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CENSZ – Critical Elements in Nonsulphide Zinc Deposits

Periodic Reporting for period 1 - CENSZ (CENSZ – Critical Elements in Nonsulphide Zinc Deposits)

Reporting period: 2015-07-01 to 2016-06-30

The definition “Critical Elements” (CE) was assigned by the “European Union AdHoc Working Group” to those raw materials that are characterized by high supply risk and their high economic importance for EU economy. The metals Gallium, Indium and Germanium (GIG) occur in a number of different natural deposits from which they are currently recovered as secondary metals. There is currently no collective database defining the distribution of these elements systematically and thus the supply of the metals is linked to the individual mine from which they are currently recovered. The central aim of this project ‘CENSZ’ is to provide a comprehensive database of GIG deportment within Nonsulphide Zinc+Lead (NSZ) deposits, that form as a result of geological processes in the Critical Zone of the Earth’s crust (The Earth’s surface crustal environment). Sulphide deposits that are progenitors of supergene NSZ mineralisation contain sphalerite which is a known host for the GIG elements. The factors determining the geochemical distribution of GIG within weathering profiles are still only poorly understood. Gaining an understanding of the mechanisms governing mineralogical residence is fundamental to unlocking potential new processing streams.
The importance of metals in society is increasingly recognised within the EU as reported in 'The Raw Materials Initiative - Meeting Our Critical Needs for Growth and Jobs in Europe', The European Technology Platform on Sustainable Mineral Resources and the European Innovation Partnership on Raw Materials. As the EU is highly dependent on imports of CEs, new strategies are now needed to enhance resource efficiency, recycling and reuse. The security of supply of CE is particularly acute, due to the lack of substitutes and because the resource presently residing in the anthroposphere is far too small to meet current demand through recycling alone, let alone address the increasing future demands. The EU recognises the need for research projects and training which promote innovative approaches to exploration, extraction and beneficiation and which enhances the knowledge base of mineral deposits both within the EU and further afield. The underpinning aim of such actions is to achieve greater efficiency in the utilization of the primary resource base which, if not addressed, will lead to a diminished effectiveness of the European mining industry and the high technology manufacturing ability of the EU.
The elements GIG have all been highlighted in studies concerning security of supply of key commodities for European High Tech industries. Gallium is used in thin layer photovoltaics, intergrated circuits and white-light LEDs. Indium is used in displays and thin-layer photovoltaics, whilst germanium is used in fibre-optic cables and IR technologies. Many of such concerns are the result of their recovery as by-products from other processes. As such they lack their own production infrastructure, making both their current and future supply vulnerable to the changing fortunes of extraction operations for another commodity that effectively determines the potential viability of CE recovery. Complicating the supply position further is the fact that these metals are in part recovered from geographically restricted regions, making their supply vulnerable to changing political and economic factors.
This project aimed to perform detailed mineralogical and geochemical studies of examples of known deposits, many within Europe. The key questions of this study were: 1) What is the fate of GIG from progenitor sulphides during the formation of NSZ deposits? 2) What is the behaviour of GIG under different geochemical conditions in the critical zone? 3) What are the climatic conditions most favourable for NSZ deposit formation? 4) Which of these deposits might be more amenable to low impact processing options?
The specific objectives of the project were:
Objective 1 - To determine the mineral residence of GIG in NSZ deposits: Targeting of Ga, In, Ge in NSZ deposits developed in the Critical Zone, and from NHM collections.
1. Sample selection from NHM Collections - Nonsulphide specimens from several localities of Europe, North America and Africa were sampled. However, the work was particularly focused on the Kabwe deposit (Zambia), which is well-known for the high Ge contents in the primary sulphides.
2. Targeted fieldwork collection - Fieldwork was conducted to sample the Rio Cristal (Bongarà) NSZ deposit (Peru), in collaboration with researchers of University of Naples (Italy).
3. Whole rock geochemical analyses - Chemical analyses of the samples from the Rio Cristal (Bongarà) deposit were obtained from a commercial lab (Bureau Veritas Commodities Canada Ltd.). Chemical analyses on the NHM Collection’s specimens were obtained from the NHM chemistry labs.
4. Whole rock mineralogical analyses - The Collection’s specimens were fully analyzed at NHM, whereas mineralogical analyses of the Rio Cristal (Bongarà) deposit were carried out at University of Naples (Italy). XRD on powdered samples and SEM-EDS analyses were carried out for mineral identification.
5. Analysis of mineral separates - Analyses on mineral separates from Kabwe and Rio Cristal (Bongarà) deposits were conducted at LA-ICP-MS. More than 400 LA-spot analyses were carried out. Bulk rock mineralogical and chemical analyses on the other specimens from the Collection have shown low GIG concentrations and the absence of favourable minerals for LA analysis.

Objective 2 - To define possible geochemical models for pathways of GIG from hypogene minerals to deposits in the Critical Zone: Mineral chemical models for NSZ deposits.
1. Targeted fieldwork collection - Fieldwork was conducted to sample the Rio Cristal (Bongarà) Zn nonsulphide deposit (Peru), in collaboration with researchers of University of Naples (Italy). The analysed drillcore samples derived from both the supergene (weathering-related) orebody and the hypogene (sulphide-rich) zone. Hypogene minerals were also collected from the NHM Collection.
2. Whole rock geochemical analyses - Chemical analyses of the samples from the Rio Cristal (Bongarà) deposit were obtained from a commercial lab (Bureau Veritas Commodities Canada Ltd.). Chemical analyses on the NHM Collection’s specimens were obtained from the NHM chemical labs.
3. Whole rock mineralogical analyses - mineralogical analyses of hypogene assemblages have been carried out by using XRD, SEM-EDS-WDS and LA.
4. Construction of geochemical models - No specific geochemical models for supergene alteration of Kabwe and Rio Cristal (Bongarà) deposits have been constructed, because our results confirm the formation of typical alteration assemblages seen elsewhere in NSZ deposits and we found the relevant critical elements (GIG) in the minerals where they were expected to be. Simple geochemical rules were used to verify the extension of the minerals stability fields and their compatibility with the GIG mobility in the supergene environment.

Objective 3 - To investigate if the mineral residence of GIG in NSZ deposits is climate-controlled: Compilation of evidence for climate control.
Oxygen isotopes data have been measured, during the secondment at SUERC (Glasgow - Scotland), on carbonates and silicates from the Kabwe deposit and on carbonates from the other districts selected from the NHM Collection. Oxygen isotopes analyses of carbonates from Rio Cristal (Bongarà) deposit were carried out in collaboration with Universities of Naples (Italy) and Erlangen (Germany). There is a climatic control on the mobility of some elements in the supergene environment.

Objective 4 - To develop new mineralogical models for the distribution of critical metals as a guide for the development of new processing technologies: Development of mineralogical models for processing.
Mineralogical models have been completed. For the leach tests on the Rio Cristal (Bongarà) deposits, to find the right way to extract Ge from the supergene minerals, we have tried different leaching agents and different leaching time period. The tests are still ongoing and form part of continuing collaboration with Professor Herrington at the NHM.

The analyses revealed that in carbonate-rich deposits GIG have not become concentrated during secondary processes in newly formed phases, whilst Ge can become strongly enriched in silicate-rich NSZ deposits. This is not surprising, since Ge has geochemical affinity to both Si4+ and Fe3+ which can lead to concentration of this element in both Zn-silicates and Fe-oxy-hydroxides. More interestingly, we discovered that this rule is not always respected: Zn-silicate-bearing deposits formed under arid or temperate climates seem to not show any Ge concentration in secondary minerals, whereas NSZ deposits formed under strongly humid environments retain Ge grades in the secondary phases when weathered from sulphides to nonsulphides.
In previous work it was evident that germanium has a geochemical affinity to Si4+ and Fe3+: this means that Ge may become concentrated in both silicate minerals and Fe-(hydr)oxides. Despite this, Ge enrichments in these minerals were rarely observed in true deposits. However, we have demonstrated that high Ge concentrations in silicates and Fe-(hydr)oxides are not uncommon, and that Ge amount in these minerals can be at a level to result in bulk rock values that can be considered of economic interest for Ge recovering as by product of Zn extraction.