PETROCHEMISTRY AND GENESIS OF THE GRANODIORITE AND ASSOCIATED IRON-COPPER SKARN DEPOSIT OF MAZRAEH, AHAR, EAST-AZARBAYJAN, IRAN

Abstract

PETROCHEMISTRY AND GENESIS OF THE GRANODIORITE AND ASSOCIATED IRON-COPPER SKARN DEPOSIT OF MAZRAEH, AHAR, EAST-AZARBAYJAN, IRAN The Iron-copper skarn deposits of Mazraeh lies on the southern slope of the Sheyviar Dagh range, which is at the northern contact of Oligocene granodiorite with Cretaceous limestone. The country rock has a general NW-SE trending formations showing the open folds. Several common joint sets have been noticed in the area which can be grouped into (1) -N-S to NNE-SSW trending, mostly vertical, prominent sets; the tongues of granodiorite and most of the quaternary volcanic and aplite veins have similar trend; (2) ENE-WSW to ESE - WNW sets with maxima lying close to E-W; some mineralized veins, aplite veins and mafic dikes follow this trend; and (3) NW-SE trend which is more common in the host rock, dipping towards NE or SW and giving rise to foliation joint or strike joint. These joints have developed fractures or plumbing system near the contact, for migration of ore bearing fluid. The chemical data obtained from Mazraeh granodiorite correspond to I-type granodiorite series with the increasing Si02, K20 and Na20 values as per the accepted principles of magmatic differentiation. Based on the coexisting feldspar of Mazraeh granodiorite, the temperature of crystallization ranges between 698°C to 754°C with the average of 720°C„ Plots of Mazraeh granodiorite on normative Ab - Or - Q diagram suggest that crystallization of magma has taken place between 700°C and 800°C at 2 to 5 kb pressure. The contact metasomatism of the metasediments has led to the formation of skarn. The intensity of skarnification is higher have been affected by the fluids and heat generated by where the contact was discordant in comparison to the case where the bedding is parallel to the igneous contact (concordant). The lithology of the host rock is another controlling parameter for the skarn formations. The main constituent minerals of skarns are, garnet, magnetite, calcite and epidote accompanied by tremolite-actinolite, chalcopyrite, pyrite, clinopyroxene, plagioclase, chlorite, quartz and some amount of sphene and apatite. On the basis of microprobe data garnet has been confirmed as largely andradite. Pyroxene lies in the diopside - hedenbergite range. Magnetite is titanium free and epidote is Mn-bearing. The Mazraeh skarn deposit can be classified on the petrological basis into Endoskarn and Exoskarn. Ore skarn is the skarn where ore minerals predominate. Each of these has further been divided on the basis of characteristic mineralogical assemblage. The bulk chemistry and spatial variation indicate that Si and Fe have magmatic source, Al has been contributed to the system from magma as well as host rock. The crystalline limestone is the source for Ca. The formation of epidote indicates post- magmatic activities. On the basis of petrography following stages of the growth of skarn deposit has been worked out: 1. Emplacement of granodioritic magma at about 750 to 670°C in contact with carbonate rocks. 2. Development of contact metamorphism due to heat generated from the magma and the contamination of granidiorite 3. The main stage of metasomatism and skarn formation. 4. First stage of mineralization (magnetite), and formation of ore skarn. 5. Second stage of mineralization (chalcopyrite- magnetite) following the wall rock alteration due to hydrothermal activity. 6. The last barren stage of hydrothermal activity. The fluid inclusions from different rock types were found to be polyphase, biphase as well as monophase. In the mineralized zones, these were predominantly polyphase. The homogenization of inclusions, both in liquid and vapour states, within a close range of temperature (300°C - 400°C) indicates the boiling condition of the ore fluid. The fluid inclusions show vide range of salinity, 10 to 60% equivalent of NaCl, indicating the mixing of fluids. The salinity versus temperature relationship of homogenization proves the normal cooling of fluid. The separation of a volatile- rich, low-density, low salinity fluid coexisting with a volatile - poor fluid of higher density and salinity was ultimately responsible for the deposition of ore minerals. Two main stages of ore genesis were thus recognised. The first stage coincided with the retrograde part of the contact metasomatism when the deposition of high grade massive magnetite, which replaces nearly all the earlier minerals, took place. The bulk chemical data and spatial relations suggest that iron has leached out from an iron-rich parent magma or its differentiate. The second stage of mineralization was the post contact metasomatic deposition of the sulfide ore. This deposition took place in the main hydrothermal alteration stage. The high value of Cu in granodiorite (85-446 ppm) in comparison to other plutonic body proves the granodiorite magma being rich in Cu and being a potential source for the Cu-mineralization. The high salinity of the biphase fluid inclusion suggests that the parent magma was rich in the mineralizers and hence, was capable of transporting the sulfide complexes and depositing copper. The post contact-metasomatic episode was thus characterized by the upward migration of the magma generated by hydrothermal ore bearing fluids through plumbing system developed during earlier skarn formation near the contact zone of the granodiorite. The deposition of ore minerals was controlled by the Fe-Cu-O-S chemistry of fluids and their interaction with Ca, Si and other host rock assemblage. The mineralization was initiated by magnetite at temperature above and about 520°C replacing Ca-bearing minerals and was followed by a large scale hydrothermal alteration and the deposition of chalcopyrite between 336°C to 520°C due to the neutralization of fluid in contact with Ca rich rock.