Mineralization and Fluid Inclusion Characteristics of Pirmardan Stratabound Copper Deposit (Manto Type), SW Shahrood)

Located in the NE Iran, the early Tertiary volcanic sequences host a vast stratabound Cu mineralization (manto type), which is likely to be significant economically (Samani, 2002). The Pirmardan copper deposit is located 130 km southwest of Shahrood and is classified as a manto type mineralization, hosted by altered andesite to trachyandesite, volcanic breccia and tuff. The host rock suffers from two kinds of local and regional hydrothermal alterations of sericite- carbonate, and propylitic, respectively. Sulfide minerals occur as disseminated vein and veinlet forms in the host rock. Manto mineralization type in the northeast and central Iran could be a new prospective for the copper deposits subsequence of the Cu porphyry deposits in Iran (Samani, 2002).Our studies focus on the mineralogy, geology, fluid inclusion and ore formation in an attempt to understand the characteristics of ore fluids and mechanisms of ore formation, and to develop exploration criteria for Pirmardan and similar occurrences in the northeast of Iran. 

Various rock types and alteration assemblages, and mineral paragenesis, were characterized by transmitted and reflected light microscopy, X-ray diffraction (XRD) and X-ray Fluorescence (XRF) analysis. Representative samples from drill holes were selected for fluid inclusion studies. Microthermometric measurements were carried out by the linkam stage for heating and freezing system at the Geological Survey of Iran (GSI).

Several mineralized zones with the width of 0.5-20 m and length of 5-300 m have been found in the Pirmardan area.  Mineralization occurs in the shape of vein and veinlet, breccia, stockwork and open space filling. The copper mineralization in the area occurs as strata bound and sulfide minerals are disseminated in the andesitic rock in a depth of 30 m up to 20 m thickness. The ore grade is about 2% and ore minerals include chalcocite, malachite, azurite, hematite, pyrite and chalcopyrite. Based on microscopic and macroscopic studies, the mineralization at the Pirmardan area is divided into two stages: hypogene stage, which is further subdivided into early and main stages; and supergene stage, which includes sulfidation and oxidation stages. Given the results of the geological and geophysical investigations carried out in the study area, six exploration bore-holes were suggested to be drilled to a depth ranging from 34 to 45 m. The results of drilling confirmed a level of mineralization containing amounts of copper. Mineralization in the depth of (0 to 28 m) occurs in the forms of vein and veinlet with a disseminated texture. Fluid inclusions in the veinlet of calcite from the main hypogene stage occur typically as isolated bodies. Homogenization temperatures are between 117 and 400 ºC. The final melting temperatures are between -0.8 and -9.2 ºC, giving apparent fluid salinities of 1.3 to 13.0 % weight NaCl equivalent.

The study area is located in the Pirmardan district along with other manto type Cu deposits in the northeast of Iran that occur mostly in the felsic-intermediate volcanic and pyroclastic rocks of Tertiary range. The geological studies in this area indicate the two main hydrothermal events of copper mineralization and alterations resulting from circulating heated waters (e.g. Oyarzun et al., 1998). It is likely that both of the hydrothermal events have contributed to extensive mineralization in shallow and deeper parts of the subsurface area. Mineralization in the shallow depths is controlled by local faults and fractures. These fractures are mineralized by chalcosite, malachite, pyrite, chalcopyrite and Fe-oxides minerals associated with veinlets of calcite and quartz. Microthermometric data indicate that the ore formation was mainly related to the low to moderate temperatures (117-400 °C) and salinities (1.3 to 13 wt.% NaCl).  The widespread salinity in the Pirmardan area could be explained by the processes of boiling. Mixing and boiling are two important processes during the ore formation in the hydrothermal systems (Oyarzun et al., 1998). The occurrence of hydrothermal breccias and coexistence of fluid-rich and vapor-rich inclusions (Oyarzun et al., 1998) in calcite indicate that boiling is one of the main ore-forming processes, especially during the main hypogene stage in the Pirmardan district. Overall, the host rocks, alteration, ore mineralogy, ore structure and texture and fluid inclusions characteristics in the Pirmardan district are similar to those deposits belonging to the Cenozoic manto type deposits in the South America (Kojima et al., 2003; Wilson and Zentilli, 2006).