The Angouran Zn (Pb) deposit, NW Iran: Evidence for a two stage, hypogene zinc sulfide-zinc carbonate mineralization

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Abstract

The unusually high grade hypogene zinc ore at Angouran in northwestern Iran (40.4% Zn, 1.9% Pb in the sulfide ore, 28.1% Zn, 4.4% Pb in the carbonate ore, and 110g/t Ag) formed from an initially highly saline, reduced, relatively acid hydrothermal brine at two successive sulfide and carbonate ore stages. The early ore stage consists of multiple phases of sphalerite dominated sulfide ore breccia with subordinate amounts of galena (±Pb sulfosalts), minor pyrite, and abundant barite. Sphalerite precipitated at moderate temperatures (≥155°C) because of pH increase in the presence of hangingwall marble. Smithsonite precipitated at a higher pH value (≥7) and at lower temperatures (≤120°C) from dilute solutions (salinities close to zero) by mixing of the Zn bearing brines with cool, HCO3- bearing waters. The first melting points of the primary (LV) fluid inclusions in sphalerite and in hydrothermal quartz are unusually low (≤-60°C), close to the eutectic point of the ZnCl2-H2O system (-62°C). Total salinities taken from the ZnCl2-H2O system as a best approximation correspond to 26-41eqmass % ZnCl2. The initial brine evolved to a CaCl2-NaCl rich solution with 27eqmass % salinity. Gas densities (≤0.1gcm-3 for water vapor and ≤0.18gcm-3 for CO2) in the fluid inclusions indicate low pressure (≤5bar for water vapor, and ≤100bar for CO2) at the entrapment temperatures. At the first carbonate ore stage sulfides continued to precipitate characteristically as arsenopyrite with minor amounts of galena and pyrite. The abrupt change of the fluid composition at the sulfide-carbonate boundary was accompanied by a change of the fluid temperature and pressure that produced brecciation of the sulfide ore matrix and an almost total dissolution of barite and replacement by the hypogene smithsonite. Alteration is restricted to ore deposition and consists of weak sericitization and silicification with local dolomitization at the sulfide ore stage, and pervasive dissolution of the hangingwall marble, in particular at the carbonate ore stage. The breccia orebody at Angouran is sited at the crestal portion of a domed antiform at the lithological and thrust boundary of Neoproterozoic-Lower Cambrian footwall schists and the hangingwall marble in rapidly uplifted and exhumated Angouran Block east of the Geynardjeh Thrust Fault. The footwall schists occupy a detachment fault zone above imbricated nappe sheets of the basement metamorphic complex of the Sanandaj-Sirjan zone. During the Pliocene, the nappe sheets were thrust toward the southwest onto the Miocene felsic volcanic rocks of the Urumieh Dokhtar Volcanic Belt that are intruded by 10. Ma late Miocene basalts. The 11.9. Ma and 18.4. Ma zircon ages of the felsic volcanic rocks indicate the lower age limit of the ore body emplacement. The associations with large scale, mid-late Miocene, felsic volcanism along the active Tethyan belt, as well as the ubiquitous presence of the volcanic rock clasts in the sphalerite ore matrix, provide strong evidence of the involvement of hydrothermal processes at Angouran. Ore fluids were successively and pulsatorily generated within the seismically active region. A following geothermal activity appears to have had a significant input in the formation of the carbonate ore of the hypogene, as well as the supergene stage. Stable isotope data suggest complex interaction of element sources and processes. Allowing a broad interpretation, the sulfur isotopic composition of the sulfides δ34S (3.9 to 7.4‰) suggests that the sulfur could be sourced from evolving, mixed magmatic-basinal brine. The isotopic composition of the hypogene smithsonites (δ13C: 2.72 to 5.51‰, δ18O: 18.4 to 22.8‰) broadly supports the local geology and field relationships, which comply with a marble wallrock source for the carbonate ores. They lend support to the assumption that smithsonite was deposited from solutions with isotopic composition similar to those involved in the hydrothermal dolomitization of the marbles. The excess of dissolved marble precipitated as large volumes of travertine and as late calcite veins (δ13C: 18.8 to 20.3‰, δ18O: 3.1 to 6.4‰) at the mineralization site. Isotope values of the travertine (δ13C: 4.5 to 6.6‰, δ18O: 20.1 to 21.1‰ V-SMOW) are consistent with the involvement of CO2 derived from thermogenic decarbonization of the host marble by waters of dominantly meteoric origin, most likely concomitantly with ore forming processes. The Angouran deposit is the only reported case of a two stage, hypogene zinc sulfide-zinc carbonate mineralization, and represents a new style of nonsulfide zinc mineralization. © 2013 Elsevier B.V.

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Daliran, F., Pride, K., Walther, J., Berner, Z. A., & Bakker, R. J. (2013). The Angouran Zn (Pb) deposit, NW Iran: Evidence for a two stage, hypogene zinc sulfide-zinc carbonate mineralization. Ore Geology Reviews, 53, 373–402. https://doi.org/10.1016/j.oregeorev.2013.02.002

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