Arfè, Giuseppe (2017) Genesis of supergene nonsulfide zinc mineralizations in the Bongará (Peru) and Skorpion-Rosh Pinah (Namibia) areas. [Tesi di dottorato]

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Item Type: Tesi di dottorato
Resource language: English
Title: Genesis of supergene nonsulfide zinc mineralizations in the Bongará (Peru) and Skorpion-Rosh Pinah (Namibia) areas
Creators:
CreatorsEmail
Arfè, Giuseppepepparfe@gmail.com
Date: 9 December 2017
Number of Pages: 267
Institution: Università degli Studi di Napoli Federico II
Department: dep20
Dottorato: phd084
Ciclo di dottorato: 30
Coordinatore del Corso di dottorato:
nomeemail
Fedi, Mauriziofedi@unina.it
Tutor:
nomeemail
Boni, MariaUNSPECIFIED
Balassone, GiuseppinaUNSPECIFIED
Mondillo, NicolaUNSPECIFIED
Date: 9 December 2017
Number of Pages: 267
Keywords: namibia; skorpion; rosh pinah; perù; bongará; cristal; mina grande; nonsulfides; supergene; stable isotopes; paleoclimate
Settori scientifico-disciplinari del MIUR: Area 04 - Scienze della terra > GEO/09 - Georisorse minerarie e applicazioni mineralogico-petrografiche
Date Deposited: 19 Dec 2017 14:43
Last Modified: 02 Apr 2019 11:04
URI: http://www.fedoa.unina.it/id/eprint/12111

Collection description

Supergene Zn(Pb) nonsulfide deposits typically form via oxidation of sphalerite- and galena-rich sulfide bodies, under favorable weathering conditions. The resulting mineral assemblage mainly consists of smithsonite, hydrozincite, hemimorphite, sauconite, willemite, cerussite and anglesite, whose relative abundances generally depend on climate regime at the time of weathering and on the nature of host rocks. The main object of this thesis is to increase the knowledge on the genetic processes of this kind of supergene ores by investigating all factors at the local scale, which may control the supergene profile genesis and development. To reach this aim, it was carried out an accurate study of the mineralogy and geochemistry of selected samples, which were collected along weathering profiles of supergene Zn deposits characterized by different geological and climatic conditions. Four deposits were considered: Mina Grande and Cristal in northern Peru (Bongará province), and Skorpion and Rosh Pinah in southern Namibia (Gariep Belt). Even if these districts show several common mineralogical features, they are completely different from the geo-climatic point of view. The genesis of nonsulfide mineralization in these deposits is probably related to similar weathering conditions that were active during the initial sulfides oxidation in both districts. Skorpion and Rosh Pinah deposits in Namibia are characterized by fossil supergene profiles that formed under humid conditions similar to those currently affecting those of the Bongará province. Subsequently, this tropical environment evolved toward the current arid conditions in southern Namibia, resulting in a different development of the supergene profiles in this area, compared to the Peruvian ones. Therefore, the identification of the features and possible genetic mechanisms in currently active supergene profiles, such as those occurring in the Bongará province, should fill the existing gaps for complete understanding of the various processes that are no more identifiable in no longer active (or fossil) supergene profiles. The Mina Grande zinc nonsulfide deposit (Amazonas region, Peru) belongs to an ample mining concession corresponding to the “Bongará Project”, which covers an area of approximately 100 km2. The deposit consists of several accumulations in karst cavities of Zn-oxide minerals, derived from the complete weathering of a Mississippi Valley-type deposit hosted in Mesozoic carbonate rocks (mainly limestones) of the Condorsinga Formation (Pucará Group). The karst cavities hosting the nonsulfide ores were developed in the Miocene, along northwest-southeast faults (avg. N40W/70SW) associated with regional structures, and locally along stratification joints (avg. N8E/26NW). Thus far, the Mina Grande Zn accumulations have been partially mined in two pits (named “Fase A” and “Fase B”) and explored in a third area, named “Fase C.” In 2008, the measured resources were as follows: Fase A = 160,000 metric tons (t) @ 21.2% Zn, Fase B = 36,400 t @ 28.9% Zn, and Fase C = 116,700 t @ 22.5% Zn. The nonsulfide mineral assemblage, consisting mostly of hydrozincite, smithsonite, and hemimorphite, is associated with few remnants of the hypogene ore (pyrite and sphalerite). Mineralogical and petrographic studies revealed several texturally distinct smithsonite and hydrozincite generations, which are characterized by extremely negative δ13C compositions, pointing to the prevailing contribution of an isotopically light carbon component from the oxidation of organic matter-derived carbon, and different δ18O isotope compositions: 26.9 to 27.2 and 26.0 to 26.3‰ δ18O VSMOW for smithsonite, and averages 24.6 and 23.7‰ δ18O VSMOW for hydrozincite. These distinct δ18O compositions indicate that smithsonite and hydrozincite precipitated at least during two depositional stages. These stages were probably related to several periods of uplift that occurred in the Bongará district since at least ~10 Ma, when the transition from the Pebas to Acre systems affected the Amazonas foreland basin in Miocene. The karst activity, to which the supergene mineralization is related, was restricted to the late Miocene and early Pliocene periods. Climatic conditions reconstructed from the ecosystems persisting in the region from Late Tertiary to Recent suggest that the Mina Grande supergene mineralization was associated with several weathering episodes that occurred under a climate resembling the present-day conditions. The Cristal prospect is located 6 km north of the Mina Grande deposit, in the northernmost part of a wide mining district corresponding to the “Charlotte Bongará Zinc Project”, which covers an area of approximately 110 km2. Differently from Mina Grande, at Cristal remnants of the primary Mississippi Valley-type mineralization are still in place. Indeed, the mineralized area of the Cristal Zn prospect contains both sulfide and nonsulfide mineralizations, which are locally mixed (sulfides + nonsulfides) or well separated (sulfides or nonsulfides). As in the case of Mina Grande, the mineralization is hosted by the lithologies of the Pucará Group (mainly dolostones), deposited in a Mesozoic extensional basin on the western margin of the Brazilian-Guyana shield. Zinc sulfides at Cristal occur in the roots of the nonsulfide concentrations, and are locally present also nearer to surface. The sulfide mineralization clearly postdates two hydrothermal dolomitization phases and the sulfides occur mainly in veins, cavity fillings or as disseminated mineralization, generally associated with sparry to saddle dolomite. They mostly consist of dark-brown sphalerite, associated with smaller amounts of pyrite. Sphalerite commonly displays Fe-zoning (it contains on average ca. 6 wt.% Fe), and is Ge-rich (mean concentration of 145 ppm, maximum of 383 ppm). Galena is rare. The Cristal sphalerite has sulfur isotopic compositions of δ34S = 14 to 15 ‰ VCDT. Oxygen isotopic compositions of dolomites are: δ18O = 24.4 to 24.7 ‰ VSMOW for early and 18.4 to 22 ‰ for late hydrothermal dolomites. The δ34S and δ18O values of Cristal sulfides and dolomites respectively are similar to those observed in two Mississippi Valley-type (MVT) deposits occurring in the region, namely the Florida Canyon and Florcita deposits. This is consistent with a single mineralization event at district scale. The Pb isotope ratios of sphalerite from two different areas of the property define two distinct data-point clusters (centered around averages of 206Pb/204Pb = 18.850 ± 0.002, 207Pb/204Pb = 15.685 ± 0.002, 208Pb/204Pb = 38.752 ± 0.004, and 206Pb/204Pb = 19.042 ± 0.002, 207Pb/204Pb = 15.712 ± 0.002, 208Pb/204Pb = 39.080 ± 0.004, respectively). This difference required distinct sulfide-bearing hydrothermal pulses in the mineralized area. The Pb isotopic compositions of the Cristal sulfides are intermediate between the compositions of galena from the San Vicente and Shalypaico MVT deposits, and record a contribution from an old crustal component. The Marañon Complex basement, which has Pb isotopic ratios roughly matching those of dolomites and sulfides from Cristal and Charlita North areas, represents the most reliable candidate to be the main end-member source of the metals (e.g. Pb and Zn) for the Cristal sulfide mineralization. The second end-member, a Mesozoic or Tertiary magmatic source contributing Pb to the hydrothermal fluid, is elusive, as none of them has been documented in the Cristal area. The Cristal nonsulfide ores are interpreted to be the product of weathering of primary MVT mineralization. The nonsulfide prospect spans over an area of approximately 2 x 1 km with nearly continuous zones of Zn enrichment that has been detected in soil and rock samples. The nonsulfide mineralization consists mainly of semi-amorphous orange to brown zinc “oxides” that include hemimorphite, smithsonite and Fe-(hydr)oxides. The most important mineralized areas are present in the Esperanza and Yolanda occurences, which were also most densely explored. In both occurrences the supergene Zn-carbonates and silicates infill solution cavities, or replace the carbonate host rocks and/or the primary sulfides forming smithsonite- and hemimorphite-rich mineralizations. The analyzed drill cores are mainly from Esperanza, where the zinc content, associated with hemimorphite-rich layers can reach ~ 53 wt.% Zn; the average Zn grade is around 20 wt.%. Germanium concentrations are significant at Cristal, with values around 200 ppm measured on bulk rock, which are mostly related to the Ge-rich sphalerite, and to products of sulfides oxidation such as hemimorphite and goethite. As already observed in the nearby Mina Grande deposit, the area experienced a prolonged phase of weathering from Miocene to Recent under tropical climatic conditions. In these climatic conditions, weathering processes affected many pre-existing sulfide deposits (e.g. Cristal, Florida Canyon, Mina Grande etc.), where supergene profiles were developed under locally different conditions, which are defined primarily on the basis of mineralogical and geochemical data. At Cristal, the occurrence of siderite paragenetically preceding smithsonite indicates that sulfide weathering took place under a transition from a reducing to an oxidizing environment. The mineralogy and geochemistry of the Bongará ore deposits is primarily conditioned at a local scale by two factors: (1) uplift rates, and (2) host rock lithology. The latter may favor the development of more (e.g. Mina Grande) or less (e.g. Cristal) alkaline supergene environments. As instance, Cd in the nonsulfide mineral assemblage of Mina Grande and Cristal deposits is respectively hosted in otavite and greenockite. The first mineral is indicative of alkaline conditions, whereas the second one is stable under weakly alkaline to near-neutral conditions. This difference in the geochemistry of the nonsulfide ores of the two deposits is also remarked by their Ge contents, which are much lower at Mina Grande than at Cristal. The absence of Ge at Mina Grande should reflect the limited capacity of some supergene minerals (e.g. goethite) to fix Ge in the lattice under highly alkaline conditions. Supergene smithsonites and calcites show isotopically different generations. The δ18O and δ13C compositions of sulfide-replacing smithsonite range from 24.6 to 25.8‰ VSMOW and from -0.6 to -11.7‰ VPDB, respectively. The host rock-replacing smithsonite has a range of δ18O values varying between 26.6 and 27.3‰ VSMOW, whereas the δ13C values are comparable with the δ13C ratios of sulfide-replacing smithsonite. Late smithsonite veins show δ18O and δ13C values that are between 27.8 and 28.3‰ VSMOW and between 0.1 and -2.4‰ VPDB, respectively. The δ13C values of supergene calcites occurring in late veins range between 0.7 to -7.4‰ VPDB, whereas the δ18O values are around 24.7‰ VSMOW. The uplift factor was mainly controlled by the activity of local faults, which allowed the exposure of sulfide protores at variable elevations in different periods of time and hydrological settings (e.g., the current superficial runoff rate is major at Mina Grande than at Cristal). Such different factors and settings may result in the precipitation of isotopically different supergene carbonates (e.g. smithsonites and calcites at Mina Grande and Cristal). Contrary to the Mina Grande deposit, the development of a karst network at Rio Cristal was quenched by a limited uplift rate, and the supergene alteration did not completely obliterate the roots of the original sulfide orebody. In both Mina Grande and Cristal deposits, different Zn-bearing phyllosilicates were found in association with the principal Zn nonsulfide minerals (smithsonite, hemimorphite and hydrozincite). The XRD and SEM-EDS investigations revealed that the Zn-bearing micas occurring in both deposits mostly consist of I/S mixed layers of detrital origin, which have been partly altered or overprinted by sauconite during the supergene alteration of the sulfides. Sporadic hendricksite was also identified in the Cristal nonsulfide mineral assemblage, whereas at Mina Grande the fraipontite-zaccagnaite (3R-polytype) association was detected. The identified zaccagnaite polytype suggests that fraipontite and zaccagnaite are both genetically related to weathering processes. The hendricksite detected at Cristal may be instead considered a hydrothermal alteration product, formed during the emplacement of sulfides. The complex nature of the identified phyllosilicates is an evidence of the multiple hydrothermal and supergene processes that occurred in the Bongará district. Skorpion and Rosh Pinah Zn(Pb) deposits are hosted in Neoproterozoic rocks that are part of a volcano-sedimentary sequence within the Gariep Belt (southwest Namibia). Skorpion is the largest Zn nonsulfide mineralization ever discovered and exploited. It mostly consists of Zn-oxidized minerals, derived from the weathering and oxidation of a volcanic-hosted massive sulfide (VMS) protore. Rosh Pinah is a hybrid Zn massive sulfide deposit, with both VMS and Broken Hill-type characteristics; it is partly weathered in the uppermost part of the massive sulfide lens. A comparison between the deep oxidation processes that occurred at Skorpion and the limited weathering of the Rosh Pinah deposit was carried out by analyzing the carbon and oxygen isotope ratios of supergene carbonate minerals. Twenty-three smithsonite samples from the Skorpion deposit and six gossanous samples (containing both the host dolomite and smithsonite) from the uppermost levels of the Rosh Pinah mine have been analyzed. The Skorpion smithsonites form botryoidal crusts overgrown by euhedral calcite crystals. At Rosh Pinah all sampled smithsonites occur in veins within the host rock dolomite. Skorpion smithsonite is characterized by δ13C values strongly variable between -9.1‰ and 0.1‰ VPDB and by a small range in δ18O from 28.0 to 29.9‰ VSMOW. Calcite shows a minor variation in δ13C, with values being generally positive (0-1.6‰ VPDB) and δ18O values slightly lower than those of smithsonite (25.4-27.1‰ VSMOW). The analyses of the Rosh Pinah samples show that the host dolomite is characterized by δ18O values ranging from 18.7 to 22.0‰ VSMOW, and by negative δ13C values (-5.9 to -2.7‰ VPDB). The carbon isotope ratios of smithsonite, as in Skorpion, are negative (-2.8 to -1.9‰ VPDB) and they partly overlap with those of the host dolomites. The δ18O values (26.7-29.0‰ VSMOW) are on average comparable with the values measured at Skorpion. The similar negative δ13C values of smithsonite and dolomite at Rosh Pinah point to the involvement of both re-oxidized organic carbon and host dolomite inorganic carbon during smithsonite formation, whereas at Skorpion a larger contribution of isotopically light organic carbon is considered more probable. The comparable δ18O compositions of smithsonite from the two deposits imply similar ore-forming fluids and/or similar temperature conditions during formation. In agreement with former studies, this research suggests that Skorpion smithsonite precipitated at an average temperature near 17°C from fluids depleted in 13C, due to a high contribution of organic carbon from the soil, either during the first (Late Cretaceous-Paleocene) or the last humid climatic stage (early-middle Miocene). Even if the similarity between the δ18O composition of Rosh Pinah and Skorpion smithsonites points to similar ore-forming fluids and/or similar conditions during formation, the relatively high δ13C values of the Rosh Pinah smithsonites suggest a minor influence of isotopically light organic carbon and the absence of soils over this deposit. Combining these data with the limited thickness of the supergene zone over the Rosh Pinah orebody, it is likely that these smithsonites, together with the gossan in which they occur, formed at the end of the early-middle Miocene semi-humid period. The δ18O and δ13C compositions of Skorpion calcite indicate that the precipitating supergene fluids remained roughly unchanged, but that the contribution of bicarbonate from the host rock became prevailing in them. This suggests that calcite formation occurred at the beginning of the late Miocene-Pliocene semi-arid period, when the host marbles were uplifted and karstified, thus promoting a higher bicarbonate contribution from dissolving host rock. One of the main components of the Skorpion ore association is the Zn-smectite (“sauconite”). In fact, in the Skorpion orebody the trioctahedral Zn-bearing smectite predominates over the other Zn-oxidized minerals (e.g. smithsonite, hemimorphite and hydrozincite). A thorough study of the clay component has been focused on microtextural observation and chemical analyses of the clay nano-particles in the supergene nonsulfide ores from the Skorpion deposit, carried out for the first time using TEM/HRTEM and AEM. 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 Zn-clays suggest they formed from fluids, meteoric and/or hydrothermal in nature, in two textural contexts: the 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. At the micro- and nanoscale, also Zn-beidellite has been detected at Skorpion, though very subordinated. Chlorite and baileychlore also occur. Detrital micas are commonly the templates for epitaxial sauconite growth. 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. In conclusion, 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. In order to compare the evolution of the weathering profiles and the genetic characteristics of supergene deposits located in arid (Skorpion and Rosh Pinah) and humid (Cristal and Mina Grande) areas, parallel studies have been carried out. Carbon and oxygen stable isotope data have been useful to record long-lasting and short-term episodes of weathering. The first are mirrored by extreme differences between the δ13C values of the host rock carbonates and those of the supergene minerals (e.g. smithsonite). The second are characterized by the homogeneity of carbon isotope ratios between the host rock and the weathering products. However, such homogeneity in supergene profiles that evolved in semi-arid conditions may also indicate long-lasting periods of weathering, during which the organic activity in the soil may have been negligible. Cristal and Mina Grande supergene carbonates, which formed and developed in uplifted areas under humid climate settings, show small variations of their isotopic compositional range that would suggest limited environmental changes. However, the extreme variation of the weathering conditions at local scale tells the exact opposite. Simultaneous changes of temperature and δ18O composition of the supergene ore-forming fluids may result in similar oxygen isotope ratios of the various supergene carbonates belonging to different generations. However, the investigation on the constraints of the depositional conditions of supergene minerals in nonsulfide deposits cannot be conducted on the mineral association only. Once again, the involvement of several factors on the local scale may control the precipitation of the mineral phases (e.g. siderite instead of smithsonite, otavite instead of greenockite, sauconite instead of fraipontite). Additionally, the identification of environmental-proxy minerals such as kaolinite and smectite group minerals, which are found in tropical and arid soils respectively, might not be straightforward, since these minerals can also be precipitated from hydrothermal fluids (e.g. fraipontite and sauconite). The importance of local scale factors in humid and tropical uplifted areas is testified by the marked difference between the weathering profiles of Mina Grande and Cristal deposits. Such differences mainly reside on the capability of host rocks to store CO2 under an acidic alteration enviroment (dolomite > calcite), which directly affects the pH of meteoric solutions, and on the altitude(uplift)-dependence of the runoff rates. The combination of these factors may result in a local preservation of sulfide minerals, as in the case of the Cristal prospect, or in a continuous supply of fresh meteoric water and deepening of the supergene profile, as recorded in the Mina Grande deposit. All these mechanisms control the genesis and development of supergene profiles in a restricted range of pH-Eh conditions (from alkaline to near-neutral, and from weakly reducing to oxidizing), where the formation of a limited suite of minerals is favoured (mainly metal-bearing carbonates). In conclusion, this study has confirmed that paleoclimatic switch-overs from seasonally humid/arid to hyperarid periods (e.g. in the Skorpion-Rosh Pinah district) might be the most favourable scenario for the formation and preservation of supergene world-class nonsulfide deposits. However, the present research has also denied some theories about the poor possibility of preserving supergene profiles under tropical weathering. The coexistence of many factors and their complex interplay during sulfide oxidation in modern tropical areas may generate local conditions of preservation (e.g. Cristal) or instability (e.g. Mina Grande). C-O stable isotope analyses are useful to roughly estimate these local conditions, but they do not provide data on the availability of water and its recharge capability. Therefore, any attempt to present paleoclimatic hypotheses, without considering the geological background (e.g. uplift and rainfall rates, host rock lithology) of the areas hosting nonsulfide deposits may result useless. The data provided by this study on this interesting topic could be applied to other (paleo)weathering profiles worldwide, in order to build more accurate geological and geochemical models. The results of this study have been published in several papers on international Journals, and other manuscripts have been presented for publication. Several parts of this PhD thesis correspond to the content of the manuscripts quoted below: Arfè, G., Boni, M., Balassone, G., Mondillo, N., Hinder, G., Joachimski, M., 2017. New C-O isotopic data on supergene minerals from the Skorpion and Rosh Pinah ore deposits (Namibia): Genetic and paleoclimatic constraints. Journal of African Earth Sciences, 126, 148-158. Contributions of G. Arfè to the paper: samples preparation, petrological characterization, part of geochemical data collection, model interpretation of the data. G. Arfé wrote the first version of the manuscript that was revised and integrated by the co-authors. Arfè G., Mondillo, N., Balassone G., Boni, M., Cappelletti, P., Di Palma, T., 2017. Identification of Zn-bearing micas and clays from the Cristal and Mina Grande - Bongará zinc deposits (Amazonas region, northern Peru). Minerals, 7, 214; DOI: 10.3390/min7110214. Contributions of G. Arfè to the paper: on-site samples collection, samples preparation, petrological characterization, part of mineralogical geochemical data collection. G. Arfé strongly contributed to the interpretation of the data and to the drafting the first version of the manuscript, then revised and integrated by other co-authors. Arfè, G., Mondillo, N., Boni, M., Balassone, G., Joachimski, M., Mormone, A., Di Palma, T., 2017. The karst-hosted Mina Grande nonsulfide zinc deposit, Bongará district (Amazonas region, Peru). Economic Geology, 112, 1089-1110. Contributions of G. Arfè to the paper: on-site samples collection, samples preparation, petrological and mineralogical characterization, part of geochemical data collection. G. Arfé elaborated the interpretation of the data, and wrote the first version of the manuscript, then revised by other co-authors. Arfè, G., Mondillo, N., Boni, M., Joachimski, M., Balassone, G., Mormone, A., Santoro L., Castro Medrano E. The Cristal Zn prospect (Amazonas region, Northern Peru). Part II: An example of supergene zinc enrichments in tropical areas. Ore Geology Reviews, (in press). DOI: 10.1016/j.oregeorev.2017.11.022. Contributions of G. Arfè to the paper: on-site samples collection, samples preparation, petrological and mineralogical characterization, part of geochemical data collection. G. Arfé elaborated the interpretation of the data, and wrote the first version of the manuscript, then revised by other co-authors. Balassone, G., Nieto, F., Arfè, G., Boni, M., Mondillo, N., 2017. Zn-clay minerals in the Skorpion Zn nonsulfide deposit (Namibia): Identification and genetic clues revealed by HRTEM and AEM study. Applied Clay Sciences, 150, 309-322. Contributions of G. Arfè to the paper: samples selection and part of the mineralogical study (e.g. preliminary interpretation of XRD patterns). Mondillo N., Arfè G., Boni M., Balassone G., Boyce A., Joachimski M., Villa I.M. The Cristal Zn prospect (Amazonas region, Northern Peru). Part I: New insights on the sulfide mineralization in the Bongará province. Ore Geology Reviews, (in press). Contributions of G. Arfè to the paper: on-site samples collection, samples preparation, part of the mineralogical and petrological characterization, C-O stable isotope analyses. G. Arfé participated to data interpretation and to manuscript drafting with other co-authors.

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