1. Introduction

2. Materials and Methods

3. Results

3.2. The Maljavr Occurrence: Geological Structure, Alteration, Ore Minerals

Gold occurrence Maljavr is located at the SW flank of the belt, where biotite gneiss of the lower sequence forms an anticline fold with the upper bend dipping SE (Figure 1). Biotite gneiss (metamorphosed sandstone, gritstone, and conglomerate) is cut by a tourmaline granite pegmatite vein of sub-meridional strike, about 40 m thick (Figure 3). Rock forming minerals in pegmatite are quartz, plagioclase (albite-oligoclase), microcline, muscovite and tourmaline, minor minerals are biotite, apatite, zircon.

The western contact of the vein is sharp linear, but the eastern contact is transitional, through a zone of migmatization and foliation ~10 m wide. In the zone of foliation, pebbles are deformed together with the cement (Figure 4A). The biotite gneiss is intensely tourmalinizated at both sides of the pegmatite vein (Figure 3). Deformation and tourmalinization intensity in the biotite gneiss decreases within ~20m distance from the pegmatite contact.

Smaller lenses of pegmatite were found up to 50 m away from the main pegmatite vein and zone of migmatization. The biggest lenses are up to 20 m long, smaller lenses are less than 20-30 cm. The pegmatite lenses cut biotite gneiss, including the pebbles in metaconglomerate (Figure 4C).

A series of lenses of altered rocks makes a zone ~10 m thick, which strikes NNE 10-15° conformably to shistosity in the biotite gneiss (Figure 3). The zone of alteration was traced for 50 m in the outcrop, farther it is covered by soil.

Lenses of metasomatic rocks reach the length of 10 m and 1 m thickness (Figure 3). The lenses clearly exhibit three alteration envelopes (Figure 5). Hornblende and hedenbergite are the principal rock-forming minerals in the outer zone, where minor minerals are epidote, quartz, and garnet. Biotite, garnet, and quartz make the intermediate biotite-garnet envelope. The central parts of the lenses consist of garnet and quartz or garnet-only (garnetite) (Figure 3 and Figure 5). The outer hornblende-hedenbergite envelope is reduced in some lenses (Figure 5).

Metasomatic rocks are massive, medium- to coarse-grained: garnet grains reach 1 cm, other minerals are 1-3 mm big. Biotite flakes and needle crystals of amphibole and pyroxene are differently oriented. Quartz veinlets penetrate all zones of metasomatic rocks, the veinlets are oriented along the strike of the lenses (Figure 5).

The spidergrams (Figure 6) illustrate change chemical composition of the rocks during alteration processes. The diagrams exhibit general loss of SiO₂ and Na₂O, and gain of FeO in the altered rocks. Al₂O₃, CaO, MgO, K₂O were re-deposited during alteration and formed metasomatic zoning. Hornblende-hedenbergite rock has high CaO, low Al₂O₃, Na₂O, and K₂O. Biotite-garnet rock contains high Al₂O₃, MgO, and K₂O, but low CaO. Garnet-quartz rock in the central part of the lenses has high FeO, and lower content of other general elements (Table S1).

Change of mineral composition of the rocks during alteration processes is chiefly limited to decomposition of plagioclase in the biotite gneiss, recrystallization of biotite and quartz and formation of garnet, amphiboles, and pyroxene. Sodium, released during plagioclase decomposition, was taken away from the altered rocks. Calcium was re-deposited in the outer hornblende-hedenbergite envelope, where it formed Ca-rich hornblende, hedenbergite, and high-Ca almandine. The residual alumina from the decomposed plagioclase together with iron, which was added to the system, formed garnet (almandine) in biotite-garnet and garnet-quartz rocks. Potassium, concentrated in the primary biotite in gneiss, was re-deposited mainly in the garnet-biotite rock.

GER diagrams [22] (Figure 7) illustrate change of both mineral and chemical composition during alteration and formation of metasomatic zoning in terms of element ratios: loss of sodium due to decomposition of plagioclase (Figure 7A,B), redistribution of potassium, connected with recrystallization of biotite (Figure 7A,B), addition of Fe and redistribution of Ca and Al, which lead to formation of Ca-rich almandine, hornblende, and hedenbergite (Figure 7C,D).

All Fe-Mg silicates in the altered rocks are Fe-rich (Table S2): pyroxene is hedenbergite, amphiboles are ferro-hornblende and grunerite, chlorite is chamosite, and mica is Fe-biotite. Garnet is almandine, but almandine is Ca-rich in the hornblende-hedenbergite rock: mole proportion of grossular is 14-28% (Table S2), increasing in the outer parts of the zoned almandine crystals.

Altered rocks, if compared with the unaltered biotite gneiss, display loss of Rb, Cs, Sr, Ba, Zr, Nb, LREE, U, Pb, and gain of As, Ag, Te, Se, Bi, Cu (Table S3, Figure 8). It is important to note high As in the unaltered biotite gneiss, arsenic is 3–15 times higher than the average for the upper continental crust [24]. Concentration of As, Te, Se, and Bi in gold-mineralized rocks is by 1-2 orders of magnitude more than in non-mineralized metasomatic rocks (Table S3).

Gold content in the unaltered biotite gneiss is close to the average in the continental crust, and increases in altered rocks. Channel sampling across one of the lenses of altered rocks gave 1.78 g/t for 0.8 m [14], but gold content in hand samples reaches 30 g/t.

Unaltered biotite gneiss (metaconglomerate, metagritstone, metasandstone) contains disseminated sulfide mineralization <1 vol.%; sulfide minerals are pyrrhotite (prevails), arsenopyrite, rarely chalcopyrite, and late pyrite; oxide minerals are ilmenite and rare magnetite. No sulfide mineralization was found in the migmatizated rocks at the contact of biotite gneiss with pegmatite.

Sulfide content increases significantly in the lenses of altered rocks, but distribution of sulfide mineralization is very uneven, varying from 1 to 25 vol.%. Sulfide-rich rocks were found in different envelopes of the lenses, texture of mineralization is disseminated, veinlet-disseminated, or nested. Mineral composition is not complicated: the main minerals are pyrrhotite and arsenopyrite, minor sulfides are chalcopyrite, löllingite, troilite, pentlandite. Oxide minerals are ilmenite and magnetite (more rare, but up to 10% in some samples).

Two generations of arsenopyrite were detected in the metasomatic rocks, the generations differ in grain form and size (Figure 9), and in chemical composition (Table 1). The early arsenopyrite-1 occurs as disseminated grains up to 0.2 mm. It contains high Co and Ni, and some grains are clearly zoned (Figure 9) with As-, Co- and Ni-rich core (As 38.2 – 39.7 at%, Co 3.12 – 4.11 mas.%, Ni 1.62 – 2.89 mas.%) and lower As, Co and Ni in the rim (As 34.6 – 35.0 at%, Co 2.23 – 2.44, and Ni 0.38 – 0.81 mas.%) (Table 1). Late re-crystallized coarse-grained (0.5 – 3 mm in size) arsenopyrite-2 is disseminated in the altered rocks, or it forms chains of euhedral grains (Figure 9 and Figure 10). Zoning in arsenopyrite-2 is complicated – zones with lower (32,7 – 33,8 at.%As) and higher (34,6-35,9 at.%As) arsenic alternate in the grains (Figure 9); Co and Ni impurities are below 0.5 mas.%.

Figure 9. Arsenopyrite from the Maljavr gold occurrence. A – zoning in arsenopyrite-1, polished section photo, crossed polarized light. В – a chain of arsenopyrite-2 grains in hornblende-hedenbergite rock, polished section photo, plane polarized light. С and D – complicated zoning in arsenopyrite-2 – back-scattered electron (BSE) images: arsenic-rich zones are lighter than arsenic-poor. Apy – arsenopyrite, Bi – native bismuth.

Figure 9. Arsenopyrite from the Maljavr gold occurrence. A – zoning in arsenopyrite-1, polished section photo, crossed polarized light. В – a chain of arsenopyrite-2 grains in hornblende-hedenbergite rock, polished section photo, plane polarized light. С and D – complicated zoning in arsenopyrite-2 – back-scattered electron (BSE) images: arsenic-rich zones are lighter than arsenic-poor. Apy – arsenopyrite, Bi – native bismuth.

Table 1. Microprobe data for arsenopyrite (Apy) and löllingite (Lo) of the Maljavr gold occurrence, mas.%.

Table 1. Microprobe data for arsenopyrite (Apy) and löllingite (Lo) of the Maljavr gold occurrence, mas.%.

Sample No.	ТУ-25-1	ТУ-25-1	ТУ-25-2	ТУ-25-2	ТУ-31	ТУ-31	ТУ-41	ТУ-41	ТУ-45	ТУ-45
Mineral	Apy-1-C	Apy-1-R	Apy-1-C	Apy-1-R	Apy-1-C	Apy-1-R	Apy-1-C	Apy-1-R	Apy-1-C	Apy-1-R
S	14,94	18,32	16,08	18,94	15,49	18,36	18,16	20,03	17,93	19,28
Fe	26,46	31,4	27,92	30,56	27,73	31,49	33,15	34,61	31,89	33,65
Co	3,76	2,32	3,12	2,44	4,11	2,23	0,47	0,13	1,28	0,48
Ni	2,89	0,64	2,25	0,81	1,62	0,38	0,24	bdl	0,29	0,02
As	51,83	47,69	50,57	47,45	50,93	46,8	47,86	45,47	48,52	46,38
Total	99,88	100,37	99,93	100,2	99,88	99,26	99,88	100,25	99,91	99,81
Atoms per formula unit
S	0,805	0,946	0,852	0,965	0,831	0,957	0,94	1,014	0,927	0,986
Fe	0,819	0,931	0,850	0,894	0,854	0,942	0,985	1,006	0,946	0,987
Co	0,110	0,065	0,090	0,068	0,120	0,063	0,013	0,004	0,036	0,013
Ni	0,085	0,018	0,065	0,023	0,048	0,011	0,007	0,000	0,008	0,001
As	1,195	1,054	1,148	1,035	1,169	1,043	1,06	0,986	1,073	1,014
at% As	39,7	35,0	38,2	34,7	38,7	34,6	35,3	32,7	35,9	33,8

Table 1. (ending).

Table 1. (ending).

Sample No.	TY-41-1	TY-41-1	TY-41-2	TY-41-2	TY-29	TY-42	TY-42	TY-41	TY-41	TY-29	TY-42	TY-42
Mineral	Apy-2-C	Apy-2-R	Apy-2-C	Apy-2-R	Apy-2	Apy-2	Apy-2	Lo	Lo	Lo	Lo	Lo
S	18,66	19,30	17,77	19,28	17,37	18,06	18,21	1,40	1,98	1,84	1,83	2,04
Fe	33,25	34,54	33,43	34,36	33,78	33,62	33,63	27,66	28,23	28,03	25,11	26,31
Co	0,34	0,12	0,39	0,09	0,07	0,24	0,17	0,08	0,13	0,10	0,49	0,42
Ni	0,49	bdl	0,38	bdl	0,17	0,26	0,11	0,09	0,10	0,13	2,07	0,68
As	47,96	46,72	48,59	45,80	48,80	47,87	48,09	70,27	69,35	69,94	70,26	70,04
Total	100,70	100,67	100,55	99,53	100,19	100,05	100,21	99,51	99,79	100,04	99,76	99,49
Atoms per formula unit
S	0,952	0,982	0,921	0,992	0,908	0,937	0,939	0,089	0,125	0,116	0,114	0,127
Fe	0,975	1,009	0,996	1,015	1,014	1,001	0,996	1,009	1,024	1,013	0,904	0,944
Co	0,009	0,003	0,011	0,002	0,002	0,007	0,005	0,003	0,004	0,003	0,017	0,014
Ni	0,014	0,000	0,011	0,000	0,005	0,007	0,003	0,003	0,003	0,004	0,071	0,023
As	1,048	1,018	1,079	1,008	1,092	1,063	1,061	1,911	1,875	1,884	1,886	1,873
at% As	34,9	33,8	35,7	33,4	36,1	35,2	35,3	63,4	61,8	62,4	63,0	62,8

Note: bdl=below the detection limit. Atoms per formula unit are calculated for As+S=2. Apy-1 and Apy-2 – arsenopyrite of the first and the second generations, correspondingly. C – data for the central part of the grain, R- data for the rim.

Löllingite occurs as inclusions in arsenopyrite-2 (Figure 11), rarer as separate euhedral grains up to 0.5 mm in size. Löllingite contains impurities of Ni (< 2 mas.%), Co (<0.5 mas.%), and S (< 2 mas.%) (Table 1).

Gold was found exceptionally in numerous monomineral and polymineral inclusions in arsenopyrite-2 and löllingite, together with native bismuth and bismuth telluride ehrigite Bi₈Te₃ (the second finding in the world after Good Hope gold mine [25]) (Table 2, Figure 11 and Figure 12). Rarer mineral phases, found in the inclusions, are maldonite, bismuthinite, joseite-B, hedleyite, and hessite. The inclusions are of irregular angular form, up to 0.2 mm sized (Figure 12). High concentration of bismuth–ehrigite–gold inclusions is reached at the contact arsenopyrite–löllingite, or these minerals form veinlets along arsenopyrite–löllingite boundary (Figure 11).

Native gold is of high- or medium-grade, it contains from 70 to 96 mas.% Au, commonly 72 – 82 mas.%, the main impurities are Ag (4 – 30 mas.%), Fe and As ( <0.2 mas.%) (Table 3). One more gold mineral in the deposit is maldonite Au₂Bi (Table 2), which occurs in the inclusions in arsenopyrite and löllingite together with gold, native bismuth, and ehrigite (Figure 13).

3.3. Zircons in the Rocks of the Maljavr Occurrence and Age of Zircons

Zircon from sample SHM-12 of biotite gneiss-metaconglomerate forms euhedral transparent pale yellow fractured crystals, with plain facets and smoothed edges. Prism {110} and di-pyramid {111} facets are predominant. The crystal size is 50–150 μm, the elongation coefficient varies from 2.0 to 3.0. Coarse zoning was noted in the outer part of some zircon crystals. Eight samples of the least altered zircon crystals, selected from different size fractions, were assayed for U and Pb isotopes (Table 4). Analytical points of isotopic composition of eight zircon fractions lie in discordia with the age 2665±22 Ma at the upper intersection with the concordia, MSWD=9.5 (Figure 14). The lower intersection with age 380±93Ma reflects the age of Paleozoic tectonic activity and formation of the Khibiny and Lovozero alkaline intrusions [26].

Zircon from sample SHM-11 of tourmaline granite pegmatite forms euhedral long-prysmatic transparent fractured crystals of brown color. Prism {110} and di-pyramid {111} facets are predominant, the crystal faces are rough, and the edges are sharp. The inner structure is nonhomogeneous, with dark and light areas of irregular form, some grains contain inclusions of a black mineral. The crystal size is 100–300 μm, the elongation coefficient varies from 3.0 to 4.0. Five samples of the least altered and free of inclusions zircon crystals were selected from different size fractions and assayed for U and Pb isotopes (Table 4). Analytical points of isotopic composition of five zircon fractions lie in discordia with the age 2508±7 Ma at the upper intersection with the concordia, MSWD=1.4 (Figure 15). The lower intersection with the concordia occurs at 358±57 Ma.

Table 4. Results of U–Pb geochronological studies of zircons from biotite gneiss-metaconglomerate and tourmaline granite-pegmatite.

Table 4. Results of U–Pb geochronological studies of zircons from biotite gneiss-metaconglomerate and tourmaline granite-pegmatite.

Sample No./ Fraction No.	Weight, mg/ Size, μm	Pb, ppm		U, ppm	Isotope ratios
					206Pb/204Pb*	207Pb/206Pb*	208Pb/206Pb*
Sample SHM-12 – biotite gneiss
SHM-12/1	0.2/<100	165		358	533	0.1999±0.0001	0.1281±0.0002
SHM-12/2	0.5/>100	130		341	702	0.1907±0.0001	0.1246±0.0002
SHM-12/3	0.5/>100	141		337	822	0.1891±0.0001	0.1291±0.0002
SHM-12/4	0.2/>50	164		363	485	0.1972±0.0001	0.3042±0.0001
SHM-12/5	0.5/<100	361		803	1178	0.1845±0.0001	0.2709±0.0001
SHM-12/6	0.3/<50	158		371	539	0.1909±0.0001	0.2773±0.0001
SHM-12/7	0.4/<50	186		432	818	0.1829±0.0001	0.2064±0.0001
SHM-12/8	0.4/<100	175		406	1242	0.1829±0.0001	0.2062±0.0001
Sample SHM-11 - granite pegmatite
SHM-11/1	0.3/<200	1218	2587		1426	0.1724±0.0001	0.03001±0.0001
SHM-11/2	0.9/<150	729	1534		1233	0.1741±0.0001	0.05311±0.0002
SHM-11/3	0.6/>100	793	1619		1723	0.1719±0.0001	0.02737±0.0001
SHM-11/4	1.7/>250	1109	2370		2347	0.1695±0.0001	0.02410±0.0001
SHM-11/5	1.2/>200	1140	2643		2091	0.1686±0.0001	0.02238±0.0001

Table 4. (ending).

Table 4. (ending).

Sample No./ Fraction No.	Weight, mg/ Size, μm	Isotope ratios		Rho	Age, Ma
		206Pb/238U	207Pb/235U		206Pb/238U	207Pb/235U	207Pb/206Pb
Sample SHM-12 – biotite gneiss
SHM-12/1	0.2/<100	0.3958±0.0015	9.669±0.038	0.91	2150±9	2404±10	2627±3
SHM-12/2	0.5/>100	0.3322±0.0049	7.9400±0.012	0.99	1849±27	2224±33	2590±2
SHM-12/3	0.5/>100	0.3639±0.0011	8.7448±0.026	0.96	2000±6	2312±7	2599±2
SHM-12/4	0.2/>50	0.3418±0.0014	8.1055±0.040	0.91	1896±8	2243±11	2577±4
SHM-12/5	0.5/<100	0.3551±0.0007	8.5313±0.017	0.94	1959±4	2289±5	2599±1
SHM-12/6	0.3/<50	0.3226±0.0016	7.6289±0.038	0.96	1834±9	2188±11	2539±3
SHM-12/7	0.4/<50	0.3457±0.0010	8.2387±0.025	0.88	1962±6	2258±7	2538±2
SHM-12/8	0.4/<100	0.3565±0.0011	8.5121±0.025	0.92	1966±6	2287±7	2588±2
Sample SHM-11 - granite pegmatite
SHM-11/1	0.3/<200	0.4633±0.0005	10.509±0.011	0.90	2463±2	2481±2	2495±1
SHM-11/2	0.9/<150	0.4449±0.0007	10.068±0.015	0.95	2373±4	2441±4	2499±1
SHM-11/3	0.6/>100	0.4715±0.0011	10.714±0.026	0.99	2490±6	2499±6	2506±1
SHM-11/4	1.7/>250	0.4529±0.0009	10.265±0.021	0.96	2408±5	2459±5	2501±1
SHM-11/5	1.2/>200	0.4182±0.0009	9.3869±0.017	0.94	2252±5	2377±5	2485±1

*Isotope ratios are corrected for the blank and common lead; Rho - the correlation coefficient of the ²⁰⁷Pb/²³⁵U-²⁰⁶Pb/²³⁸U ratios.

Figure 14. Diagram with concordia for the biotite gneiss from the Maljavr occurrence.

Figure 14. Diagram with concordia for the biotite gneiss from the Maljavr occurrence.

Figure 15. Diagram with concordia for the tourmaline granite pegmatite from the Maljavr occurrence.

Figure 15. Diagram with concordia for the tourmaline granite pegmatite from the Maljavr occurrence.

4. Discussion

5. Conclusions

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