Chapter 16 Mineral genesis

Mineral genesis and genetic mineralogy Genesis = origin Genesis = origin –Primary crystallization –Subsequent history: transitions, exsolution, reaction Mineral properties explained by forming conditions Mineral properties explained by forming conditions Genetic mineralogy: Genetic mineralogy: –Investigate principles controlling mineral formation –Quantitative: geothermometer, geobarometer, chemical studies –Modelling of crystal growth Mineral deposit: Mineral deposit: –Geological body formed under specific conditions –Contains characteristic minerals: scattered, segregated, lenses, strata, veins / veinlets

Mineral forming environments Aqueous solutions: Aqueous solutions: –2 Types of solutions: Hydrothermal solutions (endogenic) Hydrothermal solutions (endogenic) –Sources: Crystallizing magma; dehydrating sedimentary rocks; mantle degassing; migrated meteoric and seawater –Heated solution dissolve, transport and precipitate minerals from rocks along pathway - specific minerals can be concentrated and accumulated, often forming ore mineral deposits in this way –Quartz and calcite and sulphide ore deposits Surface solutions or brines (exogenic) Surface solutions or brines (exogenic) –Ground, karst and soil water – carbonates: calcite, aragonite –Lacustrine, oceanic, lagoon waters – evaporite minerals: halite, gypsum

Mineral forming environments Gas Gas –Rare environment –Hematite, native sulfur, realgar from volcanic gases –Ice crystals from vapor: dendritic snowflakes Fluids Fluids –Fluid mixtures of CO 2 and H 2 O important during formation of skarns and metamorphism of limestone –Water can cause significant alteration and dissolution of minerals especially at high P and T Eg: Quartz Eg: Quartz

Mineral forming environments Colloidal solutions Colloidal solutions –Typical in ocean floor silt rich in clay minerals, Al-, Fe-, Mn-hydroxides –Rarely in thermal springs with recent volcanic activity: amorphous opal Magma Magma –Not a simple pure melt: a mixture of substances and the compositions is not necessarily corresponding to the rocks that form from them –Liquid and solution properties –Anion groups in polyhedra ‘dissolved’ as clusters in the magma –Also large cations such as K +, Na +, Mg +, Ca +

Mineral forming environments Solid systems Solid systems –Crystalline Polymorphic transitions (no change in chemical composition) Polymorphic transitions (no change in chemical composition) –Diamond to graphite; high quartz to low quartz; opal to quartz Transform precursor mineral to new phases with different compositions Transform precursor mineral to new phases with different compositions –Pseudomorhps: pyrite replaced by limonite Replacement processes Replacement processes –Porphyroblasts: garnet growing in a gneiss replacing (and including) pre-existing minerals –Usually associated with molecular water at grain boundaries

Types of mineral crystallization Why do minerals form? Why do minerals form? –More stable at new P, T or concentration than the melt, solution or pre-existing minerals from which they are forming Types: Types: –Free space crystallization –Metasomatism –Recrystallization

Types of mineral crystallization Free space crystallization: Free space crystallization: –Grow freely in gas, melt, solution –Examples: Sulfur in volcanic gas Sulfur in volcanic gas Porphyritic feldspars in magma Porphyritic feldspars in magma Amethyst in hydrothermal solution Amethyst in hydrothermal solution –Usually euhedral crystal habits

Types of mineral crystallization Metasomatism: Metasomatism: Definition: Definition: –a metamorphic process in which the chemical composition of a rock is changed significantly, usually as a result of fluid flow –a process of simultaneous capillary dissolution and crystallization by which a new mineral completely or partially replaces an initial mineral, often changing the chemical composition Formation of compositionally diverse ores and rocks Formation of compositionally diverse ores and rocks Greisen forms from granite subject to hydrothermal solution: 3K(AlSi 3 O 8 ) + 2H +  KAl 2 (ALSi 3 O 10 )(OH) 2 + 6SiO 2 + 2K + Greisen forms from granite subject to hydrothermal solution: 3K(AlSi 3 O 8 ) + 2H +  KAl 2 (ALSi 3 O 10 )(OH) 2 + 6SiO 2 + 2K + M icrocline Muscovite Quartz M icrocline Muscovite Quartz Simultaneously cassiterite (SnO 2 ) forms when tin is added to the greisen

Types of mineral crystallization Recrystallization: Recrystallization: –New crystals replace earlier ones –Increase or decrease in grain size –Compositional changes or not –Proceeds in solid state, driven by free chemical energy or deformation defects in crystals –NB factor in diagenesis and metamorphism –Recrystallization at low temperature and polymorphic transformations at higher temperature

Types of mineral deposit Types Genetic groups Endogenic MagmaticIgneousPegmatiteSkarnHydrothermalExhalational MetamorphicMetamorphic Exogenic SupergeneVadose Weathering and oxidation zones SedimentaryMechanicalChemicalBiogenic Endogenic-exogenic Hydrothermal sedimentary