Minor and Trace Elements in Copper Tailings: A Mineralogical and Geometallurgical Approach to Identify and Evaluate New Opportunities

Abstract

Reliable information on the chemical and physical makeup of mine tailings is critical in meeting environmental and regulatory requirements, as well as identifying whether contained elements, including critical minerals, might be economically recovered in future to meet growing demands. Detailed mineralogical characterization, supported by chemical assays and automated mineralogy (MLA) data on different size fractions, underpins a case study of flotation tailings from the processing plant at the Carrapateena mine, South Australia. The study provides valuable insights into the deportment of minor and critical elements, including rare earth elements (REEs), along with uranium (U). REE-minerals are represented by major phosphates (monazite and florencite) and subordinate REE-fluorocarbonates (bastnäsite and synchysite). More than half the REE-minerals are concentrated in the finest size fraction (−10 μm). REEs in coarser fractions are largely locked in gangue, such that economic recovery is unlikely to be viable. MLA data shows that the main REE-minerals all display specific associations with gangue, which change with particle size. Quartz and hematite are the most common associations, followed by sericite. Synchysite shows a strong affiliation to carbonates. The contents of other critical elements (e.g., tungsten, molybdenum, cobalt) are low and for the most part occur within other common minerals as submicron-sized inclusions or in the lattice, rather than discrete minerals. Nevertheless, analysis of mine tailings from a large mining–processing operation provides an opportunity to observe intergrowth and replacement relationships in a composite sample representing different ore types from across the deposit. U-bearing species are brannerite (associated with rutile and chlorite), coffinite (in quartz), and uraninite (in hematite). Understanding the ore mineralogy of the Carrapateena deposit and how the ore has evolved in response to overprinting events is advanced by observation of ore textures, including between hematite and rutile, rutile and brannerite, zircon and xenotime, and the U-carbonate minerals rutherfordine and wyartite, the latter two replacing pre-existing U-minerals (uraninite, coffinite, and brannerite). The results of this study are fundamental inputs into future studies evaluating the technical and economic viability of potentially recovering value metals at Carrapateena. They can also guide efforts in understanding the distributions of valuable metals in analogous tailings from elsewhere. Lastly, the study demonstrates the utility of geometallurgical data on process materials to assist in geological interpretation.

1. Introduction

Increased demand brought about by their manifold applications in green technologies and defense applications, coupled with concerns about potential supply disruption, has led to certain commodities being defined as “critical” or “strategic” [1]. Lists of critical and/or strategic minerals and/or elements differ subtly between jurisdictions (e.g., the European Union, United States of America, United Kingdom, Australia, Japan, etc.) and continue to evolve, reflecting demand, availability, emerging uses, supply bottlenecks, and overarching geopolitical concerns. All critical mineral inventories include minor and rare metals, selected non-metals, minerals sensu stricto such as graphite and barite, as well as other non-metal commodities like potash [2,3,4,5,6].

Governments, international agencies, and the global media have amply promoted the necessity and potential economic benefits of critical mineral production in recent years, creating an awareness of critical minerals as important drivers of economic activity. Lithium, cobalt, nickel, and rare earth elements (REEs) are now widely acknowledged as essential for the transition to clean energy generation [7,8]. The search for new and alternative critical mineral resources has led to an acceleration of global exploration expenditure [9]. New resources of REE, lithium, cobalt, etc., have been identified, with dozens of these currently being developed as mines. The twenty-first century has also witnessed an explosion of research interest aimed at identifying non-conventional sources of critical minerals, as well as engineering new methods to extract value minerals from them [9].

Centuries of human mining and metal production have left a legacy of discarded mining waste, including abandoned mines, low-grade ore dumps, mine tailings, and smelter slags. Systematic re-examination of these materials has, in some cases, established the presence of critical minerals that were either not recorded at the time of mining, were considered too low-grade and/or difficult to process, or simply were of no commercial interest because no market existed for them at the time [10]. These wastes are commonly presented as ‘new types of ore’ or ‘the ores of tomorrow’, and, as such, are inferred to be an important part of the supply solution [11]. In parallel, industry and researchers continue to explore new opportunities for by-product metal production from operating mines and processing facilities in which critical minerals occur at trace to minor concentrations in ores, concentrates, tailings, or other processing materials but are not recovered [12]. Mine tailings of different types have become particularly attractive targets for research aimed at identifying potential pathways to REE recovery (e.g., [13,14,15,16]).

Knowledge of mineralogy and how a given element or group of elements is distributed in any rock, tailings site or slag—and how this may vary spatially—is an essential prerequisite for the optimization of every part of the mining–processing–refining value chain, as is assessing the viability of reprocessing [10,17,18]. Tailing characterization is also vital for management of tailings storage facilities, including control of acid mine drainage [19].

Prior to around 2010, interest in the processing of copper tailings largely focused on the residual content of precious metals (e.g., [20]). More recently, high copper prices and long-term supply concerns have led to a shift towards the recovery of contained copper, as well as a range of critical minerals. Mineralogical studies targeting the distribution of unrecovered components of the tailings are particularly important for critical minerals given their typically low concentrations, their fickle geochemical affinities, and the tendency of some to occur as substituents in one or more common minerals rather than as discrete phases. Careful characterization is thus essential and requires knowledge transcending mineralogy, geochemistry, and geometallurgy to underpin the accurate quantification of element distributions and constraints on element behavior, both within the deposit (plant feed) and during processing.

We address the distribution of REEs and other metals in fresh flotation tailings from the Carrapateena iron oxide copper–gold (IOCG) deposit, South Australia [21,22]. The study is part of a larger investigation tracking the distribution and behavior of minor elements from ore to metal in mining–processing–smelting–refinery operations in the Olympic Cu-Au Province, South Australia [23,24,25,26]. We identify which mineral species are present, their compositions, morphologies and associations, and address size distributions. Emphasis is placed on the relationships between REE- and U-bearing minerals. Traditional methods of chemical assay and mineral liberation analysis are complemented by detailed μm scale imaging of mineral textures. The results underpin an evaluation of the opportunities, but also the pitfalls, when considering the reprocessing of tailings. Aside from this, we also show how a textural examination of tailings provides valuable information on mineralogy and alteration processes in the deposit and processing plant.

2. The Carrapateena Deposit: Regional and Local Setting

The Carrapateena IOCG deposit is situated on the eastern margin of the Gawler Craton, South Australia, approximately 100 km southeast of Olympic Dam and 472 km northwest of Adelaide. Carrapateena lies within the Olympic Cu-Au Province (e.g., [27]), a metallogenic province of global significance that hosts several major Mesoproterozoic IOCG systems, including the >10 billion tonne Olympic Dam deposit [28,29], Prominent Hill, and the newly identified Oak Dam West resource (Figure 1).

The Carrapateena deposit was discovered in 2005 and mining commenced in 2019. The current 5.2 Mtpa operation was acquired by BHP in 2023 and currently produces copper sulfide concentrates. Ores are crushed, ground, and separated by froth flotation on-site at Carrapateena; copper concentrates are sold into the global concentrate market. Total ore resources, as of 23 June 2023 (measured + indicated + inferred), are 900 Mt @ 0.56% Cu, 0.24 g/t Au, and 3 g/t Ag [30].

Geologically, Carrapateena is hosted within 1860–1850 Ma Donington Suite granite [31], which is part of the Paleoproterozoic to Mesoproterozoic basement of the Gawler Craton. Mineralization has been dated at ~1585 Ma by apatite, hematite and xenotime U-Pb geochronology [22] and occurs within polylithic breccias and volcanic units. Like the Olympic Dam deposit, there is geochronological evidence for modest fluid-assisted overprinting and ore remobilization at ca. 1200–1100 Ma and again at ca. 600–500 Ma [22].

Hematite, a product of iron metasomatism, is the dominant gangue mineral. Copper mineralization is represented by chalcopyrite, bornite, and lesser abundances of chalcocite. Ores have the typical textural and geochemical characteristics of IOCG systems, including geochemically anomalous concentrations of REE and U [21,22]. Similar to the Olympic Dam deposit, Carrapateena exhibits mineralogical zonation from an inner high-grade bornite ± chalcocite zone, which is in part surrounded by a chalcopyrite–bornite zone and an outer pyrite–chalcopyrite zone. There is also a ‘barren’ zone comprising hematite-rich, sulfide-poor rocks [22]. Prior publications have identified some aspects of ore mineralogy, including the presence of rare earths such as fine-grained monazite, xenotime, and subordinate bastnäsite, florencite and synchysite [22]. A quantitative understanding of the deportment of REEs, U, or other minor elements of potential interest as by-products (Co, Ni, Mo etc.) is, however, unavailable at the present time.

3. Sampling and Methodology

3.1. Sampling and Sample Assay

The study comprises a μm scale analysis of seven size fractions of a sample of fresh flotation tailings from the Carrapateena processing plant (July 2024): CTS+106, CTS+75, CTS+53, CTS+C1, CTS+C3, CTS+C5 and CTS–C5. Particles from each fraction are mounted in a 1-inch-diameter epoxy block and polished, exposing tens of thousands of individual particles at the surface. Mass balances are included in

Table 1. Assay data for major elements and selected minor elements, including lanthanides, in the seven size fractions (in ppm, unless otherwise indicated). Mass% proportions for each fraction are also indicated.

Fraction	Mass%		Al %		Ca %		Fe %		K %		Mg %		Mn %	Na %	P %	Si %	Ti %	CO2 %	S %	LOI * %
CTS+106	8.6		2.44		1.08		14.62		0.91		0.47		0.22	0.12	0.14	30.56	0.12	1.98	0.25	1.82
CTS+75	7.7		1.78		0.75		31.90		0.60		0.32		0.21	0.06	0.16	21.58	0.10	1.75	0.26	1.39
CTS+53	10.1		1.33		0.55		41.30		0.45		0.22		0.15	0.05	0.14	15.16	0.09	1.11	0.30	1.11
CTS+C1	13.1		0.79		0.30		56.01		0.23		0.12		0.10	0.02	0.12	6.77	0.11	0.68	0.47	0.97
CTS+C3	13.8		1.39		0.69		41.57		0.46		0.26		0.18	0.06	0.15	15.46	0.10	1.38	0.44	1.27
CTS+C5	5.2		1.55		0.81		39.25		0.53		0.32		0.21	0.08	0.16	14.62	0.11	1.80	0.45	1.70
CTS-C5	33.3		4.38		0.70		38.82		1.48		0.54		0.21	0.07	0.36	12.3	0.11	1.63	0.27	3.09
Fraction	Ag		As		Bi		Co		Cu		Mo		Ni	Pb	Sn	Th	U3O8	W	Zn
CTS+106	2.1		20		1.9		111		2250		18		25	35	16	18.3	160	49	92
CTS+75	1.6		23		2.1		121		1864		28		23	38	32	21.4	169	142	73
CTS+53	3.3		23		1.9		110		1265		31		22	39	45	18.4	180	206	53
CTS+C1	2.2		33		1.9		115		1390		39		21	47	64	18.4	219	289	43
CTS+C3	1.2		26		1.6		136		775		30		24	43	40	20.3	200	187	57
CTS+C5	1.2		25		1.8		185		860		28		28	48	33	22.4	220	155	66
CTS-C5	1.5		36		3.5		298		1084		33		53	82	22	48.6	389	103	147
Fraction	Y	La		Ce		Pr		Nd		Sm		Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu
CTS+106	197	1055		1757		174		530		68.9		14.4	45.4	6.62	37.0	7.25	21.6	2.99	20.1	2.59
CTS+75	206	1216		1994		198		615		79.4		16.5	53.5	7.22	38.1	7.46	22.1	2.94	19.7	2.52
CTS+53	167	970		1607		159		484		61.6		12.3	42.7	6.14	32.3	6.25	18.3	2.46	16.7	2.12
CTS+C1	153	854		1420		144		447		59.2		11.9	39.3	5.36	29.5	5.67	16.3	2.35	15.0	2.03
CTS+C3	201	1020		1712		173		540		70.4		14.9	48.6	7.16	38.4	7.50	22.3	3.08	20.3	2.62
CTS+C5	251	1112		1870		191		588		80.5		16.8	58.5	8.59	48.9	9.19	26.7	3.66	24.5	2.99
CTS-C5	399	2627		4318		435		1328		162.8		32.9	105	14.7	78.5	14.84	42.2	6.02	39.2	5.32

Other minor elements assayed include Cr (273 ppm in CTS+C5, 30–120 ppm in all others), Sc (14 ppm in CTS–C5, 6–8 ppm in all others), Nb (16–48 ppm), Sb (<5 ppm), Se (<3 ppm in all), Ta (<1 ppm), Te (2.4 ppm in CTS–C5, <1 ppm in all others), Sr (342 ppm in CTS–C5), and Zr (54–83 ppm). Analytical methodologies: four-acid-HBr/mass spectroscopy (Ag, As, Bi, Co, Cu, Ni, Pb, Se, Te, Zn), lithium borate fusion/optical emission spectroscopy (Al, Ca, Cr, Fe, K, Mg, Mn, Na, P, Sc, Si, Ti), lithium borate fusion/mass spectroscopy (Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Mo, Nb, Nd, Pr, Sm, Sn, Sr, Tb, Th, Tm, U₃O₈, W, Y, Yb, Zr), combustion furnace/infrared spectroscopy (C, S) LOI *: 1000 °C.

.

Each of the seven individual size fractions was assayed commercially for a total of sixty-three elements at the laboratories of Intertek, Adelaide, Australia. Analytical methodologies can be found in the footnote to Table 1.

3.2. Mineral Liberation Analysis

Quantitative mineral proportion and association datasets were determined in each polished block at ALS Global, Brisbane, Australia, following procedures outlined in [32]. Data used in this contribution was obtained via the Extended Liberation Backscatter (XBSE) method. It was, however, supplemented with Grain-based X-ray Mapping (GXMAP) to help resolve sericite and K-feldspar.

3.3. Microanalysis

Each polished block was carbon-coated and examined at high magnification on a FEI Quanta 450 scanning electron microscope (SEM) housed at Adelaide Microscopy, The University of Adelaide. High-contrast imaging was performed in back-scattered electron (BSE) mode. The instrument was operated at 10 nA current and 20 kV voltage. The built-in energy-dispersive spectrometer (EDS) allowed the acquisition of compositional data for particles in the tailings. The EDS method is standardless and the total of all measured concentrations is normalized to 100%, excluding any OH or H₂O present. The carbon coating added for improved conductivity impairs accurate analysis of carbon-containing minerals; however, roughly, analyses containing more than 10%–12% C contain structurally bound carbon. Although favorably comparing with electron probe microanalysis for most common minerals, EDS cannot adequately navigate complex X-ray interferences among, for example, individual HREEs.

4. Results

4.1. Assay Data

Assay data (Table 1; Figure 2) show some significant variations in element contents across the size fractions. As expected, Cu grades are highest in the coarsest fraction, due to trapping of Cu-(Fe)-sulphides within larger gangue particles, and are thus unable to float. Elements, notably Ag, that are typically contained with Cu-sulphides, whether lattice-bound or as inclusions, follow Cu distributions. Iron and other major elements (Al, K) are highest in the finer fractions. Silicon, however, is highest in the coarse fraction—largely as quartz. Most minor elements, including REE and U₃O₈, are highest in the finest fraction, at concentrations roughly double those of the other fractions for several elements of interest such as Co, Ni, and Mo. Elements characteristically contained in inclusions within hematite, notably Sn, Mo, and W, all show a different trend with the highest concentrations in medium-sized fractions. Barium (a few hundred ppm) also follows this size distribution.

Chondrite-normalized REE fractionation trends for the seven size fractions (Figure 3) show subtle variations. The finest fraction (CTS-C5) exhibits a trend that is similar in shape to the others but at significantly higher REE concentrations than the other fractions. Fractionation patterns for all size fractions show a steep downwards trend from LREEs to HREEs but are relatively flat across the HREE segment. The relative enrichment in LREEs (La through Nd) is commensurate with the observations detailed below. The +C1 fraction shows the lowest overall REE concentrations. All size fractions display a weak negative Eu anomaly and a barely perceptible negative Y anomaly.

The importance of the fine fraction, which accounts for more than 50% of ΣREEs, is seen from the plot in Figure 3b.

4.2. Tailing Mineralogy

4.2.1. MLA Data

Quantitative modal mineralogy indicates that all six size fractions are dominated by gangue minerals (

Table 2. Modal mineralogy of the six Carrapateena tailings fractions and the 10,000-sample MLA dataset for the Olympic Dam deposit [28].

Mineral	Carrapateena Mine Tailing Abundance (wt. %)						Olympic Dam Dataset Abundance (wt. %)
	CTS+106	CTS+75	CTS+53	CTS+C1	CTS+C3	CTS+C5
Pyrite	0.02	0.202	0.489	0.669	0.722	0.708	0.4614
Chalcopyrite	0.394	0.225	0.127	0.084	0.07	0.065	0.9261
Bornite	0.111	0.087	0.041	0.068	0.026	0.029	0.3346
Chalcocite	0.005	0.001	0.008	0.051	0.004	0.003	0.1388
Covellite	0.002	0.001	0	0	0	0	0.0068
Carrollite	0.003	0.002	0.005	0.003	0.008	0.015	0.0004
Cobaltite	0	0.003	0	0	0.011	0	0.0012
Molybdenite	0.001	0.009	0	0	0.005	0	0.0015
Sphalerite	0.007	0.001	0	0	0	0	0.0115
Galena	0.001	0	0	0	0	0	0.0012
Uraninite	0.003	0	0.001	0.005	0.002	0.021	0.0049
Coffinite	0.004	0.003	0.003	0	0.010	0.007	0.0159
Brannerite	0.026	0.003	0.002	0.006	0.007	0.006	0.0119
Zircon	0.013	0.024	0.004	0.012	0.013	0.015	0.0549
Thorite	0.001	0	0	0	0	0	0.0023
Crandallite *	0.427	0.485	0.441	0.446	0.451	0.506	0.1097
Monazite	0.156	0.131	0.164	0.134	0.180	0.279	0.0184
Florencite	0.017	0.044	0.062	0.055	0.041	0.095	0.0660
Synchysite	0.008	0.016	0.023	0.015	0.006	0.024	0.0124
Bastnäsite	0.038	0.03	0.04	0.054	0.042	0.052	0.0833
Xenotime	0.02	0.019	0.021	0.028	0.052	0.044	0.0045
Fe oxide	19.318	46.092	60.596	81.185	60.511	64.218	30.5338
Quartz	58.618	39.32	28.491	11.787	27.241	22.428	30.8395
Chlorite group	3.633	2.651	1.9	1.21	1.838	1.805	1.3240
Sericite	10.856	5.836	3.823	2.103	4.301	4.625	18.1930
Orthoclase	1.309	0.734	0.525	0.15	0.592	0.465	9.8422
Albite	0.549	0.357	0.389	0.097	0.449	0.66	0.6371
Schorl	0.284	0.174	0.057	0.057	0.135	0.093	0.0521
Corundum	0.001	0	0	0	0	0	0.0001
Kaolinite	0	0	0.002	0	0	0	0.0060
Siderite	0.223	0.307	0.254	0.156	0.313	0.459	2.6574
Siderite_Mn	0.471	0.806	0.554	0.397	0.548	0.799	0.2432
Ankerite	0.601	0.551	0.402	0.256	0.414	0.4	0.1849
Dolomite	1.277	0.543	0.474	0.257	0.758	0.832	0.1327
Calcite	1.294	1.096	0.798	0.44	0.849	0.936	-
Fluorite	0.006	0.01	0.01	0.008	0.014	0.031	1.1795
Barite	0.007	0.021	0.013	0.075	0.078	0.07	1.2252
Barite-Sr	0.001	0	0	0	0.002	0.005	-
Anhydrite	0.016	0.016	0.041	0.001	0.001	0.004	0.0108
Apatite	0.103	0.09	0.089	0.085	0.162	0.132	0.1029
Rutile	0.077	0.033	0.08	0.052	0.078	0.097	0.2411
Ilmenite	0.01	0.007	0.022	0.035	0.02	0.007	0.0447
Plagioclase	0.085	0.069	0.045	0.011	0.035	0.042	-
Atacamite	0.001	0.001	0.001	0.006	0.009	0.011	-
Titanite	0	0	0	0.001	0	0.001	-

* “Crandallite” here refers to Ca-Sr-dominant aluminum-phosphate-sulfate minerals of the alunite supergroup, chiefly of woodhouseite and svanbergite composition, or members of a complex solid solution with florencite [34].

). Iron oxides are the most abundant, with their abundances increasing significantly from the coarsest to the finest fractions, reaching up to 81 wt.% in the CTS+C1 fraction. Quartz is the second most abundant gangue mineral and has a greater abundance in the coarser fractions and lower in the CTS+C1 fraction. All REE-bearing minerals are most abundant in the finest fraction, particularly in CTS+C5.

Table 2 compares these data with bulk MLA data for the Olympic Dam deposit (10,000 sample dataset of Ehrig et al. [28]). The Carrapateena tailings exhibit a higher proportion of Fe-oxides than in the Olympic Dam deposit; however, quartz and pyrite concentrations are remarkably similar. Some gangue minerals, notably sericite, siderite, baryte, and fluorite are less abundant than in Olympic Dam, whereas others (calcite, siderite, and plagioclase) are more abundant in the Carrapateena tailings. Several common REE minerals (monazite, xenotime and synchysite) appear to be more abundant in the Carrapateena tailing sample than in the Olympic Dam deposit as a whole. Major exceptions to this trend are bastnäsite and florencite, which are more prevalent in Olympic Dam than at Carrapateena.

4.2.2. High-Magnification Imaging and EDS Analysis

Imaging of many tens of thousands of particles across all size fractions revealed the complex mineralogy of the deposit and enabled the identification of fifty-one mineral phases, in addition to several unidentified phases. The study revealed key assemblages of ore mineral and gangue and information on the size, morphology, and associations of minerals containing U, REEs, and precious metals, as well as processes of alteration and replacement.

The main gangue minerals and their associations are summarized in the following paragraphs. Hematite (α-Fe₂O₃) is frequently found alongside quartz (SiO₂), as they are the most abundant gangue minerals in the Carrapateena mine tailings. In some cases, quartz was observed in association with other silicates such as sericite, KAl₂(AlSi₃O₁₀)(OH)₂, chlorite, (Fe, Mg)₅Al(Si₃Al)O₁₀(OH,O)₈, carbonates, and/or REE minerals. Sericite is the third most common gangue mineral after hematite and quartz and is usually observed as fine-grained acicular aggregates. Well-formed tabular grains comprising sericite intimately associated with hematite are rare. The association between REE-minerals and sericite was rarely observed, compared to hematite and quartz; however, fine crystals of florencite, CeAl₃(PO₄)₂(OH)₆, occur with a hematite–sericite association. Sericite also hosts, in some cases, xenotime-(Y) (YPO₄), monazite-(Ce), (Ce(PO₄)), and thorite. Chlorite is another abundant silicate, typically forming associations with quartz and hematite. In some cases, it also occurs as isolated grains, often accompanied by disseminated REE minerals such as monazite, florencite, xenotime, and bastnäsite, (Ce,La)CO₃F. Chlorite is mainly represented by chamosite, an iron-rich variety with concentrations of Fe ranging from 29 to 40 wt.%, Al between 10 and 12 wt.% Al, and Mg between 1 and 4.4 wt.%. Sub μm scale intergrowths with sericite are suggested by the persistent presence of minor K (1–3.4 wt.%).

Carbonates are relatively rare compared to hematite and quartz. Calcite (CaCO₃) is the most abundant carbonate and occurs as liberated particles, combined with hematite and/or quartz, or coexisting with other carbonates, notably siderite, (Fe,Mn,Mg)CO₃. Calcite rarely hosts tiny inclusions of monazite and xenotime. Calcite also appears intergrown with other silicates like albite and even hosts carrollite (CuCo₂S₄) or micro-sized inclusions of hessite (Ag₂Te) or clausthalite (PbSe). Calcite at Carrapateena typically contains impurities, with manganese (Mn) being the most notable, occurring at concentrations of 3–5 wt.%, in all analyzed grains. Siderite is also present in the Carrapateena tailings, typically occurring within a hematite–sericite association, potentially replacing hematite and, in some cases, occurring in association with quartz. Manganese is ubiquitous in all carbonates, but is particularly abundant within siderite, with concentrations ranging from 8 to as much as 17 wt.%. Siderite also contains traces of magnesium (Mg), ranging from 2 to 3.5 wt.%.

The principal Cu-(Fe)-sulfide minerals are chalcopyrite (CuFeS₂) and bornite (Cu₅FeS₄). Both are often locked within gangue minerals (Figure 4a,b), most commonly within hematite. Chalcocite group minerals (Cu₂-xS; x = 0–0.2) are subordinate in abundance and are occasionally observed as liberated grains (Figure 4c) or intergrown with bornite in fine-grained symplectites. Covellite (CuS) is notably absent in all the fractions. Minor cuprite (CuO₂), exhibiting dark dots enriched with aluminum, is observed as inclusions within quartz (Figure 4d). Chalcopyrite hosts small grains of carrollite and sub-micrometer inclusions of precious metal-bearing tellurides, including hessite, observed as tiny inclusions within pyrite (Figure 4e), and calaverite (AuTe₂). Carrollite, associated with hematite, is also observed enclosed within calcite, (Figure 4f). Traces of molybdenite (MoS₂) are identified (Figure 4g), reinforcing the presence of high-value metals in these tailings, albeit at low concentrations. Sphalerite (ZnS) is scarce and primarily observed as small inclusions within hematite (Figure 4h) or baryte (BaSO₄).

Pyrite (FeS₂) occurs both as liberated grains and in intimate intergrowths with chalcopyrite. Pyrite commonly hosts nanoscale inclusions of trace ore minerals, including various tellurides and clausthalite (Figure 4i). Arsenides include löllingite (FeAs₂) and domeykite (Cu₃As), both identified as bleb-like inclusions within quartz (Figure 4j,k). Only minor galena (PbS) is seen, typically occurring as tiny inclusions in hematite. Sulfates are dominated by baryte, which appears in a dense and compact form with a distinct ‘brick-like’ morphology readily recognizable on BSE images. Minor celestine (SrSO₄) and anhydrite (CaSO₄) are noted, often hosted within hematite or feldspar.

Carrapateena is a hematite-dominant IOCG deposit and hematite is thus, unsurprisingly, the most abundant mineral in all fractions. Hematite displays various textures and habits, of which the lamellar type is predominant. Hematite is commonly intergrown with other common gangue minerals including quartz, sericite and/or siderite. Acicular, porous hematite aggregates are also observed, often containing disseminations of monazite (see below). Manganese is noted in trace amounts whenever hematite is intergrown with siderite. In some cases, hematite hosts tiny, disseminated inclusions of quartz.

Although the study did not reveal evidence of grain-scale compositional zoning with respect to U, W, Sn, or Mo in hematite, as is widely reported in other IOCG systems of similar age and composition in the region [35,36], dusty inclusions of U-, Pb-, W- and Sn-bearing minerals are observed, among which uraninite is most common (Figure 5). These sub μm sized inclusions are distributed along trails or within distinct fields in which the hematite displays porosity and fracturing.

While uraninite inclusions never exceeding 1–3 µm in size are widespread in hematite, tungsten minerals or μm sized inclusions of cassiterite (SnO₂) are relatively rare. Three tungsten-bearing minerals are identified: scheelite (CaWO₄), wolframite [(Fe,Mn)WO₄], and ferberite (FeWO₄). Typically, these minerals occur as small grains ranging from <1 to 5 μm in size. Identifying tungsten minerals from EDS analysis can, however, be challenging in some cases, because of their small size and spectral interference from the host matrix.

Titanium-bearing minerals, ilmenite (FeTiO₃) and rutile (TiO₂), are common and appear restricted to associations with hematite. Ilmenite typically occurs as lamellae within hematite (Figure 6a), a texture indicative of hemoilmenite. Isolated ilmenite grains, ca. 200 μm in size, are also found in the coarsest fraction. In the same fraction, ilmenite occurs as preferentially <100> or <111> oriented lamellae within aggregates of hematite (Figure 6b), textures that are indicative of titanomagnetite breakdown. EDS analysis of this ilmenite indicates 9–10.7 wt.% Zr, raising the possibility that this is a sub μm scale intergrowth of ilmenite and baddelyite (ZrO₂). Rarely, ilmenite is also observed as micro-veinlets or inclusions within rutile.

In contrast to ilmenite, rutile occurs as patchy areas within hematite–sericite or hematite–quartz associations, showing the onset of partial replacement of rutile by hematite or, possibly vice versa. Moreover, rutile appears in a symplectite texture with hematite (Figure 6c). A developed rutile–hematite replacement stage was observed in the coarsest fraction (CTS-106), showing that approximately 80% of the hematite was superseded by rutile within a 50 μm sized grain of chamosite (Figure 6d). Nb-zoned rutile is also observed.

Surprisingly, rutile-bearing quartz was observed in all size fractions, characterized by relative coarse rutile crystals exhibiting subhedral to stubby morphology (Figure 6e,f). The crystal habit implies limited space for growth, possibly indicating crystallization under strain.

Rare earths are represented by two main mineral groups: phosphates and fluorocarbonates. Monazite and florencite are the dominant REE minerals in the Carrapateena tailings. Xenotime and synchysite [Ca(Ce,La)(CO₃)₂F] are relatively scarce in comparison. Minor concentrations of REE are also noted in EDS datasets for the uranium minerals coffinite and uraninite, and to some extent also in zircon (ZrSiO₄).

Monazite, typically occurring in differently sized, well-formed, fine-grained particles and as disseminations within acicular hematite (Figure 7a–c). These particles often display micro-fractures, which are visible in both small and larger monazite grains (Figure 7d,e). The particle size of monazite ranges from <1 µm to as much as 50 µm, distributed within various gangue minerals. Monazite is commonly associated with hematite and/or quartz or sericite with association with other rare earth minerals such as florencite and rarely with carbonates (calcite). Monazite occurs as disseminated grains in different gangue minerals, particularly hematite, and appears as liberated grains in many fractions. Moreover, monazite displays a marked association with pyrite.

Monazite concentrates LREEs, notably cerium (Ce), lanthanum (La), and neodymium (Nd). Low concentrations of samarium (Sm), praseodymium (Pr), and gadolinium (Gd) are nevertheless noted in a minority of analyzed grains. Two monazite species are observed: monazite-(Ce) and monazite-(Nd). Monazite-(Ce) contains subordinate La and Nd, with La typically elevated over Nd; however, slight variations can occur. On the other hand, monazite-(Nd) contains a wider range of rare earth elements (REEs), including measurable Pr, Sm, and even heavier REEs like Gd. In these cases, whenever Nd is dominant, the concentration of La is usually low, never exceeding 3.7 wt.%.

Florencite is the second most common REE-mineral and is ubiquitous in all size fractions. It is often found as stubby, euhedral to subhedral crystals (Figure 7f). Florencite also forms aggregates of fine-grained crystals and, in some cases, attains the appearance of massive aggregates, in which individual crystals are not easily distinguishable (Figure 7g). Florencite occurs within sericite–hematite associations (Figure 7h), along micro-veinlets, as disseminated crystals between hematite lamellae or simply associated with hematite or quartz.

Florencite is an Al-bearing phosphate that concentrates La, Ce and Nd; however, the proportions of the individual REE can vary. Two distinct species are recognized: florencite-(Ce), by far the most common, and florencite-(La). EDS analysis of individual grains shows, however, that florencite can also contain minor amounts of other REE, notably Pr. A key feature of Carrapateena florencite is that it typically contains strontium (Sr) at concentrations of up to 3.7 wt.%), although usually in more modest amounts. Monazite is also observed to be replaced partially or completely by florencite, with the original monazite preserved as relict grains (Figure 7i,j). Florencite was found, in some cases, in association with other phosphates, notably fluorapatite, Ca(PO₄)₃F (see below).

Xenotime is relatively rare compared to monazite and florencite. It is usually found in association with hematite, and, in some cases, occurs together with carbonates, notably fine-grained siderite, or within chlorite–albite associations. Xenotime also appears associated with acicular hematite, enclosed within quartz (Figure 8a), and as liberated grains, ca. 5 μm in size (Figure 8b). It often forms a thin outer mantle around zircon crystals and, in some cases, displays almost complete replacement of zircon (see below). Xenotime-(Y) has the general formula (YPO₄) but also contains significant amounts of heavy rare earth elements (HREEs), including Gd, dysprosium (Dy), ytterbium (Yb), holmium (Ho), and erbium (Er). Light rare earth elements (LREEs) such as La, Ce, Nd, Sm and europium (Eu) are typically present in much lower concentrations. Some xenotime grains analyzed here were seen to be particularly enriched in Gd, with concentrations ranging from 6.6 to 8.6 wt.%. In certain cases, however, Dy can be nearly as abundant as Gd, with concentrations reaching around 6 wt.%. Xenotime occasionally hosts minor concentrations of thorium (Th), (~2 wt.%), and may also host trace uranium (U).

Zircon is a rare mineral in the tailings and is almost always observed in association with xenotime. Typically, xenotime forms a mantle or overgrowth on zircon and, in some cases, forms micro-veinlets within zircon. Fully liberated fine grains of zircon are scarce (Figure 8c,d). BSE imaging shows subtle compositional variation indicating oscillatory crystal zoning within zircon and metamict cracks around grain margins. The marked association between zircon and xenotime is likely the result of replacement reactions which appear to be incomplete, as clearly shown in Figure 8e,f. EDS data shows zircon hosting measurable concentrations of REEs, particularly HREEs, and that it is the only mineral between all the fractions that hosts measurable concentrations of scandium (Sc). Zircon also contains low, albeit measurable, U, typically in the range of 1%–2%. A key characteristic of zircon is that it typically contains hafnium in minor concentrations.

Bastnäsite is observed in association with different gangue minerals, such as hematite, sericite and quartz. Bastnäsite also appears within composite agglomerates, together with hematite, quartz and agglomerations of chamosite with some chalcopyrite grains (Figure 9a). Bastnäsite varies in size, i.e., as micro-sized inclusions smaller than 2 µm, ranging up to grains measuring around 20 µm. Several 10 µm sized liberated grains are observed (Figure 9b). The mineral commonly forms tabular, prismatic, or stubby crystals, often with distinct faces. It can also appear as massive, granular, or compact aggregates.

Synchysite typically occurs intergrown with bastnäsite, although some larger monomineralic synchysite particles, 30 µm in size, are also seen (Figure 9c). Moreover, synchysite occurs as 40 µm sized liberated particles hosting inclusions of bastnäsite and some particles of gangue minerals (hematite, sericite and quartz (Figure 9d)).

EDS analysis of the two minerals shows that bastnäsite primarily contains Ce and La but can also include other REEs such as neodymium, Sm and traces of praseodymium. Bastnäsite exhibits variability in its chemical composition. Two types of bastnäsite are observed: bastnäsite-Ce and bastnäsite-La. The latter does not contain measurable Nd.

Like bastnäsite, the dominant rare earths in synchysite are Ce and La, even if other lanthanides, notably Nd, but also Sm and traces of Pr, are noted. Synchysite is Ca-bearing and thus darker than bastnäsite on BSE images; the measured Ca typically ranges from 10 to 13.7 wt.%. Two synchysite species are observed: synchysite-(Ce), the more common variety, typically containing more Nd than La, and synchysite-(Nd), which is much rarer.

Bastnäsite–synchysite intergrowths are common (Figure 9e–j), notably in the coarser size fractions. The two fluorocarbonate minerals occur as blocky or lamellar grains with common crystal orientation, reflecting their closely related layered crystal structures that can readily accommodate compositional changes, as recorded in REE-fluorocarbonates from Olympic Dam [37,38]. Replacement relationships between the two endmember fluorocarbonates are also suggested by patchy textures like those in Figure 9i. In most cases, the two minerals have similar proportions of major lanthanides, Ce, Nd, and La, with Ce always dominant.

Fluorapatite, Ca₅(PO₄)₃F, is moderately abundant in the Carrapateena tailings, occurring alongside various gangue minerals such as hematite and quartz (Figure 10a,b). It also forms numerous associations with REE-minerals. For example, fluorapatite is commonly intergrown with florencite in several size fractions and has been observed to contain tiny inclusions of xenotime (>1 µm) (Figure 10c) or to have xenotime forming along its boundaries (Figure 10d). In some cases, fluorapatite hosts tiny microinclusions of uraninite (Figure 10d), and may occur as liberated grains, typically around 10 µm in size.

Uranium-bearing minerals occur throughout all tailing size fractions. A range of textures are observed, as is a close association with hematite and with only minor occurrences alongside other mineral phases. Uranium is primarily found in the form of uraninite (ideally UO₂), brannerite (UTi₂O₆) and coffinite (USiO₄.nH₂O). In addition to these, two less common U-carbonate minerals, rutherfordine (UO₂CO₃) and wyartite [CaU⁵⁺(UO₂)₂(CO₃)O₄(OH).7H₂O], are also observed. Uranothorite is also present in some fractions but is relatively scarce.

Uraninite (pitchblende) is the most abundant uranium mineral. It occurs as isolated grains with irregular morphology (Figure 11a,b) in some fractions, but more typically occurs as inclusions within hematite that are <5 μm in size (Figure 11c). EDS analysis reveals the presence of lead (Pb) at concentrations varying from as little as 3 wt.% to as much as 20.5 wt.%. Additionally, uraninite hosts minor, albeit measurable REEs, notably Y, which can reach up to 10.3 wt.%, Ce (as much as 9.2 wt.%), Nd and Yb, along with Ca, Fe, Th (approaching 4 wt.%), and even niobium (Nb).

Brannerite consistently occurs in close association with rutile in all instances where brannerite is present in any fraction. Replacement of rutile by brannerite is, however, often patchy (Figure 11d). Incomplete replacement of rutile readily explains the Ti-rich zones that stand out as darker on the BSE images (Figure 11e). A 20 µm sized grain of brannerite in the coarse fraction (Figure 12a) exhibits two types of incomplete replacement. In the more advanced stage, a symplectitic texture of rutile intergrown with hematite is observed. Additionally, darker zones enriched in Ti suggest a partial to near-complete replacement of rutile by brannerite (Figure 11e and Figure 12a,b). The mineral textures show two pathways where hematite is incompletely replaced by rutile. After this initial stage, rutile becomes the host for a second replacement process, during which it is partially transformed into brannerite. EDS images highlight the areas where each replacement sequence begins, revealing the progression from hematite → rutile → brannerite. These textures capture a complex, multi-stage hydrothermal or metasomatic history.

Uranium minerals, specifically uraninite and wyartite, occur in other forms as mixtures with hematite. These mineral mixtures are consistently observed across a wide range of grain-size fractions, including both the coarsest (Figure 12b,c) and finest (Figure 12d) fractions. The ultrafine-grained mixtures may reflect complex alteration processes, late-stage precipitation events involving both uranium and iron oxides [39], or simply the recrystallization of hematite, which initially contained U within its lattice [36].

5. Discussion

5.1. Critical Minerals: Rare Earths and Other Possible Value-Add Components

Substantial effort is currently being invested in establishing the secondary prospectivity of Australia’s tailings and mine wastes [40]. The reprocessing of mine waste and tapping of by-product potential are heralded as major growth opportunities and, if successful, will represent a contribution to the circular economy [41]. This research has culminated in an Atlas of Australian Mine Waste [42], later renamed the Atlas of Australian Re-Mining Potential [43].

The IOCG deposits of the Olympic Cu-Au Province and associated tailings dams are widely cited as one of Australia’s most significant critical mineral repositories, with particular focus on the large volumes of rare earths contained within them [44,45,46]. REEs are, however, not recovered and no resource data are reported due to the extremely fine grain size and the difficulty of obtaining REE concentrates with ΣREE concentrations >3% [47,48]. Interest in potential future REE recovery centers on stored tailings at Olympic Dam and other sites, prompting efforts to secure reliable information on mineralogy and metal deportment.

The relative abundance of monazite and florencite, followed by xenotime, relative to other REE-mineral species is in sharp contrast to the Olympic Dam orebody, in which bastnäsite is the dominant REE host [28,49,50]. In the Carrapateena tailings, both phosphates show variation in grain size and morphology. A plot of % surface area exposed vs. particle size (Figure 13) reveals differences in distribution between the REE-fluorocarbonates and florencite, which are more likely to be liberated, and the finer, less exposed monazite and xenotime. Although a small number of relatively larger liberated grains are observed in the coarser fractions, monazite and florencite are dominantly <5 μm in size and are thus heavily concentrated in the finest fractions (Figure 3b).

Further differences in the size distribution and locking characteristics of each REE mineral are recognizable from the MLA association data. Figure 14 compares the associations of monazite, xenotime, bastnäsite, synchysite, and florencite with the most common gangue minerals (physically touching) in six different size fractions (+106, +75, +53, +C1, +C3 and +C5). The figure shows the importance of quartz as a host for monazite, which exceeds that of hematite. Monazite and hematite have comparable major associations; sericite is only a major host in the coarser size fractions. Quartz also has a greater association than hematite with bastnäsite. Synchysite associations are somewhat different; synchysite is the only REE-mineral for which carbonates have a significant association. The greater association of synchysite with ‘other minerals’, as seen in Figure 14, stems largely from its common association with bastnäsite. Hematite is a more important association than quartz only in the case of florencite.

Although the abundance of monazite relative to bastnäsite might provide encouragement for recovery by flotation, the ultrafine grain size and locking character make the recovery of rare earths from Carrapateena tailings an unattractive target. Abaka-Wood et al. [47,48] put forward flowsheets for the upgrading and recovery of REEs from low-grade tailings of comparable mineralogy to Olympic Dam. Using a rougher–scavenger–cleaner magnetic separation process, a magnetic REE mineral concentrate was obtained with a TREO recovery of 51% at a grade of just 1.25% and non-magnetic tails with a TREO recovery of 41% at a grade of 1.08%. Abaka-Wood et al. [51] evaluated the froth flotation of the pre-concentrates, enabling development of a potential processing flowsheet for recovery of REE minerals suitable for a feed containing florencite, bastnäsite and monazite, by magnetic pre-concentration prior to hydroxamic acid flotation. These studies emphasized the recovery challenge presented by low-grade, ultrafine-grained REE minerals in an iron-oxide/silicate-dominant matrix, compared, for example, with REE recovery from carbonatite, alluvial gold, or phosphor rock tailings. The modest enrichments achieved may nevertheless be viable first steps prior to other downstream methods and should be accompanied by efforts to minimize the recovery of iron-oxides [51].

The distributions of other critical metals in the Carrapateena tailings are also not promising for the establishment of low-cost recovery opportunities without complete dissolution. Tungsten minerals are, for example, only present as minute, disseminated inclusions within hematite. The only discrete cobalt mineral identified is carrollite, leading to the assumption that much of the Co (and Ni) endowment is found in the crystal lattice of pyrite. Tin and Mo are rare, occurring as tiny inclusions of cassiterite and molybdenite in hematite and quartz, respectively.

We conclude that although the REE-minerals stand out as the most obvious value-add target if the tailings were to be reprocessed, the residual ore minerals (chalcopyrite, bornite and chalcocite) are more likely to be potentially low-value economic targets. The Cu-(Fe)-sulfides are largely locked within gangue and concentrated in the coarsest size fractions favoring selective cyclosizing and regrinding.

The MLA association data also highlight important differences between the three main U-minerals (Figure 15). Rutile has a modest association with brannerite in the coarser fractions but not in the finer fractions and not with other U-minerals. Chlorite has a major association with brannerite; however, it does not correlate with coffinite or uraninite. Quartz has an important association with coffinite in the coarsest fraction but diminishes with reduced size. Hematite has a more important association with uraninite, especially in medium-sized fractions. Uranium minerals are mostly free in the finest fractions. Sericite is more closely associated with brannerite than the other U-minerals. Thorite is a trace component but one associated with sericite and carbonates (in the +106 fraction). Brannerite displays no significant associated with other uranium minerals in any of the fractions. Coffinite is found only in association with uraninite, and uraninite is therefore associated with coffinite.

5.2. Intergrowths and Replacement Reactions

A detailed examination of the Carrapateena tailings has provided valuable information on relationships between minerals, some of which have been documented elsewhere, and others which are either new, or which represent instructive examples complementing those reported previously. Many of the reactions are also important from an industrial perspective because they impact the distribution and mineral speciation of uranium and its daughter radioisotopes. This is a challenging topic of importance for current mining–processing operations; however, it is equally important for the future exploitation of rare earths, given the intimate relationships between lanthanides and actinides in time and space.

5.2.1. Hematite-Rutile

Hematite is the dominant mineral in IOCG ores of the Olympic Cu-Au Province and the defining product of iron metasomatism. Hematite textures in all deposits are diverse, reflecting varying degrees of grain replacement and recrystallization. These are recognized by the reworking of trace element distribution patterns and/or the nucleation of inclusions [35,36,52]. Nonetheless, a common feature is the presence of a distinct trace element signature that includes uranium, (radiogenic) Pb, W, Sn, and Mo [35,36,52,53]. Coarse hematite commonly retains primary zoning with respect to these minor components, making it an excellent geochronometer that could be successfully applied across the Olympic Cu-Au Province [22,52,54,55]. The range of textures observed in hematite from Carrapateena tailings does not include primary oscillatory zoning patterns. However, local recrystallization of hematite is observed, in which impurity elements are released from the crystal lattice and precipitate as nanoparticles within the hematite matrix or immediately adjacent.

Relationships between hematite and contained ilmenite (Figure 6a,b), whether a product of hemoilmenite decomposition or an inherited texture, are complex, sometimes contradictory, and are almost certainly the result of more than a single process. Some particles could potentially be derived from altered picrites or other mafic igneous rocks, known from Olympic Dam [56], in which the intergrowth/breakdown textures developed during cooling. Other textures reflect (partial) breakdown of hydrothermal Ti-rich hematite. We note that the ilmenite shown in Figure 6a is not homogeneous and appears to contain nanoparticles of another phase, possibly baddeleyite, or even srilankite (ZrTi₂O₆), given the high measured Zr content.

The graphic intergrowths between hematite and rutile (Figure 6c) are not only abundant but also quite stunning. Comparable textures are reported from other IOCGs in the district, including from Olympic Dam (e.g., Figure 7 in Krneta et al. [56]). Similar topotactic hematite–rutile intergrowths are described by Rečnik et al. [57] and are explained in terms of decomposition of an initial ferrous ilmenite. The presence of ilmenite in some of the patches, as thin lamellae (Figure 6a) and even trellis exsolutions (Figure 6b), lends weight to this interpretation. It is unclear, however, whether the hematite–ilmenite intergrowths are the product of mineralization (iron metasomatism) or were inherited from pre-existing lithologies and (incompletely) converted to hematite–rutile intergrowths during the ~1590 Ma event.

Hematite–ilmenite and hematite–rutile intergrowths both represent fertile ground for future work to address relationships and mechanisms of transformation. This would require nanoscale characterization of phases that are already well studied at the micron-scale (i.e., microprobe), analogous to the work on magnetite–ilmenite intergrowths in the nearby Acropolis prospect that showed how epitaxial relationships between spinel and Fe-Ti-oxides formed by exsolution from a high-temperature solid solution and were followed by replacement via mineral-buffered reactions [58].

Rutile-bearing quartz was identified across all size fractions. The rutile crystals observed are relatively coarse and display subhedral to stubby morphologies (Figure 6e,f). Although crystallographically controlled rutile needles in quartz are described in the petrologic literature and represent a valuable geothermobarometer if equilibrium can be established (e.g., [59,60]), the rutile inclusions in Carrapateena quartz appear randomly distributed and may represent various low-temperature rutile replacement and/or growth cycles. Such relationships are comparable with those reported for low P-T rutile in quartz veins interpreted as products resulting from the replacement of Ti-bearing minerals by quartz [61].

5.2.2. Brannerite-TiO₂ Polymorphs

Brannerite is well known to occur together with TiO₂, usually rutile, although anatase is also noted. According to Kopáčik et al. [62], this replacement relationship occurs through a process in which uranium is adsorbed onto Ti-rich phases, such as rutile, leading to the formation of U-Ti oxides. The transformation is facilitated by the chemical affinity between U and Ti, allowing substitution of U⁴⁺ for Ti⁴⁺ in the rutile structure, thereby forming brannerite. The incomplete replacement of rutile is, however, commonplace in Carrapateena, as also observed at Olympic Dam [63]. This thus explains the darker, Ti-rich zones on BSE images. Given that the brannerite structure comprises alternating layers of UO₂ and TiO₂, in which both cations are surrounded by six O atoms in an octahedral arrangement, the reaction may be gradual or may fail to reach completion. Brannerite itself may be poorly crystalline and metamict [64].

Brannerite from Olympic Dam displays a wide range of compositions from an effectively uraniferous rutile to a nearly stoichiometric UTi₂O₆ [63]. Textural observations and the generally low Pb contents (a few wt.%) have been taken to indicate that brannerite is, at least in part, the result of the alteration of primary uraninite or of a reaction between U-rich fluids and rutile, during fluid-assisted overprinting [63]. NanoSIMS mapping by Rollog et al. [65] revealed that brannerite from Olympic Dam contains U and ²⁰⁶Pb but appears to be largely missing intermediate members of the ²³⁸U decay chain, possibly indicative of late formation from circulating U-Pb-rich fluids.

The relationships between TiO₂, presumably rutile but requiring verification, demand investigation at the nanoscale to resolve whether the relationship is epitaxial, the extent to which the brannerite is crystalline, and potentially whether nanoparticles of rutile are preserved. This work is, however, beyond the scope of the present study, and would require careful selection of suitable grains for in situ extraction ahead of nanoscale investigation.

5.2.3. Zircon-Xenotime

Zircon and xenotime are common accessory minerals in the Olympic Dam deposit [28]. Although some zircons of igneous origin are sufficiently well preserved to be accurately dated [55], most have undergone metamictization and contain anomalous concentrations of non-formula elements [66]. Metamict zircons are also characteristically brecciated, corroded and veined. Halos of xenotime, typically 5–10 µm thick, are commonly observed, with the phosphate also forming part of the vein filling. While some textures are suggestive of xenotime nucleation on the surface of zircon, other textures are clearly replacive, as in the example illustrated in Figure 7e,f. Xenotime outgrowths on detrital zircon are widely reported from sedimentary sequences (e.g., [67]), and have, in some instances, been shown to be significantly younger than the zircon [68].

Xenotime and zircon share the same crystal structure [69], and overgrowths can be expected to be epitaxial. The relationships illustrated demand investigation at the nanoscale to resolve whether both minerals are a product of initial hypogene hydrothermal activity, as has been believed—although without evidence—or whether the xenotime overgrowths belong to a later stage of alteration. Like the ‘branneritization’ above, such work is well beyond the scope of the present study. It might, however, also encompass efforts to locate coarser grains that could be comparatively dated to elucidate their temporal relationships.

5.2.4. Coffinite-Xenotime

Much of the coffinite encountered in the tailings contains measurable Y, HREE, and P, often up to 5 wt.% HREE+Y. This represents a compositional continuum (solid solution) between coffinite and xenotime and is prevalent in all other South Australian IOCG systems, including Olympic Dam [63]. According to Förster [70], a coffinite-xenotime solid solution involves the substitution of Y and HREE for U and of phosphate for silicate within the mineral structures. This substitution forms a continuous compositional range between the two minerals, which has been studied in the context of thorite, zircon, and coffinite systems [69]. Coffinite without Y, HREE, and P content is scarce in the Carrapateena tailings and is primarily observed intergrown with and replaced by uranium carbonates. Coffinite-xenotime solid solution is significant for understanding uranium mineralization and the distribution of HREEs.

5.2.5. Coffinite and U-Carbonates

Several examples of U-carbonates are seen in the present study, in which uraninite is replaced by wyartite, rutherfordine, or mixtures thereof and preserved as relicts (Figure 11f,g). Rutherfordine has previously been suspected in copper concentrate samples from the Olympic Dam deposit; however, this has not been confirmed. Wyartite has not been observed before now in any of the IOCG ores from the Olympic Cu-Au Province or their concentrates or tailings.

Rutherfordine and wyartite are relatively common in a range of U deposits, where they occur as secondary minerals formed through the alteration of primary uranium-bearing phases like uraninite or coffinite [71]. Uranium carbonates form by oxidative dissolution of the uranyl ion, UO₂²⁺, from circulating low-temperature waters, in reaction with carbonate ions, possibly over a wide range of pH conditions [72]. In all Olympic Cu-Au Province IOCG ores, there is substantial mineralogical and isotopic evidence for multiple—possibly even continuous—episodes of fluid-assisted uranium mobility driven by radioactive heat [63,73,74,75]. Redox-controlled remobilization of uranium and radiogenic lead and their subsequent redeposition as the same mineral, or in another form, e.g., U as coffinite [63] or Pb as galena, clausthalite, and altaite [76], is recognized at scales ranging from nanometers to hundreds of meters.

A sound and adequate knowledge of the remobilization of uranium, its minerals, and the naturally occurring radioactive materials formed via decay of U and its daughter products is essential for the development of long-term health and environmental risk mitigation strategies. Establishing uranium deportments across giant IOCG deposits of the Olympic Cu-Au Province and through their respective processing plants continues to be a major focus of operators like BHP. Detailed characterization work over the past decade, involving novel nano- and microanalytical methodologies [65,77,78,79], has facilitated important, yet often surprising, advances in understanding. These include the driving role of coupled dissolution–replacement reactions across many, if not all, mineral groups and, implicitly, the importance of surface area and porosity, in turn explaining the apparent breakdown of secular equilibrium during concentrate leaching. As other deposits in the region are developed for production and others are discovered, this knowledge is critical for predicting likely radionuclide behavior. However, the subtly different geological settings of each deposit, and of distinct lithological or chemical zones within each deposit, highlight the need for generalized distribution models to be agile and adaptable. The discovery of U-carbonates in Carrapateena tailings is a good example that, on the one hand, adds to the overall complexity; however, on the other, it represents a valuable new find that helps resolve the puzzle.

Wyartite was the first U-mineral found to contain pentavalent uranium [80]. Although the mechanisms remain unclear, it is probably not coincidental that U5+ has been found to be the main oxidation state in hematite at Olympic Dam [81]. One of us has also observed rutherfordine and wyartite replacing coffinite and brannerite, commonly together with molybdenite, in samples from Oak Dam West (unpublished data). They may be more prevalent in deposits like Carrapateena or Oak Dam West, in which carbonate-bearing host rocks are more abundant. More than 40 different uranium carbonate minerals are currently recognized and named by the International Mineralogical Association [71]. Further work on these occurrences is likely to identify other species.

6. Conclusions

This study of Carrapateena mine tailings is primarily focused on identifying the mineralogy of rare earths and uranium. The approach aims to develop a basic understanding of tailing composition, mineral proportions, and grain size that could assist in evaluating the potential for future by-product recovery. Beyond the potential economic value of some contained components, these tailings also preserve valuable mineralogical and geochemical features that reflect the composition, alteration, and textures of the original ore body. Tailings, sourced from ores from different stopes across a large, zoned deposit, offer a snapshot of mineralogy that is not always available in hand specimens or drillcore samples. This study demonstrates that a wealth of detailed mineralogical information is often present in processing materials and can be readily extracted through the qualitative determination of mineral associations across size fractions like those used here. Such data, including the mineral relationships elucidated above, offer valuable insights into ore genesis and deposit evolution.

The focus on the distribution of REEs and speciation and chemistry of REE-minerals and their relationship with uranium minerals within the tailings offers insights into the history of the ore deposit itself, since they preserve complex mineral replacement and intergrowth relationships. Among these, the replacement between hematite and rutile, rutile and brannerite, zircon, and xenotime, or the formation of U-carbonates from coffinite, could be the subject of future nanoscale studies.

The >10 billion tonne Olympic Dam orebody, situated to the northwest of Carrapateena, is one of the best studied in the world in terms of deposit geology, ore genesis, geochronology hydrothermal alteration, isotope geochemistry, microstructural evolution, and the petrography and chemistry of different mineral groups. This is, in part, a reflection of its giant size and the mineralogical diversity in this world-class resource; however, it is also because optimized ore processing continues to rely on the exhaustive monitoring of ore feed composition, controls regarding the mineralogy and geochemistry of uranium and its daughter radioisotopes (e.g., [65,77]), and robust geometallurgical models that continue to evolve as new information becomes available [82]. A further reason for the strong emphasis on mineralogical research is the expanding interest in the distribution of metals beyond the four commodities exploited at present (Cu, Au, Ag, U), as well as the potential which a mine that is expected to run for many decades may have as a supplier of by-products. We anticipate that the results presented here can assist future engineers in capitalizing upon these endowments, and potentially also guide efforts to recover valuable metals from tailings elsewhere (e.g., [83,84]). Research of the kind described here, and in analogous studies (e.g., [85]) that integrate detailed mineralogy with quantitative geometallurgy, presents long-term advantages for realizing the economic potential of value minerals in complex deposits.

Compared with Olympic Dam, the other deposits in the region, Carrapateena, Prominent Hill, and Oak Dam West, are currently less well characterized. The present work can help to fill these gaps and, importantly, can contribute to the construction of holistic models for IOCG mineralization across the Olympic Cu-Au Province that explain the subtle differences between each deposit in the context of their distinct geological settings, host lithologies, and superimposed overprint events. This work also advances our understanding of the distinct associations displayed by the major minerals, as well as the intergrowth and replacement relationships among minerals, especially between REE- and U- bearing minerals, which help inform the genetic model but also have implications for processing.