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Several models have been proposed to explain the origin of a chromitite stringer located at the contact between the Mafic and Ultramafic Sequences in the Unki Mine area of the Shurugwi Subchamber of the Great Dyke, Zimbabwe. A petrographic and geochemical study of this chromitite stringer was undertaken with the aim of constraining its origin. Forty-three chromite compositions were obtained from the studied chromitite stringer, which is characterized by a chromium number between 59.9 and 62.8 and a magnesium number which ranges from 37.8 to 46.4. The chromites at the contact zone in the Unki Mine commonly contains inclusions of sulfides, orthopyroxene, plagioclase, and/or amphiboles. The chromites likely formed early in the crystallization history of the Mafic Sequence, as they are commonly partially rimmed by sulfides and they occur as inclusions in plagioclase crystals. Unlike chromites from underlying Ultramafic Sequence chromitite layers, chromites at the contact zone contain low Cr2O3 contents which range from 39.4 to 42.6 wt.%. Furthermore, these chromites are enriched in Fe compared to most Great Dyke chromitites, which is interpreted to be a consequence of subsolidus exchange of Mg into orthopyroxene and Fe into the chromite. The absence of zoning in the chromites at this contact zone, and their low Mn, Fe contents, is consistent with attainment of equilibrium because the altered chromites often contain Cr-bearing magnetite rims. Two possible models for the formation of this chromitite stringer are mixing of relatively primitive and evolved magmas (i.e., ultramafic and anorthositic magma), possibly of different oxygen fugacities, and chemical diffusion across the contact between the Mafic and the Ultramafic sequences which resulted in melting at and below this boundary. The latter would have caused preferential loss of orthopyroxene from the underlying P1 Pyroxenite Layer, accompanied by re-precipitation of chromite at this contact.

Chlorite is a common mineral in most metamorphosed mafic and ultramafic rocks and the use of chlorite as a geothermometer was applied to rocks from the southern Appalachian Piedmont for this study. In particular, chlorite-bearing rocks from the Russell Lake Allochthon (RLA) and the Soapstone Ridge Complex (SSR) are the focus of this investigation. The RLA is composed of numerous metamorphosed mafic and ultramafic bodies exposed within rocks of the Carolina superterrane which forms part of the peri-Gondwana realm of the Southern Appalachians. The SSR is one of the largest metamorphosed mafic and ultramafic bodies in the southern Appalachians; it is in thrust contact with underlying rocks of the Piedmont domain, which forms part of the Iapetan realm of the Southern Appalachians. The metamorphosed bodies that comprise the RLA are generally small, with outcrop areas generally not exceeding a few square kilometers. In the RLA and SSR, chlorite occurs together with zoned amphiboles. In the RLA, chlorite exhibits low total iron atoms per formula unit and dominantly plots in the ripidolite field. In contrast, SSR chlorites are poorer in total iron and almost all are clinochlores. Results of chlorite geothermometry from rocks of the RLA yield temperatures which range from ~ 280 to ~ 320 °C, whereas chlorites from the SSR yield temperatures which range from ~ 201 to ~ 250 °C. These calculated temperatures indicate that chlorites in the RLA were formed at slightly higher temperatures in comparison to those from the SSR, which may suggest that these bodies were subjected to different late-stage metamorphic conditions and may also have originated under different tectonic settings. The RLA and SSR are approximately 100 km apart and a terrane boundary occurs between them; these bodies were each metamorphosed, and likely evolved, in different regions and each was later thrusted onto its country rocks during the final stages of the assembly of Pangea.

The 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.

An investigation of the economically important Mashava Igneous Complex (MIC) in Zimbabwe was conducted using petrographic and silicate mineral compositions to gain some insights into its origin. The MIC, an ultramafic-mafic complex located in the western extremity of the Masvingo greenstone belt (MGSB), is composed of amphibole, serpentine, epidote, chlorite, clinopyroxene, orthopyroxene, plagioclase, and olivine. Spinel, sulfide minerals, and minor quartz as well as other opaque minerals are also present. The dominant mineral is serpentine, which displays a wide range in both FeO and SiO2 compositions, and the majority of this mineral belongs to the lizardite, chrysotile, and antigorite varieties. Amphiboles display a wide range in composition from tremolite and actinolite which are both consistent with greenschist facies metamorphism, through magnesiohornblende to tschermakite which are both consistent with amphibolite facies metamorphism. Clinozoisite is the only epidote type occurring in the MIC, whereas chlorite varies widely in composition which reflects the varying concentrations in iron and silica in this mineral. Chlorite types range from talc chlorite through penninite, diabantite, pycnochlorite, brunsvigite, ripidolite, and pseudothuringite. Plagioclase shows a wide compositional range from high albite to bytownite, with only two samples plotting in the alkali feldspar anorthoclase field. Pyroxene ranges in composition from enstatite through pigeonite and augite to diopside whereas olivine is rich in MgO with forsterite contents ranging from 80 to 89. Application of the chlorite geothermometry equation of Kranidiotis and MacLean (1987) yields temperatures ranging from ∼200–320 °C whereas temperatures ranging from 130 to 445 °C are obtained using equations of Lanari et al. (2014) for the formation of this mineral, generally consistent with low- to medium-grade metamorphic conditions. Chlorites in the North West Arm were formed at relatively lower temperatures (97–234 °C) than those at which chlorites in the Central Sector were formed (204–445 °C). Pressure estimates for the metamorphism of the MIC range from 3.8 to 7.3 kbars, which corresponds to depths of ∼13–25 km, respectively. Based on the different silicate mineral compositions of the North West Arm and the Central Sector, it is inferred that these two blocks underwent different petrogenetic and evolutionary histories but they were later juxtaposed, possibly during thrusting of the former onto its present position.

Electron microprobe analyses were performed on chromites from Mashava Igneous Complex (MIC), a Neoarchean ultramafic complex located in the western extremity of the Masvingo greenstone belt (MGSB) in Zimbabwe. In backscattered electron images (BSEs), the massive, layered, and sometimes serpentinite hosted chromite crystals are rimmed by a thin lighter-colored zone enriched in Fe relative to the much larger cores. Chromium number, Cr#, and magnesium number, Mg#, from the darker grain cores range from values of 70.0 to 75.8 and 69.0 to 80.6, whereas the narrow lighter colored rims have chromium numbers that range from 90.7 to 99.6 and 58.5 to 69.3, respectively. These Mg#s are much higher than those typical of layered intrusions, and are interpreted to be a consequence of the initial high magnesium concentrations of the bulk ultramafic magma of the MIC. On a plot of Cr2O3 versus TiO2, MIC chromites plot within the field of komatiites, whereas on plots of Cr# versus Mg# the chromites plot in the stratiform field. However, on both TiO2 (wt%) and Al2O3 (wt%) versus Cr2O3 (wt%) plots, MIC samples fall within the fields of podiform chromites. The high Cr# of MIC chromite cores suggest arc affinities based on their inferred Al2O3-TiO2 contents of parental melts to the chromites. It is therefore proposed that MIC formed at a convergent boundary and was later accreted to its present position at the margin of the MGSB, or formed entirely due to subduction processes, which may also have formed this part of the MGSB. The ferritchromite rims likely formed during subsequent metamorphism under at least greenschist facies metamorphism, which likely also resulted in the formation of chrysotile asbestos.

The Musina copper deposits are located in the Central Zone of the Limpopo orogenic belt in Limpopo Province, South Africa. We carried out a petrographic, sulfide composition, and δ34S study on samples from Artonvilla and Campbell copper deposits and a country rock granitic gneiss to Artonvilla Mine to place some constrains on the origin of these deposits. The assemblages at both Artonvilla and Campbell Mines of brecciated quartz, potassium feldspar, muscovite, chlorite, calcite, and amphibole are consistent with sericitic alteration. Quartz, amphibole, feldspars, and micas often display angular textures which are consistent with breccias. Sulfur concentrations in pyrite from Artonvilla Mine plot in a narrow range, from 50.2 wt. % to 55.7 wt. %. With the exception of a positive correlation between Fe and Cu, no well defined correlations are shown by data from the Musina copper deposits. The occurrence of sulfides both as inclusions in, or as interstitial phases in silicates, suggests that hydrothermal alteration that affected these deposits most likely helped concentrate the mineralization at the Musina copper deposits. Sulfur concentrations in chalcopyrite samples investigated vary widely whereas the copper concentrations in chalcopyrite are not unusually higher compared to those from chalcopyrite from other tectonic settings, probably indicating that either the Cu in the Musina copper deposits occurs in native form, and/or that it is hosted by other phases. This observation lends support to the Cu having been concentrated during a later hydrothermal event. One sample from Artonvilla Mine (AtCal01) yielded pyrite δ34S values of 3.1and 3.6‰ and chalcopyrite from the same sample yielded a value of 3.9‰. A country rock granitic gneiss to Artonvilla Mine yielded a δ34Spyrite value of 8.2‰. For Campbell Mine samples, one quartz vein sample has a δ34Spyrite value of 0.5‰ whereas chalcopyrite samples drilled from different areas within the same sample yielded values of 0.4 and 0.7‰. The same sample also yielded a δ34Sbornite value of 0.4‰. Another Campbell Mine quartz vein sample yielded a chalcopyrite δ34S value of −0.3‰. Sulfur isotope thermometry for one Campbell Mine quartz vein sample with coexisting sulfides yielded a Δ34Schalcopyrite-bornite value of 359 °C that is consistent with the stability of this mineral pair. Thus, δ34S values from Campbell Mine are consistent with an igneous source for the sulfur. Based on a simple two-end member isotope mixing model, contamination of the sulfur by sulfur derived from granitic country rocks likely occurred at Artonvilla Mine. Based on findings from this study and by other previous investigators, it is concluded that features displayed by the Musina copper deposits are consistent with a breccia pipe origin for the Musina copper deposits.

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.

Samples of the Soapstone Ridge Complex (SSR) in the Southern Appalachians examined for this study are composed of megacrystic orthopyroxenites, amphibole schists, and talc-chlorite-amphibole schists. The mineralogy of the SSR samples studied consists of varying proportions of amphibole, chlorite, talc, orthopyroxene, and Fe-Ti oxides. Amphibole is the domi- nant mineral in almost all SSR samples studied. The amphiboles display zoning, whereby cores that are composed of magne- siohornblende and edenite, consistent with amphibolite facies conditions, are rimmed by tremolite/actinolite, consistent with greenschist facies conditions. Oxygen isotope compositions of SSR amphiboles investigated show a wide range in 18O values, from a low value of 1.8‰ to a high value of 6.1‰. Such wide ranges in 18Oamphibole values occur in ophiolite complexes and oceanic crustal rocks. Relict orthopyroxene from the SSR has a 18O value of 3.4‰, a value lower than those from layered intrusions but comparable to some pyroxene 18O values from ophiolites. Equilibration temperatures calculated for actinolite- chlorite yield a temperature of 227 °C, consistent with greenschist facies conditions. It is proposed that the SSR amphibole cores were metamorphosed probably during terrane-terrane collision under amphibolite facies conditions, with the actinolite rims possibly being formed during thrusting of the SSR rocks onto the continental margin.