Surface and sub-surface studies on geology and characteristics of ore-forming fluids in the Niaz porphyry copper prospect, west of Meshginshahr

The Niaz Cu prospect is located 25 km west of Meshkinshahr, east of the Qaradagh metallogenic zone. The intrusion Oligo-Miocene magmatic bodies into the Paleocene-Eocene rock units has led to alteration and mineralization. The rock units of this area include batholiths Ι and ΙΙ (Khanbaz granodiorite and Khankandi granodiorite), Niaz quartz-monzonite/ quartz-monzodiorite and ore-bearing rhyodacite breccia.

In this research, the geological features, alteration, mineralization and physicochemical conditions of mineralizing fluids have been studied. In this regard, sampling from altered, mineralized zones and drilling cores were carried out and 23 samples were analyzed by XRF and 18 samples by ICP-OES methods and microthermometric measurements were performed on 8 doubly-polished thin sections.

The intrusive units have high-K calc-alkaline to shoshonitic nature, and the geochemical characteristics of their trace elements indicate similarities with subduction-related magmas. The negative anomaly of Ti and Nb in these rocks can be due to the magmatism related to the subduction processes, as well as the stability of the phases containing these elements during partial melting or their separation during the differentiation process. The enrichment of Pb, La, K, U, and Th elements and the depletion of Sr, Ti, and Nb can be attributed to crustal contamination. Hypogene alterations at Niaz include potassic, phyllic, propylitic and intermediate argillic types. Mineralization has occurred during early, middle, and late stages. Based on the mineralogy and paragenetic sequence, at least five types of veins/veinlets can be distinguished in Niaz deposit. Group A veinlets contain quartz+pyrite+chalcopyrite+magnetite, group B veins are also present in the potassic and phyllic alteration zones, veinlets of group C are mainly formed in the middle stage of mineralization, group D veins are mostly observed in the phyllic alteration zone, which are formed in the middle and later stages of hydrothermal activities and group E veins are almost devoid of sulfide minerals and mainly contain bright-color minerals (quartz and/or calcite)±tourmaline and are mostly present in the propylitic alteration zone. Studies on fluid inclusions (FIs) within the quartz veinlets showed that there are four types of FIs at room temperature, (1) mono-phase vapor, (2) liquid-rich 2-phase, (3) vapor-rich 2-phase, and (4) multi-phase solid containing daughter solid phases. The ranges of FIs are about 280-360°C in the potassic, 280-360°C and 280-300°C in phyllic and 170-330°C in propylitic alteration zones.

The petrology and petrogenesis of magmatic host rocks, mineralogy, hydrothermal alteration in the Niaz area testify to a porphyry-type Cu mineralization. The main sulfide mineralization includes vein-type pyrite, molybdenite, chalcopyrite, sphalerite and galena. Fluid inclusion microthermometry results show a Th range of 170-360 °C and salinity values of 0.2-60 wt%NaCl equiv., corresponding to the ranges of porphyry Cu deposits. Boiling and simple colling of ore-bearing fluids were the main processes for ore precipitation.