Marble-hosted sulfide ores in the Angouran Zn-(Pb-Ag) deposit, NW Iran: Interaction of sedimentary brines with a metamorphic core complex.

The Angouran Zn-(Pb-Ag) deposit, Zanjan Province, NW Iran, is located within the central Sanandaj-Sirjan Zone of the Zagros orogenic belt. The deposit has proven and estimated resources of 4.7 Mt of sulfide ore at 27.7% Zn, 2.4% Pb, and 110 g/t Ag, and 14.6 Mt of oxidized carbonate ores at 22% Zn and 4.6% Pb. It is hosted by a metamorphic core complex that is unconformably overlain by a Neogene volcanic and evaporite-bearing marine to continental sedimentary sequence. The sulfide orebody, precursor to the significant nonsulfide ores, is located at the crest of an open anticline at the contact between Neoproterozoic to Cambrian footwall micaschists and hanging wall marbles. Ar-40-Ar-39 data on muscovite from mineralized and unaltered footwall micaschists suggest a rapid Mid-Miocene exhumation of the metamorphic basement (similar to 20 Ma) and yield an upper age constraint for mineralization. The fine-grained sulfide ore is massive, replacive, often brecciated, clearly postmetamorphic and dominated by Fe-poor sphalerite, with minor galena, pyrite, anhydrite, quartz, muscovite, dolomite, and rare calcite. Sphalerite contains Na-Ca-Cl brine inclusions (23-25 mass% total dissolved solids) with homogenization temperatures of 180-70 degrees C. Fluid inclusion chemistry (Na-K-Li-Ca-Mg-Cl-Br), ore geochemistry, S, and Pb isotope data suggest that the Angouran sulfide ore formed by the interaction of modified, strongly evaporated Miocene seawater and the lithotypes of an exhumed metamorphic core complex. Minor contributions of metals from Miocene igneous rocks cannot be excluded. Mineralization occurred in a collisional intra-arc setting with high heat flow, probably during the transition from an extensional to a compressional regime. The Angouran deposit may represent a new type of low-temperature carbonate-hosted Zn-Pb ore that is distinct from Mississippi Valley type and sedimentary-exhalative deposits.