Krivolutskaya, N.
Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Kosygina, 19, Moscow, 119991, Russia e-mail: nakriv@mail.ru
ABSTRACT. The main problems for the genesis of the Noril’sk deposits are evaluated: 1) evolution of magmatism in the Noril’sk area, 2) the positions of ore-bearing intrusions in the Siberia trap, 3) geochemical classification of the intrusions, 4) magma compositions of ore-bearing intrusions, and 5) degree of assimilation in magma chambers. Based on the geology and geochemistry of igneous rocks, a long-term formation of the deposits in a rift zone is suggested.
Intrusive rocks in the Noril‘sk region are clas- sified (based on the abundance of trace elements and isotopic data) into three types in accord with the Gudchikhinsky (Dymtaleysky type of intru- sions), Nadezhdinsky (Lower Talnakh Type), and Morongovsky (Noril’sk type) Formations.
Based on geological relations between the basalts and intrusions, it is recognized that the Noril‘sk Intrusive Complex was formed after the Low Nadezhdinsky lavas. Comparison of major and trace element compositions between the ore-bear- ing intrusions and the associated lavas formed in this stage reveals some significant differences between them such as whole-rock MgO concen- trations (10- 12 wt. % in the intrusions and 6-7 wt. % in the volcanic rocks), whole-rock TiO2 con- centrations and whole-rock La/Yb ratios as well as isotopic compositions (e.g., δ34S from + 1 to -5 and 18‰ in the basalts and intrusions, respec- tively). The absence of geological evidence for a genetic relationship between the intrusions and lavas, plus their differences in geochemistry has led me to conclude that the ore-bearing intrusions and volcanic rocks are not comagmatic. As sug- gested by many researchers (Godlevsky, 1959;
Likhachev, 2006 among others) the Norilsk intru- sions were formed by a distinct magmatic pulse of activity after the Nadezhdinsky eruption. This study is the first to estimate the composition of the parental magma for ore-bearing and barren intru- sions based on study of melt inclusions trapped by olivine and pyroxene. The results show that the ore-bearing intrusions were produced by high magnesian basaltic magma (up to 8 wt. % MgO), which crystallized Ol and Pl phenocrysts at depth.
The parental magma also shows geochemical sig- natures of crust (negative Ta-Nb and positive Pb anomalies). It contains 0.5-0.7 wt. % H2O, trace CO2 and up to 0.2 wt. % Cl.
It was suggested that heavy sulfur isoto- pic composition of the sulfide ores (up to 18‰, Grinenko, 1985), which is atypical of magmatic deposits, has been contributed to the assimilation of sulfate-bearing country rocks by the magma.
Our new data of sulfur isotopes for the anhydrite are inconsistent with the hypothesis that anhy- drite from the country rocks was an external sulfur source for the Noril‘sk ores. Sulfur isotopic com- positions of basalts and some intrusions in the Tai- myr Peninsula may be used to address this issue.
The highest δ34S values among all formations were
found in the primitive rocks from Gudchikhin- sky picrites (δ34S = 8.6 ‰; Ripley et al., 2003) and Dyumtaleysky (δ34S = 12.2‰) gabbro. These data support the hypothesis of Likhachev (2006) that the heavy S isotopic composition of the Noril’sk ores is a mantle characteristics, which could have resulted from recycling of biogenic sulfides by subduction.
The following conclusions are outlined below.
1. The volcanic rocks of the Norilsk region formed by two episodes (rifting and trap) which can be further divided into four cycles.
2. The ore-bearing intrusions are not comag- matic with the tuff-lava sequence; they formed by separate magmatic activity during the develop- ment of the trap.
3. The intrusions formed by high-Mg tholeiitic magma.
4. The fluids associated with trap magmatism were dominated by H2O-CO2, the parental mag- mas of both ore-bearing and ore-barren intrusions contained similarly low H2O, CO2 and F.
5. Assimilation processes did not play a major role in the formation of the ore deposits.
REFERENCES
1 . GODLEVSKY, M .N . (1959): Traps and ore-bearing intrusions . Moscow, Gosgeoltekhizdat, 61 p . (in Russian) . 2 . GRINENKO, L .N . (1985): Sources of sulfur of the
nickeliferous and barren gabbro dolerite intrusions of the northwest Siberian platform . International Geology Reviews, 27, 695-708 .
3 . KRIVOLUTSKAYA, N .A . (2011): Formation of Pt- Cu-Ni deposits in the process of evolution of flood- basalt magmatism in the Noril’sk region . Geology of Ore Deposits, 4, 309-339 .
4 . LIKHACHEV, A .P . (2006): Platinum-Copper-Nickel and Platinum deposits . Moscow, Eslan . 586 p . (in Russian) . 5 . RAD’KO, V .A . (1991): Model of dynamic differentia-
tion of intrusive traps in the northwestern Siberian plat- form . Soviet Geology and Geophysics, 32(11), 15-20 . 6 . RIPLEY, E .M ., LIGHTFOOT, P .C ., Li, C ., ELSWICK,
E .R . (2003): Sulfur isotopic studies of continental flood basalts in the Noril’sk region: Implications for the association between lavas and ore-bearing Intrusions . Geochimica et Cosmochimica Acta, 67, 2805–2817 .
7 . NALDRETT, A .J . (1992): A model for the Ni-Cu-PGE ores of the Noril’sk region and its application to other areas of flood basalts . Economic Geology, 87, 1945-1962 .
It is commonly assumed that ultramafic-mafic intrusions and associated PGE-Cu-Ni sulfide deposits of Northern Siberia represent a small component of a major episode of mafic activity at
~250 Ma, which included formation of the most extensive flood-basalt province on Earth (Campbell et al., 1992). Recent studies, however, advocated protracted evolution of the ore-forming magmas parent to the Noril’sk-type intrusions (e.g., Malitch et al., 2010; 2012). Mafic-ultramafic intrusions and Ni-Cu-PGE (platinum-group elements) sulfide deposits in the Noril’sk-Talnakh region (Russia) are considered to be closely linked, indicating that primitive mantle-derived materials are intrinsic to their petrogenesis (Tuganova, 2000; Arndt et al., 2005; Malitch et al., 2013 among others).
This study assesses Hf-Nd-Sr-Cu-S isotope data for the same suite of lithologies and associ- ated PGE-Cu-Ni sulfide ores from 12 ultramafic- mafic intrusions of the Noril’sk province and two ultramafic-mafic intrusions (i.e., Binyuda and Dyumptalei) of the Taimyr province. Intrusions of the Noril’sk Province were subdivided into three main types in terms of sulfide mineralization style and economic significance.
Type 1 comprises the economic ore-bearing intrusion that hosts commercial reserves, including the Noril’sk-1, Talnakh and Kharaelakh intrusions, which contain well-defined horizons of plagioclase- bearing dunite and wehrlite with elevated contents of Cr and taxitic-textured rock assemblage and host disseminated, veined and massive ores. In the upper part of these intrusions a PGE-rich low-sulfide hori-
zon is hosted by leucogabbro with lenses of ultra- mafic rocks. Rocks of these intrusions have “radio- genic” initial Sr ratios (87Sr/86Sri = 0.7055–0.7075) against rather constant εNd values of ~ +1. Zircons from economic intrusions with U-Pb ages between ca. 230-340 Ma yielded εHf(t) values in the range from 2.3 to 16.3 (n = 24) at Kharaelakh, from 0.1 to 16.2 (n = 76) at Talnakh and from -4.7 to +15.5 (n = 54) at Noril’sk-1, consistent with a model that involves interaction of distinct magma sources (Malitch et al., 2010; 2013; this study).
Type 2 comprises sub-economic intrusions that host non-commercial reserves, including the Cher- nogorsk, Zub-Marksheider, Vologochan, Yuzhnoe Pyasino and Imangda intrusions. Rocks of the sub- economic intrusions have lithology, mineralogy, geo- chemistry and Nd-Sr-Hf isotope systematics broadly similar to that of the economic intrusions. The sub- economic intrusions, however, have disseminated or more rarely vein-disseminated sulfide ores and may contain small- to medium-sized Ni-Cu sulfide depos- its, and medium- to large-sized PGE deposits.
Type 3 comprises weakly mineralized mafic- ultramafic intrusions, the so-called Lower Tal- nakh type, as represented by the Nizhny Talnakh, Nizhny Noril’sk and Zelyonaya Griva intrusions.
They contain low-grade disseminated Cu–Ni ores with ~0.2 wt.% of Cu and Ni, and low Cr and PGE (~0.005 ppm, rarely up to 0.02 ppm). Rocks from the Nizhny Talnakh, Nizhny Noril’sk and Zelyo- naya Griva intrusions have more radiogenic initial Sr values (87Sr/86Sri = 0.7076 – 0.7086), average εNd values of ca. -5 and εHf values of ~0.
Nd-Sr-Hf-Cu-S ISOTOPE SYSTEMATICS OF ORE-BEARING
ULTRAMAFIC-MAFIC INTRUSIONS FROM POLAR SIBERIA (RUSSIA):
GENETIC CONSTRAINTS AND IMPLICATIONS FOR EXPLORATION Malitch, K.N.1, Badanina, I.Yu.1, Belousova, E.A.2, Griffin W.L.2, Latypov, R.M.3,
Romanov A.P.4 & Sluzhenikin, S.F. 5
1Institute of Geology and Geochemistry, Ural Branch of Russian Academy of Sciences, Ekaterinburg, 620075, Russia
2CCFS/GEMOC ARC National Key Centre, Macquarie University, Sydney, NSW 2109, Australia
3School of Geosciences, University of the Witwatersrand, Pvt Bag 3, Wits 2050, South Africa
4Krasnoyarsky Research Institute of Geology and Mineral Resources, Krasnoyarsk, 660049, Russia
5Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences (IGEM RAS), Moscow, 119017, Russia
e-mail: dunite@yandex.ru
ABSTRACT. The study evaluates the usefulness of Nd-Sr-Hf-Cu-S isotope information, providing insights into origin of mafic-ultramafic intrusions and associated Ni-Cu-PGE sulfide ores of the Polar Siberia. New indicators for the economic potential of Ni-Cu-PGE sulfide deposits are suggested.
Samples from the different intrusions in the Noril’sk Province have overall δ65Cu values ranging from -2.3‰ to 1.0‰ and δ34S values from -0.7‰
to 13.8‰ (Fig. 1). Samples from the three eco- nomic deposits have distinct mean δ65Cu values of -1.56±0.27‰ for Kharaelakh, -0.55±0.41‰ for Tal- nakh and 0.23±0.28‰ for Noril’sk-1, consistent with those for carbonaceous chondrite and iron meteorites.
The variation of δ65Cu values is interpreted to repre- sent a primary feature of the ores, although assimila- tion of external source at Kharaelakh cannot be ruled out. Sulfide ores from the three economic intrusions have distinct but restricted ranges of δ34S values with a mean of 12.7±0.5‰ for Kharaelakh, 10.9±0.6‰
for Talnakh and 9.2±1.8‰ for Noril’sk-1. The ores of the subeconomic Chernogorsk intrusion have homogenous δ34S values of 10.9±0.4‰, in contrast to the highly variable S isotopic compositions for those in the Vologochan, Yuzhnoe Pyasino and Zub- Marksheider intrusions (from 5.1‰ to 8.5‰, from 8.1‰ to 10.5‰, from -0.7‰ to 3.9‰, respectively) and sulfide accumulations from the non-economic intrusions (1.8–9.7‰). Given the Zub-Marksheider and Kharaelakh intrusions are located at the same stratigraphic level, their contrasting S isotopic com- positions indicate that the immediate country rocks have little impact on the S isotopic composition of sulfide ores of the intrusions, whereas the interac- tion of magma with host rocks that took place deeper before final emplacement might be the major control (Malitch et al., 2014).
The observed wide range of εHf and 87Sr/86Sri values combined with a restricted range of εNd
values, and the negative correlation of S and Cu isotope compositions along with a restricted range of δ34S and δ65Cu values for an individual intrusion are considered to be useful indicators of the poten- tial for hosting Ni-Cu-PGE sulfide deposits.
Acknowledgments. The study was partly sup- ported by the Russian Foundation for Basic Research (grant 13-05-00671-a) and the Ural Branch of Russian Academy of Sciences (grant 12-U-5-1038).
REFERENCES
1 . ARNDT, N .T ., LESHER, C .M . & CZAMNSKE, G .K . (2005): Magmas and magmatic Ni-Cu-(PGE) deposits . Economic Geology 100th Anniversary Volume, 5-23 . 2 . CAMPBELL, I .H ., CZAMANSKE, G .K ., FEDO-
RENKO, V .A ., HILL, R .I . & STEPANOV, V . (1992):
Synchronism of the Siberian traps and the Permian- Triassic boundary . Science, 255, 1760-1763 .
3 . MALITCH, K .N ., BADANINA, I .Yu ., BELOUSOVA, E .A .
& TUGANOVA, E .V . (2012): Results of U-Pb dating of zircon and baddeleyite from the Noril’sk-1 ultramafic- mafic intrusion (Russia) . Russian Geology and Geo- physics, 53 (2), 123-130 .
4 . MALITCH, K .N ., BELOUSOVA, E .A ., GRIFFIN, W .L .
& BADANINA, I .Yu . (2013): Hafnium-neodymium constraints on source heterogeneity of the economic ultramafic-mafic Noril’sk-1 intrusion (Russia) . Lithos, 164-167, 36-46 .
5 . MALITCH, K .N ., BELOUSOVA, E .A ., GRIFFIN, W .L ., BADANINA, I .Yu ., PEARSON, N .J ., PRESNYA- KOV, S .L . & TUGANOVA, E .V . (2010): Magmatic evolution of the ultramafic-mafic Kharaelakh intru- sion (Siberian Craton, Russia): insights from trace-ele- ment, U-Pb and Hf-isotope data on zircon . Contribu- tions to Mineralogy and Petrology, 159, 753-768 .
6 . MALITCH, K .N ., LATYPOV, R .M ., BADANINA, I . Yu . & SLU-
ZHENIKIN, S .F . (2014):
Insights into ore genesis of Ni-Cu-PGE sulfide depos- its of the Noril’sk Province (Russia): evidence from copper and sulfur isotopes . Lithos, doi: 10 .1016/j . lithos .2014 .05 .014 .
7 . TUGANOVA, E .V . (2000): Petrographic types, genesis and occurrence of Ni-Cu-PGE sulfide depos- its . VSEGEI Press, St . Petersburg, 102 pp . (in Rus- sian) .
Fig. 1. 34S-65Cu systematics of Cu-Ni sulfide ores from economic, subeconomic, prospective and non-economic intrusions of the Noril’sk and Taimyr provinces
Apatite is considered to be an important col- lector of F and Cl in igneous rocks. Both F and Cl are important in the formation of PGM. Both apatite-(CaCl) and apatite-(CaFCl) are observed in the PGM-rich horizons in large layered mafic- ultramafic intrusions in the world such as the Bushveld and Stillwater complexes (Boudreau &
McCallum, 1992; Mathez et al., 1985;Meurer &
Boudreau, 1996).
Apatite-(CaF) in the Noril’sk sulfide ores was first described by Genkin & Vasil’eva (1961). We have found that apatite is common in the oliv- ine-rich intrusive rocks and troctolites that con- tain magmatic sulfide droplets with diameter up to 50 mm in the Skalisty and Mayak (Talnakh intrusion) and Oktaybr’sky (Kharaelakh intru- sion) mines. The sulfide droplets are composed of sulfide minerals formed by the breakdown of Mss and Iss. Pneumatholitic alteration aure- oles up to 12 mm in thickness are present above the sulfide droplets. The alteration aureoles are composed of Ti-bearing biotite, apatite-(CaCl), apatite-(CaClF), anhydrite, Cl-bearing hasting- site, kaersutite, edenite and Cl-bearing alka- line sulfides such as djerfisherite and bartonite (Godlevsky 1959; Spiridonov 2010).
Micro-grains of PGM are also present in the alteration aureoles (Spiridonov, 2010). Apatite- (CaCl) and apatite-(CaClF) are associated with interstitial sulfides (Fig. 1) and sulfide droplets.
The apatite grains occur as large pristmatic crys- tals with lengths up to 4 mm (Fig. 2). The apatite
of the first generation (Fig. 3) contains high Cl and up to 2 wt.% total REE. Apatite II, which occurs most commonly as overgrowth on apa- tite I, has variable compositions from apatite- (CaFCl) to apatite-(CaF) (Fig. 3). The F-rich apatite occurs as short prismatic crystal soli- tarily. Apatite II contains up to 0.5 wt.% total REE.The compositions of apatite I and II from the Noril’sk sulfide ores indicate that in the initial flu- ids derived from highly fractionated sulfide melts were enriched in chlorine. With time the fluids became enriched in fluorine instead.
In the epigenetic metamorphic-hydrothermal mineralization zones apatites I and II have been partially or completely replaced by apatite-(CaOH) (Fig. 4). The hydroxylapatite is present together with prehnite, pumpellyite, chlorite and others low- temperature silicate minerals. Fluorine was com- pletely lost immediately during the replacement.
In contrast, chlorine was leached out gradually.
Acknowledgements. This study was financially supported by RFBR (grant 13-05-00839).
REFERENCES
1 . GENKIN, A .D ., VASIL’EVA, Z .V . & YAKOVLEVS- KAYA, T .A . (1961): Conditions for localization of apatite in sulfide Cu-Ni ores at Noril’sk deposit . Geologiya Rudnykh Mestorozhdenii 3 (2), 100-108 (in Russian) .