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.
