Mondillo, Nicola (2013) Supergene Nonsulfide Zinc-Lead Deposits: The Examples of Jabali (Yemen) and Yanque (Peru). [Tesi di dottorato]


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Item Type: Tesi di dottorato
Lingua: English
Title: Supergene Nonsulfide Zinc-Lead Deposits: The Examples of Jabali (Yemen) and Yanque (Peru)
Date: 1 April 2013
Number of Pages: 186
Institution: Università degli Studi di Napoli Federico II
Department: Scienze della Terra, dell'Ambiente e delle Risorse
Scuola di dottorato: Scienze della Terra
Dottorato: Scienze della Terra
Ciclo di dottorato: 25
Coordinatore del Corso di dottorato:
Date: 1 April 2013
Number of Pages: 186
Uncontrolled Keywords: Zn-nonsulfides, Peru, Yemen, mineralogy, geochemistry
Settori scientifico-disciplinari del MIUR: Area 04 - Scienze della terra > GEO/09 - Georisorse minerarie e applicazioni mineralogico-petrografiche
Date Deposited: 03 Apr 2013 13:10
Last Modified: 10 Nov 2014 14:16


“Nonsulfide zinc” is a very general term, referred to a group of ore deposits consisting of Zn-oxidized minerals, mainly represented by smithsonite, hydrozincite, hemimorphite, sauconite and willemite, which are markedly different from sphalerite ores, typically exploited for zinc. Locally, Ag minerals can occur too. The supergene nonsulfide deposits form from low-temperature oxidation of sulfide-bearing concentrations. Objective of this study is to increase the knowledge on the geology, mineralogy and genetic processes of this kind of supergene zinc-lead ores. Two nonsulfide deposits were taken as example: Jabali in Yemen, and Yanque in Peru, whose characteristics were compared with the peculiarities of other known nonsulfide concentrations. The Jabali and Yanque deposits are totally different from each other, both in geological setting and in mineralogical association. Therefore, the identification of their features and possible genetic mechanisms should cover several characteristics possibly occurring in other deposits of the same kind. A particular attention has been given to the mineralogy of the ores, and to the relationship between the newly formed zinc minerals and the host rock, because the mineralogical characteristics are crucial in order to develop the metallurgical processes. The Jabali deposit lies in a mountainous desert terrain about 110 km east of Sana'a, the capital of Yemen, along the western border of the Marib-Al-Jawf/Sab'atayn basin. In the Middle Age (7th - 9th century AD) Jabali was considered one of the most important mining areas for silver in the Muslim world. The Jabali nonsulfide concentration has a resource of 8.7 million tonnes at an average grade of 9.2% zinc. The ore is hosted in the Jurassic carbonate rocks of the Shuqra Fm. (Amran Gp.) and the nonsulfide mineralization derives from the low temperature alteration of the primary sulfide deposit. The primary deposit shows the general features of Mississippi Valley-type ores, and consists of sphalerite, galena, and minor pyrite/marcasite (mainly as remnants) and other minor sulfide phases. Silver, cadmium, copper, arsenic, germanium and mercury are generally contained in sphalerite. Primary ore deposition has been accompanied by several hydrothermal dolomitization phases. Smithsonite is the most abundant economic ore mineral in the secondary deposit. It occurs in two main phases: Smithsonite 1, which replaces the host dolomite and sphalerite, and Smithsonite 2, which occurs as cements and concretions precipitated in cavities and porosity of the host rock. In the gradational “bands” which mark the boundary between host dolomite and replacive smithsonite, dolomite is widely replaced by Zn-rich dolomite phases, where Zn has substituted for Mg. The ZnO content in this dolomite can reach 17-22 wt.% (~70 mol.% substitution). These phases indicate that the replacement of the host rock proceeded in stages: it started with a partial replacement of Mg by Zn in the dolomite lattice, and went on with a gradually higher amount of substitution of Mg by Zn, until a maximum measured value of 70 mol.% Zn in the dolomite. Eventually, the dolomite lattice became totally unstable and smithsonite precipitated. Zn-dolomite appears to be an intermediate replacement phase between dolomite and smithsonite. The Jabali smithsonites have variable δ18O compositions in different parts the orebody. The measured values are generally lower than those of other supergene smithsonites, whereas the C-isotope composition is in the same range of values of smithsonites from other supergene nonsulfide ores. The δ18O spread of values for the Jabali smithsonites could indicate: a precipitation from a low-temperature hydrothermal system, similarly to the Angouran (Iran) nonsulfide deposit, or a supergene precipitation from waters with variable O-isotope composition, as hypothesized also for the Coahuila-Sierra Mojada district (Mexico). The C-isotope composition is typical of supergene nonsulfide Zn deposits, and is generally interpreted as a result of mixing between host rock carbonates and soil/atmospheric CO2. In the case of a low-temperature hydrothermal genesis, these compositions could also indicate the involvement of meteoric-surficial water in a deep, thermal circulation, or organic carbon values not related to soils, e.g. organic matter from black shales. Another interesting characteristic of the Jabali deposit is the strict association of Ag-sulfides with smithsonite. Ag-sulfides are quite common at Jabali and are locally associated also to secondary greenockite; they mostly occur together with concretionary smithsonite and between lamellae of gypsum derived from weathering processes. They have never been detected in association with primary sulfides, but occur always together with the secondary phases. After literature data, the co-precipitation of Ag-sulfides and smithsonite under atmospheric conditions can happen only at neutral pH, and in an Eh range between 0 and -2 Volts, because the stability fields of the two phases are very near, though not superposed. Comparing Jabali to other nonsulfide deposits in the world, it is possible to find some similarities with the SW Sardinia “Calamines” (Italy), and with the Upper Silesian ”Galmans” (Poland), because in both districts Zn-dolomite is also fairly abundant. In particular, Jabali shows the same Zn-dolomite replacement fronts of the host dolomite like in SW Sardinia. The Yanque nonsulfide Zn-Pb deposit (inferred resources 12.5 Mt @ 3.5% Zn and @ 3.7% Pb) is located in the “Accha-Yanque Belt” (southern Peru), about 90 km southwest of the city of Cuzco, within the Andahuaylas-Yauri province. The Andahuaylas-Yauri province covers an area of approximately 25,000 km2 in southern Peru and extends for 300 km between the localities of Andahuaylas in the northwest and Yauri in the southeast. The belt hosts numerous Cu-porphyry and porphyry-related skarn deposits, spatially and temporally associated with the middle Eocene to early Oligocene (ca. 48-32 Ma) intrusion of the homonymous batholith into the sedimentary successions of Mesozoic age. The Yanque deposit occurs within a base metal mineralized district, extended for about 20 km2, which is centered on the Dolores Cu-porphyry. The zinc-lead deposit of Yanque is located 1.5 km west of Dolores Cu-porphyry. Other two small Zn-Pb mineralizations occur less than a kilometer on the east and northeast of the same porphyry. Both sedimentary and igneous rocks constitute the backbone of the Yanque-Dolores area. The sedimentary lithologies belong to the Soraya, Mara and Ferrobamba Fms. (upper Jurassic-middle Cretaceous). They are bordered by a large apophysis of the Yauri batholith, and by the intrusive porphyries hosting the Dolores mineralization. The Yanque deposit itself is hosted by a complex breccia facies, constituted by a siliciclastic conglomerate in the northern part of the deposit, interfingered with a dolomite breccia more in the south. The breccia is stratigraphically located at the transition between Mara and Ferrobamba Fms. The Yanque economic deposit consists of Zn-Pb nonsulfide concentrations, derived from the secondary alteration of primary sulfides. The Yanque protore could be considered as a distal polymetallic mineralization, genetically related to the emplacement of the Andahuaylas-Yauri batholith, like other mineral deposits in the region (porphyry copper and skarn). A possible genetical relationship with the nearest Cu-porphyry has been investigated through Pb-isotope geochemistry of ore minerals, as well as of sedimentary and magmatic rocks occurring in the district. The Pb-isotopic compositions of Yanque and Dolores sulfides (pyrite and galena) are similar, and correspond to the isotopic composition of the Tertiary magmatically derived ore bodies in this part of Peru. Looking more in detail, the three Pb-isotope ratios of the sulfides are affected by ubiquitous small but resolvable differences and disequilibria, probably because many components contributed to their final lead isotope signature. Igneous rocks show a large spread in the isotopic composition, possibly indicating variable degrees of country rock assimilation, or different isotopic compositions of the magma pulses. It is possible to resolve significant differences also for the sedimentary rocks sampled at Yanque. It is important to note that the hydrothermal cement, which impregnates these lithotypes, has the same composition of sulfides. Similar values have been detected in the hydrothermally affected igneous rock. This could indicate that the same hydrothermal fluids, which determined the phyllic/sericitic halo around the Cu-porphyry, deposited also the Yanque sulfides. However, the observed significant spread of the Pb isotopic compositions is not compatible with a simple genetic model involving just one fluid circulation episode. The primary ores have been totally oxidized, and the economic concentrations at Yanque are dominated by supergene Zn-Pb nonsulfides. The remnants of the sulfide minerals are represented by moderate amounts of galena and pyrite, and only by traces of sphalerite. The nonsulfide ore association consists of sauconite, hemimorphite, smithsonite and cerussite. Zinc is allocated mainly in sauconite (Zn-smectite), rather than in carbonates: a factor strictly related to the prevailing siliciclastic character of the Yanque host rock. Sauconite is derived from a typical process of supergene wall-rock replacement and alteration, represented by the reaction: K-feldspar (detrital) → sericite/illite and kaolinite (hydrothermal) → sauconite (supergene); sericite/illite and kaolinite are enriched in zinc at the reaction boundaries with the replacing sauconite, this pointing to an almost progressive modification process of clays. Secondary Pb-minerals are mostly linked to direct replacement of galena. The Zn-Pb Yanque deposit can be compared only with the Skorpion mineralization in Namibia, similarly hosted in silicate rocks. Both have sauconite as main economic mineral, but it is characterized by different textures. Important differences exist also on the whole mineralogical association (e.g. the occurrence of Cu-secondary minerals at Skorpion, and of secondary Ag-sulfides and enrichments in Fe-hydroxides at Yanque). Another important issue regards the different mobilization of the metals within the host rock. At Yanque Zn is mostly enriched in the topmost volumes of the orebody, whereas Pb is concentrated in the lowermost levels. Surprisingly, this corresponds to an inverse zonation, compared to usual geometry observed in many other Zn nonsulfide deposits, including Skorpion. In fact, Zn is generally concentrated in the more distal areas of the deposit, whereas Pb, that is less mobile, is accumulated in situ, near the sulfide protore. A possible explanation of the inverse zonation at Yanque can arise from the current almost total absence of sphalerite in the mineralized area. Zinc in nonsulfides could have originated from the dissolution of a primary sphalerite-rich ore, located somewhere in the eroded volumes of rock surrounding the actual site, and, after being transported by supergene solutions, migrated and precipitated in the porosity of the host conglomerates. This kind of genesis could bear the genetic concept of a Zn “exotic” mineralization, like those exploited around the Cu-deposit of Chuquicamata (Chile). The occurrence of sauconite as main ore mineral at Skorpion has been added as an evidence that the deposit formed under semi-arid to arid conditions during several paleo-weathering episodes. Semi-arid climates are generally favorable to the deposition of smectite-type clay minerals: a similar consideration could be valid also for the Yanque sauconite ores. The geological, mineralogical and geochemical investigation on the Jabali and Yanque deposits has shown that these Zn-Pb nonsulfide concentrations, though being considered to share the mainstream genetic concepts for supergene deposits, have some distinctive characteristics that could be of importance for their scientific as well as economic evaluation. The common occurrence of Zn-dolomite at Jabali is deleterious when considering the economic evaluation of the ore, because it can result in an unaccounted Zn loss. At Yanque, the issue related to the sauconite (which is mixed with other clays too) abundance is not simple to face from the processing point of view. In conclusion, the nonsulfide Zn-Pb orebodies are a type of deposits far more complex than previously considered. The research carried out on the Jabali (Yemen) and Yanque (Peru) deposits has increased the knowledge on the nature and genetic mechanisms responsible for at least part of this mineralization type. The discovery of Zn-dolomite at Jabali, and of mixed Zn-clays at Yanque, represents an interesting step for a better understanding of the wall-rock replacement process. The observation of unusual C-O-isotope compositions in the Jabali smithsonite enriches the available geochemical “database” of the stable isotopes literature on Zn- and Pb carbonates. All the results of this research work can be a great help in avoiding too many straight simplifications on the nonsulfide characteristics and genesis.

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