Physicochemical modeling of mineral formation processes at the Badran gold deposit (Yakutia)

Russian Geology and Geophysics 52 (2011) 290–306 www.elsevier.com/locate/rgg

Physicochemical modeling of mineral formation processes at the Badran gold deposit (Yakutia) A.A. Obolensky a, L.V. Gushchina a,*, G.S. Anisimova b, E.S. Serkebaeva b, A.A. Tomilenko a, N.A. Gibsher a a

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b Institute of Diamond and Noble-Metal Geology, Siberian Branch of the Russian Academy of Sciences, pr. Lenina 39, Yakutsk, 677980, Russia Received 6 July 2009; 25 December 2009

Abstract The physicochemical modeling of mineral formation processes at the Badran subthrust gold-quartz deposit was performed, based on a study of fluid inclusions in quartz by Raman spectroscopy, gas chromatography, thermometry, and freezing. The results show that at stage I, highly productive gold-bearing quartz veins (gray quartz) of the deposit formed from heterogeneous fluid at <320 ºC and 2.0–0.1 kbar with the active participation of CO2, N2, and CH4; the salinity of this solution reached 10 wt.% NaCl-equiv. At stage II (Au-productive), milky-white quartz was produced from the homogeneous medium-chloride-sulfide solution which remained after the heterogenization of the initial fluid, at 300–100 ºC and 0.1 kbar. At stage III (with low Au production), clear quartz formed from homogeneous chloride solutions with salinity of <4.5 wt.% NaCl-equiv. at <200 ºC and <0.1 kbar. The physicochemical conditions of Au concentration within the complex geochemical system Au–Fe–Cu–Pb–Zn–As–Sb–Hg–Ag–H2O–Cl–H2S–CO2 at the Badran deposit was modeled using the Chiller software. The following models were used: (1) solution–rock interaction and (2) condensation of gas phase (for stage I); (3) simple cooling of medium-chloride-sulfide solution (for stage II); (4) simple cooling and (5) mixing of low-chloride-sulfide solution with acid meteoric waters (for stage III). The models show the sequence of vein formation in the ore-producing system and the host-rock metasomatism in the deep horizons of the deposit. © 2011, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: gold deposit; fluid inclusion; composition of hydrothermal solution; thermodynamic model

Introduction The Badran gold deposit is localized in the Indigirka River basin, 134 km west of Ust’-Nera Village. It was discovered in 1961, comprehensively explored in the 1980s, and is still successfully exploited, being the largest ore object in the upper-Indigirka industrial-mining region. The geologic structure, mineral composition of ores, and localization of the deposit are described elsewhere (Amuzinskii, 2005; Nokleberg et al., 2005; Parfenov and Kuz’min, 2001). The Badran deposit occurs in the Adycha–Nera metallogenic zone covering the central and southeastern parts of the Kular–Nera schist belt and the margin of the Upper Yana fold-thrust belt bordering the Adycha–Taryn boundary fault (Fig. 1). This zone is formed by the Permian and Triassic deep-water marine blackshale strata of the Kular–Nera trough and Upper Triassic and Lower Jurassic shelf deposits of the

* Corresponding author. E-mail address: [email protected] (L.V. Gushchina)

Upper Yana fold-thrust belt. Farther northeastward, there is the Chybagalakh metallogenic zone spatially coinciding with the Major batholith belt within the Polousnyi–Debin terrane of an accretionary wedge composed mainly of turbidites. The formation of the Major batholith belt determined the metallogenic composition of the zone including rare-metal (Sn-Wquartz, Sn-quartz-vein, B-Sn-quartz) and Au-rare-metal deposits (Parfenov and Kuz’min, 2001). In contrast to the Chybagalakh zone, the Adycha–Nera zone abounds in metamorphogenic-hydrothermal Au-quartz low-sulfide deposits localized in thrust structures, which permits them to be considered typical orogenic deposits according to the concept of Groves et al. (1998). The formation of ore-controlling and ore-hosting thrust structures in the Adycha–Nera zone is related to the collision of the Kolyma–Omolon superterrane with the Upper Yana passive continental margin of the Siberian craton in the Late Jurassic–Early Cretaceous. The formation of the fold-thrust belt was accompanied by the regional metamorphism of greenschist facies. The collisional dynamometamorphism manifested locally along the thrust

1068-7971/$ - see front matter D 201 1, V . S. S obolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2011.002.003

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291

Fig. 1. A, Adycha–Nera and Chybagalakh metallogenic zones. B, Geologic section of the Upper Yana–Kolyma orogenic area, reflecting the structure of the fold-thrust belt (Parfenov and Kuz’min, 2001). 1, boundary of the Adycha–Nera metallogenic zone; 2, thrusts (AT, Adycha–Taryn, ChI, Charky–Indigirka); Au–quartz (3), Au–rare-metal–quartz (4), and Au–Sb (5) ore occurrences and deposits: 1, Burgavliiskoe, 2, Kere-Yuryakhskoe, 3, Chuguluk, 4, Nenneli, 5, Khaptagai-Khaya, 6, Sokhatinskoe, 7, Venera, 8, Badran, 9, Yakutskoe, 10, Kellyam, 11, Kokarin, 12, Talalakh, 13, Imtachan, 14, Khangalas, 15, Ergelyakh, 16, Dora-Pil’, 17, Sentachan, 18, Sarylakh, 19, Kinyas’-Yuryakh, 20, Maltan, 21, Tan; 6, Precambrian crystalline basement; 7, Carboniferous and Permian clastic deposits; 8, Mesozoic clastic deposits (UYa, Upper Yana fold-thrust belt); 9, island-arc terrane (A, Alazei); 10, terrane of accretionary A-type wedge, composed mainly of turbidites (PD, Polousnyi–Debin); 11, terrane of accretionary B-type wedge, composed mainly of oceanic rocks (K, Kenkel’dy); 12, ophiolite terrane (MN, Munilkan); 13, miogeoclinal terrane of passive continental margin (OM, Omulevka); 14, turbidite terrane of the base of continental margin (KN, Kular–Nera); 15, turbidite terrane (ArT, Arga–Tas); 16, Late Jurassic–Cenozoic postamalgmation and postaccretionary deposits (ZB, Zyryanka basin); 17, thrust.

structures was responsible for metamorphogenic-hydrothermal ore formation with mobilization and partial redeposition of gold from the carbonaceous strata of the Upper Yana complex, which are enriched in gold present in carbonaceous-substance kerogen (Au = 0.24–0.33 ppm) and marcasite (Au = 2 ppm) (Amuzinskii, 2005; Kokin, 1985; Konstantinov and Kosovets, 1996; Tatarinov and Yalovik, 2006). The rocks of the Upper Yana complex can be considered a possible source of gold during the ore formation.

The geologic structure of deposit The Badran gold deposit is a reference model type of subthrust Au-quartz low-sulfide orogenic deposits that has recently formed in Yakutia (Amuzinskii, 2005; Parfenov and Kuz’min, 2001). It is confined to the Badran–Egelyakh reverse

fault of en-echelon structure more than 20 km in length, dipping northeastward at 10 to 45º. The thickness of the dynamometamorphism zone varies from 0.6 to 16.0 m; the contacts are distinct and are accompanied by gouge. The zone is of different morphology along the dip and strike and forms flexures. The amplitude of the horizontal block displacement is 600–1300 m (Neustroev, 2003). The gold-bearing site of the Badran–Egelyakh reverse fault is recognized as the Nadvigovaya (Thrust) mineralized zone. It is traceable for 5–6 km along the surface and to depths of 360 m (by underground mining works) and 1200 m (by boreholes). According to Fridovskii (1999), the lying flank of the thrust is composed of Norian clastic rocks, and the hanging flank mainly of Carnian deposits. The relationship between the deposit and igneous rocks has not been established because the nearest granitoid massif occurs 30 km southeast of the deposit. Plate-like quartz veins and veinlets were discovered

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in a mineralized crush zone in the reverse-fault plane and are traceable for 800 m along the dip and for 6 km along the strike. Quartz veins in the fault zone form separated rod-like gold orebodies up to 4.2 m wide in swells and up to 200 m in length. The veins are accompanied by thin quartz veinlets, whose number drastically increases at the wedging-out sites. The massive and coarse-banded vein structures are changed by veinlet and breccia ones. In addition to vein and veinlet orebodies, there is disseminated mineralization—mineralized boudins and tectonites. The maximum Au concentrations are revealed in massive quartz veins (Fridovskii, 1999). Massive quartz ores contain >100 ppm Au and also brecciform and banded ones, 10–25 ppm Au. The lowest Au contents (1 ppm) were found in metasomatic disseminated ores containing no more than 1–3% sulfides, whereas in rich ores they amount to ~10% (Anisimova et al., 1998). In general, gold mineralization is unevenly distributed within the Nadvigovaya zone. Four ore columns have been recognized and studied there. According to Neustroev (2003), column I is the Au-richest. The enriched sites are confined to its axial part with thick quartz veins. In the cross section of the orebody, the gold content in quartz veins is higher by one or two orders of magnitude as compared with mineralized tectonites. The position of ore column II is controlled by a thick site of the mineralized zone. It is composed of mineralized silicified mylonites and mylonitized rocks hosting quartz veins. Ore column III is localized in the well-expressed flexure of the axial plane of the Nadvigovaya zone. Within the orebody, this zone is of great thickness and abounds in short thick quartz veins bearing gold mineralization. The veins border weakly mineralized silicified mylonites and mylonitized rocks. Ore column IV is predicted to stretch along a quartz vein stripped in the Bezvodnyi Brook valley. The thickness of the mineralized zone is 6.0–7.0 m. The sites between the ore columns are composed of barren tectonites and mylonites as well as mylonitized and foliated sedimentary rocks with veinlet silicification and, sometimes, thin goldbearing quartz veins (Neustroev, 2003).

The mineral composition of ores and mineralization stages The mineral formation in the Badran deposit proceeded in three stages. Six parageneses including about 50 minerals were produced. At stage I, an assemblage of metasomatic high-Au pyrite and arsenopyrite was generated in sedimentary wallrocks as well as high-sulfide arsenopyrite–carbonate–quartz paragenesis in veins. At stage II, the most productive mediumsulfide parageneses formed: chalcopyrite–galena–albite–dolomite–quartz and sphalerite–tetrahedrite–sericite–quartz. Late nonproductive silver–quartz and low-sulfide stibnite–quartz parageneses were produced at stage III. Hypogene ores are the sulfide-richest in the ore columns. The mineral composition of ores was considered in detail in literature (Anisimova, 1983, 1993, 2008; Anisimova et al., 1998, 2006). Note that hypergene ores are traceable along the dip of orebodies to a

depth of 310 m. In the oxidation zone, sulfides and veined carbonates are completely oxidized, and visible native gold is present. The primary ores are dominated by finely dispersed gold occurring mainly in early pyrite and arsenopyrite (Anisimova, 2008; Anisimova et al., 1998, 2006). At stage I, two mutually related mineral assemblages formed: dissemination of high-Au pyrite and arsenopyrite in ore-hosting beresites and carbonate–quartz dissemination in quartz-vein zones. Pyrite and arsenopyrite are typomorphic minerals of the early assemblage. They are in paragenesis with quartz, chlorite, and sericite. This assemblage is the earliest and is ubiquitous in the mineralized zone. The arsenopyrite– pyrite–carbonate–quartz assemblage in veins is the most abundant among all others formed at this stage and is also ubiquitous. Gold occurs in it as euhedral crystals and cloddy clusters. At stage II, two spatially conjugate mineral assemblages formed: chalcopyrite–galena–albite–carbonate and sphalerite– tetrahedrite–sericite–quartz. Galena–chalcopyrite paragenesis at the upper horizons of the deposit occurs mainly on the flanks of the Nadvigovaya zone. It was also revealed at deeper horizons in the central zones of the ore columns. Its typomorphic minerals are chalcopyrite, galena, gold, dolomite, and albite. Native gold occurs mainly as cloddy or, less often, interstitial aggregates in microdruse cavities. Sphalerite–tetrahedrite–sericite–carbonate paragenesis with inclusions of rare sulfostibnites is localized in the central, ore-richest part of the Nadvigovaya zone. The localization of these minerals is usually determined by the position of ore columns composed of mylonitized powdered quartz. The typomorphic minerals of this part of the zone are sphalerite, tetrahedrite, gold, and sericite. Native gold is present as interstitial, lenticular, and cloddy clusters in microdruse cavities. At stage III, cinnabar–stibnite–carbonate–quartz and silver– quartz parageneses formed, which are not widespread. Stibnite veinlets with clear quartz cross the earlier deposited ores. Cinnabar grains were discovered only at the subsurface horizons of the ore columns, and dickite replaces hydromicas of beresitized rocks. Native gold was not found in this paragenesis, whereas silver paragenesis was deposited at this stage. Its typomorphic minerals are freibergite, argentite, high-Ag fahlore, and low-grade Hg-containing gold. This paragenesis occurs locally in the deposit ore columns. Distinct relationships between the mineral assemblages have been established in particular objects; therefore, the paragenetic scheme shows only the general sequence of ore formation (Fig. 2). The most gold-productive are early pyrite– arsenopyrite–quartz (with finely dispersed gold), galena–chalcopyrite–albite–dolomite–quartz, and sphalerite–tetrahedrite– sericite–quartz parageneses, while the orebodies composed of them are the gold-richest. The Badran deposit abounds in zones of hypergene alteration of primary ores, which bear two main mineral assemblages: sulfur-sulfate and oxide. The former assemblage is widespread in the central part of the ore field along the Nadvigovaya zone. Its typomorphic minerals are gypsum, jarosite, covellite, and chalcosine. The drilling results show

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Fig. 2. Paragenetic scheme of the sequence of formation of main minerals at the Badran deposit, after Anisimova et al. (1998).

that these minerals are ubiquitous in the zone ores to a depth of 240 m. Gypsum is also widespread in the rocks of the hanging flank of the zone, where it usually occurs as fine veinlets and disseminations in clastics cement. Jarosite forms separate fine-grained aggregates. This paragenesis includes scorodite (replacing arsenopyrite) and redeposited high-fineness (997) native gold of spongy texture. Oxide paragenesis is more intensely developed. It includes goethite, hydrogoethite, kaolinite, malachite, azurite, and cervantite. The typomorphic minerals are usually produced as a result of the replacement of pyrite, arsenopyrite, chalcopyrite, dolomite, and mica minerals. Sometimes, pseudomorphs of goethite, hydrogoethite, and lepidocrockite develop after pyrite. Malachite and azurite replace fahlore and chalcopyrite, and cervantite develops after stibnite. Kaolinite usually forms veinlets and veinlet-like irregular-shaped segregations in the

host rocks, predominantly in carbonate, which it usually replaces. This paragenesis probably includes polianite, which was found in veins of W–E strike on the right bank of the upper reaches of the Badran Brook. Polianite occurs in chlorite–carbonate–quartz veinlets and thin veins. Quartz is present as large-columnar aggregate, dirty-gray because of the polianite impurity. Calcite in quartz forms white or creamy nests up to 10 cm in size. The hypergene minerals of the deeper horizons are iron hydroxides and oxides and gypsum.

The temperature and fluid regimes of formation of the Badran deposit Quartz is the main vein mineral in the Badran deposit (70–90% of the vein mass). We studied fluid inclusions from

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quartz from three ore columns. Twenty-two quartz samples were used to prepare 44 plates 0.3–0.5 mm in thickness, polished from both sides. Microscopic study of the plates in transmitted light showed that the quartz is of three generations: gray (I), milky-white (II), and clear (III). Gray quartz (I) usually composes the core of zonal quartz grains and is, most likely, an early mineral. It is saturated with fluid inclusions and solid particles of black substance, which determine its gray color. The solid particles are of different sizes and shapes. The gray quartz also bears fluid inclusions, either isolated or in groups, localized beyond the cracks. These are primary inclusions of negative-crystal or irregular shape or rounded, measuring 5 to 15 μm. In two samples (8 and 10), the fluid inclusions are not larger than 3 μm; therefore, we failed to carry out their thermometric studies. The secondary inclusions are localized along the healed cracks cutting the boundaries of quartz grains; they are rounded and are seldom larger than 3–5 μm. The difference between gray (I) and milky-white (II) quartz is seen only under a microscope. The fluid inclusions in gray quartz at room temperature and at <0 ºC are of three types: (1) CO2–H2O, with a varying phase proportion, (2) essentially gas (gas phase is predominant, and water is present as rims around the vacuole walls), and (3) essentially aqueous (the water phase dominates over the gas one). These types of fluid inclusions can be present both together and separately in the growth zones or in the grain cracks, which evidences their formation from a two-phase fluid. Milky-white quartz (II) occurs either between the grains of gray quartz (I) or in the peripheral growth zones of large quartz grains with the core composed of gray quartz. It seldom contains black solid particles. In the essentially aqueous fluid inclusions, the liquid phase dominates over the gas one. The liquid : gas proportion is nearly constant. Clear quartz (III) in paragenesis with carbonates occurs as fine veinlets cutting both gray and milky-white quartz. The veinlet minerals bear essentially aqueous inclusions (the aqueous phase significantly dominates over the gas one). We failed to carry out a thermometric study of fluid inclusions in quartz II because of their ultrasmall size. Methods of study of fluid inclusions. Microthermometric studies of fluid inclusions were performed on a thermal microstage designed by Osorgin and Tomilenko (1990). The accuracy of thermometric measurements was controlled by the regular calibration of the thermal and freezing stages, using standard fluid inclusions and synthetic substances. Temperatures in the range from 31 to 150 ºC were measured with an accuracy of ± 0.1 ºC, and in the range from 100 to 700 ºC, with an accuracy of ± 5 ºC. The concentration and salt composition of the aqueous phase of the inclusion were determined by the freezing method (Borisenko, 1977; Roedder, 1984). Raman spectroscopic microanalysis of the gas phase of fluid inclusions was carried out on a RAMANOR U-1000 (Jobin Yvon) unichannel Raman spectrometer with Ar laser at the Sobolev Institute of Geology and Mineralogy, Novosi-

birsk, following the technique proposed by Dubessy et al. (1989). The pressure of mineral-forming fluids was determined from the diagrams constructed using the program for the calculation of the critical parameters and trapping isochores of fluid inclusions in the system CO2–CH4–N2 in minerals (Sverdlova et al., 1999). With results of a Raman spectroscopic analysis of inclusions and the temperature of partial homogenization in the system CO2–CH4–N2, the program permits the determination of the density of mineral-forming fluid. This parameter, in turn, is used to estimate the crystallization temperatures of minerals by the homogenization method and the pressures in the inclusions (Roedder and Bodnar, 1980). The isochores for CO2 inclusions were determined from the temperature of their partial homogenization (Vargaftik, 1972). The bulk composition of volatiles in the inclusions was determined by gas chromatography, using a 300 mg quartz sample. Gases were released by decrepitation on heating the quartz fraction (0.5–0.25 mm) to 600 ºC. Pure quartz grains without visible inclusions of ore-free and ore minerals were sampled under a binocular microscope. The samples were treated with weak HCl and HNO3 and then were thoroughly washed with distilled water and dried at 105 ºC. The used gas chromatograph and analytical technique are described by Osorgin (1990). The temperature and pressure of mineral-forming fluid. The temperature of mineral formation of quartz veins in the Badran deposit was estimated from the parameters of fluid inclusion homogenization (Fig. 3). We have established that the temperature of fluids forming milky-white quartz (II) was not higher than 180–200 ºC. Most of the measured values fall into the narrow range 120–140 ºC. All inclusions homogenized into a liquid phase only (Fig. 3). Gray quartz (I) is characterized by higher homogenization temperatures of fluid inclusions, 180–320 ºC. The inclusions homogenized both into liquid and gas phases in the temperature range from 280 to 320 ºC. This fact evidences that quartz I crystallized from a heterogeneous fluid. The density of fluids determined from the homogenization temperature of CO2 in the inclusions varied from 0.064 to 0.96 g/cm3 (homogenization into gas and liquid phases, respectively). The temperatures of formation of gray quartz in ore columns I and III varied from 180 to 320 ºC, and the fluid pressure varied from 0.1 to 2.0 kbar. The freezing and Raman spectroscopic data show no presence of CO2 in the fluid inclusions in quartz II. The salinity and composition of mineral-forming fluids. According to the results of freezing studies, the compositions and concentrations of salts in the fluid inclusions in the gray and milky-white quartz are different. The salinity of fluids reached 10 wt.% NaCl-equiv. in the gray quartz and was not higher than 4.5 wt.% NaCl-equiv. in the milky-white quartz. The main salt components of fluids were Na and Mg chlorides in the gray quartz and K and Na chlorides in the milky-white quartz. The gas composition of fluids. The gas composition of fluid inclusions was studied by three methods: Raman spec-

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Fig. 3. Histograms of homogenization temperatures of fluid inclusions from milky-white (a, c) and gray (b, d) quartz from ore columns I and III of the Badran deposit. N, Number of determinations. Homogenization into liquid (1) and gas (2).

troscopy and freezing (fluid inclusions in quartz veins) and gas chromatography (bulk composition of volatiles from fluid inclusions in quartz veins, silicified mylonites, and mylonites of the hanging and lying flanks) (Table 1). We have established that the essentially gas inclusions are filled with a low-density composite fluid (CO2, CH4, N2). As the temperature of the freezing stage decreases to –180 ºC, a liquid phase is condensed in the inclusions and then homogenizes into a gas phase in the temperature range from –147.0 to –153.4 ºC. These temperatures are close to the critical temperature of nitrogen, –146.9 ºC (Vargaftik, 1972). The solid phase of these inclusions melts in the temperature range from –57.9 to –63.4 ºC, which evidences that this is CO2 with CH4 impurity. Methane, whose critical temperature is −82.6 ºC (Vargaftik, 1972), can significantly decrease the melting point of pure carbon dioxide (–56.6 ºC) (Vukalovich and Altunin, 1965). As the temperature decreases below +20 ºC, liquid CO2 appears in the gas phase of CO2–H2O inclusions, which melts at –56.9 to –62.1 ºC and homogenizes into gas at +10.0 to +18.7 ºC. Homogenization of gas CO2 into a liquid proceeds

at –3.0 to –3.8 ºC. The melting point of solid CO2 is below –56.6 ºC, which indicates the presence of methane and nitrogen decreasing the melting point of pure carbon dioxide. According to the Raman spectroscopy data, the fluid inclusions also contain N2 and CH4. As shown by the bulk analysis, the fluid volatiles are H2O, CO2, N2, and CH4 present in different contents and proportions (Figs. 4–6). The total fluid content in the quartz from mylonites of the lying and hanging flanks was not higher than 1200 ppm in ore columns I and III. In the silicified mylonites of ore column III it reached 2000 ppm, and in the quartz veins of the same column, 2600 ppm. High CO2 contents, up to 700 ppm (sample 271) and 200 ppm (ore lump 19), were established in the fluids from the quartz veins of ore columns I and III, respectively (Table 1). High N2 contents (up to 20 ppm) were detected in the fluids from the quartz veins of ore column IV in the west of the ore field of the Badran deposit, whereas the fluids from the quartz veins of the Vostochnyi site bear only nitrogen traces (Fig. 6,

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Table 1. Results of chromatographic analysis of volatiles from quartz, silicified mylonites, and mylonites from the hanging and lying flanks of the Badran gold deposit (Yakutia) Sample

Content, ppm CO2

Fluid saturation, ppm H2O

N2

CH4

Ore column I, quartz veins 4

40

620

9

3

670

50

140

760

0

2

900

51

220

610

6

4

840

168

50

530

6

2

588

170

50

500

6

2

558

173

60

730

5

2

797

175

70

630

6

2

708

178

70

670

4

4

748

180

60

3

3

0

666

186

140

800

10

4

954

263

60

720

5

2

787

265

70

330

4

3

407

267

70

780

12

3

865

269

120

600

14

3

737

270

180

590

0

3

773

271

700

740

0

4

1444

272

370

520

6

4

900

274

300

420

3

3

726

275

150

290

2

4

446

276

70

520

4

3

557

278

70

860

0

3

933

281

100

950

7

2

1059

1 (ore lump)

40

780

4

1

825

2 (ore lump)

30

1000

4

1

1035

3 (ore lump)

40

740

4

2

786

4 (ore lump)

20

500

3

1

524

5 (ore lump)

120

1100

9

3

1232

6 (ore lump)

40

1200

8

9

1257

7 (ore lump)

40

1100

6

1

1147

8 (ore lump)

20

540

5

1

566

9 (ore lump)

20

1600

4

1

1625

10 (ore lump)

20

1200

3

1

1224

11 (ore lump)

20

1300

8

1

1329

12 (ore lump)

20

710

5

8

742

Ore column I, silicified mylonites 16

180

610

4

3

797

17

220

1200

4

4

1428

18

150

580

0

4

734

19

80

620

0

3

703

20

230

590

0

3

823

49

540

680

0

6

1226

174

370

340

5

5

720

273

110

440

5

3

558 (continued on next page)

297

A.A. Obolensky et al. / Russian Geology and Geophysics 52 (2011) 290–306 Table 1 (continued) Sample

Content, ppm

Fluid saturation, ppm

CO2

H2O

N2

CH4

279

190

550

9

3

752

280

370

450

5

4

829

Ore column I, mylonites of hanging flank 5

90

670

10

2

772

171

270

500

7

6

783

172

180

400

5

3

588

177

360

830

8

6

1204

264

80

300

0

2

382

266

120

380

7

2

509

277

210

700

5

4

919

Ore column I, mylonites of lying flank 169

180

310

3

5

498

176

120

580

5

3

708

179

70

440

6

4

520

268

110

560

6

3

679

Ore column III, quartz veins 6

70

680

5

2

757

10

100

870

8

2

980

14

50

740

5

2

800

15

80

890

8

5

983

21

70

610

4

2

686

23

70

420

4

3

497

24

70

690

7

2

769

25

50

690

5

2

747

26

80

1000

5

3

1088

28

50

760

7

3

820

29

60

2500

5

4

2569

30

90

820

6

3

919

31

70

800

4

2

876

32

80

1200

7

2

1289

33

70

1100

7

2

1179

34

70

1000

8

2

1080

35

50

670

4

2

726

36

90

1100

9

3

1202

37

40

430

4

2

476

42

80

930

6

2

1018

119

80

740

5

2

827

121

80

800

4

2

886

123

90

740

6

2

838

127

90

920

6

2

1018

128

50

610

4

1

665

129

80

550

4

2

636

132

60

580

3

1

644

133

40

580

0

1

621 (continued on next page)

298

A.A. Obolensky et al. / Russian Geology and Geophysics 52 (2011) 290–306

Table 1 (continued) Sample

Content, ppm

Fluid saturation, ppm

CO2

H2O

N2

CH4

134

80

580

3

2

665

137

50

740

3

1

794

143

60

430

6

2

498

144

70

330

3

2

405

146

50

320

4

1

375

147

90

260

6

2

358

148

150

600

6

3

759

149

60

250

5

1

316

150

90

650

7

2

749

151

110

620

7

3

740

152

110

930

9

3

1052

153

110

1200

10

3

1323

154

160

940

10

4

1114

155

60

670

7

2

739

156

50

490

7

2

549

157

100

1000

9

3

1112

158

50

520

4

2

576

159

90

800

8

2

900

160

110

820

8

3

941

162

120

870

10

3

1003

163

70

420

3

2

495

164

110

750

7

3

870

165

160

880

10

3

1053

166

130

1000

9

4

1143

167

20

150

0

1

171

182

90

1100

10

3

1203

183

80

400

4

2

486

259

120

480

6

4

610

260

70

650

7

3

730

261

110

400

8

3

521

262

230

350

9

3

592

13 (ore lump)

40

880

4

1

925

14 (ore lump)

70

1300

10

2

1382

15 (ore lump)

190

1200

10

3

1403

16 (ore lump)

150

1300

10

7

1467

17 (ore lump)

150

1200

20

6

1376

19 (ore lump)

200

840

7

6

1053

20 (ore lump)

30

1230

3

1

1264

21 (ore lump)

60

1200

10

1

1271

22 (ore lump)

100

1800

20

3

1923

Ore column III, silicified mylonites 27

70

450

0

2

522

43

90

1800

0

8

1898

122

80

820

5

2

907

135

70

770

3

1

844 (continued on next page)

299

A.A. Obolensky et al. / Russian Geology and Geophysics 52 (2011) 290–306 Table 1 (continued) Sample

Content, ppm

Fluid saturation, ppm

CO2

H2O

N2

CH4

136

70

1100

4

2

1176

138

80

500

7

2

589

139

330

1000

20

6

1356

140

70

1000

5

2

1077

141

60

520

4

2

586

142

70

670

6

2

748

181

210

570

4

6

790

256

240

880

9

6

1135

258

340

720

9

7

1076

Ore column III, mylonites of hanging flank 7

70

540

5

20

635

11

160

1000

6

6

1172

12

70

790

5

8

873

13

270

630

6

6

912

120

90

910

9

3

1012

124

80

1000

6

3

1089

131

90

640

7

3

740

490

0

2

522

Ore column III, mylonites of lying flank 8

30

9

50

560

0

2

612

118

140

520

4

3

667

125

90

650

4

3

747

126

130

770

5

3

908

130

70

630

3

1

704

161

150

1000

10

4

1164

257

110

250

6

4

370

Ore column IV, quartz veins 1

80

540

20

4

644

2

80

400

20

4

504

3

120

490

10

4

624

Ore column IV, silicified mylonites 44

690

600

5

5

1300

45

160

620

7

3

790

46

340

670

3

8

1021

47

160

1400

6

5

1571

Vostochnyi site, quartz veins 38

50

1900

0

3

1953

39

50

2300

0

4

2354

40

40

1200

0

4

1244

580

0

6

976

Vostochnyi site, silicified mylonites 41

390

300

A.A. Obolensky et al. / Russian Geology and Geophysics 52 (2011) 290–306

Fig. 4. Contents of CO2, N2, and CH4 in fluids from quartz veins (a) and silicified mylonites (b) from ore columns I and III of the Badran deposit (gas chromatography data). 1, 2, ore columns I and III, respectively. The fields of the ore columns are outlined.

Table 1). Thus, the N2 content in the fluids increases in the transition from east to west of the Nadvigovaya zone. In general, the nitrogen content in the fluids from the quartz veins of ore columns I and III is higher as compared with the fluids from the mylonites of the same ore columns, whereas the methane content is lower. Thus, the microthermometric studies of fluid inclusions in the quartz showed that ore columns I and III of the Badran quartz–gold deposit formed with the participation of three types of fluids: (1) high-temperature, of CO2–H2O composi-

tion, with CH4, N2, and Na and Mg chlorides (salt content reaches 10 wt.% NaCl-equiv.); (2) essentially gas, of CO2 + CH4 + N2 + H2O composition; and (3) low-temperature, essentially aqueous, with Na and K chlorides (salt content is <4.5 wt.% NaCl-equiv.). The first two fluids formed, most likely, during the heterogenization of the initial fluid. The presence of CO2, CH4, and N2 in noticeable contents (up to 700, 9, and 20 ppm, respectively) in the fluids from the Badran deposit quartz points to the intense interaction of the mineral-forming fluid with the host rocks containing up

A.A. Obolensky et al. / Russian Geology and Geophysics 52 (2011) 290–306

301

Fig. 5. Contents of CO2, N2, and CH4 in fluids from quartz of mylonites from the hanging (a) and lying (b) flanks of ore columns I and III of the Badran deposit (gas chromatography data). Designations follow Fig. 4.

to 3–4% carbonaceous organic matter (mainly carbonates) (Anisimova et al., 1998). Molecular nitrogen found in the fluids might have been produced during the decomposition of ammonium-containing silicates of the host rocks. The presence of high-temperature fluid containing CO2, CH4, and N2, which was trapped by the inclusions of quartz I, is a probable indicator of the gold presence (Tomilenko and Gibsher, 2001). The formation of the Badran deposit at the last stage proceeded from lowly concentrated essentially aqueous solutions at <200 ºC.

Thus, the performed study of fluid inclusions in quartz from ore columns I and III of the Badran gold deposit showed that: – at the early stages, the quartz veins formed from a heterogeneous fluid at <320 ºC and 2.0 to 0.1 kbar with the active participation of CO2, N2, and CH4; the salt content of the mineral-forming solution reached 10 wt.% NaCl-equiv.; – milky-white quartz (II) formed from homogeneous chloride solutions (salt content no higher than 4.5 wt.% NaClequiv.) at <200 ºC and <0.1 kbar;

302

A.A. Obolensky et al. / Russian Geology and Geophysics 52 (2011) 290–306

Fig. 6. The N2 content in fluids from quartz veins from ore columns I, III, and IV and the Vostochnyi site of the Nadvigovaya zone of the Badran deposit (gas chromatography data). 1,Vostochnyi site; 2, 3, and 4, ore columns IV, I, and III, respectively.

– the increase in N2 content in the fluids from quartz veins in the transition from east to west of the Nadvigovaya zone is, most likely, due to collisional metamorphism in the thrust zone.

from the Badran quartz–gold deposit and the results of Raman spectroscopic, gas-chromatographic, thermometric, and freezing studies of fluid inclusions in gray and milky-white quartz from ore veins. Thus, the behavior of Au(I) was modeled within the complex geochemical system Au–Fe–Cu–Pb–Zn– As–Sb–Hg–Ag–H2O–Cl–H2S–CO2. The following models were used: (1) solution–rock interaction and (2) condensation of gas phase (for stage I); (3) simple cooling of medium-chloride–sulfide solution (for stage II); (4) simple cooling and (5) mixing of low-chloride–sulfide solution with acid meteoric waters (for stage III). The models show the vein formation in the ore-producing system and the intense metasomatism of rocks at the deep horizons of the deposit. For the production of ore parageneses from fluids with medium salt contents (mCl− = 1.7) at stages I and II of the Badran deposit formation, we used a model 1 m NaCl solution containing Mg, Ca, K, Fe, Si, As, Pb, Zn, Cu, S, and Au (2 × 10–5–2 × 10–3 m) and prevailing (according to the data of study of gas-liquid inclusions) CO2 and CH4. The solution at Table 2. Equilibrium concentrations (mole/kg H2O) of chemical species in starting model solutions Component Concentration at 300 °C

Concentration at 200 °C

The genetic model of the Badran deposit The above data on the mineral composition of ores, sequence of their formation, and thermobarogeochemical parameters of mineral production permitted us to elaborate a genetic model for the formation of the Badran gold deposit. The main model elements are: – pre-ore metasomatism of the host rocks, expressed as their beresitization and the formation of pyrite–arsenopyrite paragenesis rich in invisible gold; – formation of minerals in veins and production of arsenopyrite–pyrite–carbonate–quartz paragenesis at stage I; – formation of two spatially conjugate parageneses at stage II: chalcopyrite–galena–albite–carbonate (at the upper horizons and on the flanks of the Nadvigovaya zone) and sphalerite–tetrahedrite–sericite–carbonate (in the central, orerichest part of this zone); – formation of cinnabar–stibnite–carbonate–quartz and silver–quartz parageneses at stage III, localized mainly in the subsurface zone of the ore columns. To estimate quantitatively the mineral formation processes and the physicochemical conditions of gold-ore generation at the Badran deposit and to study the general evolution of the hydrothermal ore-forming system that produced the deposit, we performed a thermodynamic modeling of some of the above elements, using the Chiller (Solveq) software (Reed, 1982, 1998) and the Soltherm-98 thermodynamic database compiled from the SUPCRT-92 database (Johnson et al., 1992) and supplemented with the thermodynamic parameters for chloride, sulfide, and mixed antimony complexes (Belevantsev et al., 1998a,b). The composition of the model thermodynamic system was determined by the mineral composition of ores

Solution I

Solution II

Solution III

pH = 3.9

pH = 5.5

pH = 6.5

HS–

1.00 × 10−2

1.00 × 10−2

1.00 × 10−2

SO2− 4

1.00 × 10−5

1.00 × 10−5

1.00 × 10−5

Na+

1.00

1.50

7.00 × 10−1

Cl

–

1.73

Au+

1.70 −5

−3

1.00 −5

−3

2.00 × 10 –2.00 × 10

2.00 × 10 –2.00 × 10

2.00 × 10−6

1.57 × 10−2

1.60 × 10−2

1.09 × 10−2

++

1.00 × 10−2

1.00 × 10−3

1.00 × 10−3

SiO2

9.24 × 10−3

9.28 × 10−3

3.45 × 10−3

H3AsO3

1.00 × 10−3

1.00 × 10−3

1.00 × 10−4

++

2.00 × 10−6

2.00 × 10−6

2.00 × 10−6

++

2.00 × 10−6

2.00 × 10−6

2.00 × 10−6

+

2.00 × 10−6

2.00 × 10−6

1.00 × 10−6

1.00 × 10−1

5.00 × 10−2

–

NH4

1.00 × 10−4

–

–

+

Fe

Pb

Zn

Cu Ca

++ +

5.00 × 10−1

–

3.00 × 10−1

+++

1.00 × 10−2

–

–

++

Mg

1.00 × 10−2

5.00 × 10−2

–

Ti(OH)4

1.00 × 10−4

–

–

Sb(OH)3

K

Al

1.00 × 10−4

1.00 × 10−4

1.00 × 10−2

++

–

–

1.00 × 10−4

+

–

–

2.00 × 10−5

Hg Ag

303

A.A. Obolensky et al. / Russian Geology and Geophysics 52 (2011) 290–306 Table 3. Chemical composition (wt.%) of mudstone from the Badran gold deposit SiO2

TiO2

Al2O3

Fe2O3

FeO

MgO

CaO

Na2O

K2O

P2O5

CO2

H2O

Σ

52.43

1.18

18.98

2.75

3.30

2.90

0.87

2.39

4.32

0.45

2.21

7.32

99.10

300 ºC was in equilibrium (Solveq software) with quartz and had the following parameters: pH = 3.9, Eh = –0.31 V (solution I) and pH = 5.5, Eh = –0.52 V (solution II; Table 2). Since hydrothermal ore-forming systems are dynamic, using equilibrium thermodynamics for calculating chemical interactions in these systems is based on the local-mosaic equilibrium (Korzhinskii, 1969). In the nonequilibrium solution–rock system, there are spatial reservoirs with constant temperature, pressure, and bulk chemical composition plus system equilibrium. Calculation of the equilibrium composition of the heterogeneous multicomponent system solution– rock–gas at each stage is the main factor of thermodynamic modeling of hydrothermal processes. For the thermodynamic modeling of the formation of early gold-bearing pyrite–arsenopyrite paragenesis by model 1 (for stage I—pre-ore metasomatism), we used a homogeneous acid medium-chloride–sulfide solution interacting with the Badran deposit mudstone (porosity coefficient of 4%) (Table 3). The composition of metasomatic column was modeled using the flow reactor model, in which a change in the system composition is expressed as a function w/r (solution/rock), i.e., the ratio of the total content of the initial solution to the total content of the reacted rock (Reed, 1998). The ordinary flow reactor model includes several successive reservoirs in which the fluid reacts with the host rock; as a result, the compositions of the fluid and deposited paragenesis change. The interaction of acid (carbon dioxide) (pH = 3.9, Eh = –0.31 V) medium-chloride–sulfide model solution (I) at 300 ºC and 2000 bars (Table 4) with the host rock (1–10 g) results in the deposition of pyrrhotite, pyrite, arsenopyrite, and quartz, with a decrease in pH of the solution to 3.5, which leads to the deposition of gold (mainly as complex Au(HS)–2) (Fig. 7) and an increase in its content in the host rock according to the reaction − 8Au(HS)−2 + 6H+aq + 4H2O = 8Au0solid + SO2− 4aq + 15H2Saq.

Condensation (model 2) during the cooling (<100 ºC) of the gas phase leads to the deposition of methane and graphite and formation of gray quartz. For stage II (formation of vein minerals), we applied model 2—simple cooling (from 300 to 100 ºC; 100 bars) of near-neutral (pH = 5.5, Eh = –0.52 V) medium-chloride–sulfide solution (II) (Table 2) remaining after the fluid heterogenization. The cooling results in the deposition of pyrite, arsenopyrite, chalcopyrite (when pH decreases to 3.4), gold, milky-white quartz, and galena. A subsequent temperature decrease to 180 ºC leads to the deposition of sphalerite, tetrahedrite, and boulangerite (Fig. 8). Gold is present in the studied solution mainly as dihydrosulfide Au(HS)–2, and its deposition in acid medium at 300 ºC follows reaction (1). For stage III (formation of cinnabar–stibnite–carbonate– quartz and silver–quartz parageneses), we used model 4—simple cooling of weakly alkaline (pH = 6.5, Eh = –0.46 V) low-chloride–sulfide (mCl– = 1.0; mHS = 0.01) solution (III) containing Au, As, Sb, Hg, and Ag (the last three components might have been introduced on the dilution of solution II) from 200 to 110 ºC at 100 bars (Table 2). As the temperature decreases, many minerals are deposited from the solution: chalcopyrite, acanthite, siderite, galena, and native gold, by the reaction − + 8Au(HS)−2 + 4H2O = 8Au0solid + SO2− 4aq + 15HSaq + 9Haq,

(2)

silver, from AgHS and AgCl−2; mercury, as a result of the solution saturation with Hg0aq; stibnite (the main Sb species

introduced into the deposition zone are sulfides SbS−2 , HSb2S−4 , and Sb2S2− 4 and hydroxo complex Sb(OH)3); bou-

– langerite, pyrite, cinnabar (from sulfides HgS2− 2 and HgSHS ), and clear quartz (III) (Fig. 9). After the simple cooling of

(1)

As the next portions of the solution pass through the rock (up to 1000 g), the pH level of the solution increases to 5.5, and Fe chlorites (clinochlore and daphnite), dolomite, chalcopyrite, and albite are deposited (Fig. 7). The solution–rock interaction leads not only to the rock metasomatism but also to the decrease in the solution pressure to 100 bars and below, up to the gas saturation of the solution (82.75 bars) as a result of the intense fracturing in the ore deposition zone and separation of the gas phase containing CO2, CH4, H2, N2, H2S, etc. (Table 4). The passage of the solution through the rock was accompanied by its cooling, gold deposition, and wallrock alteration: silicification and carbonatization (beresitization), sericitization, and sulfidization (gold-bearing pyrite– arsenopyrite paragenesis, galena, and sphalerite) (Fig. 7).

Table 4. Concentration (–log m) of gases in gas phase produced under interaction of solution I (T = 300 °C) with mudstone from the Badran deposit Component

Pressure, bars 100

82.75

H2O

0.39

+0.12

CO2

1.17

+0.31

CH4

3.58

2.64

H2

3.95

1.73

NH3

6.21

5.62

N2

4.57

4.33

H2S

3.18

2.00

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Fig. 7. The pH and temperature dependence of Au concentration in medium-chloride–sulfide solution I (see Table 4) at mCl– = 1.73 and mHS– = 0.01 (solution–rock interaction model, where w is solution (kg) and r is rock (kg), with 1 g titration step) (a); deposition of mineral phases of gold-bearing pyrite–arsenopyrite paragenesis (b).

Fig. 8. The pH and temperature dependence of Au and Sb concentrations in medium-chloride–sulfide solution II after heterogenization (see Table 4) at mCl– = 1.73 and mHS– = 0.01 (simple-cooling model) (a); deposition of mineral phases in veins (b).

solution (III) to 110 ºC (pH = 6.5), it preserves antimony, mainly as complexes SbS−2 and Sb(OH)3, and atomic mercury Hg0aq in significant concentrations. To decrease pH, we applied model 5—mixing of the solution with cold (25 ºC) acid (pH = 5) meteoric waters, which leads to a decrease in temperature and pH (to 4.8) and the deposition of stibnite and cinnabar (Fig. 9). The reactions responsible for the introduction of silver, mercury, and antimony into the zone of deposition of native Ag and Hg, stibnite, and cinnabar are given in our previous works concerned with the physicochemical models of ore formation in Ag–Sb, Hg, Sb–Hg, and Au–Sb deposits (Gush-

china and Obolenskiy, 2003; Obolensky et al., 2006, 2007, 2008, 2009; Pavlova et al., 2004).

Conclusions Study of the regularities of the localization and conditions of formation of the Badran gold–quartz low-sulfide deposit in the Upper Yana fold-thrust belt showed a distinct metallogenic zoning in the arrangement of Au–rare-metal, Au–Sb, and Au–quartz low-sulfide deposits. The latter form mainly in the Chybagalakh and Adycha–Nera zones and in the linear Adycha–Taryn zone separating them. Gold–quartz deposits of

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305

Fig. 9. The pH and temperature dependence of Au, Sb, Ag, and Hg concentrations in low-chloride–sulfide solution III (see Table 4) at mCl– = 1.70 and mHS– = 0.01 (simple-cooling and mixing models) (a); deposition of cinnabar–stibnite–carbonate–quartz and silver–quartz parageneses (b).

the Badran type are localized in the thrust structures with the local manifestation of collisional metamorphism and formation of ore-bearing metamorphogene–hydrothermal systems, probably with the participation of mantle fluids (Ridley and Diamond, 2000) ensuring the high ore potential of these systems. The performed studies of the mineral composition of ores and the wallrock alterations and the data on the composition of hydrothermal ore-bearing solutions of the Badran quartz– gold deposit suggest its formation from the complex hydrothermal ore-forming system Au–Fe–Cu–Pb–Zn–As–Sb– Hg–Ag–H2O–Cl–H2S–CO2. At stage I of the formation of gold–ore parageneses (early highly productive), a homogeneous acid (carbon dioxide) medium-chloride–sulfide solution interacted with the host rocks. This led not only to their metasomatism but also to their cooling, gold deposition, pressure decrease, and heterogenization of the solution with the separation of gas phase, which then condensed to deposit graphite, thus imparting a gray color to the quartz. At stage II (gold-productive), simple cooling of a homogeneous near-neutral medium-chloride–sulfide solution remaining after the fluid heterogenization on cooling from 300 to 180 ºC resulted in milky-white quartz. At stage III (late lowly Au-productive), cooling of weakly alkaline low-chloride–sulfide solution III containing Au, As, Sb, Hg, and Ag (the last three components might have been introduced on the dilution of chloride–sulfide solution II) led to the formation of clear quartz. Gold in the above solutions occurs mainly as dihydrosulfide Au(HS)–2, and its deposition in acid and weakly alkaline media at 300 ºC follows reactions (1) and (2), respectively. The performed calculations show a significant dependence of the Au content in solutions on temperature, Eh, pH, and

mCl−, and mHS–, and a change in these parameters causes gold deposition during the ore formation at geochemical barriers. The proposed genetic model reflects the sequence of mineral formation at the Badran deposit, and the results of modeling permit the estimation of the gold potential of metasomatic zones at its deep horizons. We thank Prof. M. Reed from Oregon University, USA, for the provided Chiller computer program. This work was supported by grants 07-05-00803 and 08-05-00915 from the Russian Foundation for Basic Research and by grant RNP.2.1.1.702 from the Russian Ministry of Science.

References Amuzinskii, V.A., 2005. Metallogenic Epochs and Gold Potential of Ore Complexes in the Upper Yana Folded System [in Russian]. IGABM SO RAN, Yakutsk. Anisimova, G.S., 1983. The zoning of the Yakutian gold-ore field (from results of mineralogical mapping), in: Mineralogical Mapping as a Method for Study of Ore-Bearing Areas [in Russian]. UNTs AN SSSR, Miass, pp. 164–165. Anisimova, G.S., 1993. Mineralogical criteria for the local prediction for gold mineralization based on the topomineralogical dating of the Badran ore filed, in: Mineralogical and Genetic Aspects of Magmatism and Mineralization in Yakutia [in Russian]. YaNTs SO RAN, Yakutsk, pp. 49–53. Anisimova, G.S., 2008. Micromineral assemblages at gold deposits, in: Proc. Intern. Conf. Ore Genesis [in Russian]. Miass, pp. 7–11. Anisimova, G.S., Amuzinskii, V.A., Balandin, V.A., 1998. Sulfide-quartz pools in gentle faults: new type of gold deposits. Otechestvennaya Geologiya, No. 6, 65–70. Anisimova, G.S., Kondrat’eva, L.A., Serkebaeva, E.S., 2006. Native gold of the Badran deposit. Otechestvennaya Geologiya, No. 5, 38–47. Belevantsev, V.I., Gushchina, L.V., Obolensky, A.A., 1998a. Solubility of stibnite Sb2S3: expertise of known interpretations and refinements. Geokhimiya, No. 1, 65–72.

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