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Mineralogical and geochemical analyses of the Dopolan karstic bauxite ore were performed to identify the characteristics of four bauxite horizons, which comprise from top to bottom, bauxitic kaolinite, diaspore-rich bauxite, clay-rich bauxite, and pyrite-rich bauxite. Diaspore, kaolinite, and pyrite are the main minerals; böhmite, muscovite, rutile, and anatase are the accessory minerals. The main minerals of the Dopolan bauxite deposit indicate slightly acidic to alkaline reducing conditions during bauxitization. Immobile elements (Nb, Ta, Zr, Hf, and rare earth elements) are enriched in the diaspore-rich horizon, which also has the highest alumina content, whereas redox sensitive elements (e.g., Cr, Cu, Ni, Pb, Zn, Ag, U, and V) are enriched in the lowest horizon of pyrite-rich bauxite. The presence of a high content of organic matter was identified in different horizons of bauxitic ore from wet chemistry. The presence of organic matter favored Fe bioleaching, which resulted in Al enrichment and the formation of diaspore-rich bauxite. The leached Fe 2+ reacted with the hydrogen sulfur that was produced due to bacterial metabolism, resulting in the formation of the pyrite-rich horizon towards the bottom of the Dopolan bauxite horizons. Biogeochemical activity in the Dopolan bauxitic ore was deduced from the reducing environment of bauxitization, and the deposition of framboidal and cubic or cubic/octahedral pyrite crystals, with large negative values of δ 34 S of pyrite (−10‰ to −34‰ ) and preserved fossil cells of microorganisms.

Introduction Sandstone and carbonate hosted Pb-rich deposits, with Zn/ (Zn+Pb) < 1, are an unusual end member of MVT deposits (Sverjensky, 1984a; Leach et al., 2005). Correlation of metal ratio with lithology is reported by Gustafson and Williams (1981). Sverjensky (1984a) has proposed that different rates of water-rock interaction in sandstone and carbonate aquifers could form galena-and sphalerite-rich deposits from single basinal brine. In this model, low Zn/Pb ratio deposits are associated with sandstone aquifer, while high Zn/Pb ratio deposits occur in carbonate aquifers. The basinal brine model (Sverjensky, 1984a) specifically explains mineral paragenesis and the Zn/Pb ratio of MVT deposits. Some investigations on the Viburnum Trend deposits, USA, show that Pb-rich ores were deposited as a result of fluid mixing. Mixing of a metal-rich and H 2 S-poor (or sulfate-rich) brine with another less saline, H 2 Srich fluid (or organic and methane bearing) better explains Pb mineralization in this area (Rowan and Leach, 1989; Anderson, 1991; Plumlee et al., 1994). An anomalous Pb-rich fluid reported by Appold and Wenz, (2011) in sphalerite hosted fluid inclusions showed that one of the aforementioned fluids was enriched in Pb. The well-known episode of Pb-Zn mineralization in Iran took place in the Cretaceous carbonate rocks, including well-known world-class MVT deposits such as Emarat, Mehdi Abad, and Irankuh (Rajabi et al., 2012). Dixon and Pereira (1974) suggested that these deposits range from sedimentary exhalative (SEDEX) to MVT, but most of these deposits are characterized by carbonate host rock and are classified as MVT

A geochemical exploration program was applied to recognize the anomalous geochemical haloes at the Ravanj lead mine, Delijan, Iran. Sampling of unweathered rocks were undertaken across rock exposures on a 10 × 10 meter grid (n = 302) as well as the accessible parts of underground mine A (n = 42). First, the threshold values of all elements were determined using the cutoff values used in the exploratory data analysis (EDA) method. Then, for further studies, elements with lognormal distributions (Pb, Zn, Ag, As, Cd, Co, Cu, Sb, S, Sr, Th, Ba, Bi, Fe, Ni and Mn) were selected. Robustness against outliers is achieved by application of central log ratio transformation to address the closure problems with compositional data prior to principle components analysis (PCA). Results of these analyses show that, in the Ravanj deposit, Pb mineralization is characterized by a Pb-Ba-Ag-Sb ± Zn ± Cd association. The supra-mineralization haloes are characterized by barite and tetrahedrite in a BaTh -Ag-Cu-Sb-As-Sr association and sub-mineralization haloes are comprised of pyrite and tetrahedrite, probably reflecting a Fe-Cu-As-Bi-Ni-Co-Mo-Mn association. Using univariate and multivariate geostatistical analyses (e.g., EDA and robust PCA), four anomalies were detected and mapped in Block A of the Ravanj deposit. Anomalies 1 and 2 are around the ancient orebodies. Anomaly 3 is located in a thin bedded limestone-shale intercalation unit that does not show significant mineralization. Drilling of the fourth anomaly suggested a low grade, non-economic Pb mineralization.

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