💎Mineralogy Unit 12 – Mineral Associations and Paragenesis

Key Concepts and Definitions

• Mineral associations refer to the co-occurrence of minerals in a specific geological environment or rock type
• Paragenesis is the sequence of mineral formation and alteration events in a rock or deposit
• Mineral stability depends on factors such as temperature, pressure, and chemical composition of the system
• Hydrothermal alteration involves the interaction of hot, mineral-rich fluids with pre-existing rocks or minerals
• Metamorphic facies are groups of mineral assemblages that form under specific pressure and temperature conditions
• Ore minerals are those that can be economically extracted for their valuable components (copper, gold, silver)
• Gangue minerals are the non-valuable minerals associated with ore minerals in a deposit (quartz, calcite)

Types of Mineral Associations

• Igneous associations form during the crystallization of magma or lava
  • Mafic igneous rocks (basalts) commonly contain olivine, pyroxene, and plagioclase
  • Felsic igneous rocks (granites) typically include quartz, potassium feldspar, and muscovite
• Sedimentary associations develop during the deposition and lithification of sediments
  • Clastic sedimentary rocks (sandstones) often contain quartz, feldspar, and lithic fragments
  • Chemical sedimentary rocks (limestones) primarily consist of calcite or dolomite
• Metamorphic associations result from the transformation of pre-existing rocks under elevated temperature and pressure conditions
  • Low-grade metamorphic rocks (slates) may contain chlorite, muscovite, and quartz
  • High-grade metamorphic rocks (gneisses) often include garnet, sillimanite, and biotite
• Hydrothermal associations form due to the interaction of hot, mineral-rich fluids with host rocks
  • Porphyry copper deposits typically contain chalcopyrite, bornite, and molybdenite in association with potassic alteration minerals (biotite, potassium feldspar)
  • Epithermal gold deposits often include native gold, pyrite, and adularia in association with silicification and argillization
• Weathering associations develop during the chemical and physical breakdown of rocks at the Earth's surface
  • Laterites are weathering products rich in aluminum and iron oxides (gibbsite, hematite)
  • Bauxites are weathering products containing high concentrations of aluminum hydroxides (boehmite, diaspore)

Factors Influencing Mineral Formation

• Temperature affects mineral stability and the rate of chemical reactions
  • Higher temperatures promote the formation of high-temperature minerals (pyroxene, olivine)
  • Lower temperatures favor the formation of low-temperature minerals (zeolites, clay minerals)
• Pressure influences mineral stability and the solubility of gases in magmas and fluids
  • High pressures can stabilize dense mineral phases (garnet, kyanite)
  • Low pressures may lead to the formation of less dense minerals (andalusite, cordierite)
• Chemical composition of the system determines which minerals can form based on the available elements
  • Silica-rich systems favor the formation of quartz and feldspar
  • Iron-rich systems promote the formation of oxides and sulfides (magnetite, pyrite)
• Oxygen fugacity (fO2) controls the oxidation state of elements and influences mineral stability
  • High fO2 favors the formation of oxidized minerals (hematite, magnetite)
  • Low fO2 promotes the formation of reduced minerals (pyrite, graphite)
• Fluid composition and pH affect the solubility and transport of elements in hydrothermal systems
  • Acidic fluids can leach and transport metals more effectively than neutral or alkaline fluids
  • Chloride-rich fluids are efficient at transporting base metals (copper, lead, zinc)
• Kinetic factors such as nucleation and growth rates influence the size and morphology of mineral grains
  • Rapid cooling or supersaturation can result in the formation of fine-grained or poorly crystalline minerals
  • Slow cooling or near-equilibrium conditions promote the growth of large, well-formed crystals

Paragenetic Sequences

• Paragenetic sequences represent the order of mineral formation and alteration events in a rock or deposit
• Primary mineralization refers to the initial formation of minerals during rock formation or ore deposition
  • Example: the crystallization of magmatic minerals (olivine, pyroxene) in a mafic igneous rock
• Secondary mineralization involves the alteration or replacement of pre-existing minerals by later fluids or processes
  • Example: the replacement of primary sulfides by supergene enrichment minerals (chalcocite, covellite) in a porphyry copper deposit
• Crosscutting relationships provide evidence for the relative timing of mineral formation or alteration events
  • A vein that cuts across another mineral or structure must have formed after the feature it crosscuts
• Overgrowth textures indicate the sequential deposition of minerals on a pre-existing substrate
  • Example: the growth of euhedral quartz crystals on a base of earlier-formed pyrite in a hydrothermal vein
• Replacement textures result from the partial or complete replacement of one mineral by another
  • Example: the replacement of calcite by dolomite during diagenesis or hydrothermal alteration
• Exsolution textures form due to the unmixing of a solid solution during cooling or changes in pressure
  • Example: the formation of lamellae of ilmenite in magnetite grains in a slowly cooled igneous rock

Analytical Techniques

• Petrographic microscopy allows the identification of minerals and their textural relationships in thin sections
  • Transmitted light microscopy is used for transparent minerals (quartz, feldspars)
  • Reflected light microscopy is used for opaque minerals (sulfides, oxides)
• X-ray diffraction (XRD) is used to identify the crystalline structure and composition of minerals
  • XRD patterns provide a "fingerprint" of the mineral based on its unique crystal structure
• Electron microprobe analysis (EMPA) is used to determine the chemical composition of individual mineral grains
  • EMPA can provide quantitative data on major, minor, and trace element concentrations in minerals
• Scanning electron microscopy (SEM) is used to image the surface morphology and texture of minerals at high magnification
  • Energy-dispersive X-ray spectroscopy (EDS) can be coupled with SEM to obtain semi-quantitative chemical data
• Cathodoluminescence (CL) imaging is used to reveal growth zoning and alteration patterns in minerals
  • CL is particularly useful for studying the growth history of quartz and carbonate minerals
• Fluid inclusion analysis provides information on the temperature, pressure, and composition of the fluids that formed minerals
  • Microthermometry involves heating and cooling fluid inclusions to determine their phase transitions and salinity
  • Raman spectroscopy can be used to identify the composition of gases and solids within fluid inclusions

Case Studies and Examples

• Porphyry copper deposits (Bingham Canyon, USA; El Teniente, Chile)
  • Paragenetic sequence: potassic alteration (biotite, K-feldspar) → phyllic alteration (sericite, quartz) → argillic alteration (kaolinite, smectite)
  • Associated minerals: chalcopyrite, bornite, molybdenite, pyrite
• Mississippi Valley-type (MVT) lead-zinc deposits (Tri-State district, USA; Pine Point, Canada)
  • Paragenetic sequence: early dolomitization → sulfide mineralization (sphalerite, galena) → late calcite and barite
  • Associated minerals: sphalerite, galena, pyrite, marcasite, dolomite
• Banded iron formations (BIFs) (Hamersley Range, Australia; Quadrilátero Ferrífero, Brazil)
  • Paragenetic sequence: deposition of iron-rich layers (magnetite, hematite) alternating with silica-rich layers (chert)
  • Associated minerals: magnetite, hematite, siderite, chert, jasper
• Skarns (Antamina, Peru; Ertsberg, Indonesia)
  • Paragenetic sequence: contact metamorphism (garnet, pyroxene) → retrograde alteration (epidote, amphibole) → sulfide mineralization (chalcopyrite, sphalerite)
  • Associated minerals: garnet, pyroxene, epidote, amphibole, chalcopyrite, sphalerite, magnetite

Applications in Geology

• Mineral exploration and resource assessment
  • Understanding mineral associations and paragenetic sequences helps guide exploration strategies and target selection
  • Example: identifying alteration zones associated with porphyry copper mineralization (potassic, phyllic, argillic)
• Geothermobarometry and paleoenvironmental reconstruction
  • Mineral assemblages can be used to estimate the temperature and pressure conditions of formation
  • Example: using the composition of coexisting garnet and biotite to calculate metamorphic temperatures
• Provenance studies and sedimentary basin analysis
  • The types and abundances of minerals in sedimentary rocks can provide information on the source area and weathering conditions
  • Example: using the ratio of stable heavy minerals (zircon, tourmaline, rutile) to unstable heavy minerals (pyroxene, amphibole) to infer the degree of weathering and transport
• Geohazard assessment and mitigation
  • Identifying mineral associations indicative of potentially hazardous conditions (asbestos, acid-generating sulfides)
  • Example: mapping the distribution of serpentine minerals in ultramafic rocks to assess the risk of asbestos exposure
• Environmental remediation and waste management
  • Understanding mineral-fluid interactions and stability fields to design effective remediation strategies
  • Example: using iron oxyhydroxide minerals (goethite, ferrihydrite) to adsorb and immobilize heavy metals in contaminated soils or water

Review and Practice Questions

1. What is the difference between mineral associations and paragenetic sequences?
2. Describe the mineral associations typically found in mafic igneous rocks and felsic igneous rocks.
3. How do temperature and pressure affect mineral stability and formation?
4. What are the main factors influencing mineral formation in hydrothermal systems?
5. Explain the concept of crosscutting relationships and how they are used to determine the relative timing of mineral formation.
6. What analytical techniques can be used to identify minerals and their chemical compositions?
7. Describe the paragenetic sequence and associated minerals in a typical porphyry copper deposit.
8. How can mineral associations be used in geothermobarometry and paleoenvironmental reconstruction?
9. What are some applications of mineral association and paragenesis studies in mineral exploration and resource assessment?
10. Discuss the importance of understanding mineral associations in geohazard assessment and environmental remediation.