The study of mineralogy and origin of dumortierite in the Sheivardagh hydrothermal system in East Azerbaijan, NW Iran

Dumortierite appears as aggregates with blue color in the intense argillic and silicification zones of the Alijavad porphyry deposit. Based on field geology and petrography evidence, this mineral is formed as a late hydrothermal and mineralization phase. The study of dumortierite is carried out to investigate the mineralization and prospecting of the goldbearing porphyry copper deposit. Considering the relative abundance of dumortierite in the area, its easy separation and concentration of elements such as As and Mo, its capability for prospecting gold-rich porphyry system is addressed in this paper. This mineral has typical blue color in hand specimen and is fibrous to acicular with radial orientation under the microscope. It is exposed in the altered volcanic rocks of the Alijavad with porphyritic dacite to andesite rocks. Field and petrography studies, XRD, ICP-MS and EPMA analyses were done to characterize this mineral. 14 points were analyzed by EPMA in Germany. The studies show that dumortierite formation is occurred widely with mineral assemblages of pyrophyllite, alunite, sericite, pyrite and copper ore minerals. Light elements including B which cannot inter the silicate structures were accumulated in the hydrothermal fluids at with progress of crystallization of the Sheivar pluton. The fluids intruded the volcanic rocks along fractures and formed dumortierite along with alteration. When concentration of Ca, Mg, P, Sr, Na and Fe is low probably due to leaching and before addition of B, dumortierite crystallized instead of tourmaline. Although biotite and tourmaline are used to prospect some porphyry deposits, but the preference of dumortierite for gold prospecting is that, this mineral is abundant in the argillic alteration zone. Also this indicative mineral can be easily separated from the rock for further studies. Meanwhile dumortierite accumulates important elements such as As and Mo in its structure. Since dumortierite was found in the mineralization cap of the advanced argillic alteration zone down to 170m in bore holes, it can be used as a key mineral in gold exploration in the area.