649

The C anadian Mine ralo g istVol.32, pp. 649-665 (t994)

SEDIMENTARY.ROCK.HOSTED DISSEMINATED GOLD MINERALIZATIONIN THE ALSAR DISTRICT. MACEDONIA

TIMOTHYJ. PERCIVAL*

Nassau Limited, 1380 Greg Street, Suite 2lg, Sparla, Nevada gg43l, U.SA.

ARTHUR S. RADTKEcougar Metals Internetional, 2296 oberlin street, palo Alto, caWrnia 94306, u.s.A.

ABSTRACT

Disseminated Au-As-Sb-Tl-Hg mineralization in the AlSar district, Macedonia, is hosted in Triassic carbonate rocks,Tertiary interbedded volcanic tuff and dolomite, and Pliocene felsic tuffs. Hydrothermally altered, porphyritic, hypauyssaligneous rocks intrude both the Triassic and Tertiary rocks. Ore fluids followed major structural zbnes, trigtr-angie fiultsand flat-lying stratigraphic feafures in the host rocks. Results from radiometric dating of the tuffs (<4.5-5.dMa)"ttrat hostsome of the Au-As-Sb-T1-Hg mineralization place an upper limit on the age of thai hydrothermal event and indicate thatit is Pliocene iq age. Hydrothermal fluids introduced large amounts of silic4 iron and sulfur into, and removed calciumand magnesiurn from, the carbonate and volcanic rocks. These fluids also introduced arsenic, antimony, thallium, mercgry,bgunl1$ gold, resulting in the formation of marcasite, pyrite, realgar, orpiment, stibnite, cinnabar, iorandite and vanousother Tl-bearing sulfosalt minerals, plus barite and micrometric to submicrometric particles of gold. Base-metal andsilver concentrations are characteristically low, as is the case in the type locality for sedirientary-rock-hosteddisseminated gold mineralization, the carlin deposit, Nevada. Al5ar is the iirst exampie of this type of lrecious metalmineralization to be described in Europe; it differs significantly from the typical base-metal- and silver-dominatedYugoslavian ore deposits.

Keltwordl, disseminated gol.d, sedimentary-rock-hosted, Au-As-Sb_Tl-Hg mineralization, pliocene, realgar, stibnire,Tl-sulfosalts, AlIar, Macedonia.

Sovruarnr

NorJs signalons la pr6sence d'une min6ralisation d Au, As, Sb, T1 et Hg dans le district de Allar, en Mac6doine. Elley est d6veloppde dans une s6quence de roches carbonatdes triassiques, ainsique dans des tufs volcaniques et des dolomiestertiaires et des.tufs felsiques pliocdnes. Des roches ign6es sub-volcaniques, porphyritiques et alt6r6ei recoupent le socletriassique et tertiaire. La phase-fluide responsable a profito de zones struciurale; mpottrot"., des failles i pendage abrupt et!e1 1qc1ur9s stratigraphiques horizontales des roches h6tes. Les r6sultats d'une datation radiom6trique des tufs-min6ra1is6s(:4.:-5.9 Ma) limitent l'dge de la nin6ralisation, qui serait donc pliocdne. La circulation hydrotheimale a men6 i I'intro-duction-de quantitds importantes de silice, fer et soufre, et le lessivage de calcium et de magn6sium des roches volcaniques etdes carbonates. Elle a aussi impliqu6 les 6l6ments As, Sb, Th, Hg]Ba et Au, et la formation de marcasite, pyrite, rEalgar,orpiment, stibine, cinabre, lorandite et des sulfosels portews dJTl, en plus de baryte et de particules microm6triques isub-micromdtriques d'or. Les teneurs de m6taux debase et d'argent sont faibles,iout comme dans les gisements d'ordissdmine du type Carlin, au Nevada, oi les roches hdtes sont s6dimentaires. Alsar serait donc le premier exernile europeen dece type de min6ralisation en m6taux pr6cieux, qui diffdre sensiblement des gisements yougosiaves typiques, i dominancede m6taux de base et d'argent.

(Iraduit par la R6daction)

Mots-cl€s: or diss6min6, roche h6te s6dimentaire, mindralisation en Au-As-Sb-Tl-Hg, pliocbne, r6algar, stibine, sulfosels dethallium, Al5ar, Macddoine.

*hesent address: Consulting Geologist, 3645 Black Forest Lane, Sparks, Nevada g9434, U.S.A.

650 TIIE CANADIAN MINERALOGIST

LrtrnonucnoN

The Al5ar district is located in the Kozuf Mountainsof southern Macedonia (Fig. I ), a republic in what wasformerly Yugoslavia. The district was extensivelymined from 1880 to 1908 for antimony (grade 24wt.7o), arsenic (l wt.Vo) and thallium (Jankovid 1982).

A program of regional reconnaissance for gold wasconducted in Macedonia and in other Yugoslavianrepublics by Nassau Limited in 1986. This workiniluded geological and geochemical exploration ofthe Al5ar district and resulted in the delineationof anomalous concentrations of gold (>1.0 g/t) inareas of arsenic, antimony and thallium mineralization,

Ftc. 1. Location and geological map of the AlSar mining district, Macedonia,

DISSEMINATED GOLD MINERALZA'TION. MACEDONIA 651

and also in widespread silicified zones. This paper pre-sents the results of continuing studies of disseminatedgold mineralization in the Al5ar district by Nassau Ltd.

Initial results suggest that physical and chemicalfeatures of the mineralization at Al5ar are similar tothose that characterize many of the sedimentary-rock-hosted, disseminated (Carlin-type) gold depositi of thewestern U.S. (Radtke & Dickson 1974,Bagby &Berger 1985, Percival et al. 1988). We believeihatAl5ar is the fust example of this type of gold mineral-ization to be recognized and described from Europe(Percival & Rad&e 1990).

Gsot-octcAt- Sarrwc

Regional geology

The Al5ar district occurs within the yar:du Zane, aregionally extensive (700 x 80 km), north-northwest-fiending tectonic suture marking the location of anextinct subduction zone (Arsovski & Ivanov 19771rit.The zone is comprised of ophiolitic rocks and a com-plex mdlange of metamorphosed tectonic blocks ofPrecambrian(?) through Mesozoic age in a cataclasticmatrix. Tectonism was most intense during the AlpineOrogeny, but occurred from Upper Triassii toPaleogene time (Arsovski & Ivanov 19776. Gabbroand granodiorite were intruded along and within theVardar Zone during the Jurassic (Arsovski & IvanovL977 a, Spray et al. 1984).

In Macedonia, the Vardar Zone separates twoupper-greenschist-facies metamorphic complexes:1) the PelagonianZone (to the west) and 2) the Serbo-Macedonian massif (to the east). Their borders aredefined by tectonically emplaced masses of serpenti-nite, ophiolit ic pil low lavas and metadiabase(Arsovski & Ivanov L97ib). Deposition of trench-related Triassic clastic and carbonate sediments in theVardar Zone was followed by Cretaceous and Tertiaryflysch, carbonate rocks and concluded by depositionof Neogene molasse in lacustrine basins (Arsovski &lvanov 1977b).

Regionally exrensive Pliocene (1.8-6.0 Ma) felsicto intermediate volcanic rocks of calc-alkaline to alka-line affinities covered much of the Kozuf Mountainsas tuffs, flows and coeval subvolcanic intrusions(Kolios et al. 1980). Radiometric dating of the lowertuffs near AlSar yielded K-Ar biotite ages of 5.g Ma(8. Boev, written comm., 1992). Several ring-radialfeatures, suggestive of caldera-type volcanism, havebeen identified recently on Landsat imagery and byregional geological reconnaissance near the Albardistrict (Jankovid & Jelenkovid 1990.) An enrichmentin large-ion lithophile elements and light rare-earthelements and high init ial strontium ratios(0.7088-0.7090) in the tuffs suggest crusral conrami-nation of a mantle-derived parent magma (Kolioset al.1980, Boev 1990a, b).

Local geology

The western flank of the district is bordered bytectonically emplaced slivers ofJurassic mafic igneousrocks and serpentinite within a cataclastic matrix, typi-cal of the Vardar Zone. Regional geological mappingby the Macedonian Geological Survey (1980) anddetailed mapping by T. Percival indicate that thegeology of the district is composed of two distinctchronostratigraphic units: 1) Mesozoic trench sedi-ments, unconformably overlain by 2)Tertiary volcanicand carbonate rocks @g. 1).

The Mesozoic rocks include greenschist-faciesmarble, dolomite and minor amounts of schist, and areassigned a Triassic age by the Macedonian GeologicalSurvey (1980). Marble is the most abundant pre-Tertiary rock (Fig. l) and consists of gray-white tobluish gray, fine- to medium-grained equigranularcalcite, with minor dispersed and interbedded detritalmaterial. The dolomite is gray-white to tan, fine-to medium-grained and is locally recrystallized todolomitic marble. Schist is exposed along the easternmargin of the district and is widely variable in compo-sition; quartz-sericite, chloritic and graphitic schistsare the most common.

The Tertiary unit directly above the unconforrnity isa massive gray-white to tan, fine-grained, micritic tosparry dolomite (Fig. l). Variable proportions ofwater-laid volcanic ash and tuffaceous debris are dis-persed throughout, and locally intercalated as bedswithin the dolomite, clearly demonstrating a volcano-sedimentary provenance. The dolomite probably wasdeposited in a fresh-water lacustrine basin, similar todolomite described elsewhere in Yugoslavia (Ilich1974, Fallick et al. l99l). These basins also containdolomite and magnesite deposits of hydrothermal-sedimentary origin spatially associated with Tertiaryfelsic igneous rocks. Basin development occurred as aresult of Miocene extensional tectonism in areasunderlain by ultramafic and carbonate rocks (Illich1974,Fallick et al. I99l\.

Tertiary (Pliocene) tuffaceous volcanic and vol-caniclastic rocks overlie the dolomite and Mesozoicbasement rocks. The lower tuff unit is dominantlylatite in composition, with lesser amounts of andesite"trachyte and quartz latite. The latite is composedof light-colored ash, crystal and crystal-lithic tuff,and fuffbreccia. Petrographic studies indicate that thetuffs contain varying amounts of sanidine, biotite,lesser quartz, hornblende and clinopyroxene phe-nocrysts in a gray aphanitic, crystalline and glassygroundmass.

In many areas these tuffs have undergone intenseargillic alteration and locally are mineralized. Theupper tuff unit is comprised of debris flows, felsic ashtuff and thin-bedded lacustrine volcaniclastic deposits.Hydrothermal alteration is weakly developed tononexistent in the upper unit.

652 IT{E CANADIAN MINERALOGIST

A hypabyssal intrusion cuts across both the Triassicand Tertiary rocks and is exposed as small outcrops(Fig. 1) and in underground workings in the antimonymines. The intrusion is porphyritic and containsphenocrysts of quartz, variably altered sanidine'biotite, hornblende and, rarely, clinopyroxene in apyrite-bearing, fine-grained crystalline groundmass.Pervasive hydrothermal alteration throughout thedistrict makes identification of rock types difficult inmany areas. Several subvolcanic intrusions ranging incomposition from latite to andesite occur within 1 to3 km south and west of the intense district-wide enve-lope of hydrothermal alteration. These intrusions areweathered or weakly altered and exhibit textural andmineralogical features, including disseminated pyritemineralization. similar to features of intrusive rocks atAl5ar.

Structural geology

Three major steeply dipping sets of structures withdistinct trends occur in the Al5ar district: l) north,2) northwest (N40-55W), and 3) northeast (N35-50E)(Fig. 1). The structures are defined by sheared andtectonically brecciated wallrocks, gouge zones, juxta-posed stratigraphic units, topographic discontinuities,variations in intensity of hydrothermal alteration, andsulfi de mineral deposition.

The river, south and immediately west of the anti-mony mines (Fig. 1), is underlain by a major north-trending structural zone (River Fault Zone). This zoneis defined by a major topographic discontinuitybetween extensively altered and mineralized cliff-likeoutcrops on the east, in contrast to glacial-till-coveredrolling hills oflow relief on the west. Because of size,amount of movement, pervasive gouge and shearedmatrix, and overall north-south trend, this structuralzone probably is related to and formed by strike-slipmovement along the Vardar zone (Smith & Spray1984). The northwest- and northeast-trending struc-tures formed as dilational conjugate sets of faults inresponse to the north-south shearing. These faults arethe locus of intense fracturing and brecciation distal tothe main shear-zone and served as open spaces formovement of hydrothermal fluids through reactivecarbonate and tuffaceous rocks.

ALTERATION

Hydrothermal alteration features are developedin each of the rock units and are aerially extensive(1 x 2 krn) throughout the Al5ar district. Intensity ofalteration is locally variable and ranges from weak tointense. Types ofalteration include: 1) decalcification,2) silicification, 3) argillization, 4) veining, 5) dolomi-tization and 6) supergene alteration.

The hydrothermal activity apparently occurred afterdeposition of the lowermost tuffs and emplacement of

the subvolcanic intrusions, but prior to the deposition ofthe unaltered tuffaceous volcanic rocks and lacustrinesediments that overlie the altered and mineralizedrocks. The unaltered tuffs and sediments are exposed onthe northem and southem margins of the Al5ar district.

Decalcification

The process ofhypogene alteration characteristic ofsedimentary-rock-hosted precious metal depositsinvariably begins with decalcification of the carbon-ate-rich host rocks (Bagby & Berger 1985, Percivalet aI. 1988\. Decalcification involves the removal ofcalcite and dolomite from the sedimentary host-rocksby weakly acidic solutions (in a pyrite-stable field),resulting in increased porosity and permeability.Carbonaie removal is accompanied by deposition ofreplacement silica and the formation of mineralassemblages typical of argillic alteration.

At Al5ar, decalcification affected both the Triassiccarbonate rocks and the Tertiary dolomitic rocks. Inboth units, intense silicification and argillic alterationare common and exhibit a lateral transition throughvariably bleached and partially decalcified rocks tofresh unaltered rock.

Sanding, a second style of decalcification that lacksaccompanying silicification, is a common featuredeveloped in the Tertiary dolomite. Sanded dolomiteexhibits varying degrees of competence and commonlyretains primary textural features. Intragranular corro-sion by hydrothermal fluids removed the fine-grainedmahix, leaving predominantly granular dolomite sand,disseminated iron oxides, iron oxide stain, and crustsand stains of an unidentified greenish to yellowsecondary mineral. Similar textures have beendescribed in altered dolomite at the Windfall sedimen-tary-rock-hosted gold deposit in eastern Nevada(Nolan 1962).

Silicification

Extensive hydrothermal silicification occurs in botipre-Tertiary and Tertiary rocks within and south of theantimony deposits at Al5ar. The intensity of silicifica-tion varies from weak to total replacement (asperoid)in carbonate rocks of all types' Most of the silicifica-tion in dolomitic rocks is very intense, reflecting theirreactivity toward this type of alteration.

The silicified rocks are products of infoduction ofhydrothermal silica and replacement within and alongfiults and fracture zones of varying orientations(Fig. l). The silica-bearing fluids also moved from thetectonically prepared zones outward into stratigraphichorizons with favorable permeabilities and chemicalproperties in both pre-Tertiary and Tertiary carbonateand volcanic rocks. End products include both weakpervasive silicification and micro-scale stockworks infracture zones.

DISSEMINATED GOLD MINERALZATION, MACEDONIA 653

The jasperoids arc gray to black in color and consistof fine-grained, dense microcrystalline quartz thatthoroughly masks the textures of the carbonate host-rock. Features such as breccia textures, quartz veinsand quartz microveins are common. Disseminated sul-fides (pyrite, marcasite and rare arsenopyrite) occurwithin massive silica, whereas the more coarselycrystalline sulfides (stibnite and realgar) and theiroxidation products (e.9., stibiconite, cervantite,pararealgar, e/c.) occur within the abundant vtrgs, openspaces and between breccia fragments. The silicifiedrocks and jasperoids at A15ar are similar to those thatformed by hydrothermal alteration in sedimentary-rock-hosted disseminated precious metal deposits ofthe western U.S., as described by numerous authors,including Bagby & Berger (1985), Bakken & Einaudi(1986) and Percival et al. (1988).

Although pervasive development of jasperoid(especially in dolomitic rocks) is common in the AlSardistrict, much larger volumes of rocks of all types areonly weakly to moderately silicified. These rocksare lighter colored and contain varying proportions ofhost rock affected by argillic alteration mixed withintroduced silica. This type of alteration is mostcommon in the tuffaceous volcanic rocks; in both thecarbonate horizons and the tuffaceous volcanic rocks,relict host-rock textures are commonly preserved.Quartz microveinlets are ubiquitous. Sparsely distrib-uted wgs are lined with drusy quartz and iron oxides,minor amounts of disseminated pyrite and marcasite,and rare crystals of stibnite and realgar. The quantityof metallic minerals in these less intensely silicifiedrocks usually is markedly less than that found in thejasperoids.

Argillic and sericitic alteration

Both volcanic and sedimentary rocks are altered toargillic and sericitic assemblages. The alteration isaerially extensive and is of varying intensity.Tuffaceous rocks are intensely altered close to faultsand silicified zones, and the argillic and sericitic alter-ation in the tuffs is more widespread and intense thanit is in the sedimentary rocks.

The moderately to intensely argillized rocks consistof varying proportions of fine-grained light-coloredclays, sericite, qvartz, iron oxides (l imonite >hematite), jarosite, gypsum, calcite, siderite, sulfides(marcasite, pyrite, realgar, orpiment), a variety ofthallium-bearing sulfosalt minerals (Table 1), andunidentified secondary minerals containing Fe + As -rTl.

Detailed studies to determine the mineralogicalcomposition of the clay-bearing suite(s) of alterationminerals are in progress. Preliminary results of X-ray-diffraction studies of alteration assemblages suggestthat the rocks affected by argillic alteration consist ofvarying proportions of the following minerals:

TABIT 1. ETPOGENE SI'I.FIDB AND SUISOSALT MINERAIS'A$AR DlsrRrcr, MAGDoNIA

Sdfd. ni@l c@pdid@

Aeol8dsBrydtsChabaM@iIoorpld€dP9tstoeBtrten{IgasdffiB@EdibrI6@dlb.Paryircrldlo.Pt@F@IftrtRs8dfle'R!hli!e.Slrod9.VltaiL.

r Ttpg l@l!|[ A$a Disd(t. Ma€doda; tydct @ is eltin &t &altod @ @d

6.d@Dld@,l. Retatiip ahndw of Bi@rs: * At@d8r' C C@ & R@

2, Motlods oed b id@ &o s$dr @d slfc€ft mi@ls A. @grscoDilatry e@Cz$l&

h hed sEd@ B: EiodPd h Ftrhod se{d@ md Fllsh6d &tn 8e'd@ C X-Ey

dleadd: D et@-dtqsFrcbo @lyds SJ: S. J@kGdt' Elb officd@ f99l'

F€AsS c(N!Fe)S, Rcss cF*: AA&s: AAg/trTq RF6" IAsS A$f. ATAr"S" RTlArs, cT(Sb,;s),S" RIBqrgg RM , RTlr$.AssS? RTlE8As3S6 Rf,PS.$y'8sqo R

B , CsJ.B , CA B , CA B , CDA , B , c ' DA , B , CA " B

A , B , C

c

kaolinite, sericite, illite-group minerals (includingceladonite), hydromicas, pyrophyllite, subordinatechlorite, ephesite (Na, Li mica) and relict(?) biotite.

Zones of argillic alteration at Al5ar typicallyenvelop hydrotherrnally silicified zones in both verti-cal and lateral directions. The intensity of argillicalteration decreases, and the mineral assemblagesvary, with increasing distance from the silicifiedrocks. Intensely argillized kaolinite-rich rocks (e.9"qlrartz + pyrophyllite + kaolinite + sericite, quartz +kaolinite t calcite and quartz + kaolinite + illite) occurclosest to silicified and sulfide-mineralized rocks.These assemblages are bordered by and variablymixed with sericite-rich zones comprised of quartz +sericite + pyrite (+ illite + kaolinite) associated withrealgar and marcasite. These zones in turn grade out-ward into weakly to moderately "argillic-altered"rocks containing assemblages of quartz + illite +calcite + dolomite and quartz + montmorillonite -r

chlorite t hydrobiotite (in tuffacous rocks) over dis-tances of tens of meters. Similar assemblages of alter-ation minerals and spatial relations occur in Carlin-type deposits of the westem U.S. (Radtke & DicksonL974,Percival et al. 1988, Berger & Bagby 1991).

Veins

Veins are relatively uncommon and sparse in theAl5ar district. Quartz veins are most abundant withi:lthe jasperoids and silicified rocks and occur as small(<1-5 mm) gray or colorless veins or microveinlets.Thicker veins qpically contain minor amounts of dis-

654 THE CANADIAN MINERALOGIST

seminated pyrite, marcasite, crystals of stibnite andrealgar and their alteration products, formed by super-gene oxidation. Veins greater than 0.5 cm wide arerare. Some late-formed q\aftz veins (l-3 mm)are banded, contain crystals of barite, and locally haveopen spaces lined with drusy intergrown quartz andbarite, and crystals of gypsum and calcite. Late-formed veins of calcite are typically less than I cmwide and may contain crystals of stibnite, realgar andbarite.

Dolomitization

Dolomitized zones within the Triassic marble showa general spatial association with the hydrothermalmineralization. However, the dolomitization is weaklydeveloped, and the dolomite erratically distributed.The dolomitized rocks are typically lighter coloredthan the host marble, and are locally mottled, fine-grained, non-layered, highly jointed, contain man-ganese and iron oxides, and grade into undolomitizedmarble. Whether the dolomitization is of hvdrothermalorigin and associated with the mineralizati-on event" oris the result of removal of calcium carbonate fromleached dolomitic rocks by low-pH hydrothermalfluids, is unknown.

Supergene alteration

Features of supergene alteration are widespreadthroughout the entire Al5ar district. They include:l) oxidation ofpyrite and marcasite to secondary ironoxides and sulfates, 2) acid-leaching oxidation andargillic alteration from the breakdown of iron sulfideminerals, and 3) weathering of igneous and sedimenta-ry rocks to form secondary clay minerals typical ofwet and humid environments.

MnnnetzaloN

The four main types of mineralization in the districtare: 1) stibnite-bearing jasperoid gold ore (+ Au + As+ Tl ) , 2) s i l iceous gold ore (+ Au + Sb + As) ,3) arsenian gold ore (+ Tl + Hg + Sb + Au), and4) thallium ore. Elemental associations and hypogenesulfide minerals and assemblages of alteration miner-als in tlese ores are similar in manv wavs to thosefound in sedimentary-rock-hosted diiseminated pre-cious metal deposits of the western U.S. (Radtki &Dickson L914,Bagby & Berger 1.985,PercivaI et al.1988). The various types of ores at Al5ar contain bothcommon and rare sulfide and sulfosalt minerals of theAs-Sb-Tl-Hg suite of elemenrs. Identifications ofthese and other minerals were made by a combinationof plane- and reflected-light microscopy, X-ray dif-fraction, electron microprobe and SEM techniques.Rieck (1993) illustrated and described all the mineralsthat have made this a classic localiW.

Stibnite-bearing jasperoid gold ore

Stibnite-bearing jasperoids at Allar were minedextensively for their antimony content (Jankovid 1979,1982). The jasperoids are comprised of dark gray toblack, fine-grained, mosaic-textured microcrystallinequartz. Most of the jasperoids are fractured or brec-ciated and contain veinlets and irregular masses ofcoarser crystalline gray to white quartz. The matrixof microcrystalline quartz contains finely disseminatedsulfides (typically L-57o), dispersed fine-grainedsericite and silicified host-rock clasts containingsericite and sulfides. Pyrite, commonly the mostabundant sulfide, occurs as fine (<0.5 mm) subhedraldisseminated grains, larger (>0.5 mm) subhedral toeuhedral crystals, aggregates of coarse-grained crys-tals and as pyrite and quaxtz + pynte veinlets that cutacross the groundmass of microcrystalline quartz,Sparse amounts of finely crystalline arsenopyrite, mar-casite and stibnite are also present in the groundmass.Sparse minute anhedral grains of petzite occur encasedin pyrite overgrowths.

The main stage of jasperoid formation with precipi-tation of silica plus small amounts of sulfide mineralspreceded a later-stage event characterized by deposi-tion of large amounts of sulfide minerals. This laterhydrothermal event resulted in coarse-grained sulfideminerals infilling and lining open spaces in brecciaand fracture zones within the bodies of microcrys-talline quartz. Marcasite occurs in banded or layeredveins and framboidal masses and can represent up to40 vol.Vo of some jasperoid ores.

Stibnite, like marcasite, is dominantly a late-formedmineral. It occurs in irregularly shaped fine to coarsecrystalline masses, as acicular crystals filling fracturesand cementing breccia fragments, and as overgrowthson quartz crystals lining open vugs. Jasperoids com-monly contain 5-20 vol.Vo stibnite. Realgar occurs ascrystals and crystalline aggregates filling vugs andopen fractures and as an overgrowth on stibniteand marcasite. These features indicate that realgar inthe jasperoids formed very late in the paragenesis. Thejasperoids commonly contain I to 3 glt gold, and asmuch as 20 gltin zones that contain more than 17oantimony (Table 2).

Siliceous gold ore

Variably silicified sedimentary rocks and tuffsoccur soutfr of the antimony adits and form a resistantridge bordering the east side of the river @g. 2). Thetuffs and underlying marble were weakly to moder-ately silicified by the introduction of rnicrocrystallinequartz and contain variable amounts of clay mineralsand sericite. Tectonic brecciation and micro-stock-work veins of quartz are common.

The abundance of all sulfide minerals is less (typi-cally <1-3 vol.%o) than that in mineralized jasperoids

MaitEl@dC@i@ i!WebhrPs@

DISSEMINATED GOLD MINERALZATTON, MACEDONIA 655

TABLBzCHEMICALANALYSESffift ffi**ilm,Iff

DsDr/FnARY

0.052,41

10.041.5E0.020.140.90

0.450.063. t20.236.69

98.73

TiO2Al2O3F4A3FOI\r!OMBoC{ONa2oK2oP2qtot

Total

t r l1.U0.210.130.021.50

51.400.060.120.19

42.70

99.53

11.605.?10.550.010.51t a t

0.061.790.129.65

5960.4

16803<5

629

5.661

48r4342110

0.101,85

0.500.010.150.150.020.630.224.32

91,20

6037

171€<l

1 8 ?

7.963,42

0.045.547.940.031.170.32

13.10

91.41

8 10.5

8.|OEo

3,51I

101

1.533,34

I

0.036.10

12.300.100.350.u

14.65

6E.50

49980.450145<54

357

t .260.030.16t.340.01

14.

0.102.23l .130.640.010.100,15

0.510.192.31

98.57

0.074.553,360.940.020.050.35

<0.01

0.1715.61

0.240. l8

0.020.614,92I

0.038,41

13.500.030.080,21

83.40

0.2r4.552.57I

<).010.310.630.021 . 1 50.15

91.48

0.144.061.500.560.08

13.5021.00<0.010.E10.27

!5.76

9E.14

0.071.70t.310.300.06

15.6024.t0<0.010.390.23

1r o t

99.73

1.0015.040.050.0E

15.3026.100.020.090.42

0.6716.505.4E2.080.083,245.463.854.770.451.7E

99.8E

0.4816.004.t31.010.091.715.702,t63.850.286.18

97.68

1.010.04l . l62,993.244,9so.u5.08

98,82

30.200.0?0.100.1?

46.14

99.86

!uAtABaBiCo&CoHgMoNiPbRbsb

S!

TeTIwY7a7t

and consists primarily of finely disseminated and crys-tallized pyrite with lesser stibnite and minor marcasite.Acicular stibnite and minor amounts of realgar lineopen fractureso breccia zones, vugs and voids.Limonite, jarosite and secondary antimony oxide min-erals (including stibiconite) are common. Gold gradesvary from 0.5 to 10 elt(Table2).

Arsenian gold ore

Arsenian gold ore occurs primadly in argillizedtuffs, Tertiary dolomite, and rarcly in Triassic carbon-ate rocks. The best exposures are in underground mineworkings and in outcrops along the River Fault Zone(Fig. 2). This type of mineralization usually occws indistal areas with respect to zones of hydrothermal sili-ca. Host rocks are pervasively altered to fine-grainedclay minerals and sericite mixed with limonite,jarosite, manganese oxides, gypsum and minorhydrothermal quartz. Sulfides include fine1y dissemi-nated marcasite, lesser pyrite, realgar, plus sparse

58170.6

780025<5

?8I

21,022r

8lO.6Vo

1 1 320

2.89J040< lt 2

amounts of orpimenx, stibnite and minor thallium-bearing sulfosalt minerals. Lorandite is probably themost common Tl-bearing sulfosalt phase (Ivanov1965, Pavicevid 1988). Sulfide minerals typicallycomprise greater than l0 vol.Vo (up to 50 vol.Vo ) of.tfie arsenian ores.

Supergene alteration has transformed hypogenemarcasite and pyrite into mixtures of yellowish, creamand greenish sulfates including melanterite, copiapite,rhomboclase(?), among other phases, and alteredrealgar to pararealgar, pharmacosiderite and scorodite.The gold content is usually lower (<l to 3 g/t) thanthat in the siliceous ores. However, in areas of intensesilicification, both primary and oxidized arsenianores commonly contain substantial amounts of gold(3-12 glt: unpubl. data, Nassau Ltd.).

Thallium ore

High concentrations of thallium (up to 2 wt.Vo Tl)occur in strongly altered carbonate and tuffaceous

t ,

0.72.20Eo

458<J5 l8364

3.258

196

201225

725

<0.271

401 A

100130

1 00.5

386)

l 7

5.86<1231 '

6 6978

9

2.07Xg<n)192E

1266<4,22E00139<5

144t s

1,54

34t 280

5.50%<5l4

8.4J J

40<11 025

t

7E6

3,414040<120<l

Sanpte desuiffms: 1, Triassic mdtle; 2. Te{tiay dolonite; 3. Plia€ne @ 4. Pfi@ tsff h€ccia: 5. Plioc€oo $bvoksic imusim; 6. fugilic alE€d &lde; 7'

are.ig" AAei t6; f. a.gillic "frene

aO Oicineif nerblc; 9. Agitric aftered od siligifred nnbtq 10, AtSillb altacd md silicifred@ U. SlitdiF-bcdiry jasp€rod$

tZ-stticneanaue gotO'ce 13. Siticified rblodt gold dq .4. Slfcffied dgoldqq 15. Arsda gpld uei 16. Autinooy qq l?. Ars@ic 6e;I8.-lfmlized

"'nc'todty (sntesil 6E; 19. Aloned{imal nrfdtcuic luo"io. Noe$ Analyses by B@ds-€legg Is', Spdc, Nevadq US.A. Mtid oxids by Ie ttr&ot|s

scepiFeo(dtlmeftj.A[byfircs$ayn€,thodwithabdcatsspd@rnist I{g,TeadTlbf aronicdcapimneeod. Ba'Rb,Sr'Yadabyx-ratnu(r€s@

refu. A[ dn elqneds bv ICP neM-

T@ Eboelr Ab@d6G h P{B Pq Mili@ Exce,pr geld h P@ Pd Bili6

656 TI{E CANADIAN MINERAIOGIST

EXPLANATION

Alluvium(Quaternary)

Glacialdeposits(Quaternary'l

Argillic alleration

Silicification

rocks in the nortlem part of the district and accountfor a thallium resource of I x 105 tonnes of materialwith an overall grade of 0.1 w.To Tl (JankoviC 1982).The ores are hosted in light-colored, friable "argillicaltered" dolomitic carbonate rock and contain mix-tures of orpiment, realgar, pyrite, marcasite, loranditeand rare thallium-bearing sulfosalt minerals includingvrbaite, picopaulite, rebulite, simonite, bernardite,

t.Z Fault, showing relative movement and brecciation>4Frc. 2. East-west cross section through the southern AlSar disrrict.

Latite tuff(Pliocenel

Hypabyssal intrusiveporphyry (Pliocene)

Basal unconformity

Marble and dolomite(Triassic)

raguinite and parapierrotite (Percival & Radtke 1993).Sulfide and sulfosalt minerals (>5 to 30 voI.Vo ) uedisseminated throughout the rock, fill fractures, jointsand open space between breccia fragments and,locally, massively replace the rock.

The mineralization is located along or near contactzones with the subvolcanic intrusive rocks. Arsenisand thallium commonly range from 0.02 to L.25 wLEo,

A'A

DISSEMINATED GOLD MINERALZATION. MACEDONIA 657

and arsenic concentrations of up to 5 wt.Vo occurlocally (unpubl. data, Nassau Ltd.). The concentrationsof antimony usually range from l0 to 300 ppm, andthose of mercury, from <1 to 200 ppm. The thalliumores analyzed to date contain only traces of gold(unpubl. data, Nassau Ltd.).

Controls on ore deposition

Structural and stratigraphic features combine toprovide the primary controls of hydrothermal deposi-tion of ore in the Al5ar district. Structural featuresserved as channelways for the movement of thehydrothermal fluids into reactive environments. Fault-induced brecciation, fracturing, shearing and regionaltectonic disruption of the stratigraphic units signifi-cantly increased rock porosity and permeability,allowing fluids to permeate through large volumes ofrock. The presence of stibnite-bearing jasperoids atfault intersections indicates that intersections of struc-tures served to localize the mineralization.

Stratigraphic controls of mineralization were, inpart, dependent upon strucfural controls. The massive,relatively impermeable and nonporous carbonate rocksare poor hosts for mineralization, except in structurallyprepared zones. In contrast, extensive silicification andjasperoid gold-antimony mineralization (Table 2)occur along the basal unconforrnity and extend intooverlying Tertiary rocks @g. 2). Porous and perme-able clastic debris along the unconformity were silici-fied and mineralized to form a stratiform zone ofmineralization. Highly permeable and porous ash andcrystal tuffs and the reactive tuffaceous dolomiticrocks above the unconformity are extensively silicifiedand mineralized over large areas at and south of theantimony adits Gig. l).

GBocnsrvttsrRy

Results of whole-rock and trace-element analysesof samples representing each of the major types ofaltered and mineralized rock and fresh host-rock arepresented in Table 2. These samples were chosen froma suite of approximately 1500 rock samples collectedfrom surface outcrops and underground workings, allof which had been previously analyzed for gold plusAs, Sb, Hg, and Tl. The samples that were selectedwere re-analyzed for eleven major and minor compo-nents (shown as oxides) and twenty-three otherelements plus gold.

Bulk-rock chemistry

Results of whole-rock chemical analyses of fivespecimens of unaltered host-rock and fourteen speci-mens of hydrothermally altered and mineralized rockare presented in Table 2.T\e data were used to calcu-late the relative gains and losses of major components

in the altered rocks using the ooisocon" method ofGrant (1986).

Three types of isocon diagtams were constructedusing the Al5ar chemical data (Table 2): 1) best-fitisocon among all chemical components; 2) aluminum(AlzO:); and 3) titanium (TiO) as immobile compo-nents. The titanium diagram provided the best refer-ence isocon with respect to determinations of thechemical gains or losses for the A15ar samples, withchanges in bulk composition occuring within accept-able limits among the progressively more hydro-thermally altered rock-types. This is in good agree-ment with mass-balance studies by Kuehn (1989),Bakken & Einaudi (1989), and Kuehn & Rose (1992)at the Carlin gold deposit, Nevada. These investigatorsestablished titanium as an immobile element duringhydrothermal alteration and mineralization.

Table 3 shows chemical gains and losses calculatedfrom the isocon diagram treating Ti as an immobileelement for two samples of altered marble, three sam-ples of altered dolomite, and three samples of alteredtuff. The content of SiO2 and Fe2O, increased in mostof the samples of carbonate and tuffaceous parent-rocks. The brecciated carbonate rocks were an idealhost for hydrothermal replacement, resulting in sub-stantial increases with respect to SiO, (to 27x) andFerO, (to 18x). Conversely, CaO, CO2 @igh LOI incarbonates), and MgO (in dolomite and tuffl werestrongly depleted.

Although only a small number of samples wereanalyzed, the results suggest that Na, A1, P, K and Mnwere weakly to strongly depleted in dolomite and tuff,and were enriched or unchanged in altered marble(Table 3). The marble exhibits a significant increase inMg, which corresponds to the variable dolomitic com-ponent of tle marble, as documented in the field; how-ever, this increase in Mg may have resulted from acidleaching of Ca from a dolomitic marble parent-rock,as previously discussed. Regardless of the originalcomposition of the parent-rock (carbonate or tuff), inmost cases the same suites of major elements wereadded and removed from rocks that have undergonesimilar types of hydrothermal alteration.

Detailed geological and chemical studies of theCarlin deposit, where gold mineralization is hosted insilty dolomite (or dolomitic siltstone), were publishedby Dickson et al. (1979), Radtke et al. (1980), andRadtke (1985). Results of these studies suggest thatvarying amounts of SiOr, total Fe, Al, and K, wereintroduced, and Ca, Mg and CO2 were removed fromthe parent rocks during hydrothermal alteration andmineralization. However, results from more recentstudies at Carlin by Bakken (1990) and Kuehn & Rose(1992) indicate that the apparent enrichments of Al,Fe, and K are due to volume losses ofup to 50Vo as aresult of carbonate removal during the hydrothermalprocess. Their calculations indicate that Ti and Alremain constant in all altered zones, and therefore

658

TABLE 3, REI,,ATTSE ADDMONS AND DEPLETIONS OF O)ODES USINC CONSTANT TITAJ\IIUM INHYDROTIIERMALLY ALTERED AND MINERALUED R@KS COMPARED TO TIIEIRIJNALTERED PARENT ROCKS, AIJAR DISTRJCT, MACEDONIA

Dolmite

A B C

Trtr

A B C

SiO2

1io2

Al2O3

XFe

Mrlo

Mp

Cao

I.lh2o

wPzOS

27.54

0.m

o.26

18.53

t25

7.47

2L46

0.00

{.39

3.98

0.00

-0.41

{.91

0.28

3,45

-0.80

-o.vz

0.39

-0.48

3.&

{.80

{.99

4.99

{.96

{31

-0.71

-0.@

425

0.@

0.66

0.17 2.tQ

0.m 0.00

{.84 4.73

t1.% 0.r2

-0.11 {.60

434 -0.90

-0.71 -0.90

-0.90 {.98

-0.70 0.21

{.17 -0.76

4.52 t.t2

0.00 0.00

4.97 -0.51

0.36 -0.04

4.40 {58

4.63 -0J5

4g2 4,82

4,99 {.98

-0.70 4.6r

-0.54 {.04

4.16

0.00

-050

0.43

422

-0.70

{.69

{.45

4.64

r.7I

Fe as Fe2O3 No!g: Clmnges are given in p€rcent (e.g . AJ2 = l2%o can) Nd were cdculaEd using tle im methodofcnnt0936).Roc& samples used fq theabovg calqilatiors re given below and mple nmbas cmspond !o those in Table2.

were immobile, and that Ca, CO2 and Mg were strong-ly depleted (-30 to -1007o) and K and Fe were weakly(-L0Ea) depleted. Silica exhibits a weak depletion(-10 to -20Vo) in the weakly to moderately alteredzones, but \ilas strongly (+40Vo) enriched in the highlyaltered zones.

Our results suggest that the changes in major-element chemistry in carbonate rocks at AlSar are gen-erally similar to those at Carlin. Differences may beexpected because two different hydrothermal systemsreacted with compositionally different host-rocks atCarlin and Al5ar.

TR,qcs ELEN{ENTs

Mineralized samples of all three rock types arestrongly enriched in Au, As, Sb, Hg, T1" and Ba rela-tive to abundances of these elements in the unalteredparent-rocks (Tabte 2). These elements were intro-duced by the hydrothermal fluids and were depositedprimarily as stibnite - realgar - orpiment - arsenopy-rite - lorandite - cinnabar in association with quartz,pyrite and marcasite. In the assemblage of hypogeneminerals, arsenic occurs as realgar, orpiment andarsenopyrite, antimony as stibnite, mercury ascinnabar, and thallium as lorandite and other but less

Tuffs

A. Unallaed (#3) vs. alt€red(#?B. Unalered (#3) vs. altered (#10)C. Unahq€d (#3) vs. altsed (#f4)

abundant T1-As-Sb-Hg-Fe sulfosalt minerals.Barium occurs as barite, which was depo$ited latein the mineral paragenesis. Contents of base metals(Cu, Pb, Zn md Mo) and Ag are low in most of thesamples of altered and mineralized rocks (Table 2yThese mineralogical and chemical features are similarto those accompanying ore deposition in sedimentary-rock-hosted disseminated precious metal deposits ofthe western U.S. (Radtke & Dickson 1974, Bagby &Berger 1985, Percival et al.1988).

Some of tlte mineralized samples at Al5ar containanomalous amounts of Te, Se, F" Bi, Cd, Mo and Zn.Thirty samples containing more than 3 ppm Au andnot included in Table 2 were analyzed by atomicabsorption methods for selected elements typical ofigneous-magmatic environments. Results from theseanalyses showed Bi (20 to 85 ppm), Cd (5 to 15 ppm),Mo (1 to 560 ppm)" Zn (5 to 615 ppm) and F (33 to1265 ppm).

Results of chemical analyses of mineral separatesof stibnite, realgar and orpiment (Table 4) indicate thatthese three sulfide minerals can contain significantamounts of Tl, As, Au, Te and Hg (stibnite) and Tl,Sb, Au, Te, and Hg (orpiment and realgar). Similarelemental associations among arsenic and antimonysulfide minerals have been reDorted from the Carlin

!r16bls

A. Unalrercd (f r) vs. alMed (#8)B. Unaltered (#1) w al6ed (#11)

Dolomitc

A. Unaltered (#2) vs. alsed (#6)B. Unaltered (#2) vs. alt€red (lt9)C. Un lts€d (#2) vs. alt€red (*13)

DISSEMINATED GOLD MINERALZATION. MACEDONIA

TABLE4. CIIEMCALDATAONIIYPOGENEOREMINERAI.S FROMTHEALSAR DISRI T,MACtsDONIA

659

r0

Au 479

AC <O.2

As 3r3

Bi <5

C o l

Cr 15

C u 7

k(40\ 023

HS 222

M n ? 5

M o 3

Ni <1

Pb <2

s b M

Te 14.5

T1 100

M2

kt 12

v5 r84

3.9 <O,2

u 331

<5 <5

7 7

7 4 4

r52 88

0.67 0.v

16.8 3.48

165 I

2 9

<1 94

<2 <2

M M

>1@ 5.8

70 >1m

2t <m

2 9 y

81 y9

4.2 <0.2

r35 153

<5 <5

<1 <1

33 17

0.08 0.35

1.82 7.54

2 3

2 t o

< l < l

<2 <2

M M

4.2 <O.2

6 4 5

4 2 3

2 6

1240 14 61

1.0 <0.2 4.2

l 7 O M M

<5 <5 <5

4 < l < 1

4 0 1 9 I

5 3 3

0.18 0.40 1.55

2.91 4.62 0.86

145 2A 9

4 < t l

< 1 1 < 1

< 2 < 2 5

M >2000 >20s

55.6 <0.2 36.0

>1@ 15 3E

40 <m 40

2 3 1 1 4

r t < 1 a

<o.2 <o.2

M M

<5 <5

<l <1

! < l

l < l

0.2r 0.05

3.76 t5.@

31 32

<l <1

<l <2

05 2v

t2.4 <O.2

75 >100

<24 4n

2 2

Anatyss by Bmdr-Clegg, Sps*s, Nevada Gold aralyses by firc Ny methods (Eporled in ppb); Hg, Te md Tl byaromic absptiotr Ba by x-ray fluoren@, all othq malys by ICP mefiods, dnd all reported in ppm except imn(rspqt€d in p€rcsrl). M=mqior.

SAMPLE DESCRIPTIONS

I Stibniie(sutbzone)2 Stibnilo (soutb zom)3 Stibnile(suthzone)4 Sdbntte(c€ntralzoF)5 Stibnite (cemaf zcrr)

Sdbnile (entral zme)Realgar (centralzoqe)Realge (entatzone)Orpimat (qtralm)Oryiment (mrth zone)

zonation suggests that although arsenic and antimonyare concentrited in surface coatings, both elementsalso occur structurally bound in pyrite.

Electron-microprobe studies of pyrite in unoxidizedores from the Carlin and Cotrez deposits, Nevada,were frst undertaken by Wells & Mullens (1973) andRad&e (1985). Their studies showed that pyrite con-tains surface coatings and internal disseminations ofarsenic (arsenian pyrite), and lesser amounts of ant!mony, gold and mercury.

Recent studies using secondary-ion mass spectrom-etry, electron microscopy and X-ray photoelectronspectroscopy were done by Bakken et al. (1989,1991)and by Arehart et al. (1993) on pyrite in gold oresfrom the Carlin and other sedimentary-rock-hosteddisseminated gold deposits in Nevada. The results ofthe recent studies indicate that arsenic concentratesprimarily in an overgrowth (arsenian pyrite) on pyrite

6789r0

deposit (Radtke et al. L973, Dickson et al. 1975), withsolid-solution incorporation of Sb and Au in orpimentand realgar, as well as As and Au in stibnite. Detailedmineralogical studies of Al3ar minerals are needed todetermine whether the composition of these phasesrepresent solid solution or minute intergrowths orinclusions of discrete minerals.

Electron-microprobe analysis of several grains ofpyrite from a sample of gold-rich (21 glt), stibnite-bearing jasperoid from Al5ar reveals a distinct enrich-ment of As and Sb at the periphery of the grains.Three distinct zones, outer, intermediate and core,were recognized in a number of pyrite grains.Preliminary results indicate that the As and Sb contentof the zoned grains decreases from the outer zoneto the core. The outer zone contains 7.1.7o As and2.0V0 Sb. the intermediate zone" 0.97o As and 0.077oSb, and the core zone,0.56c/o As and <0.07Vo Sb. This

660 THE CANADIAN MINERALOGIST

and that these rims also contain high concentrations ofAu and Sb. Rims are typically a few (l-4) pm thickand are formed on host pyrite grains (cubes andframboids) banen of these metals. The arsenian pyritealso can @cur as na:row veinlets within, and as simpleand oscillatory compositionally zoned overgrowths onbarren pyrite (Arehart et al. L993). Similar studies byFleet et al. (1993) on pyrite from several stratiformand stratabound gold deposits also revealed thatarsenian pyrite containing Au is a feature common togold deposits of different types.

Fluid-inclusion studies and temoeratureof ore deposirton

All fluid inclusions in ore-stage quartz in stibnite-bearing jasperoids and sil iceous ores at Al5arexamined to date are too small (3 1 pm) to permitmeasurements. The inclusions are hosted in micro-crystalline quartz exhibiting a mosaic texfure and aremorphologically similar to those described by Bodnaret al. (1985) in similar materials from Carlin-typegold deposits in the western U.S. They also foundthat most inclusions of this type are too small formicrothermometry, but that those inclusions that couldbe measured commonly homogenized at 200oC orless.

Measurements of primary liquid + vapor inclusionsin realgar crystals from the Al5ar district made byBeran et al. (1990), gave temperatures of homogeniza-tion ranging from 144 to l70oC and salinities from7.9to 12.9 equivalent wt.7o NaCl. Because most of therealgar at Al5ar probably formed very late in the hlpo-gene paragenesis, it is tenuous to use these data toinfer conditions of main-stage deposition of silica +gold.

The nearly ubiquitous presence of primary marca-site in the stibnite-bearing jasperoids and siliceousores places general constraints on the temperature oflate main-stage mineral deposition, most of whichfollowed deposition of the earlier main-stage gold ore.Detailed studies on marcasite precipitation fromhydrothermal solutions by Murowchick & Barnes(1986) and Murowchick (1992) indicate that the upperlimit of marcasite deposition is 240"C (at a pH lessthan 5) and that marcasite is usually deposited below200'c.

We believe that a temperature of approximately200"C for the main-stage deposition of gold ore atAl5ar is reasonable and in agreement with results fromdetailed studies of fluid inclusions carried out atseveral Carlin-type gold deposits in the western U.S.Gold deposition at the Carlin deposit occurredbetween L75 and 235oC from fluids containingapproximately l-3 wt.Vo equivalent NaCl (Radtkeet al. 1980, Rose & Kuehn 1987, Kuehn 1989); at theCortez deposit, it occurred between 176 and 21.8"C(Rytuba 1977), and, at the Mercur deposit, between

220 and270'C from fluids containing 5-8 equivalentwt.7o NaCl (Jewell & Parry 1988).

DrscussloN

Interal zonation

Variations in mineralogy and intensity of alterationindicate that a distinct pattern of lateral zonation existswithin the Al5ar district @ercival & Boev 1990). Keyfeatures of this zonation are shown along anorth-south cross-section through the center of thedistrict (Fig. 3). Three units are recognized onthe basis offeatures and location: 1) the northern area.characterized by intense widespread argillic alteration,high concentrations of As + T1 + Hg with minor asso-ciated Sb + SiO2 + Au, 2) the central area of stibnite-bearing jasperoids with high but variable concentra-tions of Au, As, Tl and Hg, with a thick zone ofargillic alteration occuring above and lateral to thestibnite-bearing jasperoid ores, and 3) the southernarea, characterized by extensive widespread replace-ment-induced silicification containing low concentra-tions of gold and Sb + As + T1 + Hg.

Results from recent studies by Jelenkovii &Pavicevid (1990) indicate that erosion in the northernarea is less than 250 m below the paleosurface butincreases to much greater depths in tfie southern area.The central area exhibits transitional features commonto both shallow and deep erosion, with an upper zoneof argillic alteration (As + Tl + Hg) similar to thatin the north, underlain by stibnite-bearing jasperoids(+ Au + As + Tl + Hg). These relationships suggestthat the high-level zone of argillic alteration exposedin the northern area may be underlain by a zone ofreplacement silicification and possible mineralizationanalogous to that exposed by structural juxtapositionand a deeper level of erosion in the south.

Comparison with other Yugoslavian deposits

A15ar is one of a series of mining districts that liealong the Vardar Zone between northern Greece andcentral Yugoslavia. Mineralization within thesedistricts is spatially related to subvolcanic, locally por-phyritic, intrusive rocks of intermediate composition.The intrusions are bordered by mineralized skarns incarbonate terranes and are commonly enveloped byone or more metall iferous zones containing Fe,+ Cu-Mo-Pb-Zn within and marginal to the intrusivebody. The contents of Au-As-Sb-Hg increase out-ward to form distinct zones containing these metals.These zones commonly are lacking or deficient in basemetals (Jankovi i L974.1977.1982. Jankovid &Petkovi11974).

At A15ar, the Sb-Au-As-Tl-Hg mineralization ishosted by both carbonate rocks and volcanic tuffs.Zones of intense silicification containine Sb + Au

DISSEMINATED GOLD MINERALZATION. MACEDONIA 66'.t

SOUTHERN(gold+antlmony)

CENTRAL(antlmony+gold)

NORTHERN(arsenic+thalllum)

zoEtrLrJb

Slrdlg silictffettm:si&spread and p€ryaslYo

W€ak aroilllc altmtln

Rar€ dolomltlzatts

Mod€rate io strongarclllb alloratlon@srlylng Jasporoid

trlod€rate to strongalllcmetbn

Local dolomltlzatio

Modsrai€ lo strongarollllc altdation:widepr€ad and psruaslv€

t&lmxtsrtie

W€alq locallzed Ellcmcail!!-

zokNfEUJ7

Sillcflod tutfs ands€diments (+Au,As,sb)'(abundant)

Sllbmed and gokFmin. inunconiormity zof,€. (ahrdant)

Stlbnlte-bearing jaep€roldmin. (localiz€d)

StlbnhFbearhglasfBroid (+Au"As)(abundant)

Arsonlcal gold min.(locallz€dl

Ars€nic min.(localhed)

ThallhJm mln.(abundant)

Arg€nlc mln.(ah.rdant)

trFctt=IJJIooIJJo

sroilsb+A,slBaTt,Hg I sb+sto2+A8tTt,Hg,BaWusspreadgpld I Locallzedhlgh-grad€(l-3ppm) min. I gold (2-2@m) nln.

I sb:nI Sb:As

lncrasim --Itr,

As+Tl >Hg+Sb+SiO2

I;: *, ((o.rppm)

i i.SiJ@lf;"ol@

EXPLAMNON

E$ffimN".2>6

Ftc. 3. North-south longitudinal section through the Al5ar district showing geological, alteration and mineralization relation-ships among the northern, central and southem portions of the district.

Latiie iufi (Frioaarre)

llypabyseal fnttugiv€ po'iphyfy lPll@arc,

TuffaceotF dolomtte aTertlan/,

Basl undr'dmlty

Marue ard (blomhe lfirlagstc,

formed in both rock types spatially associated withsubvolcanic alkaline intrusions. The As-Sb-Tl-Au-Hg mineralization at AlSar differs markedly from

Argllllc alteratlon

Sllbmcailon

Stlhft e-b€arlrq laop€rold

Fault, atwlng relallv€ |M€nt ardbreociatlon

the base-metal- and Ag-dominated ore deposits typicalof Yugoslavia. Base-metal mineralization has not beenfound to date at A15ar.

l r r Il b . l

ml r d It l

E^21b%l

l l r l

662 TTIE CANADIAN MINERALOGIST

Genetic funplicartons

Although the mineralization ar Alsar is spatiallyrelated to intrusive rocks, no firm genetic tie has beenestablished. Several lines of indirect evidence existwhich, taken togetler, could support a possible genetictie between intrusions and mineralization at Al5ar.These are: 1) results from preliminary sulfur isotopicstudies on stibnite, realgarn orpiment and marcasite,which give 0.35 to -6.84Vo0, implying a possible par-tial magmatic origin for the sulfur (Serafimovski er al.l99l),2) presence of anomalous levels of As, Sb, Tl,Te, Se, F, Bi, Mo, Zn, Cd and Mn, described byMutschler et al. (L985) and Sillitoe (1990) as beingcharacteristic of deposits genetically related to alkalineintrusions, and 3) results from studies of several sedi-mentary-rock-hosted gold deposits in other parts of theworld [Purisima Concepci6n, Peru: Alvarez & Noble(1988); Bau, eastern Malaysia: Percival et al. (1990rBare Mountain, Nevada: Castor & Weiss (1992)l inwhich the authors provided some evidence to supporta tie between mineralization and intrusions.

We believe that spatial associations by themselvesmay be suggestive of a tie or genetic association, butprovide no conclusive evidence for it. An example ofthis is provided by Sillitoe & Bonham (1990), whoattempted to establish a genetic tie for many depositsin Nevada and elsewhere, most of which lack anv

proven genetic relationships.An alternative explanation for ore genesis at Al$ar

would be similar to the shallow-seated hot-springmodel suggested for the Carlin deposit and otherdeposits with similar features by Radtke & Dickson(L974), Dickson et al. (1979), and Radtke er al.(1980). Meteoric water heated by shallow-seatedintrusive rocks reacted with sedimentary rocks toextract ore components and moved upward alongmajor structures and outward through permeable rocksfavorable for solution penetration and ore deposition.Although the intrusive rocks could provide ore com-ponents, the only necessary role they play is to provideheat to the surrounding rocks and meteoric waters,thereby initiating rock-water interactions and settingthe waters in motion to form a convective hydrother-mal cell (Dickson et al. 1979).

Key physical and chemical features at AlIar andthose at the Carlin, Getchell and Mercur deposits inthe western U.S. are summarized in Table 5. Similarfeatures are also shared with deposits in the People'sRepublic of China (Cunningham et al. L988, Ashleyet al. 1991) and at Baun eastem Malaysia (Perclal et aI.1990).

Although many similarities in physical and chemi-cal properties exist among the various deposits of thistype, there is no guarantee that the genetic models foreach of the deposits are identical. Furthermore, in spite

TABII5. coMPARtsoNoPcEoLocIcALANDCEEMICALFEATTJRESorrmar,SanosmrcrvlrrrrsosBoFTy?IcAlCARLIN-TYPB DEP(NNs OF THE WA9TERN UNIIED STATES

Al5r, Yugoslavia Cali4NcndaEadtle 19E

Getchsll, Nsada(Bagby&Berger, l98t

M€(cu.U6h@agby A Berger, 1985)

IlosrRocks

Smclural Conficls

Asiat€dIg!rurRelG

l4{octrmicalctt-8es inninemlized ncks

H}m€reesdn€ralw

Triassicmtble; Terdryn&c@us dolomit8 mdPliocarB felsic orlfs

$€Aly dippirg huls,nd $€armes. Rglatively0ar $aigr@hiccdillolsed uconfmiry zotr€s.

Plioc€neladte!II& mdsrbvolcanic intNios(dacb! K-Arsgearhdat 5.S4.5 n.y.

hro&red Si,Fe. S. Mg;Atr As, Tl, Sb, t{9, B4 F, TeSeRmove& CaedMgtmcaton brel(s

Siluian-DevoEim Rob€dMmtain Fqmadon: $in-beddo{lmnnt€d siltyelmilg andlime.gme.

Steqply difp,ng fadrg Norrh-n€st tqdisg TuscduaAilicltus. RobqBMoEtain!hru$ &dL

Edy Gelacrous (f27 m.y.)ingnediab compositimdikagale€dadnimlized.

I,rmd|tce* Si,Fe,S,Au, Aq Hg, Sb, Ba, T1, Cuw,7^Rsnove& Ca,Mg

Canbrian Preble Fornarim;erborac€ous stnle, linesrcedqudtzite.

St€sply dipping fadtsIslinallyfoLkdsedin€olary mcle

crsEc@l|s (90 n.y.) gmo-dioribphton in &eOsgoodMmains. Dsci@rndadesitsp6phyly difs; alter€d @dmiMaliz€d

Inloduce(t Si,Fe,SAr As, Sb, Hg, T1, F, 84 TeSeRemoved: Ca, Mg

Qudtz, calrits,pyrilo,Eseoqydb, macarl4roalgd, opimqq scibnne,dfrab@,oulte,barite,dalcopyrite,Tocc: As-,T1-, mdHg.sdfosatts.

Mississippi@ Grcd BhFUmesore; thin-bedded, silty-lime$one, nudstorc. liees@eed slnle.

St€Alydilping fadtsNodnese-rending OptirAntidi[e.

Oligpc€ne Fagle ltillPrphyry (31.6 n.y.) @dBtuds Eye Porphyry (36.7m.y) as dil€s, sius admall stock&

Inrodrced. SiS,A& A& Sb, Ba, Fe, Hg, Tf,F, T1, Te, SoRmwed. C4Mg

Qualtl'cabiB,pFite!€algd, spimel marasiE,stibdre, cima!6, baite,Ituotfte, goldTraco: Tl-, As- sulfosalB

Qustz,pyrile,n6casits, Quanz,calcite,pyrits,$atgr-cpimml$ibnito, Es€Dian-ffite,&algE,loanhe,aseuayrile" opin€nlstibdto,cinnab€r,barilo,cimabr,As-,fi-, biE"Ilg-mdSbeulfmts. TE s Eliyea$enb,tatacnal@yde Ae,Tl-,andgg-sofosalr$

DISSEMINATED GOI.D MINERALZATION" MACEDONIA 663

of extensive studies done to date on "Carlin-type"deposits, many uncertainties still exist regarding theirgenesis. For example, stable isotope data from thesame samples at Carlin have been interpreted bythe same geologists to support different geneticmodels. At this time, the conditions of ore depositionand the nature of the ore fluids and sources of orecomponents at AlSar are unknown.

ACKNOWI.EDGE]VENTS

We thank Blakeney Stafford, managing partner ofNassau Limited, for supporting our research work inthe Al5ar district and allowing us to publish theseresults. An earlier version of this manuscript benefirted from the careful and thoughtful review of C.A.Kuehn. This manuscript was geatly improved by indepth reviews and constructive comments of B.M.Bakken, R.F. Martin, and an anonymous referee. Wethank Betty Wyatt for drafting the figures and BettyKrause for typing of the text and tables.

RSFERENcF,s

ALVAREZ, A.A. & Nosl-e, D.C. (1988): Sedimentary rock-hosted disseminated precious metal mineralization atPurfsima Concepci6n, Yauricocha district, central Peru.Econ. Geol. 83. 1368-1378.

Anruanr, G.B., CuRyssoulrs, S.L. & KESLER, S.E. (1993):Gold and arsenic in iron sulfides from sediment-hosteddisseminated gold deposits: implications for depositionalprocesses. Econ. GeoI. 88, 171-185.

ARsovsKI, M. & Ivenov, "t. (1977a): Neotectonics, magma-tism and metallogeny on the territory of Yugoslavia. /zMetal logeny and Plate Tectonics in the NortheastMediterranean (S. Jankovid, ed.). Univ. of Belgrade,Belgrade, Yugoslavia (47 1 -482).

& - (1977b): Geotectonic evolution of theVardarZone. InProc.6th Colloquium on Geology oftheAegean Region (G. Kallergis, ed.). Institute of Geologicaland Minirg Research, Athens, Greece (569-517).

AsHLEy, R.P., CuNNrNGHnna, C.G., BosrrcK, N.H., Dr,aN,W.E. & CHou, I-Mnc (1991): Geology and geochemistryof three sedimentary-rock-hosted gold deposits inGuizhou Province, People's Republic of China. OreGeol. Rev.6. 133-151.

Baonv, W.C. & BERcER, B.R. (1985): Geologic characteris-tics of sediment-hosted disseminated precious-metaldeposits in the western United States. 1n Geology andGeochemistry of Epithermal Systems (8.R. Berger &P.M. Bethke, eds.). Rav. Econ. Geol.2,1,69-202.

BAKKEN, B.M. (1990): Gold Mineralization, Wall-RockAlteration, and the Geochemical Evolution of theHydrothermal System in the Main Orebody, Carlin Mine,Nevada. Ph.D. thesis, Stanford Univ., Stanford,California.

& ETNAUDI, M.T. (1986): Spatial and temporal rela-tions between wall rock alteration and gold mineraliza-tion, main pit, Carlin gold mine, Nevada, U.S.A. /a Gold'86 (A.J. Macdonald, ed.). Konsult International Inc.,Willowdale. Ontario (488496).

& - (1989): Volume loss and mass trans-port dudng hydrothermal alteration and weathering,Carl in gold mine, Nevada. Geol. Soc. Am,, Abstr.Programs2l(6), M95.

FLEMNG, R.H. & HocreLLA, M.F., Jn. (1991):Higb-resolution microscopy of auriferous pyrite from thePost deposit, Carl in distr ict, Nevada. /n ProcessMineralogy. XI. Characterization of Metallurgical andRecyclable Products (D.M. Hausen, W. Petruk, R.D.Hagni & A. Vassiliou, eds.). The Minerals, Metals &Materials Soc., Warrendale, Pennsylvania ( l3-23).

HocHELrA, M.F., Jn." MARSHALL, A.F. & Tunmn,A.M. (1989): High-resolution microscopy of gold inunoxidized ore from the Carlin mine, Nevada. Ecaia.Geol. M.171-179.

BsRAN, A., GorzNGER, M. & krcr, B. (1990): Fluid inclu-sions in realgar from Allchar. /r Symp. on ThalliumNeutrino Detection @ubrovnik, Yugoslavia), 42 (abstr.).

BERGER, B.R. & BAcBY, W.C. (1991): The geology and ori-gin of Carlin-type gold deposits. /z Gold Metallogenyand Exploration (R.P. Foster, ed.). Blackie, Glasgow(210-248).

BouNan, R.J., REyNoLDs, T.J. & Kusmt, C.A. (1985): Fluidinclusion systematics in epithermal systems. /z Geologyand Geochemistry of Epithermal Systems (8.R. Berger &P.M. Bethke, eds.). Rev. Econ. Geol.2,73-97.

BoEv, B. (1990a): Microelements in the volcanic rocks of theKozuf Mountains. /ri Proc. KIth Congress on Geology ofYugoslavia (Ohdd). II. Mineralogy &Petrology. Geol.Soc. Macedonia (5-74).

(1990b): The origin of melts from which the vol-canic rocks of the Kozuf Mountains were formed. /nProc. Xtrth Congress on Geology ofYugoslavia (Ohdd).tr. Mineralogy & Petrology. Geol. Soc. Macedonia (l8l'187).

CAsroR, S.B. & Wntss, S.I. (1992): Contrasting styles ofepithermal precious-metal mineralization in the south-western Nevada volcanic field, USA. Ore Geol. Rev. 7,193-223.

Cwxncnav, C.G., AsHLEY, R.P., CHou, I-Mnc, HuaNc,ZusHU, WAN, CHaovuar & Lt, WENKANc (1988): Newlydiscovered sedimentary rock-hosted disseminated golddeposits in the People's Republic of China. Econ. Geol.83, 1462-1467.

D lcKsoN, F .W. , Ranr rn , A .S. , WatssBERc, B .G. &Hnnopoulos, C. (1975): Solid solutions of antimony,arsenic, and gold in stibnite (Sb2s3), orpiment (As2S3),and realgar (As2S2). Econ. GeoL 70,59l-594.

6U TIIE CANADIAN MINERALOGIST

Rye, R.O. & Reprrn, A.S. (1979): The Carlingold deposit as a product of rock-water interactions. 1nPapers on Mineral Deposits of Westem North America(J.D. Ridge, ed.). Nevada Bur. Mines & Geol., Rep.33,101-108.

Fellrcr, A.E., IucH, M. & Russnr,r-, M.J. (1991): A stableisotope study of the magnesite deposits associated withthe Alpine-type ultramafic rocks of Yugoslavia. Econ.Geol. 86,847-861.

Flrrr, M.E., Cm.yssoul-rs, S.L., MecLsAN, P.J., DAVDSoN,R. & WETSENER, C.G. (1993): Arsenian pyrite from golddeposits: Au and As distribution investigated by SIMSand EMP, and color staining and surface oxidation by)(PS andLMS. Can Mineral.31. 1-17.

Gnanr, J.A. (1986): The isocon diagram - a simple solutionto Gresens' equation for metasomatic alteralion. Econ.Geol.8l.1976-1982.

It-rcH, M. ( 1974): Hydrothermal-sedimentary dolomite: themissing link? Am. Assoc. Petrol. Geol., Bull. 58, 133'l-1347.

Ivarov, T. (1965): Zonal distribution of elements and miner-als in the deposit Al(ar. In Symp. on Problems ofPostmagmatic Ore Deposition II (Prague). Geol. Soc.Czechoslovakia (186-19 1).

JANKovIe, S. (1974): Metallogenic provinces of Yugoslaviain time and space (an overview). .Iz Metallogeny andConcepts of the Geotectonic Development of Yugoslavia(S. Jankovid, ed.). Belgrade, Yugoslavia (37-63).

(1977): Major Alpine ore deposits and metallo-genic units in the northeastem Mediterranean and con-cepts of plate tectonics. 1z Metallogeny and PlateTectonics in the Northeastern Mediterranean (S.Jankovid, ed.). Faculty Mining Geol., Univ. Belgrade,Yugoslavia (105-172).

(1979): Antimony deposits in southeast Europe.lnst. of Research, Geol. and Geophys., Univ. of Belgrade,Belgrade, Yugoslavia (2548).

(1 982): Yugoslavia. 1n Mineral Deposits of Europe.II. Southeast Europe. Inst. of Mining and Metallurgy,London, U.K. (1 43 -202).

& JsLENKovrC, R. (1990): General metallogenicfeatures of thallium mineralization in the Allar deposit(Yugoslavia). /z Symp. on Thallium Neutrino Detection(Dubrovnik, Yugoslavia), 32-35 (abstr.).

- & Pe'xovre, M. (1974): Metallogeny and conceptsof the development of Yugoslavia. Ia Metallogeny andConcepts of the Geotectonic Development of Yugoslavia(S. Jankovid, ed.). Faculty Mining Geol., Univ. Belgrade,Belgrade, Yugoslavia (U3 -47 7).

JrlrNrovrd, R. & PAvrcEvrC, M.K. (1990): The erosioninvestigations of the Allchar district based on the appliedgeomorphological and structro-geological methods. 1zSymp. on Thallium Neutrino Detection (Dubrovnik,Yugoslavia), 30-3 I (abstr.).

JEwELL, P.W. & Pannv, W.T. (1988): Geochemistry of theMercur gold deposit (Utah, U.S.A.). Chem. Geol. 69,24s-26s.

KolIos, N., INNocENr, F., Mararu, P., Pnccrnrllo, A. &GtulnNt, O. (1980): The Pliocene Volcanism of theVoras Mts. (central Macedonia" Greece). Bull, Volcarcl.43. 553-568.

KUEIN, C.A. (1989): Sndies of Dissemirnted GoM Depositsnear Carlin, Nevada: Evidence for a Deep GeologicSetting of Ore Forrnation Ph.D. thesis, PennsylvaniaState Univ., Unive$ity Park Pennsylvania.

& RosE, A.W. (1992): Geology and geochemistryof wall-rock alteration at the Carlin gold deposit, Nevada.Econ. Geol. 87, 1697 -1721.

MACEDoNTAN GEorocrcAl SunvEv (1980): heliminary geo-logical map of the Vitoliste Quadrangle, State FederalRepublic of Yugoslavia. (1:100,000 scale, color map), 1sheet.

Mtnowcstcr, J.B. (1992): Marcasite inversion and the petro-graphic deterrnination ofplrite ancestry. Econ Geol.87,1141-1152.

& BARNES, H.L. (1986): Marcasite precipitationfrom hydrotherrnal solutions. Geochim- Cosmochim. Acta50.261,5-2629.

Mr,'rscHLER, F.E., GRrFFnr, M.E., SrEvENs, D.S. & SHemlorv,S.S. (1985): Precious metal deposits related to alkalinerocks in the North American Cordillera - an interpreta-tive review. Geol. Soc. 5..\fr., Trans.88,355-377.

NoLAN, T.B. (1962): The Eureka mining district, Nevada.U.S. Geol. Surv., Prof. Pap.406.

PAVICEvIC, M.K. (1988): Lorandite from Allchar - a lowenergy solar neutrino dosimeter. Nuclear InstrumentsMethods Phys. Res. M7\287-296.

Prncrval, T.J., BAoBy, W.C. & RADTKE, A.S. (1988):Physical and chemical features of precious-metalsdeposits hosted by sedimentary rocks in the westernUnited States. 1n Bulk Minable hecious Metal Depositsof the Westem United States @. Schafer, J. Cooper & P.Vikre, eds.). The Geological Society of Nevada, Reno,Nevada (1 1-34).

& Borv, B. (1990): As-Tl-Sb-Hg-Au-Ba miner-alization Al5ar district, Yugoslavia. A unique type ofYugoslavian ore deposit. le Symp. on Thallium NeutrinoDetection (Dubrovnik, Yugoslavia), 36-37 (abstr.).

& Raprrr, A.S. (1990): Carlin type gold mineral-ization in the AlSar district, Macedonia, Yugoslavia.Eighth IAGOD Symp. (Oxawa),108 (abstr.).

& - (1993): Thallium in disseminatedreplacement gold deposits of the Carlin-type; a prelimi-naxy reprt. News Jahrb. Mineral. Abh. 166.65-75.

DISSEMINATED GOLD MINERALZA'TION" MACEDONIA 665

& BAGBY, W.C. (1990): Relationshipsamong carbonate-replacement gold deposits, gold skams,and intrusive rocks, Bau mining district, Sarawak,Malaysia. Mining Geol.40, L-16.

RADTKE, A.S. (1985): Geology of the Carlin gold deposir,Nevada. U.S. Geol. Sun., Prof. Pap. 1267.

& DtcrsoN, F.W. (1974): Genesis and verticalposition of fine-grained disseminated replacement-typegold deposits in Nevada and Utah, USA. 1z Problems ofOre Deposition. Fourth IAGOD Symp. (Varna) 1,71-78.

RyE, R.O. & DrcKsoN, F.W. (1980): Geology andstable isotope studies of the Carlin gold deposit, Nevada.Econ. Geol. 7 5. 641-672.

, Tavron, C.M. & HERopoulos, C. (1973):Antimony-bearing orpiment, Carlin gold deposit, Nevada.U.S. Geol, Surv. J. Res.1. 85-87.

Rtrcr, B. (1993): Famous mineral localities: Allchar,Macedonia. Mineral. Rec. 24, 437 -449.

Rosn, A.W. & KusuN, C.A. (1987): Ore deposition fromhighly acidic CO2-rich solutioirs at the Cadin golddeposit, Eureka county, Nevada. Geol. Soc. Am., Abstr.Programs 19,824.

Rvrusa, J.J. (1977): Mutunl Solubilities of Pyrite, Pyrrhotite,Quartz anl GoM in Aqueous NaCl Solutiorx f'rom 200o to500"C, and 500 to 1,500 bars, and Genesis ofthe ConezGold Deposit, Nevada. Ph.D. thesis, Stanford Univ.,Stanford. California.

SERAFrMovsKr, T., Borv, B. & Munnnrrd, C. (1991): Isotopiccomposition of the sulphur in the sulphides from As-Sbdeposit, Al3ar. Geol. Macedonica 5,165-172.

S[LnoE, R.H. (1990): Intrusion-related gold deposits. lzGold Metallogeny and Exploration (R.P. Foster, ed.).Blackie, Glasgow (165-209).

& BoNHAM, H.F., Jn. (1990): Sediment-hosted golddeposits: distal products of magmatic-hydrothermal sys-tems. Geolo gy L8, 157- 161.

Srvrmr, A.G. & Spnev, J.G. (1984): A half-ridge transformmodel for the Hellenic-Dinaric ophiolites. 1n TheGeological Evolution of the Eastern Mediterranean (J.E.Dixon & A.H.F. Robertson, eds.). Geol. Soc., Spec. Publ.t7,629-6M.

Spnay, J.G., BEsEr.r, J., REx, D,C. & Rooorcr, J.C. (1984):Age consuaints on the igneous dnd metamorphic evolu-tion of the Hellenic-Dinaric ophiolites. In The GeologicEvolution of the Mediterranean (J.E. Dixon & A.H.F.Robertson, eds.). Geol. Soc., Spec. Publ. l7, 619-627 .

WEUJ, J.D. & Mur-Lsus, T.E. (1973): Gold-bearing arsenianpyrite determined by microprobe analysis, Cortez andCarlin gold mines, Nevada. Econ. Geol.68,187-201.

Received March 16, 1993, revised manuscript acceptedDecember 17, 1993.