GEOLOGICA BALCANICA, 33. 1-2, Sofia, Jun. 2003, p. 65-74

Thermodynamic basis for distinguishing the acid-sulphate and adularia-sericite alteration types of rock in epithermal deposits and genetic analogy with the metasomatites of the formation classification

J1ilko Kanazirski

Geological Institute, Bulgarian Academy of Sciences, 1113, Sofia, Bulgaria; e-mail: Milko@geology.bas.bg 1 submitted 02.07.200 l ; accepted for publication 11.06.2002)

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Abstract. The study of mineral equilibria in the Kp­Alp3-Si02-H20-SO system permits to propose thermo­dynamic oasis for distinguishing the acid-sulphate and adularia-sericite alteration types of volcanic rakes and frequently associated subvolcanic intrusive rocks in the epithermal deposits. The fields of the acid-sulphate and adularia-sericite types of alteration are divided by the fields of the advanced argillic alteration without sulphate­bearing minerals and the sericitiztion in the prognostic paragenetic diagram of the mineral equilibria in the sys­tem studied. This study presents theoritical arguments for the existence of the two end paragenetic incompatible types of wallrock alteration in epithermal deposits.

The advanced argillc alteration of the rocks is subdi­vided of the acid-sulphate and acid-chlorine types of al­teration. The mineralogical synonyms of the acide-sul­phate type are kaolinite-alunite and alunite alteration types and of the acid-chlorine type are kaolinite, pyre­phyllite and dickite-pyrophyllite types wallrock alter­ation. The study of the system is basis for genetic inter­pretations of the advanced argillic, sericitic, greisen, K­silicate and adularia-sericite alteration.

Kanazirski, M. 2003. Thermodynamic basis for distinguishing the acid-sulphate and adu­~ ria-sericite alteration types of rock in epithermal deposits and genetic analogy with the :netasomatites of the formation classification. - Geologica Bale., 33, 1-2; 65-74.

ey words: mineral equilibria, K;P-Al20 3-Si02-H.20-S03 system, advanced argillic alteration; id-sulphate alteration type, actularia-sericite alteration type.

Geologica Balcanica, 1-2/2003 65

"The theoretical and methodological basis of understanding the metasomatic processes could be only the strict and distinct physicochemical models"

)[{apUK06 ( 1969)

Introduction

Acid-sulphate and adularia-sericite types of alterations are discussed as two contrasting types of volcanic rock alterations in epithermal deposits. Detailed studies of these alterations (Hayba et al., 1985; Bonham, 1986; Heald et al., 1987; White and Hedenquist, 1990; Rituba et al., 1990; Remley and Hunt, 1992; Arribas, 1995) allow to be suggested different criteria for their determination such as geological position, struc­tural control, the age of host rocks, depth and temperature of forming, type of fluids, associ­ated alterations, mineralogical characteristics and zonality, ore mineralogy etc. The study of wallrock alterations of the host rocks has a pri­mary importance which is underlined by Hedenquist and Lowenstern (1994): "epithermal deposits could be divided into two main types according to the characteristics of the mineral alterations". No doubt that em­pirically found criteria for determination of both types alterations and genetical informa­tion for their products are very important. The !a~k of theoretical validity for their dividing 1s m contrast to the extremely high level of tra­ditional studies about mineralogy, geological­structural and physico-chemical conditions of acid-sulphate and adularia-sericite types of alterations in epithermal deposits. The com­plexity and diversity of the metasomatical pro­cesses which are in fact physico-chemical pro­cesses needs that together with traditional geological and mineralogical studies to be per­formed as well the methods of physico-chemi­cal modelling (HsaHOB, 1984). Theoretical bases of physico-chemical modelling are de­veloped by Korzhinskii (Kop)I(MHCKMH 1955, 1965; Korzhinskii, 1959, 1970). The fundamen­tal concepts in his studies give a possi-bility to provide thermodynamic modelling of real metasomatic processes. The aim of studies of both contrast types of wallrock alterations from the position of metasomatic phy-sico­chemical petrology is: a) to propose phy-sico­chemical basis for dividing of acid-sulphate and adularia-sericite types of alterati-ons; b) to prove their genetical incompatibility; c) to determine genetical analogies of acid-sul­phate, adularia-sericite and their related types of alterations in formation classification

66

of metasomatites. This aim is reached through application of thermodynamic modelling of processes and studies of mineral equilibria in the system K

20-Al?O -Si0 7-H

20-SO

(Kanazirski and Ivanov,- 1992; Kanazirski~ 1992a) depending on temperature, acidity of the environment and chemical potentials of potassium and sulphate anions.

Equilibria mineral associations as a criteria for determination of wallrock alterations of rocks and formational belonging of metasomatites

Meyer and Hemley (1967) discussing the prob­lem for mineral associations and types of wallrock alterations mentioned that the clas­sification of hydrothermal wallrock alterations based on a group of minerals formed simulta­neously in conditions close to thermodynami­cal equilibria has many advantages compared with the other ones. These authors underline as possible suggestion that mineral phases ob­served in hydrothermally altered rocks reflect the process of rock adaptation to the new con­ditions leading to stable equilibria. The equi­libria mineral associations are taken into ac­count in dividing the types of hydrothermal wallrock alterations in the classification of­fered by Meyer and Hemley (1967). These authors discuss the complexity of the problem and point out for the necessity of precise di­agnostics of different equilibria associations as a basis of the classification and dividing the main types of wallrock alterations.

Equilibria mineral associations (mineral pa­rageneses) are the basis for facial analysis and genetical formation classification of metaso­matites (Kop)I(MHCKMH, 1955; )l(apMKOB, 1968, 1982; )l(apMKOB 11 0MenMIHeHKO, 1978). The categories in the hierarchy order of catego­ries of metasomatites on which the classifica­tion is based are determined by )l(apMKOB et al. (1992) as following: 1) zone of metasomatic column set up by definite mineral paragen­e~is (equilibria mineral association), initial smgle element of the classification; 2) metaso­matic column that is combination of metaso-

matic zones in definite sequence characteri­sing one metasomatic facies, single member of the classification and 3) metasomatic forma­tion as a combination of metasomatic facies, integral member of the classification, main clas­sification unit.

In the present study, and namely, the one of the methods of physico-chemical petrology is used - physico-chemical analysis of mineral parageneses (Kop)l(HHCKHH, 1957) and con­nected to it thermodynamic modelling of the advanced argillization and related to it pro-

esses (Kana-zirski, Ivanov, 1992; KaHa3HpCKH, 1994). The modelling is directly onnected to the traditional petrological

-mdy of hydrothermal alterations of the rocks hrough physico-chemical analysis of natural

mineral parageneses.

Prognostic topological diagram of the state of K20-Al

20

3-Si02-H20-S03 system

_0-A120 3-Si0 -H O-S03 system is chosen as a :nodal system for fuermodynamic modelling of zonal hydrothermal alterations in acid rnetasomatosis of alumosilicate rocks rich in Si0

2•

The influence of K + and SO/ activity 11BaHoB, 1974) as well the influence of the rest :nain physico-chemical equilibria factors such as chemical potentials of H+ and H 0 (T) is : ken into account during the receAt study. Following symbols and theoretical formula of :ninerals are used: Adu - adularia (Ksp - po­- ssium feldspar), KA1Si

30 ; Alu - alunite,

. A..l/S04) 2(0H)6

; And - andalusite, Al2Si0

5;

Co r - corundum, Al20

3; Di - diaspore,

.-\IO(OH); Kl - Kaolinite (dickite),

. -\LSi20;(0H)4; Mu- muscovite (Ser- sericite) - KAl ~AlSi )0 (OH)

2; Prl - pyrophyllite,

.-\l ~Si))J0(0H\; ~- quartz, Si02; "Sericite" in­-ludes tlite and ilite-smectite mixed-layered :nineral (Hayba et al., 1985).

Innert components (Kop)I(HHCKHH, 1957) in · e system studied are Al20 3

and SiO . Vari­~ ility of the multisystem studied is - ? (n = k; - 2- r = 2 + 2 - 9= - 5), where n are the de-- ees of freedom, k - number of innert com-;>onents and r is the number of mineral phases. For the graphical design of the system is nec­osary to gather 126 nonvariant points accord­" g to estimations of Kop)I(HHCKHi1 ( 1957) I - n = - (- 5) = 6 stable points, and the rest of the _ ints are determined through geometrical :nethods.

Monovariant reactions in the system stud­ied are shown in the next table.

Table 1 Mana variant equilibria in the system K 0-Al 0 -SiO -H 0-SO

2 2 3 2 2 3

Reaction N Equation of reactions in ionnic form

2 3 4 5 6 7 8 9

10 11 12 13

14 15 16 17 18 19 20 21 22 23

3Di + K+ + 2SO/' + 3H,O = Alu + 30R 2Ser + 5Hp = 3Kl + 2K+ + 20H' PrJ + Hp = Kl + 2Q 2Ser + 6Q + 2H

20 = 3Prl + 2K+ + 20R

3Kl + 2K+ + 6H++4SO 2·=2Aiu + 6Q + 3H 0 4 2

3Prl +_2K+ + 6H+ + 4S042- = 2Alu + 6Q 6Kl + 2K+ + 4SO/- = 3Prl + 2Alu + 60H' Prl + 2Di + 2Hp = 2Kl 3Prl + 6Di + 4K+ + 40H' = 4Mu + 4Hp Prl + 6Di = And + Hp 3And + K+ +OR+ Hp = Mu + 3Di 2Mu =PrJ+ 2And + 2K+ + 20H' 4And + 2K+ + 4SO/' + IOHp = Prl + 2Alu + 60H' And + 3Q + Hp = Prl 3And + 3Q + K+ + 20H- + Hp = 2Mu Cor+ Hp = 2Di 3Cor + 2K+ + 4SO/' + 9Hp = 2Alu + 60R 6And + 2K+ + 20H- + Hp = 2Mu + 3Cor 2Ksp + Hp =And + 5Q + 2K + + OR 3Ksp + 2Hp = Mu + 6Q + 2K+ + 20H-3Ksp + 6And + 6Hp + 2K + = 5Mu + 2H+ 2Ksp + 5Cor + 2H+ = 6And + 2K + + Hp Ksp + Cor + Hp = Mu

Prognostic topological diagram for the state of the system (Fig.l) is a quantitative scheme for studying mineral equilibria. Mineral para­geneses in the fields of the diagram response tci the back two-mineral zones of metasoma­tites .

Minerals without potassium such as quartz, kaolinite, pyrophyllite, diaspore and corun­dum take part in the reactions of hydratation and de-hydratation (Table 1, Fig. 1) in the field right from the monovariant lines with reac­tions 2, 9, 12, 15 and 19. This allows as tem­peratures of the reactions to be accepted the values of temperatures determine during the investigation of the system AI 0

3-Si0

2-H

20

(Remley et al. , 1980) and to graduate diagram on temperatures.

The diagram give a possibility to follow the value of activities of the entirely mobile com­ponents K +, H +, OH-M SO,t for obtaining hi­mineral pa rageneses of the mner zones in min­eral deposits.

67

0\ 00 roc

(-a.,,o)

700

600

500

400

350

300

Z50

I O+Ksp I

s9o•c @

Q l(sp MY Ol

Q Ksp Cor

Q MU

I O+Mu I

..:_,o~ ~,1-o

e,~:}?. Mu

(){ a (Sor) Dl __.__

CSG3~~J ,;_,o~

,,;_,.._o. . c.'

'?e{'

la-t.O+Serl

Q Prl

MU a IS.rl Di ...............

Prl

Kl p,<.

And ~or -Mu

JQ-Kll

Kl .O.lu - ..:,,o~ .

"\'-'1.0 ~sq'

I a-t · I

JQ-Andl Q And CDf" ~

0

Mu Cor ~ ~ '--'0 ~0

'-'"e ~ •'"'. @ '"" "'~ •'' ·-M•! .oo _.--:;-- •''"

@ And+Ce =Prl

@ And~ =Prl+Alu

Alu

jQ-AtuJ

zoo 19 a sol- · (-tg °K+ ) a'oH- aH+

Fig. l. Diagram of the phase equilibria in the system K 0-Al 0 -SiO -H 0-SO in dependence of the temperature and ratio of . . . f 1 h d . ( 2f 2 2 3 2 2 3 • d actlvtttes o su p ate an hydroxille amons a

4 • a ). l-23 numbers of monovariant lmes an reac

S9' 20H" tions on the boundaries of the bivariant fields . Adu - adularia; Cor - corundum; Di - diaspore; Kl - kaolinite; Ksp - K -feldspar; Mu - muscovite; Prl - pyrophyllite; Q - quartz; Ser - sericite. "Sericite" includes illite and illite - smektite mixed-layered mineral (Hayba et al., 1985).

Paragenetical incompatibility of acid-sulphate and adularia-sericite types of alterations of rocks in epithermal deposits

The two end types alterations in the epither­mal deposits are determined by the diffe­rence in physico-chemical environment of their appearance or strictly to say by the differen­ces in values of some of the equilibria factors - T, P, activities of K +, H +, SO/, OH- that are important for development of a definite pro­cess of wallrock alteration: advanced argillic, sericitic, moderate argillic, propilitic, potas­sium metasomatosis in porphyry copper and epithermal deposits (Meyer, Remley, 1967). Acid-sulphate type belongs to the advanced argillic alteration and adularia-sericite one belongs to the potassium metasomatosis in the epithermal deposits.

The fields in the moderate and low-tempe­rature area of the diagram are the basis for the physico-chemical validity in separation both end types of alterations of magmatic rocks - acid-sulphate and adularia-sericite and connecting them intermediate types al­terations typical for the epithermal deposits

The present study gives reasons for theo­retical existence of both contrast types taking into account the specific mineral parageneses of their typical minerals and transitions be­tween them (Kanazirski, 1992b ).

The para genetical diagram (Fig. 1) is redu­ced to a simplified scheme (Fig. 2) of the fields of acid-sulphate and adularia-sericite types of alterations with transitional between them fields of sericite and advanced argillic acid­chlorine (without sulphate-containing miner­als) alterations.

Following fields and reactions of transitions between them in the lines of monovariant equilibria could be distinguished during the decreasing of a

42- (of the acidity) and in-

so creasing of a +:

Field lA ot the advanced-argillic acid-sul­phate type alteration, separated from the nearby fields IB and II with monovariant line 1 of the reaction (Tabl.):

(1) 3AlO(OH) + K+ + 2SO z- + 3H 0 = (diaspore) 4 2

KAl (SO) (OH) + 30H-3 . 4 2 6

(alumte) Field IB of advanced-argillic type of alter­

ation without sulphate-containing minerals with reactions of the transition to the field II in monovariant lines 2 and 4:

(2) 2KA1 (AlSi )0 (OH) + 5H 0 = 2. 3 10 2 2

(sencite) 3Al Si 0 (OH) +2K+ + 20H-

(7- 21.5 . ) 4 kao mite

(4) 2KA1 (AlSi )0 (OH) + 6Si0 + H 0 = z 3 10 2 2 2 (sericite) (quartz)

3Al Si 0 (OH) + 2K++ 20H-2 2 IQ . 2

(pyrophylhte)

Field II of the sericite type of alteration lim­ited by field III of adularia-sericite type al­teration with monovariant line 20 of the reac­tion:

(20) 3KA1Si 0 + 2H 0 = ~ 8 2

(adulana) KAl (AlSi )0 (OH) + 6Si0 + 2K++ 20H

2 ~ 10 2 2 (sericite) (quartz)

Alunite and adularia do not form equilibria mineral associations. The fields in which alunite and adularia are stable are separated by the fields II and IB for which it is typical intermedi­ate type of alteration (mainly sericitic and less advanced-argillic alteration - pyrophyllitization and kaolinitization ( dickitization).

The most distinct difference in the values of activity of the sulphate anions and the acid­ity of the environment that determines two polar types of mineral parageneses of those alterations is found in acid-sulphate and adu­laria-sericite types of alterations .

The study offers theoretical basis for the sepa­ration of the two contrast by physico-chemical conditions of realisation acid-sulphate and adu­laria-sericite alterations and it is valuable con­tribution in understanding the epithermal de­posits which are in the scope of interest and are widely commented in the literature.

69

--...1 0

T°C ( -aH1o)

700

6001

sool 400

350 .--

/ . .////~/ ~ / // / / / / //'/ ~Di /// %Adu

3001-~ Ill ////~·· ~'/ ,/ / ~ <_Q.< /' ,7 ' ,\}:

250

200

IA ··. .. i .

' '

1: 1·. J.j]A [ s JJJ I I 11]8 E:ZZJIII

I

0 so3_- ( . °K• ) Lg-2-- - l g--

0ow aw ..

I

Fig. 2. Scheme of relations between acid-sulphate and adularia-sericite types of alterations. 1 - 20 number of reactions in monovariant lines. Types of alterations: A- advance-argillic type (acid-sulphate); IB- advance-argillic,type (acid-chlorine); II- sericitic type; III -adularia-sericite type

I I

I

Partition of advanced argillic and mineralogical equivivalent of acid-sulphate type alteration of rocks

Metasomatites of the facies of secondary quartzites formation without sulphate-con­taining minerals are formed at increasing aci­dity in acid-chlorine environment or low ac­tivity of sulphate anions in hydrothermal so­lution (Fig. 1, left from the line ofmonovariant equilibria Di = Alu, reaction 1 ).

This is a reason to search and implement a genetical analogue in classification ofwallrock alterations of rocks which is acid-chlorine type related to the advanced argillic alteration. Mineralogical equivalents for low to moder­ate temperature conditions of the acid-chlo­rine type should be quartz-kaolinite, pyrophy­llite and pyrophyllite-dickite (kaolinite) type of alterations. They correlate with low to mod­erate temperature acid-sulphate type of alter­ations.

The partition offered leads to development of two sub-types in the advanced argillisazion - acid-sulphate and acid-chlorine ones. Meta­llogenic specialisation of acid-chlorine type needs additional sdudies on the basis of lite­rature references while the ore-generating im­portance of acid-sulphate (kaolinite-alunite) ype is underlined in publications of leading

authors (Hayba et al. , 1985; Bonham, 1986; Heald et al. , 1987; Hedenquist, Lowenstern, 1994).

Alunite-bearing mineral parageneses typi-al for the acid-sulphate type alterations are

formed along the increasing activity of the sulphate anion (right from monovariant line Di = Alu, reaction I) at moderate and low temperatures.

Mineralizations that are products of two ontrast abyssal fluids having respectively

neutral and acid characteristics could be dis­inguished on the basis of the characteristics

of many epithermal deposits (Hayba et al., 1985; Heald et al. , 1987; Hedenquist, 1987).

Hydrothermal alterations due to the inter­action of contrast fluids and rocks a re de­scribed in different manner. Heald et al. (1987)

sed term "adularia-sericite" and " acid-sul­phate" types of alterations. Bonham (1986)

escribed them as "low-sulphured" and "high­-ulphured". Berger, Henley (1989) offered replacing of "acid-sulphate" with mineralogi­al term "alunite-kaolinite" on the basis of the

separation of "adularia-sericite" type. He­denquist (1987) suggested the terms "low­sulphidation" and "high-sulphidation" that reflect the oxi-reductional state of the sulphur in the reacting fluid. These terms we accept as the most suitable because they reflect the acidity of mineralizing fluids and they are based on chemical principle that is in the ba­sis of all widely accepted classifications of wallrock alterations and metasomatites.

"Acid-sulphate" and "adularia-sericite" ty­pes are the most often used in the literature. That is why the mineralogical equivalent of "acid-sulphate" is of interest for using the united mineralogical basement of the classifi­cation. To determine this equivalent it is used the equilibria mineral associations of alunite with kaolinite in the paragenetic diagram (Fig.1).

The paragenesis alunite+kaolinite in the hi­variant field of quartz-kaolinite facies of sec­ondary quartzites formation is typical for the aluminium rich compositions. They are formed during the decrease of the fluids acidity after forming alunite-bearing parageneses and fol­lowing precipitation of AI rich silicate miner­als or hydroxides (kaolinite, pyrophyllite, di­aspore) in conjugated forming ()KapHKOB, 0MenMIHeHKO, 1978) in the system studied.

Paragenesis of alunite with quartz is formed in the field right from the monovariant line with reaction 5 at high activity of the sulphate anions specific for the acid-sulphate type of alteration. Ore mineralizations in acid-sul­phate type of epithermal gold deposits are often related to the most intensively altered wallrocks with quartz and quartz-alunite veins and zones. This is a reason in such types of epithermal deposits in cases when in the paragenesis kaolinite is absent as a synonym of acid-sulphate type to be used mineralogi­cal equivalent "alunite type" with preference before "alunite-kaolinite" type (Berger and Henley, 1989)

It is useful as mineralogical analogue of the acid-sulphate type alteration to be accepted except alunite-kaolinite type also alunite type alteration. Such a partition is supported by the relatively limited possibility for simultaneously forming of the typical for acid-sulphate type epithermal deposits triple-mineral paragenesis - alunite + kaolinite + quartz for the silicate line of metasomatic mineral forming with excess of quartz. This paragenesis is stable only in the line of monovariant equilibria with reaction 5 of the paragenetical diagram.

71

Genetical analogs in classification of wallrock alterations and formation classification of the metasomatites

A genetical analog of the advanced argillic alteration of rocks in the metasomatites cla­ssification ()KapHKOB, OMeJIMIHeHKo, 1978, )KapHKOB, 1982 ) is the secondary quartzite formation. Metasomatites of quartz-alunite facies of the secondary quartzite formation in this classification are corresponding to acid­sulphate type alteration (Kanazirski, 1992b; KaHa3HpcKH, 1994). Adularia-sericite type al­tered rocks have for their analog metasoma­tites from the formation quartz-sericite-adu­laria metasomatites according to the classifi­cation of )J(apHKOB, 0MeJib5lHeHKO (1978).

Chosen coordinates of the diagram (fig.l) include factors of equilibria that lead to the effects of changes in the process of interre­action of the hydrothermal fluid with the wa11rocks.

Ksp typical forK-silicate type alterations (for­mation quartz-feldspar metasomatites) at in­creasing the acidity of the environment or Adu at lower temperatures (typical for adularia­sericite alteration) are destroyed (reaction 22 and 20) with fom1ing of stable at the new con­ditions And and paragenesis Q + Mu, typical for the greisen alteration (greisen formation) or in Q + Ser for lower temperatures , typical paragenesis for sericite alteration (quartz-sericite metasomatite formation).

Muscovite - (sericite) - bearing parageneses are not stable at increasing acidity and in­stead of them are formed minerals and min­eral parageneses of advanced argillic alter­ation (secondary quartzite formation).

The genetic analogs from the cJassification ofwallrock alterations (Meyer, Remley, 1967) and those from the formation classification of metasomatites ()J(apHKOB, 0MeJib»HeHKo, 1978; )J(apMKOB, 1982) are charactersised by specific for the inner zones of metasomatic column of formations bi-mineral parageneses.

Three formational types are observed at high temperatures: secondary quartzites (an­dalusite-pyrophyllite and quartz-andalusite facies), greisen and quartz-feldspar metaso­matites. Metasomatites of these formation could be formed and equivivalent p - T con­ditions in sequence reflecting the inrcreasing of ratio aK)aH+ in the solution. Three areas are distinguished respectively: area of stabil­ity of the secondary quartzites, fields of gre­isen and quartz-feldspar metasomatites.

72

These formations are in relation association - paragenic formations after )l{apMKOB ( 1982). The three types of experimental columns, re­sulting from the interaction of the acid K-bear­ing solutions and granite and other rocks (3apaiicKMi1, 1989), are the experimental con­firmation of the relation supposed. The col­umns correspond to the three natural meta­somatic formations: secondary quartzites, gre­isen and quartz-feldspar meta-somatites. Metasomatites of those three types wallrock alterations after 3apaiicKMH (1989) could be formed at equal p - T conditions changing se­quentially as a result of increasing of the ra-tio mKc/mHcJ ~n the fluid. . .

Metasomatltes from the quartz-senc1te for-mation are located in the greisen field, these from the quartz-adularia at sericite formation (after )l{apHKOB, 0MeJih5lHeHKo 1978) are lo­cated in the field of quartz-feldspar metasomatites. )J{apHKOB (1982) suggested that quartz-adularia-sericite metasomati tes are formed in the conditions of small depth (subvolcanic facies), and quartz~feldspar metasomatites are formed at significant depth (abyssal facies). The diagram gives possibility to predict some similar physico-chemical en­vironment for the temperature and presure during the forming of the formations of quartz­adularia-sericite metasomatites, quartz­sericite metasomatites and intermediate to low temperature facies for secondary quartz­ite formation - diaspore-pyrophyllite, kaolin­ite-pyrophyllite and quartz-kaolinite facies.

Conclusions

The study of mineral equilibria in the system K 20-Al,0 3-Si02-H20 -SO provides a model for physico-chemical conditions of advanced argillization of the rocks. It allows to trace relations between advanced argillic alteration and sericitic, greisen, K-silicate and adularia­sericite alterations of rocks.

A thermodynamic basis for offered partition of two contrast types of wallrock alterations in the epithermal deposits - adularia-sericite and acid-sulphate types and it is proved their paragenetical incompatibility. Simplified scheme (fig.2) design from the prognostic topological diagram of the status of the system ~0-Al203-Si0,-H O-S03 (Fig. 1) shows that the fields of acid-sulphate and adularia-sericite types of al­teration are separated by the fields of advanced argillic alteration without sulphate minerals and those of sericite alteration.

The advanced-argillic alteration includes

acid-sulphate (alunite-kaolinite) type as well acid-chlorine (kaolinite, pyrophyllite, dickite­pyrophyllite) types alteration without sulphat­bearing minerals. Acid-sulphate type alter­ation in epithermal deposits has as its miner­alogical synonym kaolinite-alunite as well as alunite types of alteration. The paragenesis quartz-alunite typical for the alunite type of alteration is a stable one in a large bivariant field at increasing acivity of sulphur anions. The equilibria mineral associations quartz + kaolinite + alunite and kaolinite + alunite de­termining kaolinite-alunite type of alteration are stable respectivly in the line of monovari­ant equilibria with reaction 5 and in the redu­cted in area field between monomineral lines with reactions 5 and 1. The paragenesis ka­olinite + alunite is characteristic for the rich in aluminia compositions of the system that are realised at relatively decrasing of the acid­ity of the environment. The forming of rich in alluminium minerals and probably precipita­tion of the ore mineralisation in acid-sulphate type epithermal deposits is related with de­creasing of mentioned above acidity of the hy­drothermal system.

The sudy offered is a basis as well for ge­netical interpretations for some main types of wallrock alterations: advanced-argillic seri­citic, greisen, K-silicate and adularia-sericitie that have an genetic analog the formations secondary quartzites, quartz-sericite metaso­matites, greisen, quartz-feldspar metasoma­tites and quartz-adularia metasomatites.

The study was financied by The National Fund "Scientific Investigations'' (projects H3-l/91 and H3-437/94)

Acknowledgments: The author is sincerely thankful to academician V. A. Zharikov (Insitute for experimental minera logy, Russian Academy of Sciences) for his guidance and support in addaptation of physico-chemi-a l basis of the mineral parageneses analysis and also to

Prof. I. P. Ivanov from the same Institute for his consulta­tion on facial analysis and cooperation in theoretical modelling of the advanced-argillic alteration. I am obli­gated to the Commission of the European Union for the grant for three months specialisation on the project "Ad­,·anced-argillic - acid sulphate alterations in epithermal type of deposits in Spain and Bulgaria" . Realisation of this project was possible with the support of the Adminis­tration of Insitute of Earth Sciences "Jaume Almeira" (High Coincil for Scientific Investigation, Barcelona) that provided me an excellent facilities for work for which I express my sincery thanks.

I 0 Geologica Balcanica, 1-2/2003

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