ISSN 10757015, Geology of Ore Deposits, 2013, Vol. 55, No. 5, pp. 320–340. © Pleiades Publishing, Ltd., 2013. Original Russian Text © Yu.O. Larionova, A.V. Samsonov, K.N. Shatagin, A.A. Nosova, 2013, published in Geologiya Rudnykh Mestorozhdenii, 2013, Vol. 55, No. 5, pp. 374–396.
Isotopic Geochronological Evidence for the Paleoproterozoic Age of Gold Mineralization in Archean Greenstone Belts of Karelia, the Baltic Shield Yu. O. Larionova, A. V. Samsonov, K. N. Shatagin, and A. A. Nosova Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 119017 Russia Received July 16, 2012
Abstract—The Rb–Sr age of metasomatic rocks from four gold deposits and occurrences localized in Archean granite–greenstone belts of the western, central, and southern Karelian Craton of the Baltic Shield has been determined. At the Pedrolampi deposit in central Karelia, the dated Aubearing beresite and quartz–carbonate veins are located in the shear zone and replace Mesoarchean (~2.9 Ga) mafic and felsic metavolcanic rocks of the Koikar–Kobozero greenstone belt. At the Taloveis ore occurrence in the Kosto muksha greenstone belt of western Karelia, the dated beresite replaces Neoarchean (~2.7 Ga) granitoids and is conjugated with quartz veins in the shear zone. At the Faddeinkelja occurrence of southern Karelia, Au bearing beresite in the large tectonic zone, which transects Archean granite and Paleoproterozoic mafic dikes, has been studied. At the Hatunoja occurrence in the Jalonvaara greenstone belt of southwestern Kare lia, the studied quartz veins and related gold mineralization are localized in Archean granitoids. The Rb–Sr isochrons based on wholerock samples and minerals from orebearing and metasomatic wall rocks and veins yielded ~1.7 Ga for all studied objects. This age is interpreted as the time of development of orebearing tec tonic zones and oreforming hydrothermal metasomatic alteration. New isotopic data in combination with the results obtained by our precursors allow us to recognize the Paleoproterozoic stage of gold mineralization in the Karelian Craton. This stage was unrelated to the Archean crust formation in the Karelian Block and is a repercussion of the Paleoproterozoic (2.0–1.7 Ga) crustforming tectonic cycle, which gave rise to the for mation of the Svecofennian and Lapland–Kola foldbelts in the framework of the Karelain Craton. The ore forming capability of Paleoproterozoic tectonics in the Archean complexes of the Karelian Craton was prob ably not great, and its main role consisted in reworking of the Archean gold mineralization of various genetic types, including the inferred orogenic mesothermal gold concentrations. DOI: 10.1134/S1075701513050048 1
INTRODUCTION
The greenstone belts in Archean cratons are among the leading sources of lode gold in the world. Most Au resources are contained in large and giant deposits of this type (Herrington et al., 1997; Groves et al., 2003). However, no large gold deposits in the Archean cra tons have been discovered in Russia to date, although granite–greenstone domains in the crystalline base ment of the East European and Siberian platforms have much in common with the Archean gold prov inces of Canada and Australia. The most paradoxical seems to be the situation for the large and comprehen sively studied Karelian region in the Baltic Shield, where prospecting for gold revealed many gold miner alization showings as early as the 1980s, among which only a few correspond to small deposits (Mineral’no syr’evaya …, 2005). In many regards, this situation is related to the insufficiently studied genesis and age of gold mineral 1 Corresponding
[email protected]
author: Yu.O. Larionova. Email: ukalarion
ization in greenstone belts of the Karelian Craton. These issues are not only of scientific interest but are also important for the forecasting of promising territo ries and the planning of prospecting and geological exploration. Information available to date is a subject of much controversy. Many authors refer all significant gold occurrences known in greenstone belts of the Karelian Craton to the orogenic mesothermal type (Kozhevnikov, 2000; Eilu et al., 2003; Geologiya …, 2006] and view gold resource potential of this territory as optimistic, (Geologiya ..., 2006) because orogenic mesothermal deposits are the most productive in the Archean greenstone belts (Groves et al. 2003). On the contrary, in the summary Mineral’nosyr’evaya …, (2005), most gold occurrences in greenstone belts of the Karelian Craton were referred to syngenetic pluto nogenic and volcanogenic deposits, which are less promising in the Archean belts. Similar discrepancies in genetic interpretation are also noted in publications concerned with particular gold deposits and occur rences (LobachZhuchemko, 2000; Ivashchenko,
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2006; Ivashchenko and Golubev, 2009; Volkov et al., 2007). The age of the most promising gold mineralization related to tectonic zones also remains a matter of debate. By analogy with Canadian and Australian cra tons, where significant economic deposits are known, the formation of gold mineralization of this type is referred to the final stage of Archean history and related to large shear zones that formed during the breakdown of the Archean collisional orogen (Kozhevnikov et al., 1998; Kozhevnikov, 2000; Geologiya …, 2006). At the same time, tectonic zones with mesothermal gold–sulfide mineralization local ized in both Archean and Paleoproterozoic complexes are known in the Karelian Craton (Kuleshevich, 1992; Ledeneva and Pakulnis, 1997; Akhmedov et al., 2001; Mineral’nosyr’evaya …, 2005). Detailed isotopic geochronological studies of gold mineralization attributed to the mesothermal oro genic type by a set of indicatots have been carried out for the Archean greenstone belts of the Karelian Cra ton only in the territory of Finland, but they did not unequivocally answer the question on mineralization age. The U–Pb dating of titanite, rutile, and monazite from orebearing rocks and Pb/Pb study of sulfides from orebodies of the Ilomantsi belt indicate a Neoarchean age of gold mineralization, which post dates plutonic rocks by 100–150 Ma (Vaasjoki et al, 1993). The Paleoproterozoic age of ~1.7 Ga obtained by the Pb/Pb method for native gold and galena (Vaas joki et al, 1993), as well as with K–Ar and Rb–Sr methods for micas (O’Brien et al., 1993), are inter preted as a result of Svecofennian remobilization of Archean mineralization. It should be noted that the authors of the aforementioned publications do not rule out a reappraisal of the significance of Archean mesothermal gold mineralization, because closure temperature of the U–Pb isotopic system used for dat ing zircon, titanite, and rutile is much higher than that of mesothermal ore formation. We carried out an Rb–Sr isotopic geochronological study of synore metasomatic rocks at the four best known gold occurrences localized in the Archean complexes of the Karelian Craton: the Taloveis in western Karelia; the Pedrolampi in central Karelia; the Faddeinkelja in southern Karelia, and the Hatunoja site of the Jalonvaara deposit in southwest ern Karelia (Fig. 1). Rb–Sr isotopic dating of ore forming hydrothermal metasomatic processes is not a fundamentally new technique. It has been successfully applied to the giant Muruntau deposit (Kostitsyn, 1993, 1996), the Shkol’noe deposit (Moralev and Shatagin, 1999), and the Sukhoi Log deposit (Chugaev, 2007). As concerns the deposits and occur rences of Karelia, the possibility of such an approach to dating ore formation has not been considered as yet, primarily due to doubt about stability of the Rb–Sr isotopic system with respect to superposed processes GEOLOGY OF ORE DEPOSITS
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that repeatedly developed in the region over the more than 3 Ga of the history of its evolution. Nevertheless, having used this method, we man aged to obtain basically new information on the age of gold mineralization. Our isotopic geochronological data will probably be helpful for opening up fresh pos sibilities for typifying the studied sites, providing insights into the formation of mesothermal orogenic deposits as a whole, and touching on the question why large gold deposits are rare in the territory of Karelia. AGE AND ORIGIN OF GOLD MINERALIZATION IN THE GEOLOGICAL HISTORY OF THE KARELIAN CRATON The Karelian Craton in the southeastern Baltic Shield is a large segment of the Archean crust sand wiched between the Paleoproterozoic Svecofennian and Lapland–Kola foldbelts (Fig. 1). The main events in the Archean history of crust formation in the Kare lian Craton and its subsequent reworking during Pale oproterozoic are summarized in Table 1. The Archean crust of the Karelian Craton consists of Paleo, Meso, and Neoarchean terranes. The old est terranes, whose cores are composed of Paleo and Mesoarchean TTG gneisses dated at 3.5–3.0 Ga, are localized in the eastern and western parts of the Kare lian Craton. The tonalite–greenstone belts that formed 2.95–2.78 Ga ago are located in the frame work of these ancient domains. The Vedlozero–Seg ozero, Kostomuksha, and East Finnish tonalite– greenstone belts localized in the inner part of the Karelian Craton demonstrate the contribution of ancient felsic material, and their origin is discussed in terms of models of active continental margins, backarc basins, or ensialic rifts (Puchtel et al., 1998; Fedchuk et al., 2003; Samsonov et al., 1997; Lobach Zhuchenko et al., 2000, 2001; Kozhevnikov, 2000; Svetov, 2005). At the outer northeastern flank of the Karelian Craton, the Sumozero–Kenozero, South Vygozero, Parandovo–Nadvoitsy, and North Karelian tonalite–greenstone belts have ensimatic isotopic and geochemical signatures and are regarded as islandarc terranes (Puchtel et al., 1999; Bibikova et al., 2003) formed at a distance from the ancient continental crust in a vast oceanic basin, which occupied the terri tory of the Belomorian Mobile Belt (Slabunov, 2008). The older blocks and adjacent tonalite–greenstone belts are separated by Neorachean (2.74–2.69 Ga) juvenile complexes of the Central Karelian Terrane with sharp predominance of granitoid batholiths per taining to the sanukitoid series. Most characteristics of these granitoids, such as late kinematic and postki nemtic structural position, conjugation with transten sional structures, and spatiotemporal links to subalka line granitoids and lamprophyres, suggest that Neoarchean sanukitoid magmatism is related to the regime of withinplate extension (Samsonov et al., 2004; Bibikova et al., 2005; Larionova et al., 2007).
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Caledonides
33° E
V
VI
66° N 7
IV 15
8 26
5
17
III
12 13
4
Timan
16
66° N
White Sea 14 25
I 6
24 11 3
18 19
10
10 9 28
1 5
29 7 30
20
Lad oga L.
1
23 21
6
L. ega On
100 km
27
22 34
2
VII II
33° E
1
5
9
13
2
6
10
14
3
7
11
15
4
8
12
16
a b c
17
18 19 20
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The available petrologic and isotopic data show that mantle material that metasomatized during preceding, probably, subductionrelated tectonomagmatic events was a source of initial melts for sanukitoid series (Kov alenko et al., 2005; Larionova et al., 2007). Melting of the metasomatized mantle at the postsubduction stage was probably related to the ascent of a mantle diapir initiated by breakdown of collisional orogen with delamination of its lithospheric root. The aforementioned data show that the Archean history of crust formation in the Karelian Craton is close to the history of Phanerozoic accretionary and collisional orogens in the compostion of rock com plexes and in the sequence of geological and tectonic events (Kozhevnikov, 2000; Samsonov et al., 2005; Types …, 2006). The bilateral symmetry of chronolog ical, compositional, and isotopic characteristics of rock complexes established for the inner part of the Karelian Craton is typical of the Phanerozoic colli sional orogens (Moore, 1995). On the contrary, the outer northeastern flank of the Karelian Craton prob ably was an accretionary orogen consisting of island arcs different in age. The continental crust of the Karelian Craton formed by the end of Neoarchean was subject to mul tiple reworking during the Paleoproterozoic. In the Early Paleoproterozoic (2.5–2.0 Ga), the reworking was related to rifting and withinplate mag matism. During the Sumian stage (2.5–2.4 Ga), this was bimodal magmatism characterized by the forma tion of mantlederived basaltic magma and felsic derivatives of intracrustal melting. The Yatulian stage (2300–2050 Ma) was a period of relative quiescence of endogenic activity with limited manifestations of basaltic magmatism (Stepanova et al., 2011). A signif icant time interval was amagmatic and characterized by intense sedimentation first under continental and then marine conditions with deposition of carbonate and evaporite sequences (Ojakangas et al., 2001; Kuznetsov et al., 2010). The Ludicovian stage (2050–1970 Ga), which marks the transition from marine to continental sedimentation, was distinguished by deposition of
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black shales. In the Karelian province, this stage of magmatism was represented by volcanic complexes, sills, intrusions, and dike swarms composed of diverse rocks, including magnesian and ferroan tholeiites, subalkali basalts, picrites, kimberlites, and carbon atites (Puchtel et al., 1998; Ushkov, 2001; Samsonov et al., 2009; Corfu et al., 2011). At the end of Middle Paleoproterozoic 2.1–2.0 Ga, the breakup of the Archean crust gave rise to opening of the Lapland–Kola and Svecofennian oceanic basins; the Karelian Craton was situated between them (Fig. 1). Subductionrelated events, which took place 1.98–1.91 Ga ago in these oceans (Daly et al., 2006; Lahtinen et al., 2008), did not develop in the Karelian Craton in any way. The collision and postcollision granitoids dated at 1.87–1.80 Ma are localized at the margins of the Karelian Craton in the Raahe–Ladoga Zone of conjugation with the Svecofennian Belt and in the Belomorian Mobile Belt as a foreland of the colli sional Lapland–Kola Orogen (Daly et al., 2006). It seems probable that just at that time, a segment of the Karelian Craton sandwiched between two orogens underwent intense compression. The stress relaxation accompanying breakdown of the framing orogen apparently gave rise to extension of crust in the Kare lian Craton and origination of a system of nearmerid ional shear zones. These folding and faulting zones have been studied in detail in the Onega structure (Onezhskaya …, 2011). The relationships of gold mineralization to the tec tonomagmatic stages of evolution of the Karelain Craton have been keenly discussed in recent years (Table 1). As follows from earlier publications and our study, the Archean Aubearing massive sulfide lodes and Cu–Mo(Au) occurrences are identified with confi dence. The spatial distribution of these occurrences and scale of gold mineralization are nonuniform. The Archean mesothermal gold mineralization is not as reliably recognized. The massive and dispersed strati form sulfide mineralization is associated with volcanic rocks and corresponds to sedex accumulation of ore matter. The small Cu–Zn–Pb(±Au) massive sulfide
Fig. 1. Tectonic scheme of northern part of East European Craton. Compiled after Slabunov (2008), Mineral’nosyr’evay … (2005), and Samsonov et al. (2009). (1) 3.5–2.9 Ga, TTG gneiss and amphibolite; (2) 2.9–2.7 Ga, TTG granitoids of the Belo morian Foldbelt reworked in Paleoproterozoic; (3) 2.9–2.7 Ga, TTG granitoids; (4) 2.9–2.7 Ga, volcanic–sedimentary com plexes of Archean greenstone belts; (5) 2.75–2.70 Ga, sanukitoids; (6) 2.75–2.65 Ga, granulite complexes; (7) tectonic mixture of Neoarchean and Paleoproterozoic complexes in framework of Lapland–Kola Orogen; (8) 2.5–2.4 Ga, complexes of Sumian and Sariolian stages; (9) 2.30–1.97 Ga, complexes of Jatulian and Ludicovian stages; (10) ~2.00 Ga, volcanic and sedimentary rocks of the Lapland Belt; (11) 1.97–1.80 Ga, complexes of Svecofennian Belt; (12) 1.97–1.80 Ga, complexes of the Tersky Ter rane, Lapland–Kola Orogen;(13) 1.78 Ga, Vepsian sandstone and dolerite; (14) Early Riphean rapakivi granite; (15) Riphean complexes; (16) Phanerozoic complexes; (17) gold deposits and occurrences: (a) massive sulfide, (b) porphyry Cu–Mo, (c) mesothermal; (18) fault, (19) thrust fault, (20) inferred fault. Gold occurrences and deposits (numerals in figure): 1, Jalonvaara; 2, Hautovaara; 3, Talpus; 4, Zolotye Popogi (Gold Rapids); 5, Hatunoja; 6, Novye Peski (New Sand); 7, Central; 8, Pedrolampi; 9, Rybozero; 10, Zalomaevsky; 11, Voitsky Mine; 12, Taloveis; 13, Berendei; 14, Lobash; 15, Moukkori; 16, Palovaara; 17, Sep ponen; 18, Pampalo; 19, Valkeasuo; 20, Faddeinkelja; 21, Meridional zone; 22, Vesenny; 23, Kosmozero; 24, Rigovarraka; 25, Shuezero; 26, Shombozero; 27, Kozhozero; 28, Pakula; 29, Svyatnavolok 2; 30, Vietukkalami; 31, Maksovo. Tectonic blocks (numerals in figure): I, Karelian; II, Vodlozero; III, Iisalmi; IV, Ranua; V, Central Kola; VI, Murmansk; VII, Shenkur. Main greenstone belts of the Karelian Block discussed in text (numerals in circles): 1, Sumozero–Kenozero; 2, VedlozeroSegozero; 3, South Vygozero; 4, Kostomuksha; 5, East Finnish; 6, Parandovo–Nadvoitsa; 7, North Karelian; 8, Central Belomorian; 9, Ilomantsi–Jalonvaara greenstone belt. GEOLOGY OF ORE DEPOSITS
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Table 1. Position of gold mineralization in Early Precambrian history of crust formation in Karelian Craton Time, Ma ~1.70
Tectonic setting
Lithotectonic complex
Breakdown of collisional Deformation and metasomatism orogen with tornoff lithos pheric root (?)
Position of gold mineralization U–V–Au, mesothermal; Au–Qz–S2–, mesothermal
1.80–1.89
Late and postcollision
1.89–1.95
Subduction and collision
Tectonics of convergent plate boundaries in Paleoproterozoic Svecofen nian and LaplandKola foldbelts. In the Karelian Block, magmatism and ore formation of these tectonic stages have not been established
1.95–2.05
Withinplate
Traps, continental sediments
Ti–Fe–V(Au) and Cu–Ni(Au) re lated to traps
2.05–2.30
Epiplatform
Traps, continental and marine sediments, evaporites
Goldbearing conglomerate
2.40–2.50
Withinplate (continental rifts)
Rift troughs: bimodal volcanism and Cu–Ni(Au) sulfide lodes in Mggab sediments; mafic layered intrusions bronorite
2.70–2.75
Late and postcollision; breakdown of orogen with torn off lithospheric root
Deformation and metamorphism. Au–Qz–S2–, mesothermal (?); por phyry Cu–Mo(Au) related to mul Subalkaline magmatism tiphase granitoid plutons
2.78–2.94
Subduction and collision
TTG–greenstone belts: adakitic and BADR magmatism; volcanic chemogenic and biogenic (?) sedi mentary associations (BIF, black shales)
2.84–3.00
Withinplate (oceanic pla Greenstone belts: tholeiitic basalts Cu–Zn–Co–Ni(±Au, ±Pt, ±Pd), teau), primitive oceanic and komatiites massive and dispersed stratiform sul arcs fide mineralization conjugated with MORBtype volcanics
3.20–3.60
Ancient weathering mantle, TTG gneisses and granitoids are tectonic setting is unclear predominant
Cu–Zn–Pb (Au) massive and dis persed stratiform sulfide mineraliza tion conjugated with BADR volcanic activity
Unknown
Compiled after Eilu et al. (2003), Golubev et al. (2008), Hollta et al. (2008), Lahtinen et al. (2008), etc.
occurrences are the most abundant in both inner and outer greenstone belts (Rybakov, 1987; Ivashchenko and Golubev, 2009; Samsonov et al., 2010). These occurrences reveal stable spatial and probably genetic links to the basalt–andesite–dacite–rhyolite (BADR) volcanic series, which are typical of mature arcs and active margins (Samsonov et al., 2005). The porphyry Cu–Mo(±Au) mineralization in the Karelian province is partially and, probably, genetically related to the Neoarchean (2.75–2.69 Ga) multiphase diorite–grano diorite–granite plutons, which intruded supracrustal rocks in the inner and outer greenstone belts. To date, the Lobash1 deposits and the Hatunoja and Hau tovaara occurrences may be referred to this type (Min eral’nosyr’evaya …, 2005; Pokalov and Semenova, 1993; Ivannikov et al., 1995; Kuleshevich et al., 2004; Ivashchenko et al., 2007; Samsonov and Larionova, 2011). The orogenic mesothermal gold–quartz–sul fide mineralization related to the large shear zones that arose during breakdown of the Neoarchean colli sional orogen, is recently regarded as a predominant type in Archean complexes of the Karelian province (Eilu et al., 2003; Geologiya …, 2006). However, as was mentioned in the Introduction, this type of mineral
ization obviously does not belong to the Archean stage and this issue is the cornerstone of our study. In the Paleoproterozoic history of the Karelian Craton, the most widespread was epigenetic meso thermal gold mineralization, which occurs as show ings, occurrences, and several small deposits hosted in the Sumian and Sariolian volcanic–sedimentary structures (Lehta, Vetrenny belt, Shombozero, etc.), as well as in the Jatulian and Ludicovian volcanics and sedimentary rocks of the SallaKuolajarvi Trough and Onega structures (Ledeneva and Pakulnis, 1997; Akhmedov et al., 2001; Mineral’nosyr’evaya …, 2005). According to the geological data, these ore objects formed at the end of the Paleoproterozoic; the Middle Padma deposit was dated at ~1.7 Ga. RESEARCH APPROACH AND METHODS The appeal of the Rb–Sr method for ore formation dating stems from the optimal (in terms of geochro nology) properties of the Rb–Sr isotopic system for the physicochemical parameters of medium and low temperature metasomatic alteration, which accom pany the formation of gold mineralization. The Rb–Sr GEOLOGY OF ORE DEPOSITS
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isotopic system of metasomatic rocks and ores at hydrothermal deposits is formed by the interaction of fluid and host rocks. This interaction redistributes Rb and Sr between phases of this system, which eventually leads to disappearance of the difference in 87Sr/86Sr ratio that initially existed between the fluid and min eral rock phases. The process occurs at a relatively low temperature, at which the rate of Rb and Sr diffusion in the crystal lattice of minerals is extremely slow, so that a new isotopic equilibrium can be achieved only as a result of reactions between fluid and rock. From the mineralogical viewpoint, the interaction between fluid and rocks is expressed in the formation of a new min eral assemblage that replaces the preceding one, e.g., magmatic or diagenetic. As soon as 87Sr/86Sr ratio has been equalized, the system comes to a new isotopic equilibrium and the time of metasomatic alteration can be dated on this basis. However, the choice of min eral assemblage suitable for successful Rb–Sr dating is not a simple problem. The complexity in fitting sam ples, the lack of methodical developments in this field, and related failures are why the Rb–Sr method is cur rently applied infrequently to dating of orebearing metasomatic rocks despite its high geochronological efficiency. This method is commonly used only for identifying a source of ore matter. In this study we used several approaches to choose samples for dating, which are characterized below from the theoretical and practical viewpoints. (1) Use of wholerock samples of metasomatized rocks and ore veins. If the lateral metasomatic zoning in the same protolith complex, e.g., in granitoids, is established, the use of samples taken across the strike of any particular zone can be attempted. Samples col lected in immediate proximity to the orebody reflect maximum intensity of metasomatism, and use of sam ples from Aubearing veins ensures dating of the ore forming process. The isochron relationship based on samples taken across the strike of the ore zone sup poses an equalized isotopic composition in rocks char acterized by samples taken at a significant distance from one another. In practice, this implies that a vig orous alteration of host rocks is required to equalize their initial isotopic heterogeneity. (2) Use of an assemblage of newly forming metaso matic minerals. Beresitefacies metasomatic rocks are predominant at most studied gold deposits. A beresite assemblage comprises minerals with different Rb/Sr ratios, e.g., sericite with a high ratio and calcite with a low ratio. When we studied the rocks, we can identify a new metasomatic mineral assemblage with confi dence. Petrographic examination in combination with local determination of chemical composition of min erals makes it possible to establish whether they are metasomatic or relict. An isochron with good statisti cal parameters based on a set of minerals from one sample gives evidence for their paragenetic links. The parameters of this isochron correspond to the age of the process that formed this mineral assemblage. A GEOLOGY OF ORE DEPOSITS
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problem in this case is limited only by fitting of sam ples with appropriate set of mineral phases. (3) Use of rockforming minerals pertaining to the magmatic assemblage but with chemical composition altered during beresitization. This is probably the most complicated case, because behavior of the Rb–Sr iso topic system is the least predictable to what stage of alteration of chemical composition of rockforming minerals must metasomatic process achieve in order to equalize their Sr isotopic composition. The mineral ogical and petrographic control is necessary for inter pretation of the data obtained, especially in the case of good isochron relationship to be sure that a new isoto pic equilibrium has been achieved just in the metaso matic process. During petrographic examination of host rocks and beresites as products of their alteration, special atten tion should be paid to identification of magmatic, metamorphic, and metasomatic mineral assemblages. If a metasomatic assemblage was actually described in the given rock, reliable dating requires analyzing all mineral phases pertaining to this assemblage and plac ing them in the isochron except for sulfides and quartz, which do not contain Rb and Sr in perceptible quantities. The isotopic study was carried out at the Labora tory of Isotope Geochemistry and Geochronology, Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences. The samples were decomposed in an HNO3 + HF mixture (1 : 5); tourmaline fractions were broken down in an HClO4 : HF mixture (1 : 5). A mixed iso tope spike (85Rb + 84Sr) was added to the samples before decomposition. Rb and Sr were separated on a Dowex 50 × 8 ionexchange resin in quartz columns filled with 2.3N HCl . The isotopic measurements were performed on a Sector 54 mass spectrometer (Micromass, England). The accuracy and precision of the measured Sr isotopic compositions were repeat edly controlled by the international SRM987 stan dard; the average 87Sr/86Sr ratio during experiments was 0.710253 ± 13 (2σ, n = 170). The average uncer tainties of 87Rb/86Sr and 87Sr/86Sr ratios in calculations were accepted as 1% and 0.003%, respectively. The isochron parameters were computed using the method proposed by York (1969) and implemented by the Isoplot 3.00 program (Ludwig, 2003). The eventual uncertainties of the initial Sr isotopic compositions and age measurements correspond to a 95% confi dence level. RESULTS The Taloveis occurrence is located in the western Karelian Craton within the inner Kostomuksha green stone belt (Fig. 1) and hosted in the diorite–granodi otite intrusion of the same name, which was emplaced 2715 ± 5 Ma ago in the Archean metabasalt–metako
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matiite sequence of the Kostomuksha greenstone belt (Samsonov et al., 2004; Larionova et al., 2004). The Taloveis intrusion is close to oval (approxi mately 500 × 1000 m) in plan view. In section, this is a W–Etrending lenticular body steeply dipping to the south. Biotite–hornblende diorite and quartz diorite of the first intrusive phase occur as an outer rim of the intrusion and as thin dikes in country metavolcanics, whereas the main body is composed of granodiorite. The intrusion cuts through basalt, basaltic komatiite, and komatiite of the Kontok Group, which metamor phosed under conditions of epidoteamphibolite facies and transformed into amphibolite, serpentinite, and chlorite–tremolite schists (Kuleshevich and Fur man, 2009). The gold mineralization localized in the intrusion is considered postmagmatic and related to the evolu tion of the diorite–granodiorite melt (Kozhevnikov et al., 1998; Ivashchenko, 2006; Volkov et al., 2007) or to emplacement of potassic granite in the framework of the Kostomuksha greenstone belt (Kuleshevich et al., 2000). The western flank of the intrusion and country metabasic and metakomatiitic sequences are localized within a regional N–Strending shear zone. At the Taloveis occurrence, the shear zone is expressed as two nearly parallel vein and veinlet zones striking in the NNE direction. Zone 1 dissects granodiorite and zone 2 is localized in metabasalts and metakomatiites close to the western contact of the intrusion (Kuleshevich and Furman, 2009). Quartz veins extend from a few meters to 25–30 m along the strike and vary from 0.1–0.5 to 1.5 m in thickness. They are accompanied by beresite after granodiorite, listvenite after metakomatiite, and propylite. Two orebodies have been contoured in zone 1 by geological exploration. Orebody 1 is an ore shoot confined to two quartz veins, which extend along the strike of the productive zone and steeply decline toward the footwall of the intrusion. The orebody has an average thickness of 10 m and is traced for 100 m down the dip and along the strike. The ore grade is 10.8 to 12.9 gpt Au. Orebody 2 as a veinlet zone up to 20 m thick is traced for 80 m down the dip and for 100 m along the strike, having an average grade of 3.5 gpt Au. The content of sulfides in veins, as a rule, does not exceed 3–5%. In the Ushakov Vein within orebody 2, the average grade is 38.1 gpt Au; the thickness is 0.15 m and the extent is 15 m (Kuleshevich and Furman, 2009). The inferred (category С2) gold reserves of this occurrence were estimated at 15.9 kg and an average grade of 11.6 gpt Au; the hypothetical resources (cate gory Р1) are estimated at 60.9 kg (average grade 4.8 gpt Au). The Rb–Sr age was determined for slightly altered diorite and granodiorite, beresitized granitoids, wall rock metasomatic rocks, and a quartz vein pertaining to orebody 1 and for monomineralic fractions of mod erately beresitized granodiorite. Slightly altered diorite and granodiorite from vari ous parts of the intrusion yielded an apparent Rb–Sr
isotopic age of 2637 ± 140 Ma, which coincides within uncertainty limits with the time of intrusion emplace ment estimated at 2715 ± 5 Ma (U–Pb zircon age) (Samsonov et al., 2004). The uncertainty of the Rb–Sr estimate is caused by the wide scattering of data points in the isochron plot (MSWD = 66) (Fig. 2a; Table 2), which could have been a consequence of inhomogene ity of granitoids in 87Sr/86Sr or a result of metasomatic alteration. Taking into account the fact that almost all granitoids at this deposit are subject to a certain degree to hydrothermal metasomatic reworking, the second inference seems more plausible. The distribution of Rb and Sr in granitoids and their beresitized varieties allows us to state that the dis turbance of the Rb–Sr isotopic system related to ber esitization is expressed as a certain increase in the Rb/Sr ratio and that this disturbance becomes obvious in immediate proximity to the orebearing veins. Unfortunately, the isotopic study of the beresitized wall rock and Aubearing quartz vein itself did not give reliable geochronological information. In the plotted isochron, the data points of the beresitized wall rock and quartz vein are arranged along a line with a lower slope (2045 ± 250 Ma) than the line of host granitoids. Because scattering of points relative to this line is rather wide, it is evident that the formation of beres ited wall rock was accompanied by a marked redistri bution of Rb and Sr, but the Sr isotopic composition has not been equalized between particular segments of the orebody. The time of metasomatic alteration that gave rise to the rearrangement of Rb–Sr isotopic system in the metasomatized granitoids has been established by Rb–Sr dating of monomineralic fractions. A sample of moderately beresitized granodiorite was taken 300 m to the east of the axial part of vein–veinlet zone 1. The effect of alteration is confirmed by replacement of biotite with muscovite, development of quartz– pyrite–carbonate–muscovite stringers and patches, variation of plagioclase composition from Ab25 to Ab8, and an increase in Or component in microcline and in elevated Mg# of biotite from 53 to 56. Plagioclase, microcline, and biotite data points, together with the wholerock sample, make up an isochron with t = 1717 ± 27 Ma; (87Sr/86Sr)0 = 0.71074 ± 12; MSWD = 1.1 (Fig. 2b). It is important that removal of the biotite data point yields the same result: 1710 ± 16 Ma; MSWD = 0.89. The petrographic examination of this sample, like other granitoids in this intrusion, does not reveal a highgrade metamorphism, which could have led to reequalization of minerals in metasomatic rock with variable temperature of Rb–Sr isotopic system closure from 200°С for biotite to 740°С for plagioclase (Brabander et al., 1995). It is more probable that the complete rearrangement of Rb–Sr isotopic system in rockforming minerals has been achieved under a rel atively low temperature with participation of fluid in the metasomatic reworking of granite. These data allow us to state that we have established age of the GEOLOGY OF ORE DEPOSITS
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ISOTOPIC GEOCHRONOLOGICAL EVIDENCE Sr/86Sr 0.75
327
87
0.74
(a) Diorite Granodiorite Beresite
Diorite + granodiorite: t = 2637 ± 130 Ma (87Sr/86Sr)0 = 0.70210 ± 93 MSWD = 66
0.73 Beresite: t = 2045 ± 250 Ma MSWD = 108 0.72
0.71
0.70 0.1
0.3
0.5
0.7
0.9
1.3 Rb/86Sr
1.1
87
(b)
87Sr/86Sr
1.0
Bt
t = 1717 ± 27 Ma = 0.71074 ± 12 MSWD = 1.1
(87Sr/86Sr)0
0.9 WR 0.728
Kfs
0.722
0.8
0.716 Pl 0.710 0.1 0.2 0.7 0.1
2
4
6
8
0.3 10
0.4
0.5 12
0.6 14 87Rb/86Sr
Fig. 2. Rb–Sr isochrons based on wholerock samples and minerals from Taloveis deposit: (a) slightly altered diorite and grano diorite (black line) and samples from exploration line across main ore zone (gray line); (b) mineral isochron of moderately ber esitized granodiorite (sample 508).
process, which was responsible for chemical alteration of minerals, i.e., the age of metasomatism. The isotopic Rb–Sr evidence makes it possible to conclude that the formation of metasomatic wall rocks and gold mineralization at the Taloveis occurrence was related to hydrothermal metasomatic alteration that accompanied Paleoproterozoic shear deformation rather than to Archean granitoid magmatism. The GEOLOGY OF ORE DEPOSITS
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same conclusion has been drawn by Vlasov and Bak sheev (2007) on the basis of studying the Sm–Nd iso topic system of ore and metasomatic minerals from this occurrence. The Sm–Nd isotopic age of ore for mation at 1.7 Ga is coeval with our Rb–Sr date. The Pedrolampi deposit is situated in the central part of the Vedlozero–Segozero belt at the northwest ern end of the Koikar–Korbozero greenstone belt in
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Table 2. Rb–Sr isotopic data on rocks and minerals from the studied deposits No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Sample 5001 119A 120 512 522 506 122 33 503 508 509 510 5052 5053 508
03T 11 03T 12 03T 13 03T 14 03T 16 03T 17 5243
25 26 27 28 29 30 31 32
6287
33 34 35 36 37 38
6181 6183 6187 6189 61810 61811
39 40 41 42 43 44 45 46 47
7645 7646 7647 7649 7642 7643 7644 7648 76410
6222
Rb, Sr, ppm ppm Taloveis deposit Diorite 82.4 628 '' 67.8 769 '' 36.3 976 '' 71.4 781 '' 41.7 927 Granodiorite 116 574 '' 125 578 '' 85. 7 499 '' 119 561 '' 129 557 '' 130 570 '' 99.2 590 Propylite 20.6 518 '' 84.5 614 Quartz vein 38.3 745 Beresite 225 1000 Sulfide–quartz vein 658 161 Beresite 94.9 488 '' 85.9 466 '' 79.5 330 Quartz vein 7.97 18.4 Beresite 85.7 408 '' 76.7 359 Sulfide–quartz vein 117 349 Pedrolampi deposit Beresite 132 52.6 Muscovite 133 40.4 Chlorite 8.40 3.95 Tourmaline 7.80 396 Beresite 75.7 88.0 Muscovite 124 67.7 Chlorite 10.1 7.40 Calcite 0.23 883 Faddeinkelja deposit Metasomatized granite 138 315 Quartz vein 70.6 6.29 Ore zone 53.7 23.9 Beresite 119 17.9 '' 237 35.6 '' 234 57.0 Hatunoja site, Jalonvaara deposit Quartz vein 1.19 1.71 '' 1.11 1.51 '' 1.65 1.81 Propylitized diorite 148 680 Propylite 334 719 Quartz vein 21.0 57.0 '' 26.1 415 Propylite 307 616 Quartz vein, an offset 11.0 21.8 Rock, mineral
87
Rb/86Sr
87
Sr/86Sr ± 2σ
(87Sr/86Sr)t
0.380 0.2551 0.1076 0.2644 0.1303 0.5834 0.6228 0.4970 0.6126 0.6724 0.6584 0.4869 0.1543 0.3972 0.1486 0.6409 1.865 0.5627 0.5338 0.6714 1.253 0.6082 0.6188 0.9695
0.716477 ± 14 0.711478 ± 41 0.705994 ± 17 0.711721 ± 40 0.707553 ± 7 0.724421 ± 9 0.725648 ± 48 0.720762 ± 30 0.725989±11 0.727345 ± 28 0.726777 ± 10 0.722146 ± 21 0.710845 ± 11 0.717758 ± 8 0.714416 ± 9 0.726464 ± 11 1.00472 ± 8 0.724327 ± 8 0.723501 ± 16 0.727370 ± 15 0.744024 ± 15 0.724433 ± 21 0.723311 ± 21 0.736094 ± 21
0.70711 0.70518 0.70333 0.70520 0.70434 0.71002 0.71028 0.70850 0.71087 0.71075 0.71053 0.71013 0.70704 0.70796 0.71075 0.71065 0.71187 0.71044 0.71033 0.71080 0.71309 0.70942 0.70804 0.71217
7.250 9.509 6.155 0.057 2.489 5.316 3.941 0.0008
0.894602 ± 27 0.945594 ± 28 0.864039 ± 21 0.711476 ± 26 0.771391 ± 23 0.840725 ± 25 0.807152 ± 24 0.709853 ± 21
0.71534 0.71048 0.71185 0.71007 0.70985 0.70928 0.70972 0.70983
0.743334 ± 21 1.52267 ± 18 0.872971 ± 19 1.18826 ± 21 1.18900 ± 22 1.00554 ± 19
0.71187 0.71642 0.71178 0.71220 0.71098 0.71090
0.762450 ± 20 0.764905 ± 21 0.777695 ± 20 0.727477 ± 20 0.750299 ± 21 0.743775 ± 20 0.721510 ± 19 0.752375 ± 18 0.752723 ± 21
0.71251 0.71221 0.71269 0.71188 0.71705 0.71742 0.71702 0.71674 0.71654
1.268 32.49 6.496 19.19 19.27 11.87 2.020 2.131 2.629 0.6308 1.345 1.066 0.1816 1.441 1.463
GEOLOGY OF ORE DEPOSITS
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ISOTOPIC GEOCHRONOLOGICAL EVIDENCE
Ar2br
Pr1jn
78
329
N
78
622/2
84
60
1
67
2 84 79 87
66 84
62
87
4
75
57 77
3
72
84
79
85
88
78
84
5 84
78 628/78
6
86 84
A B
84 70
7
55 84
78
8
77 83 84
9
71 84
79
80
A C
B
84
10
11
Turfcovered
87
629/4
12
82 77
0
3m
Fig. 3. Schematic geological map of the Pedrolampi deposit. Compiled by E.V. Sizova. (1) Paleoproterozoic metasandstone (Yangozero Formation of the Jatulian Superunit); (2) Paleoproterozoic basal conglomerate (Yangozero Formation of the Jatulian Superunit); (3) Paleoproterozoic metagravelstone with martite cement (Yangozero Formation of the Jatulian Superunit); (4) Archean metarhyodacite (Bergaul Group of Upper Lopian); (5) Archean metabasic rocks (Bergaul Group of Upper Lopian); (6) metabasic screens among Lopian felsic rocks; (7) quartz (A) and (B) quartz–carbonate veins and veinlets; (8) western ore zone; (9) contact between Paleoproterozoic metasandstone and Archean metavolcanic rocks; (10) pyrite: (A) large (>5 cm) crystals, (B) nodules, (C) pockets; (11) strike and dip symbols of schistosity; (12) sample for dating.
the contact zone of Mesoarchean sequences and Pale oproterozoic terrigenous rocks (Fig. 3). The time of accumulation of felsic volcanics marks the upper age limit of volcanic sequences. This time was established for the southern Koikar–Korbozero structure from the result of U–Pb dating of zircons: 2859 ± 15 Ma for dacite and 2876 ± 5 for rhyolite (Samsonov et al., 1997). The geological setting of the Paleoproterozoic GEOLOGY OF ORE DEPOSITS
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metasedimentary rocks indicates their Jatulian age (~2.3 Ga). Regional metamorphism related to the Neoarchean Rebolian folding proceeded under con ditions of greenschist to amphibolite facies of the andalusite–sillimanite baric type (Rybakov, 1980). The deposit is controlled by the NWtrending regional shear zone that dissects both Archean
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metavolcanics and Paleoproterozoic (Jatulian) metasedimentary rocks. The metamorphic rocks within the shear zone are completely transformed into beresites. Actinolite and biotitebearing Archean metabasalts are replaced with newly formed carbon ate, chlorite, and sericitebearing assemblages with tourmaline and pyrite. The porphyroblastic structure and massive texture of metagabbroic and metabasaltic rocks observed beyond the zone gave way to a lepi dogranoblastic structure and fissile texture. As a result, beresites occupy almost the entire area of the deposit and also occur as numerous veins, veinlets, and stringer–disseminated pockets. The orebody was con toured by a cutoff content of 1 gpt Au in the central part of the deposit and traced by boreholes for 400 m along the strike and for 350 m down the dip. The gold reserves in category С1 + С2 (indicated + inferred) have been estimated at 2.5 t and hypothetical resources in category Р1 (reserve growth) at 4.5 t and 5–6 t in category Р2 (possible resources) (Mineral’no syr’evay …, 2005). Metasomatic rocks, veins, and veinlets were formed during at least two stages controlled by the early NNWtrending and the late nearly meridional shear zones. The beresites spatially related to the NNWtrending shear zone are characterized by an assemblage of sericite and quartz with a subordinate amount of chlorite, carbonate (ankerite, calcite), and tourmaline (NNW quartz–sericite assemblage). The areal metasomatic rocks composed of calcite, quartz, pyrite, and toumaline with a small amount of sericite and chlorite make up nearmeridional quartz–car bonate assemblage. The beresites related to the second stage of shearing are characterized by the development of veinlets (up to 2 cm thick) and patches (up to 15–20 cm across) consisting of coarsegrained pyrite–calcite– quartz aggregate with a small amount of sericite and chlorite and of pyrite–chalcopyrite–chlorite–cal cite–quartz aggregate. The hydrothermal veins are subdivided into quartz, pyrite–chlorite–ankerite– calcite–tourmaline–quartz with magnetite and chal copyrite, and quartz–tourmaline types. The veins cluster mainly along the NNWtrending zones corre sponding to the first stage of shear deformation. Pyrite is the major ore mineral at the deposit. Chalcopyrite and sphalerite occur is inclusions in pyrite; pyrite– chalcopyrite intergrowths are also typical. Galena, PGM, electrum, and native gold were noted among ore minerals by Kuleshevich (2006). According to the study of fluid inclusions in quartz, the temperature of hydrothermal process ranged from 260 to 200°C. Samples 6287 and 6222 have been chosen for isoto pic dating. The first sample represents beresite of the early quartz–sericite assemblage; minerals of the late beresite quartz–carbonate assemblage dominate in the second sample. The wholerock samples and major minerals have been analyzed in both samples. The data points of muscovite, chlorite, and tour maline from sample 6287 (metasomatic rock of the
early NNW beresite after Archean felsic tuff) are approximated by an Rb–Sr isochron corresponding to t = 1732 ± 12 Ma; (87Sr/86Sr)0 = 0.71006 ± 0.00003; MSWD = 2.0 (Fig. 4a; Table 2). Addition of the wholerock point yields the same age 1736 ± 110 Ma with an increase in MSWD to 7.6. The studied rock probably contains relics of primary minerals stable at medium and low temperatures, which did not undergo disturbance of the Rb–Sr isotopic system. In regards to geochronology, the best results were produced by the study of sample 6222 from the cen tral part of the main ore zone and composed of a late nearly meridional quartz–carbonate assemblage of beresite. The data points of calcite, chlorite, musco vite, and wholerock sample are approximated by Rb– Sr isochron at 1717 ± 10 Ma; (87Sr/86Sr)0 = 0.70983 ± 0.00002; MSWD = 0.22 (Fig. 4b). Like in sample 628 7, the analyzed mineral assemblage was newly formed during hydrothermal metasomatism, but in this case, isotopic equilibrium was achieved not only between the studied minerals but also in the rock as a whole. The calculated isochron ages coincide with each other within the uncertainty limits. Such convergence of the results is good evidence for the reliability of dat ing. The obtained isotopic geochronological data, in combination with geological evidence, allow us to interpret the age of 1.72 Ga as the time of ore forma tion at the Pedrolampi deposit. The Faddeinkelja deposit is situated in the western granitegneiss framework of the Vedlozero–Segozero Belt (Fig. 1). This Aubearing copper–basemetal deposit has been known since the end of the 19th cen tury and mined for copper; it is classified now as Cu bearing type hosted in sulfidated metasomatic rocks localized in crush zones (Mineral’nosyr’evay …, 2005). The Archean granitoids and Paleoproterozoic metabasic dikes are host rocks. This allows us to sug gest that the age of metasomatism and ore formation is no older than Paleoproterozoic. As follows from a description of the Faddeinkelja deposit, four ore zones with bulges and pinches were en echelon arranged at different depths. To date, the orebodies have been mined out almost completely. Copper and base metals remain the main ore component; their hypothetical resources in categories Р1 + Р2 are estimated at 0.352 kt. The gold resources have not been estimated (Min eral’nosyr’evay …, 2005). The quartz and carbonate–barite–quartz veins with sulfide mineralization are accompanied by advanced beresitization (quartz + sericite + carbonate + sulfides) replacing Archean granitoids. The Rb–Sr isotopic study was carried out for a sample of intensely beresitized granite, two samples of Aubearing quartz veins, and three beresite samples (see Fig. 5 for sample location). The data points of all samples listed above make up an isochron with an age of 1726 ± 9 Ma; (87Sr/86Sr)0 = 0.71185 ± 0.00037; MSWD = 0.5 (Fig. 6; Table 2). GEOLOGY OF ORE DEPOSITS
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ISOTOPIC GEOCHRONOLOGICAL EVIDENCE Sr/86Sr 1.00
331
87
(a) Muscovite
0.95
Minerals and wholerock sample: t = 1736 ± 110 Ma (87Sr/86Sr)0 = 0.711 ± 0.011 MSWD = 7.6
0.90
Beresite 6287 Chlorite
0.85 Only minerals: t = 1732 ± 12 Ma (87Sr/86Sr)0 = 0.71006 ± 0.00003 MSWD = 2.0
0.80
0.75 Tourmaline
0.70 0
1
2
3
4
5
6
7
8
10
9
87Rb/86Sr 87Sr/86Sr
(b)
Muscovite
0.84 0.82
Chlorite
0.80
t = 1717 ± 10 Ma (87Sr/86Sr)0 = 0.70983 ± 0.00002 MSWD = 0.22
0.78 Beresite 6222
0.76 0.74 0.72 Calcite 0.70 0
1
2
3
4
5
6 87Rb/86Sr
Fig. 4. Rb–Sr mineral isochrons of metasomatic rocks from the Pedrolampi deposit: (a) mineral isochron, sample 6287 of meta somatic rock after Archean felsic metavolcanic rock; (b) mineral isochron, sample 6222 of metasomatic rock taken from replace ment vein after Archean protolith.
The hydrothermal process at the Faddeinkelja deposit gave rise not only to significant structural and compositional transformations (newly formed mineral assemblage and lepidogranoblastic structure) with tenfold depletion in Sr as compared with initial potas sic granite, but also to a shift in isotopic composition. GEOLOGY OF ORE DEPOSITS
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As a result, the Rb–Sr system reached a new isotopic equilibrium in the wholerock samples taken at a sig nificant distance from one another. The Jalonvaara occurrence is located in the south ern part of the inner Ilomansi–Jalonvaara greenstone belt (Fig. 1). The Archean Pampalo gold deposit—
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LARIONOVA et al. Drowned part of open pit
Turfcovered
61811 С
6188 61810
6187 6181 6186
6189
618
6183 6185
6184
5
6
~2 м 1
2
3
4
7
6185 8
Fig. 5. Open pit at the Faddeinkelja deposit with indicated localities of sampling. (1) Potassic granite; (2) intensely beresitized granite; (3) beresite; (4) quartz; (5) beresite with malachite; (6) broken selvage of the main vein with malachite; (7) crush zone; (8) point of sampling.
one of the largest gold deposits of this age not only in Karelia but also in the entire Baltic Shield—is local ized in this belt. The Jalonvaara occurrence is composed of Archean and Paleoproterozoic rock complexes. The Archean complex (~2.74 Ga) consists of intensely deformed metavolcanic and metasedimentary rocks, which are cut through by the large posttectonic mul tiphase Jalonvaara pluton of magnesian granitoids pertaining to the sanukitoid series (Ivashchenko et al., 2005). The Paleoproterozoic complexes comprise metaconglomerate and numerous mafic dikes cutting through Archean metavolcanics and granitoids. The porphyry Mo–Cu–Au–W mineralization is related to the late intrusive phases of the Jalonvaara granitoid pluton. The highfineness native gold up to 0.2–0.3 mm in grain size is associated with native bis muth, bismuthinite, and Bi sulfotellurides (Ivash chenko et al., 2007). During later shear dislocations oriented in the nearly meridional direction, quartz– arsenopyrite veins with gold were superposed on the stockwork of older quartz–pyrite–chalcopyrite– molybdenite veins with scheelite. The stockwork of quartz veins with gold mineralization is hosted in dior
ite in the contact zone of the Jalonvaara pluton. These are veins of complex morphology with bulges and pinches. Thin (
