Putzolu, Francesco
(2021)
Mineralogy and geochemistry of Ni Co in lateritic profiles.
[Tesi di dottorato]
| Item Type: |
Tesi di dottorato
|
| Resource language: |
English |
| Title: |
Mineralogy and geochemistry of Ni Co in lateritic profiles |
| Creators: |
| Creators | Email |
|---|
| Putzolu, Francesco | francesco.putzolu@unina.it |
|
| Date: |
February 2021 |
| Number of Pages: |
281 |
| Institution: |
Università degli Studi di Napoli Federico II |
| Department: |
Scienze della Terra, dell'Ambiente e delle Risorse |
| Dottorato: |
Scienze della Terra, dell'ambiente e delle risorse |
| Ciclo di dottorato: |
33 |
| Coordinatore del Corso di dottorato: |
| nome | email |
|---|
| Maurizio, Fedi | maurizio.fedi@unina.it |
|
| Tutor: |
| nome | email |
|---|
| Boni, Maria | UNSPECIFIED | | Balassone, Giuseppina | UNSPECIFIED | | Mondillo, Nicola | UNSPECIFIED |
|
| Date: |
February 2021 |
| Number of Pages: |
281 |
| Keywords: |
laterite; Nickel; Cobalt; Wingellina; Santa Fe' |
| Settori scientifico-disciplinari del MIUR: |
Area 04 - Scienze della terra > GEO/09 - Georisorse minerarie e applicazioni mineralogico-petrografiche |
[error in script]
[error in script]
| Date Deposited: |
18 Feb 2021 09:53 |
| Last Modified: |
07 Jun 2023 10:24 |
| URI: |
http://www.fedoa.unina.it/id/eprint/14031 |
Collection description
Nickel (Ni) and cobalt (Co) laterites are supergene deposits formed under tropical climatic conditions, through the weathering of olivine-bearing mafic to ultramafic rocks. The mineralogical and geochemical study of laterites has been always considered to be a knotty challenge, due to their complex mineralogical assemblage that commonly contains amorphous to poorly crystalline ore-carriers. These features also represent a challenge for mining companies when dealing with the classification, processing and metallurgy of lateritic ores. Laterite ores represent viable targets for the supplying of Co, whose vast majority of global production is related to high-risk countries (i.e. Democratic Republic of Congo). Therefore, the studies on the Co deportment and its spatial distribution within laterites are crucial to unlock thousands of tonnes of Co reserves in countries that are more geopolitically and socially stable. Accordingly, the main goals that have been pursued in this study are:
(i) The achievement of an increased knowledge of the metallogenetic processes of the diverse mineralized facies in Ni-Co laterite deposits;
(ii) The study of the challenging aspects when dealing with the evaluation of the modal mineralogy and metal deportment of laterite ores;
(iii) The assessement of the influence of the pre-lateritic and syn-lateritic alteration processes on the Co-endowment in laterite ores.
To pursue these aims, two oxide-type laterites were considered: Wingellina (Musgrave Province, Western Australia) and Santa Fe' (Goiás Province, Central Brazil) deposits. The Wingellina Ni-Co deposit is located close to the border between Northern Territory and Western Australia and likely formed during the episodic weathering events occurred in the Australian continent during Mesozoic to Tertiary. The Santa Fe' Ni-Co laterite is a major undeveloped deposit located in the Goiás State of Central Brazil, which was originated during the Eocene to Oligocene and Miocene-Pliocene stages of tropical weathering.
Significant differences in terms of the nature of parent rocks, alteration histories and ore distribution can be recognized between the selected deposits. The Wingellina deposit formed from the alteration of a Mesoproterozoic mafic to ultramafic intrusion (Giles Complex) with tholeiitic affinity, which comprehends dunite, peridotite, and pyroxenite in the primitive members, and olivine gabbro, gabbronorite, and anorthosite in the relatively more differentiated units. The Santa Fe' laterite formed from a Cretaceous ultramafic-dominated intrusion representing the plutonic portion of a large alkaline province (South Goiás Alkaline Province). The intrusive body consists of a dunite core surrounded by peridotite, pyroxenite and minor gabbro.
The pre-lateritic history of the Wingellina hill intrusion is marked by a first stage of high-pressure and high-temperature metamorphism, which was later superimposed by a lower T° hydrothermalism leading to the serpentinization of the ultramafic units. When the lateritic processes superimposed the afore-mentioned processes, in the lowermost section of the weathering profiles Ni-bearing-smectite clays developed after the alteration of ferromagnesian minerals and of former layered silicates, while in the oxide unit Co-rich Mn-oxy-hydroxides formed after clay minerals. In the Santa Fe' deposit, the early greenschist facies alteration, leading to the serpentinization of the parent rock, has been later overprinted by an amphibolite facies assemblage consisting of secondary spinels, Ni-rich chlorite, sulfide and amphibole. At Santa Fe', Ni and Co have atypical geochemical footprints, not correlated with geochemical proxy (i.e. Mn and Fe) indicating a typical a lateritic enrichment. At Wingellina Mn-oxy-hydroxides are the main Ni and Co carriers in the oxide ore, with lithiophorite and asbolane (with their intermediates) representing that main phases. The lithiophorite-asbolane intermediates occur as late replacements of former Mn-phases, which have a lower ore grade and a high content of more mobile elements (as Ba and K), indicating that the Co enrichment in Mn-oxy-hydroxides was controlled by a multistage diagenetic evolution.
Whole-rock geochemical survey enabled to evaluate the Ni-Co enrichment in the Wingellina profile, and its High-Tech metals (REE and Sc) potential. The Ni enrichment was mostly controlled by the paragenetic evolution of the laterite itself, whereas the Co enrichment was related to the formation of Mn-oxy-hydroxide-rich horizons in the oxide orebody. Significant REEs concentrations have been observed within the oxide zones, where Ce is decoupled from the other REEs, reflecting either a variable enrichment process among the different REEs and a heterogeneous mineralogy of the REEs-bearing phases. The REEs concentration was found to be higher in the gabbro-derived laterite than in the serpentinite-derived profile due to the higher REE concentration in the more differentiated units of the bedrock and to the higher LREE fractionation in the profile, which likely resulted in the formation of neoformed LREE-bearing phases. Scandium is only partially correlated with Fe2O3, having been most commonly detected in Ni- and SiO2-rich zones of the saprolite, reflecting its association with epigenetic Fe-oxy-hydroxides derived from relict olivine and/or its adsorbtion into clays.
The study of the Ni-phyllosilicates in the Wingellina saprolite ore was aimed to understand the role of diverse bedrocks (gabbro and serpentinite) and the influence of climate on saprolitization process. In the same suite of samples the mineralogical survey was refined by an investigation with TEM-HRTEM, in order to understand the minerogenetic process of Ni-bearing clays at the nanoscale. The main phyllosilicate in the gabbro-derived saprolite consists of a Al- and Ni-rich montmorillonite, derived from the weathering of pyroxene. In the serpentinite-derived saprolite, the alteration pathway followed a multistage evolution: (i) lateritic alteration of serpentine into smectites (saponite and nontronite); (ii) late precipitation of interstratified Ni-clays as a replacement of former phyllosilicates and as neo-formed minerals. The latest phenomenon is related to the late Miocene shift from humid-and-acid (tropical climate) to saline-and-alkaline conditions (arid to semi-arid climate), and represents an example of post-lateritization processes, rarely observed in laterites located on stable cratons. The aridity-driven precipitation of Ni-rich interstratified clays in the Wingellina laterite can be seen as a new model for explaining the evolution of Ni-laterites in stable cratonic settings experiencing post-lateritization aridity. The nanoscale TEM-HRTEM imaging indicates that Ni-bearing clays occur either as a replacement of the former clay suite (i.e. serpentine and chlorite) or as porous clay aggregates, with the latter occurring in close association with nanometric Co-rich Mn-oxy-hydroxides. This finding provides a better understanding of the early stage formation of Mn-oxy-hydroxides in laterite systems, which were earlier observed only onto kaolinite templates. The detection of nanometric Mn-oxy-hydroxides is also important to constrain the Co distribution and targeting within unconventional laterite ore facies.
In the Wingellina deposit, the occurrence of ore minerals characterized either by complex crystal structures (e.g. smectite and certain Mn-oxy-hydroxides) or by poor structural order (e.g. Fe-oxy-hydroxides), restrict the efficiency of XRPD. In this frame, automated Scanning Electron Microscopy (automated SEM) methods, based on the chemistry and density of various mineral species, offer breakthroughs in defining the mineralogy and the metal deportments, bypassing the issues related to the presence of phases not evaluable by using solely XRPD-based approaches. In this study is demonstrated that the main benefit of using automated SEM is the possibility to detect and identify Ni-Co-bearing Mn-oxy-hydroxides (lithiophorite, lithiophorite-asbolane and asbolane) and Ni-bearing clays, which can be hardly distinguished through XRPD because of their poorly crystalline structure and because of their very similar diffraction features. Automated SEM also allowed the determination of the Co deportment, which is essential to predict the optimum achievable metals recoveries.
The Santa Fe' deposit is characterized by both oxide- and phyllosilicate-dominated ore zones. The oxide ore consists of Ni- and Co-bearing Fe-oxy-hydroxides, while Mn-oxy-hydroxides occur as minor components. In the saprolite, the major Ni-carrying phase is chlorite. Multivariate statistical analyses conducted on the drillcore assays showed that the Ni distribution is not controlled by in situ weathering. This is because the highest Ni concentrations have been measured in laterite horizons where Ni-rich minerals (chlorite and goethite) are associated with reworked material. Cobalt has an atypical statistical distribution at Santa Fe' if compared with Wingellina, being correlated not only with Mn, but also with Cr. From microchemical analyses it was observed that residual spinels contains significant elevated Co contents. This feature is atypical for Ni-Co laterite deposits, where Co is normally associated with Mn in supergene oxy-hydroxides. In the Santa Fe' case, the Co concentration in spinels is related to magmatic and post-magmatic processes occurring prior tolateritization, specifically: (i) orthomagmatic enrichment of Co in chromite, due to its high affinity to spinels in alkaline melts; and (ii) trace elements (i.e. Co, Mn, Ni and Zn) redistribution during amphibolite facies metamorphism of chromite into ferritchromite. Based on the interpretation of metallurgical testworks, a fraction of total Co (up to 50%) is locked in spinels.
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