The use of chemostratigraphy and geochemical vectoring as an exploration tool for platinum group metals in the Platreef, Bushveld Igneous Complex, South Africa : a case study on the Tweefontein and Sandsloot farms

>Magister Scientiae - MScThe Platreef is known for its complexity and its heterogeneous lithologies, coupled with an unpredictable PGE and BMS mineralisation. The motivation behind this study was to aid mining geologists in targeting mineralisation irrespective of the farm. It is known that the Platreef generally overlies different footwall lithologies at individual farms. Thus, the aims of this study were firstly to investigate the potential of chemostratigraphy by delineating indices indicative of distinctive lithological layers. These indices were then tied to the second aim; which were to use geochemical vectoring, which is process-based, to target the PGEs at two different farms. This study included three drillcores: from the farms Sandsloot (SS339) and Tweefontein (TN754 and TN200). The footwall units at Tweefontein are shales of the Duitschland Formation and the Penge banded iron formation; and at Sandsloot it is the Malmani Subgroup dolomites. Samples included 121 quarter cores, used for petrographical and geochemical studies. The elemental rock composition was determined by XRF and ICP-OES analyses. The approach also included statistical and mass balance methods to understand the geological and geochemical controlling processes. Initially, the Platreef package at both farms was petrographically divided into three main layers: pyroxenite, and two distinctive feldspathic pyroxenites (FP-I and FP-II). However, the pyroxenites were also further separated as P-I and P-II, because of a higher notable difference in the degree of alteration within P-I. Progressive degrees of metasomatism were further observed in the lithologies, e.g. within the Platreef package, where feldspathisation was potentially the main metasomatic process. Many geochemical plots (corroborated by the petrographical and mass balance results) illustrated that the feldspathisation were linked to an increase in the content of Al₂O₃ and CaO, and coupled with a decrease in content of Fe₂O₃ and MgO. Together with other geochemical trends, geochemically distinct units of the Platreef package could be discriminated with a metasomatism index (MI; CaO + 10Na₂O / CaO + 10Na₂O + Fe₂O₃ + MgO). The ensuing MI is lowest for the P-II pyroxenite and shows a progressive increase through FP-I, P-I to the highest values in FP-II. Geochemical layering were also observed in the calcsilicates and hornfels; e.g. a progressive decrease in the content of Fe₂O₃, Al₂O₃, Ce, Co, Cu, Ni, Zn, Zr, Au, Pd and Pt from the hornfels subunits H-I, H-II to H-III and an increase in of SiO₂, Fe₂O₃, TiO₂, SO₃, Co, Cu, Ni, Rb, V and Zn content from CS-I, CS-II to CS-III. Correlating the pyroxenites and feldspathic pyroxenites spatially from one drillcore to another were hindered, hence, chemostratigraphy were not completed. In terms of vectoring, it was essential to establish a possible link between the metasomatism index and the nature and style of the PGE and/or BMS mineralisation. The Hornfels subunit H-I and calcsilicate subunit CS-III were the main carriers of BMS and PGE. The Platreef package were more complicated: P-I (low PGE, low BMS); P-II (low PGE, high BMS); FP-II (high PGE, low BMS); and FP-I (high PGE, high BMS). Element indices (e.g. Cu+Ni and Co+Zn) were developed to define a consistent gradient indicative of these ore subunits. A validation process to assess the metasomatism index (MI), base metal indices and PGE distribution within the individual drillcores (TN754, TN200 and SS339) were then undertaken. The results were that the MI ranges were similar in all drillcores, and discriminated the subunits of the Platreef package, gabbronorites and even the calcsilicates. The base metal ratios (e.g. Ni/Co and Cu/Co) were indicative of the PGE rich zones. Trends of the base metal ratios reflected a strong positive relationship with the MI within the Platreef package and the calcsilicates. However, the opposite trend is observed with the hornfels. In conclusion, the MI could potentially be a strong vector of high PGE and BMS mineralisation. It is also possible to discriminate lithologies within the Platreef package with the MI. However, it should be noted that the limitation of this study is that the results are based on three drillcores. The Platreef is heterogeneous at individual farms and extremely diverse across the northern limb. Therefore, future research could be undertaken to validate these findings, by using a bigger drillcore database.National Research Foundatio

Metasomatic Reactions between Archean Dunite and Trondhjemite at the Seqi Olivine Mine in Greenland

A metasomatic zone formed between the contact of a 2940 ± 5 Ma intrusive trondhjemite sheet in the Archean dunite of the Seqi Ultramafic Complex, SW Greenland, consists of three distinct mineral zones dominated by (1) talc, (2) anthophyllite, and (3) phlogopite. These zones supposedly resulted from a process of dissolution of olivine by silica rich fluid residual from the trondhjemite magma, with crystallization of secondary minerals along a compositional gradient in the fluid phase. A zircon crystal inclusion in a large (4 cm) olivine porphyroblast was dated in situ via LA-ICP-MS U–Pb isotope analysis, yielding a weighted mean 207Pb/206Pb age of 2963 ± 1 Ma, which coincides with granulite facies metamorphism and potential dehydration. Considering phase relations appropriate for the dunite composition, we deduced the talc forming conditions to be at temperatures of 600–650 °C and at a pressure below 1 GPa. This is supported by oxygen isotope data for talc, anthophyllite and phlogopite in the metasomatic zone, which suggests formation in the temperature range of 600–700 °C from fluids that had a δ18O of ~8‰ and a Δ’17O0.528 of about −40 ppm, i.e., from fluids that could have been derived from the late stage trondhjemite sheet

The use of chemostratigraphy and geochemical vectoring as an exploration tool for platinum group metals in the Platreef, Bushveld Igneous Complex, South Africa: a case study on the sandsloot & overysel farms

>Magister Scientiae - MScThe paucity of geochemical criteria for stratigraphic correlations and defining the styles of mineralisation pose serious problems in locating PGE-rich zones in the Platreef. This study is therefore aimed at identifying and appraising process-based mineralogical/geochemical criteria which may be useful in stratigraphic correlations and characterizing the nature and styles of PGE mineralisation. In addition, the work investigated the possible use of geochemical vectoring as a tool to locate the PGE-rich zones. Boreholes OY 482 and SS 330, drilled at the Overysel and Sandsloot farms respectively, were logged, and a total of 119 quarter cores were sampled for petrographic studies. The elemental contents in the rocks were determined by XRF and ICP-OES analyses and were evaluated using various statistical and mass balance techniques. In borehole OY 482, where the floor rock is Archaean granite, the Platreef consists of three feldspathic pyroxenite sills referred to as Lower, Middle and Upper Platreef units, from the bottom to the top, respectively. The results show that the Lower and Upper Platreef units have higher median values of Mg# (0.58 and 0.57) and Ni/Cu (0.68 and 0.75) when compared to the Middle Platreef (Mg#: 0.54 and Ni/Cu: 0.67) which may not be totally suggestive of two magmatic intrusive pulses. In borehole SS 330, where the floor rock is dolomite, the rocks consist of clinopyroxenites and olivine clinopyroxenites (variably serpentinised). These two units are intercalated with each other and are products resulting from the injection of Platreef magma sills within the dolomite floor rock. The hierarchical clustering and mass balance calculations show that when compared to the Platreef feldspathic pyroxenites, which have higher SiO2, Al2O3 and Fe2O3 median contents, the clinopyroxenites possess higher CaO median content whereas the olivine clinopyroxenites have higher MgO and LOI median contents. The PGE-rich zones (i.e. Pt+Pd) in clinopyroxenites are marked by low Ca/Mg median values, whereas in both, the olivine clinopyroxenites and the Platreef units, these zones are marked by high Mg/Fe median values. The suggested base metal index [(Cu/Zn) x (Ni/Co)] used to vector towards PGE-rich zones, which reflects the presence of the base metal sulphides (BMS), correlates with the Pt+Pd in the BMS-rich zones. This is not always the case in zones of low BMS contents which may reflect changes in the mineralogy of the BMS. In conclusion, the two boreholes studied show contrasting petrographic and geochemical attributes. This dissimilarity is mainly due to the fact that borehole OY 482 comprises Platreef magmatic rocks whereas borehole SS 330 intersected metamorphic/ metasomatic rocks

Footprints of element mobility during metasomatism linked to a late Miocene peraluminous granite intruding a carbonate host (Campiglia Marittima, Tuscany)

The Campiglia Marittima magmatic-hydrothermal system includes a peraluminous granite, its carbonatic host, and skarn. The system evolved generating a time-transgressive exchange of major and trace elements between granite, metasomatic fluids, and host rock. The process resulted in partial metasomatic replacement of the granite and severe replacement of the carbonate host rocks. The fluid activity started during a late-magmatic stage, followed by a potassic–calcic metasomatism, ending with a lower temperature acidic metasomatism. During the late-magmatic stage, B-rich residual fluids led to the formation of disseminated tourmaline–quartz orbicules. High-temperature metasomatic fluids generated a pervasive potassic–calcic metasomatism of the granite, with replacement of plagioclase, biotite, ilmenite, and apatite by K-feldspar, phlogopite–chlorite–titanite, titanite–rutile, and significant mobilization of Fe, Na, P, Ti, and minor HFSE/REE. The metasomatized granite is enriched in Mg, K, Rb, Ba, and Sr, and depleted in Fe and Na. Ca metasomatism is characterized by crystallization of a variety of calc-silicates, focusing along joints into the granite (endoskarn) and at the marble/pluton contact (exoskarn), and exchange of HFSE and LREE with hydrothermal fluids. Upon cooling, fluids became more acidic and fluorine activity increased, with widespread crystallization of fluorite from disequilibrium of former calc-silicates. At the pluton-host boundary, fluids were accumulated, and pH buffered to low values as temperature decreased, leading to the formation of a metasomatic front triggering the increasing mobilization of REE and HFSE and the late crystallization of REE–HFSE minerals

Toward an integrated geologic, geochemical, and structural model for formation of the MacLellan Au-Ag and related mineral deposits, Lynn Lake, Manitoba

The MacLellan deposit and related Au-Ag occurrences reside in the northern portion of the Lynn Lake greenstone belt, Manitoba. All occurrences within the study area contain the same lithologies, alteration styles, and were formed by similar processes. The Nisku Deposit, Main and Dot Zones contain the most abundant Au-Ag mineralization and are characterized by increased arsenopyrite, galena, and sphalerite relative to other zones. These deposits occur at intersections between D4 north-northeast striking brittle fault zones and D2 ≈ 045° striking ductile shear zones. The differences in mineralization styles between individual zones result from the presence or absence of these intersecting D2-D4 structures. Gold and silver mineralization is the result of hydrothermal fluid infiltration along zones of high strain that formed during D2 and D4 deformation events. Amphibole-plagioclase schist has been altered by biotite ± quartz ± sulphide mineral alteration (D2) and Cr-clinochlore + carbonate ± hornblende + quartz alteration (D4)

Chloritic alteration in the east flank of the Caçapava Granite, State of Rio Grande do Sul: metasomatic evolution and associated Cu-Fe sulfides

This work investigated the origin and evolution of chloritites and chlorite bearing rocks and Cu-Fe sulfides, associated with sill and apophysis of the Caçapava Granite, which intrude in the dolomitic marbles of the Passo Feio formation, Terrane São Gabriel of the Sul-Riograndense Shield. The interaction of the granite fluids with the marbles generates high- and low-temperature mineral associations. The low-temperature alteration halos on the granitoid are mainly constituted of chlorite followed by albite titanite, sulfide and rare rutile. Chlorite, calcite and, eventually, serpentine, talc and sulfides constitute the low-temperature alteration in the country-rock marbles. The alteration halos are distributed in domains along the granitoid matrix as well as in stock-works or in hydrothermals breccias and its related fractures with chalcopyrite and pyrite. Important hydrothermal metassomatic alteration processes have been identified, notably, chloritization and albitization in the granitoid protholite. The alteration is progressive in the granitoid apophyses, with modifications since incipient to the rocks mainly constituted of chlorites, culminating in the generation of chloritites. The nucleation and growth of chlorite develops from the substitution of biotite and of plagioclase, or deposits it in empty spaces of carbonatic country-rocks. The mass balance by the isocon method indicates that the chloritization process developed through the enrichment of MgO and FeO and depletion in SiO2, K2O and Na2O in the protholite granitoids. The abundance pattern and distribution of rare-earth element (REE) belonging to Complex Granitic Caçapava do Sul. Geothermometry of chlorite composition indicates an average temperature between 280 and 300ºC for the chlorite formation.Este trabalho investigou a origem e a evolução de clorititos — rochas portadoras de clorita e/ou sulfetos de Cu-Fe —, associados a sills e apófises do Granito Caçapava que intrudem nos mármores dolomíticos da Formação Passo Feio, Terreno São Gabriel do Escudo Sul-Riograndense (ESRG). A interação dos fluidos graníticos com os mármores gera associações minerais de alta e baixa temperatura. Os halos de alteração de baixa temperatura nos granitoides são constituídos dominantemente por clorita seguida de albita, titanita, sulfetos e raro rutilo. Clorita, calcita e, eventualmente, serpentina, talco e sulfetos constituem a alteração de baixa temperatura nos mármores encaixantes. Os halos de alteração distribuem-se em domínios ao longo da matriz dos granitoides, bem como em stockworks ou em brechas hidrotermais e fraturas contendo calcopirita e pirita. Identificam-se processos importantes de alteração hidrotermal-metassomática, notadamente cloritização e albitização no protólito granitoide. A alteração é progressiva nas apófises granitoides com modificações desde incipiente até rochas compostas majoritariamente por cloritas, culminando com a geração de clorititos. A nucleação e o crescimento da clorita ocorre a partir da substituição da biotita e do plagioclásio ou deposita em espaços vazios na rocha carbonática encaixante. O balanço de massa pelo método da isócona indica que o processo de cloritização evoluiu por meio do enriquecimento de MgO e FeO e do empobrecimento em SiO2, K2O e Na2O dos protólitos granitoides. Os padrões de abundância e distribuição de elementos terras raras (ETR) indicam correlação direta das rochas portadoras de clorita e dos clorititos com fácies pertencentes ao Complexo Granítico de Caçapava do Sul. Geotermometria a partir da composição da clorita indica temperatura média entre 280 e 300ºC para formação das cloritas

The komatiite-hosted Pahtavaara gold deposit, Central Lapland:geochemistry, hydrothermal alteration and statistical analyses of usability of the portable XRF data

Abstract. Pahtavaara gold-only deposit in Sodankylä, Central Lapland, is hosted by komatiites of the Sattasvaara formation. The genesis of the deposit remains unsolved. Orogenic gold or Au-rich volcanogenic massive sulphide deposits have been typically proposed as the genesis of the deposit. Both types of deposit have formed by hydrothermal processes and thus, the study of hydrothermal processes such as hydrothermal alteration, is crucial for understanding the formation of the deposit. The Pahtavaara gold deposit is comprehensively drilled but the laboratory analyses lack partially of multi-elemental analyses. The use of portable XRF analyser in geoscience has increased in the previous years due to its time- and cost efficiency features and, development of detection limits and accuracy. As such, it is reasonable to the test the usability of pXRF measurements of laboratory-returned pulp samples. The study consists of two parts: 1) statistical analyses of the pXRF data and its usability and, 2) geochemical studies comprising the komatiitic host rock and hydrothermal alteration by utilising the pre-existing, laboratory analysed multi-elemental data and previously published studies of the other komatiitic deposits in the Central Lapland. Based on the statistical analyses, the pXRF measurement of Ca, Fe, Ni, Sr, Zn, Mn, Al, and Mg showed excellent or good accuracy and repeatability. The results were satisfactory for Cr, Cu, S and Y and bad for Ti, V and Zr. Despite of the good results of statistical analyses, the use of pXRF analyser is recommended only as an indicative method in geochemical studies and it cannot replace the laboratory multi-elemental analyses. The Pahtavaara komatiites did not show a significant correlation with Peuramaa, Lomalampi and Jeesiörova komatiitic rocks. The precursor of the Pahtavaara rocks proved to be complex. Based on the varying contents of immobile elements, the precursor may not be the same for all the lithological units. The mass balance calculations of the lithological units of the study area—talc-chlorite and amphibolite schists and, actinolite-tremolite rock—showed enrichment of MgO, CaO, TiO₂, MnO, Ba and Sr and, depletion of Fe₂O₃, Na₂O, and K₂O. On the grounds of mass balance calculation and the graphs of hydrothermal alteration, the actinolite-tremolite rocks are trending towards carbonatisation. The talc-chlorite and amphibolite schists were characterised by a formation of chlorite minerals which in fact, is more likely related to carbonate alteration of tremolite-rich rocks instead of chloritisation. Due to this, the actinolite-tremolite precursor is more probable than the talc-chlorite schist suggested in the literature.Pahtavaaran komatiittinen kultaesiintymä Keski-Lapissa : geokemia, hydroterminen muuttuminen ja tilastollinen tutkimus käsikäyttöisen XRF-analysaattorin mittaustulosten käytettävyydestä. Tiivistelmä. Pahtavaaran kultaesiintymä sijaitsee Sodankylässä Keski-Lapin vyöhykkeellä ja se liittyy Sattasvaaran komatiittiseen muodostumaan. Kultaesiintymän alkuperä ei ole yksiselitteinen. Mahdolliseksi alkuperäksi on tyypillisesti ehdotettu orogeenista kultaesiintymää tai kultarikasta, merenpohjassa syntynyttä massiivista sulfidiesiintymää (VMS). Molemmat esiintymätyypit ovat syntyneet hydrotermisten prosessien vaikutuksesta. Näin ollen hydrotermisten prosessien, kuten hydrotermisen muuttumisen tunteminen on ensisijaisen tärkeää, kun halutaan ymmärtää hydrotermisten kultaesiintymien muodostumista. Pahtavaaran kultaesiintymästä on kattavasti kairasydämiä, mutta geokemiallisista analyyseista puuttuu osittain monialkuaineanalyysit. Käsikäyttöisen XRF-analysaattorin käyttö geotieteellisissä tutkimuksissa on kasvanut viime vuosina. Sen lisäksi, että se on nopea ja kustannustehokas analysointimenetelmä, myös laitteiden määritysrajat ja mittaustarkkuus ovat kehittyneet. Nämä ominaisuudet huomioiden on perusteltavaa tutkia, kuinka käsikäyttöinen XRF-analysaattori soveltuu jauhettujen laboratorionäytteiden analysointiin. Tutkimus koostuu kahdesta osasta: 1) tilastollinen tutkimus käsikäyttöisen XRF-analysaattorin mittaustulosten käytettävyydestä ja 2) Pahtavaaran kultaesiintymän komatiittisen isäntäkiven ja hydrotermisen muuttumisen geokemiallinen tutkimus hyödyntäen jo olemassa olevia monialkuaineanalyyseja ja aikaisemmin julkaistuja tutkimuksia muista Keski-Lapin komatiittisista esiintymistä. Tilastollinen tutkimus käsikäyttöisen XRF-analysaattorin käytettävyydestä osoitti, että alkuaineiden Ca, Fe, Ni, Sr, Zn, Mn, Al ja Mg mittaustulosten tarkkuus ja toistettavuus olivat erinomaisia tai hyviä. Alkuaineet Cr, Cu, S ja Y osoittivat tyydyttävää mittaustarkkuutta, kun taas alkuaineiden Ti, V ja Zr mittaustulokset korreloivat huonosti laboratorioanalyysien kanssa. Huolimatta hyvistä tilastollisen tutkimuksen tuloksista, käsikäyttöisen XRF-analysaattorin käyttäminen on suositeltavaa vain suuntaa antavana analysointimenetelmänä, eikä sillä voi korvata laboratoriossa suoritettuja monialkuaineanalyyseja. Pahtavaaran, Peuramaan, Lomalammen ja Jeesiörovan komatiittisten kivien geokemiassa ei ollut havaittavissa merkittävää samankaltaisuutta. Protoliitin löytäminen Pahtavaaran kiville osoittautui hankalaksi. Liikkumattomien alkuaineiden vaihteleviin pitoisuuksiin perustuen, Pahtavaaran eri litologisilla yksiköillä voi olla eri protoliitit. Massatasapainolaskut osoittivat, että tutkimusalueen kaikki litologiset yksiköt, talkki-kloriitti- ja amfiboliittiliuske sekä aktinoliitti-tremoliittikivi, olivat rikastuneet MgO-, CaO-, TiO₂- ja MnO-yhdisteistä sekä bariumista ja strontiumista, ja köyhtyneet Fe₂O₃-, Na₂O- ja K₂O-yhdisteistä. Massatasapainolaskuihin ja hydrotermista muuttumista mallintaviin kuvaajiin perustuen aktinoliitti-tremoliittikivet ovat karbonaattiutuneet. Talkki-kloriitti- ja amfiboliittiliuskeet osoittivat merkkejä kloriittimineraalien syntymisestä hydrotermisen muuttumisen seurauksena, mikä todennäköisemmin liittyy tremoliittirikkaiden kivien karbonaattiutumiseen eikä kloriittiutumiseen. Näin ollen aktinoliitti-tremoliittikivi saattaa olla lähempänä Pahtavaaran kivien todellista protoliittia kuin aikaisemmissa tutkimuksissa ehdotettu talkki-kloriittiliuske

Microstructure, Geometry and Hydrogeology of the fracture network of the Boda Claystone Formation

This dissertation extensively examines the brittle structural elements of the Boda Claystone Formation (BCF), which could serve as a host formation for high-level radioactive waste in Hungary. Despite the formation's low porosity and permeability, which give its retentive features, it is crucial to assess the tectonic setting of the rock body to comprehend the timing and magnitude of faults and folds. The idea behind the research is that each successive phase of the study examines the problem at an increasing scale, from microstructural observations to models of the fracture network based on more than one borehole. The main goals of this study were to identify the geometry and hydrogeological properties of the fracture network of the Boda Claystone Formation as well as the investigation of the spatial extensibility of the properties of the fracture network. The BAF–2 well was regarded as a base well in this study because it explores the maximum thickness of the formation (917 m) with almost 100% core recovery and offers unique opportunity to evaluate its brittle structural elements. In addition, the analysis of the BAF–4 well was also done to investigate the extensibility of the properties of the fracture system. The petrographic description of the sigmoidal veins in the shear zones was carried out on thin sections. The volume and density changes of the shear zones were analysed based on the geometry of the sigmoidal veins, using the isocon method and micro-computed tomography. The geometry of the fracture network was investigated using the discrete fracture network modelling method, while its hydrological properties were characterised using the flow zone indicator. This indicator designates hydraulic flow units based on porosity and permeability. The relationship between the fracture network and well log data was investigated using multiple linear regression analysis. Based on petrographic observations, the sigmoidal veins in the BCF can be interpreted as "pennant" veins whose formation was caused by conjugate Riedel fractures. Despite the formation of sigmoidal veins, a 5-8% volume loss occurred in the shear zones. Based on fracture network modelling, five fracture network geometries can be distinguished in the BAF-2 well. In some models, the fracture network forms a communicating system along the entire length of the borehole, while in other cases it breaks down into subsystems that do not communicate with each other at depths of 100, 400, or 700 meters. The fracture system can be divided into seven hydraulic units, the boundaries of which coincide with the boundaries designated by fracture network modelling. In the upper 100 meters of the borehole, the hydrological properties of the rock were affected by weathering. Based on the properties of the fracture network, the rock column of the well can be divided into two very distinct blocks with a border at a depth of 400 m. The hydrological properties of the formation at 700 meters were determined by the by fine-grained sandstone layers. Based on multiple linear regression analysis, the fracture density of the BCF can be estimated based on density and resistance well log data. Fracture density can also be estimated in boreholes without acoustic borehole televiewer data using the regression equation

The Influence of Contact Metasomatism and Fluid-Rock Interaction, on the Nature and Style of Platinum-Group Element Mineralisation in the Platreef, Northern Limb, South Africa: A Case Study from the Moordkopje Farm.

Magister Scientiae - MSc (Earth Science)The complexity of the Platreef stratigraphy and the generic position of the Ni-Cu-PGE mineralisation is a challenge to prospecting and mining companies in the Northern Limb of the Bushveld Complex, partially, as a result of various floor rock interactions with the reef. Therefore, this study evaluated the effects of contact metasomatic fluids on the nature and style of PGE mineralisation as the main event leading to the complexity of the Platreef stratigraphy from the contact zone near the floor rock. Fifty samples from boreholes MO009 and MO019 drilled at Moordkopje 813 LR farm for Akanani Project by Lonmin Plc were used for this study. The mineralogy and geochemistry of the Platreef samples were studied and associated with their mineralisation occurrences. Major, minor and trace element contents were analysed by XRF analysis using fused beads, and PGE contents (Pt, Pd) in 11 samples were determined by Fire Assay