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Rare earth element geochemistry of feldspars: examples from Fe-oxide Cu-Au systems in the Olympic Cu-Au Province, South Australia

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Abstract

Rare earth element (REE) fractionation trends in feldspars are reported from Olympic Dam (including Wirrda Well and Phillip’s Ridge) and Cape Donington (Port Lincoln), for comparison with two other igneous-hydrothermal terranes within the eastern Gawler Craton: Moonta-Wallaroo and Hillside. The case studies were selected as they represent ~ 1590 Ma Hiltaba Suite and/or ~ 1845 − 1810 Ma Donington Suite granites, and, aside from Cape Donington, are associated with Mesoproterozoic iron-oxide copper gold (IOCG)-type mineralization. Both plagioclase and alkali feldspar were analyzed within selected samples with the purpose of constraining and linking changes in REE concentrations and fractionation trends in feldspars to local and whole-rock textures and geochemistry. Two unique, reproducible fractionation trends were obtained for igneous plagioclase and alkali feldspars, distinguished from one another by light rare earth element enrichment, Eu-anomalies and degrees of fractionation (e.g. La/Lu slopes). Results for hydrothermal albite and K-feldspar indicate that REE concentrations and fractionation trends are generally inherited from igneous predecessors, however in some instances, significant amounts of REE appear to have been lost to the fluid. These results may have critical implications for the formation of world-class IOCG systems, in which widespread alkali metasomatism plays a key role by altering the physical and chemical properties of the host rocks during early stages of IOCG formation, as well as trapping trace elements (including REE).

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Fig. 1

Modified from Reid and Fabris (2015). b Geological sketch map of bedrock in the Olympic Dam region displaying sampled drillholes and other prospects and localities within the area

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Acknowledgements

Staff at Adelaide Microscopy assisted with microanalysis. Constructive comments by Panagiotis Voudouris, an anonymous reviewer and journal editor William Guenthner are gratefully acknowledged. BHP Olympic Dam kindly provided financial support and access to Olympic Dam samples and facilities. We also acknowledge the ‘FOX’ project (Trace elements in iron oxides), supported by BHP and the South Australian Government Mining and Petroleum Services Centre of Excellence.

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Authors and Affiliations

  1. School of Physical Sciences, The University of Adelaide, Adelaide, SA, 5000, Australia

    Alkis Kontonikas-Charos & Sasha Krneta

  2. School of Chemical Engineering, The University of Adelaide, Adelaide, SA, 5000, Australia

    Cristiana L. Ciobanu & Nigel J. Cook

  3. BHP Olympic Dam, Adelaide, SA, 5000, Australia

    Kathy Ehrig

  4. School of Physical Sciences, The University of Tasmania, Hobart, TAS, 7001, Australia

    Vadim S. Kamenetsky

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  1. Alkis Kontonikas-Charos
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Correspondence to Alkis Kontonikas-Charos.

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Appendix 1 - details of analytical methodology

Appendix 1 - details of analytical methodology

Electron probe microanalysis (EPMA)

Standards, X-ray lines, count times and typical minimum detection limits (mdl) for this work are given in the table below.

Element

Standard

X-ray line

Count time (s) peak/background

Average mdl

(ppm)

Na

Albite

Na Kα

20/10

120

K

Sanidine

K Kα

20/10

130

Ca

Wollastonite

Ca Kα

20/10

120

Fe

Almandine

Fe Kα

20/10

350

Al

Almandine

Al Kα

20/10

130

Si

Sanidine

Si Kα

20/10

150

Ba

Barite

Ba Kα

20/10

300

Sr

Celestite

Sr Lα

20/10

600

Laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS)

All multi-element LA-ICP-MS data was collected on a Resonetics M-50-LR 193-nm Excimer laser microprobe coupled to an Agilent 7700cx Quadrupole ICP-MS (Adelaide Microscopy). Trace element spot analysis was carried out using a uniform spot size diameter of 40 μm for plagioclase and alkali feldspars. The laser system was operated at pulse rates of 10 Hz and power levels of 50% corresponding to laser energy output around 6 – 9 J/cm-2, giving an ablation rate of approximately 1.5 μm/s-1. The following set of isotopes were monitored: 23Na, 24Mg, 27Al, 29Si, 31P, 39K, 43Ca, 45Sc, 47Ti, 48Ti, 49Ti, 51V, 53Cr, 55Mn, 57Fe, 58Fe, 59Co, 60Ni, 65Cu, 66Zn, 69Ga, 75As, 85Rb, 88Sr 89Y, 90Zr, 93Nb, 95Mo, 118Sn, 133Cs, 137Ba, 139La, 140Ce, 141Pr, 146Nd, 147Sm, 153Eu, 157Gd, 159Tb, 163Dy, 165Ho, 166Er, 169Tm, 172Yb, 175Lu, 178Hf, 181Ta, 182W, 206Pb, 207Pb, 208Pb, 232Th and 238U. Dwell times of 0.05 s were used for Y, REEs, Pb, Th and U, whereas 0.01 s was used for other elements. Average minimum detection limits for Y, REE, Pb, Th and U are given in the table below. Analysis time for each spot analysis was a uniform 90 s, comprising a 30-s measurement of background (laser-off), and 60-second analysis of the unknown (laser-on). Standard reference materials for all mineral matrices were NIST-610 using coefficients given by Pearce et al. (1997). Standards were run after each 20 – 24 unknowns; detection limits were calculated for each element in each spot analysis. Internal calibration was achieved using ideal concentration values for Al for feldspars. Data reduction was performed using Glitter software (Van Achterbergh et al. 2001).

Element

mdl (ppm)

Element

mdl (ppm)

Y

0.05

Dy

0.05

La

0.02

Ho

0.01

Ce

0.02

Er

0.04

Pr

0.01

Tm

0.02

Nd

0.08

Yb

0.05

Sm

0.06

Lu

0.02

Eu

0.03

Pb

0.09

Gd

0.09

Th

0.02

Tb

0.01

U

0.01

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Kontonikas-Charos, A., Ciobanu, C.L., Cook, N.J. et al. Rare earth element geochemistry of feldspars: examples from Fe-oxide Cu-Au systems in the Olympic Cu-Au Province, South Australia. Miner Petrol 112, 145–172 (2018). https://doi.org/10.1007/s00710-017-0533-z

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