Nundorite: a geological oddity of critical minerals significance from the Mount Arrowsmith Volcanics, northwestern NSW

Figures & data

Figure 1. Visual and chemical features of nundorite. (a) Photo of a polished cut slab of ‘outcrop’ nundorite with green aegirine crystals evident. (b) Zirconium vs Al₂O₃/SiO₂ of peralkaline igneous rocks from the global igneous geochemistry database of Gard et al. (2019). The data were filtered for intermediate to felsic composition (>50 wt% SiO₂) and evolved compositions with P₂O₅ <0.04 wt% and TiO₂ <0.46 wt% to match the characteristics of nundorite. Note, very few rocks globally compare to the composition of nundorite.

Figure 2. (a) Location, (b) geological setting of the northern Koonenberry Belt of western NSW and (c) local geological setting of the nundorite outcrop, adapted from Greenfield and Reid (2010) and Mills et al. (2015). Sample locations are presented by the coloured symbols, and nundorite sample locations are numbered (c).

Figure 3. Textural and mineralogical changes across nundorite sampling traverse from outcrop to foliated varieties from left to right. (a) Hand-sample photos of nundorite. Note, the prominent green aegirine in the outcrop nundorite samples and increasing degree of foliation across the sample series. (b) MLA mineral maps of nundorite samples (note, the MLA technique could not be used to distinguish natrolite from analcime). The colours refer to minerals shown in (c). (c) Modal mineralogy across the sample series based on MLA mapping result, EDS and WDS analysis, thin-section petrography and XRD analysis.

Figure 4. Backscattered electron images of outcrop nundorite samples. (a) Major and minor minerals in sample MA0802. (b) Monazite in interstices of aegirine, sample MA0804. (c) Euhedral schizolite and skeletal albite intergrown with matrix natrolite of sample MA0802. (d) Accessory REE and Zr phases in sample Nund-1. (e) Zr alkali silicate intergrown with aegirine in sample Nund-1. (f) Complexly zoned Zr alkali silicate associated with microcline and cerite-group minerals in sample MA0802. Mineral abbreviations: Aeg, aegirine; Crt, cerite-group mineral; Mnz, monazite; Nb, unidentified Nb-rich mineral; Ntr, natrolite; Mic, microcline; ZAS, zirconium alkali silicate.

Table 1. Representative major element oxide (wt%) composition of rare metal-bearing accessory phases found in nundorite.

Sample	MA0804	MA0807	Nund-1	MA0808	Nund-1	Nund-1	MA0807	Nund-1	MA0807	Nund-1	MA0502	Nund-1	MA0502	MA0508
Mineral	Monazite-(Ce)	Epidote–allanite	Cerite group	Cerite group	Cerite group	Galenbergite-(Ce)	Fergusonite	Natroniobite	Nb–Mn-silicate	Elpidite^a	Elpidite^a	Lemoynite^a	Lemoynite^a	Catapleite^a
SiO2	4.33	30.14	20.20	18.67	25.14	0.72	0.29	3.94	31.92	62.54	59.28	52.49	56.61	47.20
TiO2	–	0.33	0.01	0.00	0.00	–	0.03	0.23	0.06	0.04	0.01	0.02	0.03	0.03
ZrO2	–	–	–	–	–	–	–	–	–	17.74	17.69	21.33	22.11	30.67
ThO2	0.26	0.24	1.63	0.05	3.05	0.00	0.15	0.57	0.03	–	–	–	–	–
Al2O3	0.60	14.10	0.03	0.79	1.48	–	0.02	0.24	2.24	0.66	2.44	1.13	0.91	0.09
Fe2O3	–	15.50	0.00	2.26	0.00	0.72	–	–	–	–	–	–	–	–
FeO	0.02	–	–	–	–	–	0.23	1.56	5.73	0.61	0.05	0.07	0.07	0.07
MnO	0.92	1.98	0.00	1.13	3.16	–	0.30	1.24	25.44	2.75	0.39	2.35	0.96	0.18
CaO	0.20	9.12	2.44	4.41	0.97	4.36	1.46	4.41	0.25	0.63	2.52	3.56	3.96	7.74
SrO	0.10	0.17	0.30	0.42	0.00	1.99	0.00	0.43	0.00	–	–	–	–	–
Nb2O5	–	–	–	–	–	–	43.92	58.71	14.40	–	–	–	–	–
P2O5	27.24	–	–	–	–	–	–	–	–	–	–	–	–	–
La2O3	17.69	8.56	20.41	9.83	5.69	19.86	3.62	4.28	0.05	–	–	–	–	–
Ce2O3	31.15	13.74	40.53	35.13	38.86	34.04	14.82	6.70	0.04	–	–	–	–	–
Pr2O3	3.49	1.08	3.50	4.08	2.38	3.25	2.21	0.47	0.00	–	–	–	–	–
Nd2O3	10.37	2.51	7.92	11.80	7.77	8.42	10.35	1.10	0.08	–	–	–	–	–
Sm2O3	1.03	0.05	0.27	1.23	0.76	0.08	2.65	0.06	0.00	–	–	–	–	–
Y2O3	0.12	0.00	0.01	0.39	0.21	–	7.38	0.25	0.08	–	–	–	–	–
Gd2O3	0.46	0.08	0.02	0.54	0.55	0.00	2.42	0.12	0.03	–	–	–	–	–
Dy2O3	0.00	0.00	0.00	0.03	0.00	–	2.04	0.08	0.00	–	–	–	–	–
Na2O	–	0.12	1.78	0.09	0.04	–	0.13	3.78	2.88	4.80	5.42	4.55	3.18	9.97
K2O	–	–	–	–	–	–	0.03	0.12	5.03	0.54	1.89	0.42	1.30	0.08
Total	97.98	97.72	99.05	90.85	90.06	73.44	92.05	88.29	88.26	90.32	89.70	85.92	89.14	96.05

aAssignment of Zr minerals is regarded as tentative.

Low totals for Nb minerals are, in part, due to an expected component of Ta₂O₅, which was not measured here.

Figure 5. Backscattered electron images of mineral textures in foliated nundorite. (a) Poikiolitic aegirine with nepheline inclusions in sample MA0808. (b) Intergrown aegirine and aegirine-augite surrounded, and partly replaced by, hematite, sphalerite and clustered cerite-group minerals; sample MA0807. (c) Fine zircon and mica grains rimming microcline in sample MA0807. (d) Schizolite associated with zircon and cerite in sample MA0807. Mineral abbreviations: Aeg, aegirine; Aeg-Aug, aegirine-augite; Crt, cerite; Hem, hematite; Or, microcline; Sp, sphalerite; Szl, schizolite; Zrn, zircon.

Table 2. Representative major silicate mineral compositions (wt%) of nundorite samples.

Sample no.	MA0502	MA0502	Nund-1	MA0504	MA0505	MA0507	MA0508	MA0504	MA0507	Nund-1	MA0505	MA0508	MA0508	MA0502	Nund-1	Nund-1	MA0505	MA0507	MA0508
Mineral	Aegirine	Aegirine	Aegirine	Aegirine	Aegirine	Aegirine	Aegirine	Microcline	Microcline	Nepheline	Nepheline	Analcime	Analcime	Schizolite	Schizolite	Schizolite	Schizolite	Schizolite	Schizolite
SiO2	52.79	53.62	53.08	53.88	52.04	51.85	51.94	64.30	64.38	40.54	41.38	52.91	53.26	52.05	51.98	51.96	52.11	51.12	50.73
TiO2	0.06	0.04	0.01	0.05	0.09	0.01	0.03	–	–	–	–	–	–	0.00	0.00	0.00	0.00	0.00	0.00
ZrO2	0.04	0.00	0.06	0.00	0.09	0.00	0.25	–	–	–	–	–	–	–	–	–	–	–	–
Al2O3	1.99	5.32	4.92	7.38	1.36	1.41	1.62	18.69	18.40	35.24	34.70	21.14	22.52	0.00	0.00	0.01	0.03	0.02	0.12
Fe2O3	31.16	26.71	27.94	24.33	31.30	32.52	31.46	–	–	–	–	–	–	–	–	–	–	–	–
FeO	–	–	–	–	–	–		–	–	–	–	–	–	0.20	0.26	0.17	1.62	1.66	1.68
MnO	0.26	0.00	0.03	0.00	0.26	0.09	0.25	–	–	–	–	–	–	15.47	15.86	14.25	14.79	18.31	20.39
MgO	0.07	0.08	0.03	0.02	0.03	0.00	0.07	–	–	–	–	–	–	0.03	0.05	0.02	0.01	0.00	0.19
CaO	0.54	0.11	0.08	0.04	2.07	0.62	1.56	0.00	0.00	0.00	0.02	0.58	0.16	19.85	19.44	20.42	18.57	15.60	14.17
Na2O	13.50	13.64	13.69	13.86	12.51	13.12	12.78	0.12	0.17	15.89	16.91	13.41	12.75	8.97	8.83	9.10	8.80	9.01	9.07
K2O	0.01	0.01	0.02	0.00	0.00	0.00	0.01	16.27	16.59	7.86	7.29	0.01	0.64	0.01	0.00	0.05	0.01	0.02	0.07
H2O (calc)	–	–	–	–	–	–	–	–	–	–	–	7.84	7.96	2.60	2.59	2.59	2.59	2.57	2.55
Total	100.41	99.52	99.85	99.57	99.74	99.62	99.97	99.39	99.55	99.53	100.30	95.88	97.28	99.18	99.01	98.56	98.51	98.30	98.97
Stoichiometric formula
No. oxygens	6	6	6	6	6	6	6	8	8	4	4	6	6	8.5	8.5	8.5	8.5	8.5	8.5
Si	2.00	2.01	1.99	2.00	1.99	1.99	1.99	2.99	2.99	0.99	1.00	2.02	2.01	3.00	3.00	3.00	3.02	3.00	2.98
Ti	0.00	0.00	0.00	0.00	0.00	0.00	0.00	–	–	–	–	–	–	0.00	0.00	0.00	0.00	0.00	0.00
Zr	0.00	0.00	0.00	0.00	0.00	0.00	0.00	–	–	–	–	–	–	–	–	–	–	–	–
Al	0.09	0.23	0.22	0.32	0.06	0.06	0.07	1.02	1.01	1.01	0.99	0.95	1.00	0.00	0.00	0.00	0.00	0.00	0.01
Fe^3+	0.89	0.75	0.79	0.68	0.90	0.94	0.91	–	–	–	–	–	–	–	–	–	–	–	–
Fe^2+	–	–	–	–	–	–	–	–	–	–	–	–	–	0.01	0.01	0.01	0.08	0.08	0.08
Mn	0.01	0.00	0.00	0.00	0.01	0.00	0.01	–	–	–	–	–	–	0.76	0.78	0.70	0.73	0.91	1.01
Mg	0.00	0.00	0.00	0.00	0.00	0.00	0.00	–	–	–	–	–	–	0.00	0.00	0.00	0.00	0.00	0.02
Ca	0.02	0.00	0.00	0.00	0.08	0.03	0.06	0.00	0.00	0.00	0.00	0.02	0.01	1.23	1.20	1.26	1.15	0.98	0.89
Na	0.99	0.99	1.00	1.00	0.93	0.98	0.95	0.01	0.02	0.75	0.79	0.99	0.93	1.00	0.99	1.02	0.99	1.03	1.03
K	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.96	0.98	0.25	0.23	0.00	0.03	0.00	0.00	0.00	0.00	0.00	0.01
Cation total	4.01	3.99	4.00	4.00	3.99	4.00	3.99	4.99	5.00	3.00	3.01	4.00	3.97	6.00	5.99	6.00	5.97	6.01	6.03

All Fe assumed to be Fe³⁺ for aegirine and Fe²⁺ for schizolite. H₂O (calc) represent H₂O content calculated assuming perfect mineral stoichiometry.

Figure 6. Changes in endmember proportions of aegirine (in clinopyroxene) and pectolite (in schizolite) across the nundorite alteration series.

Table 3. Major and trace-element geochemistry and Nd isotope composition of samples of nundorite and associated mafic volcanic rocks.

Sample no.	MA0801	Nund-1	MA0802	MA0804	MA0805	MA0808	MA0807	MA0813	MA0814	MA1007	MA1014
Rock type	Outcrop nundorite	Outcrop nundorite	Outcrop nundorite	Outcrop nundorite	Foliated nundorite	Foliated nundorite	Foliated nundorite	Basanite	Basanite	Mafic	Basanite
Major elements (wt%)
SiO2	53.7	53.8	53.1	55.0	54.1	54.3	54.6	43.4	43.2	51.8	41.7
TiO2	<0.005	0.01	<0.005	<0.005	0.015	0.05	0.01	4.02	3.52	3.34	1.98
Al2O3	22.1	21.0	22.9	20.2	21.0	21.0	20.6	15.2	14.1	16.0	11.3
Fe2O3	3.72	3.69	3.76	3.62	3.85	3.78	3.66	14.00	13.20	12.10	9.84
MnO	0.22	0.25	0.21	0.16	0.24	0.23	0.22	0.16	0.14	0.08	0.13
MgO	0.07	0.02	0.02	0.13	0.02	0.11	0.06	3.93	2.87	3.76	4.89
CaO	0.37	0.40	0.44	0.17	0.43	0.71	0.58	6.30	8.74	1.83	17.2
Na2O	10.60	11.01	10.80	10.90	11.70	10.80	10.60	4.74	5.50	0.59	2.71
K2O	4.30	4.53	4.69	4.58	4.02	3.78	4.66	1.77	1.17	4.27	1.82
P2O5	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	0.60	0.58	0.13	0.34
LOI	5.03	5.11	4.28	5.39	5.23	5.63	5.28	5.02	7.45	5.18	8.30
Total	100.11	99.82	100.20	100.15	100.61	100.39	100.27	99.14	100.47	99.08	100.21
Trace elements (ppm)
Ba	60	<10	20	80	10	34	16	444	200	474	216
Be	13.5	nd	15.5	15.5	19	16.5	14	2	2	2	1
Co	<1	nd	<1	<1	<1	<1	<1	32	36	45	40
Cr	<10	nd	<10	<10	<10	<10	<10	<10	40	250	320
Cs	0.2	0.2	0.8	6.3	0.4	3.9	1.7	0.3	0.3	12.8	0.3
Cu	18	5	22	24	18	28	20	32	40	84	42
Ga	51	48	54	51	59	56	52	24.4	21.4	26.4	17.0
Hf	49	50	51	48	53	47	50	9	7	9	4
Li	40	80	40	<10	100	40	60	20	10	70	<10
Nb	270	255	260	245	320	260	290	55	50	50	30
Ni	<5	<5	<5	5	<5	<5	<5	5	40	115	165
P	<50	nd	<50	100	<50	50	<50	2300	2300	500	1400
Pb	21	19	23	23	25	23	24	4	2	2	2
Rb	153	178	186	150	173	118	142	24.4	17.2	105	26
Sc	<5	<5	<5	<5	<5	<5	<5	15	20	25	25
Sn	16.0	<5	18.4	15.2	18.1	16.5	17.0	2.4	2.4	2.1	1.5
Sr	36	4	34	38	28	156	60.5	816	487	39.5	427
Ta	26	22	24	24	28	24	26	4	4	4	<2
Th	33	28.1	31	29.5	37.5	31.5	33	4.5	3	3.5	2
U	8.8	7.9	9.4	7.5	9.2	8.8	8.9	0.9	0.6	3.9	0.7
V	<5	12	<5	<5	<5	<5	<5	165	270	235	225
Y	84	91	80	73	90	75	83	27	26	22	17
Zn	234	247	228	216	296	250	218	110	92	100	44
Zr	1720	1870	1880	1820	1880	1770	1860	280	210	280	120
La	146	127	144	131	176	146	162	32.2	28	24.9	18.1
Ce	268	234	290	247	328	277	297	76	69	60	36.5
Pr	29.5	30	32.3	27.3	35.6	30.4	32.5	9.45	8.55	7.7	5.4
Nd	111	98.2	111	92.1	119	103	110	38.6	35.6	32.5	22.9
Sm	18.2	20.7	18.8	16.9	21.5	19	18.6	8.4	7.85	7.9	4.9
Eu	3.7	3.7	4.05	3.35	4.25	3.8	3.9	3.00	2.9	2.45	1.75
Gd	17.8	19.6	18	16.2	20.8	18.8	19	9.0	8.6	7.6	5.6
Tb	2.64	3.0	2.80	2.50	3.06	2.82	2.82	1.18	1.14	0.94	0.72
Dy	16.3	16.9	16.4	15.1	18.1	17.1	16.7	6.4	6.15	5.0	4.2
Ho	3.10	3.4	3.10	2.88	3.46	3.28	3.22	1.14	1.12	0.90	0.76
Er	9.65	9.6	9.70	8.90	10.10	9.85	9.60	2.90	2.95	2.35	2.00
Tm	1.20	1.5	1.25	1.15	1.30	1.25	1.20	0.30	0.35	0.30	0.25
Yb	9.50	9.0	9.90	8.90	10.10	9.95	9.40	2.25	2.40	2.05	1.75
Lu	1.24	1.4	1.24	1.16	1.30	1.30	1.18	0.28	0.30	0.24	0.24
Total REE	638	578	663	574	753	644	687	191	175	155	106
Sm–Nd isotopes
Nd (ppm)	nd	80.8	102.2	nd	nd	100.1	nd	39.7	34.9	nd	nd
Sm (ppm)	nd	14.94	18.62	nd	nd	18.25	nd	8.36	7.58	nd	nd
^143Nd/^144Nd	nd	0.5125973	0.5125890	nd	nd	0.5125864	nd	0.5126402	0.5126581	nd	nd
2σ	nd	0.0000026	0.0000023	nd	nd	0.0000025	nd	0.0000028	0.0000015	nd	nd
^147Sm/^144Nd	nd	0.11176	0.11016	nd	nd	0.11024	nd	0.12718	0.13133	nd	nd
^143Nd/^144Nd (i)	nd	0.5121689	0.5121667	nd	nd	0.5121639	nd	0.5121527	0.5121547	nd	nd
εNd (i)	nd	5.56	5.52	nd	nd	5.46	nd	5.24	5.28	nd	nd
2σ	nd	0.05	0.05	nd	nd	0.05	nd	0.05	0.03	nd	nd
TDM (Ma)	nd	820	820	nd	nd	824	nd	890	902	nd	nd

nd = no data. Initial Nd isotope compositions (i) calculated to an age of 585 Ma. TDM = single-stage depleted mantle model age using ¹⁴⁷Sm λ = 6.54 × 10^–12.

Figure 7. Total-alkali vs silica diagram (from Le Maitre et al., 2002) of nundorite and basanite samples, and MAV rocks from Greenfield et al. (2011) and references therein. The data have been filtered to exclude analyses with >8 wt% LOI, and anomalously low Na₂O (<1 wt%) and K₂O (<0.10 wt%), indicative of modification via alteration. Diamond symbols are data from the literature, and circle symbol are data from this study.

Figure 8. NMORB-normalised multi-trace element plots of nundorite (n = 7) compared with; (a) syenites (n = 2) and basanites (n = 3) of the Mount Arrowsmith Volcanics, and (b) evolved, high alumina phonolites from the Apiau Alkaline Complex (AAC), Guyana Shield (n = 13; Figueiredo et al., 2022) and the Dunedin volcano (n = 11; Pontesilli et al., 2022; Price et al., 2003). Samples normalised to NMORB values of Sun and McDonough (1989).

Figure 9. εNd vs bulk rock Zr concentration on nundorite and rocks of the Mount Arrowsmith Volcanics (MAV). Note the similar εNd of nundorite and the MAV rocks. Circles are data from this study, and diamonds are data from Crawford et al. (1997), Zhou and Whitford (1994), and Greenfield et al. (2011). Symbol size is larger than measurement uncertainties.

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Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials.