Main Sulfide Zone Research Articles

Geophysical mapping of the occurrence of platinum group elements in the main sulphide zone of the Great Dyke in Zimbabwe

This work sought to establish the geophysical signature of platinum group elements in a mineralized sulphide host rock. It has been established that complex resistivity of rocks depends on frequency. The variation of resistivity magnitude and phase for the samples were taken for frequencies in the range 10−2 to 102 Hz. In the first part, the mathematical relationship between resistivity amplitude and phase with frequency was obtained based on the Cole-Cole model equation. A MATLAB Code based on the direct inversion of the apparent resistivity spectrum was used to extract the “Cole-Cole” parameters. In the second part an experimental procedure to obtain the data was designed and a discussion of the results obtained was made. Overally, the results confirm the model to be a good candidate for use in mapping the occurrence of platinum group elements in sulphide zones. Its frequency dependency parameter c is the critical anomaly detection parameter. Several efforts to solve this problem by other geophysical methods failed to yield the desired or useable results. The reason was the complex and inverse nature of the problem. The practical method chosen from a host of geophysical methods was complex resistivity measurements based on the “Cole-Cole model”. The knowledge gap being bridged in this study involves the use of this method to detect the base of the main sulphide zone.

Petrographic and mineral chemistry evidence for the origin of sulfide mineralization in the Main Sulfide Zone, Sebakwe Subchamber of the Great Dike, Zimbabwe

The Great Dike, one of the largest repositories of platinum-group elements (PGEs) and largest layered mafic intrusions in the world, is currently being exploited for its PGEs. We undertook a petrographic and mineral compositional investigation in the Sebakwe Subchamber to place some constraints on the genesis of its PGEs. Sulfides, which host some of the PGEs, occur either as inclusions within medium-grained euhedral orthopyroxenes or in association with biotite and intercumulus plagioclase. Sulfides which occur as interstitial phases or as inclusions in silicates are consistent with a magmatic origin of PGE sulfide mineralization. However, some sulfides which occur in association with biotite, are consistent with concentration of PGEs by magmatic fluids. Average sample Mg# range from 83.5–80.1 for orthopyroxenes, 93.1–84.4 clinopyroxenes, and 79.2–76.7 for phlogopite, whereas average sample An# for feldspars range from 81.3 to 5.9. Pyroxene thermometry yields temperatures of 837–986 °C that are late magmatic, with biotite thermometry yielding temperatures of 478–645 °C that are interpreted to be consistent with recrystallization of earlier primary biotite during hydrothermal alteration by late- to post-magmatic fluids. Observations from petrographic and backscattered electron images are consistent with these interpretations. Primary, magmatic PGE mineralization is inferred to have been cogenetic with sulfides which occur either as inclusions within, or as intercumulus phases between, orthopyroxenes. Sulfide minerals coexisting with biotite likely represent primary, magmatic sulfides that were remobilized by late- to post-magmatic fluids. The inferred depth of crystallization and hydrothermal alteration of the Sebakwe Subchamber of ∼10 km, is broadly comparable to other Subchambers of the Great Dike, lending support to the view that similar processes operated in the uppermost part of Great Dike ultramafic rocks in all its Subchambers. The involvement of fluid migration in the possible remobilization and concentration of primary magmatic PGE-mineralization in most layered mafic intrusions where it has been documented, must be a more significant process in concentrating PGEs in layered intrusions than currently thought.

Formation of the main sulfide zone at Unki Mine, Shurugwi Subchamber of the Great Dyke, Zimbabwe: Constraints from petrography and sulfide compositions

AbstractThe major platinum group element (PGE) occurrence in the Great Dyke of Zimbabwe, the main sulfide zone, is a tabular stratabound layer hosted in pyroxenites, and it is broadly similar in form throughout the length of the Great Dyke. We conducted a petrographic and sulfide composition study on a sulfide-enriched zone from the contact of the mafic sequence–ultramafic sequence through the main sulfide zone at Unki Mine in the Shurugwi Subchamber to its underlying footwall rocks to place some constraints on the origin of the rocks.Pyrrhotite, pentlandite, chalcopyrite, and pyrite are the base metal sulfides that were encountered during the study. Pyrrhotite, pentlandite, and chalcopyrite typically occurred as inclusions in both primary (orthopyroxene, plagioclase, and clinopyroxene) and secondary (amphibole and chlorite) silicate phases, whereas pyrite was observed in only three samples, where it occurred in association with pyrrhotite. The concentrations of PGEs in the base metal sulfides were nearly all at or below minimum detection limits. The intercumulus nature of some of these sulfides in the investigated sequence suggests that they were likely formed during the crystallization history of these rocks. The occurrence of pyrite, which we interpret to be an alteration phase, suggests that a late-stage event, likely formed during hydrothermal alteration, helped to concentrate the mineralization at Unki Mine. In some cases, however, these sulfides occur partially surrounding some chromite and silicate phases. Thus, some sulfides in the Unki Mine area were likely formed early in the crystallization history of the Great Dyke, whereas others were formed late during hydrothermal processes. Low concentrations of PGEs such as platinum (Pt), palladium (Pd), and rhodium (Rh) in base metal sulfides imply that the PGEs in the main sulfide zone and Unki Mine are hosted either in silicates and/or platinum group minerals. Very low Co contents in pentlandites in the rocks under investigation are interpreted to imply that very limited Fe substitution by Co, and also of Ni by Co, occurred. Broadly comparable trends, with minor variations of Fe in pyrrhotite, of Co and Ni in pentlandite, and of Cu in chalcopyrite, for example, likely reflect magmatic processes. The concentrations of these metals in base metal sulfides vary sympathetically, indicating that their original magmatic signatures were subsequently affected by hydrothermal fluids. The spiked pattern displayed by the variations in the percent modal proportions of the base metal sulfides across the entire investigated stratigraphic section is interpreted to reflect remobilization of the sulfides during hydrothermal alteration. Depletions in some elements, which occur near the base and at the top of the investigated succession, are likely a result of this hydrothermal alteration.

The Lower Banded series of the Stillwater Complex, Montana: whole-rock lithophile, chalcophile, and platinum-group element distributions

The principal rock types of the Lower Banded series of the Stillwater Complex are gabbronorite and norite with minor troctolite and anorthosite. The whole-rock composition is largely controlled by the cumulate minerals plagioclase and pyroxene (±olivine). The rocks are very poor in incompatible elements, at the 0.1 to 2 times mantle levels, indicating < 10% liquid fraction. The Pd and Pt are enriched in a layer known as the J-M Reef which occurs in a zone containing olivine-bearing rocks (OB I). This zone contains the same rock types as the rest of the Lower Banded series, but troctolite, olivine gabbronorite, and anorthosite are more abundant in OB I. The concentrations of lithophile elements in OB I are similar to the rocks above and below OB I. In particular, the reef rocks show no additional signs of continental crust contamination such as additional Th or LREE enrichment, and the reef rocks are not enriched in incompatible elements. The strongly chalcophile elements Cu, Ni, Se, Bi, and Au all correlate with S, indicating that these elements are controlled by sulfides. The PGE show two trends. The rocks below the J-M Reef and the reef rocks have high PGE+Au to S ratios and fall on a single trend, whereas those above the reef have low metal to S ratios. These low ratios for the rocks above the reef could arise because the magma was depleted in PGE+Au as it had already segregated some PGE+Au-rich sulfides. The relatively high PGE, Au, Cu, and Se concentrations of the rocks below the reef imply that the rocks contain ~ 0.1 wt% cumulate sulfides. However, they have very low S contents (< 300 ppm). The low S/Se ratios of these rocks (< 2000) and most of the rocks outside of the reef suggest that they have lost more than half of their S. The Pd/Pt and Pd/Ir ratios of the reef (~ 3.4 and 800, respectively) are much higher than those of the rocks below and above the reef. The high Pd/Pt and Pd/Ir ratios of the reef imply that the sulfides formed from a fractionated magma; however, there is no correlation between the Pd/Ir and Pd/Pt ratios and the host rock type. These ratios are dissimilar to the Bushveld reefs and the Main Sulfide Zone of the Great Dyke. They more closely resemble the reefs in more fractionated rocks such as the AP Reef of the Penikat Intrusion, the Roby Zone of the Lac des Iles Complex, or the disseminated sulfides of the Noril’sk I intrusion. The very high Pd/S and low Cu/Pd ratios of the J-M Reef require that the sulfides collected Pd from a very large volume of magma. None of the current models is entirely satisfactory; however, we favor a model where at depth the OB1 magma partially melted a komatiitic massive sulfide and then transported the sulfide droplets into the magma chamber at the level of the J-M reef.

Hydrothermal Alteration in the Main Sulfide Zone at Unki Mine, Shurugwi Subchamber of the Great Dyke, Zimbabwe: Evidence from Petrography and Silicates Mineral Chemistry

The main platinum-group element (PGE) occurrence in the Great Dyke of Zimbabwe, the Main Sulfide Zone (MSZ), is a tabular stratabound layer hosted in pyroxenites. A petrographic and silicate composition study across the MSZ at Unki Mine in the Shurugwi Subchamber was conducted to help place some constrains on the origin of the mineralization. The PGE-enriched zone at Unki Mine is a ~10 m thick package of rocks ranging from gabbronorites, a chromitite stringer, plagioclase websterite, plagioclase pyroxenite (pegmatitic in one narrow zone), a base metal sulfide zone and it is largely located below the contact of the Mafic and Ultramafic Sequences. Pyroxenes have been partially hydrothermally altered to amphibole and chlorite in most lithologies. In addition, sulfides tend to occur as cumulus phases or as inclusions in all the silicate phases. Two generations of sulfide mineralization likely occurred at Unki Mine with primary sulfides occurring in association with cumulus phases, and the relatively finer-grained, often lath-like, sulfides that occur in association with alteration phases of chlorite and amphibole that were likely formed later during hydrothermal alteration. Chlorite thermometry yields temperatures ranging from 241 to 390 °C, and from 491 to 640 °C, and they are interpreted to be temperatures recording the hydrothermal event(s) of magmatic origin which affected the mineralization at Unki Mine. Two-pyroxene thermometry yields temperatures that range from 850 to 981 °C, and these temperatures are interpreted to indicate a hydrothermal imprint on the minerals that constitute the MSZ.

The Snake’s Head Platinum Project – layering, PGE mineralisation and deformation in the Musengezi Subchamber of the Great Dyke, Zimbabwe

Research Article| June 01, 2016 The Snake’s Head Platinum Project – layering, PGE mineralisation and deformation in the Musengezi Subchamber of the Great Dyke, Zimbabwe M.D. Prendergast M.D. Prendergast Guesachan, Shielhill Road, Kirriemuir, Angus DD8 4PA, Scotland, United Kingdom e-mail: marprend@hotmail.com Search for other works by this author on: GSW Google Scholar Author and Article Information M.D. Prendergast Guesachan, Shielhill Road, Kirriemuir, Angus DD8 4PA, Scotland, United Kingdom e-mail: marprend@hotmail.com Publisher: Geological Society of South Africa First Online: 20 Nov 2017 Online Issn: 1996-8590 Print Issn: 1012-0750 © 2016 June Geological Society of South AfricaGeological Society of South Africa South African Journal of Geology (2016) 119 (2): 305–328. https://doi.org/10.2113/gssajg.119.2.305 Article history First Online: 20 Nov 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M.D. Prendergast; The Snake’s Head Platinum Project – layering, PGE mineralisation and deformation in the Musengezi Subchamber of the Great Dyke, Zimbabwe. South African Journal of Geology 2016;; 119 (2): 305–328. doi: https://doi.org/10.2113/gssajg.119.2.305 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Set in ca. 74 km2 of rugged terrain close to the Zambezi Escarpment, the Snake’s Head Platinum Project covers the northeastern half of the Musengezi Subchamber in the northern part of the Great Dyke. Here, the linear Great Dyke straddles the boundary between the Zimbabwe craton and the late Archaean Migmatitic Gneiss Terrain and is folded into a prominent S-shape adjacent to the Neoproterozoic-Phanerozoic Zambezi orogenic belt. Snake’s Head contains the northernmost remnant of the Great Dyke’s P1 Pyroxenite Layer, which hosts the economically-important, stratabound, PGE-rich Main Sulphide Zone (MSZ), as well as the lower grade (but petrogenetically very similar) Lower Sulphide Zone (LSZ), together containing one of the Great Dyke’s last undeveloped platinum resources (>80 m oz).The original, gently-plunging, synclinal layered structure of the Musengezi Subchamber is preserved in the western part of Snake’s Head but is replaced in the east and north by several, contiguous, kilometre-scale structural blocks in different orientations and separated by curvilinear, brittle-ductile thrust zones suggestive of sequential stacking towards the west. Shearing and associated metasomatism increase from the generally pristine western blocks to the northern and eastern blocks where the structure is overlain above another major, south-directed thrust by the Marginal Gneiss Terrain of the Zambezi belt and the underlying, well-developed magnetite gabbro unit at the top of the Great Dyke sequence is mostly converted to mafic schist.The P1 Pyroxenite displays layer-thicknesses, mineral associations and textures, and other layering features, and the MSZ shows slight systematic variations in thickness, and in sulphide and metals contents, that all indicate (1) primary magma chamber locations varying from the axis (in the case of the western blocks) to midway between the axis and margins (eastern/northern blocks), and (2) an original magma chamber width that varied along its length but was significantly greater than in other parts of the Great Dyke. The MSZ and LSZ preserved at Snake’s Head are, unlike their correlatives elsewhere in the Great Dyke, broadly similar in thickness and metals content, probably because they formed along the axial to mid-axial/marginal facies of a very wide magma chamber where the horizontal rather than the vertical gradient dominated the cooling regime to a far greater extent than elsewhere in the Great Dyke.Emplacement of the 2.58 Ga year-old Great Dyke took place towards the end of late Archaean greenstone formation, granite intrusion and deformation along the northern edge of the Zimbabwe craton at ca. 2.57 to 2.62 Ga. Thrust deformation at Snake’s Head fits only the earliest (late Archaean) stage of deformation along the northern margin of the craton and so may have occurred soon after Great Dyke emplacement during the waning phase of Migmatitic Gneiss Terrain development and the final stabilisation of the craton. Folding of the Snake’s Head thrusts and thrust blocks may reflect cross-folding during extension of the Marginal Gneiss Terrain during the earliest stage of Zambezi belt orogenesis at 0.75 to 0.85 Ma. The marked north-south shortening of the northernmost part of the Great Dyke is likely the product of south-directed thrust movements during Zambezi contractional events at 0.85 to 1.10 Ga and/or 0.50 to 0.60 Ga. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Platinum-group element concentrations in pyrite from the Main Sulfide Zone of the Great Dyke of Zimbabwe

The Main Sulfide Zone (MSZ) of the Great Dyke of Zimbabwe hosts the world’s second largest resource of platinum-group elements (PGE) after the Bushveld Complex in South Africa. The sulfide assemblage of the MSZ comprises pyrrhotite, pentlandite, chalcopyrite, and minor pyrite. Recently, several studies have observed in a number of Ni-Cu-PGE ore deposits that pyrite may host significant amounts of PGE, particularly Pt and Rh. In this study, we have determined PGE and other trace element contents in pyrite from the Hartley, Ngezi, Unki, and Mimosa mines of the Great Dyke by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Based on the textures and PGE contents, two types of pyrite can be differentiated. Py1 occurs as individual euhedral or subhedral grains or clusters of crystals mostly within chalcopyrite and pentlandite, in some cases in the form of symplectitic intergrowths, and is PGE rich (up to 99 ppm Pt and 61 ppm Rh; 1.7 to 47.1 ppm Ru, 0.1 to 7.8 ppm Os, and 1.2 to 20.2 ppm Ir). Py2 occurs as small individual euhedral or subhedral crystals within pyrrhotite, pentlandite, and less frequently within chalcopyrite and silicates and has low PGE contents (<0.11 ppm Pt, <0.34 ppm Rh, <2.5 ppm Ru, <0.37 ppm Ir, and <0.40 ppm Os). Py1 contains higher Os, Ir, Ru, Rh, and Pt contents than the associated pyrrhotite, pentlandite, and chalcopyrite, whereas Py2 has similar PGE contents as coexisting pyrrhotite and pentlandite. Based on the textural relationships, two different origins are attributed for each pyrite type. Py1 intergrowth with pentlandite and chalcopyrite is inferred to have formed by late, low temperature (<300 °C) decomposition of residual Ni-rich monosulfide solid solution, whereas Py2 is suggested to have formed by replacement of pyrrhotite and pentlandite caused by late magmatic/hydrothermal fluids.

Composition of the ultramafic–mafic contact interval of the Great Dyke of Zimbabwe at Ngezi mine: Comparisons to the Bushveld Complex and implications for the origin of the PGE reefs

The Great Dyke contains the world's second largest platinum resource after the Bushveld Complex. Isotopic and trace element data from the interval straddling the contact between the Ultramafic and Mafic Sequences of the Great Dyke indicate a less enriched composition than in the Bushveld Complex (Great Dyke: Sri 0.7024–0.7028, εNd mostly −1 to +1, Ce/Sm 2–6; Bushveld: Sri 705–0.709, εNd −5 to −7, Ce/Sm 5–15). These data are interpreted to reflect relatively moderate amounts of contamination of the Great Dyke parent magma. All analyzed isotopes show little variation across the Main Sulfide Zone and the ultramafic–mafic contact. This corroborates earlier work by other researchers that the Great Dyke crystallized from a single magma type. Mixing of compositionally distinct magmas, proposed to have caused sulfide melt saturation in the Bushveld Complex, seemingly played little or no role in the formation of the PGE mineralization in the Main Sulfide Zone, and neither did enhanced crustal contamination of specific magma batches. Instead, sulfide melt saturation of the magma was likely triggered by silicate fractionation. The mechanism of concentration of the sulfide melt remains uncertain, but theoretical considerations suggest that phase sorting in response to slumping of crystal mushes, possibly caused by chamber subsidence, played an important role. This model is consistent with the highly irregular, undulating nature of the contact between the mafic and ultramafic zones of the intrusion, in the hanging wall of the Main Sulfide Zone.

Platinum mineralization in the Kapalagulu Intrusion, western Tanzania

Low-grade copper and nickel mineralization was found near the eastern shore of Lake Tanganyika at Kungwe Bay in the early part of the twentieth century. The mineralization occurs in harzburgite at the base of a layered gabbro complex known as the Kapalagulu Intrusion, emplaced between the Paleoproterozoic Ubendian basement and overlying Neoproterozoic Itiaso Group metasediments. Several mining and exploration companies continued the geophysical and drilling exploration for base metals throughout the last century culminating in the discovery of high-grade platinum-group element (PGE) mineralization associated with chromitite and sulfide-bearing harzburgite within the southeastern extension of the Kapalagulu Intrusion (known as the Lubalisi Zone) that is covered by a layer of nickel-rich laterite regolith. The poorly layered southeastern harzburgite forms part of the >1500 m-thick Lower Ultramafic Sequence and resembles a dike-like body that flares upwards into a succession of well-layered gabbroic rocks of the Upper Mafic Sequence. No PGE mineralization has been found in the layered gabbro; all the mineralization is associated with the chromite- and sulfide-rich harzburgite of the Lower Ultramafic Sequence and the laterite regolith overlying the mineralized harzburgite. The Lubalisi Zone harzburgite is underlain by basal dunite and overlain by an interval of layered harzburgite and troctolite and this ultramafic sequence is folded into a syncline that plunges towards the northwest that has been modified by major dolerite-filled faults orientated subparallel to the fold axial surface. Extensive deep drilling in the Lubalisi Zone of the Kapalagulu Intrusion shows that the folded harzburgite can be subdivided into a lower feldspathic harzburgite, a harzburgite containing chromitite seams and intervals of sulfide and chromite mineralization known as the Main Chromite Sulfide Succession (MCSS), an overlying sulfide-rich harzburgite, and an upper feldspathic harzburgite. Impersistent, stratiform PGE mineralized horizons occur within the MCSS harzburgite from which drill core samples were taken for platinum-group mineral (PGM) characterization from two drill holes. Where the PGE reefs reach the surface there is residual PGE mineralization within the laterite regolith from which drill core samples were taken from various laterite lithological units for PGM characterization. As the harzburgite PGE reefs contain significant concentrations of both sulfide and chromite (including chromitite seams) they resemble the PGE-rich chromitite seams of the Bushveld Complex rather than the PGE-bearing Main Sulfide Zone of the Great Dyke and Main Sulfide Layer of the Munni Munni Complex. The dominant Pd PGM in three PGE reef samples varies, ranging (n = 164, relative wt%) from bismuthides (63 %), bismuthtellurides (19 %), and tellurides (6 %), to tellurides (39 %), bismuthtellurides (24 %), stannides (14 %), and alloys (13 %), and to antimon-arsenides (33 %), stannides (21 %), bismuthides (17 %), tellurides (13 %), and alloys (10 %). From 13.5 % to 21.0 % of the total Pd occurs as a solid solution in pentlandite. The three samples have similar Pt PGM modal distributions (n = 172, relative wt%); the dominant Pt mineral is sperrylite (79, 58, and 47 %) followed by tellurides (15, 17, 21 %), alloys (2, 1, 1 %), and sulfides (2, 1, 0 %). Comparison of Pd/Pt ratios from assays to those calculated from minerals show that the data for the Pt and Pd PGM are very robust, confirming the concentration methodology and characterization. Study of samples from a shallow drill hole penetrating the laterite regolith shows that the primary Pd mineralization has not survived oxidation, is mainly dispersed, but some was reconstituted to form secondary minerals: cabriite, unnamed tellurides, a selenide, a Pd-Te-Hg mineral, alloys and Pd-bearing secondary sulfides (millerite and heazlewoodite). The primary Pt minerals are more resistant to oxidation and dissolution, especially sperrylite and isoferroplatinum, but it is likely that other Pt alloys (tetraferroplatinum and tulameenite) are of secondary origin after dissolution of Pt tellurides.

Improving recoveries of platinum and palladium from oxidized Platinum-Group Element ores of the Great Dyke, Zimbabwe, using the biogenic siderophore Desferrioxamine B

This study presents results of batch leaching experiments conducted on oxidized (weathered) Platinum-Group Element (PGE) ores from two platinum mines at the Great Dyke, Zimbabwe. The Great Dyke mafic/ultramafic layered intrusion is, after the Bushveld complex in South Africa, the second largest PGE deposit worldwide. Current mining operations focus on the recovery of PGE from pristine and unweathered ore material from the Main Sulfide Zone (MSZ). Besides pristine ores, there are significant resources of oxidized MSZ material at or near the surface. These oxidized PGE ores are currently not mined because insufficient recoveries using conventional processing techniques render this process option uneconomical. The complex, polymodal mineralogical distribution of the PGE in the oxidized MSZ aggravate to the processing issue. The data presented in this study show that a mild hydrochloric acid leach combined with a subsequent leach with the biogenic siderophore Desferrioxamine B (DFOB) in an aqueous solution efficiently extract Pt and Pd from these ores. Although Pt and Pd extraction during hydrochloric acid pretreatment is rather heterogeneous with a maximum Pt recovery of 30%, it nevertheless facilitates mobilization of Pt and Pd in the subsequent siderophore leaching step. Hydrochloric acid pretreatment of the oxidized PGE ores is a prerequisite for liberating the PGE-bearing host phases from weathering products and for the reduction of the amount of easily-available cations such as Fe in hydroxide minerals, which may compete with Pt and Pd for complexation in the subsequent siderophore leach step.This two-step approach results in a combined extraction of up to 80% of total Pt. Our results further show that Pt and Pd extraction during siderophore leaching is strongly pH-dependent and appears to be most efficient at near-neutral to slightly alkaline conditions, with increasing extraction efficiencies at higher pH. Based on the results of this study, siderophore leaching of oxidized (PGE) ores represents a promising approach for the hydrometallurgical extraction of Pt and Pd from oxidized ores.

DETRITAL PLATINUM-GROUP MINERALS IN RIVERS DRAINING THE GREAT DYKE, ZIMBABWE

The present work focuses on the description of the assemblage of detrital platinum-group minerals (PGM) found in rivers draining the Great Dyke. This PGM assemblage distinctly contrasts with the suite of PGM in the pristine, sulfide-bearing Main Sulfide Zone (MSZ) of the Great Dyke, the assumed source of the detrital PGM. Specifically, PGE-bismuthotellurides and -sulfarsenides, common in the MSZ ores, and PGE-oxides or -hydroxides present in the oxidized MSZ, are missing in the assemblage of detrital PGM in the fluvial environment. Instead, conspicuously high proportions of grains of Pt-Fe alloy are common in the sediments, followed by sperrylite, cooperite, braggite, laurite, rare Pd-Sb-As compounds, and Os-Ir-Ru alloys. Possibly, some of the Pt-Fe alloy grains originate from the MSZ, but the majority appear to represent true neo-formations that formed in the course of weathering of the MSZ ores and concomitant supergene redistribution of the ore elements. Sperrylite, cooperite/braggite, and laurite appear to be direct descendants from the primary MSZ. Rare Pd-dominated minerals (Pd-Hg ± As and Pd-Sb ± As) are considered to be neo-formations that probably formed from dispersed elements during supergene processes. Os-Ir-Ru alloy grains, present in the samples from the Umtebekwe River in the Shurugwi area, possibly originate from the Archean chromitite deposits of the Shurugwi greenstone belt and not from the Great Dyke. Geochemically, the Pt/Pd proportions increase from pristine via oxidized MSZ ores to the fluviatile environment, corroborating earlier findings that Pd is more mobile than Pt and is dispersed in the supergene environment. Detrital PGM can be expected to be present in rivers draining PGE-bearing layered intrusions, and economic placers may form under particular sedimentological conditions. Therefore, the work also highlights the fact that simple field methods have their value in mineral exploration, especially if they are combined with modern micro-analytical methods. Furthermore, it is established that the PGE-bismu thotel lurides, PGE-sulfarsenides, and PGE-oxides, dominating the PGM assemblages in the pristine and oxidized MSZ, are components that are unstable during weathering and mechanical transport. Including the genetically somewhat disputed Pt-Fe alloys, the order of decreasing stability in the supergene environment is as follows: (1) Pt-Fe and Os-Ir-Ru alloys (very stable) → (2) sperrylite (stable) → (3) cooperite/braggite (variably stable/”metastable”) → (4) PGE-bismuthotellurides, PGE-sulfarsenides, and PGE-oxides (unstable).

The Enrichment Mechanism of PGE and Characteristic of MSZ in HW Mining Area in Great Dyke Zimbabwe

Zimbabwe Great Dyke is a mafic-ultramafic lithosome intruded the Zimbabwe Craton. MSZ(Main Sulfide Zone) is the most important layer which contains substantial amount of PGE(Platinum Group Elements). PGE are concentrated in bottom of MSZ layer because of the intimate relationship between the content of Cu&amp;Ni and enrichment of PGE. Acid vein rocks did not formed in the same period with the ore and adjacent rocks. Plenty of sulfide can be found in MSZ in pyroxenite, Sulfide in ore was owe to homogeneous of geochemistry process in the magamtic segregation cycle, while them in gabbro and fractures were simply by the post magmatic thermal solution activities.

Erratum to: Platinum-group element distribution in the oxidized Main Sulfide Zone, Great Dyke, Zimbabwe

Erratum to: Platinum-group element distribution in the oxidized Main Sulfide Zone, Great Dyke, Zimbabwe

Platinum-group element distribution in the oxidized Main Sulfide Zone, Great Dyke, Zimbabwe

In the Great Dyke mafic/ultramafic layered intrusion of Zimbabwe, economic concentrations of platinum-group elements (PGE) are restricted to sulfide disseminations in pyroxenites of the Main Sulfide Zone (MSZ). Oxidized ores near the surface constitute a resource of ca. 400 Mt. Mining of this ore type has so far been hampered due to insufficient recovery rates. During the oxidation/weathering of the pristine ores, most notably, S and Pd are depleted, whereas Cu and Au are enriched. The concentrations of most other elements (including the other PGE) remain quite constant. In the oxidized MSZ, PGE occur in different modes: (1) as relict primary PGM (mainly sperrylite, cooperite, and braggite), (2) in solid solution in relict sulfides (dominantly Pd in pentlandite, up to 6,500 ppm Pd and 450 ppm Pt), (3) as secondary PGM neoformations (i.e., Pt–Fe alloy and zvyagintsevite), (4) as PGE oxides/hydroxides that replace primary PGM as the result of oxidation, (5) hosted in weathering products, i.e., iron oxides/hydroxides (up to 3,600 ppm Pt and 3,100 ppm Pd), manganese oxides/hydroxides (up to 1.6 wt.% Pt and 1,150 ppm Pd), and in secondary phyllosilicates (up to a few hundred ppm Pt and Pd). In the oxidized MSZ, most of the Pt and Pd are hosted by relict primary and secondary PGM; subordinate amounts are found in iron and manganese oxides/hydroxides. The amount of PGE hosted in solid solution in sulfides is negligible. Considerable local variations in the distribution of PGE in the oxidized ores complicate a mineralogical balance. Experiments to evaluate the PGE recovery from oxidized MSZ ore show that using physical concentration techniques (i.e., electric pulse disaggregation, hydroseparation, and magnetic separation), the PGE are preferentially concentrated into smaller grain size fractions by a factor of 2. Highest PGE concentrations occur in the volumetrically insignificant magnetic fraction. This indicates that a physical preconcentration of PGE is not feasible and that chemical, bulk-leaching methods need to be developed in order to successfully recover PGE from oxidized MSZ ore.

Textural, mineralogical and stable isotope studies of hydrothermal alteration in the main sulfide zone of the Great Dyke, Zimbabwe and the precious metals zone of the Sonju Lake Intrusion, Minnesota, USA

Stratigraphic offsets in the peak concentrations of platinum-group elements (PGE) and base-metal sulfides in the main sulfide zone of the Great Dyke and the precious metals zone of the Sonju Lake Intrusion have, in part, been attributed to the interaction between magmatic PGE-bearing base-metal sulfide assemblages and hydrothermal fluids. In this paper, we provide mineralogical and textural evidence that indicates alteration of base-metal sulfides and mobilization of metals and S during hydrothermal alteration in both mineralized intrusions. Stable isotopic data suggest that the fluids involved in the alteration were of magmatic origin in the Great Dyke but that a meteoric water component was involved in the alteration of the Sonju Lake Intrusion. The strong spatial association of platinum-group minerals, principally Pt and Pd sulfides, arsenides, and tellurides, with base-metal sulfide assemblages in the main sulfide zone of the Great Dyke is consistent with residual enrichment of Pt and Pd during hydrothermal alteration. However, such an interpretation is more tenuous for the precious metals zone of the Sonju Lake Intrusion where important Pt and Pd arsenides and antimonides occur as inclusions within individual plagioclase crystals and within alteration assemblages that are free of base-metal sulfides. Our observations suggest that Pt and Pd tellurides, antimonides, and arsenides may form during both magmatic crystallization and subsolidus hydrothermal alteration. Experimental studies of magmatic crystallization and hydrothermal transport/deposition in systems involving arsenides, tellurides, antimonides, and base metal sulfides are needed to better understand the relative importance of magmatic and hydrothermal processes in controlling the distribution of PGE in mineralized layered intrusions of this type.

Platinum-Group Element Distribution in Some Ore Deposits: Results of EPMA and Micro-PIXE Analyses

The distribution of platinum-group elements (PGE) in the Main Sulfide Zone of the Great Dyke of Zimbabwe, in the arsenide ores of Vammala and Kylmakoski mines in Finland, and in the chromite-Ni arsenide ores from the Ronda and Beni Bousera massifs (Spain and Morocco) has been studied by means of different optical and microanalytical techniques. The results show a bimodal distribution of PGE in the ores from the Great Dyke and the Vammala and Kylmakoski mines where Pd, Pt and, to lesser extent, Rh occur both as discrete platinum-group minerals and in solid solution in sulfides, arsenides and sulfarsenides. In the ores from Ronda and Beni Bousera, platinum-group elements occur only in solid solution in arsenides and sulfarsenides.

Geochemistry and mineralogy of platinum-group elements at Hartley Platinum Mine, Zimbabwe

The behaviour of platinum-group elements (PGE) in the exogenic cycle was examined in profiles of oxidized Main Sulfide Zone (MSZ) ores, in which the general metal distribution patterns of the pristine MSZ are grossly preserved. However, at similar Pt grades, significant proportions of Pd have been lost from the system. This indicates that Pd is more mobile than Pt and is dispersed in the supergene environment. Sperrylite and cooperite/braggite are stable in the oxidized MSZ. In contrast, the (Pt,Pd)-bismuthotellurides, common in pristine MSZ ores, have disintegrated, and ill-defined (Pt,Pd)-oxides or (Pt,Pd)-hydroxides have formed. The assemblage of detrital PGM present in the Makwiro River close to the Hartley Platinum Mine indicates further mineralogical changes. Sperrylite largely remains stable whereas most cooperite/braggite grains have been partly altered or completely destroyed. Grains of Pt-Fe alloy are ubiquitous in the alluvial sediments. Most likely, these grains are neo-formations that formed either from pre-existing, unstable PGM or via a solution stage under low-temperature conditions.

Potential for bulk mining of oxidized platinum-group element deposits

The market for platinum-group elements (PGE) is profitable, but difficult to enter for newcomers. The main difficulties are in extraction technology, economics and marketing. Novel metallurgical processing technology may offer a route for the development of currently sub-economic PGE deposits that are amenable to surface mining. The merits of various pyro- and hydrometallurgical process routes are discussed. Before more detailed process development work can proceed it is necessary to understand the mineralogical balance of the PGE in oxidized mineralization. An example is presented from the Main Sulphide Zone (MSZ) mineralization of the Great Dyke, Zimbabwe. The PGE in the sulphide zone occur dominantly in the form of discrete bismuthotelluride, arsenide and sulphide minerals, whereas in the oxidized zone they occur as oxide or hydroxide minerals and aggregates as well as relics of the primary minerals. Such mineralogical contrasts have important implications for potential open-pit mining and processing.

PLATINUM-GROUP MINERALS AND OTHER DETRITAL COMPONENTS IN THE KAROO-AGE SOMABULA GRAVELS, GWERU, ZIMBABWE

Heavy-mineral concentrates were obtained from a recent exploration campaign for diamond in the Somabula gravels, a paleoplacer of Upper Karoo (Triassic, ~200 Ma) age near Gweru, Zimbabwe. The polymict spectrum of heavy minerals in the Somabula placers comprises staurolite, zircon, monazite, columbite–tantalite, cassiterite, chromite, native gold, platinum-grou p minerals (PGM), and other components, which suggests derivation from a variety of source lithologies. Age determinations revealed that most of the detrital grains of zircon, monazite and columbite–tantalite originate from Pan-African source rocks. Altogether, 622 detrital PGM grains were identified. Pt and Pt alloys, mainly with Fe, Rh, Pd and Ru, predominate (79%), followed by Os–Ir–Ru–Rh–Pt alloys (10%), rustenburgite [Pt 3Sn] (9%), and rare Pt-rich phases (1%). The compositions of the alloys suggest high-temperature crystallization from a complex Pt–Pd–Rh–Os–Ir–Ru–Fe solid solution in a sulfur-poor magmatic environment, probably mafic–ultramafic rocks. The suite of PGM markedly differs from that of detrital PGM originating from the Great Dyke and from the primary platinum-group element mineralization of the Main Sulfide Zone of the Great Dyke. The mineralogical evidence and Os isotope data obtained for the Os–Ir–Ru alloy rule out the possible derivation of the PGM from the Great Dyke. Instead, they are likely derived from late Archean greenstone belts of the Zimbabwe craton.

Platinum-Group Element Mineralisation in the Great Dyke, Zimbabwe, and its Relationship to Magma Evolution and Magma Chamber Structure

The platinum-group element (PGE) resources of the Great Dyke are second only to those of the Bushveld Complex and principally comprise the potentially economic Main Sulphide Zone (MSZ) and the thicker, but lower grade, Lower Sulphide Zone (LSZ). Located a few tens of metres stratigraphically apart within the uppermost P1 Pyroxenite of the Ultramafic Sequence, both mineralised zones are present in all remnants of the P1 Pyroxenite along the entire length of the Great Dyke. Other less significant PGE mineralised zones include chromitite C1d, in dunites and harzburgites a few tens of metres beneath the P1 Pyroxenite, the Middle Mafic Unit of the Mafic Sequence and the Mineralised Marginal Zone of the extreme marginal facies and Border Group. This paper describes the general geology of the Great Dyke, and the stratigraphic and structural setting and internal characteristics and lateral variations of the main PGE mineralised zones. The stratigraphic location, complex internal layering and systematic transverse variations of the MSZ, LSZ and chromitite C1d are here related to the emplacement and the chemical and physical evolution of the magma body and to the structure of the magma chamber.