apatite-rich iron deposits of the avnik (bingol) region, southeastern...

Economic Geolom Vol. 79. 1984. pp. 354-371

Apatite-Rich Iron Deposits of the Avnik (Bingol) Region, Southeastern Turkey

C. HELVACIO

Mineralogisk-Geologisk Museum, Sarsgate I , Oslo 5 , Norway

Abstract

In the Avnik area, apatite-rich iron deposits are interbedded with gneisses and intermediate to felsic calc-alkaline metavolcanics (450 m.y. old) which show some well-preserved porphyritic, spherulitic, and volcaniclastic textures. They are intruded by the heterogeneous Avnik granitoid and the homogeneous Yayla granite (350 m.y. old). Iron deposits and associated gneisses, metavolcanics, granitoids, and granites are unconformably overlain by mica schist and Permian marble, which were folded and metamorphosed during the Alpine orogeny. The metavolcanics, the granitoids, and the overlying mica schists have been extensively feldspathized and silicified.

Iron ores are banded, massive, or disseminated and are located in a gradational zone between the gneisses and better preserved parts of the metavolcanic rocks. The massive ores are lensoidal, and the banded ore zones show magnetite-apatite laminations. Although disseminated magnetites are regionally widespread in the metavolcanic rocks, they are commonly concentrated adjacent to massive ore zones.

Magnetite, fluorapatite, and actinolite are dominant minerals in the three types of deposits; accessory minerals are feldspar, quartz, mica, diopside, hornblende, crossite, and sphene. Chlorite, talc, epidote, allanite, calcite, hematite, ilmenite, and rutile are low-temperature retrograde minerals. At and near the surface, magnetite is always martitized. The final alteration is a transformation to goethite. Original fluorapatite is partly altered to hydroxyfluorapatite and hydroxyapatite.

Evidence suggests that the apatite-rich iron deposits formed initially during volcanism, . and it is concluded that they were immiscible liquids which had separated from strongly

fractionated magmas. Similar rare earth element values in coexisting apatite and magnetite and in the associated metavolcanic rocks support this conclusion. The apatite-rich iron deposits were remobilized into stockworks containing large crystals of magnetite, apatite, and actinolite where these have been intruded by the Avnik granitoid.

Introduction

THE Avnik (Bingol) apatite-rich iron deposits occur within the Bitlis massif in southeastern Turkey. The Bitlis massif is a large area of Paleozoic metamorphic rocks in the interior of the Eastern Taurus fold belt of southeastern Turkey. The Avnik (Bingol) area is in the western part of the massif. The study area is approximately 30 km southeast of the city of Bingol and 20 km west of the town of Genq (Fig. 1). The iron deposits are massive, banded, disseminated, and stockworks of magnetite, apatite, and actinolite within calc-alkaline metavolcanic rocks of intermediate to felsic composition. The metavolcanic rocks are part of the Bitlis massif along with dominantly granitic intrusive rocks. These rocks range in metamorphic grade from greenschist to amphibolite facies.

The Paleozoic age of the deposits and their deformation and metamorphism is based on geologic

" Present address: Dokuz Eyliil ~n ivers i t es i , Miihendislik-Mi- mar l~k Fakultesi, Jeoloji Miihendisligi Boliimu, Bornova, Izmir, Turkey.

and paleontologic evidence (Alt~nli, 1966) and ra- diometric dating (Yilmaz, 1971; Helvaci and Griffin, 1983a). The southern edge of the Bitlis massif is the southeast Anatolian thrust fault at the boundary between the Anatolian and the Arabian plates along which the Bitlis massif has been thrust southward over sedimentary rocks of the Arabian foreland (Altinll, 1966; Ketin, 1966; Yilmaz, 1971; Hall and Mason, 1972; Aykulu and Evans, 1974; Hall, 1976).

All the iron deposits belong to the state, and extensive exploratory trenching and drilling of the deposits was done by the Mineral Research and Explo- ration Institute ofTurkey from 1976 to 1981 on behalf of the state-owned Iron and Steel Company ofTurkey. The Avnik area was mapped and sampled by the author in the summers of 1978, 1979, and 1980.

Proven ore reserves in the Avnik area are 104 mil- lion metric tons. There are several qualities of ore with average iron contents of 14 to 58 percent.

The aim of the present paper is to report geologic, mineralogic, and geochemical aspects of the deposits that are critical in interpreting their genesis. The

APATITE-RICH Fe DEPOSITS, TURKEY

FIG. 1 . Simplified geologic map of the Avnik region (based on B. Erdogan, unpub. data, 1982)

weight of evidence supports the immiscible liquid hypothesis of origin, while many features are incom- patible with other hypotheses.

General Geology of the Avnik Area

The rocks of the Avnik area are divided into lower and upper units (Figs. 1 and 2). The lower unit is a series of intermediate to felsic calc-alkaline metavolcanic rocks dated by Rb-Sr as 454 + 13 m.y. (Hel- vacl and Griffin, 1983a), and interbedded banded and massive apatite-rich iron deposits, which are intruded by the Avnik granitoid and the Yayla granite dated by Rb-Sr as 347 f 52 m.y. (Helvac~ and Griffin, 1983a). When the Avnik granitoids are compared with the less altered Yayla granites, the Yayla granites are obviously more homogeneous than the Avnik

granitoids. The latter show a slightly greater spread in an AFM diagram (Helvacl and Griffin, 1983b); this may be due to the observed assimilation of metavolcanic rocks. Several samples from the middle of the body, at some distance from the metavolcanics, have the K/Na ratios and K concentrations of typical granites (Helvac~ and Griffin, 1983b).

The lower unit can be divided into four formational groups: quartz-feldspar gneiss (strongly foliated felsic metavolcanics), amphibole-rich gneiss and amphibolite (basic to intermediate metavolcanics), metavolcanics-metatuffs, and metavolcanics-meta-agglomerates.

The quartz-feldspar gneiss, the lowest group observed in the Avnik sequence, consists mainly of quartz and feldspar with variable amounts of am-

1 1 1 LITHOLOGY I

Frc. 2. Stratigraphic section of.the Avnik region (modified from B. Erdogan, C . Helvaci, and 0.0. Dora, unpub. rept., 1981).

phibole, muscovite, magnetite, and secondary chlorite after biotite and amphibole. Lepidoblastic texture is common. This gneiss commonly alternates with amphibole-rich gneiss and amphibolite and is migmatized along its contact with the Avnik granitoid (Figs. 3 and 4). Its chemical composition and petrographic evidence suggest that this gneiss is strongly foliated, recrystallized, and albitized felsic metavolcanics.

The amphibole-rich gneiss and amphibolite consist of amphibole (actinolite, rarely hornblende and crossite), diopside, albite, epidote, apatite, magnetite, biotite, muscovite, and minor quartz, talc, chlorite, calcite, sphene, and hematite. They have mainly schistose textures and less commonly granoblastic textures.

Metavolcanics-metatuffs include a variety of rocks and chemical compositions ranging from felsic to mafic. They are dominantly intermediate in composition, but in the upper part of the sequence felsic metavolcanics are relatively abundant. The mafic to

intermediate metavolcanics-metatuffs consist mainly of albite, amphibole, mica, magnetite, chlorite, and talc; K-feldspar is a minor component. Some meta- agglomerate is intercalated with these rocks.

Metavolcanics-meta-agglomerates also include a variety of rocks and chemical compositions, but they are dominantly felsic and are characterized by 1- to 5-mm megacrysts of quartz and K-feldspar in a fine- grained groundmass having very irregular micro- structures; porphyritic and rarely relict spherulitic textures are preserved locally. Meta-agglomerates are lensoidal or form thin lavers and consist of volcanic debris (phenocrysts, gro;ndmass 'material identified only in thin section) in a fine-grained, strongly foliated micaceous matrix. Cataclastic texture is common.

The lower unit is intruded by the Avnik granitoid, a heterogeneous and strongly albitized rock which has intrusive and gradational contacts with the u

metavolcanic rocks, and by the Yayla granite, which is a homogeneous body that has sharp contacts with the surrounding rocks (Figs. 1-3).

The Avnik granitoid (Avnik albitite) is foliated and recrystallized at its margins, which are transitional with the surrounding, partly assimilated metavolcanics. The granitoid body consists principally of quartz, albite, K-feldspar, and amphibole; muscovite, biotite, chlorite, zircon, sphene, magnetite, and hematite are minor components. It has porphyritic and granoblastic textures at its margins, which commonly are overprinted by a strong foliation, and granitic texture in its central art. Silicification is common. and dikes of quartz up to 5 0 cm thick have been dbserved in the granitoid.

The Yayla granite is coarse grained and equigran- ular, and consists of 4- to 5-mm quartz, orthoclase, microcline, pertite, amphibole, and biotite. Chlorit- ization and sericitization have been observed in some parts of the granite, but albitization is rare. Aplite and pegmatite dikes and veins are abundant within the bodv.

In the Avnik region, the upper unit consists of the following succession grading upward: garnet-biotite mica schist, gray fossiliferous Permian marble, marble- schist intercalations, and white marble. The garnet- biotite mica schist rests with angular disconformity on the metavolcanics, the Avnik granitoid, and the Yayla granite. Lenses of quartz-marble locally inter- vene between the mica schist and both the metavolcanics, granitoids, and the granite of the lower unit. The lower part of the quartzites is mainly me- taconglomerate; they contain pebbles of the lower unit rocks and are interpreted as a basal conglomerate (Figs. 1 and 2). There is a local unconformity between the garnet-biotite mica schist and the gray marble, and they are also separated by a lensoidal quartzite.

The metavolcanics and the granitoids of the lower

APATITE-RICH Fe DEPOSITS, TURKEY 357

FIG. 3. Geologic map of the Miskel and Murdere deposits.

unit have been subjected to extensive metasomatic the intrusion of the granitoids into the volcanic pile feldspathization and silicification (Helvacl and Griffin, (Helvacl and Griffin, 1983a). 198313). The same metasomatism has produced albite In the Avnik area, the metamorphic rocks of the porphyroblasts in the overlying mica schists. Rb-Sr Bitlis massif are in a large anticline overturned to the analyses of the metavolcanics suggest that this meta- south (Fig. 1). The lower and upper units have been somatic event occurred 91 + 9 m.y. ago, long after affected by several stages of deformation, and several


imbricated thrusts occur within the massif. After Miocene time the Bitlis massif of the Avnik region was thrust along nearly horizontal thrust planes to the south over the Miocene flysch (Fig. 1).

The rocks and associated apatite-rich iron ore deposits of the Avnik region have been affected by several stages of alteration and metamorphism. A regional deformation, and metamorphism probably to the amphibolite facies, has affected the lower unit before ore during the intrusion of the Avnik and Yayla granites. The intrusive event was followed by uplift, folding, and faulting that has not affected the upper unit. The lower unit then was subjected to a second metamorphism which also affected the upper unit. During the second metamorphism, previous amphibolite fa- c i e ~ mineral assemblages were overprinted by greenschist facies and, in places, epidote-amphibolite facies assemblages. Isotopic data (Helvac~ and Griffin, 1983a) and the field and petrographic observations suggest three possible metamorphic stages: (1) contact metamorphism of the volcanics during intrusion of the granitoids, accompanied or preceded by folding, prior to deposition of the overlying garnet-biotite mica schists; (2) folding and regional metamorphism in Eoalpine time, affecting both lower and upper units; and (3) late Alpine retrograde metamorphism.

Sample Collection and Analytic Methods

All samples were collected from newly opened road cuts and trenches, drill cores, and a series of seven sample traverses. Each series renresents collections made at a separate location; these are shown in Figs. 1, 3, and 16 (below), together with their identifying numbers. The samples weighed approximately 2 kg each, half ofwhich was prepared for chemical analysis.

Major elements in the iron ores were determined by wet chemical analytic methods. Mineral analyses of polished thin sections were done using a LINK

<,

energy dispersive system (ZAF-4 correction program) mounted on an ARL-EMX microprobe at 15 kV ac- celerating voltage, under a focused beam. Each re-

G. - ported analysis represents an average of 5 to 10 spots. Repeated tests on standard minerals have shown that no loss of alkalies or other volatiles occurred during analysis.

Mineral fractions were purified by means of heavy liquids and a magnetic separator. Rare earth elements were analyzed by instrumental neutron activation methods, using a modification of the instrumental neutron activation analysis technique of Gordon et al. (1968). The U. S. Geological Survey reference sample BCR-1 was used as the standard for calibration. For apatite, rare earth elements were determined nondestructively by Ge (Li) and LEPD y-ray spec- trometry with thermal neutrons using a 2-min irra- diation, as described by Brunfelt and Roelandts

(1974). The Odegaarden and Durango apatites were used as the standards. The methods of Helvac~ and Griffin (1983a) were used for the isotopic analyses.

Ore Types and Mineral Paragenesis

Ore types

The iron orebodies are of three different types: massive-banded, disseminated, and stockworks of complex vein networks. Their distribution varies considerably in individual deposits.

Massive and banded ores occur as lenses that are generally concordant with the wall' rocks, and they vary in thickness from 5 to 10 cm to 2 to .5 m (Figs. 5 and 6). The massive ore lenses, which are commonly fine grained (from 0.06 to 5 mm), are laminated in- ternally and consist of several smaller lenses. Locally, folding has produced thick lensoidal bodies which contain boudinage structures (Fig. 3). Banded iron ore contains laminations of magnetite, apatite and, commonly, actinolite, from 1 to 2 mm to a few centimeters thick (Figs. 7 and 8). Chemical analyses of selected massive and banded iron deposits are given in Table 1.

Disseminated iron ore is widespread in the gneiss and the metavolcanic rocks but commonly is found adjacent to massive ore zones in rock. However, in some of the deposits this type of ore is variably dominant (Figs. 1 and 3). Disseminated magnetite is relatively homogeneously distributed through the iron- stone (Fig. 9) and makes up 20 to 30 volume percent of the rock. It forms euhedral to subhedral grains (to 1 cm in diameter), which in places coalesce in irregular aggregates. Inclusions of apatite, actinolite, and rarely, crossite occur within the magnetite grains. A partial analysis of the disseminated iron occurrence is given in Table 1.

Stockwork iron ore occurs as irregular cross cutting vein networks of magnetite and apatite, which commonly cut the other iron-rich rocks as well as the associated metavolcanic rocks. The veins varv in thickness, from a few millimeters to at least 5 m and in places form trellises. From field and textural evidence, it is clear that the magnetite-apatite stockworks formed wherever the massive-banded and the disseminated ores were intruded and remobilized by the Avnik granitoid. The stockwork veins generally consist of large magnetite, apatite, and actinolite crystals ranging from 2 to 15 cm across (Fig. 10). Partial analyses of the stockwork iron occurrences are shown in Table 1.

Mineral paragenesis

Magnetite, apatite, and actinolite are the dominant minerals in the three types of iron deposits; feldspar, quartz, mica, diopside, hornblende, crossite, and

360 C. HELVACI

FIG. 5 . Massive iron ores interbedded with metatuffs and FIG. 6. Massive ore lenses intercalating with metatuffs, metavolcanics, Murdere deposit. mrtavolcanics, and meta-agglomerates. M i ~ k e l deposit.

FIG. 7 . Banded iron ore, showing magnetite (black), apatite FIG. 8. Banded iron ore showing magnetite and apatite lam- (white), and minor actinolite (gray) laminations. Length of hand inations with minor actinolite and other gangue minerals. Crossed magnet is 7.5 cm. nicols.

FIG. 9. Finely dispersed euhedral magnetite within the rock FIG. 10. Large recrystallized magnetite (black), apati te fabric; associated phases are actinolite, apatite, albite, talc, and (white), and actinolite (dark gray) minerals from the stockwork calcite. Ordinary light. iron ore.


T A ~ L E 1. Partial Analyses of the Apatite-Rich Iron Deposits from the Avnik Region

Massive and banded iron deposit Disseminated Stockwork iron deposit

iron deposit 1 2 3 4 5 6 7

Fel SiOz TiOp A1203 MgO CaO Na20 Kg0 PlO5

I Total iron as Fe 1. Massive iron ore consisting of dominant magnetite and apatite with minor actinolite, hematite, quartz, and chlorite (Mi~kel deposit) 2. Magnetite and apatite laminations, and minor actinolite, quartz, talc, chlorite, and hematite (Miskel deposit) 3. Magnetite and apatite laminations with minor actinolite, quartz, albite, talc, calcite, epidote, and hematite (Miskel deposit) 4 Dominant magnetite, albite, quartz, and muscovite, and minor crossite, chlorite, hematite, and Ti hematite (Hamek deposit) 5 . Recrystallized magnetite, apatite, and actinolite; minor biotite, chlorite, quartz, hematite, and Ti hematite (Miskel deposit) 6. Recrystallized magnetite, apatite, and actinolite; with minor chlorite, muscovite, epidote, hematite, and Ti hematite (Miskel deposit) 7. Recrystallized magnetite, apatite, and actinolite; with quartz, chlorite, talc, epidote, and hematite (Miskel deposit)

sphene are accessory minerals. Chlorite, talc, epidote, allanite, calcite, hematite, Ti hematite, ilmenite, and rutile are low-temperature retrograde minerals.

Magnetite is the dominant iron mineral in the three typesc& deposit. It occurs as euhedral to subhedral grains, varying from 0.06 to 5 mm in diameter in the massive and banded ores. to 1 cm in diameter in the disseminated deposits, and 1 0 cm in diameter in the stockwork iron deposits. Magnetite recrystallized during the metamorphism, but electron microprobe analyses show that it has homogeneous compositions in the three types ofiron deposit (Table 2). Magnetite crystals are free of exsolution intergrowths and have no detectable concentrations of other elements.

Magnetite grains contain small inclusions of apatite, actinolite, and rarely crossite and other gangue minerals. Magnetite is the primary iron oxide in the deposits and was probably Ti rich before the meta- m o r ~ h i s m and alteration Drocesses. In near-surface deposits, the magnetite commonly has been martitized along grain boundaries, fractures, crystal faces (Fig. 1 l ) , and octahedral cleavage faces (Fig. 12).

T h e first stage in the alteration of magnetite is - u

martitization, and the final stage is a nearly total transformation of earlier formed martite into goethite and rutile. Goethite is resent onlv near the land surface, but the martitization has taken place at depths of as much as 200 m below the topographic surface.

Hematite. Ti hematite. ilmenite. and rutile formed by oxidation of Ti-bearing magnetite during retrograde metamorphism. The hematite and Ti hematite oxidation products of magnetite commonly show la- mellar twinning (Fig. 13). They rarely consist of il-

menite exsolution lamella. Major element analyses of hematite and ilmenite are given in Table 2. T h e variation in the vanadium content of magnetite and he- " matite is relatively small, but the latter always has a higher vanadium content (Table 2).

No T-fo, estimates have been obtained for the rocks of the Avnik region because of the considerable low-temperature alteration and martitization that has affected the magnetite in these rocks. Remnant magnetite in the cores of grains is not sufficientlv tita- niferous to apply the oxide thermobarometry method of Buddington and Lindsay (1964).

A ~ a t i t e is the second most abundant mineral after magnetite in the iron deposits of the Avnik region. The Avnik deposits are characterized by a fluorapatite containing small amounts of hydroxyl but no chlorine or carbonate (Table 3). Fine- and coarse-grained apatite occur in varying proportions with magnetite, ranging from sporadic grains to bodies of solid apatite rock. The apatite is in the form of subhedral to euhedral grains in the massive ore zones, mostly 0.02 to 0 .5 mm long (Fig. 14) and u p to 5 mm across in places in the banded ores (Fig. 7). Apatite in the stockwork zones occurs as prismatic crystals, commonly forms crystals as much as 15 cm long (Fig. lo) , and may contain small inclusions of magnetite and actinolite.

The apatite has a rather wide compositional variation which is related to the ~ a r t i a l alteration of the primary fluorapatite to hydroxyfluorapatite and hydroxyapatite, forming almost pure end members (S3- 5A and S3-5B in Table 3) .

Actinolite is the most important silicate mineral

t- t- - - 9 - 0 * - w

-r 0 N LO?? - m o 10

m 0 w w t- o!? 1% O S z z 2

m

O t - - m w - '? a0 O 2 FS! 2 ? I i

N m w m 2 - o ? ? d: ? c? 2 o m - 0 0 - (Dm 13

- N N - 2 2 2 2 '4 w m 2

m w m - m - 7.: 2 o! % 6 m - w m 0 0

2

0 m m o! kc? 2 3, 0 00-

(Dm 0 E

- NCQN - cp n! -kc? o o m -

2 2 w c-3 2

Dresent in the iron de~osi ts . It occurs as subhedral ;o euhedral grains upLto 10 to 15 cm long in the stockwork iron deposits (Fig. 10). Actinolite crystals commonly recrystallized in radial growths during metamorphism. Hornblende and crossite occur rarely in the deposits.

Most of the other gangue minerals occurring in the deposits are diopside, quartz, albite, K-feldspar, biotite, chlorite, talc, epidote, and allanite. Xeno- morphic chlorite fills cracks and cleavage planes and commonly replaces biotite. Calcite, sphene, rutile, and goethite occur sporadically within the iron deposits. Chemical analyses of the gangue minerals are given in Table 4.

Detailed Geology of the Iron Deposits

The Avnik apatite-rich iron deposits interbedded within the metavolcanic sequence are massive, banded, or disseminated. The position of the deposits is stratigraphically controlled in the gradational contact zone between the gneiss and the better preserved metavolcanic rocks (Figs. 1 and 2). The disseminated iron occurrences are widespread in the gneisses and the metavolcanics but generally are adjacent to the massive ore zones. The banded iron deposits have magnetite-apatite laminations 1 to 2 mm to a few centimeters thick. Where the massive and banded iron deposits are intruded by the Avnik granitoid, the deposits were remobilized into stockworks containing large crystals of magnetite, apatite, and actinolite.

In the Avnik region, the metamorphic rocks are in a large anticline overturned to the south. The Mur- dere and Mi~kel deposits are in the northwest-plung- ing nose of the anticline; the Haylandere, Gonaq Tepe, Kelme Tepe, and Kllhaz deposits are along the south-

, western overturned limb of the anticline; and the c m - Villik and Harabe deposits are along the northeastern a" normal (upright) limb. The Hamek and Kas~man de- Y

P posits are in the northeastern part of a zone of im- s bricate thrusts which cut the Bitlis massif in the Avnik J

TI region (Fig. 1). m Study of the regional setting of the iron deposits, m - z combined with detailed study of the Mi~kel and Mur- 8 LI

dere deposits, which are the deposits of principal - m economic interest, has resulted in new ideas about 2 and a reasonable explanation of the origin of the de- M .- ,. posits. C 2 Murdere deposit a

-0 - The magnetite concentrations of the Murdere area m - occur within basic metavolcanics, amphibolites, and 3 - m gneisses (strongly foliated felsic metavolcanics). Mas- m .- sive magnetite lenses are interbedded with metatuffs 0 + and metavolcanics (Fig. 5) and wedge out with depth 2 and toward the contact with the granitoid. The deposit -

contains two main iron-rich lenses, one, 1 t o 5 m


FIG. 11. Magnetite (gray), partly m;irtitiz.etl (white) along the FIG. 12. Magnetite (gray) martitized regularly along the oc- crystal surfaces and cracks. Polished section, ordinary light. tahedral faces and replaced by hematite (white). Polished section,

ordinary light.

FIG. 13. Strained hematite, showing translation shearing and FIG. 14. Apatite (gray and dark gray), magnetite (black), and twin lamellae. Polished section, crossed nicols. actinolite (white) occurring together in the recrystallized apatite-

rich iron ore. Crossetl nicols.

3 ) . A major part of the iron deposits was intruded and assimilated by the Avnik granitoid and now occurs as layers within dark patches of assimilated volcanic rock.

Parts of the Murdere deposit wall rocks have been . feldspathized, silicified, chloritized, and seriticitized, I.-- + ,!: , ,:\? .- . - v i h and the magnetite has been martitized. ' . .L . ;p.,:, \

, 1 % , " 7 . # , t 6. ,."J%A ,yL- ,

m" _. , : . . I \ % .,. . :.' .g Migkel deposit ? - *LSY .- :,uJ*' < # 1, <

The Mi~kel iron deposit is a continuation of the ;<. *;; G~ I

- * Murdere deposit and is about 500 m southwest of it.

r ' k, .: *& +. '& \, , ! 4 2 , - ? . , ,; ',. ,

i a The main magnetite body is located in the gradational contact zone between the gneisses (strongly foliated

nc. 15. Better preserved metavolcanlc rocks alternntmg 1)asic- feisic metavolcanics) and the metavolcanic rocks intermediate to felsic in compo~it ion, Mi~kel deposit.

(better reserved than gneisses) (Figs. 2 and 15). Boreholes show that the same massive and banded

thick and the other, 50 cm to 8 m thick, and several deposits extend to approximately 250 m in depth, thin short lenses (Fig. 4). The disseminated iron de- but some lenses pinch out laterally and downdip (Figs. posits occur mainly around the massive deposits (Fig. 3 and 4).

364 C. HELVACl

TABLE 3. Electron Microprobe Analyses of Apatite from the Apatite-Rich Iron Deposits, Avik (Bingol) Region

S3-5 S3-5A S3-5B S3-15 S1-2 S1-3 S1-4 S1-6 S1-7 S1-8 --

SiO, 0.23 0.14 0.16 0.17 0.13 0.42 0.26 0.28 0.29 0.18 TiO, 0.10 A1203 FeO' 0.15 0.15 0.10 0.13 MnO MgO 0.15 0.10 0.10 CaO 56.29 55.87 56.08 56.01 56.32 55.32 55.76 55.24 55.16 56.39 Na20 0.13 0.15 0.15 0.11 0.10 0.15 0.21 0.17 0.10 K2O pzo5 4 1.78 41.64 42.24 41.49 41.91 41.63 41.63 41.26 41.38 42.18 Sr 0 0.10 0.16 0.12 0.11 0.10 so, 0.20 0.17 0.22 0.15 0.15 0.19 0.13 0.13 0.12 0.10 yzos 0.62 0.13 0.12 0.10 F 3.08 4.02 3.99 3.08 4.22 2.96 4.12 3.01 4.18 CI 0.10 H20' 0.22 0.04 0.23 0.16 0.53 0.07 0.09 0.08 co:! 0.07

102.03 101.99 99.16 101.96 102.91 102.27 101.75 101.63 100.55 103.21 0 = F, CI 1.30 1.69 0.0 1.68 1.30 1.78 1.27 1.73 1.27 1.76

Total 100.73 100.3 99.16 100.28 101.61 100.49 100.48 99.90 99.28 101.45

Total iron as FeO

TABLE 4. Electron Microprobe Analyses of Minerals from the Apatite-Rich Iron Deposits, Avnik (Bingol) Region

Pyroxene Crossite Biotite Actinolite Hornblende

Si02 TiO, A1203 FeO' MnO MgO CaO Na20 KzO p205

Total

Albite Epidote Allanite Sphene Talc K-feldspar Chlorite

S1-11 S1-14 S1-11 S1-11 S6-6 Sl-8 S3-15 SI-15 S6-7 S1-13 S1-1 S1-3

SiO, TiO, A1203 FeO1 MnO MgO CaO Na20 K20 p205

Total

' Total iron as FeO


The disseminated iron occurrences are widespread in amphibolite, gneiss, and other metavolcanic rock but generally are most common adjacent to the massive magnetite deposits. The stockwork iron deposits are located mainly adjacent to or within the granitoids in the eastern and southeastern part of the Mi~ke l deposit (Figs. 3 and 4) and consist of complex networks of veins containing large crystals of magnetite, apatite, and actinolite. The stockwork deposits also occur above and below the main orebody.

The main deposit crops out in a belt 650 m long and 1 0 to 15 m wide. It is a lens of massive laminated magnetite which laterally splits into several lenses having maximum thicknesses of 2 to 5 m. Amphibolite, metatuff and other metavolcanic rock, and rarely meta-agglomerate are intercalated with the massive magnetite lenses (Fig. 6). The banded iron deposits within the Mi~kel deposit consist of magnetite-apatite laminations 1 to 2 mm to a few centimeters thick (Fig. 7). In places, apatite-rich aggregates also occur within the massive orebodv.

Magnetite is the dominant mineral, and apatite and actinolite are the most important gangue minerals of the denosit.

The average phosphorus content is 0.8 percent; however, within the northeastern and southeastern parts of the deposit the average phosphorus content can be as high as 1.46 percent. In these phosphorus- rich parts, the apatite typically is in bands within magnetite or broadens into veinlets and has crystals up to 1 5 cm in length. The titanium content of the deposit is 0.12 to 0.61 percent.

Haylandere deposit

The Haylandere deposit occurs 1 km south of the Mi~kel deposit. The Murdere, Mi~kel, and Haylandere all lie approximately at the same stratigraphic horizon, but the Haylandere deposit is not a continuation of

FIG. 16. Cross section of the Haylandere orebody, showing the host rocks and the granitoid intrusion. Description as in Figure 3.

FIG. 17. Massive and bantled iron ores, interbetldetl with the rnetavolcanics, are intrucletl by the Avnik granitoid (white) on the left.

the other deposits, as traced from the regional map (Fig. 1). The northern part of the deposit is covered by alluvium, and the whole body is enclosed within the Avnik granitoid. The wall rocks, gneiss, and amphibolite were intruded and assimilated by the granitoid (Figs. 16 and 17). The massive magnetite lenses strike northeast-southwest and dip to the northeast (Fig. 16). Massive, banded, and disseminated iron mineralizations occur within the Haylandere deposit, and at the surface these are grouped around a point marked by borehole H-9 (Fig. 16). Magnetic studies and drilling (borehole H-7) have proven the presence of an iron deposit approximately 6 0 m below the ground which does not reach the surface. The Hay- landere deposits are connected to the Gonaq deposits bv small ferruninous lenses. u

Magnetite is the dominant mineral, and the most - important gangue minerals are apatite, actinolite, and epidote.

The average titanium content is 0.79 percent, and phosphorus content is 0.78 percent.

G o n a ~ Tepe, Keb t~e Tepe, and Kzlhaz deposits

These deposits are corinected to each other by small magnetite lenses along the southwestern overturned limb of the regional anticline. The ferruginous lenses strike approximately northwest-southeast witliin the Avnik granitoid body, which pervasively assimilated and rernobilized the iron devosits. Tliese deposits are characterized mainly by stockworks of crosscutting veins consisting of large crystals (up to 1 0 cm in length) of magnetite, apatite, and actinolite. Boreholes in the Gonaq Tepe deposit penetrate to the granitoid, after cutting iron-rich lenses 5 to 10 m thick. Most of these deposits are small arid are economically unimportant. Magnetite of the Gonaq deposit coexists with apatite and actinolite. The average phosphorus content of the Gonaq deposit is about 1.41 percent; titanium is 0.36 percent.

' O w o r \ T I I I 7 Hamek and Kaszman deposits

Sl-2 S1-3 Miskel deposll Sl-4

""I ;, ~ 1 . 6 7 SI.7 Harabe deposll Ionic radii (A)

Sl -8 Duranoo aoalcle

FIG. 18. Chondrite-normalized REE patterns of the Avnik apatites. The numbers in the figure correspond to those in Table 5, and they are also shown on Figures 1 and 3.

Villik and Harabe deposits

These deposits are located within strongly foliated amphibole gneisses that are enclosed and extensively assimilated by the granitoid. Quartz-feldspar gneiss and migmatite occur near the contact of the granitoid and amphibole gneiss. The deposits are located along the northeastern normal limb of the anticline and are characterized by dominant apatite and subordinate magnetite. The Harabe deposit, especially, is a po- tential apatite ore, with apatite concentrations of 5 0 to 70 volume nrecent in some lenses. Banded. disseminated, and crosscutting veins of apatite are common, and magnetite appears mainly in disseminatiorls in this denosit.

Quartz, actinolite, sphene, allanite, and ilmenite occur in minor amounts. A close association of apatite with sphene and allanite is commonly observed.

These deposits consist mainly of disseminated magnetite within gneiss, amphibolite, and other metavolcanic rocks located above the imbricate thrust plane which cut the metamorphic rocks in the central part of the Avnik area (Fig. 1). Magnetite concentrations reach 20 to 3 0 volume percent of the rocks, but massive magnetite lenses are rare.

Magnetite is the dominant mineral, and the most important gangue minerals include quartz, feldspar, mica, chlorite, crossite, hematite, and Ti hematite. The average phosphorus content of the Hamek deposit is 0.26 percent; titanium content is 0.40 percent.

Rare Earth Elements

The rare earth element (REE) distribution in the apatite separated from the iron deposits, in the massive-banded iron deposits, in the stockwork iron deposits, and in the associated metavolcanic rocks is presented in Figures 1 8 to 22. All concentrations were normalized against a set of chondritic values given by Haskin et al. (1968).

Apatites

Samples S3-5, S3-15, S 1-6 have been separated from the recrystallized banded-massive iron deposits, and the rest of the samples are from stockwork deposits. The REE of the apatites are listed in Table 5.

Apatite of the Avnik ores shows dominant light rare earth elements (LREE) and subordinate heavy rare earth elements (HREE), a feature similar to that of many magmatic apatites. The REE patterns for the Avnik apatites are totally different from those of apatites of marine origin, such as apatite from the Vay- rylankyla deposits, Finland (Laajoki, 1975).

The Avnik apatites have high ZREE and those from the Murdere-Mi~kel and Harabe deposits have

TABLE 5. Rare Earth Element Abundances (ppm) in Apatites from the Avnik (Bingiil) Region

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

Avnik apatites

Durango apatite

2,900 4,519 1,190 132.5 13.6 120.1 18.0 69.2 9.5 28.7 31.3 4.4

APATITE-RICH Fe DEPOSITS, TURKEY

very similar patterns, except for a large negative Eu anomaly in the REEs of the Murdere-Mi~kel and Har- abe apatites (Fig. 18). All of these samples have larger negative Eu anomalies than do those from the associated metavolcanics (Fig. 19). The REE patterns of the metavolcanic rocks are typical of a magmatic dif- ferentiation series in which there has been fractionation of plagioclase (Helvac~ and Griffin, 1983b). In general, the Murdere-Mi~kel apatites have higher LREE contents than the Harabe apatites. The Harabe apatites are depleted in LREEs because of the presence of coexisting allanite and sphene, which take up most of the LREEs.

Negative Ce anomalies may indicate sedimentary deposition (Laajoki, 1975), but the Avnik apatites do not have negative Ce anomalies. The similarity of the REE patterns of the apatites from the iron deposits to the REE patterns of the metavolcanics suggests a magmatic origin for the iron deposits.

The Durango apatite (Young et al., 1969) resembles the Murdere and Mi~kel apatites, but its ZREEs is much higher than the ZREEs of the Avnik apatites. The REE patterns of the Mi~kel and Murdere apatite also resemble those of the apatites from the Kiruna magnetite deposit, northern Sweden, studied by Par& (1973) (Fig. 20).

Massive and banded ores-whole-rock rare earth elements

The REE analyses for the massive and banded iron deposits are given in Table 6. The REE patterns of the massive and banded deposits are mainly controlled by apatite, and the absolute REE concentration in the ores is largely related to the proportion of apatite. The REE content of some of the deposits is very high, and the patterns show a relatively great enrichment in the LREEs, flat HREEs, and large negative Eu anomalies (Fig. 21). The patterns of the deposits also are similar to those of the associated metavolcanics,

Metavolcanic 0 S2- 8 S2-15 a S2-12 A S2-16 n 52-13 . S2-17 r S2-14 o S2-18

52-19

FIG. 19. Chondrite-normalized REE patterns of the Avnik metavolcanics. All samples are from section 2 in Figures 1 and 3.

u Murdere and Mlskel Harabe 0 K~runa (Parbk 1973) lonlc radn (A)

FIG. 20. Chondrite-normalized REE patterns of the Avnik apatites compared with the Kiruna apatites.

which in turn are similar to those of many modern calc-alkaline volcanic series (Dostal et al., 1977). Some of the ores show a lower ZREE and the lowest one has a positive Eu anomaly.

Stockwork ores-whole-rock rare earth elements

The REE patterns of the stockwork iron deposits (Table 6) are identical to those of the massive and banded deposits, and reflect a variation in the ZREE varying with apatite concentration, enriched in the LREEs, and depleted in the HREEs (Fig. 22). Re- mobilization and recrystallization of the stockwork deposits during intrusion of the granitoid and the subsequent metamorphism evidently have not dis- turbed the REE patterns with the exception of the Eu anomalies. This suggests that the REE patterns are representative of the original rocks. The negative Eu anomaly of the stockwork deposits is not as large as in the massive and banded deposits, which may suggest that some Eu was picked up from the other coexisting material during remobilization of the massive and banded deposits.

Discussion: Genesis of the Iron Deposits

In interpreting the field, petrographic, and geochemical data about the apatite-rich magnetite deposits from the Avnik region, it must be stressed that the deposits occur in a series of mafic to felsic calc- alkaline metavolcanic lavas and tuffs. There is no evidence of the volcanic materials having been rede- posited and reworked in water. The iron deposits are stratiform within the volcanic pile, which has been later deformed.

The apatite, massive-banded deposits, stockwork deposits, and the associated metavolcanics show similar REE patterns, which are very different from those of sedimentary environments and suggest a genetic

TABLE 6. Rare Earth Element Abundances (ppm) in Massive-Banded and Stockwork Iron Deposits from the Avnik (Bingol) Region

Sample number La Ce Nd Sm Eu T b Tm Yb Lu

Massive and banded iron ores

Stockwork iron ores

relation between the iron deposits and the volcanics, felsic magmas. Roelandts and Duchesne (1977) have as also was noted for the Damberg deposits in central shown that apatite in the Rogaland anorthosites has Sweden (Arvanitidis and Rickard, 1981). larger negative Eu anomalies than do the associated

The large negative Eu anomalies of the apatites liquids, because of simultaneous fractionation of pla- and the coexisting iron deposits are another link be- gioclase during crystallization. The same mechanism tween them and the host volcanic series. Each shows may apply to the Avnik apatites. a marked negative Eu anomaly and no Ce depletion, Graf (1977) suggested that the large positive Eu consistent with extreme fractional crystallization of

Massive and banded iron ore

1 , v , Ionic radii (A) ' 1 I ~a Ce Nd Sm Eu Gd Tb Tm Yb Lu

Stockwork iron ore \I!;! '

I

lonic radii (A)

FIG. 21. Chondrite-normalized REE patterns of the massive FIG. 22. Chondrite-normalized REE patterns of the stockwork and banded iron ores. T h e numbers in the figure correspond to iron ores. The numbers in the figure correspond to those in Table those in Table 6, and they are also shown in Figures 1 and 3. 6, and they are also shown in Figures 1 and 3.


anomalies in the New Brunswick iron-formations and massive sulfide deposits are typical of hydrothermal sulfide deposits. The massive sulfide deposits are be- lieved to form as chemical sediments when hyclro- thermal solutions which derived metals from inter- action with the volcanic rocks flow out into seawater. It was shown that the REE patterns of the chemical sediments could be related to water-rock interactions within a hydrothermal system and that interactions between a hydrothermal solution and a felsic feldspar porphyry could produce a positive Eu anomaly in the solution (Graf, 1977). The feldspar-rich metavolcanics from the Avnik region have negative Eu anomalies (Fig. 19), indicating that the volcanic rocks of the Avnik deposits have not interacted with seawater during their eruption.

Negative Ce anomalies are good evidence of the effect of seawater with REE patterns similar to those of modern seawater (Fryer, 1977a). Metalliferous sediments on the East Pacific Rise and modern sea- floor deposits show negative Ce anomalies (chondrite- normalized) inherited from seawater (Graf, 1978). The REE patterns of the Avnik iron deposits and the associated volcanics do not show negative Ce anomalies, which also suggests that the deposits and the volcanics have not been influenced by seawater. Chondrite-normalized REE patterns in metalliferous and other oc-ean ridge sediments show small negative Eu anomalies and large negative Ce anomalies (Fryer, 1977a, 1977b). This suggests that the Avnik deposits have not equilibrated with seawater and thus are not chemical sediments or even volcanic exhalative-sedimentary deposits.

The only apatite-rich iron deposits yet studied that do show seawater REE patterns are the Precambrian iron-formations in the Vayrylankyla deposits in northern Finland (Laajoki, 1975), which have large negative Ce anomalies and essentially no Eu anomalies. Laajoki (1975) argues that an apatite-rich iron deposit would be very resistant to metamorphic dis- turbances of the REE patterns, which seems reasonable; and his interpretation suggests that the REE

analyses of the Avnik apatites and the iron deposits can be treated as representative of the original rocks.

The formation of remarkably pure and mineral- ogically distinct apatite and iron-rich deposits from a felsic melt means that separation of the iron fraction must have occurred during movement and consoli- dation of the host melt rather than under stagnant conditions. The most attractive mechanism is one of immiscibility, which is a common phenomenon within alkaline and phosphorus-rich magmas provided there are high concentrations of Fe, P, and volatile components. Watson (1976) showed that when a melt separates into immiscible mafic and felsic liquids, the REEs are concentrated in the mafic melt by a factor of 4, and P is enriched there by a factor of 10. The mafic melt, with further fractionation, could then give rise to an immiscible apatite-magnetite melt, as pro- posed by Philpotts (1967).

On the basis of this model, it is possible to calculate the REE pattern of apatite that would crystallize from an immiscible mafic liquid coexisting with the Avnik rhyolitic magma as follows:

1. Felsic melt X 4 = mafic melt (immiscible). Wat- son's (1976) data suggest that a mafic melt would be enriched in La, Sm, and Lu by a factor of 4 to 5 relative to a felsic melt, but differences are within error of measurements. Two-liquid partitioning numbers are taken from Watson's (1976) paper (p. 125- 126 and table 2).

2. Mafic melt X apatite-basalt partition coefficient = REE apatite, using an average of partition coefficients at 950°C, for four basic compositions studied by Watson and Green (1981). Averages of apatite- basalt partition coefficient numbers are taken from Watson and Green's (1981) paper (p. 411 and table 2).

3. Watson and Green (1981) give KD data for Dy, for which the Avnik metavolcanics have not been analyzed (Table 7). These coefficients have been used for Tb, which has very similar ionic radius and thus, expectably, similar partitioning.

T A ~ L E 7. Rare Earth Element Abundances (ppm) in Metavolcanics from the Avnik Region

Sample number La Ce Nd Sm Eu Tb Tm Y b Lu

370 C. HELVACI

o S2-13 Calculated Apal~te '! S2-15 from tmmlsclble baslc liquid -coex~sl~ng..

4 S2-19 1 wlth rhyollllc magmas Murdere and Mtskel IOIIIC r a d ~ ~ (A) Harabe 10Lamw Rare earth elements

FIG. 23. Chondrite-normalized REE patterns of the Avnik apatites and the calculated apatites from hypothetical immiscible basic liquid coexisting with rhyolitic magmas in the Avnik deposits.

The calculated REE att terns of a ~ a t i t e are parallel to those of apatites from the deposits, which is consistent with the hypothesis that the Avnik apatite- rich massive and banded iron de~os i t s are formed bv separation of an immiscible apatite-magnetite liquid from m d c melts cogenetic with felsic volcanics (Fie. 23). \ " ,

Sr isotopes for the separated apatites from the iron deposits have been analyzed (Table 8). All of the H7Sr/86Sr ratios are so high that they cannot have any relation to seawater. Mi~kel apatite samples have identical 87Sr/A6Sr despite the large distance between them. These samples are all from the stockwork iron deposits (Fig. 3), which suggests that they equilibrated with an isotopically homogeneous fluid phase during metamorphism and recrystallization. Samples from other iron deposit types are heterogeneous, which suggests they may have equilibrated with the local rocks only during metamorphism. The s7Sr/"Sr values of the stockwork deposits are close to those estimated for the initial ratio of the Avnik granitoid (Helvac~ and Griffin, 1983a). This is consistent with the stockwork iron deposits being remobilized by fluids from the Avnik eranitoid when it intruded the " volcanic rocks and iron deposits.

The El Laco magnetite-apatite deposits in Chile (Frutos and Oyarzun, 1975), and the Mertainen and Painirova deposits, associated with Kiruna area in Sweden (Lundberg and Smellie, 1979; Smellie, 1980), have been interpreted as fusion products from itab- irite iron-formation and greenstones, respectively, which are assumed to be closely related to these deposits. By analogy, the iron deposits of the Avnik region may also have formed from a magma that has assimilated iron-rich materials from depth. It is ten- tatively suggested for Avnik deposits that assimilation of iron-rich material at depth from underlying paragneisses and amphibolites, which are observed in the Cacas area to the east of the Bitlis massif (Yllmaz, 1971), has resulted in an unusually iron-rich calc-alkaline primary magma. Extreme fractionation of this magma may locally have given rise to a residual silicic melt relatively rich in iron and phosphorous. At a certain stage of maximum iron enrichment, the separation of an immiscible liquid rich in magnetite, apatite, and volatiles may have taken place.

Conclusions

1. The Avnik magnetite-apatite deposits are in- timately associated with a dominantly intermediate to felsic calc-alkaline volcanic sequence, and there is little field evidence to suggest either an intrusive or a sedimentary origin. The deposits have been remobilized to stockworks by the intrusion of granitoids, and Sr isotope data are consistent with remobilization by fluid derived from the granitoids.

2. The high REE contents, negative Eu anomalies, and lack of negative Ce anomalies suggest that the ore have not equilibrated with seawater. Therefore a sedimentary or volcanic-exhalative origin is unlikely.

3. The REE patterns of the deposits are similar to those of the metavolcanic rocks, suggesting a genetic relationship. REE patterns in the apatites of the iron deposits are very similar to those of the apatites that would crystallize from a hypothetical mafic magma in immiscible liquid equilibrium with the Avnik volcanic~.

TABLE 8. Rb-Sr Data on Apatites

Sample number Deposit Type of apatite Rb (pmm) Sr (ppm) Rb/Sr "Sr/=Sr

S3-5 Murdere Recrystallized massive banded <1 46.5 0.000 0.71372 f 12 S1-2 Mi~kel Stockwork <I 194 0.004 0.71149 f 12 S1-3 Mi~kel Stockwork <1 222 0.000 0.71 157 + 12 S1-4 Mi~kel Stockwork 4.9 31 1 0.016 0.71151 + 12 S1-6 Harabe Recrystallized massive banded < 1 138 0.000 0.71048 f 12 S1-8 Harabe Apatite-quartz association <1 141 0.001 0.71139 k 12

APATITE-RICH Fe DEPOSITS, TURKEY 37 1

4. The geochemical data therefore suggest that the magnetite-apatite deposits formed from immiscible liquids that separated during the fractional crystallization of the magmas that produced the Avnik volcanic rocks.

Acknowledgments

This paper was written while the author was on sabbatical leave at the Mineralogical-Geological Mu- seum, Oslo, Norway, and the use of their facilities and technical services, and discussions with the staff, are appreciated. A special acknowledgment goes to William L. Griffin, who helped the author to focus on the main thrust of this paper and who provided constructive criticism of the manuscript. 0 . 0 . Dora of the Dokuz Eylul University, Jens A. W. Bugge and Odd Nilsen of the University of Oslo, Arild 0 . Brunfelt and Tom V. Segalstad of the Mineralogical-Geological Museum, Oslo, and John A. T. Smellie of the Geo- logical Survey of Sweden are thanked for helpful comments and criticism. The author is indebted to Feruze N. Helvacl for considerable help, particularly with the analyses. He also thanks Magnus Ranheim for drafting assistance, Borghild Nilssen for mineral separation assistance, and Bjorn Elgvad and Per E. Aas for photographic assistance. Field work was supported by Dokuz Eylul University and by the Mineral Research and Exhloration Institute in Ankara and the local division in Diyarbaklr, and the author thanks the management and technical staff for their assistance. He was supported by a postdoctorate fellow- ship from the Royal Norwegian Council for Scientific and Industrial Research during his stay at the Min- eralogical-Geological Museum, Oslo, Norway.

March 4, September 13, 1983

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apatite-rich iron deposits of the avnik (bingol) region, southeastern...

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