فهرست مطالب n. rashidnejad-omran

The garnet calc-silicates at Baba-Nazar are located in the NE of Takab, Sanandaj- Sirjan zone. Anhedral to euhedral green colored garnets is a characteristic feature of these Calc-silicates. Mineral chemistry shows that garnets are Andradite- Grossular and contain more than 13% uvarovite. The high chromium content garnet, Cr-Spinel that have relatively Cr rich core than the rim, sulfide opaque minerals such as pyrite and chalcopyrite in this calc-silicate unit are regarded as evidence of the ultramafic protolith which can be considered the source of chromium for these garnets. The similarity in the geochemical composition of whole- rock between these garnet calc-silicates and ultramafic rocks is also consistent with ultramafic protolith. These rocks were affected by hydrothermal alteration that has caused high Ca and low Mg contents in these garnets and the host rocks compare to those from mantle source and ultramafic rocks. The introduction of CO2 rich fluids caused the carbonatization of ultramafic protolith and preferential removal of Mg. The faults, as channels for circulation of the hydrothermal fluids, play a key role during the process of hydrothermal alteration.

Miocene to Pliocene volcanic rocks in the Raveh region have exposed northern part of the Urumieh - Dokhtar Magmatic Belt. Neogene volcanic rocks are situated on Upper Red Formation. Magmatic activity separated in two phase. The first phase is composed of basic to intermediate volcanic rocks such as basaltic-andesite to andesitic lavas and pyroclastics (Ng 1).The second phase has intermediate to acidic rocks as well as andesite, quartz andesite and dacite (Ngv2).According to geochemical data, these samples show magmatic affinities to the calc-alkaline series. REE and trace element patterns show LREE enrichment relative to HREE, depletion in Nb, Ta and Ti, and also high Th/Yb and Th/Nb ratios relative to MORB and OIB. The parent magma of the Raveh volcanic rocks, with calc-alkaline compositions, has been originated probably from the metasomatized mantle with effective of fluids and sediment resulted from the Neothytian subducting slab. After collision between Arabian plate and Central Iran Block in Early Cenozoic, region experienced of crustal thickening and volcanism activity formed in relation to localized extensional basin in Early Miocene to Early Pliocene. The volcanism postdates continental collision, occurring in transtentional tectonic setting.

Gomish-Tappeh Zn-Pb-Cu (Ag) deposit is located 90 km southwest of Zanjan, in northwestern part of Urumieh-Dokhtar volcano-plutonic zone. Exposed rocks at the area include Oligo-Miocene volcano-sedimentary and sedimentary sequences as well as Pliocene volcano-plutonic sequence (andesite porphyry dykes, dacitic subvolcanic dome and rhyodacitic volcanics). Alteration in the deposit developed as silicic, silicic-sulfidic, sericitic, carbonate, argillic and propylitic. Main mineralization at the Gomish-Tappeh deposit is observed as veins occurring in a steeply-deeping normal fault defined by an NE-SW trend in host rocks such as dacitic crystal litic tuff, dacitic subvolcanic dome, specifically the rhyolitic tuff. Paragenetic minerals in the ore veins consist of pyrite, arsenopyrite, chalcopyrite, bornite, low-Fe sphalerite, galena, tetrahedrite and specularite. Gangue minerals accompanying the ores include quartz, calcite, chlorite, sericite and clay minerals. Based on geochemical data, average grades for samples from the ore veins at the Gomish-Tappeh deposit are: 4% Pb, 6% Zn, 2% Cu and 88 ppm Ag. Moreover, REE distribution patterns for altered samples of the dacitic subvolcanic dome and acidic tuff when campared with fresh samples, show enrichment in LREE, while HREE demonstrate various bahaviours. The negative Eu anomaly in chondrite-normalized REE patterns for these rocks is related to the increase in fluid/rock ratio and destruction of those grains of plagioclase enriched in Eu. REE distribution patterns for the silty tuff (footwall to the ore) campared with acidic tuff represent enrichment in all REE as well as positive Eu anomalies. However, the ore samples indicate more enrichment in LREE/HREE ratios and higher Eu contents when campared with wallrock of the ore veins (silty tuff). This is due to the influence of chloric magmatic-hydrothermal fluids that caused alteration along the ore zone, releasing LREE and Eu from the host rocks and finally, concentrating and transporting these elements in the ore fluid.