GEOL 543 2024 Lecture 5 Ore Deposits Classification PDF

Lecture 5: Classification & Ore deposit types: An Overview GEOL 543 Ore Deposits Fall 2024 Courtesy to Basem Koheir Ore Deposits in the Arabian-Nubian Shield https://doi.org/10.1007/978-3-030-72995-0_23 Journal: The Geology of the Arabian-Nubian S...

Lecture 5: Classification & Ore deposit types: An Overview GEOL 543 Ore Deposits Fall 2024 Courtesy to Basem Koheir Ore Deposits in the Arabian-Nubian Shield https://doi.org/10.1007/978-3-030-72995-0_23 Journal: The Geology of the Arabian-Nubian Shield Regional Geology Reviews, 2021, p. 585-631 Publisher: Springer International Publishing Author: Nagy Shawky Botros et al. Classification of ore deposits Classification of ore deposits Ore deposits are classified based on their: 1. Genetic classification: origin, Magmatic ore deposits: mineralogy, Formed from the crystallization of magmas. host rocks, Examples: Chromite, magnetite, and and the processes that formed them. platinum group elements in layered mafic intrusions. Hydrothermal ore deposits: Formed from hot, metal-rich fluids circulating through rocks. Subtypes include: Porphyry deposits: Large, disseminated deposits of copper, molybdenum, and gold. Epithermal deposits: High- and low- sulfidation deposits rich in gold and silver. Volcanogenic massive sulfides (VMS): Copper, zinc, and lead deposits formed at mid-ocean ridges. Classification of ore deposits Sedimentary ore deposits: Formed by chemical precipitation or sedimentary processes. Examples: Banded iron formations (BIFs), placer gold deposits, and sediment-hosted copper deposits. Metamorphic ore deposits: Formed by metamorphic processes, typically involving fluid movement during metamorphism. Examples: Gold in orogenic belts, skarn deposits. Supergene ore deposits: Formed by the weathering and enrichment of primary mineral deposits near the Earth's surface. Examples: Secondary copper deposits, bauxite (aluminum ore). Classification of ore deposits 2. Morphological classification (based on shape and form): Vein deposits: Ore minerals fill fractures or cracks in rocks, forming veins. Disseminated deposits: Ore minerals are spread diffusely throughout the host rock (e.g., porphyry copper deposits). Massive deposits: Large, homogeneous bodies of ore (e.g., VMS deposits). Layered or stratiform deposits: Ore minerals are concentrated in layers or strata within the host rock (e.g., chromite, BIFs, coal seams). Classification of ore deposits 3. Host rock classification: 4. Economic classification Igneous-hosted deposits: (based on commodity): Ore bodies associated with Precious metal deposits: igneous rocks (e.g., chromite in Gold, silver, platinum group ultramafic rocks, rare metal metals (e.g., epithermal gold granites). deposits). Base metal deposits: Sedimentary-hosted deposits: Copper, lead, zinc, nickel (e.g., Ore bodies associated with porphyry copper deposits). sedimentary rocks (e.g., Industrial mineral deposits: Non- Mississippi Valley-type lead-zinc metallic minerals such as deposits). limestone, phosphate, gypsum (e.g., sedimentary phosphorite Metamorphic-hosted deposits: deposits). Ore bodies associated with Energy mineral deposits: metamorphic rocks (e.g., Coal, uranium, oil shale (e.g., orogenic gold deposits in coal seams, uranium roll-front greenstone belts). deposits). Classification of ore deposits 5. Tectonic Setting Classification: Orogenic deposits: Formed in orogenic (mountain-building) belts, typically associated with convergent plate boundaries (e.g., orogenic gold). Mid-ocean ridge deposits: Formed at divergent plate boundaries, typically VMS deposits. Intracratonic deposits: Formed within stable continental interiors (e.g., diamond-bearing kimberlites).  Ore deposits classification based on the tectonic setting tectonic setting deposit type deposit sub-type metal association geologic setting; associated rocks Porphyry Cu±Au±Mo Continental and island arc; Au-Ag-As-Hg- intermediate calc-alkaline to alkaline Epithermal (Pb-Zn) Continental and island arc; Fe, Cu, Au, Zn, intermediate calc-alkaline some back Porphyry systems Skarn W-Sn, Mo arc (sulphide- poor), bimodal felsic- Carbonate mafic Convergent replacement / Zn-Pb-Ag Au-As Continental and island arcs; carbonate margin Silty carbonate rocks hosted (Carlin) Iron-oxide Cu-Au Fe±Cu-Au± Transpressional to extensional settings Many variations (IOCG) U±REE (complex craton margins in older deposits; variety of host rocks Fore-arc, Orogenic Au Au-As Au-As back-arc, accretionary wedge; greenschist facies) Mid-ocean ridge (Cyprus); bimodal Spreading center Volcanic-hosted Cu-Pb-Zn ± Ag, mafic, mafic and convergent- massive sulphide Cyprus / Kuroko Au Back-arc (Kuroko); bimodal felsic, margin extension (VMS) siliciclastic Mississippi Valley- Post-collision foreland basin; platform Foreland basin Pb-Zn±Ba±F type (MVT) carbonate host  Ore deposits classification based on the tectonic setting tectonic setting deposit type deposit sub-type metal association geologic setting; associated rocks Intercraton rift basin; red beds, Sediment-hosted Cu±Co±Ag carbonaceous units, evaporites stratiform Cu Sedimentary basin; redox front, contacts Unconformity Uranium U±Au±Co±Mo± Closed basin; redox front, contacts Sandstone Rifts, sag basins, Se±Ni U±V±Mo Passive margin, back-arc and passive margins Clastic-dominated Pb-Zn±Cu± continental rift, sag basin; shale and Zn-Pb (or SEDEX) Ag±As±Bi carbonate rocks Banded Iron Passive margin, deep basin; carbonate (BIF) Fe-(P) Formation and silica facies Mn-rich sediments Mn-(Fe) Open shelf Passive margin P-(U, REE, Se,Mo, Phosphorites Platform ; epeiric sea platform Zn, Cr) Large igneous Komatiite Ni±Cu±PGE Greenstone belt; ultramafic Plumes; province (oceanic Ni sulphide Mafic intrusion Ni-Cu-PGE±Co±Au sedimentary basin or continental) Craton or craton Layered intrusion PGE-Ni±Cu; Cr Plume, craton margin Diamond diamond Cratons; kimberlites Al Granite-gabbro, arkose Lateritic Al, Ni Ni-Co Ultramafic Land surface Placer Au±U; Zr-Ti; Fluvial, marine (palaeoplacer) diamond Ore deposit types Magmatic ore deposits Magmatic metasomatic deposits Rare metal granites Mineralized pegmatites Hydrothermal ore deposits Porphyry deposits Epithermal ore deposits VMS deposits Sediment-hosted sulfide deposits Skarn deposits Orogenic gold deposits Carlin-type gold deposits Iron Oxide Copper-Gold (IOCG) deposits Banded Iron Formations (BIF) Magmatic ore deposits Magmatic ore deposits are formed directly from the crystallization of magma. These deposits are typically associated with igneous rocks and form in various geological environments, primarily in intrusive bodies like mafic and ultramafic complexes. Types of magmatic ore deposits: 1.Layered mafic-ultramafic intrusions: These deposits form through the crystallization of mafic and ultramafic magmas in large, layered intrusions. Heavy minerals like chromite, magnetite, and platinum group elements (PGE) may settle out of the magma due to their higher density. Examples: 1. Bushveld complex (South Africa): Hosts significant deposits of chromite, platinum, and palladium. 2. Stillwater complex (USA): Known for platinum group metals (PGMs) and nickel-copper deposits. 3. Great dyke (Zimbabwe): Contains large reserves of chromite and PGMs. Magmatic ore deposits 2. Anorthosite-hosted deposits: These deposits are associated with large, anorthosite intrusions. Titanium-rich minerals like ilmenite and magnetite can accumulate during the cooling of the magma. Examples: 1. Lac Tio (Canada): A major source of titanium from ilmenite deposits within anorthosite. 2. Tellnes (Norway): Another significant ilmenite deposit within anorthosite complexes. Magmatic ore deposits 3. Nickel-copper sulfide deposits: These deposits form from magmas that are sulfur-saturated, causing sulfide minerals (rich in nickel, copper, and often PGMs) to segregate from the silicate melt and concentrate as dense immiscible droplets. Examples: 1. Sudbury Basin (Canada): One of the largest nickel-copper sulfide deposits, likely formed from an impact-generated melt. 2. Norilsk-Talnakh (Russia): Hosts major nickel, copper, and PGE deposits associated with Siberian flood basalts. Magmatic ore deposits 4. Diamond-bearing kimberlites and lamproites: Diamonds form at high pressures deep within the Earth's mantle and are brought to the surface by kimberlite or lamproite magmas during explosive volcanic eruptions. Examples: 1.Kimberley (South Africa): Famous for its diamond- bearing kimberlites, the source of the world’s first diamond rush. 2.Argyle Mine (Australia): A major source of diamonds, particularly pink diamonds, associated with lamproite pipes. Magmatic-metasomatic deposits Magmatic-metasomatic deposits are mineral deposits that form through the interaction of magmatic fluids with the surrounding rocks, leading to the metasomatic alteration of these rocks and the concentration of valuable minerals. These deposits are typically associated with the late stages of magmatic activity, where volatile-rich fluids exsolve from the crystallizing magma and react with the country rocks. Formation process: Magmatic Fluids: During the cooling and crystallization of a magma body, residual fluids enriched in volatiles (e.g., water, carbon dioxide, fluorine, chlorine) and metals (e.g., copper, molybdenum, tin) are released. These fluids can migrate away from the magma chamber and interact with the surrounding rocks. Metasomatism: The interaction between these magmatic fluids and the surrounding country rocks (often referred to as the host rocks) leads to metasomatism, a process where the chemical composition of the host rocks is altered due to the introduction of new elements from the fluids. This process can result in the formation of new minerals and the concentration of ore minerals. Magmatic-metasomatic deposits Types of magmatic-metasomatic deposits: Skarn: Skarns form when magmatic fluids interact with carbonate rocks (such as limestone or dolomite), resulting in the replacement of these rocks by a suite of calc-silicate minerals (e.g., garnet, pyroxene, wollastonite). Skarns can host significant deposits of metals like copper, iron, tungsten, and zinc. Greisen: These are associated with granitic intrusions and form when magmatic fluids rich in fluorine and other volatiles interact with the surrounding granitic rocks, leading to the formation of greisen, a type of rock characterized by quartz, muscovite, and tourmaline. Greisen deposits can be significant sources of tin, tungsten, and rare metals like lithium. Rare metal (specialized) granites Rare metal granites and pegmatites are important sources of rare metals, including elements such as Li, Ta, Nb, Sn, W and various rare earth elements (REEs). These rocks form in specific geological environments and have distinct mineralogical characteristics. Key features: Formation: Rare metal granites are evolved, high-silica, and alkali-rich granitic rocks. They typically form in the late stages of the crystallization of a granitic magma, where incompatible elements become concentrated in the residual melt. Composition: These granites are enriched in rare metals such as Sn, W, Mo, Ta, Nb, Li, and REEs. They often contain minerals like cassiterite (SnO₂), wolframite Textures: These granites are typically coarse- ((Fe,Mn)WO₄), columbite-tantalite grained and may show evidence of late- ((Fe,Mn)(Nb,Ta)₂O₆), and spodumene stage hydrothermal alteration. They can also (LiAl(SiO₃)₂). host mineralized veins and greisen zones. Mineralized pegmatites Mineralized pegmatites are coarse-grained igneous rocks that are known for their exceptionally large crystals and are often the source of economically important minerals. They form during the late stages of magma crystallization, where the residual melt becomes enriched in water, volatiles, and rare elements, leading to the crystallization of large mineral grains. Key features: Formation: Pegmatites form from the final fraction of magma that is rich in volatile components like water, boron, and fluorine, which lower the crystallization temperature and increase the mobility of ions. Mineralogy: Common minerals in pegmatites include quartz, feldspar, and mica, but mineralized pegmatites are particularly significant because they can contain high concentrations of rare metals. These include lithium (spodumene, lepidolite), tantalum (tantalite), niobium (columbite), beryllium (beryl), tourmaline, and various gemstones. Geological setting: Pegmatites are typically found in orogenic belts, associated with granitic intrusions, and can also occur in metamorphic terrains. They are often emplaced as dikes or veins cutting through surrounding rock. Zoning: Pegmatites often display internal zoning, with different minerals concentrated in distinct zones within the pegmatite body. The outer zones might consist of large feldspar and quartz crystals, while the inner zones could be enriched in rare- element minerals and gemstones. Hydrothermal ore deposits Hydrothermal ore deposits are formed by the action of hot, mineral-rich fluids circulating through fractures and porous rocks in the Earth's crust. These fluids, which can originate from magmatic, metamorphic, or meteoric (surface) water sources, dissolve minerals and transport them to new locations, where they precipitate out to form concentrated deposits of valuable metals and minerals. Hydrothermal deposits are among the most important sources of metals like gold, silver, copper, lead, zinc, and molybdenum. 1. Porphyry deposits 2. Epithermal deposits High-sulfidation epithermal deposits Low-sulfidation epithermal deposits 3. Volcanogenic massive sulfides (VMS) 4. Sediment-hosted sulfide deposits 5. Skarn deposits 6. Orogenic gold deposits 7. Carlin-type gold deposits 8. Iron-oxides-Copper-Gold (IOCG) Porphyry deposits Porphyry ore deposits are one of the most significant sources of copper, molybdenum, gold, and other valuable metals. These deposits are typically formed in large, disseminated systems, where metals are distributed in low concentrations throughout large volumes of rock, making them a major target for bulk mining operations. Porphyry deposits are usually associated with magmatic-hydrothermal systems, in tectonic settings where magma rises from the mantle or lower crust and intrudes into the upper crust, often near convergent plate boundaries where subduction occurs. Porphyry deposits Large size and low grade, with ore bodies extending over several kilometers, but they have relatively low metal grades (< 1% Cu). Alteration zonation: concentric zones of alteration, with a central potassic zone often containing the highest metal grades, surrounded by phyllic, argillic, and propylitic alteration halos. Stockwork veins: the ore is often concentrated in stockwork vein systems, where the metals are hosted within a network of thin, interconnected veins. Porphyritic host rocks: typically porphyritic intrusive rocks, characterized by large phenocrysts in a fine-grained groundmass. Formation of porphyry ore deposits Key processes involved in their formation include: Magma generation and intrusion: Magma, often of intermediate to felsic composition (such as andesite, dacite, or granite), forms deep within the Earth's crust. This magma is rich in volatiles, such as water, sulfur, and chlorine, which are critical for the transport and concentration of metals. As the magma rises, it cools and begins to crystallize, leading to the exsolution of a volatile-rich fluid phase. Hydrothermal alteration: The volatile-rich fluids exsolve from the crystallizing magma and start to ascend through fractures and permeable zones in the surrounding rocks. These fluids are often hot (~300°C to 700°C) and can cause extensive hydrothermal alteration of the host rocks. Porphyry deposits Common alteration zones include potassic, phyllic, argillic, and propylitic assemblages, which are critical for understanding the distribution of mineralization. Metal deposition: As the hydrothermal fluids move away from the cooling magma, they begin to cool and react with surrounding rocks. Metals such as Cu, Mo, and Au are transported by the fluids as complexes (e.g., chloride complexes) and precipitate out of the solution due to changes in temperature, pressure, or chemical environment. The result is the formation of disseminated mineralization within the host rock, with metal concentrations often increasing around the stock or core of the intrusion. Formation (what controls porphyry deposits formation) o Subduction zone tectonics Plate convergence Arc magmatism o Magmatic processes Magmatic differentiation Volatile exsolution Intrusive activity (porphyritic intrusions) o Hydrothermal fluid flow Hydrothermal circulation Temperature and pressure fluctuation o Host rock characteristics Fracture systems (permeability) Reactive host rocks (e.g., limestone) o Metal precipitation mechanisms Cooling of hydrothermal fluids Chemical reactions (pH, redox changes) o Tectonic uplift Post-magmatic tectonics (uplift, erosion) Metal zoning in porphyry deposits Core zone: High concentrations of Mo and W, near the central intrusion Intermediate zone: Rich in Cu and sometimes Au, typically the most economically valuable zone Outer zones: Dominated by Pb, Zn, and Ag, farther from the core Distal zones: Includes As, Sb, and Hg, at the outermost edges, possibly transitioning into epithermal systems Exploration for porphyry deposits 1. Geological Mapping 3. Geophysical Surveys Magnetic surveys: identifying magnetic Identifying intrusive rocks and alteration anomalies associated with porphyry zones and extensive structures intrusions Induced polarization (IP): detecting 2. Geochemical Surveys chargeability and resistivity anomalies Soil Sampling: analyzing surface soils for indicative of sulfide mineralization Gravity surveys: identifying large Cu, Mo, Au, and other pathfinder intrusions based on density contrasts elements Stream Sediment Sampling: detecting 4. Drilling Reverse circulation (RC) drilling: Faster, metal anomalies in drainage systems less expensive method for initial Rock Sampling: analyzing rock outcrops exploration for metal concentrations Core drilling: extracting continuous rock samples to assess mineralization at depth Drilling helps define the size, grade, and continuity of the deposit Exploration for porphyry deposits 5. Alteration and zoning analysis Studying the distribution of hydrothermal alteration zones (e.g., potassic, phyllic) to locate the ore center Understanding metal zoning (e.g., Mo in the core, Cu in the intermediate zone) 6. 3D modeling Creating geological models using drilling data and geophysical survey results Modeling the ore body helps in resource estimation and mine planning 7. Resource Estimation Calculating the tonnage and grade of the deposit based on drill results Estimating economic viability through feasibility studies Epithermal ore deposits Epithermal deposits form in shallow crustal environments, typically within 1 to 2 km of the Earth's surface, and are associated with low to moderate temperature hydrothermal systems (50°C to 300°C). These deposits are often related to volcanic (particularly andesite and rhyolite) activity, and are a significant source of Au and Ag, along with other metals. Epithermal ore deposits 1.High-sulfidation (HS) epithermal deposits Form from acidic fluids Associated with intense alteration (e.g., advanced argillic alteration) Commonly enriched in Au, Ag, Cu, As, Sb 2. Low-sulfidation (LS) epithermal deposits Form from neutral to slightly acidic fluids Associated with quartz-adularia-sericite alteration Enriched in Au, Ag, and occasionally base metals (e.g., Pb, Zn) Epithermal ore deposits: formation controls Hydrothermal fluids: Derived from magmatic or meteoric sources Boiling: A key process in gold and silver deposition, as boiling reduces the solubility of metals in the fluids, causing them to precipitate Structural control: Faults, fractures, and breccias often serve as conduits for the hydrothermal fluids Epithermal ore deposits: study and exploration strategies Fluid inclusion studies Examine fluid inclusions in quartz and other minerals to understand the temperature, pressure, and composition of the mineralizing fluids. Use fluid inclusion data to assess the depth and conditions of ore formation. Boiling zones: Identify areas where hydrothermal fluids boiled, as this is a common process for gold and silver deposition. Structural controls: Faults and fractures are critical to guiding hydrothermal fluids; understanding these structures is essential for targeting ore zones. Volcanogenic massive sulfide (VMS) deposits Volcanogenic Massive Sulfide (VMS) deposits are formed on the seafloor, usually near volcanic activity. Economic importance: VMS deposits are economically important because of their high concentrations of base and precious metals. They are major sources of copper, zinc, lead, gold, and silver globally. These deposits are also valued for their relatively simple metallurgy, which makes the extraction of metals more straightforward compared to other types of deposits. VMS deposits and their environments The formation process of VMS deposits involves complex geological and hydrothermal processes that concentrate valuable metals into massive sulfide bodies. Tectonic Setting VMS deposits are primarily associated with submarine volcanic settings, particularly at mid-ocean ridges, back-arc basins, and island arcs. These tectonic environments provide the heat source necessary for hydrothermal circulation. Spreading centers and subduction zones create the necessary volcanic activity and fracture systems that allow seawater to penetrate the oceanic crust. VMS deposit formation 1. Hydrothermal circulation 2. Metal transport The formation of VMS deposits The hydrothermal fluids, now enriched in begins with seawater penetrating metals and sulfur, rise back to the surface the oceanic crust through fractures along faults and fractures. created by tectonic activity. As the fluids ascend, they remain under high pressure and temperature conditions, As the water descends deeper into keeping the dissolved metals in solution. the crust, it is heated by underlying magma chambers, reaching 3. Precipitation of sulfides temperatures of 200°C to 400°C. The rapid drop in temperature and pressure This hot water becomes a causes the dissolved metals to precipitate as hydrothermal fluid, capable of sulfide minerals. This typically forms massive leaching metals such as Cu, Zn, sulfide mounds or chimneys on the seafloor, and Pb from the surrounding rocks. composed of minerals and over time, these The fluids also dissolve sulfur, silica, sulfide minerals accumulate and form large, and other elements. lens-shaped ore bodies, which can be several meters to hundreds of meters thick. VMS deposit characteristics Deposition environment Alteration & zonation The deposition of VMS Around the VMS deposits occurs in deposits, the rocks are volcanic or often highly altered sedimentary basins by the interaction with where volcanic activity hydrothermal fluids. is ongoing or recent. This alteration typically These environments are results in the formation often characterized by of chlorite, sericite, the presence of and quartz in the exhalative vents (black footwall rocks smokers) that discharge beneath the deposit. metal-rich hydrothermal fluids into the ocean. VMS deposits often show metal zonation, with different metals concentrated in different parts of the deposit. Typically, Cu is found near the vent (proximal zone), while Zn and Pb are concentrated in the more distal areas. Exploration of VMS deposits Exploration strategies for VMS deposits: Geochemical sampling: Soil and rock geochemistry to detect metal anomalies and Geological mapping: Identify volcanic alteration haloes. stratigraphy and fault structures linked Drilling: Targeted drilling to confirm to VMS formation. mineralization and analyze core samples for Remote sensing: Use multispectral, sulfide content and alteration. hyperspectral imaging, radar, and radiometric data to identify surface alteration. Geophysical techniques: Electromagnetic (EM) surveys: Detect conductive sulfide bodies. Magnetic surveys: Map geological features and magnetite-bearing rocks. Gravity surveys: Identify dense, sulfide-rich ore bodies. Exploration of VMS deposits Gossan asociated with VMS deposits Gossans are oxidized surface expressions of underlying sulfide mineralization, making them key exploration targets for VMS deposits. They form through the natural weathering of sulfides at the surface. Color: Red, brown, or yellow, due to iron oxides like hematite, goethite, and limonite. Texture: Porous or spongy, resulting from sulfide leaching and silica enrichment, often forming a silicified cap. Mineral composition: Rich in iron oxides with secondary minerals like jarosite and alunite; sulfides like pyrite and chalcopyrite are typically leached out. Supergene alteration and metal enrichment: Weathering processes may cause supergene enrichment, concentrating valuable metals (e.g., copper, silver) in the gossan zone, enhancing the economic potential of the deposit. Gossan Gossans, often referred to as "iron hats," are oxidized, weathered portions of sulfide mineral deposits that are exposed at the Earth's surface. They are characterized by their distinctive rusty, reddish-brown color, which is primarily due to the presence of iron oxides such as hematite and goethite. Gossans form when sulfide minerals like pyrite are exposed to oxygen and water, leading to the breakdown of the sulfides and the formation of oxides, hydroxides, and sulfates. Key features: Formation: The process involves the chemical weathering of the sulfides, leading to the release of sulfur and the formation of iron oxides and other secondary minerals. Indicator of mineralization: Gossans are important exploration guides because they often overlie or are associated with significant sulfide mineralization, such as massive sulfide deposits, which may contain valuable metals. Gossan Geochemical zoning: Gossans often display vertical and lateral zoning of minerals, which can provide information about the underlying mineral deposit. For example, the presence of certain secondary minerals in the gossan, such as jarosite, limonite, or anglesite, can indicate the type of primary sulfide mineralization below. Economic significance: While gossans themselves are not typically mined, they are valuable exploration tools. Their presence can lead to the discovery of economically important metal sulfide deposits beneath them. Sediment-hosted sulfide deposits Sediment-hosted sulfide deposits are significant sources of base metals like lead, zinc, and copper. These deposits form in sedimentary basins, often through the interaction of metal-rich hydrothermal fluids with sedimentary rocks, such as shales, limestones, and sandstones. Key characteristics: Host rocks: Typically found in fine- grained sedimentary rocks such as shales, siltstones, and carbonates, which provide a favorable environment for the precipitation of sulfides. Sulfide minerals: Common sulfide minerals include galena, sphalerite, and chalcopyrite. Other associated minerals may include pyrite and marcasite. Sediment-hosted sulfide deposits Stratiform/stratabound: These deposits often form as layers or lenses parallel to bedding planes, indicating that the mineralization occurred during or shortly after sediment deposition. Ore genesis: Metal-rich fluids typically migrate through permeable sediments and precipitate sulfides due to chemical reactions with organic matter or other reducing agents in the host rocks. Textures: Sediment-hosted sulfides often exhibit fine-grained, laminated textures, reflecting the depositional environment of the host sediment. Sediment-hosted sulfide deposits Types of sediment-hosted sulfide deposits: 1.Sedimentary exhalative (SEDEX) deposits: Formed by hydrothermal fluids exhaled onto the seafloor, leading to the precipitation of sulfides within sedimentary layers. Lead-zinc sulfide deposits are typical, often associated with black shales and fine-grained sediments. 2.Mississippi Valley-type (MVT) deposits: Hosted in carbonate rocks (limestone, dolostone). Formed by low-temperature fluids. Lead and zinc are the primary metals, with associated barite and fluorite. Sediment-hosted sulfide deposits 1. SEDEX (sedimentary exhalative) deposits Tectonic Setting: SEDEX deposits typically form in extensional tectonic settings, often in rifted continental margins, back-arc basins, and intracontinental rift basins. These environments are characterized by the following features: Extensional basins: SEDEX deposits are often found in large, subsiding sedimentary basins where extensional tectonics have created space for thick accumulations of fine-grained sediments like black shales, which act as host rocks. Anoxic conditions: The formation of SEDEX deposits requires deep marine environments with stagnant, anoxic bottom waters, which prevent the oxidation of metal-rich fluids exhaled onto the seafloor. Hydrothermal activity: Hydrothermal fluids, enriched in lead, zinc, and silver, migrate through fault systems and are exhaled onto the seafloor, precipitating sulfides within the sediments. Sediment-hosted sulfide deposits 2. MVT (Mississippi Valley-Type) deposits Tectonic Setting: MVT deposits are typically associated with stable continental interiors and foreland basins within passive tectonic settings. Formation of MVT deposits through the movement of metal-rich basinal brines into carbonate platforms. Foreland basins: Formed by crustal loading from nearby orogenic belts, these basins accumulate large volumes of sediment. The resulting overpressure drives metal-rich brines into adjacent carbonate platforms, where MVT deposits form. Stable cratonic margins: MVT deposits also occur in stable cratonic regions, where regional tectonic compression or basin subsidence causes low- temperature brines to circulate through carbonate rocks like limestones and dolostones. Low-Temperature Brines: These cooler brines dissolve metals as they migrate through the sedimentary column and precipitate sulfides. Skarn deposits Skarn deposits form at the contact zones between igneous intrusions and carbonate- rich sedimentary rocks. They are important sources of metals such as Fe, Cu, Au, and W, arising from metamorphic (metasomatic) processes driven by magma. They are often found in convergent tectonic settings where subduction and associated magmatic activity create the conditions necessary for their development. Formation processes Metamorphic reactions: The heat and fluids from the intruding magma induce metamorphic reactions in the carbonate rocks, producing minerals like garnet, pyroxene, wollastonite, and epidote. Elemental enrichment: The specific minerals formed depend on the magma's composition. Skarn deposits Mineralogy and textures Skarn deposits exhibit a wide range of mineralogical compositions and textures, which are influenced by the original composition of the host rock, the nature of the intruding magma, and the conditions of metamorphism. Key minerals in skarns include: Garnet: Often present in high-temperature skarns, garnet forms as a result of the metamorphic reaction between Ca-rich rocks and magmatic fluids. Pyroxene: Typically associated with high- temperature skarns, pyroxene forms from the reaction of magnesium and iron with carbonate minerals. Wollastonite: Common in calc-silicate skarns, wollastonite forms through the reaction of calcium- rich fluids with carbonate rocks. Epidote: Frequently found in lower-temperature skarns, epidote forms from the reaction of Al-rich fluids with carbonate minerals. Orogenic gold deposits Orogenic gold deposits are a significant type of gold deposit associated with tectonic processes in orogenic belts. These deposits form in orogenic (mountain-building) settings and are typically found in metamorphic rocks. They are characterized by gold mineralization in quartz veins and host rocks. They are commonly associated with: Mountain-building events: They form during orogeny, where tectonic forces cause significant deformation and metamorphism of rocks. Metamorphic terrains: These deposits are typically hosted in metamorphosed rocks, such as schists and gneisses, within orogenic belts. Faults and shear zones: Gold mineralization often occurs along major faults and shear zones, which act as conduits for gold-bearing fluids. Orogenic gold deposits Formation processes Fluid flow and mineralization: Gold is transported by hydrothermal fluids that circulate through the crust during tectonic activity. As these fluids cool or react with surrounding rocks, gold precipitates, forming quartz veins and other mineral assemblages. Metamorphism and deformation: The high-pressure, high-temperature conditions associated with orogenic processes facilitate the formation of gold deposits. Deformation and metamorphism create spaces for fluid movement and mineral deposition. Gold precipitation: Gold typically precipitates from the fluids as changes in temperature, pressure, or chemical conditions occur. Common mineral associations include quartz, pyrite, arsenopyrite, and other sulfides. Carlin-type gold deposits Carlin-type gold deposits are a distinctive class of gold deposits known for their unique geochemical characteristics and association with sedimentary rocks. These deposits are characterized by fine-grained gold mineralization and a significant presence of arsenic. Geological setting Carlin-type deposits are typically found in: Sedimentary rocks: These deposits are hosted in Paleozoic sedimentary rocks, often within carbonate or silicate-rich rocks. Basin and range province: They are most commonly associated with the Basin and Range province of the western United States, but they can also occur in other tectonic settings with similar geologic features. Carlin-type gold deposits Formation Processes Hydrothermal alteration: Carlin-type deposits form from hydrothermal fluids that interact with sedimentary rocks. These fluids are often low-temperature, acidic, and rich in carbon dioxide and sulfur. Gold precipitation: Gold in Carlin-type deposits is typically found in microscopic particles disseminated throughout the host rock, rather than in visible veins. The gold is often associated with arsenic, forming compounds such as arsenian pyrite and arsenian pyrrhotite. Common alteration types include silicification, carbonate alteration, and argillic alteration (formation of clay minerals such as illite and kaolinite). Carlin-type gold deposits Mineralogy and Textures Gold: Fine-grained, often occurring as submicroscopic particles within the altered rock. Arsenian pyrite and pyrrhotite: Commonly associated with gold and indicative of the mineralization process. Quartz and carbonates: Altered rock can include quartz, calcite, and dolomite as a result of hydrothermal activity. Textures: Gold is disseminated throughout the host rock rather than concentrated in veins, leading to a disseminated ore texture. Iron Oxide Copper-Gold (IOCG) deposits Iron Oxide Copper-Gold (IOCG) deposits are a type of large, low-grade ore deposit characterized by the presence of iron oxides (magnetite and/or hematite), along with copper, gold, and sometimes uranium and rare earth elements. Key characteristics: Host rocks: They are typically found in a variety of host rocks, including granitic to felsic intrusions, volcanic rocks, and sedimentary rocks. Alteration: IOCG deposits are associated with extensive hydrothermal alteration, including sodic-calcic alteration (albite, scapolite), potassic alteration (K-feldspar, biotite), and iron-oxide alteration (magnetite, hematite). IOCG deposits Mineralization: The main ore minerals are copper sulfides (chalcopyrite, bornite) and gold, often accompanied by iron oxides, and sometimes uranium and rare earth elements. Geological setting: These deposits typically form in regions with a history of tectonic activity, often related to large-scale fault systems, magmatic arcs, or rift settings. Fluid source: The ore-forming fluids are typically high-temperature, Sodic-calcic alteration: albite and actinolite. saline, and rich in iron, derived from Potassic alteration: K-feldspar, biotite, and magnetite. deep crustal or mantle sources, Iron oxide alteration: magnetite and hematite. often associated with magmatism. Propylitic alteration: chlorite, epidote, and carbonates, IOCG deposits Mineralogy: Iron oxides: Magnetite and hematite often massive, disseminated, or in veins. Copper sulfides: Chalcopyrite is the main copper mineral, with bornite and chalcocite also present. Gold: Native gold and electrum are typically found in association with sulfides and iron oxides. Uranium minerals: Uraninite and coffinite are found in some IOCG deposits, like Olympic Dam. REE minerals: Monazite and allanite are common in REE-enriched IOCG deposits. Gangue minerals: Quartz, feldspar, carbonates, and apatite often occur as gangue minerals. Banded Iron Formations (BIF) Banded Iron Formations (BIFs) are distinctive sedimentary rocks composed of alternating layers of iron-rich minerals (usually hematite or magnetite) and silica-rich minerals (like chert or quartz). These formations are typically Precambrian in age, dating back over 2.5 billion years, and are one of the primary sources of iron ore. Key features: Layering: Alternating bands of iron oxides and silica, which can be millimeters to centimeters thick. Composition: Iron-rich layers are primarily hematite or magnetite, while silica-rich layers consist of chert or quartz. Origin: BIFs are thought to have formed in ancient oceans, where dissolved iron precipitated out of seawater in response to increasing oxygen levels, a process linked to the Great Oxidation Event. Banded Iron Formations (BIF) Tectonic Settings for BIF Formation: Cratonic basins: Stable, ancient continental regions that hosted large, shallow seas during the Precambrian. Mid-Ocean Ridges and back-arc basins: Tectonically active settings where hydrothermal activity was prevalent. Passive margins: Large continental shelves where upwelling and sedimentation occurred over long periods. Models of formation of BIF deposits Hydrothermal model: Volcanic-sedimentary model: Formation from iron-rich hydrothermal fluids Formation from volcanic activity releasing at mid-ocean ridges or back-arc basins. iron, with subsequent sedimentation in In tectonically active regions like mid- shallow marine environments. ocean ridges or volcanic arcs. In island arcs, continental rifts, or other volcanically active regions. Upwelling model: Formation from upwelling of iron-rich deep Chemocline model: waters into oxygenated surface waters. Formation at the boundary between In passive continental margins or large oxygenated surface waters and anoxic epicontinental seas. deep waters (chemocline). In large, stable marine basins with strong Oxidation model (Great Oxidation Event): stratification. Formation linked to the rise of atmospheric oxygen, leading to iron oxidation and precipitation. In various settings, dependent on global changes in ocean chemistry. Banded Iron Formations (BIF) Mineralogy of BIFs: 1.Iron oxides Magnetite (Fe₃O₄): often occurs in thin layers or bands. Hematite (Fe₂O₃): forms fine-grained layers or intergrown with magnetite. Goethite (FeO(OH)): a secondary iron mineral that can form through the weathering of hematite and magnetite. 2.Silicate Chert (microcrystalline quartz, SiO₂): usually found as thin layers alternating with iron oxides. Jasper: a red, iron-rich variety of chert, sometimes present in BIFs. 3.Carbonates (in some BIFs): Siderite (FeCO₃): in certain BIFs, particularly in more reduced environments. Ankerite (Ca(Fe,Mg,Mn)(CO₃)₂): mixed carbonate that can occur in BIFs. 4.Other minerals: Clay minerals: minor amounts of clay minerals like kaolinite and chlorite Sulfides: in trace amounts, indicating reducing conditions during deposition. Banded Iron Formations (BIF) Textures of BIFs: 1.Banded Alternating layers: alternating iron-rich and silica-rich layers. These bands can range from millimeters to several centimeters in thickness. Rhythmic layering: layers often exhibit rhythmic, cyclic patterns, which may reflect changes in environmental conditions during deposition. 2.Granular Oolitic or granular: some BIFs show granular textures, where iron minerals form small, rounded grains (ooids) within a chert matrix. 3.Laminated Fine lamination: millimeter-scale laminations of iron oxides and chert are common, indicating slow deposition in a quiet marine environment. Banded Iron Formations (BIF) 4. Recrystallization: Metamorphic alteration: BIFs often undergo low-grade metamorphism, leading to recrystallization of iron oxides and chert. This can result in coarser-grained textures and the development of foliation in the rock. 5.Microbanding: Sub-millimeter bands: Very fine alternating layers of iron and silica, visible under a microscope, often characterize BIFs and provide clues to the depositional environment. Banded Iron Formations (BIF) Banded Iron Formations (BIFs) are categorized into different types based on their geological setting, age, and depositional environment. Here’s a summary of the main types: 1. Algoma-type BIFs: Geological Setting: Associated with volcanic and sedimentary sequences, typically in greenstone belts. Age: Mostly Archean (2.5 to 3.8 billion years ago). Depositional environment: Formed in small, deep marine basins, often near volcanic centers. Iron content: Typically lower iron content than Superior-type BIFs. Examples: BIFs in the Canadian Shield and Pilbara Craton, Australia. Banded Iron Formations (BIF) 2. Superior-type BIFs: Superior Lake Geological setting: in large, stable cratonic basins. Age: Predominantly Paleoproterozoic (2.5 to 1.8 billion years ago). Depositional environment: Formed in shallow, continental shelf settings with extensive, stable marine environments. Iron content: High iron content, making them economically significant. Examples: Mesabi Range (USA), Hamersley Basin (Australia), and the Transvaal Supergroup (South Africa). Banded Iron Formations (BIF) 3. Rapitan-type BIFs: Geological Setting: associated with glacial deposits, often linked to Neoproterozoic "Snowball Earth" events. Age: Neoproterozoic (about 750 to 600 million years ago). Depositional environment: Formed in glaciogenic environments, often interpreted as dropstones within marine sediments. Examples: Rapitan Group in Canada, Chuos Formation in Namibia. Banded Iron Formations (BIF) 4. Ironstone-type (Clinton-type) BIFs: Geological setting: associated with shallow marine and continental shelf environments, often in association with oolitic ironstones. Age: Primarily Phanerozoic, especially Ordovician to Silurian periods. Depositional environment: Formed in shallow, nearshore environments with high sedimentation rates. Iron content: Lower iron content compared to older BIFs. Examples: Clinton Ironstones in the eastern United States.