Application of Raman spectroscopy to distinguish adularia and sanidine in drill cuttings from the Ngatamariki Geothermal Field, New Zealand

Figures & data

Figure 1 Map showing the location of the Ngatamariki Geothermal Field (NGF), with wells Ngatamariki-5 (NM5) and Ngatamariki-6 (NM6) in the south of the field. The approximate near surface (c. 500 m) extent of the hydrothermal system is marked by the 10 and 20 ohm resistivity contours from Stagpoole et al. (1985). Mapped faults are from Leonard et al. (2010), and the Whakamaru caldera boundary is the approximate boundary as proposed by Wilson et al. (1986).

Figure 2 Simplified stratigraphic column showing the subsurface stratigraphy encountered in wells NM5 and NM6. The stratigraphy reported by Ramirez & Rae (2009) and Rae et al. (2009) is simplified following the naming conventions of Gravley et al. (2006), in which the Tahorakuri Formation and any locally named ignimbrites or interbedded lava bodies form the Reporoa Group. Age data have been modified after Leonard et al. (2010).

Figure 3 Cross-polarised light photomicrographs and BSE image of potassium feldspars and alteration products. A, TVZ adularia from Ohakuri Dam (FB11-OD8a) used for reference; B, TVZ sanidine from the Whakamaru Group ignimbrites (SB-2028a) used as for reference; C, a typical potassium feldspar observed in the cuttings of NM5 with no obvious twinning; D, pseudomorphs of adularia with fine twinning associated with alteration products such as illite are sometimes observed in NM5; E, photomicrograph; and F, BSE image of a crystal (sample NM5-1600g) partially altered to adularia (medium grey in BSE image), albite (dark grey) and carbonate (light grey). The circles show the approximate region the EMP measurements were taken and analysis numbers correspond to those given in Table 1.

Table 1 Major element analysis for alteration products in NM5.

Sample	NM5-1300b/1	NM5-1300b/2	NM5-1600b/1	NM5-1600b/2	NM5-1600f/1	NM5-1600f/2	NM5-1600g/1	NM5-1600g/2	NM5-1600g/3
Type	Adularia	Illite?	Adularia	Albite	Adularia	Calcite?	Adularia	Albite	Ankerite?
SiO2	64.17	51.07	64.20	67.37	63.17	0.00	62.53	66.40	1.57
Al2O3	18.12	30.82	18.28	19.51	18.00	0.00	17.71	18.92	1.21
FeO	0.13	1.24	0.00	0.00	0.11	1.26	0.00	0.00	15.42
MgO	0.00	1.83	0.00	0.00	0.00	0.25	0.00	0.00	8.39
CaO	0.00	0.13	0.00	0.08	0.00	54.42	0.00	0.07	29.37
Na2O	0.11	0.07	0.41	10.98	0.35	0.00	0.45	9.78	0.00
K2O	16.70	8.70	16.32	0.24	16.40	0.02	16.33	2.36	0.43
BaO	0.04	0.00	0.03	0.00	0.08	0.00	0.02	0.04	0.02
Total	99.27	93.86	99.24	98.18	98.11	55.95	97.04	97.57	56.41
An	0.00		0.00	0.40	0.00		0.00	0.34
Ab	0.99		3.68	98.19	3.14		4.02	86.01
Or	99.01		96.32	1.41	96.86		95.98	13.66

Figure 4 A, Representative spectra showing major peak positions for epoxy resin, TVZ adularia (FB11-OD8), TVZ sanidine (SB-2028), and quartz. B, Averaged Raman spectra for the TVZ adularia and sanidine showing major peak positions in the 100–620 cm⁻¹ spectral region.

Table 2 Raman peak positions for samples outside the adularia field from PCA.

Sample	Peak positions (cm^−1)										Classification
Sanidine^*	123	nil	155	162	180	282	408	456	476	514	–
Orthoclase^*	122	141	157	176	198	282	403	455	475	513	-–
Sanidine^‡	–	–	160.3	–	–	284.7	–	456.6	474.7	512.9	–
Orthoclase^‡	–	–	156.5	–	–	282.1	–	454.9	477	512.7	–
SB-2028	122.3	nil	159.1	172.8	196	283.9	402.2	–	473.6	514.2	TVZ Sanidine
FB11-OD8	118.5	138.1	155.7	176.1	196.1	282.3	394.7	456.9	475.3	513	TVZ Adularia
1125a	120.1	139.3	156.2	c.175	196.2	282.8	405.2	455.6	475.2	513.3	Adularia
1125b	120.5	c.140	157.1	c.175	196.1	282.2	406.2	455	474.7	513.3	Adularia
1125c	119.2	–	153.7	175.4	c.195	281.7	405.4	c.455	475.1	513.1	Adularia
1175b	119.9	137.8	157.2	c.173	195.8	283.1	403	453.8	474.7	513.5	Adularia
1225a	120.7	c.138	156	c.178	196.2	282.2	405.2	455.1	474.9	513.1	Adularia
1225b	119.1	139	158.6	c.178	196.2	282.7	405.7	c.457	474.9	513	Adularia
1300a	118.2	139.5	156.2	c.174	195.8	283.3	404.4	457.6	474.7	513.2	Adularia
1350a	120.5	c.140	154.3	c.175	c.195	283	405.8	c.455	475.4	512.8	Adularia
1450f	127	-–	–	177	–	287	409	465	479	509	Plagioclase
1450g	121.7	139.3	157.6	c.172	196.4	281.8	398.6	454.8	475.1	513.6	Adularia
1500c	119.2	c.141	155.2	172.8	c.195	283.2	403.9	c.454	474.5	513.3	Adularia
1550e	120.4	c.137	152.8	176.3	196	280.3	405	454.9	475.2	513	Adularia
1600a	120.3	c.139	157	175.2	c.196	286.9	406.4	455.3	477	512.7	Adularia
1600e	119.7	c.138	153.5	176.2	196.4	280.7	405.7	455.2	475.2	513.1	Adularia
1600g	118.7	138.2	157.3	c.173	c.192	281.2	392.3	453.8	474.5	513	Adularia
1600h	119.2	c.141	155.4	c.170	c.190	280.6	393.4	c.455	475	513.2	Adularia
1650a	116.2	c.140	155.2	c.174	c.194	282.9	404.2	c.455	474.6	513.2	Adularia
1650d	111.1	c.139	153.5	c.175	c.196	281.3	404.2	c.454	473.7	512.3	Adularia
1700a	120.2	c.138	154.6	c.173	c.195	280.5	404.8	c.455	474.4	512.8	Adularia
1700b	c.122	–	152.7	175.7	195.1	280.8	404.7	453.9	474.8	512.6	Adularia
1700f	119.9	c.138	154	174.3	195.9	281.4	403.2	c.454	474.4	513	Adularia
1700g	119.2	c.139	155.7	c.172	c.195	282.9	403.4	454.6	474.4	513.3	Adularia
1750c	118.7	138.3	156.6	174.9	196	282.2	403.2	454.2	474.7	513	Adularia
1800c	120.9	139.8	156.2	c.173	196.7	282.7	405.3	465.4	475	513.3	Adularia

Main peaks used for identification are shown in bold. Published analyses for sanidine and orthoclase, as well as sanidine and adularia standards are shown for comparison.

* Mernagh (1991); ^‡Freeman et al. (2008).

Figure 5 A, Scores plot showing all samples after PCA. The unknown samples from Ngatamariki-5 plotting in a field containing adularia and sanidine were analysed further, those outside this field all plot in the quartz field.B, Scores plot showing PC-4 against PC-5 for the feldspar samples are shown. Taupo Volcanic Zone adularia is shown in blue, and TVZ sanidine in red, with the unknown NM5 drill-cutting samples, shown in grey, plotting in and around the adularia field. The NM5 samples within the blue field are assumed to be adularia, whereas those samples outside this field were analysed further using peak position data.

Figure 6 Loadings plots of PC-4 and -5 are shown. Peaks associated with adularia spectral features are given in blue and those associated with sanidine are given in red.

Figure 7 Ternary plot showing compositions for all feldspars analysed in this study, as well as TVZ sanidine and adularia from the TVZ reported elsewhere. Inset shows the orthoclase end member in a larger scale. ^*Hedenquist & Browne (1989); ^‡Brown (1994).

Table 3 Major element analysis for TVZ sanidine and adularia and representative NM5 samples.

Sample	SB-2028a	SB-2028c	FB11-OD8b	FB11-OD8d	NM5-1125b	NM5-1300a	NM5-1450a	NM5-1500c	NM5-1700c
Type	Sanidine	Sanidine	Adularia	Adularia	Adularia	Adularia	Adularia	Adularia	Adularia
SiO2	64.16	63.15	64.60	64.05	64.08	64.45	64.23	63.57	63.91
Al2O3	18.45	18.53	18.28	18.07	18.33	18.30	18.41	17.95	18.32
FeO	0.00	0.06	0.00	0.00	0.05	0.00	0.00	0.00	0.00
MgO	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
CaO	0.09	0.14	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Na2O	3.25	3.15	0.08	0.11	0.16	0.28	0.28	0.47	0.41
K2O	11.45	11.35	16.69	16.84	16.37	16.42	16.53	16.43	16.73
BaO	0.47	0.78	0.00	0.01	0.24	0.01	0.02	0.05	0.00
Total	97.87	97.16	99.65	99.08	99.23	99.46	99.47	98.47	99.37
An	0.46	0.72	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ab	30.00	29.45	0.72	0.98	1.46	2.53	2.51	4.17	3.59
Or	69.54	69.82	99.28	99.02	98.54	97.47	97.49	95.83	96.41

Appendix Electron microprobe results

Sample	SB-2028a	SB-2028b	SB-2028c	SB-2028d	FB11-OD8a	FB11-OD8b	FB11-OD8c	FB11-OD8d	NM5-1125a
Type	Sanidine	Sanidine	Sanidine	Sanidine	Adularia	Adularia	Adularia	Adularia	Adularia
SiO2	64.16	62.63	63.15	62.56	63.11	64.60	63.82	64.05	63.43
Al2O3	18.45	17.94	18.53	18.21	17.57	18.28	17.76	18.07	18.07
FeO	0.00	0.14	0.06	0.00	0.00	0.00	0.00	0.00	0.00
MgO	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
CaO	0.09	0.11	0.14	0.12	0.00	0.00	0.00	0.00	0.00
Na2O	3.25	3.30	3.15	3.13	0.07	0.08	0.09	0.11	0.25
K2O	11.45	11.59	11.35	11.42	16.91	16.69	16.70	16.84	16.63
BaO	0.47	0.40	0.78	0.69	0.02	0.00	0.02	0.01	0.09
Total	97.87	96.11	97.16	96.13	97.68	99.65	98.39	99.08	98.47
An	0.46	0.55	0.72	0.62	0.00	0.00	0.00	0.00	0.00
Ab	30.00	30.04	29.45	29.22	0.63	0.72	0.81	0.98	2.23
Or	69.54	69.41	69.82	70.16	99.38	99.28	99.19	99.02	97.77
Sample	NM5-1125b	NM5-1125c	NM5-1125d	NM5-1300a	NM5-1300b/1	NM5-1300b/2	NM5-1450a	NM5-1450b	NM5-1450d
Type	Adularia	Adularia	Adularia	Adularia	Adularia	Illite?	Adularia	Adularia	Adularia
SiO2	64.08	63.37	63.62	64.45	64.17	51.07	64.23	64.16	64.19
Al2O3	18.33	18.29	18.14	18.30	18.12	30.82	18.41	18.52	17.99
FeO	0.05	0.00	0.03	0.00	0.13	1.24	0.00	0.00	0.00
MgO	0.00	0.00	0.00	0.00	0.00	1.83	0.00	0.00	0.00
CaO	0.00	0.00	0.00	0.00	0.00	0.13	0.00	0.00	0.00
Na2O	0.16	0.29	0.21	0.28	0.11	0.07	0.28	0.26	0.17
K2O	16.37	16.50	16.63	16.42	16.70	8.70	16.53	16.43	16.54
BaO	0.24	0.06	0.03	0.01	0.04	0.00	0.02	0.03	0.00
Total	99.23	98.51	98.66	99.46	99.27	93.86	99.47	99.40	98.89
An	0.00	0.00	0.00	0.00	0.00		0.00	0.00	0.00
Ab	1.46	2.60	1.88	2.53	0.99		2.51	2.35	1.54
Or	98.54	97.40	98.12	97.47	99.01		97.49	97.65	98.46

Table

Sample	NM5-1450g	NM5-1500c	NM5-1500g	NM5-1600b/1	NM5-1600b/2	NM5-1600d/1	NM5-1600d/2	NM5-1600e
Type	Adularia	Adularia	Adularia	Adularia	Albite	Adularia	Albite	Adularia
SiO2	63.72	63.57	63.83	64.20	67.37	63.73	67.67	63.91
Al2O3	18.21	17.95	18.16	18.28	19.51	18.29	19.41	18.11
FeO	0.00	0.00	0.00	0.00	0.00	0.11	0.00	0.00
MgO	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
CaO	0.00	0.00	0.00	0.00	0.08	0.00	0.13	0.00
Na2O	0.34	0.47	0.45	0.41	10.98	0.51	11.03	0.40
K2O	16.41	16.43	16.26	16.32	0.24	16.24	0.17	16.34
BaO	0.07	0.05	0.02	0.03	0.00	0.05	0.00	0.00
Total	98.75	98.47	98.72	99.24	98.18	98.93	98.41	98.76
An	0.00	0.00	0.00	0.00	0.40	0.00	0.64	0.00
Ab	3.05	4.17	4.04	3.68	98.19	4.56	98.36	3.59
Or	96.95	95.83	95.96	96.32	1.41	95.45	1.00	96.41
Sample	NM5-1600f/1	NM5-1600f/2	NM5-1600g/1	NM5-1600g/2	NM5-1600g/3	NM5-1700e	NM5-1700f	NM5-1700g
Type	Adularia	Calcite?	Adularia	Albite	Ankerite?	Adularia	Adularia	Adularia
SiO2	63.17	0.00	62.53	66.40	1.57	64.02	63.66	64.34
Al2O3	18.00	0.00	17.71	18.92	1.21	18.15	18.22	18.31
FeO	0.11	1.26	0.00	0.00	15.42	0.00	0.07	0.00
MgO	0.00	0.25	0.00	0.00	8.39	0.00	0.00	0.00
CaO	0.00	54.42	0.00	0.07	29.37	0.00	0.00	0.00
Na2O	0.35	0.00	0.45	9.78	0.00	0.29	0.59	0.46
K2O	16.40	0.02	16.33	2.36	0.43	16.66	16.09	16.35
BaO	0.08	0.00	0.02	0.04	0.02	0.03	0.05	0.04
Total	98.11	55.95	97.04	97.57	56.41	99.15	98.68	99.50
An	0.00		0.00	0.34		0.00	0.00	0.00
Ab	3.14		4.02	86.01		2.58	5.28	4.10
Or	96.86		95.98	13.66		97.42	94.72	95.90

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