Occurrences of Pd–Pt Bismuthotellurides and a Phosphohedyphane-Like Phase in Sulfide Veins of the Monchepluton Layered Complex, Kola Peninsula, Russia

Abstract

We describe occurrences of platinum-group minerals (PGM) and an uncommon mineral enriched in Cl, and provide a brief review of Cl-bearing minerals associated with basic–ultrabasic complexes. An unusual phosphohedyphane-like phase (~30 µm), close to CaPb₄(PO₄)₃Cl, occurs in one of the PGM-bearing veins of massive sulfides in the Monchepluton layered complex, Kola Peninsula, Russia. These veins consist of varying amounts of pyrrhotite, pentlandite, chalcopyrite, pyrite and accessory grains of galena; they are fairly abundant in the heavy-mineral concentrate, as are small (<0.1 mm) grains of PGM: michenerite, sperrylite, Bi-enriched members of the merenskyite–moncheite series and kotulskite, also rich in Bi. The PGE mineralization is attributed to a low-temperature deposition at the hydrothermal stage. The pyromorphite–phosphohedyphane solid solution likely formed as a secondary phase under conditions of a progressive build-up of oxygen fugacity via oxidation reactions of a precursor grain of galena and involving Ca, as an incompatible component of the sulfides, in a medium of residual fluid enriched in Cl.

1. Introduction

A detailed investigation of ore veins sampled at the site of long-abandoned mine adits in the Monchepluton layered basic–ultrabasic complex has led to the discovery of a member of the pyromorphite–phosphohedyphane solid solution among the ore constituents. As a chlorine-bearing member of the apatite supergroup, its presence in a vein of pyrrhotite, pentlandite, chalcopyrite, pyrite, galena and platinum-group minerals (PGM) raises questions about the role of Cl and other volatiles in mineralized ultrabasic suites. Our aims are to describe the occurrence, documented for the first time in a vein of massive Cu–Ni sulfides bearing platinum-group elements (PGE) at Mount Nittis, in the Monchepluton layered mafic–ultramafic complex of Paleoproterozoic (~2.5 Ga) age [1,2]. We explore what is known about the pyromorphite–phosphohedyphane solid-solution series and document the associated PGM. We believe that this phase is significant not only as a new representative of Cl-bearing minerals associated with ore zones, but also as a monitor of a progressive build-up in levels of oxygen fugacity (fO₂) during ore formation.

Geological Background

The Monchepluton layered complex belongs to an extensive group of Paleoproterozoic layered intrusions in the Fennoscandian Shield, e.g., [2]. It is located at the boundary of the Imandra–Varzuga zone of the Polmak–Pasvik–Pechenga–Imandra–Varzuga rift of Paleoproterozoic age with the Archean Kola–Norwegian terrane of the Kola Province (Figure 1).

The Monchepluton complex consists of sequences of dunite–harzburgite–orthopyroxenite–gabbronorite [4,5,6,7]. Seven divisions are recognized: the Dunite block, mounts Nittis, Kumuzhya, and Travyanaya along a northeasterly trend, and mounts Sopcha, Nyud, and Poaz on an easterly trend (Figure 2). The Dunite block, which hosts the Sopcheozyorskoe chromite deposit [8], may correspond to the location of the magma-feeder channel [5]. This block represents a podiform or core-like zone of olivine and olivine–chromite cumulates formed close to the nexus of the boomerang-shaped structure [5].

2. Materials and Methods

Our materials consist of ore samples and heavy-mineral concentrates obtained from three samples (10–15 kg each) of Ni–Cu–PGE ores collected at old mine adits (67°55′18.4″ N 32°46′46.6″ E; 67°55′25.1″ N 32°46′27.3″ E; 67°55′31.7″ N 32°46′44.9″ E) known historically and built during the years 1933–1936 (Figure 2). The fragments of ore are dense, massive, and coated by an oxidized rind (Figure 3a,b). They consist of pyrrhotite, pentlandite, chalcopyrite and less-abundant pyrite, in varying proportions. The heavy-mineral concentrates studied consist of ~10 to 100 micrometer-sized fractions of galena, grains of platinum-group minerals (PGM), followed by minor amounts of barite, and remnant grains of base-metal sulfides, mostly pentlandite. By interpolation, the density of our member of the apatite supergroup is approximately 6.5 g/cm³.

The phosphohedyphane-type phase occurs in a single subhedral grain that is small (~30 µm) and cracked due to its brittleness (Figure 4a). It was found in one of the veins sampled (Figure 2). Bismuthotelluride species of Pd and Pt and sperrylite, present in the concentrates, are consistent with the findings of a previous study [9].

We employed scanning-electron microscopy (SEM) and energy-dispersive analysis (EDS) conducted at an accelerating voltage of 20 kV and a beam current of 1.6 nA, with a beam size of ~1 µm, using a MIRA 3 LMU (Tescan Orsay Holding, Brno, Czech Republic) SEM with an attached INCA Energy 450 XMax 80 (Oxford Instruments Nanoanalysis, Wycombe, UK) microanalysis EDS system at the Institute of Geology and Mineralogy (the Analytical Center for Multi-Elemental and Isotope Studies), Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia. The standards used were FeS₂ (S), PtAs₂ (As), PbTe (Pb and Te), wollastonite (Ca), synthetic GaP (P), NaCl (Cl) and pure metals (Fe, Co, Ni, Cu, Rh, Pd, Pt and Bi). For the analytical signal of S, Fe, Ni, Cu, Ca, P and Cl, the K family of peaks was used, and for the other elements, the L family. The use of the L family for Pt and Bi avoids the mutual overlaps of their M peaks. Minimum detection-limits (3σ criterion) of the elements (wt.%) were found to be 0.1–0.2 for S, Fe, Co, Ni, Cu, Ca and Cl; 0.2–0.4 for As, Pd and Te; and 0.4–0.7 for Pt and Bi. The analytical error for the main components did not exceed 1–2 relative % and satisfied the requirements for a quantitative analysis.

3. Results

The Pyromorphite–Phosphohedyphane Solid Solution

Phosphohedyphane, ideally Ca₂Pb₃(PO₄)₃Cl, a member of the apatite supergroup [10], was discovered in the Capitana mine, Atacama Province, Chile, where it is commonly encountered in zones of oxidized Pb-bearing ores [11]. Phosphohedyphane and pyromorphite, ideally Pb₅(PO₄)₃Cl, seem to form a complete solid solution, as demonstrated by a compilation of literature data [11,12]. Values of the ratio 5 × Ca/(Ca + Pb) range from 0 to 2 over that interval, and our specimen has a value of 0.78. Values between 2 and 5, i.e., intermediate compositions between phosphohedyphane and chlorapatite, Ca₅(PO₄)₃Cl, are not found in nature [11]. Although we do not have supporting structural data, we can assume that our phase, (Pb_4.13Ca_0.76)_Σ4.90(PO₄)₃Cl_0.95 (

Table 1. Composition of the phosphohedyphane-type phase from the Monchepluton layered complex, Kola Peninsula.

	Wt. %	apfu
CaO	3.54	Ca	0.76
PbO	76.74	Pb	4.13
P2O5	15.67	Σ	4.90
Cl	2.81
O≡Cl	0.64	P	2.65
Total	98.12	Cl	0.95

Note: Results of quantitative SEM/EDS analysis are expressed in weight %. Values of atoms per formula unit (apfu) are based on a value of 12 oxygen atoms per formula unit.

), (Pb_1.13Ca_0.76)_Σ1.89Pb₃(PO₄)₃Cl_0.95 or, in general terms, (Pb,Ca)₂Pb₃(PO₄)₃Cl, is a member of the phosphohedyphane–pyromorphite series, in which Pb²⁺ enters the Ca site in accordance with the formula (Ca,Pb)₂Pb₃(PO₄)₃Cl. It displays a characteristic EDS spectrum (Figure 5). A complete solid-solution series probably exists between phosphohedyphane and hedyphane, Pb₃Ca₂(AsO₄)₃Cl. Phosphohedyphane is the phosphate analog of hedyphane, i.e., the long-established arsenate [13], and the Cl analog of fluorphosphohedyphane [11]. In an arsenic-bearing system, intermediate members are complex solid solutions involving components of phosphohedyphane, hedyphane and mimetite, Pb₅(AsO₄)₃Cl [14,15,16]. Members of such a series are developed, for example, in the oxidized zone at the Preguiça mine, Portugal [17]. As in that case, the phase found at Monchepluton is a secondary mineral formed by oxidation of primary Pb-rich precursors.

The Platinum-Group Minerals

We have identified five species of PGM in the samples investigated (

Table 2. Compositions of platinum-group minerals in sulfide veins of the Monchepluton layered complex.

		Pt	Pd	Te	Bi	As	S	Total	Pt	Pd	Pt + Pd	Te	Bi	Te + Bi	As	S
		Wt.%							apfu
1	Spy	56.38	bdl	bdl	bdl	44.22	bdl	100.6	0.99	–	0.99	–	–	–	2.01	0.00
2		55.45	bdl	bdl	bdl	43.77	bdl	99.22	0.98	–	0.98	–	–	–	2.02	0.00
3		56.45	bdl	bdl	bdl	44.29	bdl	100.74	0.99	–	0.99	–	–	–	2.01	0.00
4		54.93	bdl	bdl	bdl	43.65	0.53	99.11	0.96	–	0.96	–	–	–	1.98	0.06
5		55.18	bdl	bdl	bdl	42.97	0.36	98.51	0.98	–	0.98	–	–	–	1.98	0.04
6		54.47	bdl	bdl	bdl	43.80	0.48	98.75	0.95	–	0.95	–	–	–	2.00	0.05
7		54.93	bdl	bdl	bdl	42.54	0.53	98.00	0.98	–	0.98	–	–	–	1.97	0.06
8		54.59	bdl	bdl	bdl	42.88	0.63	98.10	0.96	–	0.96	–	–	–	1.97	0.07
9	Ktu	Bdl	38.92	30.25	29.22	bdl	bdl	98.39	–	0.98	0.98	0.64	0.38	1.02	–	–
10	Mch	Bdl	24.56	32.80	41.78	bdl	bdl	99.14	0.00	1.01	1.01	1.12	0.87	1.99	–	–
11		Bdl	24.52	33.66	41.03	bdl	bdl	99.21	0.00	1.00	1.00	1.15	0.85	2.00	–	–
12		Bdl	24.72	33.22	41.53	bdl	bdl	99.47	0.00	1.01	1.01	1.13	0.86	1.99	–	–
13		3.82	22.58	28.29	44.49	bdl	bdl	99.18	0.09	0.96	1.04	1.00	0.96	1.96	–	–
14		4.54	21.31	28.04	45.37	bdl	bdl	99.26	0.11	0.91	1.02	1.00	0.99	1.98	–	–
15		3.09	22.56	28.16	45.24	bdl	bdl	99.05	0.07	0.96	1.03	1.00	0.98	1.97	–	–
16		3.41	22.35	28.42	46.12	bdl	bdl	100.30	0.08	0.94	1.02	1.00	0.99	1.98	–	–
17		1.86	23.99	32.29	40.59	bdl	bdl	98.73	0.04	0.99	1.03	1.11	0.85	1.97	–	–
18		bdl	24.41	33.53	41.18	bdl	bdl	99.12	0.00	1.00	1.00	1.14	0.86	2.00	–	–
19		bdl	24.05	28.48	45.60	bdl	bdl	98.13	0.00	1.02	1.02	1.00	0.98	1.98	–	–
20		bdl	23.71	28.76	45.85	bdl	bdl	98.32	0.00	1.00	1.00	1.01	0.99	2.00	–	–
21		bdl	24.34	31.82	42.66	bdl	bdl	98.82	0.00	1.01	1.01	1.10	0.90	1.99	–	–
22		5.59	20.50	27.09	45.61	bdl	bdl	98.79	0.13	0.89	1.02	0.98	1.00	1.98	–	–
23		2.06	23.71	29.87	43.39	bdl	bdl	99.03	0.05	0.99	1.04	1.04	0.92	1.96	–	–
24		5.76	20.91	26.83	46.13	bdl	bdl	99.63	0.13	0.90	1.03	0.96	1.01	1.97	–	–
25	Mon	31.89	5.50	45.19	16.49	bdl	bdl	99.07	0.76	0.24	1.00	1.64	0.37	2.00	–	–
26		36.29	1.76	40.65	19.33	bdl	bdl	98.03	0.91	0.08	0.99	1.56	0.45	2.01	–	–
27		35.47	1.94	41.01	20.01	bdl	bdl	98.43	0.88	0.09	0.97	1.56	0.47	2.03	–	–
28	Mrk	9.39	21.04	52.96	16.58	bdl	bdl	99.97	0.20	0.80	1.00	1.68	0.32	2.00	–	–
29		6.05	22.64	52.16	17.54	bdl	bdl	98.39	0.13	0.87	0.99	1.67	0.34	2.01	–	–
30		5.36	23.55	52.88	16.52	bdl	bdl	98.31	0.11	0.89	1.01	1.68	0.32	1.99	–	–

Note: Results of quantitative SEM/EDS analyses are listed in weight %. The atomic proportions are based on a total of 3 atoms per formula unit (apfu) for sperrylite (Spy), michenerite (Mch), moncheite (Mon) and merenskyite (Mrk), and on a total of 2 apfu for kotulskite (Ktu). bdl: below detection limit.

). Sperrylite (ideally PtAs₂), both S-bearing (up to 0.63 wt.% S) and essentially devoid of S, is relatively common, as well as michenerite (PdBiTe). Members of the moncheite Pt(Te,Bi)₂–merenskyite Pd(Te,Bi)₂ series are subordinate, and kotulskite Pd(Te,Bi) is rare in our samples.

A relative enrichment in Bi is noteworthy; it is observed in compositions of all species of the Pd–Pt bismuthotellurides (Table 2; Figure 6 and Figure 7). Note that Te occupies almost entirely the Te site, and makes up a substantial portion (up to ~20%) of the Bi site (Figure 6).

4. Discussion

The presence of pyromorphite–phosphohedyphane solid solution in association with zones of massive PGE-bearing Cu–Ni sulfide ore provides a direct indication of the role of Cl at the hydrothermal stage in the Monchepluton cumulates. The abundance of galena, Bi-, Te, and As-enriched species of Pd and Pt is consistent with deposition from hydrothermal fluids late in the crystallization history of the complex. The observed assemblages enriched in Bi imply a relative decrease in temperatures of crystallization of the bismuthotelluride species. Experimental results [18] indicate that michenerite is stable below 500 °C, whereas the melting point of the end member of the merenskyite series (740 °C) decreases to 500–525 °C for the Bi-enriched phases having greater extents of Bi-for-Te substitution. Sperrylite is also known to form at a postmagmatic hydrothermal stage in layered intrusions [19]. We therefore presume that the phosphohedyphane-like phase represents a part of the late Pb–S–Te–Bi–As-enriched assemblages deposited hydrothermally during the formation of the sulfide veins.

In

Table 3. A review of various minerals and phases enriched in Cl from basic–ultrabasic complexes.

Minerals	Complex, Locality	Occurrence	Comment	References
Chlorapatite	Bushveld, South Africa	Lower zone		[20,21]
	Stillwater, Montana, USA			[21]
	Koitelainen, Keivitsa- Satovaara, Finland		Cl-rich Amp	[22]
	Lukkulaisvaara, northern Karelia, Russia		Clap in Ol cumulates	[23]
	Kläppsjö, Sweden	Gabbro	Clap–PGM	[24]
	Kivakka, northern Karelia, Russia	Lower zones	Clap–PGM	[25,26]
	Monchepluton, Kola Peninsula, Russia	Mounts Nittis, Kumuzh’ya, Travyanaya, Sopcha, Nyud and Poaz	Clap in ultramafic-mafic cumulates	[6]
	Noril’sk, Siberia, Russia	Oktyabr’sky deposit	Pd–Pt, Au–Ag atypical ore	[27]
Cl-bearing Ap	Yoko-Dovyren, Baikal region, Russia	PGM-bearing horizon	≤2.42 wt.% Cl	[28,29]
Cl-bearing mica	Yoko-Dovyren, Baikal region, Russia	PGM-bearing horizon	≤0.58 wt.% Cl	[28,29]
Cl analog of ferrokinoshitalite	Yoko-Dovyren, Baikal region, Russia	Pl-bearing peridotite		[30]
Cl-rich annite	Noril’sk, Siberia, Russia	Oktyabr’sky deposit	≤7.54 wt.% Cl atypical ore	[27]
Cl-rich hastingsite	Noril’sk, Siberia, Russia	Oktyabr’sky deposit	≤4.06 wt.% Cl atypical ore	[27]
“Ferro-chloro-pargasite”	Lukkulaisvaara, northern Karelia, Russia	Pegmatitic orthopyroxenite	Fcprg–PGM	[31,32]
	Tudor, Ontario, Canada	Gabbro		[33]
	Monchepluton, Kola Peninsula, Russia	Gabbronorite, Mount Poaz		[6]
Pd–Bi chlorides	Noril’sk, Siberia, Russia	Ore-bearing zone		[34]
	Sudbury, Ontario, Canada	Strathcona Deep Copper zone		[35]
Hibbingite	Sudbury, Ontario, Canada	Strathcona Deep Copper zone		[36]
	Duluth, Minnesota, USA	Troctolite		[37,38]
	Noril’sk, Siberia, Russia	Ore zone	Solid solution with kempite	[39]
Pyrosmalite-(Fe)	Noril’sk, Siberia, Russia	Oktyabr’sky, massive sulfide	Pys-Fe–Tty	[40]
Penfieldite-type phase	Bushveld, South Africa	Merensky Reef	With PGM in orthocumulate	[41]
Djerfisherite	Noril’sk, Siberia, Russia	Cu–Ni ores		[42]
	Guli complex, Polar Siberia, Russia	Dunitic rocks		[43]
	Yoko-Dovyren, Baikal region, Russia	Magnesian skarn	Included in Spl, Tro	[44]
Thalfenisite	Noril’sk, Siberia, Russia	Cu–Ni ores		[45]

Note: Symbols used: Amp: amphibole, Clap: chlorapatite, Fcprg: ferro-chloro-pargasite, Ol: olivine, Pl: plagioclase, PGM: platinum-group minerals, Spl: spinel, Tty: tatyanaite, Tro: troilite.

, we review [6,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45] the occurrences of Cl-rich minerals known in basic–ultrabasic complexes emplaced in different geological settings worldwide. These phases include hydroxychlorides, such as (Fe,Mn)(OH)Cl and hibbingite (Fe₂(OH)₃Cl) from the Sudbury complex, Canada, hibbingite in the Duluth layered complex, USA, members of the hibbingite–kempite (Mn₂(OH)₃Cl) series, djerfisherite (K₆(Fe,Cu,Ni)₂₅S₂₆Cl) and thalfenisite, its thallium analog (Tl₆(Fe,Ni)₂₅S₂₆Cl), pyrosmalite-(Fe) in the Noril’sk complex of Russia, djerfisherite in the Guli and Yoko-Dovyren complexes, Russia, a penfieldite-type phase (Pb₂Cl₃(OH)) in the Bushveld complex, South Africa, and unnamed Pd–Bi chlorides from Noril’sk and Sudbury.

On the basis of findings in the reviewed occurrences (Table 3), we infer that Cl-rich compounds typically form at relatively low temperatures as a consequence of build-up in levels of Cl attained locally during late or final stages of formation, or during stages of postmagmatic alteration of the complexes. Chlorapatite, which is the most abundant expression of Cl buildup, crystallized from pockets of intercumulus melts [6,21,25] or is a result of phase transformations involving fluorapatite as a precursor phase [46]. In addition, Cl was important in PGE remobilization in the Duluth complex, Minnesota, USA [47]. The amphibole “ferro-chloro-pargasite” likely precipitated from a late phase of hydrous Cl-bearing fluids (e.g., micro-inclusions hosted by PGM), or formed as a result of transformation reactions, which involved a primary plagioclase during deuteric (autometasomatic) events of alteration [6,31,32]. Many of the uncommon compounds rich in Cl likely formed from residual droplets of highly fractionated melts or crystallized from remaining microvolumes of aqueous hydrochloric solutions [41].

The Behavior of Cl during Crystallization of the Monchepluton Complex and the Origin of Calcian Pyromorphite

A recent evaluation of geochemical trends and their link to compositions of rock-forming silicate and oxide minerals at Monchepluton indicates that variations in apatite differ from trends reported for other layered intrusions [6]. Chlorapatite (>6 wt.% Cl) is invariably abundant, and compositional variations in apatite are generally similar in all of the geological divisions of the complex (i.e., the Mounts Nittis, Kumuzhya, Travyanaya, Sopcha, Nyud and Poaz). No relationship exists between the Cl content of apatite and the degree of magnesium enrichment in the coexisting magmatic silicates. Two fields of apatite compositions are recognized: ≤3 wt.% and >6 wt.% Cl, which overlap extensively, consistent with two generations of apatite. The first nucleated early during crystallization of pockets of H₂O-bearing intercumulus melts. An inferred stage of degassing of the crystallizing melt likely caused a decoupling of Cl and F. Fluorine at Monchepluton essentially remained in the melt, whereas Cl was partitioned efficiently to an H₂O-bearing fluid phase. As a result, at the early stage, the apatite incorporated combinations of the components hydroxylapatite and fluorapatite, with a low content of Cl. At a late stage, chlorapatite precipitated from a Cl-rich fluid. Consequently, “ferro-chloro-pargasite” (4.1 wt.% Cl) formed via reactions of that fluid with plagioclase and pyroxene during an event of autometasomatic alteration occurring in relatively evolved cumulates of gabbronoritic sequences of the Poaz massif (Figure 2) [6].

Features of the geochemical behavior of Cl [6] are useful in recognizing a mode of formation of the phosphohedyphane-like phase occurring in a sulfide vein of the Monchepluton complex. The investigated veins occur close to the nexus (Figure 2); they are representative examples that belong to the entire system of steeply dipping veins of massive sulfide ores (0.1 to 1.4 km long and up to 0.5 m in thickness, locally up to 2–3 m thick). These veins presumably formed by remobilization of early occurrences of magmatic sulfide melts, accumulating and filling the space along the system of early brittle faults developed in the nexus region of the complex: [6] and references therein. In accordance with previous observations at Monchepluton [6], we presume that Cl was locally abundant in a late fluid during the formation of the sulfide Cu–Ni–PGE mineralization of the veins. We propose a secondary origin, related to a low-temperature oxidation of a Pb-bearing precursor, presumably galena, to account for the presence of a pyromorphite–phosphohedyphane solid solution at Monchepluton. This mechanism and mode of formation are consistent with findings on related Cl-rich phosphates of Pb and Ca from other localities [11,17].

We believe that the lead-bearing phosphate likely formed at elevated levels of fO₂ via two-stage reactions of precursor grains of galena with components of the residual fluid saturated with respect to an apatite-supergroup mineral and containing Ca (and minor Ba) as incompatible components during crystallization of the sulfide ore:

2PbS + 3O₂ → 2PbO + 2SO₂

(1)

and

4CaO + 16PbO + 2P₂O₅ + Cl + 9O₂ → 4[CaPb₄(PO₄)₃Cl]

(2)

Grains of barite, also present in the heavy-mineral concentrate of the veins, could well form according to the following reaction: Ba + SO₂ + O₂ → BaSO₄.

The inferred regime of progressive increase in fO₂ is further corroborated by the common occurrence of highly oxidized varieties of ochreous and porous ores, which commonly accompany the primary sulfide ores in the veins at Monchepluton. A progressive rise in intrinsic fO₂ is also inferred for some other ultrabasic complexes in the Serpentinite belt of the Kola Peninsula [48,49,50].

5. Conclusions

(1)

We describe the occurrence of a pyromorphite–phosphohedyphane solid solution in zones of massive PGE-bearing Cu–Ni sulfide mineralization in the Monchepluton complex. An elevated level of Cl developed in the cumulates at the hydrothermal stage and migrated along the system of early brittle faults.

(2)

The phosphohedyphane-like phase is a member of the late Pb–S–Te–Bi–As-enriched assemblages, which include Pd–Pt bismuthotellurides and sperrylite, all deposited hydrothermally, from ~750 to below 500 °C, during the formation of the sulfide veins.

(3)

A secondary origin is inferred, involving a low-temperature oxidation of a Pb-bearing precursor, presumably galena, to form grains of the phosphohedyphane-like phase. This mechanism and mode of formation are consistent with findings on related Cl-rich phosphates of Pb and Ca from other localities, which are reviewed.

(4)

There was a progressive increase in fO₂, in agreement with the abundance of oxidized varieties of ores in association with primary ores in the veins at Monchepluton and at other ultrabasic complexes of the Kola Peninsula.