of the Mount Isa inlier were subject to extensive infiltration by highly saline, CO2-rich fluids during orogenesis. Because both fluorite and carbonate dissolution increase exponentially with increasing salinity (Newton and Manning, 2002; Tropper and Manning, 2007), we suggest that this orogenic fluid infiltration event promoted dissolution of fluorite and REE fluorocarbonates in the sheared granite margin, allowing concurrent CO2 and F complexation with U, Zr, and REEs.

Given that the internal high-Th/U lineaments and granite margin shear zone are connected to the extensive regional shear system, and thereby to the U occurrences (Fig. 1), we suggest that the Sybella batholith is the primary source of the U, Zr, F, and REEs in the deposits. Although regional metasedimentary rocks contain detrital zircon, these are likely to be weakly radioactive zircons that do not become metamict over time (detrital minerals are robust and therefore not metamict at the time of sedimentation), and so would be resistant to dissolution by the orogenic fluids. The clustering of fluorite, U-Th-Zr–silicate microveinlets, and REE fluorocarbonates in the granite is ideal for fluid complexation. We suggest that the fundamental reason why the Mount Isa inlier formed orogenic uranium rather than gold deposits is that the regional fluids were highly saline. Other regions that went through orogenesis after accumulating sediments in a similar evaporitic setting should be considered prospective for orogenic uranium deposits, provided that appropriate metal sources exist. Furthermore, McLaren et al. (1999) demonstrated that the Sybella batholith contributed to regional metamorphism ∼100 m.y. after emplacement through radiogenic heating. Our study suggests that this process was enhanced by fluid-driven transfer of K and U from the granite into the surrounding rocks during orogenesis.

Tomkins and McGloin were supported by the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australian Synchrotron (Victoria, Australia), the Australasian Institute of Mining and Metallurgy (AUSIMM), and the Society of Economic Geologists (SEG) Hugh E. McKinstry grants. Part of this research was undertaken on the XFM beamline at the Australian Synchrotron. Summit Resources are thanked for access to drill core and logistical support. We thank N. Wilson and A. Torpy for assistance with EPMA mapping. S. Vollgger is thanked for assistance with Leapfrog. F. Bea, N. Oliver, and J. Hanchar are thanked for their constructive reviews.

1GSA Data Repository item 2016004, Table DR1 (selected whole-rock geochemistry of the Kitty Plain Microgranite), is available online at www.geosociety.org/pubs/ft2016.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.