Hi Greg. Sorry, I didn't see you follow up comment a couple of days ago. I am not able to answer myself, so I asked my "new best friend", Google AI Studio.
Question: Can you explain the relationships and differences between hydrothermal, mesothermal, and epithermal as it relates to ore deposits, particularly gold?
Reply: Okay, let's break down the relationships and differences between hydrothermal, mesothermal, and epithermal ore deposits, focusing specifically on their relevance to gold mineralization.
The Big Picture: Hydrothermal Systems
The term "hydrothermal" simply refers to hot, aqueous fluids circulating within the Earth's crust. These fluids are incredibly important because they act as a transport medium for dissolved metals, including gold. As these fluids interact with rocks, change temperature and pressure, or encounter different chemical environments, they can deposit their dissolved metals, forming ore deposits.
All three of the terms – hydrothermal, mesothermal, and epithermal – fall under the umbrella of hydrothermal systems. However, they describe different subtypes based on the temperature and depth at which the ore deposits form.
1. Hydrothermal (General Term)
• Meaning: This is the broadest term, encompassing any ore deposit formed by hot water solutions.
• Characteristics:
o Source of Fluids: Can be magmatic (from magma), metamorphic (from the dehydration of rocks), meteoric (from rainwater that has seeped into the Earth), or a mix of these.
o Wide Range of Temperatures and Depths: Hydrothermal deposits can form at a wide range of depths and temperatures, from near the surface to deep within the Earth's crust.
o Diverse Mineral Assemblages: The specific minerals present in hydrothermal deposits vary dramatically depending on the fluid source, temperature, pressure, and rock types.
o Gold Potential: Gold is a common metal found in many hydrothermal systems.
• Key Takeaway: While all mesothermal and epithermal deposits are hydrothermal, not all hydrothermal deposits are either mesothermal or epithermal. It's a parent term.
2. Mesothermal (Intermediate Temperature and Depth)
• Meaning: Specifically refers to hydrothermal deposits formed at intermediate temperatures and moderate depths in the Earth's crust. Sometimes also called orogenic gold deposits.
• Characteristics:
o Temperature Range: Generally between 250-400°C (482-752°F).
o Depth: Typically form several kilometers below the surface (1-5 km or more).
o Geologic Setting: Often associated with compressional tectonic environments, like mountain ranges.
o Fluid Source: Typically metamorphic fluids, derived from the dewatering of deeply buried rocks.
o Vein-Style Deposits: Often form as quartz veins and shear zones within faults.
o Mineral Assemblage: Common minerals include quartz, pyrite, arsenopyrite, and gold. Gold often occurs in its native form, or associated with pyrite/arsenopyrite. Often, there's a lack of extensive alteration.
o Gold Grades: Can have very high gold grades.
• Gold Mineralization:
o Gold Form: Gold usually occurs as "free" gold within quartz veins and associated with sulfide minerals, particularly pyrite and arsenopyrite.
o Deposition Mechanism: Gold precipitates as a result of changes in temperature, pressure, fluid chemistry and wall-rock interaction.
• Examples: The Mother Lode district in California, the Abitibi greenstone belt in Canada, many deposits in the Victorian gold fields in Australia.
• Key Takeaway: Mesothermal gold deposits are generally older, often occurring in metamorphic terranes and form deeper and hotter than epithermal deposits. They tend to be more structurally controlled and form vein-style deposits.
3. Epithermal (Shallow Temperature and Depth)
• Meaning: Refers to hydrothermal deposits formed at shallow depths and relatively lower temperatures in the Earth's crust.
• Characteristics:
o Temperature Range: Generally between 100-250°C (212-482°F).
o Depth: Form relatively near the surface (typically less than 1 km, often only a few hundred meters).
o Geologic Setting: Often associated with volcanic and geothermal systems, particularly in areas of recent volcanism.
o Fluid Source: Typically a mixture of magmatic fluids and meteoric water.
o Alteration: Often accompanied by extensive hydrothermal alteration of the surrounding host rocks, with characteristic clay minerals and silica enrichment (silicification).
o Vein, Stockwork, and Disseminated Mineralization: Can form as veins, stockwork zones (a network of small veins), and disseminated deposits where metals are spread throughout the rock.
o Mineral Assemblage: Common minerals include quartz, calcite, adularia, clay minerals, pyrite, and various sulfides like galena, sphalerite and chalcopyrite. Gold can occur as native gold, electrum (a gold-silver alloy), or in association with tellurides.
o Gold Grades: Can vary greatly, with some deposits being high-grade but often more variable than mesothermal deposits.
• Gold Mineralization:
o Gold Form: Gold is generally associated with pyrite and other sulfide minerals. It can also occur as "electrum," an alloy with silver, particularly at lower temperatures.
o Deposition Mechanisms: Deposition is caused by processes such as boiling, cooling, fluid mixing, and wall-rock interaction.
o Subtypes: Epithermal deposits can be further categorized into "high-sulfidation" (formed from highly acidic, sulfur-rich fluids) and "low-sulfidation" (formed from neutral to slightly alkaline fluids). These have different characteristic mineral assemblages and alteration styles.
• Examples: The Comstock Lode in Nevada, the Porgera mine in Papua New Guinea, many deposits in the Andes mountains.
• Key Takeaway: Epithermal gold deposits are generally younger, associated with volcanic activity, and form at shallower depths and lower temperatures than mesothermal deposits. They are also characterized by extensive alteration of the host rock.
Alteration Variable Generally Less Extensive Often Extensive
Gold Form Variable Native, Associated with Sulfides Native, Electrum, Sulfides, Tellurides
Age Variable Generally Older Generally Younger
In the Context of Gold Exploration:
Understanding these differences is crucial for gold exploration:
• Mesothermal: Exploration targets include metamorphic terrains, fault zones, quartz veins, and shear zones. Geologists look for indicators of metamorphic fluids and characteristic sulfide minerals.
• Epithermal: Exploration focuses on volcanic regions, areas of geothermal activity, and zones of intense alteration. Geologists look for altered rocks, characteristic epithermal minerals, and structural controls like faults and fractures.
In Conclusion
Hydrothermal systems encompass a wide range of processes that result in ore deposits, with mesothermal and epithermal systems representing two key end-member types. While all three involve hot fluids, mesothermal deposits form at intermediate depths and temperatures in compressional tectonic settings while epithermal deposits form at shallow depths and lower temperatures, often in volcanic regions. Understanding these distinctions is vital for exploration, mineral deposit genesis models, and resource assessment.
Is that mesothermal gold ?
More likely hydrothermal origin Greg.
Mesothermal is a type of hydrothermal processes
Hydrothermal gold is mesothermal or epithermal - I was asking for a clarification, if possible.
Hi Greg. Sorry, I didn't see you follow up comment a couple of days ago. I am not able to answer myself, so I asked my "new best friend", Google AI Studio.
Question: Can you explain the relationships and differences between hydrothermal, mesothermal, and epithermal as it relates to ore deposits, particularly gold?
Reply: Okay, let's break down the relationships and differences between hydrothermal, mesothermal, and epithermal ore deposits, focusing specifically on their relevance to gold mineralization.
The Big Picture: Hydrothermal Systems
The term "hydrothermal" simply refers to hot, aqueous fluids circulating within the Earth's crust. These fluids are incredibly important because they act as a transport medium for dissolved metals, including gold. As these fluids interact with rocks, change temperature and pressure, or encounter different chemical environments, they can deposit their dissolved metals, forming ore deposits.
All three of the terms – hydrothermal, mesothermal, and epithermal – fall under the umbrella of hydrothermal systems. However, they describe different subtypes based on the temperature and depth at which the ore deposits form.
1. Hydrothermal (General Term)
• Meaning: This is the broadest term, encompassing any ore deposit formed by hot water solutions.
• Characteristics:
o Source of Fluids: Can be magmatic (from magma), metamorphic (from the dehydration of rocks), meteoric (from rainwater that has seeped into the Earth), or a mix of these.
o Wide Range of Temperatures and Depths: Hydrothermal deposits can form at a wide range of depths and temperatures, from near the surface to deep within the Earth's crust.
o Diverse Mineral Assemblages: The specific minerals present in hydrothermal deposits vary dramatically depending on the fluid source, temperature, pressure, and rock types.
o Gold Potential: Gold is a common metal found in many hydrothermal systems.
• Key Takeaway: While all mesothermal and epithermal deposits are hydrothermal, not all hydrothermal deposits are either mesothermal or epithermal. It's a parent term.
2. Mesothermal (Intermediate Temperature and Depth)
• Meaning: Specifically refers to hydrothermal deposits formed at intermediate temperatures and moderate depths in the Earth's crust. Sometimes also called orogenic gold deposits.
• Characteristics:
o Temperature Range: Generally between 250-400°C (482-752°F).
o Depth: Typically form several kilometers below the surface (1-5 km or more).
o Geologic Setting: Often associated with compressional tectonic environments, like mountain ranges.
o Fluid Source: Typically metamorphic fluids, derived from the dewatering of deeply buried rocks.
o Vein-Style Deposits: Often form as quartz veins and shear zones within faults.
o Mineral Assemblage: Common minerals include quartz, pyrite, arsenopyrite, and gold. Gold often occurs in its native form, or associated with pyrite/arsenopyrite. Often, there's a lack of extensive alteration.
o Gold Grades: Can have very high gold grades.
• Gold Mineralization:
o Gold Form: Gold usually occurs as "free" gold within quartz veins and associated with sulfide minerals, particularly pyrite and arsenopyrite.
o Deposition Mechanism: Gold precipitates as a result of changes in temperature, pressure, fluid chemistry and wall-rock interaction.
• Examples: The Mother Lode district in California, the Abitibi greenstone belt in Canada, many deposits in the Victorian gold fields in Australia.
• Key Takeaway: Mesothermal gold deposits are generally older, often occurring in metamorphic terranes and form deeper and hotter than epithermal deposits. They tend to be more structurally controlled and form vein-style deposits.
3. Epithermal (Shallow Temperature and Depth)
• Meaning: Refers to hydrothermal deposits formed at shallow depths and relatively lower temperatures in the Earth's crust.
• Characteristics:
o Temperature Range: Generally between 100-250°C (212-482°F).
o Depth: Form relatively near the surface (typically less than 1 km, often only a few hundred meters).
o Geologic Setting: Often associated with volcanic and geothermal systems, particularly in areas of recent volcanism.
o Fluid Source: Typically a mixture of magmatic fluids and meteoric water.
o Alteration: Often accompanied by extensive hydrothermal alteration of the surrounding host rocks, with characteristic clay minerals and silica enrichment (silicification).
o Vein, Stockwork, and Disseminated Mineralization: Can form as veins, stockwork zones (a network of small veins), and disseminated deposits where metals are spread throughout the rock.
o Mineral Assemblage: Common minerals include quartz, calcite, adularia, clay minerals, pyrite, and various sulfides like galena, sphalerite and chalcopyrite. Gold can occur as native gold, electrum (a gold-silver alloy), or in association with tellurides.
o Gold Grades: Can vary greatly, with some deposits being high-grade but often more variable than mesothermal deposits.
• Gold Mineralization:
o Gold Form: Gold is generally associated with pyrite and other sulfide minerals. It can also occur as "electrum," an alloy with silver, particularly at lower temperatures.
o Deposition Mechanisms: Deposition is caused by processes such as boiling, cooling, fluid mixing, and wall-rock interaction.
o Subtypes: Epithermal deposits can be further categorized into "high-sulfidation" (formed from highly acidic, sulfur-rich fluids) and "low-sulfidation" (formed from neutral to slightly alkaline fluids). These have different characteristic mineral assemblages and alteration styles.
• Examples: The Comstock Lode in Nevada, the Porgera mine in Papua New Guinea, many deposits in the Andes mountains.
• Key Takeaway: Epithermal gold deposits are generally younger, associated with volcanic activity, and form at shallower depths and lower temperatures than mesothermal deposits. They are also characterized by extensive alteration of the host rock.
Here's a table summarizing the key differences:
Feature Hydrothermal (General) Mesothermal Epithermal
Temperature Wide Range Intermediate (250-400°C) Low (100-250°C)
Depth Wide Range Moderate (1-5 km+) Shallow (<1 km)
Geologic Setting Variable Orogenic Belts, Metamorphic Volcanic/Geothermal Systems
Fluid Source Variable Metamorphic Magmatic, Meteoric, Mixed
Mineralization Variable Vein-Style, Structurally Controlled Vein, Stockwork, Disseminated
Alteration Variable Generally Less Extensive Often Extensive
Gold Form Variable Native, Associated with Sulfides Native, Electrum, Sulfides, Tellurides
Age Variable Generally Older Generally Younger
In the Context of Gold Exploration:
Understanding these differences is crucial for gold exploration:
• Mesothermal: Exploration targets include metamorphic terrains, fault zones, quartz veins, and shear zones. Geologists look for indicators of metamorphic fluids and characteristic sulfide minerals.
• Epithermal: Exploration focuses on volcanic regions, areas of geothermal activity, and zones of intense alteration. Geologists look for altered rocks, characteristic epithermal minerals, and structural controls like faults and fractures.
In Conclusion
Hydrothermal systems encompass a wide range of processes that result in ore deposits, with mesothermal and epithermal systems representing two key end-member types. While all three involve hot fluids, mesothermal deposits form at intermediate depths and temperatures in compressional tectonic settings while epithermal deposits form at shallow depths and lower temperatures, often in volcanic regions. Understanding these distinctions is vital for exploration, mineral deposit genesis models, and resource assessment.