Geochemical discrimination of volcanic rocks associated ...kisi.deu.edu.tr/cahit.helvaci/Geochem.pdf · Geochemical discrimination of volcanic rocks associated with borate deposits:

JOURNAL OF GEOCHEMICAL E~PlORI \ I IO#

ELSEVIER Journal of Geochemical Exploration 60 (1998) 185-205

Geochemical discrimination of volcanic rocks associated with borate deposits: an exploration tool?

P.A. Floyd C. Helvaci b, S.K. Mittwede " Departnzent o f Earth Sciences, Unioersih o f Keele, Keele, Staffordshire. ST5 5BG, UK

Dokuz Eyliil iinicersitesi, Jeologi Miihendisligi Bolrtmii, 35100 Rornoua-Izmir, Tlirkey Mutqferriku Consulting Senvices Ltd., P.K.290, 06443 Yeniseliir, Ankara. Turkey

Received 15 July 1997; accepted 25 September 1997

Abstract

The Miocene borate deposits of western Turkey are associated with extensive medium- to high-K calc-alkali ignimbritic volcanism and a differentiated comagmatic alkaline trachybasalt-trachydacite lava suite. Ignimbritic air-fall and reworked pumicebus clastic materials are intimately associated with lake sediments that host the borate deposits. Local ignimbritic volcanism is considered the primary source of the B for the Kirka borate deposit in this area. Comparison of the geochemical composition of Turkish ignimbrites associated with borates ('fertile' ignimbrites) with those that do not ('barren' ignimbrites), exhibit a number of features that might prove useful in the exploration for borates in similar volcanic domains. In particular, 'fertile' ignimbrites are (a) generally a high-K calc-alkali suite, well-evolved and fractionated (K/Rb is low, < 200) with a high-silica rhyolitic bulk composition, (b) exhibit a combined high content of B, As, F, Li and Pb, with high B/La (> 1) and B /K (> 0.001) ratios, and (c) a mildly fractionated REE pattern (La,/Yb, - 2) and large positive Eu anomaly (Eu/Eu* - 0.1). Other apparent discriminants involving both compatible and incompatible elements (relative to major silicate phases) are largely a function of different degrees of partial melting and fractionation. It is suggested that the initial source of the B (and other associated elements) was from LIL-rich fluids released by the progressive dehydration of altered oceanic crust and pelagic sediments in a subduction zone. The absence or presence of sediments in a segmented subduction zone may influence the variable lateral distribution of borates in active margins on a global scale. Once the crust has become enriched in B via previous or contemporary subduction-related calc-alkali magmatism, the effect of tectonic environment, climate and hydrothermal activity influence the local development of the deposits. O 1998 Elsevier Science B.V.

Keywords: ignimbrites: geochemical discrimination; boron; borates; Turkey

1. Introduction under arid climatic conditions in playa lakes. Borate minerals (colemanite, ulexite, borax etc.) are the

Borate deposits may often be an important con- major source of commercial boron (B) and are largely stituent of economic nonmarine evaporites formed concentrated in continental Tertiary deposits of west-

em Anatolia (Turkey) and the American continent (e.g. western USA, central Andes) (Helvaci, 1978;

Corresponding author. Fax: +44 1782 715261; E-mail: Kistler and Smith, 1983; Alonso et al., 1988, 1991; [email protected] Helvaci et al., 1993).

0375-6742/98/$19.00O 1998 Elsevier Science B.V. All rights reserved. PI1 S0375-6742(97)00047-2

186 P.A. Floyd et a/. / Jol~rnal o f Grockernicol E,rploru/ion 60 (1998) 185-203

Apart from the restricted range of environmental and tectonic conditions under which Tertiary borate deposits accumulated, one other consistent feature is their close geological association with volcanogenic rocks. It has long been believed that borate deposits may be genetically related to volcanic activity especially in the light of associated boron-rich thermal waters and boric acid-bearing steam fumaroles (e.g. Foshag, 192 1 ; Clarke, 1924; Gale, 1964; Jensen and Bateman, 1981). In particular, the borate deposits of western Turkey are closely associated in space and time with Miocene tuffs and lavas (Helvac~ et al., 1993; Helvac~, 1995). While both lavas and tuffs commonly surround the borate-bearing playa deposits, it is the tuffs that appear to be consistently interbedded with the borates or constitute a major component of bedded epiclastic materials. The source of the boron is thus often linked to this volcanic activity via the agency of hydrothermally generated B-rich springs that mix with the playa waters and precipitate borates (e.g. Inan et al., 1973; Helvac~, 1995).

2. Objectives and data sets

The object of this work is to test the presumed relationship by examining the chemical composition of silicic volcanic rocks which occur in the vicinity of a known borate deposit, such as the Kirka borax deposit, near Afyon, central Anatolia, Turkey (Fig. 1). If it is possible to geochemically fingerprint or differentiate volcanic rocks associated with borates (termed 'fertile' volcanics) from those without (termed 'barren' volcanics) this feature might prove to be useful in the exploration of large silicic volcanic provinces as to their potential for borate deposits. Previous geochemical work has largely concentrated on the borate minerals themselves and the volcaniclastic rocks intimately associated and interbedded with the borates (e.g. Inan et al., 1973; Castor, 1993; Helvacl et al., 1993; Helvacl, 1995). However, these volcanic rocks have been invariably penetrated by migrating B-rich hydrothermal solutions such that high concentrations of B and other associated mobile elements (As, Li, Sr; Castor, 1993) are to be expected, the host rock having been effec- tively metasomatized and mineralized. However, we

are concerned with the volcanic suite as a whole and suggest that if the B in the deposits is genetically related to the volcanic rocks, then they may display a distinctive geochemical signature overall, as well as being specifically enriched in B. Preliminary data suggest that this may indeed be the case in Turkey (Anderson et al., 1995; Floyd et al., 1995). This proposal is based on the assumption that the vol- c a n i c ~ have a special composition governed largely by their genesis and that subsequent borate deposition reflects a later mobilization and enrichment caused by essentially secondary processes (such as hydrothermal activity and evaporation). The tectonic environment within which the volcanics initially developed may also play an important part in the generation of their specific geochemical characteristics.

In order to test the above proposition a number of volcanic rocks were analysed that are associated with borates from western Turkey, but to avoid contamination by mineralization all samples were collected > 10 km away from the actual deposits themselves. The Turkish data set includes compositionally variable Miocene volcanics (lavas and tuffs) that strati- graphically span one of the main commercial borate deposits at Kirka (see below, and Fig. 1). In particular, the Kirka tuffs (the 'fertile' group) are compared with lithologically similar tuffs from central Anatolia that do not host borates (the 'barren' group). A third set of tuffs that are associated with borate mineralization from other parts of the world (Samos, Greece; Nevada, USA; Tincopalca region, Peru; Tujuy and Salta provinces, Argentina) have also been analysed to see if similar geochemical characteristics are dis- played as the 'fertile' Kirka tuff^. However, as the data set for this last group is small and geographi- cally restricted, attempts to provide a satisfactory global discrimination at this stage must be considered preliminary and premature. Representative anal- yses are shown in Table 1 (ignimbrites associated with borates, Kirka deposits, Turkey), Table 2 (Kirka alkalic lavas), Table 3 (ignimbrites associated with selected borate deposits), and Table 4 (central Anato- lian ignimbrites not associated with borate deposits).

All samples were analysed for major oxides and most trace elements at the University of Keele (UK) using an ARL 8420 X-ray fluorescence spectrometer, calibrated against both international and within-house

P.A. Floxd rt 01. / Jour~ial of Grochrmical Evplorcrtion 60 (1998) 185-205 187

geostandards (analytical method and precision in Floyd and Castillo, 1992). Various subsets of tuff samples were also analysed for B, Li, F, Hf, Sc, Ta, Th, U and REE by Activation Laboratories Ltd., Canada.

3. Stratigraphic relationships

The economic borate deposits of western Turkey are situated in five districts, Bigadic, Emet, Kestelek, Kirka and Sultancayiri, and are geologically bounded within a series of Tertiary NE-SW-trending rift basins or half-grabens (Fig. I). The deposits were all formed during the Miocene in a lacustrine environment in which B is thought to have been enriched by the activity of geothermal springs. The borate deposits formed by evaporation in shallow playa lakes (Helvaci, 1995; Helvac~ and Yagmurlu, 1995). Al- though each locality shows differences in detail, the Kirka succession is typical, with borates intimately interbedded with and penetrating marls, claystones and zeolitized volcanic tuff (Helvaci et al., 1993). In addition to tuffaceous epiclastic material in the lacustrine sedimentary rocks, the main volcanic components in the Kirka s~~ccession sandwich the borate horizons (Fig. 1). The volcanic rocks comprise three groups: a lower unit of (a) intermediate lavas penecontemporaneous with (b) voluminous tuffs, and an upper nit of (c) basaltic lavas. Tuffs and tra-

chytic lavas from the lower unit have similar K-Ar biotite ages (tuffs 19.0 f 0.2 Ma and lavas 18.5 + 0.2 Ma), whereas the upper basalts are clearly younger (16.1 f 0.2 Ma; K-feldspar) (Helvaci, 1995). Field, sedimentological and petrographic evidence link the tuffaceous epiclastic material in the lake sediments with the voluminous tuffs surrounding the borate deposits, and suggest that volcanism was largely contemporaneous with borate formation. For example, a characteristic feature of the tuffs are ubiquitous dark quartz clasts that are also a common feature of ash-fall (and reworked) deposits interbedded with lake epiclastic sediments. Further details of the stratigraphy, lithologies and depositional models for the Kirka and associated deposits in western Anato- lia are given in Inan et al. (1973), Helvaci et al. ( 1993), Helvaci and Yagmurlu (1 995), and Helvac~ (1995).

The nowTurkish 'fertile' tuffs also show stratigraphic association with tuffaceous materials both within and surrounding the depositional lake envi- ronments. For example, the Late Miocene Samos deposits (Greece) are found in tuffs that form part of a saline-alkaline lacustrine sequence of claystones and limestones (Stamatakis, 1989; Helvaci et al., 1993). Neogene nonmarine evaporite deposits in the central Andes contain borate deposits of about 7 Ma, which are encompassed by volcanism (that deposited the intercalated tuffs) over a period of 15 to 0.3 Ma

Fig. I. Location of Miocene borate deposits in extensional rifts in western Turkey, and the stratigraphy of the Kirka area. Hachured areas are Neogene and Quaternary basins (after Ketin, 1983), some of which are bounded by normal faults (thick lines).

188 P.A. F l o ~ d rr a/. / Jortrnol of Geochetnicrrl E.rploration 60 (IYYHl 185-205

Table I Geochemical composition of ignimbrites from the Kirka region, western Turkey

Sample KIR-3 KIR-4 KIR-5 KIR-6 KIR-7 KR-8 KIR-9 KIR-19 KIR-20 KIR-21 KTR-22

Major oxides (wt.96) SiO, 74.72 TiO, 0.09 AI,O, 13.21 Fe,O, 1.40 MnO 0.04 MgO 0.12 CaO I .04 Na?O 2.24 K 2 0 4.88 PzOr nd LO1 2.7 1 Total Tom5

Trace elements (ppm) As 32 25 3 1 30 28 33 39 2 1 60 B 57 76 68 7 1 97 I44 7 1 160 134 Ba 104 49 52 34 31 36 60 70 35 Ce (XRF) 52 36 49 46 33 37 42 40 32 CI 240 28 45 20 49 49 121 43 144 Cr 14 15 15 14 16 15 16 2 I Cu nd nd nd nd nd nd 1 nd I F 440 710 520 560 760 560 540 240 560 Ga 15 17 18 17 18 18 18 16 17 La (XRF) 27 - 8 13 19 13 13 13 17 15 Li I0 7 7 9 4 5 7 6 4 Nb 27 38 37 36 37 38 42 24 39 Nd (XRF) 24 23 32 23 25 18 24 16 29 Ni 6 4 3 5 3 4 5 3 3 Pb 90 60 78 80 59 73 91 104 90 Pb 318 376 392 376 39 1 390 347 294 393 S 151 115 82 68 82 80 85 64 76 Sr 64 33 24 27 26 29 44 46 30 Th 52 41 38 43 4 1 39 37 50 37 V 3 1 I 1 2 3 5 18 8 Y 40 79 65 79 53 52 62 33 58 Zn 34 27 25 38 33 22 36 36 40 Zr 100 83 85 85 86 82 87 95 85

REE (ppm) La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu

Fe,O; =total Fe as Fe,O,: nd = not detected

P.A. F l o ~ d rt 01. / Jortuiul of Grochrmicol E.rploru/ion 60 (19981 185-205 189

Major oxides (wt.7~) Si02 77.49 TiO, 0.05 AI,O, 12.68 Fe,O; 0.48 MnO 0.02 MgO 0.01 CaO 0.68 Na,O 3.01 KzO 4.55 PZOC nd LO1 0.8 1 Total ')9.78

Trace elements (ppm) As 13 53 B 134 167 Ba 32 79 Ce (XRF) 46 40 C1 62 233 Cr 3 7 Cu nd nd F 380 580 Ga 14 19 La (XRF) 14 20 Li 20 I I Nb 29 44 Nd (XRF) 23 24 Ni 3 6 Pb 98 l I0 Pb 357 346 S 63 67 Sr 25 39 Th 4 1 43 V 4 18 Y 50 70 Zn 20 39 Zr 84 101

REE (ppm) La 16.67 Ce 34.69 Pr 4.43 Nd 17.81 Sm 5.66 Eu 0.20 Gd 6.39 Tb 1.26

DY 8.21 Ho 1.76 Er 5.35 Tm 0.88 Yb 6.0 1 Lu 0.94

190 P.A. Floyd et a/ . /Journal of Geochenzicnl Exploration 60 (1998) 185-205

Table 2 Geochemical composition of representatives of the alkali lava suite in the Kirka region, western Turkey

Sample KIR-I KIR-2 KIR-I6 KIR-17 KIR-I8 KIR-I0 KIR-I I KIR-I2 KIR-13 KIR-I4 KIR-15

Major oxides (wt.%)

SiO, 62.29 62.76 60.72 60.39 62.54 53.47 52.50 52.25 50.04 51.53 51.49 TiO, 0.67 0.70 0.75 0.78 0.68 1.32 1.38 1.30 1.40 1.40 1.35 A1,0, 15.71 16.14 15.32 15.13 15.39 14.38 15.21 14.27 15.07 15.39 14.80 Fe,O; 4.96 4.99 5.38 5.57 4.75 7.38 7.72 7.35 7.64 7.68 7.00 MnO 0.10 0.10 0.09 0.12 0.09 0.16 0.14 0.14 0.13 0.13 0.12 MgO 2.61 1.75 2.96 3.32 2.37 5.58 5.33 6.19 5.78 6.43 8.65 CaO 4.67 3.87 5.12 5.1 1 4.25 8.43 7.16 8.63 9.49 8.04 6.89 Na,O 3.55 3.52 3.20 2.97 3.17 2.36 2.28 2.25 2.50 2.50 2.1 1 K,O 4.67 4.94 5.17 5.10 5.03 5.10 5.05 4.87 3.84 4.41 4.61 p2 0 5 0.47 0.47 0.51 0.52 0.47 0.70 0.72 0.68 0.72 0.7 1 0.56 LO1 0.29 0.66 0.70 0.79 0.82 1.59 2.03 2.33 - - - - - - - 3.14 - - 1.88 2.25 - - Total 99.99 99.90 99.92 99.80 99.56 100.47 99.52 100.26 99.75 l00.10 99.83

Trace elements (ppm) Ba 1519 1580 1488 Ce (XRF) 173 187 175 CI 147 135 I81 Cr 41 45 80 Cu 30 23 30 Ga 16 20 20 La (XRF) 84 89 84 Nb 26 27 26 Nd (XRF) 56 62 54 Ni 15 ' 19 39 Pb 35 42 34 Rb 171 182 199 S 113 95 76 Sc 16 - -

Sr 1289 1276 1177 Th 37 39 41 V 109 117 118 Y 24 25 26 Zn 5 1 67 5 1 Zr 313 313 333

REE (ppm) La 102.35 Ce 193.33 Pr 20.98 Nd 76.82 Sm 11.71 Eu 2.85 Gd 9.86 Tb 1.11

DY 5.00 Ho 0.89 Er 2.48 Tm 0.33 Yb 2.09 Lu 0.32

Geochemical composition of ignimbrites associated with selected borate deposits; Nevada,'USA (sample I), Samos, Greece (2 and 3). Peru (4-6) and Arccntina (7-15)

Number I 2 3 4 5 6 7 8 9 10 I I 12 13 14 IS Sample NEV-I GRE-l GRE-2 PERU-I

Major oxides (wt.%'c) Si02 70.90 63.81 64.65 71.81 TiO, 0.29 0.22 0.31 0.10 AI203 11.48 16.49 16.25 13.20 Fe,O3' 2.46 2.35 2.26 0.58 MnO 0.04 0.02 0.01 0.13 MgO 0.46 1.81 1.26 0.08 CaO 1.55 1.98 1.89 0.43 Na,O 2.13 1.85 1.54 2.47 K,O 4.53 5.08 6.48 5.85 p205 0.02 0.04 0.06 0.01 LO1 6.45 6.45 5.74 5.06 ---- Total 100.3 I 100. I 1 100.44 99.72

Trace elements (ppm) As 5 17 32 17 B 10 168 296 6 1 Ba 927 75 278 221 Ce(XRF) 206 290 229 24 CI 109 59 196 315 Cr 8 13 14 I I Cu 4 7 5 I F 820 900 1100 190 Ga 20 24 2 1 21 La(XRF) 89 150 96 4 Li 33 128 76 8 Nh 37 57 66 44 Nd(XRF) 59 80 67 I9 Ni 2 6 9 2 Pb 34 57 46 25 Rb 237 325 345 262 S 72 74 73 67 Sr 95 1333 900 44 Th 34 68 63 18 v 12 7 8 7 Y 66 45 39 36 Zn 72 67 52 30 Zr 432 773 587 46

PERU-' PERU-3 ARG-I ARG-2 ARG-3 ARG-4 ARG-5 ARG-6 ARG-7 ARG-9 ARG-I0

192 P.A. Floyd rr (11. / Juunial o/Geoc.heriiic.rrl E.~pIom~io~ i 60 f 1998) 185-205

Tahle 4 Geochemical composition of selected "barren" ignimbrites from Central Anatolia, Turkey (data from Yurtmen, 1993; Kuscu. 1997)

Sample I 2 3 4 5 6 7 8 9 10 I I 12

Major oxides (wt.C/c) SiO, 74.23 73.59 73.21 75.64 81.00 78.05 TiO, 0.13 0.14 0.13 0.08 0.06 0.10 A1,0, 12.44 12.45 12.45 12.32 10.24 12.46 Fc,O; 0.97 0.96 0.97 0.64 0.53 0.32 MnO 0.01 0.01 0.02 0.02 0.04 0.02 M_eO 0.71 0.85 0.56 0.18 0.06 0.50 CaO 1.68 1.92 2.1 1 1.40 0.25 0.62 Na,O 1.65 1.52 1.80 2.18 1.84 1.07 K,O 4.36 3.89 4.26 4.53 4.92 4.66 p2 0 5 nd nd ntl nd nd nd LO1 4.21 4.71 4.21 3.73 0.90 2.34 - - - - - - Total 100.39 100.04 99.72 100.72 99.84 100.14 Trace elements (ppm) As 3 2 5 5 I I B 8 10 7 7 8 4 Ba 353 373 301 293 58 50 Ce (XRF) 42 50 33 33 25 43 CI I 18 27 97 39 18 Cr 6 6 5 9 10 8 Cu I I I I I I F 500 350 270 200 200 1000 Ga 14 14 13 16 14 16 La (XRF) 13 26 19 7 2 12 Li 10 12 5 15 3 1 19 Nb 23 24 23 4 1 37 46 Nd (XRF) 14 25 6 16 2 1 21 Ni I 1 2 I I 1 Pb 7 I I I 16 19 25 Rb 15 1 131 189 278 338 326 S 49 62 54 147 8 I 101 Sr 1400 2315 472 338 I9 53 Th (XRF) 22 19 28 25 23 3 1 V 15 13 5 I 16 7 Y 7 9 8 20 24 22 Zn 23 27 25 27 27 29 Zr 75 67 88 78 65 76

FezO; = total Fe as Fc,O,: nd = not detected

(Alonso et al., 1991; Vandervoort et al., 1995). Similarly, Middle Miocene tuffaceous volcanism is an associated feature of borate deposits in California and Nevada; in the later case tuffaceous sediments dated at between 17 and 12 Ma are contemporary with nearby volcanism (Castor, 1993).

In each of these localities calc-alkali and alkaline volcanism is common and although lavas, minor intrusives and tuffs are present, it is commonly ash-flow or air-fall tuffaceous materials that are intimately associated with the borates. Because of this

important general association we have concentrated on the geochemical features of the tuffs in this work.

4. Field and petrographic outline of Kirka volcanic~

The petrography, composition and replacement relationships of the borates (principally borax, with lesser amounts of colemanite and ulexite) and associated authigenic minerals in the Kirka deposit have been described by Inan et al. (1973) and Helvac~

P.A. Floytl et nl./Jorrrrrnl qf Grochernic~nl Erplorrrtiorl 60 (1998) 185-205 193

(1978). Boron isotope data for the different borate minerals suggested that they were not coprecipitated, but were deposited from different brines at variable pH (Palmer and Helvac~, 1995). Apart from the borate minerals, authigenic B-rich K-feldspar and clays, together with volcanogenic sanidine, albite, anorthoclase and quartz are common constituents of the deposits in the lake sediments (Helvac~ et al., 1993).

The surrounding volcanics show a wider range of compositions and, petrographically and chemically

70.37 0.35

15.36 2.68 0.08 0.42 2.13 3.88 4.45

ncl 0.70

(see below) constitute two main groups: (a) an alkali lava suite and (b) a voluminous series of ignimbritic tuffs. It is the altered and partly replaced pyroclastic representatives of the latter group that are associated with the borate deposits.

4.1. Alkali laua suite

This comprises various thick lava flows domi- nated mainly by basaltic trachyandesites and trachy-

194 P.A. Floyrl rt 01. / Jottrnul q f Geocher~~rcal Exploration 60 (1998) 185-205

dacites. The trachyandesite members are often highly porphyritic (up to 20% phenocrysts) and are characterised by numerous small granular-textured tra- chytic inclusions. The typical phenocryst assemblage is composed of large zoned plagioclases, diopsidic clinopyroxene and red-brown amphibole and biotite set in a fine-grained felsic matrix. Large plagioclases are generally zoned and may display internal thermal dissolution surfaces truncating the generally regular zonation. Some may also display rounded and cor- roded margins rather than euhedral outlines and, in rare cases, enclose inclusions of glass. Both amphibole and biotite are strongly pleochroic from yellow to deep red and invariably show magmatically cor- roded margins peppered with minute opaque gran- ules. Pale green pyroxene forms long prismatic sub- hedral phenocrysts, but is more granular in the matrix. Apatite is a common accessory mineral. Large composite, stressed, irregular quartz grains probably represent xenocrysts derived from the local basement, rather than juvenile phenocrysts. The most mafic rocks in the suite are vesicular olivine-clinopyroxene-phyric trachybasalts, the more evolved members of which contain minor biotite in the granular matrix. The olivine phenocrysts are generally oxidized or altered to smectitic clays; some clinopy- roxenes are zoned.

In the Afyon area just south of Kirka, more acidic rock types (trachyandesite and trachydacite) characterize the earliest representatives of the alkaline suite, whereas the more basic lavas (trachybasalt) are generally later in the succession capping the Miocene lake sediments (Fig. 1). A similar magmatic sequence is recorded in the nearby Bigadic borate province (Helvacl, 1995) where the least evolved members of the alkalic suite erupted last following borate formation.

4.2. Ignimbrites

Although often referred to simply as 'tuffs' in previous literature, these regionally important volcanic deposits are the products of a series of ignimbrite eruptions. Most exposures are composed of the upper pumice-rich layer 2b of a characteristic ignimbrite section (Sparks et al., 1973) with relatively small amounts of lithics (basement-derived argillites, quartzites, schists) being present. Well-be-

dded Plinian deposits exhibiting reverse graded pumice layers are present, although base surge deposits have not been identified. In the field, the ignimbrites are characterised by numerous small crystals (feldspar and black quartz) and elongate brownish pumice clasts (from a few mm to 100 mm in length) which often are aligned and partly flattened although the flows are not welded. The black quartz crystals observed in the ignimbrites are also a significant and readily identifiable compodent of the pumice-rich epiclastic lake deposits that host the Kirka borates. The distinctive colouration of the quartz is not due to any secondary coating of the grains, but is probably a consequence of irradiation (Deer et al., 1963); under the microscope many grains have a cloudy appearance due to minute solid inclusions not dissimilar to U- and Th-enriched, inclusion-bearing calcite (Williams and Floyd, 198 1).

The ignimbrites are dominantly crystal-vitric tuffs with a very minor lithic component (about 1%) that is composed of basement lithologies and quenched vitric rhyolites. In thin section the ignimbrites exhibit a fine-grained, glassy, but variably recrystallized, granular matrix, and abundant flattened and con- torted (but not welded) pumice clasts, some of which contain quartz and feldspar phenocrysts. The overall crystal component of the ignimbrites is similar to that found in the pumice, being largely embayed quartz and plagioclase, but with the addition of dark biotite and some K-feldspar.

5. Geochemical features of the Kirka volcanics

5.1. Cornparison of lauas and tc/ffs

In terms of the total alkali-silica classification (LeBas et a]., 1986) the two main petrographic groups define an alkali basalt suite (representing the pre- and post-borate lavas) ranging from trachybasalts to trachydacites, whereas the ignimbrites have bulk compositions of sub-alkaline rhyolites (Fig. 2) . Rep- resentative samples from other borate locations in western Turkey show a fuller range of compositions for the alkaline suite (unlike the apparently bimodal distribution at Kirka), and the ignimbrites also in- clude some dacitic compositions (e.g. around the Bigadic deposits; Helvacl, 1995). Trace element

P.A. F loy l et 01. / Jo i~ rnn l of Geochemirrrl E.r[>lorotion 60 (1998) 185-205

Fig. 2. Classification of local Kirka volcanics into a differentiated alkalic suite and a calc-alkaline group of ignirnbritic tuffs with a rhyolite bulk composition.

10 Na20

+ 8 - K20

4

2 -

compositions are also distinctive (Figs. 3 and 4); again data from other locations extends the alkali suite range to lower Nb, Zr, and Ba values. Rare earth element (REE) patterns (Fig. 4) show the typical enrichment of light REE for the alkalic suite, whereas the ignimbrite pattern, with a large negative Eu anomaly, is characteristic for the involvement of feldspar in their genesis. The trace element trends and parallel REE patterns indicate that the Kirka alkali lavas represent a single comagmatic differentiated suite influenced by the fractionation of mainly olivine, pyroxene and minor feldspar.

There is no genetic relationship between the two magmatic groups at Kirka, although in terms of the initial volcanic source of the B for the borate deposits, both can show high B values. Very high values (range 700-1400 ppm B) are recorded for

Kirka alkali lava suite

40 45 50 55 60 65 70 75 Si02wt%

A lgnirnbritic tuffs Alkali lava suite -

T r a c h y d a d t e s . KIR-I . KIR-17

Basaltic trachyandew'tes o KIR-10

KIR-15

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Trn Yb Lu

- - Basan Andesne Dacne Picro- basal, Basatiic

andeane

Fig. 4. Chondrite-normalized rare earth element patterns for Kirka alkali suite and calc-alkali rhyolitic ignimbrites.

'OoO 7

0 .- - (d z 100, - .- L

u c 0 .c 0 2 I 0 7 - a

E V)

1

some members of the alkalic suite at Bigadic, which probably reflect local hydrothermal contamination, as additional data for Emet and Kirka only average

Kirka ignirnbritic tuffs o KIR.5 . KIR.8

o KIR-19

KIR-24

* KIR.28 a KlR-31

1 1 1 1 1 1 1 1 1 1 1 l 1 1

58 ppm B (Helvac~, 1995). The Kirka ignimbrites have generally higher contents (average 95 ppm B) and in view of their volume and stratigraphic associ-

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 3. Incompatible element distinctions between the Kirka calc-alkaline ignirnbritic tuffs and the alkali trachybasalt-trachydacile suite.

P.A. Floytl rt 01. /Jo~~rncrl of Geoclieriiictrl E,r/)Iorcrtior~ 60 (1998) 185-205

subalkal~ne---$ ,.;;;.,::+ .. .. . . ./,.

. : .. ; .E PPm

10 .. :, .. :...:

Medium lo high-K

Law-K dc-alkal~ne tholeiitic ..:..:..: . .. ..,

Fig. 5. Classification of 'fertile' ant1 'barren' ignimbrites as cheractcristic of the high-K calc-alkalinc suite with a bulk composition ranging from dacitic to rhyolitic. K,O-SiOl diagram with nomenclature fr-om Rickwood (1989). and Nb-Zr diagram with fields from Leat et al. ( 1986).

ation with the actual borate deposits are more likely to be the primary source of the B.

5.2. Class~ficatiori and alteration eflects

It is sometimes considered that the bulk chemical composition of ignimbrites are unreliable (e.g. Walker, 19721, not only because of their heteroge- neous nature, but due to the effects of devitrification, hydration of glass and post-consolidation alteration. For example, hydration of rhyolitic glass can cause a progressive loss of Na, with a gain in K, Sr and Ba (Loften, 1970; Scott. 1971), whereas Na, Mg, Sr, Cs, Y and REE can be mobile during the crystallization of peralkaline silicic lavas (Weaver et al., 1990). Post-consolidation silicification of the glassy matrix

can increase the SiO, content beyond the maximum (77.4 wt.%) suggested by Hildreth (1981) for unal- tered rhyolites. The Kirka ignimbrites (and the other ignimbrites used in this study) have clearly undergone variable degrees of alteration, many having a siliceous recrystallized matrix with SiO, values > 77 wt.% (Tables 1 and 3). As it is likely that other elements were probably mobile also, not only during crystallization/devitrification, but possibly during the circulation of hydrothermal (mineralizing) solutions, the classification and composition of the ignimbrites is not straight forward. As seen in a stan- dard K,O-SiO, diagram (Fig. 5) many of the ignimbrite samples apparently have high-K calc-alkaline dacite-rhyolite bulk compositions, although the scat- ter in K z O contents is considerable and reflects

wmm-pare granues

Volcanic arc +

synooll~siona. ganltes granttes

" I

0 1 2 3 4 1 10 100 Gaxl O ~ / A I Y PPm

Fig. 6. Tectonic and granitoicl-lype classification of' 'fertile' and 'barren' ignimbrites. Nh-Ga diagram from Whalen ct al. (1987). and Nb-Y diagram from Pearce et al. (1984).

P.A. Floyd rr a/. /Jounicrl ofGrocliernica1 Explortrtion 60 (1998) 185-205

Kirka 'fertile' ignimbrilles 7 1 + Nan-Turkish Yenilem ignimbrites I

Central Analolian 'barren' ignirnbriie

Fig. 7. Effect of variable post-depositional alteration, as measured by loss-on-ignition (LOT), on As and B contents of 'fertile' and 'barren'

alteration. However, utilizing elements such as Zr and Nb that are generally considered immobile during alteration (e.g. Pearce et al., 19841, most samples have compositions that broadly confirm their calc-alkaline character (Fig. 5).

Further classification of the ignimbrites, utilizing the chemical discrimination of their tectonic environment. shows that the Kirka 'fertile' ignimbrites have characteristics comparable with typical A-type and within-plate granitoids (Fig. 61, and in this sense differ from the control group of 'barren' ignimbrites from elsewhere in Turkey.

One additional feature concerning ignimbrite alteration is the degree to which post-depositional effects can influence, and in particular enhance, the abundance of potentially mineralizing elements such as B in any ignimbrite. The total 'loss-on-ignition' (LOI) can be used as an approximate measure of the degree of alteration, whose range (generally 0-7 wt.%) indicates that secondary alteration is a common feature amongst the ignimbrites sampled (Ta- bles 1 and 3). However, as illustrated in Fig. 7, with the exception of some non-Turkish 'fertile' ignimbrites, the distribution of B or As is not markedly influenced by the degree of alteration; the Kirka 'fertile' ignimbrites are systematically enhanced in B and As relative to the 'barren' ignimbrites at all levels of LOI.

5.3. Main geochemical characteristics

The Kirka ignimbrites, in addition to being representative of a medium- to high-K, calc-alkali suite with an overall high-SiO, rhyolite bulk composition, exhibit the following chemical characteristics: (a) a typically low peralkaline index ( - 0.45); (b) are highly evolved for acidic rocks (K/Rb I loo), forming a coherent group with only minor internal differentiation; (c) variable relative enrichment (K, Rb, Th, Nb, Y) and depletion (Ba, Sr, Zr) of selected elements; and (d) minor light REE enrichment (La,/Yb, - 2) with a strong negative Eu anomaly (Eu/Eu* - 0.1). In summary they have many of the characteristics of within-plate A-type granitoids produced by crustal anatexis (Pearce et al., 1984; Whalen et al., 1987; Eby, 1990).

6. Geochemical comparison and discrimination

As noted above, the Kirka ignimbrites are considered to be the primary source of B as they are relatively B-rich throughout and closely associated in time and space with the lacustrine-developed borate deposits. In this section we compare their trace element characteristics with a control group of lithologically similar ignimbrites ('barren' ignimbrites)

198 P.A. Floyd er (11. /Journnl'nf Geocherrricnl Esplorntion 60 (1998) 185-205

from central Anatolia, Turkey (Yurtmen, 1993; Kuscu, 1997), together with a small selection of ignimbrites that are closely associated with borate deposits ('fertile' ignimbrites) from elsewhere (e.g. Samos, Nevada, central Andes).

Trace element diagrams have been constructed to characterize and compare the 'fertile' and 'barren' groups of ignimbrites.

6.1. B and associated nob bile elernerits (Fig. (9)

B is a highly incompatible element and is reported to be mobile along with Be, Li, As, Pb, and Sb in the subduction environment where calc-alkaline melts can be generated (Ryan and Langmuir, 1987, 1993; Noll et al., 1996). There is a gross correlation between B and As, Li, F and Pb in the sample sets, with varying degrees of enrichment in all these elements in the 'fertile' ignimbrites relative to the 'barren' group (Fig. 8). In terms of the Turkish data only, high B, As, F and Pb provide a reasonable

discrimination parameter for the 'fertile' ignimbrites. However, some of the high values for these elements exhibited by the non-Turkish 'fertile' group may be a consequence of hydrothermal enrichment by mineralizing fluids closely associated with the borate deposits themselves. While this is probably not the case for the Kirka group, as all samples were collected well removed and apparently unaffected by the development of the borate deposits, it is recog- nized that hydrothermal reservoirs can be very large and far-reaching. Thus, subsequent pervasive hydrothermal alteration could negate any suggested primary relationship between the ignimbrites and the borates. However, we consider that enrichment is primary (in the melt source region) as the degree of alteration suffered by both the 'barren' and 'fertile' groups completely overlap (Fig. 7) and is similar, both chemically and mineralogically. It is suggested that the 'potential' for borate formation is primarily a consequence of the initial enrichment of the volcanic~, followed by near-surface mobility generated by associated circulating hydrothermal solutions.

o Kina 'tenile'ignimbciws + Non.Twh 'lerlle' lgnimiles

Cened Anatdian 'barren-lgnmbriles + -

- ++

1 11111111 1 l l l l l t l l l 1 llU

1 10 100 1000

As PPm Li ppm

Fig. 8. Characteristic enrichment of B. As, Li. F and Pb in 'fertile' ignimbrites closely associated with borate deposits.

P.A. F lo~d er a/. / Jo~trnnl qf Geochetrtical E.r,ulorntion 60 (1998) 185-205 199

KlRKA IGNlMBRlTES "fertile" group

la00 K i i igninbne o KIRS

KIR-8 KIR-19 . KIR-24

r KlR-28 n KIR.31

6

0.1 CsRhBalh U K NbTaLaCeSf P NdknZr HI EuTiGd Y Yb

Fig. 9. Chondrite-normalized patterns for REE and incompatible eleme from central Anatolia.

6.2. Normalized element patterns (Fig. 9)

REE patterns between the Turkish sample sets are clearly distinct, whereas multi-element plots only differ in minor detail (Fig. 9). The Kirka ignimbrites are characterised by relatively 'flat' light and heavy REE patterns and a large negative Eu anomaly, which are characteristic of silicic volcanics from both continental interiors and margins (Macdonald et al., 1992). This pattern differs from the strongly fractionated REE patterns of the 'barren' group, although again both patterns could be influenced by the differing nature of their sources and degree of melting involved. Both multi-element patterns are characterized by prominent negative anomalies at Sr-P and Eu-Ti. The main discriminatory features are (a) the size of the Eu anomaly, (b) relative enrichment of the heavy REE and Y, and (c) enhanced (Th),, in the 'fertile' group.

6.3. Possible discrinzination diagrams (Fig. 10)

On the basis of some of the features described above, a degree of discrimination can be achieved

CENTRAL ANATOLIAN IGNIMBRITES "barren" group

Ozan igninbrite 0 22 . 0 2 4

nts in the 'fertile' Kirka ignilnbrites relative to 'barren' ignilnbrites

with the Turkish samples (and possibly on the global scale) using the B/K ratio. In the binary diagrams (Fig. lo), both the K/Rb ratio and TiO, are used as indices reflecting the degree or range of chemical evolution exhibited by the suites; both functions decrease with progressive fractionation. A multi-element diagram normalized against a 'barren' ignimbrite (Fig. 10) does not produce a definitive pattern except for the strong positive B-As-Li anomalies generated by 'fertile' ignimbrite samples. Better discrimination can be produced by employing simple binary plots involving B with As and Li (as in Fig. 8).

At this preliminary stage discrimination can probably be achieved for Turkish ignimbrite samples, but wider global discrimination involving non-Turkish 'fertile' ignimbrites is more in doubt.

7. Effects of magmatic processes on discrimination

Because common magmatic processes can cause variation in the absolute abundances of many trace

P.A. Floyrl et 01. / Jo~~rricil of Geochoniical E.rplorurion 60 (1998) 185-205

Kirka 'fertile' ignirnbriims

+ Mn-Turkish 'fedlea lgnlrnbrltes 1 CanVal AnalOlian 'barren' lgnirnbrlles +

0.1 Yertile" + i

WRb Ti02 wt.%

0 Kirka ignimbrite (average of 22 samples)

o Peru ignimbrite (sample PERU-3) (c)

Fig. 10. Binary diagrams (top) exhibiting possible discrimination of 'fertile' ignimbrites with high B/K ratios relative to 'harren' analogues. K/Rb and TiOz are fractionation indices, indicative of overall suite val-iation. The multi-clement diagram (bottom) shows the pronounced and typical positive B-As-Li anomalies of 'fertile' ignimbrites when normalized relative to an average 'barren' ignimbrite from central Anatolia.

elements, ratios are probably more reliable as potential discriminants, although they can also be suscepti- ble to differential effects, such as degree of partial melting and extent or type of fractionational crystallization. Ideally these magmatic effects should be understood so that real chemical differences between ignimbrite groups at about the same degree of magmatic evolution (here, broadly silicic) can be deter- mined. As seen in Fig. 11, the Kirka 'fertile' samples are apparent1 y characterized by low (Ce/Y), , Zr/Y, K/Rb, and Ba, uniform Zr/Nb, and high Rb/Sr and K/Li, which broadly separate them from the Turkish 'barren' samples. However, because

these ratios can be influenced to varying degrees by magmatic processes they may only reflect the degree of magmatic evolution of the suite, rather than provide useful discrimination. For example, consider the range in (Ce/Y), and Zr/Y ratios which can be modelled by variable partial melting and differential phase fractionation (Fig. 1 I). Some of the ratio variation within the 'barren' ignimbrites could be accounted for by the variable partial melting of a lower-crustal garnet-bearing granulite source (K- feldspar,,-plagioclase,-orthopyroxene,,- clinopyro xene,,-quartz,,-garnet,,), whereas the Kirka ignimbrites could represent higher-degree crustal melts

P.A. Floyd et a/. /Journal of Geochemical Explorcttion 60 (1998) 185-205

100 Garnet granullle partid mslllng

>N\

WRb WLi Fig. I I. Apparent discrimination of 'fertile' and 'barren' ignimbrites utilizing various element ratios that are probably more related to various masmatic processes (degree of partial melting, variable phase fractionation) than to source differences; see text for discussion.

from a different source composition with lower ratios. Note that fractionation of clinopyroxene, horn- blende and biotite (mineral vectors for 60% fractionation, F = 0.4; D values from Rollinson, 1993) could also account for much of the internal variation of each Turkish group, although not their overall differences. In other words, the range in (Ce/Y), and Zr/Y ratios within the Turkish 'fertile' and 'barren' ignimbrite groups could be accounted for by the variable extent of magmatic processes, although the difference behveen the groups is largely a function of different source compositions. Modelling sug- gests that variation in the Zr/Nb ratio (Fig. 11) is not effected by closed-system fractional crystallization of major or assessory phases (such as zircon, apatite) and the Y/Nb-Zr/Nb linear trend for the 'barren' ignimbrites is more likely a function of variable partial melting coupled with sediment contamination (PAAS or greywacke). The other dia- prams in Fig. 11 that involve variation in ratios of LIL elements are probably the result of differential magmatic differentiation involving mainly felsic and

mica fractionation and as such are not good discriminants. For example, the trend of a concomitant decrease of K/Rb with Ba (from 'barren' to 'fertile' Turkish ignimbrites) is largely a function of progressive K-feldspar fractionation (Shaw, 1968).

In summary, many ratios of either HFS or LIL elements apparently show variable discrimination between the Turkish 'fertile' and 'barren' ignimbrite groups, although in some cases these are largely a consequence of differences in the degree or type of magmatic process (especially partial melting and fractional crystallization) involved. The similarity between the Turkish 'barren' group and the non-Turkish 'fertile' ignimbrites also indicates that global discrimination is not possible using these ratios.

8. Source and initial enrichment of boron

Although many borate deposits are situated in continental rift zones, the eruptive environment of the associated volcanics may be either (a) a contemporaneous subduction-related active margin, or (b) a

202 P.A. F lo~d et a/. / Jo~~riinl of G~ochemical Exploration 60 (19981 185-205

continental collision compressional regime. For example, Miocene calc-alkali volcanism in western Turkey is considered the consequence of continental collision and thickening after ocean closure (Y~lmaz, 1990), derived from a subcontinental lithosphere enriched by subduction-related processes (Seyitoglu et al., 1997). The central Andean ignimbrites are situated on a continental high plateau elevated by active subduction of the Nazca plate throughout the Miocene (Jordan and Gardeweg, 1989) and are petrogeneti- cally related to subduction processes (Thorpe and Francis, 1979). In both cases, subduction processes may have played a part in either the direct generation or subsequent development of the borate-associated silicic volcanism. It is therefore suggested that the subduction environment holds the key to the initial concentration of B that will eventually produce continental borate deposits.

Much of the recent work on volcanic arcs has been directed towards determining the 'slab component' of arc volcanics, and in particular the contribution of subducted sediments to the compositional variation exhibited by arc basalts (e.g. Tera et al., 1986; Morris and 'Tera, 1989; Morris et al., 1990; Leeman et al., 1994). In this context B (together with B and Be isotopes) has been used as a tracer for slab input to arc melting processes because it is both highly incompatible (Shaw and Sturchio, 1992) and readily mobile under both low- and high-temperature hydrothermal alteration (Seyfried et al., 1984; Thompson, 1991). Comparison of volcanics in different eruptive settings has demonstrated that B is enriched in arc basalts relative to both mid-ocean ridge (MORB) and ocean island basalts (OIB) (Ryan and Langmuir, 1993; Chaussidon and Jambon, 1994; Dostal et al., 1996; Ryan et al., 1996). Apart from B, other mobile trace elements such as Li, Be, As, Sb and Pb, are also considered to be enriched in the subduction environment via a fluid contribution into the overlying mantle wedge from the downgoing dehydrating slab (Morris et al., 1990; Palmer, 1991; You et al., 1993; Noll et al., 1996). The B is derived from two main sources: (a) ocean crust which has undergone submarine alteration which causes an enrichment in B (and Li); and (b) pelagic sediments which have adsorbed B from seawater (Spivack and Edmonds, 1987; You et al., 1993). In both cases, B is largely hosted by secondary phyllosilicates

(Domanik et al., 1993) which on subduction of the oceanic segment undergo progressive metamorphism and dehydration, releasing a B-rich hydrothermal fluid (with other mobile LIL elements) into the arc mantle.

9. Discussion

From the above it is clear that subduction processes produce enrichment in the same range of elements (B, As, Li, Pb) that are concentrated in ignimbrites associated with borate deposits in western Turkey and elsewhere. We suggest that at some stage in the development of calc-alkali volcanics associated with borate deposits, subduction of a lithospheric slab composed of some pelagic sediment and altered basaltic crust was involved. Without the sediment (with the highest proportion of B, but smaller volume) the degree of B-enrichment is un- likely to be large and thus the likelihood of subsequent borates is reduced. In this context it is interest- ing that B and Be isotope data commonly exhibit a considerable range, both between arcs and within single arcs (Tera et al., 1986), and could reflect the variable contribution of a sedimentary component. A range of geochemical tracers indicate that subducted sediments leave their mark in arc volcanics (e.g. Hickey et al., 1986; Plank and Langmuir, 19931, although on a gross scale Rea and Ruff (1996) suggest that there is no correlation between the amount of sediment being subducted and bulk arc composition. This later feature probably reflects the generally small proportion of sediment involved and does not take account of the high fluid flux generated by the progressive dehydration of the volcanic segment of the slab.

Although initial B enrichment could be a consequence of a sediment component and subduction dehydration processes, the western Turkish ignimbrites are not directly related to an active subduction zone, but largely generated in a post-collisional setting by crustal melting. However, as demonstrated by Seyitoglu et al. (1997) the Early Miocene silicic melts were derived from a lithospheric source enriched via subduction-related processes. This is one fundamental difference between the Turkish ignimbrites (largely continental rift-related) and the

P.A. Floyd e/ a/. / Jo~ fn ia l of Geochrmical Esplor-a/ion 60 (1998) 185-205 203

non-Turkish samples (which are developed in con- current arc-related active margins). In this case it is suggested that the Kirka ignimbrites were derived from a continental crust already enriched in B, via the agency of previous subduction-related magmatism. Tourmalinites are found in metasedimentary units in southwestern Anatolia (Mittwede et al., 1995) indicating that B was available in the continental basement.

It is suggested that the Kirka borate deposits represent the end-product of four stages of trans- portation for B: (a) initial concentration in subduction-derived fluids; (b) incorporation in continental crust via arc magmatism; (c) melting of B-enriched continental crust to produce ignimbrites; and (d) selective mobilization of B from ignimbrites by local hydrothermal activity and precipitation in alkaline lakes. Finally, although the initial enrichment of B in volcanic source rocks is considered a necessary feature for the development of borates, the local climatic, tectonic and volcanic conditions are important secondary features in their eventual genesis.

10. Conclusions

(1) Ignimbritic calc-alkali volcanics of rhyolite composition associated with the Turkish Kirka borate deposits ('fertile' group) can be distinguished from lithologically similar ignimbrites that do not host borates ('barren' group). Because of limited geographical coverage the use of the chemical discrimination parameters as a global exploration tool should be treated with caution, although borate-associated ignimbrites from elsewhere in the world have some chemical features that are broadly similar to those of the Turkish 'fertile' ignimbrites.

(2) Relative to 'barren' ignimbrites, the 'fertile'

plot only satisfactorily discriminates Turkish ignimbrites (Fig. 11). The distribution of many LIL and HFS elements or their ratios are largely governed by magmatic processes involving variable melting and/or fractionation, and are not reliable discriminants on the global scale.

(d) A 'flat' REE pattern with little light-to-heavy REE fractionation (La,/Yb, - 2) and characterized by a large negative Eu anomaly (see Fig. 9); note that this pattern is also typical of many active margin rhyolites.

(3) The initial source and enrichment of B was in a subduction environment via the release of B-rich fluids caused by the progressive dehydration of the altered oceanic slab and a sediment component. Arc calc-alkali volcanism was the main carrier of B that became available for the subsequent deposition of borates via associated hydrothermal activity. Borate- associated volcanism not related to a contemporary subduction zone can release B by partially melting previously arc-generated silicic magmatic rocks within the continental crust.

(4) It is suggested that the variable lateral distribution of borate deposits along active margins related to subduction zones is primarily governed by the availability of pelagic sediments and/or altered oceanic crust being subducted, coupled with the local volcanic and tectonic situation.

Acknowledgements

Dr. R. Kistler (US Borax Inc.) and Dr. R.N. Alonso (Universidad Nacional de Salta, Argentina) are thanked for discussion and comments on the general thesis of this paper at the IESCA meeting, Gulluk, Turkey (1995).

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