Nonsulfide Zinc Deposits Research Articles

A global assessment of Zn isotope fractionation in secondary Zn minerals from sulfide and non-sulfide ore deposits and model for fractionation control

We investigated extent and direction of Zn isotope fractionation in secondary zinc minerals formed during low temperature hydrothermal and/or supergene oxidation of primary sulfide deposits. Zinc isotope data have been obtained from non-sulfide zinc mineral separates (willemite - Zn2SiO4, smithsonite - ZnCO3, hemimorphite - Zn4(Si2O7)(OH)2·H2O, hydrozincite - Zn5(CO3)2(OH)6, and sauconite - Na0.3Zn3(Si,Al)4O10(OH)2·4H2O) collected from several Zn deposits in Ireland, Belgium, Poland, Namibia, Peru, Yemen and Zambia. The data are compared with Zn isotope compositions measured on Zn sulfides collected in the same areas and/or derived from the existing literature, to establish the controls of direction and likely extent of any fractionations. We find that willemite has the greatest compositional variability, with measured δ66ZnJCM-Lyon values ranging from −0.42 to 1.39‰, spanning the entire range of terrestrial variation in Zn isotopes recorded to date. Overall, significant fractionations in positive and negative directions are recorded relative to the precursor phase (primary sphalerite or an earlier secondary phase), with primary sphalerite falling in a relatively narrow range of isotopic values (approximately −0.1 to +0.4‰). Most of the data observed on willemite, hemimorphite and hydrozincite can be explained with a model of isotopic fractionation, in which partial dissolution of primary sphalerite is followed by precipitation of an initial secondary phase that preferentially incorporates heavy Zn isotopes. Smithsonite, instead, preferentially incorporates light Zn isotopes. This reflects the variation in the Zn-x bond strengths of these secondary phases with respect to the original sulfides. We also observed that isotope compositions do not depend only on the difference between the fractionation factors of the involved phases but also on the amount of the secondary mineral precipitated after dissolution of primary sulfide, and that the greatest fractionations occur when only small amounts of secondary mineral are precipitated. Progressive precipitation from migrating fluids that form phases enriched in heavy zinc isotopes would lead to a gradual decrease in the δ66Zn values of such phases, and the fluids involved, in time and space. Strong negative isotopic shifts are almost only observed for late crystallizing phases, such as those in vugs. These are interpreted to reflect precipitation from residual, isotopically-light fluids that are the inevitable highly-fractionated product of the above-described process. Where a more complete replacement of primary sulfide has occurred, such as in the high-grade core of non-sulfide zinc orebodies, there is limited net isotopic fractionation because dissolved primary zinc is nearly quantitatively reprecipitated locally. In addition, in only one case (Yanque, Peru) we observed that the fringes of non-sulfide zinc deposit were characterized by isotopically fractionated compositions, with highly negative values implying extensive precipitation (earlier, or elsewhere) of isotopically heavy secondary phases. The higher-grade ore zones, where complete breakdown of primary sulfides and quantitative reprecipitation of zinc have occurred, show instead less fractionated compositions.

Formation of secondary nonsulfide zinc ore in Cho Dien Pb-Zn deposits

In Viet Nam, non-sulfide zinc ore in the Cho Dien deposit has been exploited for a long time. Up to the present, zinc ore remains the major exploited ore in Cho Dien. There are numerous studies of Pb-Zn ore in Cho Dien. However, most of the studies have dedicated only to description of mineralogical and chemical composition of Pb-Zn ore. There has been no publication on this non-sulfide zinc ore. Based on the mineralogical studies, the content of Pb and Zn in groundwater determined by reflective microscope, SEM, EPMA and ICP-MS methods, the study explained the formation of secondary non-sulfide zinc ore in the Cho Dien deposit. Strong weathering process makes the upper part of ore bodies completely oxidized. Difference in geochemical behavior of lead (Pb) and zinc (Zn) in the oxidation process of Pb-Zn ore is the reason to form non-sulfide zinc ore in the Cho Dien deposit. Oxidation of primary Pb-Zn ore is mainly sphalerite, pyrite, galena minerals which creates a low pH environment and transforms of zinc from immobile (sphalerite - ZnS) to mobile (Zn2+) and retained in solution under acid pH conditions whereas lead has the tendency to form soluble minerals (anglesite, cerussite). The acid neutralization actions of the surrounding rocks make zinc precipitate, to form secondary non-sulfide zinc minerals.ReferencesAndreas Nuspl, 2009. Genesis of nonsulfide zinc deposits and their future utilization (www.geo.tu-frei berg.de/oberseminar/OS_09/Andreas_Nuspl.pdf.Boland M.B., et al., 2003. The Shaimerden supergene zinc deposit, Kazakhstan: Economic Geology, 98(4), 787-795.Chau N.D., Jadwiga P., Adam P., D.V. Hao, L.K. Phon, J. Paweł, 2017. General characteristics of rare earth and radioactive elements in Dong Pao deposit, Lai Chau, Vietnam, Vietnam J. Earth Sci., 39(1), 14-26.Dao Thai Bac, 2012. 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Nature and origin of the nonsulfide zinc deposits in the Sierra Mojada District, Coahuila, Mexico: constraints from regional geology, petrography, and isotope analyses

The Sierra Mojada District comprises multiple types of near-surface mineral concentrations ranging from polymetallic sulfide zones, “nonsulfide Zn” (NSZ) deposits, and a silver-rich Pb carbonate deposit hosted by lower Cretaceous carbonate strata. Hypogene concentrations of Fe-Zn-Pb-Cu-Ag sulfides and sulfosalts are locally preserved and are associated with hydrothermal dolomite and silica. Alteration mineralogy and sulfur isotope data suggest primary Zn-Pb-Ag mineralization from circa 200 °C hydrothermal fluids. The NSZ deposits dominantly consist of smithsonite and hemimorphite associated with local Mn-Fe oxides. The Red Zinc Zone consists of strata-bound zones dominantly of hemimorphite that fills pores in residual and resedimented Fe oxides. The White Zinc Zone shows local dissolution features, including internal sediments interbanded with and cemented by smithsonite. Similar Pb isotopic compositions of smithsonite, hemimorphite, and cerussite to Sierra Mojada galena document that the NSZ deposits originated from polymetallic carbonate-replacement sulfide deposits, with flow of metal-bearing groundwater being controlled by local topography and structural features in this extensional terrane. Oxygen isotope values for Sierra Mojada smithsonite are relatively constant (δ18OVSMOW = 20.9 to 23.3‰) but are unusually low compared to other supergene smithsonites. Using δ18OVSMOW (− 8‰) of modern groundwater at nearby Cuatrocienegas, smithsonite formational temperatures are calculated to have been between 26 to 35 °C. Smithsonite precipitation was favored by near-neutral conditions typical of carbonate terranes, whereas hemimorphite precipitated by reaction with wallrock silica and locally, or episodically, more acidic conditions resulting from sulfide oxidation. Transition to, and stabilization of, the modern desert climate over the past 9000 years from the Late Pleistocene wetter, cooler climate of northern Mexico resulted in episodic drawdown of the water table and termination of local supergene metal mobilization.

C–O Stable Isotopes Geochemistry of Tunisian Nonsulfide Zinc Deposits: A First Look

A preliminary C–O stable isotopes geochemical characterization of several nonsulfide Zn-Pb Tunisian deposits has been carried out, in order to evidence the possible differences in their genesis. Nonsulfide ores were sampled from the following deposits: Ain Allegua, Jebel Ben Amara, Jebel Hallouf (Nappe Zone), Djebba, Bou Grine, Bou Jaber, Fedj el Adoum, Slata Fer (Diapir Zone), Jebel Ressas, Jebel Azreg, Mecella (North South Axis Zone), Jebel Trozza, Sekarna (Graben Zone). After mineralogical investigation of selected specimens, the C–O stable isotopic study was carried out on smithsonite, hydrozincite, cerussite and calcite. The data have shown that all the carbonate generations in the oxidized zones of Ain Allegua and Jebel Ben Amara (Nappe Zone), Bou Jaber, Bou Grine and Fedj el Adoum (Diapir Zone), Mecella and Jebel Azreg (North South Zone) have a supergene origin, whereas the carbonates sampled at Sekarna (Graben Zone) (and in limited part also at Bou Jaber) precipitated from thermal waters at moderately high temperature. Most weathering processes that controlled the supergene alteration of the Zn-Pb sulfide deposits in Tunisia had probably started in the middle to late Miocene interval and at the beginning of the Pliocene, both periods corresponding to two distinct tectonic pulses that produced the exhumation of sulfide ores, but the alteration and formation of oxidized minerals could have also continued through the Quaternary. The isotopic characteristics associated with the weathering processes in the sampled localities were controlled by the different locations of the sulfide protores within the tectonic and climatic zones of Tunisia during the late Tertiary and Quaternary.

C–O Stable Isotope Geochemistry of Carbonate Minerals in the Nonsulfide Zinc Deposits of the Middle East: A Review

Zinc nonsulfides are well represented in the Middle East, with occurrences in Turkey, Iran, and Yemen. Their genesis can be constrained by using carbon and oxygen isotope systematics applied to carbonate minerals. The δ13C ratios of smithsonite and hydrozincite in Iran and Turkey are comprised in the typical interval of supergene Zn carbonates (−0.4 and −7.1‰ V-PDB). The oxygen isotope geochemistry is more complex. Oxygen isotope compositions of smithsonite of the Hakkari deposit (Turkey) (δ18O from 24.2 to 25.6‰ V-SMOW) point to precipitation temperatures between ~4 and ~18 °C, corresponding to a normal weathering environment at these latitudes, whereas δ18O of smithsonite from other Middle East deposits (Angouran in Iran, Jabali in Yemen) point to the precipitation from low- to medium-temperature hydrothermal fluids. The C–O isotopic compositions of hydrozincite from the Mehdi Abad, Irankuh, and Chah-Talkh deposits can be only partially compared with those of smithsonite, because the oxygen isotopes fractionation equation for hydrozincite-water is not known. A comparison between the geochemical characteristics of all Zn-nonsulfide ores in the Middle East indicates that, even though several mineral deposits are derived from supergene weathering processes, other ones have been deposited from fluids associated with magmatic activity (Angouran, Iran) or with hydrothermal systems (Jabali, Yemen). This suggests that it is not possible to apply a common interpretative model to the genesis of all nonsulfide deposits in the Middle East.

Zn-clay minerals in the Skorpion Zn nonsulfide deposit (Namibia): Identification and genetic clues revealed by HRTEM and AEM study

Zn-clays worldwide occur in various supergene nonsulfide (Zn-Pb) ores, where they can be the prevailing economic minerals or represent minor concentrations in the mineral assemblage. The world-class Zn smectite-rich Skorpion mine (Namibia) is considered one of the most important supergene nonsulfide zinc deposits in the world. At Skorpion, the trioctahedral Zn-bearing smectite predominates over the other Zn-oxidized minerals. This work is focused on microtextural observation and chemical analyses on the clay nano-particles of the supergene nonsulfide ores from Skorpion, carried out for the first time using TEM/HRTEM and AEM investigations. This approach helped to better understand the formation mechanism of the Skorpion Zn-clays and related phases down to the nanoscale. The microtextures of the Skorpion Zn-clays suggest they formed from fluids, meteoric and/or hydrothermal in nature, in two textural contexts: smectites can grow on previously deposited phyllosilicates (mica) (CCP texture), and/or directly nucleate from Zn-rich solutions (PCA texture). The Skorpion sauconite is chemically characterized by a greater homogeneity if compared with natural sauconites from other occurrences; it is quite stoichiometric, with Ca as interlayer cation and limited quantities of Mg and Fe, with an average composition of Ca0.14K0.02(Zn2.7Mg0.09Al0.14Fe0.10)(Si3.4Al0.6)O10(OH)2·nH2O. Contrary to Peru nonsulfide ores (Accha and Yanque), at the micro- and nanoscale the occurrence of Zn-beidellite at Skorpion is very subordinated. Chlorite and baileychlore have been also detected. Detrital micas are commonly the template for epitaxial sauconite growth. Typical supergene processes at ambient temperatures should be considered for the genesis of the Skorpion sauconite-bearing deposit, with some local contribution of low-T hydrothermal fluids. The micro- and nano-features of the Skorpion mineral assemblage confirm the complex mineralogical nature of the smectite-rich nonsulfide (micro)systems, with remarkable implications for mineralogical evaluation and processing.

A review of major non-sulfide zinc deposits in Iran

The numerous non-sulfide zinc ore deposits were the historical basis for the development of zinc mining in Iran. They include the Mehdiabad, Irankouh and Angouran world-class deposits, as well as the Zarigan and Haft-har deposits. These deposits were formed by supergene oxidation of primary sulfide minerals during the complex interplay of tectonic uplift, karst development, changes in the level of the water table, and weathering. Zn (Pb) carbonates, Zn-hydrosilicates and associated hydrated phases directly replace the primary ore bodies or fill cavities along fractures related to uplift tectonics. Direct replacement of primary sulfides is accompanied by distal precipitation of zinc non-sulfide minerals in cavities or internal sediments filling. The mineralogy of the non-sulfide mineralization in all six deposits is generally complex and consists of smithsonite, hydrozincite, and hemimorphite as the main economic minerals, accompanied by iron and manganese oxy-hydroxides and residual clays. Commonly, non-sulfide minerals in these deposits 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. Typical minerals of the RZO are Fe-oxyhydroxides, goethite, hematite, hemimorphite, smithsonite and/or hydrozincite and cerussite. Common minerals of the WZO are smithsonite or hydrozincite and only minor amounts of Fe-oxyhydroxides and hemimorphite.

Gahnite composition as a means to fingerprint metamorphosed massive sulfide and non-sulfide zinc deposits

Gahnite occurs in and around metamorphosed massive sulfide (e.g., Broken Hill-type Pb–Zn–Ag (BHT), volcanogenic massive sulfide Cu–Zn–Pb–Au–Ag (VMS), sedimentary exhalative Pb–Zn (SEDEX)), and non-sulfide zinc (NSZ) deposits. In addition to occurring in situ, gahnite occurs as a resistate indicator mineral in unconsolidated sediments (e.g., glacial till) surrounding such deposits. The spatial association between gahnite and metamorphosed ore deposits has resulted in its use as an empirical exploration guide to ore. Major and trace element compositions of gahnite from BHT, NSZ, SEDEX, and VMS deposits are used here to develop geochemical fingerprints for each deposit type.A classification tree diagram, using a combination of six discrimination plots, is presented here to identify the provenance of detrital gahnite in greenfield and brownfield terranes, which can be used as an exploration guide to metamorphosed massive sulfide and non-sulfide zinc deposits. The composition of gahnite in BHT deposits is discriminated from gahnite in SEDEX and VMS deposits on the basis of plots of Mg versus V, and Co versus V. Gahnite in SEDEX deposits can be distinguished from that in VMS deposits using plots of Co versus V, Mn versus Ti, and Co versus Ti. In the Sterling Hill NSZ deposit, gahnite contains higher concentrations of Fe3+ and Cd, and lower amounts of Al, Mg, and Co than gahnite in BHT, SEDEX, and VMS deposits. Plots of Co versus Cd, and Al versus Mg distinguish gahnite in the Sterling Hill NSZ deposit from the other types of deposits.

Non-sulfide zinc deposits of the Moroccan High Atlas: Multi-scale characterization and origin

To understand the processes and timing of their formation, six non-sulfide Zn–Pb ore deposits were investigated in the Moroccan High Atlas. Sulfide and non-sulfide ores are hosted in Lower Jurassic reefal to para-reefal limestone. Zn (Pb) carbonates, Zn silicates and associated hydrated phases directly replace the stratabound primary ore bodies or fill cavities along fractures related to the Atlasic compression. Field observation has been complemented by a multidisciplinary approach (e.g. XRD, Raman, SEM, EPMA) for the mineralogical characterization. All six ore deposits present similar paragenesis revealing three successive stages for ore deposition: 1) formation of the protore sulfides, 2) early supergene weathering with formation of Zn–Pb-bearing carbonates and iron oxi-hydroxides and 3) late supergene weathering with deposition of Zn-carbonates, Zn-silicates and hydrated phases. Direct replacement of primary sulfides is accompanied by precipitation of zinc non-sulfide minerals in cavities or internal sediments filling. The proposed three-step scenario can be placed within the tectonic evolution of the Moroccan High Atlas belt. Deposition of primary sulfides is contemporaneous with opening of the Tethyan and Atlantic oceans. During the Tertiary, intracontinental deformation has given rise to the High Atlas fold-and-thrust belt and to regional uplift. As a result, Zn–Pb sulfides, hosted in carbonates experienced oxidation under an arid climate to form karst-related Zn–Pb non-sulfide ore bodies.

Stable isotope (O and C) geochemistry of non-sulfide Zn–Pb deposits; case study: Chah-Talkh non-sulfide Zn–Pb deposit (Sirjan, south of Iran)

The study of oxygen and carbon isotopic ratios has gained importance to determine the origin of ore-bearing fluids, carbon origin, and also to determine the formation temperature of non-sulfide Pb and Zn minerals. In order to determine the origin of fluids and carbon existing in Zn carbonate minerals in Chah-Talkh deposit, initially the amounts of δ18OSMOW and δ13CPDB changes in various zinc minerals in important deposits in Iran and the world were studied, and then by comparing these values in Chah-Talkh deposit with those of other deposits, the origin of fluids responsible for ore forming, carbon, and formation temperature of Chah-Talkh deposit was determined. The range of δ18OSMOW changes in smithsonite mineral in non-sulfide lead and zinc deposits varies from 18.3 to 31.6 ‰, and δ18OSMOW in hydrozincite mineral varies from 7.8 to 27 ‰. Due to the impossibility of smithsonite sampling from Chah-Talkh deposit (due to it being fine-grained and dispersed), hydrozincite minerals which have high isotopic similarities with smithsonite are used for the isotopic analysis of carbon and oxygen. The range of δ18OSMOW changes in hydrozincite mineral of Chah-Talkh deposit varies from 7.8 to 15.15 ‰, which places in the domain of metamorphic water. The extensiveness of δ18OSMOW changes in Chah-Talkh indicates the role of at least two fluids in the formation of non-sulfide minerals. The obtained formation temperature of non-sulfide minerals (hydrozincite) in Chah-Talkh deposit is 70 to 100 °C, which indicates the role of metamorphic fluids in the formation of deposit. Complete weathering of sulfide minerals to a depth of 134 m confirms the role of rising metamorphic fluids in the formation of non-sulfide minerals. The δ13CPDB values of Chah-Talkh deposit are set in the range of atmospheric CO2 and carbonate rocks, in which the existence of atmospheric CO2 indicates the role of atmospheric fluids, and the existence of carbonate carbon rock indicates of the role of metamorphic fluids in the precipitation of non-sulfide Zn minerals.

The Hakkari nonsulfide Zn–Pb deposit in the context of other nonsulfide Zn–Pb deposits in the Tethyan Metallogenic Belt of Turkey

The Hakkari nonsulfide zinc deposit is situated close to the southeastern border of Turkey. Here both sulfide and nonsulfide Zn≫Pb ores are hosted in carbonate rocks of the Jurassic Cudi Group with features typical of carbonate-hosted supergene nonsulfide zinc mineralization. The regional strike extent of the mineralized district is at least 60km. The age of the supergene deposit has not been determined, but it is probable that the main weathering happened during Upper Tertiary, possibly between Upper Miocene and Lower Pliocene. The Hakkari mineralization can be compared to other carbonate-hosted Zn–Pb deposits in Turkey, and an interpretation made of its geological setting. The zinc mineral association at Hakkari typically comprises smithsonite and hemimorphite, which apparently replace both sulfide minerals and carbonate host rock. Two generations of smithsonite are present: the first is relatively massive, the second occurs as concretions in cavities as a final filling of remnant porosity. Some zinc is also hosted within Fe–Mn-(hydr)oxides. Lead is present in cerussite, but also as partially oxidized galena. Lead can also occur in Mn-(hydr)oxides (max 30% PbO). The features of the supergene mineralization suggest that the Hakkari deposit belongs both to the “direct replacement” and the “wall-rock replacement” types of nonsulfide ores. Mineralization varies in style from tabular bodies of variable thickness (<0.5 to 13m) to cross-cutting breccia zones and disseminated ore minerals in pore spaces and fracture planes. At Hakkari a As–Sb–Tl(≫Hg) geochemical association has been detected, which may point to primary sulfide mineralization, quite different from typical MVT.

In search of the lost zinc: A lesson from the Jabali (Yemen) nonsulfide zinc deposit

The Jabali nonsulfide zinc deposit, located northeast of Sana'a (Yemen) contains a geological resource of 12.6 million tonnes of ore grading 8.9% zinc, 1.2% lead and 68 g/t silver, with a projected recovery of ca. 80% zinc. The primary sulfide deposit shows features of both Mississippi Valley and Carbonate Replacement types, and is believed to have been formed by circulating hydrothermal fluids, either associated with Mesozoic rifting, or generated from Tertiary igneous activity, developed in the area during the Red Sea crustal extension. An extension of this phenomenon should have also triggered the late uplift, which favored the oxidation of sulfides. Ore deposition has been accompanied by several dolomitization phases, some of which have been considered strictly hydrothermal. A complete quantitative (Rietveld) mineralogical and geochemical study of mineralized full-length core samples, carried out with the aim of possibly increasing zinc recovery, shows a discrepancy between the zinc grades recorded in the chemical assays, and those calculated from the sum of the ore minerals occurring in the same samples. The difference between the assayed and calculated zinc amounts in various parts of the deposit is due to the presence of Zn-rich dolomite phases (up to 20% Zn in the lattice), as well as of Mg-smithsonite (up to 12% Mg), both phases replacive of the earlier dolomites in the weathering environment. The Zn-enriched dolomite phases could be the “missing link” between pure dolomite and smithsonite. Zinc occurring in dolomite cannot be processed economically with today's methods. Analysis of the total zinc amount contained in Zn-dolomite, when compared with the zinc occurring in the processable ore minerals shows that there is a significant proportion of unrecoverable zinc. This explains why at Jabali the projected metallurgical recovery of around 80% is unlikely to be improved upon, due to the trapped zinc within the “supergene” dolomite phases. The extensive development of the Zn-dolomite bodies, which occur throughout the whole mining area, may be highly significant for the evaluation of nonsulfide zinc ores at Jabali and for the exploration philosophy of the region. The possible occurrence of Zn-dolomite has to be kept in mind when exploring for supergene Zn-nonsulfides in other mining districts where the ore is also dolomite-hosted, which may feature a significant non-recoverable phase.

CHARACTERIZATION AND GRAVITY SEPARATION OF NONSULFIDE ZINC DEPOSITS FROM UM GHEIG MINE, RED SEA COAST, EGYPT

This paper aims to beneficiation of Um Gheig nonsulfide Zinc ore from its associated gangues using the gravity separation “shaking table” technique. The different operating parameters affecting of the efficiency of the separation process is evaluated. The zinc locality of Um Gheig represents one of the important nonsulfide zinc deposits along the coastal plain of the Red Sea Coast, Egypt. These zinc ores are called “calamines” and consist mainly of a mixture of zinc carbonates (smithsonite, hydrozincite) and zinc silicates (hemimorphite), with a variable content of calcite, goethite and Pb bearing minerals. Extensive mineralogical (using XRD, XRF) and petrographic studies have been carried out on the selected samples from Um Gheig Mine. Theses investigations show that, the nonsulfides mineral association in the deposit consists mainly of smithsonite (50.74%), hydrozincite (18.78%) and hemimorphite (14.78%) replacing both primary ore minerals and carbonate host rocks. Smithsonite occurs in two generations, cryptocrystalline mixed with goethite and rhombohedral crystals, precipitate in vugs and open space of the high grade ore. The main gangue minerals are calcite and goethite. XRF analysis indicates that the sample contains 47.50% ZnO, 1.55% PbO and about 29.28% L.O.I. Liberation and mineralogical investigation showed that higher degree of liberation (over 90%) between the valuable zinc bearing minerals and their gangues can be achieved below the size 0.106 mm. The results of gravity separation showed that the best operating parameters to obtain a concentrate with high assay and recovery are at inclination angle (4 degree), stroke length (2.5 cm), feed rate (150 gm / min), and water flow rate (25 l / min). At these optimum conditions a concentrate assaying 60.85 % ZnO was obtained with a recovery of ~90 %.

Numerical modeling of hydrothermal zinc silicate and sulfide mineralization in the Vazante deposit, Brazil

AbstractThe Vazante zinc deposit in central Brazil is currently the world’s largest known example of a hypogene nonsulfide (i.e. willemite‐dominant) zinc deposit. The mineralization is hypothesized to have formed as a result of mixing between a hot, acidic, reducing, metal‐rich brine and a cool, more basic and dilute, metal‐poor meteoric fluid. The present study sought to investigate this scenario by quantifying the individual effects of temperature, pH, salinity, and oxidation state on willemite and sphalerite solubility, and modeling their combined effects during mixing through reaction path and reactive transport modeling. Solubility calculations showed that in an initially hot, moderately acidic, reducing, metal‐rich ore fluid saturated with respect to silica, willemite solubility is relatively insensitive to changes in temperature and log , but highly sensitive to changes in pH and salinity. In contrast, sphalerite solubility was highly sensitive to changes in temperature and log , as well as salinity, and was less sensitive than willemite to changes in pH. Reaction path models sought to extend these observations by modeling the geochemistry of mixing. The results show that mixing is able to produce most of the major zinc ore and gangue minerals observed in the field, though not necessarily at the same paragenetic stages, and that both compositional and temperature changes from mixing are needed. Reactive transport models were formulated to investigate spatial patterns of mineralization. The results showed that sphalerite deposition was strongly controlled by temperature and concentrated in the regions of greatest temperature change. Willemite deposition was concentrated along the interface between the metal‐rich ore fluid and the surrounding meteoric fluid. The more rapid transport of solute than heat, in conjunction with the higher concentration of silica than sulfide in both fluids meant that willemite mineralization developed over a broader region and in greater concentrations compared with sphalerite.

The Nonsulfide Zinc Deposit at Accha (Southern Peru): Geological and Mineralogical Characterization

The Accha-Yanque zinc belt is located in the southern Altiplano of Peru, a major zinc-rich metallogenic province hosting a number of economic mineral deposits (porphyry copper and skarn ores). Several nonsulfide-type occurrences, showings, and mineral deposits are situated in a belt, peripheral to the northern, northeastern, and northwestern edge of the Oligocene-(Miocene?) Yauri-Apurimac batholith. Mineralization is hosted in breccias of both sedimentary and tectonic origin in the limestones of the Middle to Upper Cretaceous Ferrobamba Formation. Primary ores belong to the carbonate replacement deposit type and are at least in part structurally controlled. Currently, the Zn mineralization is almost fully oxidized: the Accha deposit can be assigned to both direct replacement and wall-rock replacement types. The mineralized zone (indicated resources 5.1 Mt @ 8.2% Zn and 0.9% Pb) occupies the hinge of an anticlinal dome that has been exposed by erosion. The southern limb of the structure dips about 55° to the south-southwest, whereas its northern limb is truncated by faults. The nonsulfide concentrations, consisting of a mineralized zone 5 to 20 m thick, are continuous along strike to the west for at least 700 m. The mineralogy of the Accha deposit shares many characteristics with that of the typical carbonate-hosted calamine-type nonsulfide Zn ores. The nonsulfide mineral association consists mainly of smithsonite and hemimorphite replacing both primary ore minerals and carbonate host rocks. Hydrozincite has been detected only in samples near the surface. Smithsonite occurs in zoned concretions with goethite, Mn (hydr)oxides and Zn clays, as well as replacive cement in the limestone intervals. One of the most peculiar nonsulfide Zn minerals at Accha is a sauconite-like, zincian smectite, variably concentrated throughout the deposit. Locally sauconite occurs as replacement of detrital feldspars and/or detrital fragments occurring in marly sediments or in infills of karst cavities. It also replaces both hemimorphite and smithsonite deposited during earlier stages. The age of the supergene products in the whole belt is poorly constrained, although there is geomorphologic evidence that the formation of supergene minerals postdates by more than 10 m.y. the last large-scale secondary enrichment event that terminated with central Andean climatic desiccation at ~15 Ma. The age of the Accha deposit may be consistent with a Pliocene K-Ar date of 3.3 ± 0.2 Ma obtained for supergene alunite from the top part of the leached cap in the nearby Cotabambas Cu deposit.

Nonsulfide zinc deposits in the Silesia–Cracow district, Southern Poland

The Silesia–Cracow district in Poland has been one of the world’s principal sources of zinc from nonsulfide zinc ore (Polish: galman). The still remaining nonsulfide ore resources can be estimated at 57 Mt at 5.6% Zn and 1.4% Pb. Nonsulfide mineralization is mainly hosted by Lower Muschelkalk (Triassic) limestone and is associated with different generations of the hydrothermal ore-bearing dolomite (OBD I, II, III). A fundamental ore control is believed to have been exerted by the basement faults, which were repeatedly reactivated during the Alpine tectonic cycle, leading to the formation of horst-and-graben structures: these dislocations may have caused short periods of emersion and the circulation of meteoric waters during the Cenozoic. Nonsulfide ores show a wide range of morphological characteristics and textures. They occur as earthy masses, crystalline aggregates, and concretions in cavities. Breccia and replacement textures are also very common. The most important mineral phases are: smithsonite, Fe–smithsonite, Zn–dolomite, goethite, and Fe–Mn(hydr)oxides. Minor hemimorphite and hydrozincite have also been detected. Two distinct nonsulfide ore types occur: the predominant red galman and the rare white galman. In the white galman, Fe–smithsonite and Zn–dolomite are particularly abundant. This ore type is commonly considered as a peripheral hydrothermal alteration product related to the same fluids that precipitated both the OBD II–III and the sulfides. In contrast, a supergene origin is commonly assumed for the red galman. Evidence of the petrographic and mineralogical difference between white and red galman is also found in stable isotope data. Smithsonite from red galman shows a limited range of δ13CVPDB values (−10.1 to −11.4‰), and δ18OVSMOW values (25.3‰ to 28.5‰, mean 26.8 ± 0.3‰). The uniform and low carbon isotope values of red galman smithsonite are unusual for supergene carbonate-hosted deposits and indicate the predominance of a single organic carbon source. Smithsonite from white galman has a more variable, slightly more positive carbon isotope (−2.9‰ to −7.4‰), but broadly similar oxygen isotope composition (26.8‰ to 28.9‰). The relationship of the white galman ore with the hydrothermal system responsible for OBD II and sulfide generation is still uncertain. The most important paleoweathering events took place in both Lower and Upper Silesia during Late Cretaceous up to Paleogene and early Neogene time. During this period, several short-lasting emersions and intense weathering episodes facilitated the formation of sinkholes in the Triassic carbonate rocks and the oxidation of sulfide orebodies through percolating meteoric waters. These phenomena may have lasted until the Middle Miocene.

Lead Isotope Constraints on the Origin of Nonsulfide Zinc and Sulfide Zinc-Lead Deposits in the Flinders Ranges, South Australia

Hypogene nonsulfide zinc deposits represent a mode of zinc enrichment in the Earth’s crust that until recently has largely been overlooked relative to supergene nonsulfide zinc and sulfide zinc-lead deposits. The Flinders Ranges, a deformed and uplifted portion of the northern Adelaide geosyncline in South Australia, host numerous prominent examples of hypogene nonsulfide zinc mineralization and thus provide an excellent setting in which to investigate the origin of this deposit type. The Flinders Ranges also host numerous syngenetic and Mississippi Valley-type sulfide zinc-lead deposits, allowing a possible genetic relationship between sulfide and nonsulfide mineralization to be examined. In the present study, lead isotope compositions of zinc and lead minerals from nonsulfide zinc and sulfide zinc-lead deposits from throughout the Flinders Ranges were analyzed to investigate these issues. All the lead isotope compositions measured are anomalous with respect to the Holmes-Houtermans and Stacey-Kramers models. Compositions range from 18.332 to 21.125 for 206Pb/204Pb, from 15.729 to 16.022 for 207Pb/204Pb, and from 38.264 to 40.967 for 208Pb/204Pb. The anomalous, highly radiogenic lead isotope compositions probably reflect input from the highly uranium and thorium enriched basement rocks in the region. The data also form a linear trend in both 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb space, indicating that the data either define an isochron from one of multiple possible stages of lead evolution, or that the deposits all contain varying mixtures of the same two end-member leads, in either case implying that the deposits are genetically related. For a mixing scenario, the present data provide little further constraint on the origin of a hypothetical less radiogenic end member. However, further constraints on the origin of the more radiogenic end member can be obtained by considering the slope of the regression line through the data. Based on an instantaneous model of lead growth, the maximum age of formation of the more radiogenic lead component would be between 910 and 1036 Ma. These ages fall within a period of tectonic quiescence in the region but allow for the possibility that the lead may be associated with a younger event, such as the 850 to 600 Ma rifting event that led to the opening of the Adelaide geosyncline. Based on a continuous model of lead growth, the more radiogenic lead component could have evolved in a source with an age between 1313 and 1523 Ma before it was sequestered in the mineral deposits at about 435 Ma. These 1313 to 1523 Ma dates correspond with the age of much of the younger igneous rocks (e.g., the Hiltaba Suite granites) in the Gawler craton that flank the Adelaide geosyncline on the west and may comprise part of the basement of the Adelaide geosyncline. The lead isotope data also show a strong regional pattern consisting of two distinct centers of radiogenic lead, one located in the southeast corner of the study area and a second, broader zone lying near the west-central part of the study area. This pattern may in part be a function of depth to the basement, in which the more radiogenic lead occurs in deposits located near the edges of the basin, where sediment thicknesses are lower, although heterogeneity in basement composition and proximity to sources of less radiogenic lead may also have affected the pattern. Finally, the lead isotope compositions also show some correlation with deposit size, in which the largest deposits in the Flinders Ranges also contain the most radiogenic lead.

Nonsulfide and sulfide-rich zinc mineralizations in the Vazante, Ambrósia and Fagundes deposits, Minas Gerais, Brazil: Mass balance and stable isotope characteristics of the hydrothermal alterations

The Vazante Group hosts the Vazante nonsulfide zinc deposit, which comprises high-grade zinc silicate ore (ZnSiO 4), and late-diagenetic to epigenetic carbonate-hosted sulfide-rich zinc deposits (e.g. Morro Agudo, Fagundes, and Ambrósia). In the sulfide-rich deposits, hydrothermal alteration involving silicification and dolomitization was related with ground preparation of favorable zones for fluid migration (e.g. Fagundes) or with direct interaction with the metalliferous fluid (e.g. Ambrósia). At Vazante, hydrothermal alteration resulted in silicification and dolomite, siderite, jasper, hematite, and chlorite formation. These processes were accompanied by strong relative gains of SiO 2, Fe 2O 3(T), Rb, Sb, V, U, and La, which are typically associated with the nonsulfide zinc mineralization. All sulfide-rich zinc ores in the district display a similar geochemical signature suggesting a common metal source from the underlying sedimentary sequences. Oxygen and carbon isotope compositions of hydrothermally altered rocks reveal a remarkable alteration halo at the Vazante deposit, which is not a notable feature in the sulfide-rich deposits. This pattern could be attributed to fluid mixing processes involving the metalliferous fluid and channelized meteoric water, which may control the precipitation of the Vazante nonsulfide ore. Sulfide deposition resulted from fluid–rock interaction processes and mixing between the ascending metalliferous fluids and sulfur-rich tectonic brines derived from reduced shale units.

100th Anniversary Special Paper: Sedimentary Mineral Deposits and the Evolution of Earth's Near-Surface Environments

The nature of sedimentary mineral deposits has evolved during Earth's history in concert with changes in the oxidation (redo) state of the ocean-atmosphere system, biological evolution, and the growing importance of geologically young accumulations of ore-grade material. There is now strong evidence that the atmosphere and the oceans were anoxic, or essentially anoxic, before 2.4 Ga. Banded iron formations (BIF) and the detrital uranium ores formed prior to 2.4 Ga are consistent with such a state. The period between 2.4 and 2.0 Ga is called the Great Oxidation Event by some. Its ores bear unmistakable marks of the presence of atmospheric O{sub 2}. Between 1.8 and 0.8 Ga the Earth system seems to have been remarkably stable. Sedimentary ore deposits of this period were influenced by the presence of O{sub 2}. BIF, sedimentary manganese, and phosphorites disappeared ca. 1.8 Ga, but sedimentary exhalative (SEDEX) deposits and unconformity-type uranium deposits flourished, and nonsulfide zinc deposits put in an appearance. The period between 0.8 Ga and the end of the Proterozoic at 0.54 Ga was as turbulent or more so than the Paleoproterozoic. BIF returned, as did sedimentary manganese deposits and phosphorites. A further rise in the O{sub 2} content of themore » atmosphere and an increase in the sulfate concentration of seawater during this period brought the composition of the atmosphere and of seawater close to their present redox state. The last 540 m.y. of Earth's history have seen the system pass through two supercycles of roughly equal length. Climate, the redox stratification of the oceans ocean mixing, and the nature of sedimentary ores were influenced by tectonically and volcanically driven changes during these supercycles. The evolution of the higher land plants gave rise to coal deposits and sandstone-type uranium ores and was important for the formation of bauxites.« less

Isotope geochemistry of the mafic dikes from the Vazante nonsulfide zinc deposit, Brazil

The Vazante Group, located in the northwestern part of Minas Gerais, hosts the most important zinc mine in Brazil, the Vazante Mine, which represents a major known example of a hypogene nonsulfide zinc deposit. The main zinc ore is represented by willemite and differs substantially from other deposits of the Vazante-Paracatu region, which are sulfide-dominated zinc-lead ore. The age of the Vazante Group and the hosted mineralization is disputable. Metamorphosed mafic dikes (metabasites) that cut the metasedimentary sequence and are affected by hydrothermal processes recently were found and may shed light on the geochronology of this important geological unit. Zircon crystals recovered from the metabasites are xenocrystic grains that yield U–Pb conventional ages ranging from 2.1 to 2.4 Ga, so the basement of the Vazante Group is Paleoproterozoic or has metasedimentary rocks whose source area was Paleoproterozoic. Pb isotopes determined for titanite separated from the metabasites have common, nonradiogenic Pb compositions, which prevents determination of their crystallization age. However, the Pb signatures observed for the titanite crystals are in agreement with those determined for galena from the carbonate-hosted Zn–Pb deposits hosted by the Vazante Group, including galena from minor sulfide ore bodies of the Vazante deposit. These similarities suggest that the metalliferous fluids that affected the metabasites may have been those responsible for galena formation, which could imply a similar lead source for both nonsulfide and sulfide zinc deposits in the Vazante–Paracatu district. This common source could be related to deep-seated, basin-derived, metalliferous fluids associated with a long-lived hydrothermal system related to diagenesis and deformation of the Vazante Group during the Neoproterozoic.