Supergene nonsulfide Zn–Pb mineralization in the Mehdiabad world-class sub-seafloor replacement SEDEX-type deposit, Iran

The world-class Mehdiabad nonsulfide deposit consists of several accumulations of Zn–Pb–Fe nonsulfide minerals, derived from supergene of a vent-proximal sub-seafloor replacement SEDEX-type deposit hosted in lower Cretaceous clastic–carbonate rocks of the Taft and Abkuh Formations. The main sulfides are galena, sphalerite, pyrite, and minor amounts of chalcopyrite, which have been partially or completely transformed into nonsulfide minerals by supergene processes. Currently, the Zn mineralization in the Abkuh Formation (ore horizon II) is almost fully oxidized: the Mehdiabad deposit which can be assigned to both direct replacement and wall-rock replacement types. The time of the formation of the nonsulfide deposit has not been determined, but it is probable that the main supergene metallogenesis happened during Tertiary, possibly between post-Cretaceous to present. The mineralogy of the nonsulfide mineralization is generally complex and consists of smithsonite, hydrozincite, and hemimorphite as the main economic minerals, accompanied by cerussite, anglesite, iron–manganese oxy-hydroxides, sauconite, and Zn-rich clays. Commonly, nonsulfide minerals in this deposit consist of two types of ore: red zinc ore (RZO), rich in Zn, Fe, Pb-(As), and white zinc ore (WZO), typically with very high zinc grades but low concentrations of iron and lead. The proposed three-step scenario for the deposition of nonsulfide mineralization in the Mehdiabad deposit comprises: (1) the deposition of primary sulfides contemporaneous with lower Cretaceous clastic–carbonate host rocks to form SEDEX-type mineralization; (2) during the Late Cretaceous to present, regional uplift leads to deformation, folding, and thrusting of sulfide-bearing lower Cretaceous sequence; and (3) as a result, Zn–Pb–Fe sulfides hosted in carbonates experienced oxidation under an arid-warm climate to form the supergene nonsulfide ore mineralizations.

Keywords: Nonsulfides; Supergene; SEDEX type; Mehdiabad; Iran

"Nonsulfide" is a term, which comprises a series of oxidized Zn-(Pb) ore minerals, mainly considered as related to supergene alteration of Zn-(Pb) sulfide concentrations. They can also be due to hypogene processes: (1) structurally controlled replacement deposits (Appold and Monteiro [5]; Borg [14]; Slezak et al. [52]), (2) stratiform bodes in highly metamorphosed terrains (Brugger et al. [16]; Peck et al. [40]), or (3) hydrothermal overprints of sulfide deposits (Boni et al. [11]; Boni [8]). Supergene alteration processes can cause the modification and/or destruction of hypogene sulfide ores, which may result in the precipitation of supergene nonsulfide orebodies (Anand [3]). The precipitation of new-formed nonsulfide mineralization depends on (1) the tectonic evolution of the area, uplift, and supergene oxidation of sulfide ores, and (2) the climatic conditions existing at the time of sulfide oxidation, which favor the precipitation of certain oxidized nonsulfide minerals, rather than others (Hitzman et al. [23]; Boni and Mondillo [9]; Borg [15]). Despite the widespread occurrence of surficial supergene zinc minerals in gossans and soils (Jacquat et al. [24]), economic nonsulfide deposits are far less common than sulfide deposits. Nonsulfide deposits have experienced a significant exploration over the recent years, as a consequence of new developments in hydrometallurgical acid leaching, solvent extraction, and electrowinning techniques (Boni et al. [12]). The supergene nonsulfide zinc ore, locally called "calamine" by the miners, includes hydrated carbonates (hydrozincite), silicates (willemite), carbonates (smithsonite), and hydrated silicates (hemimorphite, Zn clays) locally associated with Fe-oxy-hydroxides and clays.

These deposits may result from the supergene modification of primary sulfide deposits such as Mississippi Valley-type (MVT), sedimentary exhalative (SEDEX), Irish type, veins, and less commonly skarns (Heyl and Bozion [22]; Hitzman et al. [23]; Borg [13]). In the case of supergene processes, subtypes have been distinguished, including direct replacement deposits and wall-rock replacement deposits (karst deposits) (Heyl and Bozion [22]; Hitzman et al. [23]; Borg [15]). Mineralogy of nonsulfide zinc deposits is significantly dependent on host-rock composition. Those deposits in carbonate rocks tend to be dominated by smithsonite and hydrozincite, due to the interaction of low-pH zinc-rich meteoric fluids with host-rock carbonates, whereas deposits in clastic rocks (where Al and Si are available) tend to contain hemimorphite- and sauconite-bearing minerals (Hitzman et al. [23]). The mineralogy of the primary sulfide ore mineralization usually determines the paragenesis of the supergene secondary ore mineralizations (Scott et al. [51]), although some metals like copper and zinc can be very mobile under oxidizing acidic conditions ([4]).

Iran possesses a large range of Zn–Pb deposits including SEDEX, Irish-type, and MVT that occur in carbonate and siliciclastic rocks (Fig. 1) (Borg [13]; Maghfouri [29]; Maghfouri et al. [31]; 2018a, b; 2019; 2020). There are more than 350 Zn–Pb deposits and occurrences in Iran, including world-class deposits such as Mehdiabad, Angouran, and Irankouh (Fig. 1) (Borg [13]; Maghfouri et al. [32], [30], [33]) (Figs. 2, 3). Only a few of them, however, have actually been explored and exploited. The Mehdiabad deposit, located 115 km southeast of Yazd city, can be subdivided into four ore zones (Fig. 4): (1) the Black-Hill ore (BH), the Central Valley Orebody (CVOB), the East Ridge ore (ER), and the Calamine Mine (CM) (Fig. 2). This deposit is the largest accumulation of clastic–carbonate-hosted zinc in the world (Leach et al. [28]; Maghfouri [29]; Maghfouri et al. [33]), with a total resource of 394 Mt, grading 4.2% Zn, 1.6% Pb, and 36 g/t Ag. In addition, the barite content of this deposit is unusually large (> 40 Mt). Early studies at Mehdiabad (Ghasemi [20]) described the initial geologic framework of the deposit. This work showed that the Mehdiabad deposit bears geologic similarities with other sediment-hosted deposits of the southern Yazd Basin. The formation of secondary oxidized Zn–Pb–Fe mineral species in the Mehdiabad deposit by oxidation and low-temperature conversion of hypogene sulfide ores by meteoric fluids has been explained in publications by Reichert ([43]) and Maghfouri et al. ([32]). Maghfouri et al. ([33]) studied the Mehdiabad deposit, which is located in the southern Yazd Basin (Fig. 3), and classified it as a vent-proximal sub-seafloor replacement SEDEX deposit model. They further proposed that the formation of SEDEX deposits in this basin is related to the evolution of a Cretaceous extensional tectonic environment that affected the Central Iranian microcontinent. Considering that the Mehdiabad deposit is an important vent-proximal sub-seafloor replacement SEDEX-type mineralization of world and Iran (Maghfouri [29]; Maghfouri et al. [32]; [33]), the objective of our research is to provide a comprehensive mineralogical, petrographic, and geochemical characterization of the supergene nonsulfide mineralization in this deposit and to better constrain its origins in the frame of the climatic and tectonic evolution of the area.

Graph: Fig. 1 Distribution map of sediment-hosted Zn–Pb deposits in the main tectonic elements of Iran. The Mehdiabad deposit occurs in the Yazd-Anarak metallogenic belt. Al Alborz zone; CIGS Central Iranian geological and structural gradual zone; E East Iran ranges; K Kopeh-Dagh; KR Kermanshah Radiolarites subzone; KT Khazar-Talesh-Ziveh structural zone; L Lut block; M Makran zone; Oph ophiolite belts; PB Posht-e-Badam block; SSZ Sanandaj-Sirjan zone; T Tabas block; TM tertiary magmatic rocks; UDMA Urumieh-Dokhtar magmatic arc; Y Yazd block; Z Zabol area; Za Zagros ranges (tectonic and structural map of Iran modified after Alavi [2]; Aghanabati [1])

Graph: Fig. 2 a Schematic columnar section of the Lower Cretaceous sedimentary sequence hosting the Mehdiabad deposit. The lower unit of the Taft Formation and the middle unit of the Abkuh Formation host the Zn–Pb–Ba mineralization. b Geological map of the Mehdiabad deposit showing the location of the barite deposit in the east of Black-Hill Fault and of the oxide Zn–Pb orebody in the northeast of the map. CVF Central Valley fault; NTF North Thrust fault; FF Fourozandeh fault; BHO Black-Hill ore; CVOB Central Valley orebody; ER East Ridge; CM Calamine Mine (modified after Nosratian 1991)

Graph: Fig. 3 a General view of the two ore horizons (OH I and OH II) with different ore facies, OHI is located within the Taft Formation, whereas OH II corresponding to the CM orebody that occurs within the Abkuh Formation. b W–NE cross-section of the ore sequence and extension of ore facies in the Mehdiabad deposit (Maghfouri [29]). The mineralization is located to the east of the Black-Hill synsedimentary fault

Graph: Fig. 4 a General view of the supergene nonsulfide mineralization in the ore horizons I and II Mehdiabad. The massive oxide ore facies in the ore horizon I is capped by the barite ore mineralization. b Widespread non-sulfide ore and Fe-oxide ore (red) forming the wall of pit. c View of the ER nonsulfide mineralization between the Sangestan and Taft Formations. d Detailed view of the ER (red zinc ore) nonsulfide mineralization between Sangestan and Taft Formations. e Non-sulfide ore of the OH I (ER) covering the Sangestan Formation

The Mehdiabad deposit is hosted by sedimentary rocks of the Lower Cretaceous sequence (Fig. 2a) (Maghfouri et al. [32],[33]). The maximum thickness of the Lower Cretaceous Sedimentary Sequence (LCSS) at the mine site is > 1200 m. The LCSS can be subdivided, from bottom to top, into the Sangestan Formation, the Taft Formation, and the Abkuh Formation (Fig. 2a). In the south of the Mehdiabad deposit (Tale Madar-Bacheh area), the Sangestan Formation overlies the Shirkuh granite. The thickness of the Sangestan Formation is 350 m, which grade into the Taft Formation. The lithotype of the Sangestan Formation in the Mehdiabad mine consists mainly of conglomerates, sandstones, calcareous sandstone, oolitic limestone, shale, and siltstone (Fig. 2b). The Sangestan Formation is conformably overlain by C-rich shale, silty shale, sandy limestone, dolomite, and siltstone of the Barremian to probably Aptian Taft Formation (Fig. 2a) (Nabavi [38], [38]), consisting of thick-bedded to massive rudists, algae, and orbitolinid-bearing shallow-water facies (e.g., Bucur et al. [17]; Schlagintweit et al. [49], [50]). The Taft Formation includes two units (Fig. 5a). The lower unit (70–85 m thick) comprises organic carbon matter-rich shale, silty shale, silty limestone, dolomite, and siltstone (Fig. 2a), whereas the upper unit (315–330 m thick) includes limestone and thick-bedded dolomite (Fig. 2a). The main ore horizon of Zn–Pb-Ba (Cu ± Ag) sulfide and nonsulfide mineralization (ore horizon I) in the Mehdiabad deposit occurs within the lower unit and at the base of the upper unit of the Taft Formation (Fig. 2). The Taft Formation is a thick layer of clastic–carbonate rocks, which gradually transforms into the Abkuh Formation (with a thickness of 420 m). The Abkuh Formation is locally subdivided into seven units (members), which from the base of the succession are (Maghfouri et al. 2018a) (Fig. 2):

• -Unit 1: laminar limestone with bands of chert (20–30 m);
• -Unit 2: dolomitic shale, gray limestone with intercalation of chert (30–40 m);
• -Unit 3: reef limestone and porous limestone (20–30 m);
• -Unit 4: limy shale with intercalations of laminated limestone (60–80 m). Ore horizon II (nonsulfide ore) is located in the upper part of this unit (Fig. 2a);
• -Unit 5: ore horizon II, overlain by chert-bearing bedded limestone (50–60 m);
• -Unit 6: massive dark gray limestone (25–35 m);
• -Unit 7: interbedding limy shale and bedded gray limestone. This unit is exposed at the top of the Abkuh Formation and is strongly affected by folding (80 m).

Graph: Fig.5 a View of the CM mineralization hosted by the Abkuh Formation to the east of Black-Hill fault. (b, c and d) Schematic W–E, N–S, and SSE–NNW sections of the folded and faulted strata at the Calamine Mine area (OH II) in the Mehdiabad deposit. e Detailed view of the Calamine Mine nonsulfide mineralization in the chert-bearing limestone of the Abkuh Formation

The Mehdiabad deposit in the southern Yazd basin is the largest vent-proximal sub-seafloor replacement SEDEX-type deposit in Lower Cretaceous sedimentary rocks of Iran (Maghfouri et al. [31], [32],[33]). The orebody consists of an oxide/hydroxide cap underlain by a mixed oxide/hydroxide-sulfide mineralization that grades downward into a sulfide body. The sulfide and nonsulfide Zn–Pb–Ba (Cu–Ag) mineralization of the Mehdiabad deposit occurs along two horizons that extend over a length of at least 3.4 km (Fig. 3a). The sulfide and nonsulfide ore of the ore horizon I (OH I) is hosted by the organic carbon-rich shale, silty limestone, dolomite, and siltstone of the Taft Formation, whereas nonsulfide ores of the ore horizon II (OH II) are hosted in the limy shale and thinly bedded limestone of the Abkuh Formation (Fig. 3b) (Reichert et al. [46]). The main part of the orebody or OH I, the so-called Black hill ore (BH), Central valley orebody (CVOB), and East Ridge (ER), is located in a depression surrounded by hills and mountains (Fig. 3). The second ore horizon (OH II) is only observed at the Calamine Mine (CM) (Fig. 3). The BH, ER, and CM expose completely oxidized ore (Figs. 4, 5), whereas the CVOB mainly consists of sulfide mineralization. Detailed geological mapping of host rocks and synsedimentary structures in the Mehdiabad deposit shows that Zn–Pb–Ba (Cu ± Ag) mineralization occurs to the east of the Black Hill synsedimentary fault in the LCSS (Fig. 3). The BH vein and brecciated mineralization occur in the fault zone of Black-Hill fault (Fig. 3b). The BH ore consists of irregular veins of barite, quartz, chalcopyrite, pyrite, sphalerite, galena, Zn–Pb carbonate, and dolomite, but also limonite and hematite, so that it is called the 'Black-Hill gossan'. Toward the east, the BH mineralization changes into the CVOB (Fig. 3b). The most abundant ore minerals within the CVOB are sphalerite, galena, barite, pyrite, and rare chalcopyrite. The ER orebody mainly consists of stratabound sulfide and nonsulfide mineralization developed within the lower unit (calcareous shale, siltstone, and silty limestone) of the Taft Formation (Figs. 3, 4c). The ER occurs between the sandstone and oolitic limestone of the Sangestan and the silty limestone of the Taft Formation. The topographic elevation of the CM (or ore horizon II) represents the highest parts of the exposed mineralization of Mehdiabad (Fig. 5a). In contrast to the mixed main sulfide/nonsulfide orebody of the CVOB, the CM contains no zinc and lead sulfides.

To assess the nonsulfide distribution patterns in the ore horizon I Mehdiabad deposit, 85 drill core/outcrop samples were collected from 20 boreholes (Fig. 2b). These samples collected from BH, CVOB, and ER parts of this deposit (The location of the 20 boreholes is shown in Fig. 2b). Sampling was mainly focused on the nonsulfide mineralization in the different levels of boreholes. Representative samples (20 samples) of ore horizon II were collected from the surface outcrops, and from tunnels (Fig. 2b). A combination of field studies and microscopic examination was also carried out by preparing about 40 thin sections at the Department of Earth Sciences of University of Tabriz, Iran.

Twenty-seven thin sections were selected and prepared for SEM analyses on the nonsulfide minerals in the Mehdiabad deposit. Seventeen thin sections of ore horizon I and ten thin sections of ore horizon II were analyzed by SEM. Samples were investigated by scanning electron microscopy using a Scanning Electron Microscope (SEM) at the FEMTO-ST Institute (Université de Bourgogne Franche-Comté, France). Backscattered Electron (BSE) imaging operating at 15 kV in low vacuum conditions was combined with semi-quantitative analyses using Energy-Dispersive Spectrometers (EDS).

In the Mehdiabad deposit, the East Ridge part (from ore horizon I) and Calamine Mine (from ore horizon II) expose completely oxidized ore (Figs. 3b, 4, 5), whereas the Black Hill and CVOB (from ore horizon I) parts mainly consist of mixed sulfide-oxide mineralization. The nonsulfide mineralization is lithologically and structurally controlled (Figs. 4d, 5). Lithological control is reflected in the stratiform-to-tabular geometry of the ore bodies, and the structurally control occurring along fractures and faults. The CM ores (Fig. 5a) are hosted in the higher part of the Abkuh Formation. The ore is almost completely oxidized (Fig. 5), even though primary sulfides have been locally preserved in the interval underlying the ore horizon. The potentially economic deposit at CM and ER consists of nonsulfide Zn > Pb concentrations (> 37%) (Figs. 4, 5), associated with abundant iron oxihydrooxides (Figs. 4, 5), all formed from the supergene of presumed primary hypogene sulfide ores. Gossans in the ore horizon I (Fig. 4), occurred not only on the surface of the primary sulfide ores, but they also display the infill of deeper breccia's cavities of CVOB in Taft carbonates (Maghfouri et al. [32]). Oxidation of sulfides in this horizon generally extends from the surface (BH and ER) to an average of 80–120 m in depth (CVOB) (Fig. 4). The supergene nonsulfide orebody in the CM is located on thrust faults, which could have facilitated the karstification process and later also channeled meteoric fluids (Fig. 5). There are no obvious mineralogical changes in the ore horizons of the Mehdiabad deposit.

The supergene nonsulfide zinc products were called "calamine" by the local miners. This term describes a mixture of minerals such as zinc carbonate and zinc silicate, or the assemblage of hemimorphite, smithsonite, hydrozincite and cerussite, locally associated with Fe-oxy-hydroxides and clays. Calamine mineralization in the Mehdiabad deposit was yellow, white, red, or even gray in color and occurred as concretionary, oxide boxworks, open space filling (cavities and fractures), and stalactitic shapes (Figs. 6, 7). Their aggregates were coarse-grained or microcrystalline, but also displayed vuggy textures or were brecciated. The ore grade of the supergene nonsulfide is recorded to have been highly variable throughout the ore horizons, ranging from a few percent of combined zinc-lead to more than 68% (Zn + Pb) (Maghfouri [29]). The supergene nonsulfide ores are particularly enriched in zinc in the deeper parts of the oxidation profile, whereas upper levels of nonsulfide ore zones are generally not economic.

Graph: Fig. 6 Outcrop photographs of the RZO in the Mehdiabad deposit. (a, b and c) Radial and euhedral cerussite (Cer) and anglesite (Ang) growing on the Fe oxy-hydroxides-bearing dolomite. (d) Crystalline cerussite (Cer), anglesite (Ang) and hydrozincite on the Fe oxy-hydroxides-bearing dolomite

Graph: Fig. 7 Hand specimen photographs of the RZO in the Mehdiabad deposit. a Hand specimen photograph of Fe-oxy-hydroxide lamina in the ER. b Hematitic Fe oxides with white Mn oxy-hydroxides. c Barite (Ba) ore mineralization in the ore horizon I cemented by Fe–Mn oxy-hydroxides. d Underground exposure of the RZO showing white hydrozincite (Hyd) crystals in vugs within smithsonite (Sm) and Iron oxide-rich matrix. e Irregular bands of Fe–Mn oxy-hydroxides in the BH. f Irregular vuggy cave-fill with smithsonite (Sm) and Fe-oxy-hydroxides

The nonsulfide zinc ore of the Mehdiabad deposit can be subdivided into a red zinc ore and a white zinc ore (Figs. 6, 7, 8 and 9). The red zinc ore is rich in Zn (up to approximately 30%), iron-hydroxides (Fe app. 17%), and other metals such as Pb—(As) (Maghfouri [29]). The white zinc ore, in contrast, shows typically high zinc grades (up to 40%) but low concentrations of iron (< 7%) and lead and arsenic. Most common minerals of this ore type (WZO) are hydrozincite, smithsonite, and hemimorphite (Figs. 8, 9). Iron-bearing minerals are rare compared with the red zinc ore. Hemimorphite is less common compared with the red zinc ore.

Graph: Fig. 8 Outcrop photographs of the WZO in the Mehdiabad deposit. (a, b, c and d): Nonsulfide mineralization of the Mehdiabad deposit, showing the development of smithsonite (Sm), cerussite (Cer), anglesite (Ang), hydrozincite (Hyd), and hemimorphite (Hem) of the WZO over the red aggregate of the RZO. (f): Crystalline hemimorphite (Hem) and smithsonite (Sm) covering hematitic and goethitic limestone

Graph: Fig. 9 Hand specimen photographs of the WZO in the Mehdiabad deposit. a Smithsonite (Sm) that replaces hydrozincite (Hyd) in the CM. b Hydrozincite (Hyd) crust with concretionary texture. c Replacement of smithsonite (Sm) by hydrozincite (Hyd). d Hydrozincite (Hyd) mineral coating RZO minerals. e Botryoidal texture of smithsonite (Sm), hemimorphite (Hem), and hydrozincite (Hyd). f: Hand sample of smithsonite (Sm), hemimorphite (Hem), and hydrozincite (Hyd)-rich specimen from the ore horizon II. g Botryoidal texture of hydrozincite (Hyd). (H): Hydrozincite (Hyd) crystals around a core of crystalline colloform hemimorphite (Hem)

The red zinc ore is dominant in the ore horizon I, whereas the white zinc ore is characteristic of the ore horizon II. The nonsulfide ore of the BH, CVOB, and ER (ore horizon I) is predominately Fe-(Mn)-rich, and shows no distinct tendencies of metal separation and differentiation into a red zinc ore and a white zinc ore. The red zinc ore occurs over the full spatial range of the BH and ER area as lenses or irregular-shaped bodies with varying dimensions that range from several meters up to several tens of meters (Fig. 4). The amount of red zinc ore compared with the white zinc ore increases at the upper levels. Common minerals of the red zinc ore are Fe-oxy-hydroxides, goethite, hematite, hemimorphite, hydrozincite, smithsonite, and cerussite (Figs. 6, 7). Hemimorphite is one of the most important zinc minerals of this ore type. In this horizon, red zinc ore consists dominantly of pore-filling hemimorphite and smithsonite in Fe oxide–hydroxide matrix. In general, iron oxides with orange color due to goethitic contents and with red color are due to hematitic contents (Figs. 6, 7). The CM (ore horizon II) of Mehdiabad shows two main types of nonsulfide ore, but white zinc ore is dominant. In the ore horizon II, smithsonite is more abundant than Fe-oxide-hydroxides (Figs. 8, 9).

The principal nonsulfide zinc minerals of economic significance recognized in the Mehdiabad mining district are smithsonite, hydrozincite, and hemimorphite. Cerussite, anglesite otavite, Zn-bearing dolomite, and Zn-rich clays have been locally observed in the Mehdiabad deposit (Fig. 14) (Maghfouri et al. [32]). Among the gangue minerals, calcite is quite ubiquitous, followed by minor quartz, Fe–Mn oxy-hydroxides, and barite. The main assemblages are described below.

In the supergene nonsulfide mineralization, smithsonite (Sm) is the prominent phase and has been identified by SEM/EDS (Fig. 10a–g). Sm is widespread in different types of aggregates ranging in color from grayish white to a dark gray. An extensive SEM study performed on a large number of smithsonite-bearing samples revealed the following two types of smithsonite (Sm 1 and Sm 2). The early smithsonite (Sm 1) generally replaces the host rock (Fig. 10a–g) and is replaced in turn by hydrozincite. Collomorphous, concretionary structures of smithsonites (Sm 1) possibly formed in open space filling cavities of the carbonate rocks. These Sm 1 aggregates are encrusted by bands of rhombohedral smithsonite (Sm 2), which is locally colored by Fe oxides (Fig. 10a–g). SEM images of smithsonite show that this mineral occurs as rhombohedral crystals (Fig. 10a–g), in concretions consisting of rhombohedral individuals or concretions (Fig. 10a–g), or as massive agglomerates. In siliciclastic bands, it also occurs as cement to detrital quartz clasts in association with alumosilicates, such as mica group minerals.

Graph: Fig.10 (a–g): BSE images and EDS spectrum of smithsonite; (a) Smithsonite 1 replaced with zoned concretions of smithsonite 2. (b) Replacing of smithsonite 1 with smithsonite 2 and hydrozincite 2. (c) BSE image of destroyed smithsonite 1 within Zn-rich clays. (d) EDS spectrum of smithsonite 1. (e) Zoned smithsonite 2 concretion. (f) EDS spectrum of smithsonite 2. (g) Smithsonite 1, 2 and otavite concretions. (h–n) BSE images and EDS spectrum of hydrozincite; (a) Replacing of zoned hydrozincite 2 with hemimorphite 2. (b) Quartz and Zn-rich clays fill the cavity of hydrozincite 2. (c) Crustiform layers of hydrozincite 2. (d) EDS spectrum of hydrozincite 1. (e) Radial hydrozincite 1 replaced with smithsonite 1. (f) EDS spectrum of hydrozincite 2. (g) Replacing of zoned hydrozincite 1 with hemimorphite 2 and cerussite 2. Sm smithsonite; Hem hemimorphite; Hyd hydrozincite; Cer cerussite; Qtz quartz; Otv otavite

Hydrozincite (Hyd) has been detected in the core samples and surface specimens of Mehdiabad deposit. Hydrozincite is commonly associated with smithsonite. At the hand-specimen scale, Hyd commonly appears as whitish crustiform masses, locally growing directly on Sm crystals (Fig. 10h–n). Locally, Hyd formed to form open space filling textures between Sm crystals and even replace them (Fig. 10h–n). As observed on hand samples, Hyd is very common and forms concretions (Hyd 2) with a fibrous texture that developed on Sm aggregates (Fig. 10h–n). The rhythmical succession (Hyd 2) of these concretions locally gives a laminated appearance to the calamine. In the high levels of the orebody, Sm is replaced by hydrozincite, which forms rhythmical masses. Hydrozincite is usually white, but can also be colored by Fe oxides. Hydrozincite occurs with textures from spongy (Hyd 1) to rhythmical (Hyd 2) in concretions (Fig. 10h–n).

Hemimorphite (Hem) generally precipitated as elongated fine fibrous-shaped crystals in dissolution cavities (Fig. 11a–g), or it is replacing patchily smithsonite and/or other carbonates. Two generations of hemimorphite have been observed (Hm 1 and Hm 2) (Fig. 11a–g). The early phase (Hm I) is fairly abundant and fills fractures in the carbonate rock together with Zn-rich clays (Fig. 11a–g). Locally, hemimorphite 1 can also occur as zoned, globular-shaped concretions in association with iron oxides, or filling cavities and veinlets in the supergene nonsulfide ore. The second-generation hemimorphite (Hm 2) appears as clear elongated crystals growing in cavities and veins together with Mn-oxides (Fig. 11a–g).

Graph: Fig. 11 a–g BSE images and EDS spectrum of hemimorphite; (a) Hemimorphite 2 zoned rhombohedron. (b) Replacing of hydrozincite 2 with hemimorphite 2. (c) Hemimorphite 1 and Zn-rich clays replaced by hemimorphite 2. d EDS spectrum of hemimorphite 2. e Radial-shaped hemimorphite 2. f EDS spectrum of hemimorphite 1. g Replacing of hydrozincite 2 with hemimorphite two. h–n BSE images and EDS spectrum of cerussite; a Smithsonite 1 and hydrozincite two replaced by cerussite 2. b Replacing of hydrozincite 1 with cerussite 2. c Cerussite 1 in hemimorphite. d EDS spectrum of cerussite 1. e Smithsonite 1 and otavite growing around remnants of cerussite 2. f EDS spectrum of cerussite 2. g Smithsonite 1 replaced by cerussite 2. Sm smithsonite; Hem hemimorphite; Hyd hydrozincite; Otv otavite; Cer cerussite; Ba barite

Cerussite and anglesite minerals are generally less common than Zn nonsulfide supergene minerals. Cerussite (Cer) and anglesite also occur, generally formed from supergene process of galena sulfide mineral. These nonsulfide minerals are generally associated with hydrozincite and smithsonite (Fig. 11h–n). Cerussite occurs as aggregates of small and prismatic crystals filling cavities of carbonates, and as fibrous crystals variably replacing galena (Fig. 11h–n). At Mehdiabad, Cerussite and smithsonite are intergrown (Fig. 11h–n) or cerussite is replaced by smithsonite and hydrozincite (Fig. 11h–n). Two main generations of cerussite are observed at Mehdiabad supergen nonsulfide horizons (Fig. 11h–n). The first generation (Cer 1) typically forms thin alteration rims around galena. The second-generation (Cer 2) forms euhedral prismatic crystals filling cavities and fractures.

The Otavite (CdCO3) is the first documented Cd carbonate mineral in Iran. The rare mineral otavite was found at the nonsulfide zones of Mehdiabad deposit. Otavite occurs in the oxidized gossan, in association with smithsonite and Zn-rich clays (Fig. 12a–g). SEM imaging shows that otavite is composed of tiny rhombohedral crystals (Fig. 12a–g). It also fills vugs and fractures in smithsonite 2 (Fig. 12a–g).

Graph: Fig. 12 (a to g) BSE images and EDS spectrum of otavite; (a) Otavite zoned rhombohedron. (b) Otavite and Zn-rich clays growing around remnants of cerussite two. c Rhombohedra-shaped otavite and calcite. (d and f) EDS spectrums of otavite. e Growing of otavith with smithsonite 2. g Rhombohedra-shaped otavite. (h to n) BSE images and EDS spectrum of clays; (A): Hydrozincite two growing around remnants of Zn-rich clays (sauconite). b BSE image of mica-type clay and Zn-rich clays. c Sauconite in Zn-rich clays. d EDS spectrums of sauconite. e Zn-rich clay fills the cavity of hydrozincite two and hemimorphite 2. f EDS spectrums of Zn-rich clay. g BSE image of illite, As-rich clay, and Zn-rich clays. Sm smithsonite; Hem hemimorphite; Hyd hydrozincite; Sau sauconite; Otv otavite; Cer cerussite; Cal calcite

In addition to smithsonite, hydrozincite, hemimorphite, cerussite, and anglesite, the supergene nonsulfide mineralization includes Zn clays (mainly illite and sauconite) associated mainly with calcite and Mn–Fe oxides (Fig. 12h–n). Within cavities of the calamine ore, clayey material occurs. The matrix of nonsulfide minerals locally is cemented by Zn clays (mainly illite and sauconite). Zn-bearing clay minerals have been observed with smithsonite and hydrozincite in most white zinc ore (Fig. 12 h–n). The dominant clay mineral is tentatively identified as sauconite, the Zn-dominated illite. Sauconite typically has a powder-like texture and variable colors. Some Zn-rich clays appear to be intergrown with smithsonite and hydrozincite, but typically smithsonites appear to be fragments that are cemented by Zn clays (Fig. 12 h–n).

Calcite (Cal) is commonly observed, both as gangue mineral, which was deposited with the hypogene ore sulfides, and as a newly precipitated phase associated with supergene nonsulfide ores. Calcite stalagmites and stalactites, showing vertical orientations relative to today's morphological surface, also occur in dissolving karsts. The walls of the karsts are covered by concretions of calcite and hydrozincite, followed in turn by Fe–Mn–(hydr) oxides. A calcite phase is associated with hydrozincite and mainly occurs as late cement completely filling the porosity (Fig. 13). Some calcite layers are also interbedded with hydrozincite. Calcite, the main component of the carbonate host rock, is locally replaced by saddle dolomite and Zn-rich dolomites (Fig. 13). Saddle dolomite contains zinc, manganese, and iron, and is commonly replaced by smithsonite (Fig. 13). At the boundary between smithsonite 1 and saddle dolomite, the latter is widely replaced by broad, irregular bands of Zn-bearing dolomite, where Zn has been substituted for Mg. Zinc-bearing dolomite is always associated with remnants of the original dolomite (Fig. 13).

Graph: Fig.13 BSE images and EDS spectrum of carbonates. a Hydrozincite two growing around calcite and smithsonite 1. b Zn-dolomite replacing dolomite in the crystal cores. c Zoned Zn-rich dolomite. d EDS spectrums of calcite. e BSE image of calcite and hydrozincite two. f Mn-oxide and Zn-rich dolomite replacing dolomite in the crystal cores. Sm smithsonite; Hyd hydrozincite; Cal calcite; Dol dolomite; Ba barite

The red zinc ore consists dominantly of a Fe–Mn oxy-hydroxide matrix (Figs. 6, 7). The abundance of iron oxy-hydroxides is a specific feature of the red zinc ore. A complex association of iron and manganese oxy-hydroxides, with a characteristic red–brown staining (goethite, lepidocrocite, and hematite), and residual clay minerals host the nonsulfide ore. Fe–Mn oxy-hydroxides also display concentric, colloform textures (Fig. 7) that may represent former sulfide-rich zones. Fe–Mn oxy-hydroxides fills geodes within smithsonite and occurs as rhythmical concretions, with concentric textures. Fe-oxide pseudomorphs after pyrite (FeS2) are associated with zinc-lead nonsulfide supergene minerals, which locally present evidence for oxidation (Fig. 14).

Graph: Fig.14 Paragenesis of the main mineralogical phases observed at Mehdiabad, framed in the geological evolution of the region

Geochemical, lithological, and structural controls of supergene Pb and Zn mineralization are discussed by many authors including Takahashi ([53]), Sangameshwar and Barnes ([47]), Williams ([55]), Reichert and Borg ([45]), Choulet et al. ([18]), and Borg ([15]). Some of the key controls on the formation of carbonate-hosted nonsulfide Zn–Pb deposits are the nature and availability of near-surface sulfide protore, suitable lithotype, tectonic uplift, climate, and favorable hydrology (Large [26]; Hitzman et al. [23]; Borg [15]). As elaborated above, the oxidation of hypogene protore, liberation of metals from primary sulfides, mobilization, transport, and reprecipitation is caused by oxidizing fluids of meteoric origin.

The texture of the carbonate host rocks and the preexisting fractures/faults may facilitate the permeation of the meteoric oxide waters and, therefore, represent the fundamental control of oxidizing fluid flows (Borg [14]; Choulet et al. [18]). In the Mehdiabad deposit, one of the most main factors contributing to the deep supergene/oxidation was the vertical dip of primary sulfides coupled with a strong cavity in the lower Cretaceous limestone rocks acquired during post-Cretaceous tectonics, which made the primary sulfide ores accessible to deep infiltration of oxide meteoric fluids. The Mehdiabad area has been subjected to intense tectonic activity, which is responsible for the local thrusting and folding of the host rocks (Maghfouri [29]). The latter processes resulted in a vast dissolution of the higher units of the Abkuh Formation. Through oxidation of sulfide and the resulting increased acidity, these waters also dissolved the host carbonate rocks (especially in the ore horizon II). As a result, most limestone host rocks in the Mehdiabad area show enhanced corrosion at various depths in the karstic system, resulting in a permeability network of huge interconnecting cavities (Maghfouri [29]). Detailed studies of the distribution of sedimentary ore facies suggest that the Black-Hill fault played a fundamental role for the localization of primary sulfide mineralization and hypogene hydrothermal alteration at Mehdiabad (Fig. 15a) (Reichert et al. [46]; Reichert [43]; Maghfouri et al. [31], [33]; Pourfaraj et al. [42]; Maghfouri [29]). The maximum thickness of sulfides is found close to the Black-Hill Fault, suggesting that this synsedimentary structure acted as the most important conduit for hydrothermal fluid flow (Fig. 15b). Post-mineralization (primary sulfide) laramide orogeny (Late Cretaceous) compressive tectonics affected these normal faults, which were reactivated as thrusts and strike slips (Fig. 15c) (Pourfaraj et al. [42]). Therefore, the Black-Hill fault is now observed as a strike-slip fault. These structures have controlled the localization of narrow part of nonsulfide mineralization in the Mehdiabad deposit. Fracturing, faulting, and karstification are important in enhancing the depth and intensity of the supergene oxidation of primary sulfide minerals in the Mehdiabad deposit. Main faults (such as Black-Hill fault) represent conduits for oxygenated fluids and permit oxidation to depths exceeding 100 m.

Graph: Fig.15 Development of the Mehdiabad nonsulfide Zn–Pb deposit during the evolution of tectonic. a Venting of the exhaling hydrothermal fluid on the seafloor and sub-seafloor would have led to the precipitation of SEDEX-type sulfide mineralization in the different ore facies. This deposit occurs in Lower Cretaceous sedimentary rocks, also the Black-Hill synsedimentary normal fault is playing a role of conduits to drive Zn–Pb rich fluids from depth to surface (Maghfouri et al. [33]). b Formation of ore horizon I in the Taft Formation and ore horizon II in the Abkuh Formation. c Since post-sulfide mineralization, the area has recorded compressive tectonics, leading to the formation of the fold-and-thrust. Inversion tectonics affected the normal faults, which were reactivated as thrusts. In this period, is formed stage 1 nonsulfide mineralization of Mehdiabad deposit. d During stage D, regional uplift and erosion contributed to exhume the host rocks and the ore sulfides. When the water table has fallen below the sulfides of protore, their supergene oxidation may have started; it was accompanied by the simultaneous extension of the karst network in the carbonate formations and the coeval to subsequent replacement of sulfide ores and nonsulfide minerals one by nonsulfide minerals two

Since the Mehdiabad deposit is a typical vent-proximal sub-seafloor replacement SEDEX-type deposit and ore horizon I of this deposit contains high amounts of pyrite minerals; it contains abundantly layered and disseminated framboidal and euhedral pyrite. The oxidation of pyrite and the formation of Fe oxy-hydroxides are important factors in the development of supergene ore mineralization in the Mehdiabad deposit, because these processes lower the pH of the supergene oxidizing fluids (Choulet et al. [18]). The released acids can transport elements and react with host-rock carbonates causing the constant consumption of H+. The released metal ions may react with carbonate species and precipitate secondary supergene minerals. Such complete leaching results in the formation of a gossan with iron oxy-hydroxides, smithsonite, cerussite, hemimorphite, and copper carbonates in the BHO and ER.

The mechanisms of metal separation and the formation of two distinct types of nonsulfide zinc ore (red zinc ore and white zinc ore) are highly dependent on the iron/pyrite content of the sulfide protore (Choulet et al. [18]). Sulfide ore without pyrite would result predominantly in white zinc ore. On the other hand, a sulfide ore, which fulfills the ideal conditions and consists of pyrite and sphalerite/galena but is influenced by a slow oxidation process, associated with large quantities of water will most likely result in a red zinc ore dominated nonsulfide deposit. The red zinc ore as a result of an in situ oxidation of a sulfide ore. The white zinc ore is the result of a complex dissolution, remobilization, and reprecipitation process of zinc, which led to a white nonsulfide zinc ore with traces of iron.

High-pressure CO2 (g) increases the stability of Sm due to the increased activity of HCO3− (aq) and CO32− (aq) in the oxidized fluids. A minimum pressure CO2 (g) of 0.4 kPa is required for Sm formation, which is higher than atmospheric pressure CO2 (g) (Reichert and Borg [45]; Kyle et al. [25]). If the pressure of CO2 (g) is greater than 0.4 kPa, Sm formed will be favored instead of hydrozincite (Takahashi [53]). In the unsaturated zone, with greater communication with the atmosphere, hydrozincite would be more stable than smithsonite, because the stability of smithsonite requires higher pressure CO2 (g) than atmospheric pressure CO2 (g) (Takahashi [53]). However, at the beginning of the supergene process in a carbonate system of Mehdiabad deposit, acidic fluids are readily neutralized and buffered, and CO2 (g) is added to the environment from dissolving the carbonate host rock. This increment results in a temporary increase of the pressure CO2, which, if the availability of silica is low, can initiate smithsonite deposition (Reichert and Borg [45]; Arfè et al. [6], [7]). If geochemical conditions are subsequently re-equilibrated, the pressure of CO2 returns to atmospheric amounts, smithsonite is rapidly hydrated to form hydrozincite (Reichert [44]; Arfè et al. [6]). Furthermore, hemimorphite may precipitate together with hydrozincite if SiO2 is available in the system. However, the high content of hemimorphite in the Mehdiabad nonsulfide deposit implies a source for the silicate, which is productive enough to deliver this quantity. Two sources of SiO2 are capable to deliver these quantities. One possible source for the silicate may be the chert-bearing members of the Abkuh Formation, which hosts the nonsulfide ores of the ore horizon II (Fig. 15b). Another source is probably the lower unit of the Taft Formation, which is underlying the Abkuh and Taft Formation and is laterally located to the southeast of the Black-Hill Fault (ore horizon I) (Fig. 15). The lower unit of Taft Formation consists, among others, of fine-grained quartzitic–feldspatic sandstones, siltstone, and sandy shales. The origin of the silica from the lower unit of Taft Formation might imply ascendant or lateral fluid systems. However, the chert-bearing member of the Abkuh Formation is the favored and more probable source of the silica in the hemimorphites of ore horizon II. Hemimorphite occurrences in the late phase of supergene nonsulfide deposits suggest a lower buffering capacity of the host carbonates (Hitzman et al. [23]) and a strong mobility of SiO2 (Moore [37]).

Another main condition to form supergene nonsulfide mineralization is the climatic parameters. It is important to note that the climate as well as the local geology (fragmentation, karstification) of the carbonate host rocks influence the O2 and CO2 concentrations of the descendent fluids, and thus, the pH and the ability of the fluid to dissolve the carbonate host rock. The best condition to form a supergene nonsulfide ore concentration would be an arid to tropical modern climate with variable rainfall rates and a marked seasonality. Studies show that this type of deposits in the world is formed in a wide range of climate conditions (Hitzman et al. [23]). Recent studies (Reichert and Borg [45]; Reichert [44]; Boni et al. [11]) suggested that arid and warm climate conditions (like at the Mehdiabad deposit) are the most favorable for formation and oxidation of supergene mineralizations in Iran. Paleogeographic information is used to the reconstruction of sedimentary basins (Reichert [44]; Boni et al. [11]), but this information has not been available from Cenozoic period of central Iran. Under an arid/warm climate, enhanced sulfide oxidation has been attributed to scarce biogenic activity within the soil and to the low rates of dispersion, dilution, and removal of elements. Such features are mostly related to the deep levels of the water table in arid/warm districts. In the arid climatic environment of Mehdiabad, the dissolved O2 reaches its maximum concentration, compared to other climates, and will not be consumed by biological activities within soils (Reichert [43]; Reichert and Borg [45]; Maghfouri et al. [32]). The groundwater table in arid climates is commonly low. This will lead to an opening of the water-filled pores and joints after an individual rainfall event and will thus provide an inward flow of gasses (O2, CO2) to any available sulfide orebody (Reichert [43]; Reichert and Borg [45]; Choulet et al. [18]; Maghfouri et al. [32]). This system will commonly change to a highly permeable system due to (karstic) dissolution processes of the carbonate host rock. Due to the limited availability of water in arid and hyper-arid climates, the fluids, which have been generated during the oxidation process, would be highly enriched in zinc and other metals (Fig. 15c, d). These high metal concentrations support an effective precipitation of nonsulfide base metal minerals. These high element concentrations support an effective precipitation process within the carbonate host rock. Thus, an arid or semi-arid climate of Mehdiabad environment provides the best conditions for the oxidation of a sulfide ore and also provides the best conditions for the preservation of a nonsulfide orebody (Fig. 15).

On the basis of this study and considering the existing literature on other nonsulfide zinc-lead deposits in the world (e.g., Hitzman et al. [23]; Boni et al. [10], [12]; Borg [13], [15]; Coppola et al. [19]; Pirajno et al. [41]; Santoro et al. [48]; Choulet et al. [18]; Mondillo et al. [36]; Paradis et al. [39]; Arfè et al. [6], [7]; Kyle et al. [25]), we propose the following model for the supergene nonsulfide mineralization process at Mehdiabad deposit (Fig. 15). In a paper that focuses on the type and geneses of Mehdiabad deposit, Maghfouri et al. ([33]) reported that the Zn–Pb–Ba (Cu–Ag) primary ores occurring in the Mehdiabad deposit are stratiform, stratabound, and clastic-carbonate-hosted, and are contemporaneous with Lower Cretaceous host rocks (Fig. 15a). Maghfouri et al. ([33]) suggested that this is vent-proximal sub-seafloor replacement clastic–carbonate-hosted SEDEX-type deposit, related to Black-Hill synsedimentary normal fault (Fig. 15b). The metal zonation patterns, the presence of synsedimentary breccias and debris flows, the increase of hydrothermal alteration and that of the temperature of the ore fluids towards the fault, suggest that this fault was active during ore deposition and provided major conduits for the metalliferous hydrothermal fluids, similar to other SEDEX-type deposits described by Large et al. ([27]), Leach et al. ([28]), and Maghfouri et al. ([30],[34]).

The Mehdiabad Zn–Pb nonsulfide mineralization postdates the deposition of the hypogene sulfides. The nonsulfide Zn–Pb mineral association typically consists of smithsonite, hydrozincite, hemimorphite, cerussite, anglesite, and Zn-rich clays (Fig. 14). The Mehdiabad Zn–Pb nonsulfide orebody originated from the supergene/oxidizing of hypogene sulfide ores (consisting of sphalerite, galena, pyrite, chalcopyrite, and barite), occurring in the Lower Cretaceous carbonates. Therefore, supergene nonsulfide mineralization mainly occurs as replacement bodies, mostly replacing the preexisting sulfides and the carbonate host rock (Fig. 15c, d). For this reason, one can consider Mehdiabad as belonging both to the "wall-rock replacement" and "direct replacement" nonsulfide Zn deposit types (Heyl and Bozion [21]).

The ages of nonsulfide mineralization in the Mehdiabad area were not determined by geo-chronological dating techniques. However, they may be evaluated from geologic features in the southern Yazd basin (Maghfouri et al. [32]a, [30]; [33]; [34]). According to Maghfouri ([29]), the oldest tectonic structures (post-sulfide mineralization) in the southern Yazd basin (strike-slip faults and related folds) are related to the Laramide Orogeny (Late Cretaceous) when much of the present-day mountain topography in the southern Yazd basin was created. As a consequence, the area has recorded compressive tectonics, leading to the formation of the folding and thrusting structures (Fig. 15c). Reactivation of Black-Hill and other faults (Abkuh fault, Central valley fault, and Fourozandeh fault) into thrusts has controlled the localization of supergene nonsulfide mineralization (Fig. 15c, d). This compression is responsible for the present topography, intense erosion, and subsequent exhumation of Zn–Pb sulfide mineralizations. Alternatively, the nonsulfide-forming process at Mehdiabad may have begun during times of supergene that happen instantly after the Laramide Orogeny and continued intermittently to the present.

The faulted and folded structures (for example: Black-Hill fault) facilitate the oxidation and penetration of meteoric water to the carbonate rocks and protore sulfides (Fig. 15c, d). Wide karst systems formed along fault and fracture zones. In this deposit, the level of oxidation is highly variable in different areas of the mining district. During the oxidation of a sulfide body by percolating groundwater and meteoric waters, the aqueous solutions can carry metals (Zn2+) only if they have an acid pH; in this setting, the acidity of waters is mainly produced by the oxidation of pyrite, which releases sulfate anionic groups into solution. Acidic solutions deriving from sulfide oxidation can contain high amounts of dissolved metallic (Zn2+, Pb2+, Mn2+, Cd2+, and Fe2+) and bicarbonate (HCO3−) ions. Sphalerite is preferentially dissolved relative to galena in an oxidizing primary sulfide mineralization, and Zn is highly mobile in the supergene/weathering acidic fluids created by sulfide hydrolysis (Thornber and Taylor [54]; Choulet et al. [18]; Paradis et al. [39]; Arfè et al. [6], [7]). Less-mobile Pb is more likely to stay in the leached gossan and to be removed by erosional activities. Fe contents in the nonsulfide supergene ore are higher than in most of the observable sulfide orebody. The relatively high iron content of the supergene ore may indicate that the precursor sulfide body was more pyritic in the Mehdiabad. The high acid-generating capacity of oxidizing pyrite would assist in leaching zinc from the precursor sulfide body. In the Mehdiabad deposit, the acid Zn-bearing solutions altered and partially dissolved the carbonate of the host rock, releasing Ca2+ and Mg2+. The presence of the carbonate anionic groups in solution together with Zn2+ cations favored first the precipitation of smithsonite (Sm 1). This situation would favor the formation of minerals stable at acidic to neutral conditions [smithsonite (Sm 1) rather than hydrozincite (Hyd 1)]. In this deposit, the Sm-rich parts occurring at different depths in the drill cores within carbonate host rock are always related to pseudomorphic or sulfide remnant structures. This suggests that metal-bearing fluids did not migrate so much from the sulfide bodies, probably because the acidic fluids mostly forming from pyrite alteration were immediately buffered by the limestone/dolomite host rock. This counteraction favored the formation of Sm 1 for direct replacement of sulfide ores and the only local replacement of the limestone/dolomite host rock. Water-saturated settings and/or deep supergene environments favor the maintenance of high CO2 (g) levels, enabling Sm 1 preservation and precipitation. Due to the fact that Pliocene conglomerate rocks have been folded at a distance of one kilometer from Mehdiabad mine, this shows that the process of uplift and tectonics has been active in the region at least until the time of Pliocene. Uplift rates increased during over time and karstification of carbonatic host units would have been enhanced along fractures. Early smithsonite (Sm 1), the first product of sulfide oxidation at Mehdiabad, was then partially replaced by early hemimorphite (Hm 1) and hydrozincite (Hyd 1). Smithsonite (Sm1) associated with hemimorphite (Hm 1) and a first hydrozincite phase (spongy hydrozincite, Hyd 1) (Fig. 15c).

The second oxidation stage determined through paragenetic studies and confirmed by the transposition of the various generations of smithsonite, hemimorphite, otavite, hydrozincite, and Zn-rich clays, probably formed after a renewed and more intense uplift phase that lasted to present (Fig. 15d). Widening and deepening of uplift-related fractures/faults contributed to increasing the permeability of host rocks and favored a deeper alteration of sulfide ores and a large-scale dispersion of the highly soluble zinc cations (Borg [14]). Wide cavity systems formed along fractures and faults, and the first-stage supergene nonsulfide mineral assemblages were brecciated into soluble karsts (Fig. 15d). Late smithsonite (Sm 2), hemimorphite (Hm 2), hydrozincite (Hyd 2) generations, sauconite, and Zn-rich clays deposited as cements of the karst breccias, which can be considered products of this second uplift and oxidation (Fig. 15d). If the buffering effect is produced by Si-rich host rocks, or if Si is present in the fluid, hemimorphite or Zn-bearing clays prevail in the supergene assemblage. Near-surface conditions and/or under water-undersaturated, characterized by pressure CO2 (g) rebalance to atmospheric values (log fCO2(gas) = − 3.5), the pressure CO2 (g) values in the meteoric fluids are low and hydrozincite and/or hemimorphite can rapidly replace smithsonite deposited during earlier oxidation phases (Hitzman et al. [23]; McPhail et al. [35]; Brugger et al. [16]; Reichert and Borg [45]; Reichert [44]; Arfè et al. [6], [7]). Hydrozincite, which is a typical nonsulfide mineral of the post-oxidation phase (Reichert and Borg [45]), is extremely abundant in the ore horizon II Mehdiabad deposit. This eventuated in a downward migration of the supergene front, with an accumulation of smithsonite 2 in the deepest levels of the profile and the formation of hydrozincite 2 in near-surface horizons (ore horizon II). Hydrozincite (Hyd 2) was the last nonsulfide to form, coating smithsonite and/or hemimorphite (Fig. 15d). It precipitated as the partial pressure of CO2 (g) of the system reached equilibrium with the atmosphere and as pH increased. One of the peculiarities of the nonsulfide paragenesis in the Mehdiabad deposit is the presence of sauconite and Zn-rich illites, which can occur partly as replacement of the detrital minerals in fracture filling and as cavities in the limestone host rock. Zinc in sauconite originated from the supergene alteration of sphalerite, whereas K-feldspars of clastic rocks of Abkuh and lower parts of Taft Formations were the main source of alumina and silica.

The world-class Mehdiabad deposit consists of several parts, where mineralization occurs as zinc-lead-iron nonsulfide ores. The primary mineralization is considered to have been a vent-proximal sub-seafloor replacement SEDEX-type deposit hosted by clastic–carbonate rocks of the Taft and Abkuh Formations. The nonsulfide mineralization is considered to be the result of in situ replacement and oxidation, locally with limited transport, of the primary sulfide ores, brought about by meteoric high oxide waters circulating in a deep karstic network in Lower Cretaceous carbonate rocks. The redistribution of zinc and other elements from the sulfide mineralization was facilitated by topography-controlled meteoric fluid flow focused by local structural features that resulted in different types of supergene mineralization. Smithsonite is the most abundant economic mineral in the secondary deposit, where it is associated with hemimorphite, hydrozincite, cerussite, anglesite, and Zn-rich clays. Smithsonite occurs locally in zoned concretions with goethite, Mn (hydr) oxides, and hydrozincite, as well as replacive cements in the Taft and Abkuh Formation limestones. At least two distinct generations of smithsonite, hydrozincite, and hemimorphite have been recognized. These ore-forming phases are interpreted to be related to distinct periods of supergenes and uplift. The soluble karsts, to which the nonsulfide minerals are related, can be constrained to a time between Late Cretaceous to present. The abundance of hemimorphite indicates that silica was mobile during supergene alteration. Weakly acidic fluids generated from sulfide oxidation and buffered by silica-rich carbonate host rocks would have favored hemimorphite precipitation. The secondary enrichment of the Mehdiabad mineralized bodies can be assigned to both the wall-rock replacement (of carbonate host rock by nonsulfides) and direct replacement (of sulfide by nonsulfide minerals) types after the Hitzman et al. ([23]) classification. However, in contrast to other typical supergene nonsulfide deposits in Iran, the Mehdiabad nonsulfides have a strong association with clastic sediments, which locally dominate in the Abkuh and lower parts of Taft Formations. These sedimentary rocks with their abundance of K-feldspar have a reactive behavior in regard to the supergene metal-rich fluids, leading to deposition of sauconite and other Zn-rich clays. The type of host lithology (carbonate), the possibility to have primary sulfide-rich zones and a continuous recharge of oxide meteoric water determined the buffering of the acid metal-carrying fluids and triggered the formation of a suite of nonsulfide minerals. At Mehdiabad, these local-scale factors resulted in the precipitation of supergene nonsulfide ores that may locally result in mineralized bodies of prospective economic interest Table 1.

List of the analyzed ore samples in the Mehdiabad deposit

ER East Ridge; CVOB Central Valley Ore body; BH Black Hill; CM Calamine Mine; RZO red zinc ore; WZO whit zinc ore; Cer cerussite; Hm hemimorphite; Hz hydrozincite; Sm smithsonite; Ang anglesite; Otv otavite; Sau sauconite; Dol dolomite

The authors would like to thank Mehdiabad Mining Company and Chrono-Environnement, Université de Franche-Comté Lab. French for providing the field access to the area, accurate, and precise sampling analysis and SEM geochemical data. Particular thanks go to H. Pourfaraj and M. Hajghasem for helping the first author during data collection.