mineralogy and formation conditions of ore from the biksizak silver–base_metal occurrence of the...

ISSN 1075�7015, Geology of Ore Deposits, 2010, Vol. 52, No. 5, pp. 392–409. © Pleiades Publishing, Ltd., 2010.Original Russian Text © O.Yu. Plotinskaya, E.O. Groznova, A.I. Grabezhev, K.A. Novoselov, 2010, published in Geologiya Rudnykh Mestorozhdenii, 2010, Vol. 52, No. 5, pp. 439–456.

392

INTRODUCTION

The Biksizak base�metal occurrence is situated 35 kmsouthwest of Chelyabinsk in the Birgil’da–Tomino orecluster. The Tomino and North Tomino porphyry cop�per deposits are located three kilometers east of theBiksizak occurrence; the Bereznyakovskoe epithermalAu–Ag deposit is located a few kilometers south of thisoccurrence (Fig. 1a). Despite the fact that the depositsand occurrences of the Birgil’da–Tomino ore clusterhad been discovered and started to study not so longago (Romashova, 1984; Grabezhev et al., 1998, 2000;Lehmann et al., 1999; Puzhakov, 1999; Plotinskayaet al., 2008, 2009), they immediately attracted atten�tion as ore objects untraditional of the Urals. Detailedinvestigation of the deposits in this region providesinsight into the relationships between gold–silver,base�metal, and porphyry copper mineralization inPaleozoic volcanic–plutonic belts.

The Biksizak occurrence is so far a single base�metal ore object in this territory, and its genesisremains vague. On the basis of its spatial associationwith the Tomino porphyry copper and the Bereznyak�ovskoe epithermal deposits and the character of wall�rock metasomatic alteration, Grabezhev et al. (1998,

Corresponding author: O.Yu. Plotinskaya. E�mail: plo�[email protected]

2000) classified it as a base�metal occurrence formedat the periphery of porphyry copper system. Snachevand Kuznetsov (2009) called it stratiform, withoutspecifying this definition in genetic terms.

In this paper, we make an attempt to specify theplace of the Biksizak occurrence in the epithermal–porphyry ore�forming system on the basis of mineral�ogy and geochemistry of the ore, typomorphic featuresof the ore and gangue minerals, physicochemical con�ditions of their formation, and localization.

GEOLOGICAL OVERVIEW

The geology of the Birgil’da–Tomino ore cluster iscontrolled by its location at the western margin of theEast Ural volcanic zone (Grabezhev et al., 1998;Snachev and Kuznetsov, 2009). In the northern part ofore cluster, the Lower–Middle Ordovician basalts ofthe Sargazy Formation are conformably overlain bythe Middle Ordovician–Silurian limestone, which is,in turn, unconformably overlapped by the UpperDevonian–Lower Carboniferous andesitic dacites andsedimentary rocks of the Bereznyaki Formation,which occupy the most territory and host the Berezn�yakovskoe epithermal Au–Ag ore field in the south(Fig. 1a). Dioritic and andesitic intrusive bodiesbelong to the Late Devonian–Early Carboniferous

Mineralogy and Formation Conditions of Ore from the Biksizak Silver–Base�Metal Occurrence in the South Ural, Russia

O. Yu. Plotinskayaa, E. O. Groznovaa, A. I. Grabezhevb, and K. A. Novoselovc

a Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 119017 Russia

b Zavaritsky Institute of Geology and Geochemistry, Ural Division, Russian Academy of Sciences,Pochtovyi per. 7, Yekaterinburg, 620151 Russia

c Institute of Mineralogy, Ural Division, Russian Academy of Sciences, Miass, Chelyabinsk oblast, 456317 RussiaReceived February 11, 2008

Abstract—The Biksizak silver–base�metal occurrence is situated in the Birgil’da–Tomino ore cluster of theEast Ural Zone. The mineralization is hosted in the Silurian marble (Eastern site) and limestone interbeds inandesitic dacite (Western site). Four mineral assemblages have been established: the earliest hematite–mag�netite, the subsequent pyrite–arsenopyrite and chalcopyrite–sphalerite occurring only in the Eastern site,and fahlore–chalcopyrite known only from the Western site. The closest positive correlation links Cu–Zn–Ag, Zn–Pb, Cu–Ag, and Zn–Au. The correlation between chemical elements varies depending on the local�ization of the ore. Correlation pairs Au–Ag, Au–Cu, Pb–Ag, and Pb–Cu are characteristic of ore from theEastern site and are not established in the Western site. In the Eastern site Cu/(Zn + Pb) in ore is < 1, whereasin the Western site this ratio is markedly higher than unity. As follows from fluid inclusion study and mineralgeothermometry, ore minerals at the Bilsizak occurrence were formed at a temperature of 300 to 150°C fromlow� and moderate�saline chloride fluids with 1–9 wt % NaCl equiv. The data obtained show that the Bik�sizak occurrence was localized at the margin of a porphyry system characterized by hydrothermal and skarnprocesses.

DOI: 10.1134/S1075701510050053

GEOLOGY OF ORE DEPOSITS Vol. 52 No. 5 2010

MINERALOGY AND FORMATION CONDITIONS 393

Birgil’da–Tomino Complex. Porphyry copper depos�its in the north of the district are related to these intrusions(Grabezhev et al., 1998). Granitoid plutons (Fig. 1a) aredated at Early Carboniferous–Early Permian. Itshould be noted that no reliable isotopic datings ofrocks pertaining to the Birgil’da–Tomino Complexare available. Romashova (1984) and Grabezhev andKrasnobaev (2009) suggest that this complex and thecomagmatic Bereznyaki Formation are Silurian inage.

The Biksizak occurrence is localized in the centralpart of the Birgil’da–Tomino ore cluster at the north�ern end of the near�meridional Michurino Zn–Cu–Au–Ag metallogenic zone (Grabezhev et al., 1998).The area of this occurrence is composed of andesiticdacitic tuffs of the Bereznyaki Formation underlain bylimestone of the Middle Ordovician–Lower SilurianBiksizak Formation. The base of the section consists

of aphyric basalt of the Sargazy Formation. The strat�ified rocks are cut through by porphyritic andesite anddiorite intrusive bodies of the Birgil’da–Tomino Com�plex (Puzhakov, 1999; Rykus et al., 2006; Snachev andKuznetsov, 2009). In the southwest, limestone isskarnified at the contact with porphyritic intrusiverocks (Puzhakov, 1999).

Stratal and lenticular orebodies are conformable tohost rocks. These are Middle Ordovician–Lower Sil�urian limestone and limestone interlayers in the over�lying Upper Devonian–Lower Carboniferous andes�itic dacitic tuff in the western part of the occurrence.The ore lode 700–1000 × 400–800 m in plan viewgently plunges to the west. Disseminated, stringer–disseminated, and occasionally massive ores are com�posed of pyrite, sphalerite, magnetite, hematite, chal�copyrite, and arsenopyrite.

61°E55°N

(а)

Chelyabinsk

Ma

in U

ral

Fa

ult

6

5

43

2

1

20 km

(b)

400 m

4

21

528

I

I

c)1

3

5

7

9

2

4

6

8

10

1

a b4

–100

–200

–300

0m

I

400 m

4 5 28 I

Tr

oi t

sk

Fa

ul t

Fig. 1. (a) Geological scheme of a fragment of the East Ural Zone; the Birgilda–Tomino ore cluster is contoured by the dashedline; (b) schematic geological map of the Biksizak occurrence; (c) geological section along line I–I. Modified after the data ofthe Poletaevsky Geological Exploration Party, Lehmann et al. (1999); Herrington et al. (2005); Grabezhev et al. (1998; 2000). (1)Upper Devonian and Lower Carbobiferous sedimentary and volcanic rocks; (2) Middle Ordovician–Silurian limestone and mar�ble; (3) Lower–Middle Ordovician basaltic lavas and tuffs; (4) Precambrian basement; (5) Late Devonian–Early Carboniferousporphyritic andesite and diorite; (6) Paleozoic granitic plutons; (7) fault; (8) ore deposits and occurrences (numerals in circles):1, Biksizak; 2, Bereznykovskoe (Au–Ag); porphyry copper: 3, North Tomino; 4, Birgil’da; 5, Zelenodol’skii; 6, Novotroitskii; (9)orebody; (10) borehole and its number: (a) on map and (b) in section.

394


PLOTINSKAYA et al.

Two sites—Eastern and Western—that are calledseparate occurrences by Snachev and Kuznetsov(2009) are distinguished by localization conditionsand modal composition of ores.

In the Eastern site, mineralization is confined tothe roof of Silurian carbonate rocks (Grabezhev et al.,1998) and composed of disseminated and massivechalcopyrite–pyrite–sphalerite ore (Fig. 2a) crossedin their typical appearance by Borehole 5 (Figs. 1b, 1c,2a, 3a, 3b). Several interlayers of disseminated andmassive ores up to a few meters in thickness are sepa�rated by barren intervals up to 10 m and higher inthickness (Fig. 2a). Limestone of barren interlayers arebrecciated and cemented by pyrite–quartz–sericiteaggregate (Grabezhev et al., 1998). Ores are spotty,vaguely banded (Fig. 3a), and occasionally massive(Fig. 3b). The dispersed stringer–disseminatedpyrite–arsenopyrite mineralization at margins of ore�bodies is penetrated by Borehole 21.

Sphalerite—the major ore mineral—makes upalmost monomineralic aggregates impregnated withpyrite and chalcopyrite. The Zn content in hand spec�imens reaches 28 wt % (Fig. 2a; Table 1) but com�monly does not exceed a few percents, decreasing to afew tens ppm in barren interlayers. The Cu content insome cases is above 1 wt % but on average is lower than0.1 wt %. The Pb content varies from 100 to 1000 ppmin ore intervals and is as low as 40–70 ppm beyondtheir limits. The Au and Ag grades range from traces to3.4 and 78 gpt, respectively. The average Cu/(Pb + Zn)ratio is 0.2 and attains a maximum (0.9) withinbetween�ore intervals.

The barite–base�metal mineralization in the West�ern site is localized in the upper part of a limestonelens among andesitic dacitic tuff, which is penetratedby Borehole 4 (Figs. 1a, 2b). Limestone is silicified

and replaced with ankerite. The orebody is distinctlyzonal. Chalcopyrite and fahlore along with frequentbarite pockets are widespread in the central zone(Figs. 3c, 3d); the Cu content up to 4.5 wt % and Agcontents up to 110–157.5 ppm are noted in particularsamples (Table 1); Zn and Pb contents do not reach0.03 wt %. Pyrite, chalcopyrite, hematite, and galenaoccur at margins of the orebody, which are distinguishedby elevated Zn and Pb contents (0.61 and 0.11 wt %,respectively; Cu and Ag contents decrease down to<0.01% and a few ppm, respectively (Fig. 2b; Table 1)).The Au content commonly does not exceed a few gpt;gold is distributed irregularly. The high Ag/Au ratio isdetermined by variation of the Ag content. TheCu/(Pb + Zn) value in the central zone is alwayshigher than unity and occasionally exceeds 150,whereas in the marginal zone this ratio is, as a rule,below unity (Fig. 2b; Table 1).

In general, the Eastern site is characterized byhigher Zn and Pb contents, whereas the Western site ischaracterized by elevated Cu and Ag contents.

MAIN MINERAL ASSEMBLAGESAND THE SEQUENCE

OF MINERAL FORMATION

Four mineral assemblages: (1) hematite–magne�tite, (2) pyrite–arsenopyrite, (3) chalcopyrite–sphalerite, and (4) fahlore–chalcopyrite are estab�lished at the occurrence (Fig. 4).

The stringer–disseminated mineralization pertain�ing to the hematite–magnetite assemblage occurs inboth Eastern and Western sites (Fig. 3a). In marble ofthe Eastern site, hematite and magnetite are over�grown by sphalerite (Fig. 5a), while in silicified lime�

Table 1. Contents of major ore components, ppm at the Biksizak occurrence

ComponentBorehole 4, Western site (n = 28) Borehole 5, Eastern site (n = 48)

min max average min max average

Au 0.0 2.4 0.4 0.0 3.4 0.48

Ag 0.0 157.5 21.0 0.1 78.0 8.7

Cu 30 45 000 8 299 10 11 700 755

Zn 30 1 750 481 20 288 000 18 331

Pb 3 1 100 94 15 6 300 261

S 2 400 72 000 20 553 200 307 800 40 720

As 50 400 156 0 300 48

Ba 400 10 000 3 405 0 1 500 181

Mn n.a. n.a. n.a. 600 10 000 4 798

Ag/Au 2.0 787.5 133.3 1.8 207.5 27.0

Cu/(Pb+Zn) 0.1 154.3 13.6 0.0 0.9 0.2

Zn/Pb 1.6 50.0 12.2 0.4 491.4 79.8

Note: n, number of analyses; n.a., not analyzed. The analyses of core samples obtained by N. S. Kuznetsov and B.A. Puzhakov (Chelyabin�skgeols"emka OAO) are used.



420

290

280

270

260

250

240

100000100.1 100001001 1000.10.01(a)

m

Au

Ag

Cu

Pb

Zn Cu/(Pb + Zn)Ag/Au

1000ppmppm

г/т 1.0 10

420 100000100.1 100001001 1000.10.01

Au

Ag Cu

Pb

ZnCu/(Pb + Zn)

Ag/Au

1000 1.0 10

120

130

140

(b)

m

1 2 3 4 5

Fig. 2. Structure of orebodies in (a) Eastern site, Borehole 5 and (b) Western site, Borehole 4 at the Biksizak occurrence and con�tents of main ore elements, modified after Grabezhev et al., (1998). (1) Marble, (2) carbonate paragonite–sericite–quartz meta�somatic alteration after volcanic rocks; (3) sulfide disseminations, 1–8 wt %; (4) dense sulfide impregnations up to 35 wt %;(5) massive pyrite–sphalerite–chalcopyrite ore.

stone of the Western site, quartz with acicular hematiteis crosscut by chalcopyrite–carbonate veinlets at themargin of the orebody (Fig. 6a). Thus the hematite–magnetite assemblage is the earliest.

The pyrite–arsenopyrite assemblage develops inthe Eastern site in the form of quartz–carbonate–arsenopyrite veinlets. Pyrite and fahlore occur rarely,in addition to arsenopyrite.

396


PLOTINSKAYA et al.

The chalcopyrite–sphalerite assemblage comprisessphalerite, chalcopyrite, pyrite, and sporadic native gold,and is established only in the Eastern site (Figs. 5c, 5d).These minerals occur as dense impregnations in mar�ble and occasionally as massive ore (Fig. 3b). Arse�nopyrite aggregates are overgrown by chalcopyrite andfahlore (Fig. 5b); relics of fragmented large pyritecrystals are cemented by sphalerite (Fig. 5e). Thisindicates that the pyrite–arsenopyrite assemblagepostdates the hematite–magnetite assemblage andpredates the chalcopyrite–sphalerite assemblage.

The fahlore–chalcopyrite assemblage (abundantfahlore, pyrite, chalcopyrite, galena and sporadicsiegenite and arsenopyrite) occurs in the central zoneof the orebody in the Western site. In addition togangue quartz and carbonates, barite fills the pocketsup to a few centimeters in size, which are overgrown bylater ore minerals (Fig. 3d). We failed to observe cross�cutting relations between chalcopyrite–sphalerite andfahlore–chalcopyrite assemblages. It cannot be ruledout that such relations are lateral rather than temporal.

TYPOMORPHIC FEATURES OF MAINORE AND GANGUE MINERALS

More than 20 ore and gangue minerals have beenidentified at the Biksizak occurrence. The most abun�dant ore minerals are pyrite, chalcopyrite, magnetite,hematite, fahlore, and sphalerite; galena, siegenite,and arsenopyrite are less frequent; pyrrhotite andnative gold are sporadic. In addition, Grabezhev et al.(1998) mentioned enargite; Snachev and Kuznetsov(2009) pointed out bornite, marcasite, and polydymite(Ni2S3); Puzhakov (1999) desribed rare silver sulfos�alts: miargyrite (AgSbS2), stromeyerite (AgCuS), andxanthoconite (Ag3AsS3). Gangue minerals mainlyinclude carbonates, quartz, less frequent barite, andoccasional chlorite and amphibole.

The chemical composition of ore and gangue min�erals was determined on a Jeol JSM�5300 SEM equippedwith a Link�ISIS EDS (analyst N.V. Trubkin, Instituteof Geology of Ore Deposits, Petrography, Mineralogy,and Geochemistry, Russian Academy of Sciences)under the conditions indicated below—sphalerite:

(a) 1 cm

Car

Car

Py + Sp

Mt + Hem

Mt + Hem

Py

CarCp

Td

Brt

Cp

Qtz + Car

1 cm

0.5 cm1 cm

(b)

(c) (d)

Fig. 3. Ores from the Biksizak: (a, b) Eastern site: (a) hematite (Hem). magnetite (Mt), and pyrite (Py) disseminations in carbon�ate; (b) massive pyrite–sphalerite ore with carbonate; (b) massive pyrite–sphalerite ore with carbonate (Car); (c, d) Western site:(c) massive base�metal ore (Cp, chalcopyrite; Td, tetrahedrite) and carbonate; (d) barite (Brt) overgrown by quartz–carbonateaggregate with sulfides.



analytical lines Kα (Zn, S, Fe), standards were syn�

thetic FeS2 for S and chemically pure metals for Feand Zn; detection limits, wt %: 1.4 S, 0.4 Fe, and 2.6Zn; siegenite: analytical lines K

α, standards were syn�

thetic FeS2 for S and chemically pure metals for Fe,Co, Ni, and Cu; detection limits, wt %: 0.5 S, 1.1 Fe,1.5 Co, 1.7 Ni, and 0.9 Cu; fahlore: analytical lines L

α

(Ag, Sb) and Kα for other elements; standards were

synthetic FeS2 for S, InAs for As, and chemically puremetals for Fe, Cu, Zn, Ag, and Sb; detection limits, wt%: 1.2 S, 0.5 Fe, 1.8 Cu, 1.1. Zn, and 1.1 Ag; nativegold: analytical lines L

α (Ag) and M

α (Au), standards

were chemically pure metals; detection limits, wt %:0.7 Ag and 1.4 Au; carbonates: analytical lines К

α,

standards were chemically pure metals for Mn and Fe,olivine for Mg and wollastonite for Ca; detection lim�its, wt %: 0.75 Mg and Ca, 0.45 Mn, and 1.0 Fe; chlo�rites: К

α for all elements, standards were nacaphite for

Na, olivine for Mg, Al2O3 for Al, SiO2 for Si, wollasto�nite for Ca, chemically pure metals for Mn and Fe;detection limits, wt %: 0.6 Na, 0.3 K, 0.9 Mg, 0.45 Al,

0.4 Si, 0.9 Ca, 0.45 Mn, and 1.1 Fe. Analysis of nativegold and some sphalerite and fahlore grains was car�ried out on a REMMA�202M SEM/EDP (analystV.A. Kotlyarov, Institute of Mineralogy, Ural Division,Russian Academy of Sciences) at accelerating voltage20 kV, current in sample 30–10 nA; standards werechemically pure Au, Ag, and Fe and synthetic ZnS.

Pyrite (FeS2) is ubiquitous in ore and characterizedby diverse morphology. In the Eastern site, pyrite per�taining to the pyrite–arsenopyrite and chalcopyrite–sphalerite assemblages commonly occurs as perfectcubic metacrystals 10–300 µm in size (Fig. 5c) orsmall euhedral crystals incorporated into sphalerite(Fig. 5a). Pyrite of the next generation overgrows sphaler�ite as irregular fine�crystalline aggregates (Fig. 5d). Insilicified limestone of the Western site, euhedral crys�tals are supplemented by aggregates of fine�grainedand dusty pyrite (Fig. 6b), which are recrystallizedwith the formation of complex ringlike structures(Figs. 6c–6e), when replaced with later minerals. The

Mineral

Hematite

Magnetite

Pyrrhotite

Arsenopyrite

Pyrite

Fahlore

Chalcopyrite

Siegenite

Sphalerite

Gold

Galena

Quartz

Barite

Ankerite

Chlorite

Calcite

1 2 3 4? ?

?

Fig. 4. Generalized sequence of mineral formation at the Biksizak occurrence. The thickness of lines corresponds to the relativeabundance of minerals. (1–4) Mineral assemblages: (1) hematite–magnetite, (2) pyrite–arsenopyrite; (3) chalcopyrite–sphaler�ite, (4) fahlore–chalcopyrite.

398


PLOTINSKAYA et al.

diameter of such pyrite rings varies from 10 to 30–40µm.

Sphalerite (ZnS) is the most abundant mineral inthe Eastern site, where it makes up densely impreg�nated and massive ores together with pyrite and chal�copyrite. As a rule, this is greenish, light yellowcleiophane with extremely low Fe admixture (0.17–1.0 wt %) (Table 2; Figs. 5a, 5c, 5d). Central parts ofsome sphalerite grains contain fine chalcopyrite emul�sion, which occasionally enlarges and acquires anirregular star�shaped form, probably indicating subse�quent recrystallization of sphalerite. The above�described sphalerite varieties do not differ in chemicalcomposition. Grabezhev et al. (1998) mentionedsphalerite with the Fe content reaching 7.5 wt %. Thetemporal relationships between Fe�enriched and Fe�free sphalerite are unknown, and it is not ruled out thatthey are of lateral character.

Galena (PbS) is much less abundant than sphaleriteand is largely identified in the base�metal ore of theWestern site as small (no larger than 50 µm) anhedralgrains (Fig. 6d). Galena contains up to 2.8 wt % Ag.

Arsenopyrite (FeAsS) occurs mainly in thestringer–disseminated ore of the Eastern site (Fig. 5b)and is noted in the Western site as small inclusionswithin intergrowths of pyrite, chalcopyrite, and siege�nite (Fig. 6e). Arsenopyrite from the Eastern site con�tains 36.59 wt % (34.43 at %) Fe, 42.76 wt % (30.00 at%) As, and 21.71 wt % (35.58 at %) S.

Three generations of fahlore[(Cu,Ag)10(Fe,Zn)2(As,Sb)4S13] are distinguished.They belong to different mineral assemblages anddiffer in chemical composition. The first generationwas established in association with arsenopyrite inmarble of the Eastern site (Fig. 5b); this is Fe�richtennantite containing no higher than 1.12 wt % Sb,about 3.2 wt % Zn and corresponds to formula

200 µm

(a)

Car

Sp

Cp

Car

Qtz

Sp Car

Py

PyPy

Py

Py

Au

Sp

Sp

Cp

Hem

CarPy

Cp

Mgt

Asp

Asp

Qtz

Asp

Tn

100 µm 200 µm

Mgt

Au

SpCar

0 2 4 6 8 10 12

Py 200 µm

10 µm

Py

(b)

(c) (d)

ZnZn

Au

AuS

SAu

Au

Zn ZnAgAg

Fig. 5. Main ore minerals in the Eastern site of the Biksizak occurrence, reflected light: (a) sphalerite (Sp) with pyrite (Py) andmagnetite (Mgt) and carbonate (Car) overgrow quartz (Qtz) containing hematite (Hem); (b) arsenopyrite (Asp), fahlore (Tn),and chalcopyrite (Cp) in a quartz–carbonate veinlet; (c) native gold (Au) in pyrite, which is overgrown by chalcopyrite and thencemented by sphalerite; inset, a particle of native gold in BSE image; (d) native gold in sphalerite with magnetite, pyrite, and car�bonate and energy dispersive spectrum of area with Au.



[Cu10.2(Fe1.3Zn0.7)2.0(As3.9Sb0.1)4.0S12.8] when recalcu�lated to 29 atoms (Fig. 7). Fahlore of the second gener�ation occurs in chalcopyrite–sphalerite assemblage ofthe Eastern site, where this mineral together withchalcopyrite replaces large pyrite crystals alongfractures and pores or forms small grains intergrownwith sphalerite. The latter grains contain, wt %:38.44–41.32 Cu, up to 0.92 Ag, 1.61–5.27 Fe,5.43–7.03 Zn, 13.65–15.92 As, 6.63–8.85 Sb, and27.37–27.77 S. Their averaged formula[(Cu9.5Ag0.1)9.6(Fe1.0Zn1.4)2.4(As3.0Sb0.9)3.9S13.2] corre�sponds to Sb�tennantite with somewhat lowered Cucontent and elevated contents of bivalent metals incomparison with stoichiometric composition. Fahloreof the third generation was identified in silicified lime�stone of the Western site in association with chalcopy�rite, low�Fe sphalerite, and galena. This mineral is anintermediate member of tenantite–tetrahedrite (Fig. 7),with Sb contents varying from 11.7 to 18.8 wt % (3.1–1.5 f.c.) and As contents varying from 2.5 to 10.8 wt %(0.7–2.4 f.c.). Zn (1.6–1.9 f.u.), appreciably prevail�ing over Fe. No systematic correlation is established

between Zn and Sb (Fig. 7a). Fahlore of the secondgeneration is distinguished by enrichment in Ag (4.3–9.8 wt %); Ag reveals distinct positive correlation withSb (Fig. 7b).

A mineral from the linnaeite group (Figs. 6d, 6e)close to siegenite (CoNi2S4) in composition was identi�fied in the central zone of silver–base�metal ore of theWestern site. This mineral overgrows pyrite–chal�copyrite aggregate and contains numerous ringlikepyrite segregations 10–20 µm in diameter; central partsof these rings are filled with chalcopyrite (Fig. 5e). Siege�nite is characterized by admixture of Fe (2.9–14.3 wt %),which chemically supersedes Ni. The averaged com�position of the mineral corresponds to the formula[Co(Ni1.6Fe0.4)2.0S4.0] calculated on the basis of 7 apfu(Table 3; Fig. 8). This composition is close to mostpublished compositions of siegenite from variousdeposits, which are also characterized by a smalladmixture of the greigite end member (Fig. 8).

Native gold occurs extremely rarely and only in theEastern site as minute (from 1–2 to 20 µm) inclusions

200 µm(a)

Cp

Qtz

Hem

QtzCp

Car

Car

CpQtz

Cp

Td

Cp

Py

Td

Py

Cp

100 µm

Hem

Py

CpPy

Cp

Sg

Gn Sg

50 µm 200 µm 50 µm

Sg

CpAsp

Py

(b)

(c) (d) (e)

Fig. 6. Main ore minerals in the Western site of the Biksizak occurrence, reflected light: (a) chalcopyrite (Cp) and carbonate (Car)overgrow hematite (Hem) and quartz (Qtz); (b) tetrahedrite (Td), pyrite, and chalopyrite in quartz; (c) chalcopyrite overgrowsringlike pyrite segregations in carbonate; (d) siegenite (Sg), galena (Gn), chalcopyrite, and pyrite in quartz; (e) close�up of pho�tomicrograph (d) shows arsenopyrite (Asp) and ringlike pyrite with chalcopyrite inside overgrown by siegenite.

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PLOTINSKAYA et al.

in pyrite, chalcopyrite, and sphalerite. A single nativegold microinclusion identified in pyrite (Fig. 5c) con�tains 6.09–6.56 wt % Ag (6.29 wt %, on average overfour analyses) corresponding to the fineness 934–939.We failed to determine the exact chemical composi�tion of native gold intergrown with sphalerite becauseof too small particles. EDS examination of a site wheresphalerite contains gold inclusions showed that Znand S were added by 52 wt % Au and 12 wt % Ag. Onthe basis of these data, fineness of gold is estimatedat 820.

Carbonates are the most abundant gangue mineralsat the Biksizak occurrence and include calcite[CaCO3], dolomite [CaMg(CO3)2], and ankerite[Ca(Mg,Fe,Mn)(CO3)2] (Fig. 9; Table 4).

The pyrite–arsenopyrite mineralization is accom�panied by quartz–calcite–ankerite veinlets. Ankeriteis characterized by high Fe content (higher than0.6 apfu) and appreciable Mg content (up to 0.3 apfu);a contribution of Mn does not exceed 0.75 wt % (Fig. 9;Table 4, analyses 1–3). Calcite from this assemblagecontains up to 2.4 wt % Fe (Table 4, analyses 10, 11).

The sphalerite–chalcopyrite assemblage in theEastern site is hosted in marble, where primary calciteis replaced almost completely with quartz and ankerite(Fig. 9; Table 4, analyses 4–8). Ankerite is occasion�ally characterized by eleveted Ca content in compari�son with the stoichiometric composition (Table 4;analyses 4–6), probably due to embedding of calciterelics from the marble. The Mg and Fe contents aremarkedly variable (0.35–0.73 and 0.4–0.2 apfu,respectively); up to 2 wt % Mn are detected constantly.Dolomite with appreciable Mn admixture (up to1.7 wt %) often also occurs together with ankerite.

The fahlore–chalcopyrite mineralization in theWestern site is confined to calcite veinlets that crosscutlimestone superseded with ankerite and quartz. Cal�cite always contain appreciable amounts of Fe (0.7–2.4 wt %) and a small admixture of Mn (up to 0.5%)(Table 4, analyses 12–16). Ankerite is identified spo�

radically as relics in calcite; its composition is close toankerite from pyrite–arsenopyrite assemblage (Table 4,analysis 9).

Thus, a temporal trend is traced in the chemicalcomposition of ankerite from Fe�enriched in thepyrite–chalcopyrite assemblage to more magnesian inchalcopyrite–sphalerite assemblage. The occurrenceof the fahlore–chalcopyrite assemblage in the late cal�cite veinlets indicates that this assemblage is the latestat the occurrence.

Chlorites. Minerals of the chlorite group have beenidentified in chalcopyrite–sphalerite and pyrite–arse�nopyrite assemblages of the Eastern site. In the chal�copyrite–sphalerite ore, chlorites occur as minute(often smaller than 5 µm) particles in ankerite. In thepyrite–arsenopyrite ore, chlorites are noted in cal�cite–ankerite–arsenopyrite veinlets and make upaggregates as large as 100 µm. All studied chlorites areclose in chemical composition (Table 5). The siliconcontent varies from 4.98 to 5.97 apfu; Fe/(Fe + Mg) =0.76–0.83. Taking into account these parameters,chlorites from the Biksizak occurrence can be definedas ripidolite in the classification proposed by Hey(1954). Analysis 5 is the only exception; it is closer tobrunsvigite in composition. In addition to Fe and Mg,Ca is contained up to 1.8 wt % in two analysis; Na andK were established in one sample (0.9 and 0.3 wt %,respectively). The Me2+/Al ratio (Me2+ = Fe + Mg +Ca + Mn) in chlorite ranges from 1.04 to 1.93, owingto a variation of Fe contents.

GEOCHEMISTRY OF THE ORE

The correlation between the main ore elements atthe Biksizak deposit was analyzed on the basis of bulkchemical sampling of the mineralized core intervals inBorehole 5 (Eastern Site) and Borehole 4 (Westernsite). The minimal significant coefficients of pair cor�relation determined from Fisher–Yeats criterion for a0.05 level of confidence are 0.29 for 47 analyses from

Table 2. Chemical composition of sphalerite from the Biksizak occurrence

No.Zn Fe S Total Zn Fe S

wt % apfu

1 66.91 0.17 32.65 99.73 1.00 0.00 1.00

2 66.47 0.29 33.03 99.79 0.99 0.01 1.01

3 66.89 0.32 32.74 99.95 1.00 0.01 1.00

4 67.80 0.57 32.91 101.28 1.00 0.01 0.99

5 67.76 0.57 32.95 101.28 1.00 0.01 0.99

6 64.63 0.71 32.32 97.66 0.98 0.01 1.00

7 65.16 1.00 31.95 98.11 0.99 0.02 0.99

Note: Analyses 1–3 were performed on a REMMA�202M SEM/EDP (analyst V.A. Kotlyarov, Institute of Mineralogy, Ural Division, RAS);analyses 4–7, on Jeol JSM�5300 SEM/Link�ISIS EDS (analyst N.V. Trubkin, Institute of Geology of Ore Deposits, Petrography, Min�eralogy, and Geochemistry, RAS).



Borehole 5 and 0.38 for 28 analyses from Borehole 4.Correlation coefficients above 0.8 were deemed ashigh (Table 6). Some common relationships wereestablished in both holes, i.e., in both sites. These arestrong positive correlation between Cu and Ag (Fig. 10a;Table 6), weak positive correlation between Au andZn, and appreciable positive correlation between Pband Zn (Fig. 10c). The first relationship is caused by afrequent occurrence of Ag as an admixture in fahlore,while the second and third relationships reflect super�position of native gold on base�metal mineralizationconsisting of sphalerite, galena, and Zn�bearing fahl�ore.

The strong positive correlation of Zn with Ag andCu was established in the Eastern site, whereas in theWestern site such correlation is close to the signifi�cance limit. At the same time, Fig. 10d shows that inthe interval of low Cu contents (<1000 ppm), the Cuand Zn concentrations in holes 4 and 5 are almostidentical. Indeed, such concentrations in Hole 4reveal a positive correlation between Cu and Zn (r =0.61 for n = 18 rmin = 0.47), but for Cu concentrationsabove 1000 ppm such correlation disappears. This dif�

ference is probably caused by incorporation of Cu andZn into fahlore in the Western site localized in mar�ginal zone of the orebody, while in high�grade ore ofthe central zone chalcopyrite is the major carrier ofcopper. Another important difference between bothsites is the development of Au–Ag and Au–Cu corre�lation pairs in the Eastern site and their absence in theWestern site (Fig. 10e, 10f). The positive Au–Ag cor�relation is caused by a marked contribution of Ag tonative gold, whereas in the Western site Ag is oftenincorporated into galena and fahlore as an admixture.The Au–Cu correlation probably resulted from super�position of gold on copper mineralization and theoccurrence of gold in pyrite–chalcopyrite inter�growths. In addition, a positive correlation of Pb withAg and Cu was established in the Eastern site and notrevealed in the Western site, because of development ofdistinct zoning in this site, i.e., spatial separation ofcopper and base�metal mineralization.

In general, ores and altered wall�rocks in holes 4and 5 are close to each other in terms of the correlationof main ore components. Their mineral compositions

1.0

0.8

0.6

0.4

0.2

0 0.80.60.40.2

Fe/(Fe + Zn)

Sb/(Sb + As)

(а)

1

2

3

1.0

0.8

0.6

0.4

0.2

0 0.20.1

Ag/(Ag + Cu)

Sb/(Sb + As)

(b)

Fig. 7. (a) Fe/(Fe + Zn) vs. Sb/(Sb + As) and (b) Ag/(Ag + Cu) vs. Sb/(Sb + As) in fahlore from the Biksizak occurrence. Mineralassemblages: (1) pyrite–arsenopyrite, (2) chalcopyrite–sphalerite, (3) fahlore–chalcopyrite.

Table 3. Chemical composition, wt % of minerals from the linneite group at the Biksizak occurrence

No.Fe Co Ni S Total Fe Co Ni S

wt % apfu

1 7.48 20.56 31.4 43.57 103.01 0.39 1.03 1.57 4.01

2 2.92 20.12 35.98 42.48 101.50 0.16 1.02 1.84 3.98

3 9.89 20.07 26.41 41.71 98.08 0.55 1.05 1.39 4.02

4 3.96 18.57 33.82 41.50 97.85 0.22 0.98 1.79 4.02

5 14.34 18.06 26.08 44.32 102.80 0.75 0.90 1.30 4.05

Note: Cu and As were not detected.

402


PLOTINSKAYA et al.

differ only in relative abundances of the same mineralsrather than in sets of major ore minerals.

PHYSICOCHEMICAL CONDITIONSOF MINERAL FORMATION

Fluid Inclusion Study

To study fluid inclusions, the double�sided pol�ished platelets 0.3–0.5 mm in thickness have been pre�pared for 12 samples characterizing all stages of hydro�thermal activity. Inclusions suitable for study wererevealed only in two platelets: in preore barite andsphalerite of a chalcopyrite–sphalerite assemblage. Incompliance with the known criteria (Roedder, 1972),all observed inclusions were classified as primary. Theyare isometric and spherical in shape and 10–20 µm indiameter. At room temperature all inclusions are two�phase and contain an aqueous solution and a gas bub�ble that occupies 10–30% of vacuole volume.

Microthermometric study of fluid inclusions wasperformed on a measurement complex combining aTHMSG�600 heating�and�cooling stage (Linkam)and an Amplival microscope (Carl Zeiss) with a high�resolution Olympus objective (magnification 80). The

temperature of phase transitions were measured from–196 to +600°С with an accuracy of ±0.2°С within aninterval of +20 to –20°C and no less than ±1.0°Сbeyond this interval.

The temperature of homogenization (Thom) of two�phase inclusions was determined by the disappearanceof a gas bubble. No correction to pressure was made;therefore, the measured temperature may be lowerthan the true temperature of mineral formation. Thecomposition of solution and salt concentrations wereestimated by freezing. The salt composition of thesolution was determined from the eutectic meltingtemperature (Teut) (Borisenko, 1977). The salt con�centration was calculated from the temperature of icemelting (Tm) using a diagram of the Н2О–NaCl system(Bodnar and Vityk, 1994). The measurements werecarried out for groups of inclusions with similar rela�tionships of phases to avoid the errors related to frag�mentation of vacuoles after the heterogenization offluid (Roedder, 1984). The results of measurementsrelated to heating and cooling are shown in Table 7 andFig. 11.

Fluid inclusions in barite were homogenized at atemperature of 248–280°С. The temperature of ice

Fe3S4 Cu3S4 Fe3S4

Co3S4 Ni3S4

CarrolliteCu(Co, Ni)2S4

Linnaeite

SiegeniteCoNi2S4

FletcheriteCu(Ni, Co)2S4

Polydymite

ViolariteFeNi2S4

GreigiteFe3S4

Fe3S4

1

2

3

Fig. 8. Chemical composition of siegenite and other minerals of the linnaeite group from the Biksizak occurrence plotted in coor�dinates Cu3S4–Fe3S4–Co3S4–Ni3S4, mol %. (1) siegenite from the Biksizak occurrence; (2) theoretical composition of siege�nite; (3) data from other localities (Wagner and Cook, 1999).



melting varied from –4.7 to –6.0°С and correspondedto the salinity of fluid 7.5–9.3 wt % NaCl equiv. Therange of Teut from –26.0 to –24.9°С indicates thatNaCl dominated in the fluid; insignificant contents ofsalts of bivalent metals (Mg or Fe) cannot be ruled out.Fluid inclusions in sphalerite are characterized bymarkedly lower Thom = 148–156°C; Tm = –0.5…⎯1.5°C points to much lower salinity (1.0–2.6 wt %NaCl equiv); Teut = –24.0 to –21.8°C is characteristicof sodium chloride solutions.

Despite the fact that the data on fluid inclusions arefragmentary and do not allow characterization of fluidregime at the ore occurrence, it can nevertheless beconcluded that minerals were formed within a ratherwide temperature interval (>150°C) from low andmoderately saline fluids mainly sodium chloride incomposition.

Mineral Geothermometers

The temperature of chlorite formation was esti�mated using empirical geothermometers based on therelationship of Al content in octahedral coordination(AlIV) versus temperature (Cathelineau, 1988) and(Kranidiotis and MacLean, 1987): Т1 and Т2, respec�tively (Table 8). The calculated temperatures for asample of chalcopyrite–sphalerite assemblage (Т1 =425°С and Т2 = 338°С) are much higher than homog�enization temperature of fluid inclusions.

If a depth of mineral formation is accepted to be1 km (maximal thickness of the Bereznyaki Formationoverlapping mineralized marble (Grabezhev et al.,1998)), then the pressure will be about 300 bar underlithostatic conditions. Correction to pressure forhomogenization temperature (Roedder, 1984) in thiscase does not exceed 10–20°С and does not compen�sate a difference between the temperatures obtainedfrom fluid inclusions and chlorites. Probably, overesti�mated temperature values are caused by the occur�rence of Ca in chlorite (sample 5/276), whose effecton calculation results has not been investigated. If theanalysis containing Ca are eliminated, the tempera�ture obtained for chlorite from pyrite–arsenopyriteassociation will be Т1 = 265–385°С and Т2 = 233–312°С, that is, typical of hydrothermal conditions.This temperature range does not contradict the dataon the formation temperature of arsenopyrite (higherthan 250°С). The relatively low As content (30 at %) inarsenopyrite in association with pyrite indicates thatthe upper limit of the formation of this assemblage isabout 350°С (Scott, 1983).

DISCUSSION

The results obtained make it possible to ascertainthe origin of the Biksizak occurrence. The data onfluid inclusions and mineral geothermometry showedthat minerals crystallized from low�saline solutions ata sufficiently high temperature reaching 300°С. Thisallows us to rule out the Mississippi�type stratiform

MgCO3

60

70

80

90

40

30

20

10

10 20 30 40CaCO3 FeCO3

MnCO3

1

2

3

Fig. 9. Chemical composition of carbonates from the Biksizak occurrence plotted in coordinates CaCO3–MgCO3–FeCO3–MnCO3. (1–3) Mineral assemblages: (1) pyrite–arsenopyrite, (2) sphalerite–chalcopyrite, (3) fahlore–chalcopyrite.

404


PLOTINSKAYA et al.

Table 4. Chemical composition, wt % of carbonates at the Biksizak occurrence

No. AssemblageCa Mg Mn Fe Total Ca Mg Mn Fe

wt % apfu

1

Pyrite–arsenopyrite

20.12 3.11 0.74 18.96 42.93 0.94 0.24 0.03 0.64

2 19.64 3.08 0.65 18.19 41.56 0.95 0.24 0.02 0.63

3 19.34 3.47 0.57 17.86 41.24 0.93 0.28 0.02 0.62

4

Chalcopyrite–sphalerite

21.62 4.24 2.01 11.22 39.09 1.08 0.35 0.07 0.40

5 21.55 5.97 1.34 10.82 39.68 1.02 0.47 0.05 0.37

6 25.18 10.91 1.62 7.16 44.87 1.02 0.73 0.05 0.21

7 23.58 9.19 1.49 10.7 44.96 0.99 0.64 0.05 0.32

8 22.41 8.97 1.43 10.67 43.48 0.98 0.64 0.05 0.33

9 Fahlore–chalcopyrite 21.04 3.13 1.13 14.18 39.48 1.06 0.26 0.04 0.51

10Pyrite–arsenopyrite

36.56 n.d. n.d 2.37 38.93 0.95 0.00 0.00 0.04

11 36.67 '' '' 1.09 37.76 0.97 0.00 0.00 0.02

12Chalcopyrite–sphalerite

23.43 12.85 1.66 3.29 41.23 0.97 0.88 0.05 0.10

13 25.66 14.57 1.16 1.49 42.88 0.99 0.93 0.03 0.04

14

Fahlore–chalcopyrite

35.00 n.d n.d 1.42 36.42 0.97 0.00 0.00 0.03

15 34.73 '' '' 1.36 36.09 0.97 0.00 0.00 0.03

16 36.41 '' 0.45 1.00 37.86 0.97 0.00 0.01 0.02

17 35.09 '' n.d 0.67 35.76 0.96 0.00 0.00 0.01

18 40.18 '' 0.52 1.72 42.42 0.93 0.00 0.01 0.03

Note: 1–9, ankerite; 10, 11 – calcite, 12, 13, dolomite, 14–18 calcite. The composition was calculated on the basis of 10 apfu for ankeriteand dolomite and 5 apfu for calcite; n.d., not detected.

deposits and their analogues, because such depositswere formed from low�temperature (70–200°С) andmuch more concentrated brines (up to 35 wt % NaClequiv) (Leach et al., 2005).

The Biksizak occurrence is close in its characteris�tics to metasomatic lead–zinc deposits in carbonatesequences (Rudnye …, 1978), for example, to the Bla�godatsky deposit in the eastern Transbaikal region. Atthe same time, no early magnetite–hematite assem�blage (Radkevich et al., 1963; Dobrovol’skay andShadlun, 1974) widespread at the Biksizak occurrenceis known at those deposits.

The close spatial relations of base�metal mineral�ization with porphyritic diorite intrusions of the Bir�

gil’da–Tomino Complex (Grabezhev et al., 1998)show that this occurrence is a marginal part of a por�phyry or porphyry–epithermal system (Sillitoe, 2010).The stratal morphology of orebodies, localization ofore mineralization in carbonate rocks, and lack ofcharacteristic fill structures do not allow us to refer theBiksizak occurrence to the typical silver–base�metalepithermal deposits similar to those in the Eastern andCentral Carpathians (Banska Stiavnica or Baia Maredeposits).

The localization of ore mineralization in closeproximity to skarnified rocks and abundant magnetiteand hematite in early mineral assemblages probablycould be evidence for skarn�type attributes of the Bil�


MINERALOGY AND FORMATION CONDITIONS 405 Table 5. Chemical composition of chlorites at the Biksizak occurrence

Component 1 2 3 4 5 6 7

Wt %

Si 9.79 10.82 11.47 10.38 13.02 11.16 10.97

Al 9.72 12.23 11.25 11.59 13.87 12.00 11.67

Fe 30.19 30.36 30.67 28.91 24.76 29.21 26.76

Mg 2.71 2.97 2.73 2.50 2.22 3.08 2.78

Ca 1.78 1.38 n.d. n.d. n.d. n.d. n.d.

Na n.d. n.d. '' '' 0.86 '' ''

K '' '' '' '' 0.34 '' ''

Total 54.19 57.76 56.12 53.38 55.07 55.45 52.18

Atoms per formula unit calculated on the basis of 20 cations

Si 4.98 5.02 5.51 5.22 5.97 5.35 5.53

Al [IV] 3.02 2.98 2.49 2.78 2.03 2.65 2.47

Al [VI] 2.10 2.91 3.12 3.27 4.56 3.31 3.64

Fe 7.68 7.05 7.37 7.28 5.67 6.99 6.75

Mg 1.59 1.59 1.51 1.45 1.17 1.70 1.62

Ca 0.63 0.45 0.00 0.00 0.00 0.00 0.00

Na 0.00 0.00 0.00 0.00 0.48 0.00 0.00

K 0.00 0.00 0.00 0.00 0.11 0.00 0.00

KFe 0.78 0.78 0.83 0.83 0.76 0.80 0.81

KMg 0.17 0.18 0.17 0.17 0.17 0.20 0.19

Note: KFe and KMg are ratios of Fe and Mg to the total bivalent metals; (1) chalcopyrite–sphalerite and (2–7) pyrite–arsenopyrite assem�blages; n.d., not detected.

Table 6. Coefficents of pair correlation (r) between main ore elements of the Western and Eastern sites

Western site, n = 28; rmin = 0.38 Eastern site, n = 47, rmin = 0.29

Au Ag Cu Zn Au Ag Cu Zn

Au 1 1

Ag 0.09 1 0.60 1

Cu 0.22 0.91 1 0.55 0.96 1

Zn 0.43 0.42 0.41 1 0.52 0.82 0.84 1

Pb 0.03 0.24 0.13 0.69 –0.01 0.69 0.68 0.69

Note: Statistically significant correlation coefficients are boldfaced; high values are underlined.

406


PLOTINSKAYA et al.

1000

100

10

110001001010.1

105

104

103

102

10

11001010.1

1

2

Cu

Ag

(a)

105

104

103

102

101001010.1

Zn

Ag

(b)

104

103

102

10

110510310210

Pb

Zn

(c)

104

105

104

103

102

10510310210

Zn

Cu

(d)

104

100

10

1

210

Ag

Au

(e)

3

105

104

103

102

2110

Сu

Au

(f)

30.1

0

(g)Ag/Au

Pb/Zn

1000

100

10

110010.10.010.001

(h)Ag/Au

Cu/(Pb +Zn)10

Fig. 10. Correlation between (a–e) concentrations of main ore elements and (g, h) their ratios at the Biksizak occurrence. (1)Borenole 5, Eastern site; (2) Borehole 4, Western site.



sizak occurrence. The Zn or Pb–Zn skarn deposits areformed at various depths and in various tectonic set�tings (Meinert et al., 2005). They are characterized bythe remoteness of base�metal orebodies from contactswith intrusive rocks. The ore mineralization is oftenseparated from skarn in time and formed by the com�bination of meteoric and magmatic fluids close in theircharacteristics to the fluids at epithermal deposits. Thebase�metal orebodies of deposits in the Ertsberg dis�trict of Indonesia (Prendergast et al., 2005) or in theKadaya ore field of the eastern Transbaikal region(Polyakova, 1963) are examples. The base�metal skarndeposits of Karamazar have similar features, particu�larly some orebodies of the Altyn�Topkan deposit,which are classified as a special type of galena–sphalerite mineralization in ankeritized limestone(Zharikov, 1959).

CONCLUSIONS

(1) Four mineral assemblages are established at theBiksizak occurrence. The hematite–magnetite assem�blage is the earliest and is followed by pyrite–arse�nopyrite (arsenopyrite, pyrite, tennantite), chalcopy�

rite–sphalerite (sphalerite, chalcopyrite, pyrite, nativegold), and fahlore–chalcopyrite (tetrahedrite, pyrite,chalcopyrite, siegenite) assemblages with uncertainchronological relationships.

(2) The strongest correlation is characteristic forCu–Zn–Ag, Zn–Pb, Cu–Ag, and Zn–Au ore ele�ments. The ore localized in marble (Eastern site) ischaracterized by a low Cu/(Zn + Pb) ratio; in the orehosted in limestone interlayers among tuffs of theWestern site this ratio is much higher.

(3) The mineral formation developed in a widetemperature range from 300°С, when the pyrite–arse�nopyrite assemblage was formed, to 150°С, when mas�sive and disseminated sphalerite ore was depositedfrom low�saline (1–9 wt % NaCl equiv) sodium chlo�ride fluids.

(4) In general, the data obtained allow us to regardthe Biksizak occurrence as a marginal part of a epith�ermal–porphyry system, probably with participationof skarnification.

300

250

200

150

0 108642100

1

2

C, wt % NaCl equiv

Thom, °C300

250

200

150

–26 –21–22–23–24–25100

Thom, °CC, wt % NaCl equiv

Teut, °C

NaC

l�K

Cl

NaCl

10

8

4

2

–26 –21–22–23–24–250

Teut, °C

NaC

l�K

Cl

NaCl6

(a) (b) (c)

Fig. 11. Relationships: (a) Thom–C (salinity), (b) Thom–Teut, and (c) C–Teut of solutions that fill fluid inclusions in (1) sphaleritefrom the Eastern Site and (2) barite from the Western site.

Table 7. Results of heating and cooling experiments with primary fluid inclusions in gangue and ore minerals at the Biksizakoccurrence

Site, mineral No. n Thom, °C Teut, °C Tm, °C C, wt % NaCl equiv

Western, barite 1 3 248…258 –26.0…–24.9 –5.6…–4.7 8.7…7.5

2 5 268…280 –26.0…–25.3 –6.0…–5.2 9.2…8.1

Eastern, sphalerite

3 3 148…152 –23.1…–22.0 –1.4…–0.5 2.1…0.9

4 3 155…156 –24.0…–21.8 –1.5…–1.0 2.6…1.0

408


PLOTINSKAYA et al.

ACKNOWLEDGMENTS

We thank N.V. Trubkin and V.A. Kotlyarov formicroprobe analyses; V.A. Kovalenker, S.S. Abramov,and E.V. Belogub for discussion; and N.S. Bortnikovfor critical comments. The study was supported by theState contract no. 02.515.11.5089 at 26.06.2008, theRussian Foundation for Basic Research (project nos.09�05�00697, 10�05�00354, 10�05�96015), and Divi�sion of Earth Sciences, Russian Academy of Sciences(programs nos. 2 and 5).

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Table 8. Temperature calculated from chlorite geothermometers

Parameter

Chalcopyrite–sphalerite assemblage Pyrite–arsenopyrite assemblage

1 2 3 4 5 6 7

Al [IV] 3.02 2.98 2.49 2.78 2.03 2.65 2.47

T1 425 417 339 385 265 365 335

T2 338 333 282 312 233 299 280

Note: T1, temperature, after Cathelineau (1988); T2, after Kranidiotis and MacLean (1987). Analyses numbers correspond to Table 5.



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mineralogy and formation conditions of ore from the biksizak silver–base_metal occurrence of the...

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