Podcast
Questions and Answers
What are the main minerals found in porphyry copper deposits?
What are the main minerals found in porphyry copper deposits?
What is a characteristic feature of hydrothermal ore deposits that form due to magmatic activity?
What is a characteristic feature of hydrothermal ore deposits that form due to magmatic activity?
Which of the following types of hydrothermal deposits lacks widespread or temporally related magmatic activity?
Which of the following types of hydrothermal deposits lacks widespread or temporally related magmatic activity?
What percentage of the world's copper is sourced from porphyry copper deposits?
What percentage of the world's copper is sourced from porphyry copper deposits?
Signup and view all the answers
Which process causes metals to crystallize from hydrothermal magmatic fluids?
Which process causes metals to crystallize from hydrothermal magmatic fluids?
Signup and view all the answers
What is the primary reason for the explosive activity at the surface of a kimberlite magma eruption?
What is the primary reason for the explosive activity at the surface of a kimberlite magma eruption?
Signup and view all the answers
Which of the following best describes where diamonds are found in relation to the kimberlite magma?
Which of the following best describes where diamonds are found in relation to the kimberlite magma?
Signup and view all the answers
What structure serves as the primary channel for kimberlite melts to reach the surface?
What structure serves as the primary channel for kimberlite melts to reach the surface?
Signup and view all the answers
How does the local geology affect the emplacement of a kimberlite diatreme?
How does the local geology affect the emplacement of a kimberlite diatreme?
Signup and view all the answers
What is the minimum depth at which kimberlite melts originate from mantle partial melting?
What is the minimum depth at which kimberlite melts originate from mantle partial melting?
Signup and view all the answers
What is a key characteristic of the fluids involved in hydrothermal mineral deposition?
What is a key characteristic of the fluids involved in hydrothermal mineral deposition?
Signup and view all the answers
What temperature must kimberlite magma typically exceed for significant geological activity?
What temperature must kimberlite magma typically exceed for significant geological activity?
Signup and view all the answers
What secondary effect occurs when hot kimberlite magma interacts with groundwaters?
What secondary effect occurs when hot kimberlite magma interacts with groundwaters?
Signup and view all the answers
What is the primary reason why most volatile components in magma do not become part of crystallizing minerals?
What is the primary reason why most volatile components in magma do not become part of crystallizing minerals?
Signup and view all the answers
Which elements are concentrated in escaping magmatic-hydrothermal fluids?
Which elements are concentrated in escaping magmatic-hydrothermal fluids?
Signup and view all the answers
What factor does NOT influence the concentration of trace ore metals in an exsolved fluid from felsic magma?
What factor does NOT influence the concentration of trace ore metals in an exsolved fluid from felsic magma?
Signup and view all the answers
How does pressure affect the solubility of volatiles in a melt?
How does pressure affect the solubility of volatiles in a melt?
Signup and view all the answers
What is First Boiling in the context of magmatic processes?
What is First Boiling in the context of magmatic processes?
Signup and view all the answers
What happens during Second Boiling in magmatic activity?
What happens during Second Boiling in magmatic activity?
Signup and view all the answers
What is a hydrothermal magmatic brine?
What is a hydrothermal magmatic brine?
Signup and view all the answers
What role does fractional crystallization play in magmatic ore deposit formation?
What role does fractional crystallization play in magmatic ore deposit formation?
Signup and view all the answers
Which factor is least likely to affect the migration of magmatic-hydrothermal fluids into country rock?
Which factor is least likely to affect the migration of magmatic-hydrothermal fluids into country rock?
Signup and view all the answers
What is NOT true about the concentration of metals during the process of exsolution?
What is NOT true about the concentration of metals during the process of exsolution?
Signup and view all the answers
What is a key factor that can lead to the dropping of metals from hydrothermal fluids?
What is a key factor that can lead to the dropping of metals from hydrothermal fluids?
Signup and view all the answers
Which of the following scenarios could lead to hydrothermal alteration?
Which of the following scenarios could lead to hydrothermal alteration?
Signup and view all the answers
Which type of hydrothermal ore deposit forms closely associated with magmatic activity?
Which type of hydrothermal ore deposit forms closely associated with magmatic activity?
Signup and view all the answers
What does 'hydrothermal alteration' typically signify in geological processes?
What does 'hydrothermal alteration' typically signify in geological processes?
Signup and view all the answers
Which component is NOT part of a hydrothermal system's summary?
Which component is NOT part of a hydrothermal system's summary?
Signup and view all the answers
In which areas are mafic magmatism typically found?
In which areas are mafic magmatism typically found?
Signup and view all the answers
Which of the following types of deposits lack widespread magmatic activity?
Which of the following types of deposits lack widespread magmatic activity?
Signup and view all the answers
What is a primary mechanism leading to metal remobilization in hydrothermal systems?
What is a primary mechanism leading to metal remobilization in hydrothermal systems?
Signup and view all the answers
Which change would likely NOT affect the equilibrium conditions of hydrothermal fluids?
Which change would likely NOT affect the equilibrium conditions of hydrothermal fluids?
Signup and view all the answers
Which of the following factors is crucial for the transport of metals in hydrothermal systems?
Which of the following factors is crucial for the transport of metals in hydrothermal systems?
Signup and view all the answers
What is the primary process by which ore elements concentrate in magmatic systems?
What is the primary process by which ore elements concentrate in magmatic systems?
Signup and view all the answers
Which of the following accurately describes kimberlites?
Which of the following accurately describes kimberlites?
Signup and view all the answers
Which mineral is predominantly found in kimberlite?
Which mineral is predominantly found in kimberlite?
Signup and view all the answers
What characterizes the texture of kimberlite in hand specimens?
What characterizes the texture of kimberlite in hand specimens?
Signup and view all the answers
What results from the alteration of olivine in kimberlite upon reaching the Earth's surface?
What results from the alteration of olivine in kimberlite upon reaching the Earth's surface?
Signup and view all the answers
What is one of the main features of primary magmatic diamond deposits in kimberlites?
What is one of the main features of primary magmatic diamond deposits in kimberlites?
Signup and view all the answers
Which process does not contribute to the formation of ore deposits in magmatic systems?
Which process does not contribute to the formation of ore deposits in magmatic systems?
Signup and view all the answers
What type of magma forms from larger degrees of partial melting of mantle peridotite?
What type of magma forms from larger degrees of partial melting of mantle peridotite?
Signup and view all the answers
Which mineral is often found altered in kimberlite to form serpentine?
Which mineral is often found altered in kimberlite to form serpentine?
Signup and view all the answers
What does the term 'serpentinisation' refer to in kimberlite geology?
What does the term 'serpentinisation' refer to in kimberlite geology?
Signup and view all the answers
Study Notes
Ore Deposit Formation in Magmatic Systems
- Five processes for ore formation in magmatic systems are identified:
- Concentration of ore elements due to low degrees of partial melting.
- Accumulation and concentration of ore minerals in magma chambers during progressive crystallization and fractionation of mafic and ultramafic magmas (e.g., chromitite deposits).
- Separation of immiscible melts in a magma chamber (e.g., Ni-Cu-PGE sulfide deposits).
- Extreme fractionation during progressive crystallization of a magma
- Incorporation of a mineral that occurs at a specific depth in the Earth into an ultramafic melt (e.g., primary magmatic diamond deposits in kimberlites).
Kimberlites
- Kimberlites are alkaline, volatile-rich potassic, low-density ultramafic rocks.
- They form from small degrees of partial melts of carbonate-bearing and hydrous mantle peridotite.
- Larger degrees of partial melting would result in a basaltic magma.
- They are important because they can host diamonds.
- Kimberlites have an inequigranular texture with fragmented minerals and clasts in a fine-grained groundmass.
- Main minerals in kimberlites include olivine (often altered to serpentine), diopside, pyrope garnet, enstatite, ilmenite, and chromite.
- Alteration of olivine to serpentine promotes diamond liberation from the kimberlite host rock.
- Diamonds are accidental passengers in kimberlite melts, meaning they do not form during kimberlite melt crystallization.
- Historically, diamonds were mined from secondary alluvial deposits, such as river environments before kimberlites were discovered.
- Primary magmatic diamond deposits are mainly found in Precambrian terranes/cratons, particularly within the Archean.
- Major diamond-producing countries include Russia, Canada, Botswana, Angola, and South Africa.
Diamond Production
- Estimated diamond production in 2021, by country, is presented in a pie chart.
- Additional data on diamond production for specific countries (e.g., Botswana, Russia, Australia, South Africa, Canada) are available for additional years (e.g., 2008, 2009). Note that values are in the thousands of carats, a unit used for precious stone weight.
Kimberlite Plumbing System
- Kimberlite melts originate from a substantial depth (>150 km).
- They reach the surface as volcanic eruptions via diatremes (volcanic pipes).
- They can also occur as diatremes, pipes, dykes, and sills, and less commonly as volcanic tuffs.
- Diatremes form to ~1 km depth from the root system.
- The high volatile content in the magma at ~1 km depth results in eruptive "blowout"
- Interaction of hot kimberlite magma (>900°C) with groundwater triggers explosive activity, forming crater facies at the surface
- Diamonds are transported upward by the melt.
- The kimberlite (including diamond) is only within the diatreme, not in the surrounding rocks.
- Emplacement is discordant with the local geology, typically as a vertical pipe, dyke, or sill.
Morphology of Kimberlite Bodies/Pipes
- World-wide kimberlite bodies and pipes are typically small (<1000 m diameter) and characterized as diatremes.
- They consist of volcanic pipes filled with broken volcanic fragments (volcanic breccia).
- They are divided into 2 parts:
- Volcaniclastic kimberlite, or fragmental rocks
- Hypabyssal kimberlite, or non-fragmental rocks
Kimberlite Eruption
- Kimberlites have magmatic plumbing systems at depth, which can be cylindrical (pipe), planar vertical (dyke), or planar horizontal (sill).
- The high volatile content in the magma at shallow levels causes an eruptive "blowout" leading to a volcanic crater.
- Fluid eruption is driven by the interaction of the magma with surface water.
- Many kimberlite complexes display multiple eruption stages that have evolved over millions of years.
- Kimberlites often appear in clusters
Global Distribution of Primary Magmatic Diamond Deposits
- Deposits are concentrated in Archean crustal blocks of Precambrian terranes/cratons.
- Specific locations are mapped on a global distribution diagram.
Two Types of Kimberlites
-
Group 1 kimberlites are widespread, dominated by olivine, and intruded 80-90 million years ago. They have low percentages of matrix calcite, apatite, and diopside.
-
Group 2 (orangites) kimberlites are restricted to southern Africa, characterized by megacrysts of phlogopite (mica) with lesser olivine and mica-olivine groundmass. They have high percentages of matrix calcite, apatite and diopside and intruded 120 to 200 Ma
Different Types of Kimberlite Pipes
- Volcaniclastic kimberlite (VK), characterized by fragmental rocks
- Hypabyssal kimberlite (HK), characterized by non-fragmental rocks
Pegmatites
- Pegmatites are felsic igneous rocks with very coarse-grained crystals.
- Crystals form from a felsic melt during the late stages of fractional crystallization when the melt is rich in incompatible elements and volatiles.
- Main minerals in pegmatites are quartz, K-feldspar, micas (biotite and muscovite), and rare minerals rich in lithophile elements and/or volatiles (e.g. beryl, tourmaline, apatite, Li-minerals).
- Pegmatites are enriched in lithophile, incompatible elements and volatiles from granite crystallization.
- They can be mined for industrial minerals (quartz, feldspar), minor metals (e.g. Be, Ta, Nb, Sn-W, Cs, Li), and uranium.
Economic Rare-Element Pegmatites
- Economic rare-element pegmatites are subdivided into two groups depending on the presence of rare metals - LCT (Li, Cs, Ta) types and NYF (Nb, Y, F) types.
- The dominant process for concentration of elements of interest in pegmatites is the last stages of fractional crystallisation.
- Pegmatites are some of the most fractionated felsic igneous rocks.
- The source of rare metals in pegmatites is from pre-existing continental crust.
Granites and Volatiles
- Most granitic melts are hydrous (3-6 wt% H20 and >1000 ppm CO2).
- When a granitic melt ascends toward the surface and cools down, rock-forming minerals crystallize.
- The solubility of H₂O in silicate melt decreases with decreasing pressure, and the residual felsic melt becomes progressively more hydrated and enriched in incompatible elements and volatiles.
- Compatible and non-volatile elements enter the common rock-forming silicates during crystallisation.
- Non-volatile elements concentrate in the residual melt.
- Incompatible volatile elements concentrate, and this leads to extreme concentrations of incompatible elements (e.g. Cs, Li, Nb, Ta) and volatiles in residual felsic melts.
- During crystallisation the volatiles and H₂O concentrate into a magmatic fluid phase (liquid or vapour). These magmatic fluids will be important for the formation of hydrothermal ore deposits.
Rare Metal Enrichment by Fractionation of a Felsic Melt
- Residual felsic melt becomes progressively more saturated in H20 and enriched with incompatible elements and volatiles.
- The aqueous residual felsic melt leaves the granite intrusion and migrates into the country rock.
- Mineral crystallisation occurs as the melt cools, continuously richer in incompatible elements and volatiles.
- Zoning occurs in crystallisation moving outward from the original granitic intrusion.
Pegmatites & Rare Metals Genetic Model
- The source of rare metals in pegmatites is pre-existing continental crust.
- Metals are transported to the surface in dissolved incompatible lithophile elements in granitic magmas
- Extreme fractional crystallisation leads to extreme enrichment in incompatible (Ta-Nb-Li-Cs) and volatile elements (H-Cl-F-B-P).
- The trap is the pegmatite body where crystals have unusual chemistry and size.
- Crystallisation of minerals proceeds by progressive enrichment with increasing distance from the source.
Hydrothermal Ore Deposits
- Hydrothermal ore deposits result from the transport of metals in hydrothermal fluids (aqueous fluids).
- Fluids can originate from depth or the Earth's surface.
- Some important fluids sources are diagenetic, metamorphic and magmatic fluids, meteoric water and connate water.
- The dissolving and transport of elements is often facilitated by ligands.
- Solubility of metals is greater in fluids with ligands.
- Fluids need to be undersaturated in a certain element to allow the dissolution or leaching of metals from rocks. The fluid needs to become oversaturated of a certain element to induce the precipitation of metals.
- The fluids, metal concentration and the geochemical conditions of the fluid drive the processes of mineral precipitation.
Important hydrothermal mineral deposits
-
The physical and chemical characteristics control the formation of these fluids.
-
Various types of hydrothermal systems.
-
Three main types of hydrothermal ore deposits:
- Deposits closely associated with magmatic activity
- Deposits formed during periods of regional magmatism and tectonism, but not clustered near magmatic centers
- Deposits lacking widespread or temporally related magmatic activity
-
Main requirements for the formation of hydrothermal ore deposits.
-
Source for the ore components
-
Process for transporting the elements to the ore deposits site and allowing appropriate concentrations
-
Depositional mechanism to fix the elements in the ore body as ore minerals and associated gangue - geological process, which preserves the ore
Alteration
- Hydrothermal fluids cause mineralogical, chemical, and textural changes in the country rocks, often referred to as alteration.
- The alteration often extends over a large area compared to the ore body.
- Temperature, pressure, fluid composition, permeability of the host rocks, host-rock composition, fluid/rock ratio control the alteration.
- The alteration is often mineralogically and chemically zoned, and recognising the zoning patterns can be important for exploration.
Veins, Breccias and Alteration
- Vein formation is associated with mineral deposition in fractures.
- Veining can involve a single or multiple generations.
- Breccias form from fragmentation of rock due to a pressure or collapse gradient.
- Alteration involves mineralogical and chemical changes induced in the host rock from hydrothermal fluids.
Classification of Epithermal Deposits
-
High-sulfidation (HS): characterised by oxidized sulfur species (SO2, SO42−, HSO4−) and magmatic fluids.
-
Low-sulfidation (LS): characterized by reduced sulfur species (HS−, H2S) and meteoric fluids.
Types of Ore Deposits
- Porphyry Copper: large-tonnage low-grade copper deposits associated with felsic intrusions, often related to subduction zones.
- Epithermal Gold: shallow-level gold deposits commonly found in island arcs and continental arcs, associated with hydrothermal fluid flow from volcanic systems.
Studying That Suits You
Use AI to generate personalized quizzes and flashcards to suit your learning preferences.
Related Documents
Description
This quiz explores the essential characteristics and mineral composition of porphyry copper deposits, as well as the processes involved in their formation. Test your knowledge on hydrothermal ore deposits and their relation to magmatic activity. Ideal for geology students and enthusiasts!
