Geochemistry of argillic alteration: a case study from the Jizvan area, Tarom-Hashtjin zone

Intrusion of quartz-monzodioritic igneous bodies of Oligocene age into the Eocene lithic crystal tuffs and trachy-basalts resulted in occurrence of widespread argillic alteration zone in the Jizvan area (Tarom-Hashtjin zone). Mineralogical studies indicate that this alteration zone includes kaolinite, quartz, smectite, pyrophyllite, muscovite-illite, alunite, rutile, calcite, feldspar, chlorite, hematite and goethite minerals. Hypogene ore minerals within the silicic-carbonatic veins and veinlets of argillic alteration zone contain chalcopyrite, galena and pyrite accompanied by goethite, malachite and azurite of supergene origin. Mass changes calculations of elements with assumption of Al as a monitor immobile element indicate that elements such as Ti, P, Th, Nb, Ta, Y and Zr have suffered leaching during argillization of lithic crystal tuffs. This abnormal behaviour is in relation to low pH of altering solutions, high water/rock ratio, abundance of complexing ions and suitable drainage system. The distribution pattern of REEs normalized to chondrite shows differentiation and enrichment of LREEs relative to HREEs and occurrence of negative Eu and Ce anomalies during argillization of lithic crystal tuffs. Geochemical investigations reveal that occurrence of negative Ce anomaly (0.49-0.92) is in relation to the destruction of zircon by acidic-oxidizing fluids. Negative Eu anomaly (0.23-0.73) and mass loss of elements such as Si, Fe, K, Rb, Cs, Sr and Ba indicate destruction of plagioclase and hornblende by strongly acidic hydrothermal fluids and high oxygen fugacity of environment. The correlation coefficients between elements display the controlling role of Mn-oxides in distribution and concentration of REEs, Pb, Zn and Cu. Mineralogical and geochemical evidence such as presence of pyrophyllite, alunite and rutile, enrichment of LREEs relative to HREEs, low values of La৯, negative Ce anomaly and strong positive correlations between (LREEs/HREEs)N-LOI and (La/Lu)N-P suggest that the development and evolution of argillic alteration zone in the Jizvan area is affiliated to hypogene processes.