BRGM - Our Work

This part of the work was realised in association with the team of the Institute of Geology and Geochemistry (IGG, Ekaterinburg).

Uralian-type (or Alaskan-type) complexes are mafic-ultramafic complexes, generally interpreted as cumulates formed in a magma chamber, and notably characterised by the presence of platinum group element (PGE) mineralisation. A large number of zoned ultramafic complexes have been identified in the central and north Urals as the source of the platinum placers known since the 19th Century and of primary platinum mineralisation at the end of the 19th Century. The Pt placer production is estimated at 160 t. Primary mineralisation was mined until 1935 but the production is unknown. Today exploration is still active in this area.

These complexes occur in the Tagil zone north of Ekaterinburg, close to the Main Uralian fault. Primary PGE mineralisation is invariably associated with chromite, with no base metal sulphide and Pt dominant over the other PGE, restricted to the core dunite of complexes and interpreted as being of magmatic origin. Contrary to the relatively simple evolution of the host dunite-chromitite, the PGE mineralisation seems to have undergone a complex evolution. It is generally accepted that PGE concentration in chromitites results from the accumulation of platinum-rich alloys that segregated directly from the melt at an early stage in the evolution of the complex, followed by metasomatic replacement and remobilisation of primary platinum alloys. Some Russian workers invoke a major role played by metasomatic processes in the formation of the PGE-rich chromitite, processes that also affected the host dunite.

Many papers have been devoted to the Pt mineralisation of the Alaskan type complexes, and especially of the Urals Pt belt. They deal mainly with mineralogy of the Pt placers, some comparing placer and primary mineralisation. Our approach of the problem was meant to be broader, i.e. to understand the mineralisation in its magmatic and/or hydrothermal context. We thus focused on the two Nizhny Tagil and Kachkanar complexes, studying their PGE content and the mineral chemistry of various dunites and chromite occurrences, and the characteristics of the PGM.

• Analytical techniques

For whole-rock samples, PGE and Cr were analysed by ICP/MS. Polished sections were examined under the microscope in reflected light and all the grains presenting a high reflectivity were checked with a SEM equipped with an EDS. Subsequently, the PGM grains were analysed with a Cameca SX50 microprobe. The following work has been realised:

The Nizhny Tagil massif has been mined for placer deposits but also for primary mineralisation. The massif has a general elliptical shape, and consists of a dunite core with a narrow rim of wehrlite, and an outer rim of magnetite-bearing pyroxenite.

Primary mineralisation is associated with chromitite concentrations. The primary ore was mined from three main mines: Alexandrovsky (open-pit and underground workings), Krutoy Log (type locality) where several veins were mined from open pits and underground, including the Siribrikov vein, and Goshachta (eluvial and underground mine). Sirkov Log was mined for eluvial concentrations, but also for primary ore with surface working. Chromitite recently was discovered in the Solovyovsgork dunite quarry.

The best outcrops are found in the Solovyovsgork quarry, whereas only rare chromitite-rich samples were collected in the Alexandrovsky open pit, in the Siribrikov vein, in the Goshachta quarry and in the Sirkov Log.

Some nuggets were also collected from the Sirkov Log and a placer concentrate from the Chaush River.

Thus 16 samples where prepared for PGM study. PGMs were identified in eight polished sections. Nuggets and placer concentrates were studied with a SEM and then the grains were polished and studied.

• The Kachkanar massif

The Kachkanar massif is well known for huge magnetite deposits associated with hydrothermally altered pyroxenite and gabbro. Two dunite bodies are known in the Kachkanar massif, though apparently tectonically disconnected from the magnetite-bearing gabbro: the northern, Veressovoborsky, and southern, Svetloborsky, ones both were the source of placer and eluvial platinum concentrations. A restricted number of chromite schlieren, source of the PGM, have been sampled; primary schlieren are exploited in trenches in the Veressovoborsky dunite. Two schlieren from Svetloborsky were studied (Svietly Bor locality), but no PGM were found, whereas many PGM occur in the presently mined vein of the Veressovoborsky dunite.

• Petrology of host rocks and Results for PGE in whole rock samples

A total of 15 dunites (Cr2O3 <0.55 wt.%), 7 chromitiferous dunites (1.0<Cr2O3<10.0 wt.%), 4 chromite ores (Cr2O3>10 wt.%) and 3 pyroxenites from different mines in the Nizhny Tagil and Kachkanar massifs were analysed for PGE and Cr₂O₃.

The host rocks are dunite and peridotite with the following characteristics:

• Dunite: Olivine Fo 89.8-92.1, CaO 0.25 wt.%; no fractionation; accumulation of olivine from a mafic magma in an open-system magma chamber
• Pyroxene peridotite (rim): Olivine Fo 77.9 - 88.8; differentiation starts with the formation of the rim, corresponding to the closure of the magmatic system, after solidification of the dunite core.

The Pt content in dunite varies between 0.3 and 35 ppb, except in one sample (98 ppb) collected within the mineralised zone. For chromitiferous dunite, the Pt content is very heterogeneous, varying between 1.4 and 3248 ppb Pt, and not at all correlated with the Cr₂O₃ content. The same observation is made for chromite ore where Pt content varies between 96 and 7902 ppb. This confirms the data by Volchenko (pers. comm., 2001) mentioning the existence of "barren" chromitite, in terms of PGE. The content of the other PGE is also extremely low in dunite and in chromitiferous dunite. Chromitites show only slightly higher values in Ir, Ru and Rh, but there is no correlation in the PGE contents either.

The chondrite-normalised PGE pattern (Figure 1) suggests that the chromitiferous dunites and the Pt-rich chromitites underwent the same mineralising process and that the Pt-rich dunite belongs to the mineralised system.

• Platinum-group minerals

All PGM have been found in chromite-rich samples, and not only in major chromitite concentrations like those that are mined, but also in isolated small schlieren, very common in the dunite unit. The main minerals encountered are Pt alloys, Ir alloys and Rh minerals.

Pt alloys are the most common PGM encountered and represent more than 90% of the mineral species identified. There is a very large range of composition for PGM alloys having Pt as major element, and some are obviously the result of replacement process of pre-exiting composition. Moreover, some of them contain tiny inclusions and exsolutions of other PGE composition, rendering the interpretation of EPMA results sometimes difficult. Five types of Pt alloys have been determined: isoferroplatinum, tetraferroplatinum, Pt₂ (Fe,Ni,Cu), PtFeNi₂, PtCu₅.

Iridium occurs in solid solution in most Pt alloys in various proportions (from 0.2 to 10 wt.%). Ir has also been found in placer deposits where it occurs as inclusions in Pt-Fe alloys, but also as discrete grains, commonly adjacent to a Pt-Fe alloy containing Ir inclusions. Composition of the discrete grains shows the systematic presence of Pt (6.1 to 9.7 at.%), Fe (1.5 to 11.55 at.%) and Cu (around 2.5 at.%) and also of Os and Ru in varying proportion.

Two distinct Rh minerals have been recognised. One corresponds to a sulfarsenide, close to ideal hollingworthite RhAsS or with presence of iridium (up to 5.61 at.%). The second composition corresponds to an arsenide, without sulphide, strongly enriched in Ni compared to hollingworthite. Its general formula can be written as RhNiAs.

• Conclusion - Model of formation

Three modes of occurrence have been recognised for the PGM. They are either:

1. Totally included in chromite crystals, and therefore must have been protected from secondary processes. There is a difference of composition between the Nizhny Tagil (mostly tetraferroplatinum) and Kachkanar massif (isoferroplatinum);
2. At the grain boundary between chromite and silicate or in chromite fractures,
3. In the silicate matrix, where their composition is partly determined by their mode of occurrence. Note that in the Kachkanar massif, where only four samples have been studied, the PGM are included in chromite crystals.

Types 2 and 3 belong to the same Pt (Fe, Ni, Cu) type and present a large range in the Fe-Ni-Cu proportions. Many grains show traces of destabilisation and transformation

The primary mineralogy of PGE-rich facies and their host is relatively simple: olivine, chromite, and rare clinopyroxene, and is considered to be magmatic-derived. The only superimposed events are serpentinisation and carbonation, with abundant serpentine minerals and veins of calcium, magnesium and iron carbonates. There is no evidence of any other hydrothermal event in the samples studied, as was described for example by Garuti et al. (2002). The PGE and PGM data suggest that, after early crystallisation, the PGM were trapped in chromite crystals. The accumulation of chromite crystals (and associated PGM) occurred in conduits where chromite and PGE became concentrated (work in progress). Serpentinisation provoked the release of Ni and Cu that partly reacted with the PGM to form the complex assemblage described here.

The following steps are considered for the formation of the PGM accumulations:

1. Ultramafic core : accumulation of olivine and minor chromite (to form the schlieren ore) in a magma chamber permanently replenished.
2. Pyroxenite rim: once the chamber begins to solidify, the magma tends to be injected at the periphery and crystallizes in a more closed system,
3. Chromitite: dynamic accumulation (textures and composition) of chromite crystals in cavities along magma conduits or "mini-chambers" inside the dunite body. The cavities in the consolidated part of the dunite body were created by a magma injection in the system, according to a model largely inspired from the ophiolite podiform chromitite.

It is clear that there is a systematic association of chromite and PGM and that both crystallised from the same magma. The crystallisation of PGM and chromite is interpreted as occurring in "mini chambers", with chromite crystals trapping the PGM (Figure 2).

To increase the efficiency of this mechanism (mechanical trapping) a high amount of fresh magma (supplying the PGE) in equilibrium with the chromite is required. This condition can easily be obtained in the ‘mini chambers’. It is possible to define a CR factor: volume of magma (passing in the chamber) / volume of chromite deposited in the chamber. A high CR factor is associated with PGE-rich chromite deposits, where the mechanical collection of the PGE by chromite is maximum. A low CR factor is associated to PGM diluted in the chromitite, and low PGE concentration due to a ‘dilution’ effect.