Geochemistry of a New Zealand nephrite weathering rind

Figures & data

Table 1  EPMA analyses and formulae* of actinolite in nephritic greenstone

Analysis No Grain habit	1 Gr	2 Gr	3 Gr	4 Fgr	5 Ac	6 Ac	7 Fb	8 Fb	9 Fb	10 Fb	11 Fb	12 Bfb	13 Bfb	14 Bfb	15 Bfb
SiO2	57.53	57.62	57.02	57.00	57.50	57.64	57.56	57.25	57.42	57.80	57.92	57.36	57.85	57.05	57.16
TiO2	0.02	0.00	0.00	0.02	0.04	0.03	0.02	0.01	0.03	0.01	0.00	0.00	0.00	0.00	0.00
AlsO3	0.61	0.61	0.43	0.93	0.60	0.56	0.39	0.47	0.46	0.45	0.44	1.23	0.47	0.42	1.22
Cr2O2	0.24	0.36	0.18	0.17	0.26	0.35	0.13	0.08	0.11	0.15	0.20	0.15	0.26	0.19	0.15
FeO**	4.94	5.00	4.68	5.15	4.97	4.87	4.69	4.81	4.93	4.62	4.86	5.37	5.24	4.79	5.36
MnO	0.10	0.18	0.20	0.06	0.09	0.13	0.10	0.24	0.05	0.17	0.12	0.11	0.21	0.12	0.11
MgO	20.44	20.75	20.98	20.19	20.98	21.08	20.69	20.92	20.78	21.13	20.91	20.58	21.08	21.19	20.46
NiO	0.22	0.17	0.15	0.25	0.09	0.22	0.20	0.13	0.15	0.22	0.17	0.16	0.08	0.12	0.16
CaO	13.46	13.20	13.37	13.38	13.43	13.42	13.33	13.43	13.27	13.31	13.43	13.04	13.10	13.42	13.04
Na2O	0.04	0.02	0.07	0.05	0.01	0.01	0.07	0.05	0.06	0.03	0.02	0.01	0.02	0.05	0.03
KsO	0.01	0.02	0.02	0.01	0.00	0.02	0.02	0.03	0.02	0.00	0.01	0.01	0.01	0.03	0.01
HsO#	2.16	2.17	2.16	2.16	2.17	2.18	2.15	2.16	2.16	2.17	2.17	2.18	2.18	2.16	2.17
Total	99.77	100.10	99.26	99.37	100.14	100.51	99.35	99.58	99.44	100.06	100.25	100.20	100.50	99.54	99.87
Si	7.981	7.966	7.953	7.949	7.947	7.939	8.006	7.96	7.986	7.980	7.991	7.902	7.945	7.926	7.883
Al	0.019	0.034	0.047	0.051	0.053	0.061		0.04	0.014	0.020	0.009	0.098	0.055	0.069	0.117
TET	8.000	8.000	8.000	8.000	8.000	8.000	8.006	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000
Al	0.081	0.066	0.024	0.102	0.045	0.030	0.064	0.037	0.061	0.053	0.063	0.101	0.021	0.081
Ti	0.002			0.002	0.004	0.003	0.002	0.001	0.003	0.001
Cr	0.026	0.039	0.020	0.019	0.028	0.038	0.014	0.009	0.012	0.016	0.022	0.016	0.028	0.019	0.016
Fe3+	0.000	0.009	0.000	0.000	0.000	0.001	0.000	0.000	0.000	0.000	0.000	0.024	0.028	0.057	0.035
Fe2+	0.573	0.569	0.546	0.601	0.574	0.560	0.546	0.559	0.573	0.533	0.561	0.595	0.474	0.500	0.583
Mn	0.012	0.021	0.024	0.007	0.011	0.015	0.012	0.028	0.006	0.020	0.014	0.013	0.024	0.014	0.013
Mg	4.227	4.277	4.362	4.197	4.323	4.328	4.29	4.336	4.308	4.349	4.301	4.226	4.316	4.388	4.254
Ni	0.025	0.019	0.017	0.028	0.010	0.024	0.022	0.015	0.017	0.024	0.019	0.024	0.009	0.028	0.018
M[1]–[3]	4.946	5.000	4.993	4.956	4.995	4.999	4.950	4.985	4.980	4.996	4.980	4.999	4.900	5.006	5.000
Ca	2.001	1.995	1.998	1.999	1.989	1.980	1.986	2.001	1.977	1.969	1.978	1.975	1.928	1.992	1.989
Na		0.005	0.002	0.001	0.003	0.003	0.014		0.016	0.016	0.008	0.005	0.021	0.008	0.005
M[4]	2.000	2.000	2.000	2.000	1.992	1.983	2.000	2.001	1.993	2.000	1.986	1.980	1.949	2.000	1.994
Na	0.011		0.017	0.013			0.005	0.013						0.008
K	0.002	0.004	0.004		0.004	0.004	0.004	0.005	0.004		0.002	0.002		0.005	0.002
A	0.013	0.004	0.021	0.017	0.000	0.004	0.009	0.018	0.004		0.002	0.002		0.013	0.002
OH	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000
Total	16.964	16.980	17.013	16.971	16.987	16.987	16.964	17.005	16.977	16.98216.967	16.98216.951	17.013	16.996
XMg	0.88	0.88	0.88	0.87	0.88	0.89	0.89	0.89	0.88	0.89	0.88	0.88	0.90	0.88	0.88
*Recalculated on the basis of 23(O) and sum cations=13 (∑TET+M1-3 site occupancies) and only Ca+Na in M(4)
**All iron as FeO
#Calculated H2O
Gr=granular; Fgr=fine-grained granular; Ac=accicular; Fb=fibrous; Bfb=brown-stained fibrous

Fig. 1  Map of the north Westland, South Island of New Zealand (inset map provides location) showing distribution of weathered, slightly weathered and little weathered glacial outwash gravels and till that contain rare nephrite boulders; the distribution of streams that drain the glacial gravels; major rivers (Taramakau, Arahura, Hokitika) in which nephrite boulders have been found; and location of the Pounamu Ultramafics within biotite-garnet zone schist of the Southern Alps from which nephrite is derived (after Warren 1967; Nathan et al. 2002). Cross section A–B shows glacial outwash gravels and till deposited on Tertiary basement rocks during the Nemona (c. 360 ka), Waimaunga (300–248 ka) and Waimea (190–130 ka) Glacial periods is modified from Suggate & Watters (1991).

Fig. 2  Section through weathering rind of a nephrite boulder described in this study. Numbers refer to analyses listed in Table 2.

Fig. 3  Actinolite compositions (Table 1) plotted in terms of Si versus Mg/(Mg+Fe²⁺). Amphibole nomenclature after Leake et al. (1997).

Table 2  XRF analyses* of nephritic greenstone and weathering rind

Analysis No Thickness (cm) Colour	1 Core dk grn	2 1.1 v.pale grn	3 0.3 white	4 0.5 pale grn	5.0.2 white	6 0.2 v.pale grn	7 1.2 pale grn	8 0.5 rust bwn	9 1.5 pale bwn	10 Margin 0.6 bwn
SiO2	56.72	57.05	57.04	56.66	56.83	56.92	57.00	56.47	56.66	56.38
TiO2	0.01	0.01	0.01	0.01	0.02	0.01	0.01	0.01	0.01	0.02
Al2O3	0.50	0.51	0.57	0.69	0.76	0.65	0.75	1.03	1.09	1.00
Fe2O3	0.31	0.67	0.69	0.84	0.86	0.94	0.90	1.19	1.21	1.23
FeO	4.62	4.51	4.57	4.50	4.66	4.49	4.49	4.41	4.35	4.55
MnO	0.10	0.11	0.11	0.11	0.11	0.11	0.11	0.11	0.10	0.09
MgO	21.11	21.23	21.20	21.00	20.49	21.06	20.94	20.66	20.75	20.59
CaO	13.15	13.14	13.07	12.96	13.13	13.00	12.98	12.78	12.77	12.82
Na2O	0.68	0.33	0.33	0.50	0.29	0.39	0.31	0.41	0.32	0.53
K2O	0.05	0.02	0.02	0.04	0.02	0.02	0.02	0.02	0.02	0.03
P2O5	0.00	0.00	0.00	0.00	0.00	0.00	0.02	0.02	0.02	0.04
H2O_	1.84	2.38	2.55	2.51	2.54	2.47	2.41	2.50	2.62	3.02
H2O_	0.07	0.12	0.10	0.18	0.16	0.16	0.24	0.25	0.58	0.37
Total	99.16	100.08	100.26	100.00	99.87	100.22	100.18	99.86	100.50	100.67
Fe2O3 (total iron)	5.45	5.68	5.76	5.84	5.93	5.93	5.89	6.09	6.04	6.29
Oxidation ratio**	5.9	11.3	12.3	13.7	15.1	16.2	15.1	19.6	19.9	19.6
ppm
Cr	1420	1430	1400	1220	1436	1420	1620	1780	1890	1710
Ni	1411	1367	1246	1395	1414	1199	1469	1484	1465	1417
Sc	6	8	6	6	7	7	6	7	8	8
V	17	14	16	18	15	13	11	13	16	20
Zn	148	158	162	172	171	170	170	172	176	181
Density (g/cm ^3 )	3.03	3.01	2.96	2.96	2.95	2.97	2.95	2.87	2.83	2.83
*FeO determined by titration
H2O-=water lost at 1108C
** mol.2Fe O×100/(2Fe O+FeO)
bwn=brown; grn=green; v=very; dk=dark
nd=not determined

Fig. 4  (A) Plots of unweathered nephrite and weathering rind compositions with respect to colour zone thickness indicated in Fig. 2 and Table 2. Values are numbered as in Table 2 (analyses 1–10) and are normalised to 100 wt %. (B) Plots of unweathered nephrite and weathering rind compositions, oxidation ratios and bulk densities with respect to colour zone thickness indicated in Fig. 2 and Table 2. Values are numbered as in Table 2 (analyses 1–10).

Fig. 5  Isocon diagrams for unweathered composition and average compositions of bleached zone (upper diagrams) and oxidized zone (lower diagrams) of weathering rind of nephrite boulder. An isoconcentration line (isocon) is drawn assuming constant Al₂O₃ during weathering. Oxides are plotted in wt % (analyses in Table 2 normalised to 100 wt %) and elements in ppm (scaled together with minor oxides in right-side enlargement diagrams). Total iron as FeO is shown as FeO(T). Displacement of the data points from the isocon indicates relative gains (above the isocon) and losses (below the isocon); i.e. the change in concentration of a component relative to its concentration prior to weathering assuming constant Al₂O₃ (see text).

Fig. 6  Plots of wt % SiO₂, FeO, MgO, CaO, Fe₂O₃, Al₂O₃, ppm Cr, bulk oxidation ratio and density versus H₂O^– for unweathered (open square) and weathered (filled circles) nephrite. Analyses in Table 1 normalised to 100 wt %.

Fig. 7  Plot of H₂O⁺ versus density of unweathered (open square) and weathered (filled circles) nephrite.

Fig. 8  Plot of variously coloured north Westland nephrite compositions from this study (open circles), Wilkins et al. (2003) (filled circles) and unpublished analyses by R. Grapes (open squares) in terms of oxidation ratio (mol.2Fe₂O₃×100/[2Fe₂O₃+FeO]) versus total iron (wt % Fe₂O₃+FeO) content. Tie lines connect compositions of weathering rinds developed on unweathered nephrite.

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