Mineral and ore deposits Skarn-type Fe-Cu mineralization at eastern contact zone of Kamtal monzonitic stock, nw Iran

Hallaj Kheljani B.¹, Simmonds V. ²

1Geology Department, Islamic Azad University of Ahar, Iran; 2Research Institute for Fundamental Sciences, Tabriz University, Tabriz, Iran

Bijan.xax@gmail.com

Quartz monzonitic intrusive stock of Kamtal is located in NW of Iran, 75 km east of Jolfa town at southern bank of Arax river on the Iran-Armenia border. This stock is part of Ghareh-dagh batholith, which intruded in Alborz-Azarbaidjan zone during Oligocene period and hosts many vein-type, porphyry and skarn-type Cu, Mo, Au and Ag mineralizations, not only in NW Iran, but also in neighboring territories of Armenia and Nakhjavan Autonomus Republic.

Based on various structural classifications of Iran, the study area is part of Alborz-Azarbaidjan zone, which was magmatically active during Cenozoic, the pick of which was at Eocene and Oligocene [3].

The oldest rock units outcropped in the area are upper Cretaceous flyschic units, composed of micritic limestone, siltstone, shale and claystone, which are laterally changed to limestone-marl with intercalations of tuff and intermediate lavas. The intrusion of Kamtal stock into these units has led to occurrence of contact metamorphism, skarnification and Cu-Fe mineralization at eastern and southern margins of the intrusive stock.

The monzonitic to quartz-monzonitic stock is composed of plagioclase (40-50%), akali-feldspar (orthoclase) (30-35%), amphibole (5-10%), biotite (0-5%), quartz (5-15%) and opaque minerals (0-10%) with lesser amounts of clinopyroxene, displaying heterogranular and monzonitic texture. The accessory minerals are sphene, apetite and zircon. Based on the geochemical data provided by Mokhtari [4] this intrusive body displays High-K calc alkaline affinity, meta-aluminous nature and belongs to I-type ACG granitoids, intruded in an active continental margin volcanic arc setting, related to subduction of Neotethyan oceanic slab beneath the lesser Caucasus plate.

Endoskarn zone is narrowly developed within quartz monzonitic stock with maximum thickness of 2 m, characterized by presence of epidote, actinolite and tremolite ± garnet ± clinopyroxene ± sphene.

Exoskarn is the main zone in this skarn and contains Fe- and Cu-sulfide minerals. This zone is composed of garnet-skarn, epidote-skarn, pyroxene-skarn and marble zones.

Epidote is the index mineral of epidote-skarn zone which mainly occurs as mosaic-like and fibrous aggregates. Some of it, along with quartz and claclite is considered as the products after the alteration of garnets. Numerous epidot-, quartz- and calcite-bearing veinlets have been formed in this zone. Tremolite and actinolite are also abundant in this zone.

Andradite is the main calc-silicate mineral in garnet-skarn zone. Other minerals present in this zone are epidote, clinopyroxene, quartz, calcite and opaque minerals. Ore minerals of this zone are pyrite, chalcopyrite, limonite and goethite.

Pyroxene is present in all skarn zones and in some parts, taking into account its abundance, pyroxene-skarn zone is distinguishable. Based on mineralogical analysis, studied pyroxenes belong to diopside-hedenbergite series and mainly diopside. Epidote is also present with maximum abundance of 10%. Calcite occurs as veinlets, as well as interstitial fillings.

Hypogene sulfide minerals are composed of pyrite (5-10%), chalcopyrite (0-5%), pyrrhotite (0-2%), sphalerite (0-2%), and lesser amounts of Galena (0-1%). Pyrite is the most abundant sulfide mineral, occurring as disseminations and veinlets. The main copper-sulfide mineral is chalcopyrite, occurring as disseminations, veinlets (accompanied by pyrite) and open-space fillings, partly replacing pyrite crystals. Sphalerite mainly replaces chalcopyrite along its margins and fills the open spaces between them. Due to the presence and abundance of sulfide minerals, especially pyrite, supergene processes have been caused oxidation of hypogene sulfides and production of acidic fluids, leading to the formation of Fe-oxides and hydroxides (mainly goethite), as well as Cu-carbonates (malachite).

Based on mineralogical and petrological investigations and field evidence, Kamtal skarn has been formed in 3 stages: a) primary or prograde stage, b) early-retrograde stage, c) late-retrograde stage.

During the prograde stage, concurrent with and after the emplacement of intrusive body, isochemical pyrometamorphism has taken place, leading to formation of marble and lesser hornfels within the carbonaceous host rocks.

Transition from prograde stage to early-retrograde stage is marked by retrograde alteration of early-stage anhydrous calc-silicate minerals by low-temperature fluids derived from intrusive stock and formation of epidote, tremolite, actinolite, calcite and quartz.

Finally, the late-retrograde stage is characterized by alteration and conversion of previously formed anhydrous and hydrous mineral assemblages by low-temperature fluids to a fine-grained assemblage composed of chlorite, calcite, quartz and clay minerals.

Considering the presence of andradite, quartz, calcite and lack of magnetite, this mineral assemblage has been formed at temperature range of 430-550^°C and the estimated fO₂ is about 10^-23-10^-26 [1]. Taking into account the low sulfide abundance, accompanied by higher amounts of andradite, the sulfidation state was low and moreover, considering the coexistence of andradite-pyrite, as well as inferred temperature range of 430 to 550^°C, fS₂ is estimated about 10^-4-10^-7 [1]. Finally, considering the mineral assemblage in equilibrium and inferred pressures of about 0.5-2 kb, XCO₂ value may fall in 0.1-0.2 range [1] and so, the metasomatising fluid had dominantly been aqueous. Meanwhile, the dominant presence of andraditic garnet and rather scarcity of biotite and hornblende reflects an oxidized skarn type, which can be formed in shallow depth setting [2].

References:

1. Einaudi, M. T. (1982) General feature and origin of skarns associated with porphyry copperplutons.Advanced in geology of porphyry copper deposits, S.W. North America, 185-209

2. Newberry, R. (1987) Use of intrusive and calc-silicate compositional data to distinguish contrasting skarn types in the Darwin polymetallic skarn district, California, USA, Mineralium Deposita, 22, 207-215.

3. Mehrparto, M., Emami, M.H., Mirzai, M., Alai, S., 1997. Petrography and petrology of Ghoulan batholith (North of Azarbaidjan), 24 congress of Earth Science proceedings, Geologic Survey and Mineral Exploration of Iran.

4. Mokhtari, M.A.A., Moein Vaziri, H., Ghorbani, M.R., Mehrparto, M. (2009) Investigation of petrology, geochemistry and tectonic setting of Kamtal intrusive body (N Varzeghan, East Azarbaidjan), Earth Science, 71, 123-128 (in persian).