Stratigraphy, Mineralogy, and Genesis of the Bigadi

Stratigraphy, Mineralogy, and Genesis of the Bigadi~Borate Deposits, Western Turkey Doktrz Eyliil Ui~ioenitasi,A4iihenrli.dik Fakiiltesi, Jeokgi MiilzerzdisliRi Riiliirnii 35100, ~ o r n o o o - f z m i rTurkey ,

Abstract The Bigadif borates are the largest colemanite and ulesite deposits in the wvorlcl and the high-grade respectively) should supply a substantial proportion of the colemanite and ulexite ores (30 and 29% BB203, \vorld's needs for many years. The Bigadif deposits formed within Neogene perennial saline lakes sediments located in a northeast-southwest-trending basin. The volcano-sedimentary sequence in the deposits consists of (from bottom to top) basement volcanics, lower limestone, lower tuff, lower borate zone, upper tuff, upper borate zone, and olivine basalt. The borate deposits formed under arid conditions in perennial saline lakes fed by. hydrothermal springs associated with local volcanic activity. The deposits are interbedded with tuffs, . clavs. and limestones. . ~ ~ Borate minerals formed in two zones separated by thick tuff beds that have been transformed to montmorilIonite, chlorite, ancl zeolites (mainly heulandite) during diagenesis. Cole~naniteancl ulexite predominate in both borate zones, but other borates, inclr~clinghowlite, probertite, and hydroboracite are present in the lower borate zone; whereas inyoite, meyerhofferite, pandermite, tertschite (?), hydroboracite, howlite, tunellite, and rivadavite are for~nclin the upper borate zone. Calcite, anhydrite, gypsum, celestite, K feldspar, analcirne, heulandite, clinoptilolite, quartz, opal-CT, montmorillonite, chlorite, and illite are also found in the deposit. Colemanite nodules in both borate zones probably formed directly from solution, within unconsolidated sediments just below the sediment-water interface, and continued to grow as the sediments were compacted. It is unlikely that the colemanite formed by dehydration of inyoite and/or by replacement of ulexite after burial. Later generations of colemanite and ulexite are found in vugs and veins ancl as fihrous margins of early formed nodules. Other diagenetic changes include the partial replacement of colemanite by howlite and hydroboracite and ulexite by tunellite. N6clular-shaped colemanite ancl ulexite minerals predominate in both borate zones. Colemanite and ulexite show alternating horizons, and the transformation of one mineral to another has not heen observed ancl the and boundarv behveen them is alwavs shala. Because these minerals are readilv dissolved. secondarv Dure l transparent colemanite and ulexite are often encountered in cavities of nodules and cracks. Some colemanite ancl l~lexiteis \\leathered and completely replaced by calcite. Probertite bands are founcl in some ulexite horizons, especially in the lower borate zone. It forms in the same chemical environment as ulexite and indicates a period of more extreme desiccation and possibly subaerial exposure within the lakes. Euheclral tunellite formed during dissolution and recrystallization of some Sr-rich ulexite horizons. In the Bigadic deposits, hydroboracite formed by replacement of colemanite, with M$+ ions supplied from adjacent tuffs ancl clays by ion exchange. Howlite grew in clays alternating with thin colemanite bands and coincided with periods of relatively high Si concentrations. Diagenetic processes also produced small howlite nodules e~nbecldedin unconsolidated colemanite nodules. The initial solutions that formed the alkaline perennial saline lake(s) were low in Cl- and SO;- and high in boron and Ca2+,with subordinate Na+. ~

~

~~

2

'

i

Introduction TIIEBigadig borate district is located in the southeast of the known borate deposits of Turkey, -37 Im southeast of Balikesir province (Fig. 1).After the discove~yof borate deposits around Rigadig in 1950, mines were opened in the noitheast and southern parts of the area. Subsequently, there have been a number of stuclies of the deposits. The first minerals recorded were coleinanite and tertschite (Ca,. RloOlo 20H20); subsequently inyoite, meyerhofferite, and ulexite were recorded from the district (Meixner, 1952, 1956). Yilmaz (1977) dated the basaltic volcanic rocks associated with the Rigadiy borate deposits and obtained age of 13 Ma for the basaltic volcanics. Helvaci (1983) compared the Turkish borate deposits with other borate deposits in the world and de~ailedtheir mineralogy. Helvac~and Alaca (1991) surveyed the geology of the Bigachq deposits ancl explained the mode and occurrence of borate minerals in the area. Helvaci et al. (1987) reviewed the geoloo of Turkish borate deposits as part of a survey of the stratigraphy and econo~nicpotential

of West Anatolian Neogene sediments and correlated the stratigraphic sequences of the various borate deposits. Giindoidu et al. (1989) measured the chemical and isotopic composition of sedimentary smectite and clinoptilolite from the Bigadig basin. Details of these studies ancl the present work are discussed below. Recent exploration has shown that the Bigadig borate deposits formed in lower and upper borate zones separated by thick tuff beds. They are mineralogically more cornplex and the borate potential of the area much greater than previously thought. The Bigadiy deposits are now known to be the \vorlcl's largest colemanite ancl ulexite deposits. Proven borate mineral reserves are 532 million metric tons and total reserves are estimated as 987 nill lion metric tons (estimates published by permission of the Etibank State Mining Company). Several private companies operated mines in the area until 1978, when the deposits were nationalized ancl taken over by the Etibank Company. Extensive exploratory trenching and drilling of the deposits was undertaken by Etibank behveen

0

Bore hole

FIG. 1. Locality and geological map of the Bigadiq borate district.

1979 and 1985. The deposits are mined by open-pit and underground mining. This study covers the stratigraphic units in the Bigadiq volcano-sedimentary basin and surrounding area and presents a detailed investigation of the geology, textural aspects, and genesis of the borate-bearing units, together yith their mineralogy and ore grades.

Sample Collection and Analytical Methods All samples were collected from newly opened opencast mines, underground mines, drill core, and a series of sample traverses. Approximately 2 kg of material was collected from each site to ensure a representative sample. Borate and other minerals were characterized by direct-

BlGADlC BORATE DEPOSITS, W TURKEY

recording X-ray diffractometer analysis with standard powder and oriented sample techniques. Debye-Scheuer and Guinier cameras were also used for mineral identification. Mineral fractions were purified by handpicking, heavy liquids, and magnetic separation. B, Ca, Mg, Na, Li, and Sr from the borate sanlples were determined by the atomic absorption method. All the other major and trace elements of the borate samples were analyzed by the XRF method which is basically similar to that described by Leake et al. (1969/1970). All samples were run in duplicate. Major elements within the volcanics were determined by electron microprobe analysis of fused rock powders (Helvaci and Griffin, 1983). The results are comparable in precision and accuracy to XRF analyses for major and minor elements using a ~ h i l l i ~ 1410 s XRF spectrometer. The major elements of tuff samples were determined by a Phillips 1410 XRF spectrometer, used fused glass beads, and G-2 and BCR-1 standard rocks for calibration. Trace elements were analyzed using pressed powder pellets and G-2 and BCR-1 standard rocks for calibration. All samples were run in duplicate. Na and B from the volcanic and tuff samples were determined by atomic adsorption. U.S. Geological Survey reference samples G-2, BCR-1, and DTS-1 and a colemanite standard1013 (Helvaci, 1977, 1984) were used as the standards for calibration. Mineral analyses of polished thin sections were done using a LINK energy dispersive system (ZAF-4 correction program) mounted on an ARL-EMX electron microprobe at 15 kV accelerating voltage, under a focused beam. Each reported analysis represents an average of five to ten spots. Repeated tests on standard minerals have shouin that no loss of alkalies or other volatiles occurred during analysis. The K-Ar age dating method used in this study was presented by Damon et al. (1983).

.

1239

-

Colvso grained vitric tuff containing pumice

FF UNIT : Coarse agglomeratic in lower part

and partly dolomitic

Tectonic Setting and General Geology of the Bigadiq Area

FIG.2. Stratigraphic column section of the Bigadi~borate basin.

The Bigadiq borates were formed in two different zones separated by a thick tuff unit, elongated in a northeast-southwest direction. The basin is limited on the east and west bv extensive volcanics and on the northwest by the basement complex. Neogene sediments of the Bigadiy volcano-sedimentary basin were deposited in a number of individual or interconnected basins. bordered bv extensive outcrons of basement rocks. ~ h e s ebasins ' also t;end northeast-soutLwest and they may have been formed at the begining of the Neogene. The lateral variation in the thickness of the Neogene sedi0 ments probably reflects deposition either in separate perennial lakes or in an interconnected chain of lakes. The sediments, including the borate deposits, have been moderately folded and faulted. The total thickness of the Neo~enesequence exceeds 1,200 m. The lower and upper borate zones vary in thickness between 35 to 130 and 20 to 110 m, respectively (Figs. 2 and 3). The depositional basins and sedmentary formations of the borate deposits are aligned northeast-southwest. Their dip ranges from 20" to over 60" due to compaction, foldng, and faulting. Estimates of the subsurface extent

of the borate-bearing horizons are based on information from drilling and the distribution of older rocks exposed at the surface (Fig. 3). In the Bigadiq area the Neogene volcano-sedmentary rocks have formed several small symetric anticlines and synclines, which trend northeast-southwest (Fig. 1). The borate deposits and associated sediments are dislocated by the northwest-southeast-and northeast-southwest-trending normal faults, which occurred after deposition of the upper borates. The predominant faults are normal, with dips ranging from 40" to 90". Both vertical and horizontal displacement has taken place along some faults, such as the one located in the southeast of the Av~ardeposit (Fig. 1). The basement complex of the area was subjected to block faulting which occurred as pre-Miocene growth faults and dislocations. This resulted in the formation of sedmentary basins where the Miocene sediments were deposited. Largescale faults, which gave rise to the generation of the sedimentary basins, may also have served as conduits for exhalative boron-rich hydrothermal solutions during deposition of the borate-bearing beds. Recent faults in the basin have offset

0

Upper tuff unit Lower borate zone and lower ore Lower tuff unit

Frc. 3. Correlations and distributions o f the lower and upper borate zones, Bigadiy area.

the borate zones and have altered some of the borate to secondary calcite within the fault zones. Thermal springs, which at present deposit travertine, are active to the northeast of the area and are believed to have been important sources of boron deposition of the borate beds when they were probably related to the extensive volcanic and tectonic activity during the existence of the borate perennial lakes.

Pre-Neogene 2~nit.s The basement of the Bigadiq borates is comprised of Paleozoic and Mesozoic rocks. The borates are part of Neogene perennial lake sediments which rest unconformably on Paleozoic metamorphic rocks, consisting of marble and mica schists, and a Mesozoic ophiolite complex, consisting of the ophiolite itself, together with radiolarite, limestone, and graywacke (Figs. 1 and 2). These basement rocks are overlain unconformably by the basal volcanic unit in most of the study area, but north of Giivem~etmivillage, the lower boratebearing unit lies directly on the basement rocks. Neogene units The Neogene volcano-sedimentary rocks of the &strict consist of (in ascending order): basement (basal) volcanics,

the lower limestone (partly crystalline and dolomitized), the lower tuff unit (coarsely crystalline, rhyolitic-dacitic, and agglomeratic in the lower part), the lower borate zone (comprised of claystone, tuff, marl, chert, and thin bedded limestone containing the borate deposits), the upper tuff unit (coarse-grained vitric tuff containing pumices in the lower part and fine-grained vitric tuff and diagenetic zeolites in the upper part), the upper borate zone (claystone, tuff, marl, and thin bedded limestone containing the borate deposits), and olivine basalt (Figs. 2 and 3). Volcanic activity in the Bigadiq area began in the early Tertiary and continued until the beginning of the Quaternary. The earliest recorded lava flows are of rhyolites, dacites, trachytes, and andesitic basalts. These were followed by rhylites, dacites, and trachyandesites, which produced the tuff units present within the volcano-sedimentary sequence. The most recent flows were olivine-rich andesitic basalts and include some that are younger than the upper borate bed (Fig. 2). The volcanic rocks of the Bigadiq district are of mixed crustal and upper mantle origin. The initial felsic to intermediate calc-alkaline volcanics are dominantly crustal in origin, whereas the later volcanism are shoshonitic-alkaline-basic in character and more indicative of a mantle origin.

1241

BIGADlC BORATE DEPOSITS, W TURKEY TABLE1. Whole-Rock Analyses for Major Oxide Compositions in Volcanics and Tuffs from the Bigadi~Region Sample no. Weight percent SiOz Ti02 A1203 Fe203 FeO MnO M ~ O CaO Na20 K20 pzos B203 L.O.I. Total Trace elements in ppm Rb Sr Rb/Sr

Y Zr Nb Ni KINa

A-1

L-1

L-3

L-5

L-7

L-8

B-1

B-2

B-3

B-4

B-5

R-6

B-10

68.26 0.06 11.36 0.78 0.22 0.03 1.88 2.88 2.62 2.96 0.03 0.77 7.84

58.59 0.86 18.23 2.99 3.09 0.05 2.50 5.76 3.04 2.90 0.49 0.00 0.24

65.93 0.49 16.80 2.10 1.65 0.15 1.28 2.66 2.28 4.97 0.23 0.00 0.61

76.65 0.14 12.38 0.64 0.14 0.02 1.00 2.61 0.46 4.42 0.19 0.44 0.86

60.75 1.01 17.56 3.05 3.16 0.13 2.40 6.21 2.91 1.20 0.37 0.00 1.27

53.73 1.10 13.56 4.21 4.10 0.14 8.48 7.98 1.84 3.67 0.75 0.00 0.84

57.42 0.96 15.94 4.68 1.87 0.12 3.82 6.68 2.69 4.26 0.58 0.45 2.41

62.06 0.81 16.36 3.76 1.22 0.13 3.17 4.92 3.42 3.43 0.39 0.19 0.66

58.56 0.94 15.48 6.56 0.29 0.12 3.38 5.81 2.69 4.62 0.49 0.23 0.86

60.42 0.86 18.42 5.89 0.29 0.09 2.27 4.99 3.15 4.06 0.35 0.23 1.02

62.36 0.75 16.24 3.52 1.22 0.12 2.65 5.07 3.13 3.76 0.43 0.31 1.66

74.96 0.13 12.56 0.94 0.29 0.03 1.07 0.99 2.62 4.69 0.06 0.28 1.02

65.71 0.59 17.05 2.81 0.22 0.08 0.75 3.71 4.03 3.84 0.19 0.23 0.74

99.69

98.74

99.15

99.95

100.02

100.4

101.88

100.52

100.03

102.04

101.22

99.M

99.95

207 454 0.46 29 93 18 5 1.13

87 599 0.15 33 258 19 16 0.95

151 264 0.57 23 180 15 11 2.18

210 1,437 0.15 11 91 16 3 9.61

137 427 0.32 30 143 10 7 0.41

113 703 0.16 29 193 16 151 1.99

182 579 0.31 24 164 15 10 1.58

119 663 0.18 29 173 13 22 1.00

170 663 0.27 25 175 13 20 1.72

137 553 0.25 29 163 14 13 1.29

179 638 0.28 26 167 14 12 1.20

196 43 4.59 23 234 18 8 1.79

122 477 0.26 28 246 14 6 0.95

Samples: A-1, tuff (upper tuff unit); L-1, basalt (basement-basal volcanic unit): L-3, felsic tuff (lower tuff unit); L-5, felsic tuff (upper tuff unit); L-7, basalt (basement volcanic unit); L-8, basalt (olivine basalt, youngest basalt in the sequence); B-1, andesite (basement volcanic unit); B-2, trachycite (basement volcanic unit); B-3 and B-4, trachyandesite (basement volcanic unit); B-5, trachycite (basement volcanic unit); B-6, felsic tuff (upper tuff unit); B-10, dacite (basement volcanic unit) L.O.I. = loss on ignition

Basement (basal)volcanic unit This unit may exceed 250 m in thickness and is comprised of rhyodacite, dacite, trachycite, trachyandesite, basalt, agglomerate, and tuff. Rhyolite, rhyodacite, and dacite of the basement volcanics (the lowest rocks observed in the volcanosedimentary sequence) consist mainly of quartz, plagioclase (albite, oligoclase, and andesine), biotite, hornblende, and augite, with lesser amounts of sanidine and apatite. These rocks often alternate and coexist with tuff and agglomerates, which predominate in the lower part of the sequence. The rocks are white-gray or pale green. Porphyritic and spherulitic textures are common with a chloritzed matrix consisting of volcanic glass shards and small quartz microliths. The dacite is composed of quartz, feldspar, mica, and amphibole and has characteristic flow structures and a hyalopilitic texture. Feldspar, quartz, biotite, and to a lesser extent, hornblende microliths with rugged crystal edges occur within a vitreous matrix, showing both flow and crystalline structure. Cleavage surfaces of biotite show opacity due to alteration. These rocks contain a very small amount of opaque minerals. Trachyite and trachyandesite in the area occur together with tuff and agglomerate. The rocks are dirty yellow to gray and are composed of 3- to 4-cm-sized euhedral and subhedral sanidne crystals in a vitreous and crystalline matrix. Trachyandesites are composed of plagioclase, biotite, and hornblende phenocrysts in a vitreous matrix, and random quartz crystals. The plagioclase is oligoclase-andesine. Minor amounts of a fine-grained opaque mineral are present in the

matrix. These rocks have interstitial and porphyritic textures with a matrix containing microliths of plagioclase. The andesite is pink and cames macroscopically dlstinguishable feldspar and biotite. The rock is composed of biotite, hornblende, plagioclase (andesine, rarely labradorite), and quartz phenocrysts in a calcified and clayey, vitreous groundmass. Hornblende crystals are rare and partly chloritized. The rock is porphyritic, with a matrix of glass and plagioclase. Fresh surfaces of the basalt are green-black, whereas the altered parts are red-brown. Plagioclase, pyroxene, basaltic hornblende, magnetite, and hematite have been identified. Plagioclase is idiomorphic to hypidiomorphic, sometimes having a zoned structure, and is composed of labradorite with 55 percent An. Augite is hypidiomorphic and coarsely crystalline. The rock shows mainly intergranular, less commonly hyalophitic and ophitic textures. Volcanic tuffs and agglomerates occur in the region in varying amounts. The tuffs are generally composed of a finegrained vitreous matrix with abundant feldspar, quartz, and biotite crystals in the matrix. The feldspar is albite, oligoclase, and andesine. Some clay mineral weathering products are observed. The agglomerates are made up of andesite and dacite fragments, cemented by tuff. Pebbles are rounded to subangular with tuffaceous horizons as interbeds. Table 1 gives the chemical analyses of volcanic rocks and tuffs from the region. Hot water springs are present in andesite and trachyandeA

A

,

TABLE2. Chemical Analysis of the Hisarkoy Thermal Springs, Bigarlic Area Cations Na

K Mg Ca

B Li As Sb Sr

(mgA) 640 71 14 29

Anions C1 F

s0.t

HCOo

Trace element analysis (ppm) 12.38 Ra 1.96 44 1.62 Au \Y 0.12 0.96 Ccl

(mgA) 220 8.2 395 1,200

0.06 0.01 0.02 0.30 0.01

Analyst: Acme Analytical Laboratories Ltd., Vancol~ver,B.C., Canacla M~tliod:atomic absorption spectrophotometer and flame photo~net~y

usually occurring in a vitreous matrix. The terrestrial facies of the lower tuff unit is well exposed in parts of the region, especially around Koteyli village, north of Balikesir. Samples from this area are andesitic crystalline tuff, comprised of ashsized volcanic components cemented with plagioclase microlites. The lower terrestrial tuff contains local thin coal seams, especially around Koteyli village. Biotite from the lower tuff unit has been dated by K-Ar as 19.0 f- 0.2 Ma (University of Arizona, Tucson, pers. commun.). The lacustrine facies of the lower tuff unit overlies the lower limestone unit conformably. In places where the lower limestone unit is absent, it overlies the basement (basal) volcanic unit with a sharp unconformable contact. The lower borate-bearing unit rests on this unit with a concordant and gradational contact. The chemical analyses of tuffs me given in Table 1.

Lozuer borate zon.e (Lozuer borate-bearing un,it) site at Hisarkoy in the east of the study area (Fig. 1).The spring waters contain 10 to 13 ppm boron and have a temperature of 90°C. The chemical composition of the thermal springs is given in Table 2. The basal volcanic unit overlies the basement rock complex unconformably. This unit is in turn generally overlain unconformably by the lower limestone unit. The lacustrine volcanosedimenta~ysequence of the Bigacli~area is early to middle Miocene in age, with K-Ar values for the biotite and hornblende of the basement (basal) volcanic rocks showing an ~ 1989). average age of -22 Ma ( G u n c l o ~ det~al.,

Lower limncstone ,unit This unit is composed of white, yellow-white, green, creamy, ancl beige, thin bedded, ancl laminated marl, limestone, claystone,~dolomiticlimestone, and tuff. The lowest beds are white to creamy, thin bedded dolomitic limestone with abundant cracks and fractures. Over this lies a platy limestone-marl with alternating tuff beds. A banded claystone-limestone-tuff-marl constitutes the top section of the unit. Limestones in the lower section of the unit generally have a micritic and locally sparitic character. The thickness of the beds ranges from 5 t o 20 cm. Fractures and solution cavities are common and sometimes filled by calcite. The tuffs are yellow-green ancl composed of volcanic ash. The tuffaceous bands intercalated within the unit suggest volcanic activity continued during deposition. Dolomitic limestone at the bottom of the unit suggests that sedimentation started as chemical precipitation and became progressively more elastic with an influx of epiclastic and pyroclastic material.

This yellow-white unit is medium to well compacted with thin to intermedate bedding and lamination. The borate beds are in the lower part of the unit and alternate with limestone, clayey limestone, marl, claystone, mudstone, and tuff. Within the borate zone sedimentation begins with carbonate, followed by tuff, claystone, marl, and limestone with interbedded borates and returns to claystone, marl, and limestone. Sedimentary structures (sole marks, slump structures, and varcelike laminations) are common within the borate zone. Varvelike layers are formed by clay and carbonaceous layers, due to changes in the influx of epiclastic sediments and water level in the basin or changes in climatic conditions (Fig. 4). The lower borate-bearing unit is well exposed at the Tulu open pit (Fig. 5) and the Yenikiiy underground mine. The unit starts with a thin-bedded, occasionally laminated, alternating marl-limestone-tuff. Above these banded rocks lie the lower borate beds, which are 0.2 to 65 m thick. Borate minerals occur predominantly as nodules, thin beds, and lenses within the borate zone. They are composed of colemanite, ulexite, howlite, probertite, and hydroboracite, with colemanite and ulexite being the minerals of economic significance. Various

Lotuer tuff unit The lower tuff unit is dominantly rhyolitic and rhyodacitic and less colnmonly dacitic in composition. The unit is coarsely crystalline and agglomeratic in the lower part. The yellowwhite to gray unit was formed by active volcanism around the basin. Successive terrestrial and lacustrine phases were denositecl. Fresh surface samnles of the lacustrine facies are I I dark gray and contain abundant feldspar, quartz, biotite, rare hornblende, and diagenetic zeolites (heulandite and clinoptilolite), in addition to a small amount of volcanic fragments,

4, \rawelike clav alrd car170naccollslaminations oT the associ>lted secliments, lower borate zone, Bigadi~area, (crossed nicols), wicltll of pl~oto ca. 2.5 m m . ,

1243

BIGADlC BORATE DEPOSITS, W T U R E Y

green pumice fragments and a porous stn~ctureare typical features. The upper part of the unit is composed of very fine grained, light vitric tuff with conchoidal fracturing. The upper tuff unit is dominantly rhyolitic and rhyodacitic in composition, and less commonly dacitic. It is coarse grained and vitric with ~ u m i c ein the lower IDart.' whereas it is fine grained and vitric with dagenetic zeolites in the upper part. It is composecl of quartz, plagioclase, sanidine, K feldspar, biotite, chlorite, diagenetic zeolites (heulandite and clinoptilolite), smectite, and pyrite. Volcanic glass and shards are usually altered to authogenic minerals. Petrographic investigation of the upper tuff unit showed high amounts of pumice, heulandite, clinoptilolite, and analcime, with lesser amounts of sanidine, quartz, plagioclase, and biotite. Pumice with a fibrous texture is distributed irregularly within the rock. Some horizons are sufficientlv dominated b\7 zeolite minerals as to be considered economic zeolite deposits (Fig. 6). The absence of carbonaceous and epiclastic interbeds FIG.5. Lonar borate zone, section of Tiilii opencast mine, Rigadi~. within the tuff (which was denosited bv renewed volcanism that had been inactive during deposition of the lower borate unit) indicates that volcanism continued without interuption amounts of nonborate minerals (calcite, dolomite, anhydrite, d u r i n ~denosition of the tuff. The thickness of the unit in" I and gypsum) also occur within the zone. The most common creases toward the north within the basin and extends up to clay minerals are montmorillonite and illite (Helvac~,1983). 410 m. This may be due to variations in the intensity of Bands of gray tuff, platy claystone-limestone, and interlay- volcanic activity, the proximity to the source or the depth of ers of thin bedded limestone ancl claystone also occur within the basin at the time of denosition. The lower contact of the the borate ore zone. The borates are overlain by a banded unit with the lower borate-bearing unit is conformable and section composed of laminated brown claystone and gray- gradational. The upper contact with the upper borate zone white limestone. It is overlain by alternating beds of medium- is also conformable, but not gradational. bedded cherty limestone, claystone, and limestone. Around Yenikoy village, chert bands 40 to 60 cm thick and porous Upper borate zoize (upper borate-bearing unit) limestone are present in the upper parts of this unit. The This unit is composed of alternating beds of limestone, total thickness of the whole unit varies between 35 and 130 claystone, clayey limestone, marl, and tuff and included lenses m (Fig. 2). Limestone within the unit is composed of sparite, biospar- of borate beds at intermediate levels. Within the borate zone, ite, fossiliferous sparite, microsparite, and micrite. Clayey sedimentation begins with carbonate, followed by tuff, limestones are composed essentially of calcite and clay miner- claystone, and marl with interbedded borates ancl returns to als with minor quartz. Locally they become a dolomitic marl carbonate (Fig. 7). Sole marks, slump structures, mud cracks, composed of dolomite, clay, calcite, and feldspar. Mudstones are generally made up of carbonaceous mud with weak compaction and containing a few volcanic fragments. Tuff occurs as thin lenses or interbeds within the unit. The lower borate-bearing unit generally rests on the lower tuff unit conformably and shows gradation into the latter. The thickness of the gradation zone is variable, ranging between 2 and 4 m. The unit sometimes overlies the basement (basal) volcanics or the basement complex unconformably, as at Yenikoy village. At the top, a conformable and gradational contact exists between the lower borate-bearing unit and the upper tuff unit. The area covered by the tuff unit is extensive. The depositional environment consisted of alkaline prennial lakes probably interconnected with one another. These basins were deeper as perennial lakes during limestone deposition and shallow as ephemeral lakes during borate deposition. i

i

L >

Upper tuff unit This unit is extensive in Ihe central pa* of the area' The upper tuff unit begins with coarse-grained vitric tuff at the base and is characterized by its yellow-green color. Dark

to a,~tog(xlli~ ~ ~ ~ i l i c r di ~ol s~, ~ ~ i FIG.6. \folciillic d;iss alld shards i~ltel.~.(l nantly to zeolites, upper tuff unit, Eigadiy. Crossed nicols, \\idth of photo cn. 2.5 mm.

1244

HELVACI

Flc. 8. Ulexite ore lenses (upper orebody) intercalated \\it11 associated sediments, Kurtp~nandeposit, Rigatli~.

FIG.5. Upper Loratr zone consisting of claystonc, thin limestone, and tuff alternations, Silnav opencast mine, 13igadiy.

and varvelike laminations are present within the clay and carbonaceous layers of the borate zone. The upper borate zone starts with alternating thin beds of tuff, banded claystone, limestone, and marl. Over these beds lie a waxy claystone and the borate zone. The borates are overlain by red-brown laminated claystone. Alternating beds of claystone and limestone, with thin to medium thick beds of mixed tuff, marl, and limestone cover the former lithologies. The 11nitends with a thick bedded limestone containing occasional chert bands. The limestones of the upper borate zone are white to creamy. They are partly siliceous ancl contains dissolution cavities. The thickness of the beds varies between 0.2 and 40 cm. Thin-bedded marl is yellow to creamy whereas the tuff is yellow-green due to alteration. The latter are medium beddecl and contain macroscopic biotite and feldspar. Clays are red-brown, green, and gray and have a waxy appearence. Platy claystones at the top and bottom of the borate zone form recognizable marker beds. The borate beds occur in the middle of this unit. The thickness of the borate zone varies between 0 and 30 m, whereas the thickness of the unit varies between 20 and 110 m. Economically exploited borate minerals are colemanite ant1 ulexite. Ulexite is dominant at the Acep, Kire~lik,Kurtplnari, Arka, and o n Giinevi deposits, whereas colemanite predominates in the Avgar and Si~navdeposits. Other borate minerals that occur in the ore zone of the upper boratebearing unit are meyerhofferite, pandermite, probertite, howlite, tunellite, hydroboracite, and inyoite. The thickness, number, and distribution of these borate beds are variable. since the borate beds generally occur as lenticular bodies within the claystones (Fig. 8).

Laminated carbonaceous and clastic rocks of the unit reflect seasonal climatic changes during precipitation of the unit, as well as facies changes and water level fluctuation in the basin. Tuff bands within the unit indicate that volcanic activity continued in short intervals during sedimentation. The unit occurs in a limited area, so it appears that the deposition basin was small as ephemeral lakes during the time the upper borate was laid down.

Olivine basalt This fine crystalline black and gray-black unit has intruded all the other units older than itself. The total thickness of the unit is maximum 50 m. It occurs as dikes and flows, most notably in the Boz Tepe and Camkoy area (Fig. 9). The olivine basalt contains plagioclase, augite, biotite, and olivine (usually altered), and small amounts of hornblende and diopside. The matrix has an ophitic texture due to the irregularity of plagioclase microliths ancl the augite and olivine crystals that occur between the plagioclase crystals. Porphyrtic and intergranular textures are also present. K-Ar dating of the olivine basalt yields an age of 18.3 + 0.2 Ma. Table 1also gives the chemical analysis of olivine basalt.

FIG.9. Olivine basalt intn~dedin the older units, and occurring as dikes and flows near Camkiiy, Bigadiy.

1245

AIGADIC BORATE DEPOSITS, W TURKEY TABLE3. Borate ancl Nonhorate Minerals Occurring in the Bigadi~Deposits Mineral name

Formula

Inyoite Meyerhofferite

Ca2B6011.lnFI20 Ca2B6oL1-7H20

Colemanite Tertschite Pandermite (priceite) Ulexite Probertite

Ca2B6Ol1-5H20 C ~ R l o O 20H20 I,~ Ca4BloOla 7H20 NaCaRs09. 8HeO NaCaB50p 5 H p 0

Hydroboracite

CaMgBGOll.6H20

Tunellite Howlite

SrB60104 H 2 0 CanB5SiOl,.(OH)5

Rivadavite Calcite Aragonite Dolomite Anhydrite

N ~ M ~ B 2 4 022H20 40 CaCO:, CaC03 CaMg (COJ2 CaSO,

Gpsum

CaS04 2H20

Celestine Heulandite Clinoptilolite Analcime Potassi~~~n feldspar Quartz Opal-CT Montmorillonite Chlorite Illite

SrS04 (Ca,Na2)N2SiiO18.6H20 (Na2,K2,C&. AIRSim072. 241-120 NaAISip06 H 2 0 KA1Si30:I Si02 Si02.n H 2 0 (Mg,A1)2Si,010(OI-I)n 111-IpO (SiAl),(Mg,Fe)(,Opo(OkI)4 (K,H,O)A~~(A~S~,OI,,)(OH)~

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-

Lower borate deposits

All deposits All deposits Exploratoly bore holes (EIS-1,2,4) Ex~loratolybore holes (EIS-2) Exploratory bore holes (EIS-1,2,4,7,15,17,19) All deposits Nl deposits All deposits Exploratory bore holes (EIS-2) Ex~lorato~y bore holes (EIS-1,2,3,4,15,16) All deposits All deposits All deposits All deposits All deposits A11 deposits All deposits All deposits

Recent fluvial sediments, consisting of conglomerate, siltstone, and claystone, and more recent alluvium, were deposited on top of the Neogene sequence in the central part of the area. The fluvial sediments start with conglomerate beds, which are overlain by sandstone, siltstone, and claystone intercalations. The thickness of the unit ranges from 5 to 180 m. These recent sediments overlie the upper borate-bearing unit unconformably and are in turn overlain unconformably by alluvium. The alluvium covers the whole unit and is composed of pebbles, sand, and clays of the basement to Neogene and Quaternary rocks. The alluvium exceeds 70 m in thickness (Figs. 1 and 2).

Upper borate deposits Gunevi, Acep, Salmanl~,Simav, Kurtp~nan,Kireqlik Sirnav, Regendikler, Giinevi, Kurtpinan, Camkoy, Kireqlik, Acep, Simav, Borekqi, Avgar All deposits Kurtp~nan M e m r b q ~Simav, , Avpr All deposits Giinevi Giinevi, hfezarbq~,Kireclik, Iyklar Giinevi, Kireqlik Kurtp~nan,Domuz Deresi, Simav, Avgar Tiiluovas~ All deposits All deposits All deposits Gunevi, Avgar Giinevi, Av~ar,Acep Giinevi, Simav All deposits All deposits Mezarbag~,Kireqlik, Domuz Deresi All deposits All deposits All tleposits All deposits All deposits All deposits

The mineral assemblages in the borate zones vary at different levels in the sequence (Fig. 10). There is also an overall difference between the mineral assembla~e of the lower and 0 upper borate zones. Inyoite, meyerhofferite, pandermite, and tunellite are all restricted to the upper borate zone. Chemically this implies that the brines that formed the upper borates were relatively rich in Ca and Sr (favoring precipitation of Ca and Sr borates) compared to the lower borate zone (Fig. 10). Borate minerals in the Bieadic devosits occur as nodular 0 ' 1 forms with radiating structures, fibrous layers surrounding nodules, thin layers interbedded with clay and tuff (sometimes brecciated), disseminated crystals in a clay matrix, massive borate. thin veins crosscuttine sedmentarv lavers. and 9 could have existed in the center of the lake during the final stage of evaporation and diagenesis (Helvacl et al., 1993). The pH values at the margins of the lake J

may have been lower (7 < pH < 8). At some mines in the Bigadiy area, the presence of clinoptilolite and opal-CT only is indicative of a pH of 8 to 9 (Surdam and Parker, 1972; Helvac~et al., 1993). If, as seems likely, the borates formed either earlier or at the same time as these authigenic minerals, then this evidence indicates that borates precipitate over a range of pH values > 8 and are not restricted to a specific pH (Helvaci et al., 1993).The clay mineralogy of the Western Anatolian borate deposits indicates that the clays formed at different pH values in the Emet, Kirka, and Bigadiy deposits (Ataman and Baysal, 1978). The formation and paragenetic association of authigenic clinoptilolite, analcite, and K feldspar requires pH values in the range 8 to 10. Overall, this suggests that precipitation of the borates started at a pH of around 8 and continued at higher alkalinities. In conclusion all the mineralogical variations of evaporitic and authigenec minerals in the Bigadi~deposits were also controlled by the type and density of the volcanic products as suggested by Helvac~et al. (1993). The genesis of colemanite by the breakdown of ulexite was suggested by Foshag (1921) for deposits in California, but does not appear to be applicable to the Bigadiy colemanites for the following reasons. Nowhere is colemanite observed to be replacing ulexite; nowhere have cores of colemanite been found in indurated masses of cotton ball ulexite as at Kramer, California (Bowser, 1965; Bowser and Dickson, 1966): Colemanite and ulexite always occur as separate nodules and layers and the contact between colemanite and ulex-

1256

HELVACI TABLE7. Chemical Analyses of Ulexite Ores from the Bigadi~Deposits

Oxide percent B203 CaO Na20 MgO SrO SiOz Ale03 Fez03 So3 As L.O.I. Total Elements in ppln Li Rb Sr Ba Analyses: 1,massive and banded ulexite ore with trace hyclroboracite, tunellite, and clay (On Giinevi deposit); 2, nodular and fibrous illexite ore consisting of dominant ulexite with trace celestite and clay (Simav deposit); 3, nodular nlexite ore with minor celestite, gypsum (?), and clay (Simav deposit); 4, fibrous ulexite ore with minor celestite and clay (Simav clepsosit); 5, nodular and fibrous rllexite ore with minor clay fraction (Simav deposit); 6, banded ulexite ore consisting of dominant ulexite uith minor colemanite, tunellite, and clay (Kireclik deposit); 7, massive and fibrous ulexite ore with minor celestite and clay (Avaar deposit); 8, massive and fibrous ulexite ore with celestite and clay fractions (Av~ardeposit); 9, nodular and fibrous ulexite ore with colemanite, gypsum, celestite, and clay (Av~ardeposit); 10, massive and fibrous ulexite ore with hyclroboracite, clay, and quartz fraction (Arka Giinevi deposit) Total iron as Fe203; L.O.I. = loss on ignition

ite is always sharp. Interbedded clays at Bigacbq are notably defficient in Na+ and not enriched as they should be if exchange between the ulexite and clays had taken place occording to the model of Ozpeker (1969):

the Bigachy deposits, where no borax, or any other Na-bearing borate mineral other than ulexite and probertite, is present. Alternative suggestions that metasomatic replacement of limestone by colemanite (Gale, 1913) can also be dismissed since the colemanite beds do not pass laterally into limestone and no partially altered limestone has been found. Rogers (1919) suggested that the higher hydrate inyoite = 5Ca[B304(0H)3]. H 2 0 + 3Na+ + H+. (Ca[B30J(OH)s]4H20)was the first formed Ca borate min(colemanite) eral, which on burial and diagenesis was dehydrated to the Interbedded sediments and borates at Bigadiq are deficient denser meyerhofferite or, more commonly, colemanite. The in Na and C1, and not enriched as they should be in the presence of drusy cavities (see Fig. 16), often containing wareaction suggested by Foshag (1921) and also postulated to ter, and of rare septarian cracks filled with clear colemanite occur at temperatures >70°C by Kemp (1956), in the pres- similar to those in the Emet deposits (Helvaci and Firman, ence of percolating NaCl solutions: 1976; see Fig. 13), suggest that reduction in volume occurred in some coleinanite nodules. However, negligible inyoite has 2NaCaBs0,. 8H20 + A q (ulexite) been found in the Bigadi~deposits and no pseuclomorphs of colernanite and meyerhofferite after inyoite have been re= Ca2B6011 5 H 2 0 + [Na2B40i] + Aq. corded (Meixner, 1952, 1953, 1956; Ozpeker, 1969; Helvac~ (colernanite) (borax) and Alaca, 1991). Shrinkage cracks and d n ~ s ycavities need Suggestions that alteration of ulexite leads to the form at'ion not be due to dehydration of inyoite. In addition, Bowser and of the mineral pair colemanite plus borax are inapplicable to Dickson (1966) concluded that the coleinanite in nodules from Kramer and Death Vallev. which are similar to those from Bigadi~,grew from solution in unconsolidated sediments and need not have formed by alteration of preexisting ulexite or dehydration of inyoite as suggested by Barker and Barker (1985). Experiments have shown that colemanite and calcite form when the evaporation products of 1:l and 1:2 mixtures of Calcium -Borate Basemant ~ o c k colemanite and calcite dissolved in HC1 are exposed to the Facies + + Calcium-Sodium atmosphere for several months (Helvaci, 1977: 1986). This Borate Facies reaction takes place at atmospheric pressure, laboratory temFIG. 29. Schematic section from the Bigadiq deposits. peratures of -30°C, and a pH of 8 to 9, indicating that deep

-

i '

1+1

1257

BIGADIC BORATE DEPOSITS, W TURKEY

burial and high " Dressures are not necessarv for the solution and reprecipitation of colemanite under natural conditions (Helvaci, 1986). Ulexite nodules. like colemanite.' amear to have develo~ed I1 contemporaneously within unconsolidated sechments. ?hey developed from waters enriched in Na as a consequence of Ca carbonate and Ca borate precipitation. Within the Bigadig de~osits.colemanite and ulexite mav occur as com~letelv separate masses or they can occur together indcating that they formed contemporaneously. Layers and nodules of inyoite, mayerhofferite, pandermite, howlite, and probertite also appear to have developed within the sediments. Their sporadic occurrence indicates that preci~itation of these minerals was limited to certain IDarts of I the deposit, possibly around localized hot springs or other restricted geochemical environments. Propertite forms in a chemical environment similar to ulexite. b;t its lower water content indicates that it