Magmatic Ore Deposits — Page-by-Page Notes

Page 1

• Topic: Magmatic ore deposits form during cooling and crystallization of magmas emplaced in crust (continental or oceanic).

• Deposits are spatially found within rock types derived from the parent magma.

• These are typically referred to as orthomagmatic deposits.

Page 2

• Magmatic deposits are genetically linked with magma evolution when magma intrudes as plutonic (intrusive) or volcanic (extrusive) events.

• Reiterates: ore deposits formed by magmatic processes are orthomagmatic.

• Mineralization is located within rock types derived from crystallization of the parent magma.

Page 3

• Commonly concentrated elements/minerals include: nickel, chromium, copper, titanium, vanadium, platinum-group elements (PGEs), and iron.

• Major types of magmatic ore deposits: Chromite; PGEs (Pt, Pd, Rh, Ru, Os, Ir); Cu–Ni–Fe sulfides; Fe–Ti–V oxides; Rare earth elements (REE).

Page 4

• The most significant magmatic deposits relate to three rock families: felsic (granite, rhyolite), mafic (gabbro, norite), and ultramafic (peridotite, dunite).

Page 5

• In mafic and ultramafic rocks, magmatic ore deposits occur in several settings:

  • Layered mafic intrusions: chromite and PGE (Bushveld Complex, Great Dyke)

  • Cu–Ni–Fe sulfide deposits (Sudbury, Norilsk)

  • Ophiolites: podiform chromite (Turkey)

  • Komatiites: Ni–Cu–Fe sulfide (Kambalda, Australia)

  • Kimberlites: Diamonds (Kimberley, SA)

• In felsic intrusions: Carbonatites (Cu ore in Phalabora, SA); Nephelinite intrusions (REE, P, Nb, Li, Be, etc.).

Page 6

• Ore minerals crystallize by the separation of metal sulfides and oxides within a molten igneous melt prior to crystallization from felsic, mafic, and ultramafic magmas.

Page 7

• Concentration of ore elements/minerals can arise from: 1) Physical behaviour of magma pulses:

  • Fractional crystallization, crystal settling (gravitational)

  • Liquid immiscibility

  • Magma mixing

  • Assimilation

  • Partial melting
    2) Chemical processes:

  • Partition coefficients of ore elements between coexisting phases.

• Generalization: placer-like removal of crystals changes melt composition, enriching residual melts in compatible/incompatible elements depending on partitioning.

Page 8

• Assimilation (magma contamination): compositional modification of preexisting magma by incorporation of country rock material.

• Country rock is at a different (usually lower) temperature, so heat exchange occurs across the country rock–magma boundary.

Page 9

• Partial melting: occurs when rocks are heated enough to melt certain minerals; different minerals melt at different temperatures.

• Typical partial melting range: ~600–1300 °C.

• Early-melting minerals (e.g., quartz if present) melt first; olivine melts last (if present).

• Generally, more siliceous magmas form at lower degrees of partial melting; increasing degree of partial melting yields less siliceous compositions.

Page 10

• Magma mixing: coalescence of two or more distinct magmas to create a chemically/physically homogeneous magma with a composition different from the original melts.

Page 11

• Liquid immiscibility: segregation of two coexisting liquid fractions from an originally homogeneous magma during cooling.

• During crystallization, residual melt can separate into two immiscible melts:

  • One liquid is silicate-rich and larger in volume.

  • The other is typically smaller in volume and rich in metal oxides, sulfides, or carbonates.

Page 12

• Fractional crystallization (crystallization fractionation): as magma cools, minerals nucleate/grow and separate from the residual melt, changing the melt composition over time.

Page 13

• Gravitational Settling (Crystal Settling): a type of fractional crystallization where crystals settle due to density differences with the melt.

Page 14

• Gravitational settling depends on density contrasts:

  • Heavier crystals sink to the chamber floor.

  • Lighter crystals float toward the top.

• Result: effective segregation of dense crystals from residual melt.

Page 15

• Viscosity controlled by silica content: higher silica → higher viscosity → slower segregation.

• Low-viscosity magmas (ultramafic, mafic: basaltic) favor settling; felsic magmas are more viscous and hinder settling.

• Densities: mafic minerals and oxides (~>3 g/cm^3); mafic/ultramafic magmas typically have densities < 2.6 g/cm^3.

Page 16

• Magmatic deposits form with slow cooling/crystallization allowing gravity settling to occur.

• Mafic/ultramafic intrusions cool slowly enough for settling; felsic magmas cool quickly, inhibiting segregation.

• High-temperature crystallizing minerals: chromite (FeCr2O4), magnetite (Fe3O4), ilmenite (FeTiO3) crystallize at high temperatures.

Page 17

• Table of minerals with densities and melting points (illustrative):

  • Augite/pyroxene: density ~3.3 g/cm^3, mp ~1150 °C

  • Olivine: density ~3.4 g/cm^3, mp ~1450 °C

  • Plagioclase feldspar: density ~2.7 g/cm^3, mp ~1100 °C

  • Magnetite: Fe3O4, density ~5.7 g/cm^3, mp ~1600 °C

  • Chromite: FeCr2O4, density ~4.6 g/cm^3, mp ~1500 °C

Page 18

• Mineral development and layering in basaltic magmas: Fe-Ti oxides or chromite crystallize first, then olivine, pyroxene, plagioclase.

• Layering forms cumulates: basal chromitite, dunite, pyroxenite, norite, leuconorite, anorthosite.

• Layers are typically perpendicular to chamber walls and can extend over kilometers.

Page 19

• Chromite deposits can form by gravitational settling: chromium-bearing spinel (FeCr2O4) crystallizes early and sinks to form cumulate in the chamber bottom.

Page 20

• Two main magmatic chromite deposit types:

  • Stratiform chromite deposits (≈75% of resources): massive chromitite bodies or disseminated chromite in layered intrusions, typically in lower ultramafic parts.

  • Podiform chromite deposits (≈25%): small chromite bodies in ultramafic section of ophiolites.

Page 21

• Cumulate rocks formed by crystal settling are called cumulates; layering arises from differential settling and floating of crystals; cemented by residual magmatic fluids.

Page 22

• Stratiform chromite deposits form as stratiform chromitite seams in large layered mafic–ultramafic intrusions in cratonic regions.

• Chromitite seams are typically at the base of ultramafic parts of the intrusion.

Page 23

• Characteristics of chromitite seams:

  • Lateral continuity across layered intrusions; generally conform to intrusion geometry (sill-like, saucer/funnel shapes).

Page 24

• Host rocks for stratiform chromite deposits include varying lithologies (norite, gabbronorite, dunite, harzburgite, lherzolite, pyroxenite, troctolite, anorthosite, orthopyroxenite, gabbro).

• Lithological variability is significant between deposits and within a single layered intrusion.

Page 25

• Chromitite seam dimensions: seams are thin relative to overall intrusion size; thickness ranges from <1 cm to 5–8 m, often cyclic with silicate layers and laterally continuous across the intrusion.

Page 26

• Slide heading: Bushveld Complex (context for stratiform chromite and related deposits).

Page 27

• Stratiform chromite deposits are generally associated with PGE deposits (PGE reefs). Examples include Stillwater Reef (Ni–Cu–PGE) and Bushveld reefs (Merensky, UG2).

• PGE deposits can occur within chromitite seams such as UG2.

Page 28

• Worldwide examples: major stratiform chromite resources are in SA, Zimbabwe, Canada, Stillwater (USA); Bushveld Critical Zone is the most well-known stratiform chromite deposit globally.

• Zimbabwe hosts both stratiform and podiform chromites.

Page 29

• Podiform chromite deposits occur in ultramafic sections of ophiolites in oceanic crust.

• Ophiolite definition: a section of oceanic crust and upper mantle thrust onto continental crust.

Page 30

• Ophiolite sequence represents fragments of oceanic crust and mantle emplaced on continental crust.

Page 31

• Podiform chromite morphology/host rocks: chromitites occur in dunite, serpentinized harzburgite, serpentinized dunite; rarely longer than ~100 m.

Page 32

• (Illustrative slide showing chromitite pods in a serpentinized ultramafic mantle sequence.)

Page 33

• Global distribution of podiform chromite: known in Kazakhstan, Russia, the Philippines, Zimbabwe, Cyprus, Greece.

• Zimbabwe is unique in exploiting both stratiform and podiform chromites.

Page 34

• Typical chromitites composition: fine-grained cumulus chromite with interstitial olivine, orthopyroxene, plagioclase.

• Grade depends on Cr2O3 content and Cr:Fe ratio.

• High-grade chromite for metallurgical uses typically has Cr/Fe > ~2.0 (often ~2.8) and Cr2O3 ~46–48%.

Page 35

• Stratiform vs podiform chromite grade differences:

  • Podiform chromites have higher Cr2O3 (>55 wt%) and higher Cr/Fe ratios (Cr/Fe ~ 2–3.3) but smaller reserves.

  • Stratiform chromites generally have Cr2O3 around 43–47% with Cr:Fe ratios ~1.26–1.60 depending on subunit.

Page 36

• General grade trends in Bushveld chromitites: Cr content decreases upward from LG6 to UG2; Cr:Fe ratio also decreases upward (LG6 ~1.56–1.60; MG ~1.35–1.50; UG2 ~1.26–1.40).

Page 37

• Metallurgical-use distinctions: high-grade chromite (Cr:Fe > 2.0) suitable for ferrochrome production; lower-grade chromite suitable for refractory applications.

Page 38

• Liquid immiscibility (revisited): reaffirmed concept of segregation of two coexisting liquids during cooling.

Page 39

• Specific immiscibilities documented/possible in magmas:

  • Silicate–sulphide immiscibility

  • Silicate–oxide immiscibility

  • Silicate–carbonatite immiscibility (carbonate-rich liquids, carbonatites)

  • Silicate–oxide immiscibility can produce Fe–Ti–V oxide-rich deposits.

Page 40

• Chemical control in magmas: magmas are multi-phase systems; multiple melts and minerals may coexist; minor/trace elements partition between coexisting phases during crystallization.

Page 41

• Nernst Partition Coefficients (K values):

  • Ki = ci(a) / c_i(b) where a and b are coexisting phases (minerals, melts, etc.).

Page 42

• Uses of partition coefficients:

  • Petrogenesis: modeling crystallization of minerals from magma and resulting rock composition.

  • Trace element behavior: understanding how trace elements concentrate in rocks.

  • Geochemical modeling: essential for simulating magmatic processes.

Page 43

• Partitioning between a mineral and melt: explains distribution of an element between a crystallizing mineral and the melt.

• KD > 1 implies a element is compatible (prefers mineral); KD < 1 implies it remains in melt (incompatible).

• Examples of compatible elements in olivine/pyroxene: Ni, Cr, Sc.

• Examples of incompatible elements: Rb, Ba, U, La.

Page 44

• Compatibility example: a given mineral assemblage with Sm, Ce, Nd shows variable compatibilities across amphibole, pyroxene, magnetite, and plagioclase

  • Sm: Amphibole 2.99, Pyroxene 1.76, Magnetite 0.65, Plagioclase 0.061

  • Ce: Amphibole 0.98, Pyroxene 0.56, Magnetite 0.35, Plagioclase 0.12

  • Nd: Amphibole 2.1, Pyroxene 1.1, Magnetite 0.0, Plagioclase 0.07
    (Illustrative partitioning coefficients to show variable compatibility across minerals.)

Page 45

• (Partitioning between minerals and melts continued: practical example table shows how different elements distribute across minerals.)

Page 46

• Partitioning between two melts (concept): differential distribution of elements between interacting melts affects final magma composition.

Page 47

• Liquid immiscibility types relevant to magmatic ore formation:

  • Silicate–sulphide immiscibility

  • Silicate–oxide immiscibility

  • Silicate–carbonate immiscibility

• This framework helps explain Ni–Cu–PGE sulfide deposits, Fe–Ti–V oxide deposits, and REE-carbonatite associations.

Page 48

• Silicate–sulphide immiscibility: Ki = Ci-sulphide / Ci-silicate.

• Higher Ki means a given element prefers the sulphide phase (more chalcophile).

• Representative Ki values: Pt/Pd > 10,000; Cu ≈ 250; Ni ≈ 100; Co ≈ 40.

• This immiscibility is central to the formation of Ni–Cu–PGE magmatic deposits.

Page 49

• Magmatic sulfide deposits form when a fractionating magma saturates in sulfur, exsolving a separate immiscible sulfide liquid at end of fractional crystallization.

• Sulphur behaves as an incompatible element under sulphide-undersaturated conditions; S increases in the melt during fractional crystallization.

• Ni, Cu, and PGEs are chalcophile and become concentrated in the sulfide liquid, forming economic Ni–Cu–PGE accumulations.

Page 50

• The sulfide liquid acts as a collector for chalcophile elements, concentrating them by factors of 10 to 100,000 times relative to silicate liquids.

• Immiscible sulfide droplets settle to form stratigraphic layers or reefs at the base of the intrusion or lava flow.

Page 51

• Reef-like accumulations: sulfide droplets settle into stratigraphic layers or reefs, ranging from millimeters to meters in thickness.

• These reefs host Ni–Cu–PGE mineralization and are potentially economically mineable.

Page 52

• (Diagram-heavy page showing periodic table arrangement; note on elemental distribution not required for textual notes.)

Page 53

• Principal ore minerals in magmatic sulfide deposits: sulfide minerals dominated by pyrrhotite (FeS), pentlandite ((Fe,Ni)9S8), and chalcopyrite (CuFeS_2).

• PGEs are present as small grains in PGE-bearing sulfides, arsenides, antimonides, bismuthinides, and tellurides.

• Host rocks are predominantly mafic to ultramafic.

Page 54

• Age and distribution: many deposits are very old (Proterozoic–Archaean, 4.0–2.5 Ga).

• Major deposits include: Norilsk (Russia), Merensky Reef (South Africa), Kambalda (Western Australia), Sudbury (Canada).

Page 55

• Silicate–oxide immiscibility: Fe-rich basaltic magmas may separate into two liquids – felsic (SiO2-rich) and mafic (FeO-rich).

• Oxide melts are magnetite-rich with high Ti and V; Ti and V are siderophile and partition into oxide melts.

Page 56

• Silicate–carbonate immiscibility: carbonatites form via separation into carbonate-rich and silica/alkali-rich silicate liquids.

Page 57

• Carbonatites: igneous rocks with >50% modal carbonate minerals (calcite, dolomite, ankerite); often associated with alkaline magmatism.

Page 58

• Carbonatites: typically pipe-like bodies, dikes, sills, small plugs up to several kilometers in diameter.

• Host rocks can include calcite-rich, dolomite-rich, iron-rich carbonatites, and silico-carbonatites.

Page 59

• Illustrative cross-section showing relationships among metallic/industrial mineral deposits and lithological units.

Page 60

• Carbonatites are categorized by predominant carbonate mineral (calcite, dolomite, ferrocarbonatite).

Page 61

• Origin of carbonatites: formed via partial melting of carbonate-bearing mantle rocks; direct mantle source melting.

Page 62

• Liquid immiscibility in carbonatites: separation of carbonate liquid from CO2-saturated alkaline magmas (nephelinite/phonolite).

• Alkaline magmas contain CO2, alkalis, halides, and phosphorus as intrinsic components of mantle-derived magmas.

Page 63

• Immiscible silicate–carbonate liquids: Ki = Ci-carbonate / Ci-silicate.

• Carbonate melts can be rich in Nb, Ta, REE, Cu, Th, and P.

Page 64

• Separation of carbonate liquid leads to strong fractionation between carbonate and silicate phases.

• Fractionation commonly enriches LREE (Sc, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd), HFSE (Zr, Ti, Nb, Ta), and radioactive elements (U, Th).

• These elements are characteristic of alkaline magmas and are incompatible (i.e., preferentially excluded from early silicate crystallization).

Page 65

• REE distribution and periodic table context (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu shown).

Page 66

• In carbonatites, REE and Nb occur prominently; REE minerals include Bastnäsite (Ce, La, Pr)CO3F, Monazite (Ce, La, Nd, Th)PO4, Pyrochlore ((Na, Ca)2Nb2O6), Xenotime (YPO4).

Page 67

• Fenitization in carbonatites: alkali metasomatism of country rocks caused by fluids from crystallizing carbonatite; introduces Na/K-rich fenites.

• Fenites can be Na-rich or K-rich depending on dominant alkali.

Page 68

• (Diagram page repeated; cross-section similar to page 59; emphasis on carbonatite mineralization contexts.)

Page 69

• Transition to Bushveld Complex geology and mineralization overview.

Page 70

• Bushveld Complex comprises four main suites:

  • Lebowa Granite Suite

  • Rashoop Granophyre Suite

  • Rustenburg Layered Suite (RLS) – mafic-ultramafic rocks

  • Rooiberg Group (early ultramafic–mafic sills)

Page 71

• Rustenburg Layered Suite (RLS) details:

  • World’s largest layered intrusion; 7–9 km thick layered sequence of mafic–ultramafic rocks.

  • Extends ~450 km east–west and ~350 km north–south; ~8 km thick; underlies ~65,000 km².

Page 72

• Surface exposure of RLS occurs in four limbs (Western, Eastern, Northern, Far Western). A fifth limb (Southeastern Bethal) inferred from gravity and boreholes.

Page 73

• Regional geography: Pilanesberg, Lebowa Granite, Villa Nora, Rashoop Granophyre, Rustenburg Suite, Rooiberg Group, Transvaal Supergroup, Pre-Transvaal rocks; major faulting boundaries (e.g., Thabazimbi).

Page 74

• RLS stratigraphy and subdivision: thickness 7.5–9.0 km; subdivided into five zones from base upward:

  • Marginal Zone (MZN)

  • Lower Zone (LZ)

  • Critical Zone (CZ)

  • Main Zone (MZ)

  • Upper Zone (UZ)

Page 75

• Cross-section of Rustenburg Layered Suite showing zones and major reefs: Merensky Reef, UG2, MG1–MG4, LG1–LG7, Main Zone, Upper Zone sequences; thicknesses ranging from hundreds to thousands of meters.

Page 76

• Marginal Zone: contact between cumulate sequence and country rocks; ~300 m thick; evidence of rapid crystallization and fractional differentiation.

Page 77

• Lower Zone: olivine-rich and orthopyroxene-rich layered cumulates; thickness ~800–1300 m.

Page 78

• Critical Zone (CZ): up to ~1500 m thick; economically the most important; carries Merensky Reef and UG2 chromitite; large chromite reserves.

Page 79

• CZ subdivided into CZL (Lower Critical Zone) and CZU (Upper Critical Zone).

  • CZL: thick (≈800 m) pyroxenitic cumulates with harzburgite/dunite components.

  • CZU: thick (≈800 m) norite and anorthosite sequences.

Page 80

• Main Zone: 1600–3500 m thick; consists of norite and gabbronorite with minor anorthosite/pyroxenite.

Page 81

• Upper Zone: 1000–2700 m thick; Fe-rich; contains at least 30 magnetite seams interlayered with gabbronorite.

Page 82

• Northern Limb stratigraphy: thinner overall; CZL and CZU thinner or absent in this limb.

Page 83

• Sectoral variations (Northern Central Southern sectors) with maps showing limb-specific geology and mineralization contexts.

Page 84

• Geological map highlights Main Zone, Platreef, Lower Zone pyroxenites, Timeball Hill Formation, Duitsland Formation, Malmani Subgroup, Granitic basement, faults, and limb boundaries.

Page 85

• Platreef: basal contact near the country rock; thickened basal sequence; stratigraphic footwall and hanging-wall relationships; lithologic transitions from Transvaal Supergroup to quartzites, banded ironstones, dolomites, and Archaean granites.

Page 86

• Platreef geology map: illustrates subdivisions and cross-strata in northern limb sectors; emphasizes lithologic units and faults.

Page 87

• Cross-sectional stratigraphy: detailed magnetite-rich Upper Zone (Main Magnetite Layer) and the distribution of magnetite seams across Western, Eastern, and Northern limbs; Merensky Reef and UG2 are key reefs in CZ with chromitites, pyroxenites, and norites as footwalls.

Page 88

• Platreef geology: same map/labels reinforcing structural geometry for the Platreef area.

Page 89

• Platreef geology field observations: boreholes, hanging wall lithologies (gabbronorite, mottled anorthosite), Platreef lithologic units (feldspathic pyroxenite, peridotite reefs), footwall lithologies (serpentinite, clinopyroxenite, serpentinized xenoliths).

Page 90

• Chromite mineralization in Bushveld: chromitites are largely CZ-restricted, with minor occurrences in Lower Zone in the northern limb; three chromitite groups delineated across the CZ: Lower Group (LG), Middle Group (MG), Upper Group (UG).

Page 91

• Correlation across western/eastern Bushveld shows three chromitite groups (LG, MG, UG) with varying thickness; the CZ start is marked by chromitite layers.

Page 92

• MG2 and MG3 chromitites are intercalated with anorthosite, norite, and feldspathic pyroxenite; middle-group anorthosite serves as a persistent “marker” in the CZ (Tweefontein).

Page 93

• Lower Group chromitites (LG): up to seven layers (LG1–LG7) in pyroxenite; LG6 is the main mining layer; each layer can be ~1 m thick.

Page 94

• MG2 and MG3 intercalations and marker relationships reiterated; MG2, MG3, and MG4 chromitites appear within the CZ with markers such as anorthosite.

Page 95

• Depth distribution of chromitite groups relative to Merensky Reef and UG2: ordering typically LG7/LG6 near CZL and deeper MG/UG sequences above/below CZU depending on the section; Merensky Reef sits within CZU in some sections.

• Illustrative cross-sections show placement of LG, MG, UG chromitites, Merensky Reef, UG2, UG1, MG4, MG3, MG2, MG1, LG5–LG7.

Page 96

• Middle Group chromitites (MG1–MG4): four chromitite layers between CZU and CZL, located at the pyroxenitic/anorthositic junction.

Page 97

• Cross-section summary: Merensky Reef and UG2 occur at key stratigraphic horizons; alignment of MG and UG chromitites with major reef zones.

Page 98

• Upper Group chromitites (UG1–UG2; UG3 in parts of the eastern Bushveld); chromitite layers near the upper CZ.

• Merensky Chromitites: two-to-four thin chromitite layers (1–20 mm each) define the upper and lower limits of the Merensky reef, with peak PGE concentrations near the upper chromitite.

Page 99

• Chromitite zoning and reef geometry summarized: LG, MG, UG layers arranged with deeper Merensky Reef and UG2 deposit contexts; cross-cutting relationships shown in diagrams.

Page 100

• Cr2O3 contents across chromitite groups show upward depletion: LG6 ~46–47% Cr2O3; MG ~44–46%; UG2 ~43% Cr2O3.

• Corresponding Cr:Fe ratios trend downward upward: LG6 1.56–1.60; MG 1.35–1.50; UG2 1.26–1.40.

Page 101

• Platinum-Group Elements (PGE) mineralization in Rustenburg Layered Suite:

  • PGE mineralization occurs in stratiform sulfide-bearing horizons (Merensky Reef, Platreef, Bastard Reef) and in chromitite seams (LG1–LG7, MG1–MG4, UG1–UG2).

  • PGE concentration increases upward from the lower chromitites to upper layers.

  • Discordant dunite pipes in CZ of eastern Bushveld also host PGEs.

Page 102

• The three main economic PGE orebodies in Bushveld: UG2 chromitite (UG2), Merensky Reef (CZU in eastern/western limbs), and Platreef (basal contact in northern limb).

• Discordant dunites in eastern Bushveld are largely mined out.

Page 103

• PGE mineralization grades/contents:

  • PGM g/t (Pt+Pd+Rh) and (Ru+Os+Ir) shown for West/East sections across reefs.

  • Merensky Reef, UG2, MGs, and UG1/UG2 show variable PGM contents; typical reef-grade distributions are presented across sections.

Page 104

• Grade distributions: UG2 grade histogram (mainly Pt, Pd, Rh; and other PGEs) shows localized peaks in chromitite layers; width distributions presented for several UG2 grade profiles.

Page 105

• UG2 composition: UG2 is predominantly chromite-rich (60–90% by volume) with minor interstitial silicates (orthopyroxene 5–30%, plagioclase 1–10%). Sulfides are very low (<0.1%).

• Typical PGE-bearing silicate associations are described with extremely low sulfide contents in UG2.

Page 106

• PGE systematics in UG2:

  • Total PGE values up to ~10 ppm (g/t) PGE+Au; typical range 4–7 g/t.

  • PGEs in UG2: Pt ~49.5%, Pd ~22.5%, Ru ~15%, Rh ~8.7%, Ir ~3.7%, Au ~0.6% (proportions vary regionally).

  • PGMs occur as PGMs in sulfide/semi-metal minerals, with grain sizes ~12 μm on average.

  • Common PGMs include Pt, Pd, Rh, Ru, Ir, Os, with various sulfide, arsenide, and telluride hosts.

Page 107

• Common PGMs families shown in diagrams; PGMs include sulfides, arsenides, sulpho-arsenides, tellurides, and alloys.

Page 108

• High-resolution micrographs illustrating PGE minerals in sulfides and associated alloys/arsenides; details of Pt-Pd-Ni-S, Pt-Te-Bi composites observed.

Page 109

• Micrographs illustrating Pt-Te-Bi and related PGE mineralogy in sulfide and alloy contexts.

Page 110

• Main chromitite: grade histogram for UG2 showing concentration distribution across the chromitite seam.

Page 111

• Merensky Reef mineralization details:

  • Hanging-wall: poikilitic pyroxenite to feldspathic pyroxenite (1–2 m thick), grading into norite.

  • Hanging-wall sulfidic zones extend about 50 cm from basal contact.

  • Footwall: norite or anorthosite with a thin chromite stringer.

Page 112

• Spatial variability: pegmatoidal phase strongly developed in western/southwestern sections; in eastern/southern sections, pegmatoidal phase may be absent or barren.

Page 113

• Diagrams illustrating single chromitite Merensky Reef cross-sections with various lithologies (Merensky Reef, Merensky Pyroxenite, Chromitite Layer, Pegmatoid, Anorthosite, Norite).

Page 114

• Merensky Reef grade profile concentrates around pegmatoidal feldspathic pyroxenite; highest PGE concentrations associated with the chromitite stringers.

Page 115

• Sulphide mineral assemblage of the Merensky Reef (~3–4 vol%): predominantly pyrrhotite, pentlandite, chalcopyrite; smaller amounts of pyrite or cubanite.

Page 116

• PGE occurrence/distribution: PGEs dominated by PGE sulfides, tellurides, arsenides, and other PGE-bearing phases; includes sulfides and alloys.

Page 117

• PGMs grain counts in Merensky Reef show diverse PGM assemblages: sulfides, sulpho-arsenides, arsenides, bismuthotellurides, and alloys with significant representation of various grain types.

Page 118

• Platreef revisited: geographic setting (northern limb); variability in PGE mineralization styles; irregular distribution with Cu and Ni co-occurrence; hydrothermal contributions suspected.

Page 119

• PGE occurrence in Platreef: variable, complex styles along strike; inconsistent mineralization along lithological packages; interaction of magmatic processes, footwall contamination, and hydrothermal activity.

• Platreef generally shows lower PGE grade than Merensky Reef and UG2 (typical ~4 g/t in some sections; often <1–2 ppm elsewhere).

• Base metal sulfides (pyrrhotite, chalcopyrite, pentlandite) commonly associated with PGMs; Pt:Pd ratio near 1.

Page 120

• General Platreef metallogeny: lower PGE grades but significant base metal sulfides; PGMs show strong spatial association with Cu and Ni sulfides; overall economic potential variable across sections.

Page 121

• Platreef mineralization continues to show heterogeneous distribution, with some pockets of higher PGE concentration but typically lower than UG2/Merensky Reef equivalents.

Page 122

• Summary statements: Platreef grades often lower than major reefs; PGE distribution correlates with sulfides; base metal sulfides are major carriers of Ni–Cu–PGE in Platreef contexts.

Page 123

• Upper Zone contains about 20 m of pure magnetite distributed within ~2000 m of magnetite-bearing gabbroic rocks; Main Magnetite Layer is the most prominent magnetite seam.

• Magnetite layers are vanadium-rich, with V2O5 increasing toward the base (0.3% at the top to 2% at the base).

Page 124

• Main Magnetite Layer geology: extent ~120 km in the east, ~200 km in the west, ~100 km in the north; thicknesses range from 0.1 to 10 m; magnetite layers are vanadium-rich, contributing to V mineralization.

Page 125

• Vanadium mineralization in the Bushveld: Main Magnetite Layer sits ~130 m above the base of the Upper Zone; ~2 m thick; currently mined for V content in eastern/western limbs and accounts for a large share of world V supply.

Page 126

• Summary: Bushveld lithostratigraphy shows concentrations of mineral deposits (Merensky Reef, UG2, Platreef) associated with the Rustenburg Layered Suite and related lithologies, including the Rooiberg Group and Lebowa Granite Suite; major ore bodies are tied to distinct stratigraphic horizons (CZ, MZ, UZ) and chromitite seams (LG, MG, UG).

Key formulas and concepts to remember

• Partitioning coefficients (Nernst): Ki = \frac{ci^{(a)}}{c_i^{(b)}} where a and b are coexisting phases (e.g., mineral and melt).

• Sulphide/melt partitioning (immiscibility): Ki^{\text{(sulphide)}} = \frac{ci^{\text{sulphide}}}{c_i^{\text{silicate}}}; Pt, Pd often have very large values (>10^4 in sulphide phase).

• K values define compatibility: high KD (KD>1) means preference to be in the mineral; low KD (KD<1) means preference to stay in melt.

• Immiscibility types important for ore genesis: silicate–sulphide, silicate–oxide (Fe–Ti–V), silicate–carbonatite (carbonate melts), and silicate–oxide/silicate–carbonate combinations.

• Common ore-forming temperatures and physical controls:

  • Partial melting: 600–1300 °C

  • Crystallization rates and viscosity influence crystal settling; low silica content (low-viscosity magmas) favor faster settling; high silica magmas are more viscous and hinder settling.

• Typical ore-related mineralogy:

  • Chromite (FeCr2O4) as a high-temperature mineral; magnetite (Fe3O4); ilmenite (FeTiO3)

  • Sulfide assemblage in magmatic sulfide deposits: pyrrhotite, pentlandite, chalcopyrite; PGEs hosted in sulfides, arsenides, tellurides, and alloys.

  • Chromitite seams are key stratiform hosts; Merensky Reef and UG2 chromitite layers are major PGE-bearing horizons.

• Economics and grade ranges (examples):

  • UG2 chromitite: Cr2O3 around 43% in UG2; Cr:Fe ratios ~1.26–1.40; PGE total 4–7 g/t (variable); Pt:Pd proportions vary regionally.

  • Merensky Reef: high PGE concentrations near the upper chromitite stringer; base-metal sulfides enriched with PGE; hanging-wall and footwall lithologies defined.

  • Platreef: variable PGE, Cu, Ni; generally lower PGE grade than Merensky/UG2 but can host significant base metal sulfides.

• Carbonatites and REE: carbonatites are notable REE and Nb hosts; fenitization is a common associated late-stage alkali metasomatic process.

If you want, I can split these notes into even more granular per-page subtopics or add a compact index at the end to help with quick review.