AEGC 2019: From Data to Discovery – Perth, Australia 1

Mineralization signatures of the magnetite-dominant Acropolis prospect, Olympic Dam IOCG district, South Australia Marija Dmitrijeva*,1 Cristiana L. Ciobanu1 Kathy J. Ehrig2 marija.dmitrijeva@adelaide.edu.au cristiana.ciobanu@adelaide.edu.au kathy.j.ehrig@bhp.com

Andrew V. Metcalfe3 Max R. Verdugo-Ihl1 Jocelyn McPhie4

andrew.metcalfe@adelaide.edu.au max.verdugoihl@adelaide.edu.au j.mcphie@utas.edu.au

Nigel J. Cook1 nigel.cook@adelaide.edu.au 1School of Chemical Engineering, The University of Adelaide 2BHP Olympic Dam, Adelaide 3School of Mathematical Sciences, The University of Adelaide 4CODES, The University of Tasmania

INTRODUCTION

Acropolis is a magnetite-dominant hematite-bearing prospect located ~20 km southwest from the giant Olympic Dam (OD) deposit. Displaying a range of mineralogical-petrological

similarities to OD, Acropolis is considered to be broadly cotemporaneous with mineralisation at OD (Mortimer et al., 1988). Nevertheless, unlike the OD deposit, which displays disseminated Cu(-Fe)-sulphides within hematite breccias hosted in Roxby Downs Granite of Hiltaba Suite (HS) affiliation, the Acropolis prospect represents a vein-style mineralization with weakly-developed brecciation (Cross, 1993) within felsic Gawler Range Volcanics (GRV) (Mortimer et al., 1988). Both magmatic suites, HS and GRV, form the Large Igneous Province emplaced at ~ 1.6 Ga in the Gawler Craton, South Australia (Allen et al., 2008). At Acropolis, the GRV units are comprised of rhyolitic, dacitic and andesitic lavas, as well as minor ignimbrites (McPhie, 2016). Within the area, the basement is composed of Donington Suite granitoids (~1850 Ma; Jagodzinski, 2005) which, also host mineralization. Weakly-brecciated Hiltaba Suite-affiliated granite (drillhole ACD7), is also as a relatively minor host rock for mineralization (McPhie, 2016).

Because of the proximity of Acropolis and evident similarities with the styles of alteration at Olympic Dam (Ehrig et al., 2017), this case study aims at obtaining geochemical signatures of this magnetite-rich end-member of the IOCG clan, using statistical analyses of whole rock data. Multivariate statistical analyses are effectively applied to find structures and patterns within geochemical data which lack pre-defined categorical variables (Caciagli 2016). Such an approach allows identification of mineralized domains with distinct geochemical characteristics, combining the chemistry of protolith lithologies and superimposed geochemical signatures of the mineralization. Coupled with an implicit 3D geological model of Acropolis, these techniques allow visualization of the associations of key elements and mineralization within 3D space. The results obtained here provide insight into the development of IOCG systems from a magnetite- to hematite-dominant end-member.

METHODS

In this case study, principal component analysis (PCA), hierarchical and k-means clustering are applied to a large multi-element data set (4,864 samples, total of 36 elements), collected from fourteen drillholes at the Acropolis prospect (Figure 1). Hierarchical clustering was applied to a variation matrix containing the variances of log-ratios of every possible element pairs:

SUMMARY

The Acropolis prospect is a vein-style magnetite (±apatite ±hematite) system located ~20 km southwest from the giant Olympic Dam iron-oxide copper gold (IOCG) deposit, South Australia. A whole rock dataset comprising 4,864 core samples from fourteen drillholes was analysed using multivariate statistics to understand and identify geochemical signatures of mineralization at Acropolis. Statistical analyses included principal component analysis, hierarchical and k-means clustering. The results of statistical analyses are overlaid and interpreted relative to a 3D implicit geological model of the prospect, and encompass a projection of mineralization signature as PC1. The mineralization footprint of Acropolis is multi-element and typified by a distinct ‘magnetite’ signature of Fe-V-Ni-Co. Such a signature is developed in the western part of the prospect and represents samples containing >60 wt% Fe. In contrast, the ‘hematite’ signature displays an association of REE, W, Sn, Sb, U, Th, Ca, and P and is present throughout the prospect. Furthermore, kriged values of Cu (> 200 ppm) demonstrate an offset from Fe-rich veins, thus supporting a genetic model in which (earlier) Cu-Au-deficient veins in which magnetite is the dominant Fe-oxide are subsequently overprinted by Cu-bearing hematite-dominant mineralization. Results obtained provide insights into the evolution from magnetite- to hematite-dominant IOCG systems and may offer a proxy for exploration and discovery of economically significant IOCG deposits at shallower levels in the eastern Gawler Craton.

Key words: Acropolis, magnetite-dominant IOCG, statistical analysis, implicit modelling

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AEGC 2019: From Data to Discovery – Perth, Australia 2

𝜏"# = 𝑣𝑎𝑟(ln(𝑥"/𝑥#) (1)

The compositional properties of the geochemical data were respected by obtaining isometric log-ratio (ilr) and centred log-ratio (clr) coordinates, which were further utilized for k-means clustering and PCA respectively. In the k-means procedure, the bootstrapping algorithm was used to test the stability of each cluster by calculating a Jaccard similarity value. The optimal number of 𝑘 clusters was determined as seven based on the sufficient minimal value of the within-cluster sum of squares.

Figure 1. Drillholes and corresponding lithologies at the Acropolis prospect.

This study incorporates 3D modelling of the major faults, intrusions and lithological groups at the Acropolis prospect using the fault interpretations of McPhie (2016; Figure 2), and careful correlation of felsic GRV units using the Ti/Zr and Ti/Al ratios. Using the 3D model, we are able to overlay the lithologies of Acropolis with the interpolated results of statistical analyses as PC1 scores, the most geochemically meaningful clusters obtained from k-means algorithm and the outlines of 25 wt% Fe and 200 ppm Cu. Such a holistic approach allows us to understand the extents of mineralization signatures and relate these to the lithologies present within the Acropolis prospect.

Figure 2. Implicit 3D geological model of the Acropolis prospect based on the fourteen drillholes created in Leapfrog Geo software. The legend is as in Figure 1.

RESULTS

The hierarchical dendrogram (Figure 3) provides a summary of the associations among the elements and suggests four main geochemical groups. Group 1, comprising Ba, Zr, Al, Si, Na, K and Rb represents a geochemical signature of granites and GRV felsic units and is also indicative of silicate minerals which are the products of

hydrothermal alteration within the host rocks. In contrast, the mineralization signature is typified by Groups 2, 3 and 4, whereby Group 4 is associated with Fe-dominant mineralization, thus reflecting the strong co-dependence among Fe, V, Ni and Co, i.e. ‘magnetite’ signature, along with sulphide signature of Cu, Mo and Bi. Group 3, which encompasses W, Sb, Zn, Ti, Sc, Mg, Li, Ca and P, is evidently associated with Group 4, and represents a range of indicator and pathfinder elements typically present within a deposit of IOCG sensu stricto type (Groves et al., 2010). Combined, Groups 3 and 4 form an indicative IOCG-signature within magnetite-dominant mineralization at Acropolis.

Figure 3. Dendrogram of 36 elements grouped according to their geochemical affinities. The dissimilarity measure was obtained from variation matrix based on Equation. 1. The seven clusters identified by the 𝑘-means algorithm are used to define geochemical domains which have direct implication for characterization of data, as they reflect the original lithology along with the superimposed alteration and mineralization (Table 1). The most geologically significant clusters are Cluster 2, 3 and 6. Cluster 2 represents early sodic alteration within the Donington Suite granite and diorite in the western part of the prospect (Figure 4). Cluster 6 represents Fe-rich veins, predominantly composed of magnetite. Finally, Cluster 3 is comprised of rhyolites and dacites with relatively higher Fe content of 30 wt% and is distributed throughout the prospect independently from the Fe-rich veins. The interpolation of Fe 25 wt% and Cu 200 ppm provides additional comparison with the lithologies and clusters at Acropolis and indicates a Cu mineralization offset from Fe-rich veins (Figure 4). A plot of magnetic susceptibility values over clusters also suggest that Cluster 3 is predominantly hematite-bearing (Figure 5). The projection of 4,864 samples on PC1 and PC2 demonstrate geochemical affinities of Clusters and significantly complements the result of hierarchical and 𝑘-means analyses. The PC1-2 projection distinguishes Cluster 6 and its strong association with IOCG-signature elements: Fe-V-Ni-Co and with less pronounced loadings of Ti, Mn and Zn on PC1. (Figure 6). Similarly, Cluster 3 shows a strong correlation with Group 2 and Group 3 elements, thus likely representing altered and/or partially mineralized/vein samples, i.e. ‘hematite’ signature rather than the magnetite-dominant mineralization itself. Finally, the spatial projection of PC1 scores in 3D indicates mineralisation signatures within the western zone of the prospect, namely in drillholes ACD1, ACD9 and ACD7. Additionally, a mineralization signature is present within the Donington Suite granite in the western zone (Figure 7).

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Figure 4. Spatial projection of Clusters 2, 3 and 6 in oblique view, overlaid by the outlines of Cu > 200 ppm and Fe > 25 wt%.

Figure 5. Magnetic susceptibility versus density, colour-coded according to a cluster membership and outlined by 0.95 confidence ellipses for Cluster 2, 3 and 6.

Figure 6. Projection of clr-transformed data demonstrating seven clusters obtained from k-mean algorithm and corresponding element loadings. Each circle represents a single data point. Elements are colour-coded according to the Groups (as in Figure 3).

DISCUSSION

Many magnetite-dominant IOCG systems are known to be deficient with respect to Cu and Au (Barton, 2014). Similarities among alteration types, ore and gangue mineralogy as well as the presence of early magnetite-apatite assemblages within many IOCG deposits has prompted a genetic relation between IOCG and Kiruna-type magnetite-apatite deposits (Hitzman et al., 1992), in spite of the lack of an evident transition between

the two. Within the eastern Gawler Craton, and specifically in the immediate proximity of Olympic Dam, magnetite-apatite (± siderite ± chlorite) assemblages correspond to low-grade Cu-U-Au and are regarded as the envelopes of larger IOCG systems (Ehrig et al., 2012). Analogous to the clan of IOCG systems, which are known to display elevated concentrations of many elements (Groves et al. 2010), the manifestation of the Acropolis IOCG signature is multi-element (Figure 5). The U-W-Sn-Mo signature in hematite at Olympic Dam (Verdugo-Ihl et al., 2017) is recognised in geochemical studies as a distinct W-Mo-Sn subset besides the dominant Fe, Cu, Au association in the IOCG signature obtained from PCA analysis (Dmitrijeva et al., 2019). Although Acropolis is deficient in terms of Au (> 70% of values are below the minimum detection limit) and contains geochemically anomalous, but low concentrations of Cu, it demonstrates a mineralisation signature analogous to that at Olympic Dam, with high loadings of W, Sn, Sb, U, Th and REE (Cluster 3), albeit at trace concentrations. At Acropolis, sulphide minerals, particularly chalcopyrite, are predominantly associated with hematite-sericite-chlorite assemblages, which replace early magnetite-apatite (Krneta et al., 2017). An analogue mineral assemblage of early sulphide-poor magnetite-apatite overprinted by later sulphur-enriched Fe-oxide-Cu-Au mineralization is recorded at other IOCG systems outside the Gawler Craton (Ernest Henry, Queensland; Mark et al., 2006). The results of statistical analyses demonstrate that although Fe-oxide and Cu(-Fe)-sulphide mineralization are associated, they offset each other in some measure, reaffirming that assemblages observed in petrographic records correspond to patterns within the whole rock data. Cluster 3, which is characterised by Ca-P-Li-Mg-Sc-Ti-Zn-W-Sn-Sb, along with REE, Sr, U and Th is evidently associated with hematite-dominant alteration (Figure 5), and likely represents its geochemical footprint, overprinting early Cu-deficient magnetite-apatite assemblages. The projection of Cluster 3 in Leapfrog (Figure 4), indicates that such a signature does not form haloes around Fe-rich veins (Cluster 6), but is actually ubiquitous throughout the Acropolis prospect, confirming that a transition from magnetite- to a hematite dominant system involves significant metal endowment.

Figure 7. Spatial projection of PC1 scores in oblique view, displaying zones of most intense mineralization at Acropolis.

Overall, these results suggest that recognition of magnetite-dominant members of the IOCG clan are of critical importance in establishing the spatio-temporal model of entire IOCG systems and can facilitate discovery of metal endowed hematite-dominant IOCG deposits. This is particularly relevant in productive mineral districts, such as the Olympic Cu-Au province, where basement and the mineralization contained within is buried beneath a thick, barren sedimentary cover. Therefore, an improved understanding of the genesis and full

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extent of zonation in IOCG systems is fundamental for effective exploration, both at the local and regional scales.

CONCLUSIONS

Application of statistical analyses combined with implicit 3D geological modelling is an effective way to identify patterns and structures within the data in the absence of detailed petrographic and mineralogical data. Despite being magnetite-dominant, the Acropolis prospect displays a multi-element mineralisation signature, which is partially similar to that of Olympic Dam, with W, Sn, Mo, and REE distributed throughout the prospect. Within Acropolis, Fe and Cu mineralization are slightly offset, thus being consistent with observation of Cu-deficient magnetite-apatite veins in the western part of Acropolis and overprinting Cu-endowed hematite-dominant assemblages that extend throughout the prospect area, represented as Cluster 3. This conclusion is supported by the low magnetic susceptibility of Cluster 3 and a range of indicator elements which are typically hosted within a hematite-dominant assemblage: U, Th, REE, W, Sn and Sb. In contrast, magnetite mineralization (Cluster 6) is typified by a Fe-V-Ni-Co signature with less pronounced loadings of Ti, Mn and Zn. The magnetite veins may represent the upper part, or alternatively, the roots of an eroded or undiscovered Cu-Au mineralization, which, although partially coincident, is not centred on the HS granite. The Fe-V-Ni-Co signature is an indicator of relatively early IOCG mineralization and is a significant proxy for exploration of fertile hematite-dominant IOCG deposits at shallower crustal levels.

ACKNOWLEDGEMENTS

This work is a contribution to the FOX project ‘Trace elements in iron-oxides: deportment, distribution and application in ore genesis, geochronology, exploration and mineral processing’, supported by BHP Olympic Dam and the South Australian Mining and Petroleum Services Centre of Excellence. N.J.C. acknowledges additional support from the ARC Research Hub for Australian Copper-Uranium (Grant IH130200033).

REFERENCES

Allen, S.R., McPhie, J., Ferris, G., and Simpson, C., 2008, Evolution and architecture of a large felsic Igneous Province in western Laurentia: the 1.6 Ga Gawler Range Volcanics, South Australia. Journal of Volcanology and Geothermal Research 172, 132–147. Barton, M.D., 2014, Iron oxide (–Cu–Au–REE–P–Ag–U–Co) systems: In: Holland, H.D., and Turekian, K.K. (Eds.), Treatise on Geochemistry, 2nd ed., Elsevier, Oxford, 515–541. Caciagli, N., 2016, Multielement geochemical modelling for mine planning: case study from an epithermal gold deposit: In: Martín-Fernández, J.A., and Thió-Henestrosa, S. (Eds.), Compositional Data Analysis. Springer Proceedings in Mathematics and Statistics, 187, 45–61. Cross, K.C., 1993, Acropolis and Wirrda Well: In: Drexel, J.F., Preiss, W.V., and Parker, A.J. (eds.), The Geology of South

Australia. vol. 1, The Precambrian. Geological Survey of South Australia, South Australia, 138 pp. Dmitrijeva, M., Ehrig, K., Ciobanu, C., Cook, N. Verdugo-Ihl, M., and Metcalfe, A., 2019, Defining IOCG signatures through compositional data analysis: A case study of lithogeochemical zoning from the Olympic Dam deposit, South Australia: Ore Geology Reviews, 105, 86-101. Ehrig, K., McPhie, J., and Kamenetsky, V., 2012, Geology and mineralogical zonation of the Olympic Dam iron oxide Cu-U-Au-Ag deposit, South Australia: Economic Geology, Special Publication 16, 237-267. Ehrig, K. Kamenetsky, V.S., McPhie, J. Apukhtina, O., Ciobanu, C., Cook, N., Kontonikas-Charos, A., and Krneta, S. 2017, The IOCG-IOA Olympic Dam Cu-U-Au-Ag deposit and nearby prospects, South Australia: Proceedings, 14th Biennial SGA Meeting ‘Mineral Resources to Discover’ Québec City, Canada, vol. 3, p. 823-826. Groves, D.I., Bierlein, F.P., Meinert, L.D., and Hitzman, M.W., 2010, Iron Oxide Copper-Gold (IOCG) deposits through earth history: implications for origin, lithospheric setting, and distinction from other epigenetic iron oxide deposits: Economic Geology, 105, 641–654. Hitzman, M.W., Oreskes, N., and Einaudi, M.T., 1992, Geological characteristics and tectonic setting of Proterozoic iron oxide (Cu-U-Au-REE) deposits: Precambrian Research, 58, 241–287. Jagodzinski, E.A., 2005, Compilation of SHRIMP U–Pb geochronological data – Olympic Domain, Gawler Craton, South Australia, 2001–2003: Geoscience Australia Record 2005/20, 197 pp. Krneta, S., Cook, N.J., Ciobanu, C.L., Ehrig, K.J., and Kontonikas-Charos, A., 2017, The Wirrda Well and Acropolis prospects, Gawler Craton, South Australia: Insights into evolving fluid conditions through apatite chemistry: Journal of Geochemical Exploration, 181, 276-291. Mark, G., Oliver, N.H.S., and Williams, P.J. 2006, Mineralogical and chemical evolution of the Ernest Henry Fe oxide–Cu–Au ore system, Cloncurry district, northwest Queensland, Australia: Mineralium Deposita 40, 769-801. McPhie, J., 2016, Acropolis Report (unpublished): BHP Billiton Olympic Dam. Mortimer, G.E., Cooper, J.A., Paterson, H.L., Cross, K., Hudson, G.R.T., and Uppill, R.K., 1988, Zircon U–Pb Dating in the Vicinity of the Olympic Dam Cu–U–Au Deposit, Roxby Downs, South-Australia: Economic Geology, 83, 694–709. Verdugo-Ihl, M.R., Ciobanu, C.L., Cook, N.J., Ehrig, K., Courtney-Davies, L., and Gilbert, S., 2017, Textures and U-W-Sn-Mo signatures in hematite from the Cu-U-Au-Ag orebody at Olympic Dam, South Australia: defining the archetype for IOCG deposits: Ore Geology Reviews, 91, 173–195.

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Table 1. Characteristics of seven clusters obtained from k-means cluster analysis.

Cluster 𝑛, samples

Jaccard similarity value, 𝑗"

Predominant cluster lithology and median values of Fe, K and Cu

Median density, kg/m3

Median magnetic susceptibility,

×10-3 SI 1 779 0.57 Altered, least-veined rhyolites and minor dacites with 5% Fe,

5-6% K and 35 ppm Cu. 2.6 0.86

2 171 0.74 Altered Donington granite and diorite with pronounced sodic alteration; 8% Fe, 3.5 % K and 85 ppm Cu

2.7 0.94

3 619 0.84 Altered and intensively veined rhyolites and dacites with 30% Fe, 2.7% K and 137 ppm Cu

3.1 1.52

4 1384 0.69 Altered and moderately veined rhyolites, dacites, HS and Donington granite with 10% Fe, 5% K and 180 ppm Cu

2.7 1.91

5 712 0.74 Altered Donington granite and diorite with minor dacites; 20% Fe, 4% K and high Cu content (530 ppm)

2.9 28

6 441 0.93 Fe-rich veins; Fe >60%, K <0.1% and high Cu content (430 ppm)

4.2 306

7 759 0.75 Altered and veined rhyolites, dacites and ignimbrites with Fe 25% and K 2% and 280 ppm Cu

3.1 28