Petrology of the Plutonic Rocks at the XIV Iron-Oxide Prospect, Bafq Mining District, Central Iran

The XIV iron-oxide prospect in the northern part of the Bafq mining district is hosted by intrusive rocks and demonstrates similar characteristics with Kiruna mineralization. The plutonic complex of the XIV prospect is composed of two major types of intrusive rocks, including: 1) a granitic intrusive body which is itself composed of two types; a pale-pink leucogranite and a white-gray aplite, as well as 2) a gabbrodiorite intrusive body. Geochemically, these rocks are tholeitic-calcalkaline and metaluminous in nature. Geochemical characteristics of the plutonic rocks of the study area support a non-orogenic continental setting.


Introduction
The Bafq mining district in Central Iran is the most important iron metallogenic province in the region and a significant district on a worldwide basis ([1]- [9]). The Bafq district, extending from Bafq to Saghand, is part of the central Iranian microplate which is now embedded within the Alpine-Himalayan orogenic system. The Central Iranian Terrane is an assemblage of several Precambrian fragments of Gondwanaland which covers an approximately 230,000 km 2 of moderate relief surrounded by Mesozoic-Cenozoic fold-and-thrust belts of the Alpine-Himalayan system.
The Central Iranian Terrane consists, from east to west, of three major crustal domains: the Lut Block, Tabas Block and the Yazd Block. These blocks are separated by a series of intersecting regional-scale faults. The Tabas and Yazd blocks are separated by a more than 1000 km long and up to 80 km wide arcuate and structurally complex belt composed of variably deformed supracrustal rocks, i.e. the Kashmar-Kerman Tectonic Zone [7] which hosts the Bafq mining district. The Kashmar-Kerman Tectonic Zone provides remarkable exposures of the deeper sections of the Central Iranian platform strata, among which the Late Neoproterozoic and Early Paleozoic rocks are abundant [7] [8].
The ΧІV iron prospect (geographic coordinate: 55˚28'E -55˚30'E and 32˚04'N -32˚07'N) is located at 8 km north of Chah Gaz magnetite deposit (anomaly ΧІVA) and 60 km north of Bafq city ( Figure 1). This prospect is divided into three ore zones (northern, central and southern orebodies). According to the data obtained from drilling cores, the main orebody (northern zone) is continued down to 300 m below surface. The magnetite consists of 62% iron and is phosphorus-poor. The northern ore zone mainly occurs in an aplite dome (caused by late differentiation of granite and called "leuco-metasomatite" in mining terminology) and is composed of the massive magnetite and hematite ore bodies together with actinolite, all of which are located in the northern and southern parts of the area. The origin of the Bafq district deposits has been the subject of longstanding debate and remains controversial. Also, the ΧІV ironprospect has received little attention so far. This paper presents the outcome of the preliminary petrological investigations on the plutonic rocks of the prospect which partly hosts the mineralization. In this regard, the different rock types of the so called Zarigan granite at the prospect have been studied using petrographic and geochemical investigations and have been interpreted accordingly the tectonic setting.

Research Methodology
During the field observations, 120 rock samples were collected from different parts of the study area, of which 80 thin sections and 34 polished thin sections were prepared and studied using polarizing microscope. 12 samples from granite and gabbrodiorite intrusive bodies were analyzed for their geochemistry (including REEs) using ICP-MS and XRF. The petrographic and geochemical results were then interpreted for understanding the petrology of the intrusive rocks in the study area.

Granites
The Zarigan intrusion is the largest intrusive body exposed in the study area. This intrusion is a shallow, leucocratic body that ranges in texture and color from a typical medium-crystalline granite to a sub-volcanic granite porphyry (aplite) (Figure 2   Eutectic intergrowth of quartz with sodic feldspar is often of the granophyric and rarely of the micrographic type and has just been observed sporadically. In some Leucogranite samples of the XIV prospect, the granophyric texture is developed and the whole rock is composed of this type of intergrowth. Alkali feldspar in granophyres is typically of albite type that has been altered to sericite. The second type of granite is a white to gray aplite (sub-volcanic granite porphyry orlate differentiation phase of Zarigan granite which is called "leuco-metasomatite" in mining terminology) (Figure 3(b)). The major rock forming minerals of the aplite of the XIV prospect consist of quartz (70% -85%) and sodic feldspar (15%), while the accessory minerals includezircon, apatite, hornblende and opaque minerals. Quartz crystals are seen as individual texture and comprise two distinct textural components: 1) a fine grained (up to few millimeters) broadly equigranular alkali feldspar and plagioclase component; 2) a fine to medium grained component (Figure 3(c)); sometimes intergrown with albite and rarely with pertite. These types of quartz crystals have formed simultaneously.

Gabbrodiorites
The modal mineralogical composition of representative gabbrodiorite samples (Figure 4 This gabbrodiorite indicates an inter-granular and hypidiomorph granular texture with fine-grained euhedral-subhedral magnetite scattering with pyroxene and plagioclase. The petrographic investigations show that most of theamphibole has converted to tremolite and actinolite. Also undeveloped granophyric quartz-albite intergrowth is observed in the gabbrodioritic rocks which indicate mixing between granite and gabbrodioritic rocks.

Geochemistry
The result of major element analyses of granite and gabbrodiorite samples are listed in Table 1 and Table 2. The alumina saturation index values [16] are plotted in Figure 5 and show that the least altered Zarigan granites are both Al-oversaturated (peraluminous) and Al-undersaturated (metaluminous), but most samples have been plotted in the peraluminous region. The felsic plutonic rocks of the region plot in the granite and alkali granite field, while the mafic plutonic rocks plot in the gabbro field of the plutonic rock classification diagram [17] (Figure 6). To distinguish between calcalkaline and tholeiitic suites, the samples were plotted in an AFM diagram (Figure 7) which shows       Table 1).
The XIV granites have relatively high contents of LILE such as Ba and Rb, and low HFSE contents with marked negative Nb, Sr and Ti anomalies on MORB normalized spider diagram (Figure 8 and gabbrodiorite of the XIV prospect. REE concentrations in the granite are higher than those in gabbrodiorites (Figure 8(a)) and the granite has a higher Th/U ratio (2.5 -7.4) than that of the gabbrodiorite (0. 7 -3.52 (Figure 8(a)). They are depleted in HREE and relatively enriched in LREE with LREE/HREE ratios of 4.3 to 42, and have narrow compositional variation with negetive Eu anomalies (Figure 8(a)).

Tectonic Setting: Anorogenic or Post-Orogenic?
The host rocks of the XIV prospect area, according to this study and the study conducted by Ramezani and Tucker [13], vary from granite to gabbrodiorite, indicating an alkaline-sub-alkaline or a bimodal composition. The tectonic setting diagrams ( Figure   9) illustrate volcanic arc granites.
Boron is an interesting trace element, as it is strongly enriched in seawater and marine sediments compared with its concentration in most crust and mantle rock types [19]. Therefore, boron can be used as a tracer to indicate the presence of subducted sediment or altered oceanic crust in magma's source regions. Boron concentration gener- ally varies from 1 to 2 ppm in mid-ocean ridges and ocean island basalts, but, frequently reaches to more than 10 ppm in volcanic arc basalts and andesites. No boron was detected in this study or those conducted by Ramezani and Tucker [13] and Isfahani and Sharifi [21] probably indicating an orogenic continental rift setting for the Bafq region.
The plutonic rocks of the Bafq region are diverse and range from granite to gabbrodiorites (bimodal composition). Many are identified as syenites and/or red granite (e.g. Sorkh granite in the vicinity of Chadormalu paleocrater) (Figure 1). These rocks generally display either potassic or sodic alteration and are pervasively replaced by hydrothermal hematite and magnetite. Extensive sodic, potassic, sericitic and silicic alterations are present at the XIV iron-oxide prospect and its associated iron deposits.
A conceptual conclusion by Hitzman et al. [

Conclusions
The Zarigan granite is one of the most important plutonic rocks in the study area. This intrusion is a shallow, leucocratic body that ranges in texture and colors from a typical medium-crystalline granite to sub-volcanic granite porphyry (aplite). Mineral assemblage in these rocks is quartz, K-feldspar, plagioclases, biotite, chlorite and opaque minerals. The alumina saturation index values [19] show that Zarigan granites are Alundersaturated (metaluminous), while these felsic plutonic rocks of the region plot in the granite and alkali granite field of the plutonic rock classification diagram [18].
Plutonic rocks from the XIV prospect are similar in their REE patterns. They are depleted in HREE and relatively enriched in LREE with LREE/HREE ratios of 4.3 to 42, and have narrow compositional variation with negative Eu anomalies.
The collected evidence in this study shows that the host rocks (bimodal series) contain voluminous intermediate-felsic intrusive bodies which have occurred in a nonorogenic environment.