The Chargar Au-Cu deposit: an example of low-sulfidation epithermal mineralization from the Tarom subzone, NW Iran

摘要

The Tarom subzone, to the west of the Cenozoic Alborz magmatic belt in NW Iran, hosts a large number of high-, intermediate- and low-sulfidation epithermal deposits and prospects, associated with voluminous Eocene-Oligocene volcanism and plutonism. The Charger deposit is an example of epithermal Au-Cu mineralization from the southern part of Tarom. The Tertiary felsic-basic volcanic and volcaniclastic country rocks in the Charger area consist of rhyolitic to basaltic units. Whole-rock geochemistry and geochemical modeling show calc-alkaline to high-K shoshonitic and metaluminous characteristics for the volcanic rocks. Silicification is the main alteration type in the area. Mineralization is structurally and lithologically controlled and is hosted by Eocene volcanic-volcaniclastics of the Tarom belt. Gold is hosted mostly within a lithic-lapilli crystal tuff unit. Two sets of faulting cut the area in NW and NE directions; mineralization occurs along an NW trend. The gold in Charger appears as microscopic and submicroscopic grains in quartz, base-metal sulfide (chalcopyrite) and sulfoarsenide (lautite). Part of the gold occurs in nanoparticles in these minerals. Chalcopyrite is the main sulfide mineral. Two generations of pyrite (Py1 and Py2) were distinguished at the Charger deposit. As content in framboidal pyrite (Py1) varies from 0.23 to 0.33 wt.%, containing up to 0.79 wt.% Pb. Py2 is Pb-rich forming the most dominant type with Pb contents ranging from 0.14 to 0.9 wt.%; the As content reaches up to 0.61 wt.%. Ore mineralogical study of Chargar shows the occurrence of variable quantities of Cu-As-Sb-Ag-Bi-Pb sulfosalt minerals. Distribution of Au and Ag contents imply that Charger plots in the epithermal field. Mineralogical and chemical compositions of the sulfosalts at Chargar indicate low-temperature epithermal mineralization.