Hydrothermal Ore Deposits

Introduction

A mineral deposit precipitated from hot, aqueous solution is called hydrothermal solutions. These solutions can originate from various sources, including:

• Magmatic water and vapour
• Heated meteoric water
• Seawater
• Diagenetic and metamorphic fluids
• Evaporative and salt-solution brines

Sources of Hydrothermal Fluids

The hydrothermal fluids can originate from various geological processes and locations.

• Magmatic Fluids: These fluids are derived directly from cooling magmas. As magma crystallizes, incompatible elements and volatiles, including water, become concentrated in the remaining melt. Eventually, these components are exsolved as a separate fluid phase, often carrying significant concentrations of metals.
• Meteoric Water: Meteoric water refers to groundwater originating from precipitation. This water can be heated by interaction with hot rocks at depth, particularly in areas with high geothermal gradients like volcanic regions and active tectonic zones. Heated meteoric water can circulate through the crust, dissolving metals and forming ore deposits.
• Oceanic Water: Similar to meteoric water, ocean water can percolate through the ocean floor, later heated by interaction with magma chambers. This heated seawater can then circulate through the crust, leaching metals and contributing to the formation of hydrothermal deposits, particularly those found in seafloor environments like VHMS deposits.
• Connate Water (Formation Water): The water trapped in the pore spaces of sedimentary rocks during their deposition is called connate water. As sediments are buried and compacted, this water can be heated and modified by diagenetic processes, potentially acquiring dissolved metals.
• Diagenetic and Metamorphic Fluids: Diagenetic fluids are generated during the compaction and lithification of sediments. Metamorphic fluids are released during the recrystallization of rocks under high temperature and pressure conditions. Both of these fluid types can contribute to the formation of hydrothermal deposits.
• Evaporative Brines: In arid environments, evaporation can lead to the formation of concentrated brines. These brines can interact with existing groundwater systems, potentially carrying dissolved metals and contributing to hydrothermal ore formation.

The Process of Hydrothermal Ore Deposit Formation

The formation of a hydrothermal ore deposit is a complex process that involves several key stages, which are interconnected and can overlap.

1. Hydrothermal Fluid Generation:

The first step in the formation of any hydrothermal ore deposit is the generation of a hydrothermal fluid capable of dissolving and transporting metals. As discussed above, the source of these fluids can be diverse. For instance, magmatic fluids are derived from cooling magma bodies, while metamorphic fluids are released during metamorphic reactions in rocks undergoing heat and pressure changes. Meteoric water, originating from precipitation, can also become hydrothermal solution when heated at depth. Connate water, trapped in sedimentary rocks during their formation, can be mobilized by tectonic or thermal events.

2. Dissolution of Ore Constituents:

Once a hydrothermal fluid is generated, it needs a source of ore constituents to form an ore deposit. Ore constituents can be leached from the surrounding rocks as the fluid migrates through them. The composition of the host rocks plays a crucial role in determining the types of minerals that can be dissolved and transported. For example, fluids circulating through mafic and ultramafic rocks can dissolve significant amounts of nickel, copper, and platinum group elements (PGEs). Conversely, fluids interacting with granitic rocks might become enriched in tin, tungsten, and molybdenum.

3. Fluid Migration:

Structures like faults, fractures, and shear zones provide pathways for fluid migration. These structures act as conduits, allowing the hydrothermal fluids to move through the Earth’s crust. The permeability of the host rocks also influences fluid flow, with fluids preferentially flowing through more permeable units. Migration of hydrothermal fluid from a large volume of rock into a smaller area is essential for creating an economic ore deposit.

4. Precipitation due to changes in Physicochemical Conditions:

As hydrothermal fluids migrate, they encounter changing physical and chemical conditions. These changes can trigger the precipitation of ore minerals from the solution. Key factors influencing mineral solubility and, consequently, precipitation include:

• Temperature: Generally, mineral solubility decreases as the hydrothermal fluid cools. As the hot hydrothermal fluid ascends and interacts with cooler rocks, the solubility of the dissolved metals decreases, leading to their precipitation.
• Pressure: A decrease in pressure can also induce mineral precipitation. As fluids rise towards the surface, the confining pressure decreases, which can lead to a decrease in mineral solubility and the formation of deposits.
• Fluid Mixing: Mixing of two chemically distinct fluids can lead to supersaturation and mineral precipitation. For example, mixing of a hot, metal-rich fluid with a cooler, oxygenated fluid can cause the precipitation of sulfide minerals.
• Fluid-Rock Interaction: Chemical reactions between the hydrothermal fluid and the surrounding rocks can alter the fluid’s composition and trigger mineral precipitation. For instance, reactions with carbonate rocks can increase the pH of the fluid, leading to the precipitation of carbonate minerals and base metal sulfides.

5. Ore Deposition:

The final stage in hydrothermal ore deposit formation is the deposition of ore minerals. This can occur in various forms, depending on the specific geological setting and the physicochemical conditions of the system. Some common forms of hydrothermal ore deposits include:

• Veins: Veins are tabular mineral deposits that form when fluids fill fractures or other open spaces in rocks.
• Disseminated Deposits: Disseminated deposits occur when ore minerals are distributed as fine grains throughout the host rock. Porphyry copper deposits are a prime example of this type.
• Replacement Deposits: Replacement deposits form when hydrothermal fluids dissolve pre-existing minerals and replace them with ore minerals. Skarn deposits, where ore minerals replace carbonate rocks, are a common example.
• Massive Sulfide Deposits: Massive sulfide deposits are characterized by high concentrations of sulfide minerals, often formed in submarine volcanic settings.

Classification of hydrothermal mineral deposits

Based on the concept of a fluid continuum from high, magmatic temperatures at depth to lower temperatures near the surface, Lindgren (1933) classified hydrothermal mineral deposits in terms of the depth of emplacement into three groups:

• Hypothermal, formed at great depths (300°-500°C, very high pressure);
• Mesothermal, formed at intermediate depths (200°-300°C, high pressure);
• Epithermal, formed at shallow depths (50°-200°C, moderate pressure).
• Telethernal, Graton (1933) proposed the term telethernal for deposits formed at the low temperature-pressure end of the spectrum.
• Xenothermal, Buddington (1935) introduced the term xenothermal for deposits formed at shallow depths but at relatively high temperatures.

The formation of a hydrothermal deposit requires a favorable combination of: (a) source(s) of water; (b) source(s) of ore constituents; (c) dissolution of ore constituents in sufficient concentrations in the fluids to render them potentially ore-forming; (d) migration of the enriched fluids to sites of deposition; and (e) focused discharge of a large volume of fluids into an appropriate environment, leading to precipitation of ore constituents in large quantities in a relatively limited volume of rock.

Examples of Hydrothermal Deposits

Various types of hydrothermal deposits have been identified over the period, each with its unique set of characteristics, reflecting variations in fluid sources, geological settings, and depth of formation:

• Volcanic-Hosted Massive Sulfide (VHMS) Deposits: These deposits, frequently rich in copper, zinc, and lead, form near submarine volcanic vents where hot, metal-rich fluids interact with seawater. The Cyprus-type deposits, found in ophiolites, serve as prime examples of ancient VHMS deposits, providing valuable insights into the formation processes of modern seafloor sulfide deposits.
• Sediment-Hosted Massive Sulfide (SMS) Deposits: Occurring in sedimentary basins, SMS deposits form when hydrothermal fluids, often carrying metals leached from surrounding rocks, encounter favorable conditions for precipitation within the sedimentary layers. These deposits are further categorized based on the specific sedimentary host rock, with examples including the HYC deposit in Australia and various deposits in Ireland.
• Sedimentary-Exhalative (Sedex) Deposits: Sedex deposits result from hydrothermal fluids venting directly onto the seafloor, leading to the precipitation of metals within sediments. The Ballynoe baryte deposit in Ireland exemplifies this type.
• Porphyry Deposits: Characterized by their large size and low grade, porphyry deposits typically form around felsic to intermediate intrusions. These deposits are distinguished by their stockwork veining and disseminated mineralization, resulting from the circulation of magmatic-hydrothermal fluids. They often contain economically significant quantities of copper, molybdenum, and gold.
• Epithermal Deposits: Forming at shallow depths, typically less than 1 km below the surface, epithermal deposits are intimately associated with volcanic activity. These deposits are classified into high- and low-sulfidation types based on the acidity and oxidation state of the involved hydrothermal fluids.
  • High-sulfidation epithermal deposits are characterized by their acidic fluids, intense wall rock alteration, and the presence of minerals such as alunite and kaolinite. These deposits are often associated with volcanic caldera settings.
  • Low-sulfidation epithermal deposits are generally associated with less acidic fluids and frequently manifest as vein-hosted mineralization. They are often found further from volcanic centers compared to high-sulfidation deposits.
• Skarn Deposits: Formed through the metasomatic replacement of carbonate rocks, primarily limestone, near igneous intrusions. The interaction of hot magmatic-hydrothermal fluids with carbonate rocks produces characteristic calc-silicate minerals, accompanied by the deposition of various metals, including iron, copper, tungsten, tin, gold, and lead-zinc.
• Vein Deposits: These deposits form when hydrothermal fluids fill fractures and faults in diverse host rocks. The hydrothermal vein deposits frequently exhibit spatial zonation of minerals, reflecting changes in temperature and fluid chemistry along the fracture system.
• Mississippi Valley-Type (MVT) Pb-Zn Deposits: MVT deposits are epigenetic deposits rich in lead and zinc sulfides found in carbonate host rocks. Their formation is attributed to the migration of basinal brines through sedimentary basins, often far from igneous activity.
• Diagenetic-Hydrothermal Carbonate-Hosted Pb-Zn (F-Ba) Deposits: Representing a specific subset of carbonate-hosted lead-zinc deposits. The Mississippi Valley type (MVT) is highlighted as a well-studied example of this category, where diagenetic processes play a crucial role in concentrating metals prior to or during hydrothermal fluid flow.
• Metamorphic and Metamorphosed Deposits: Metamorphic processes, while not directly forming large ore deposits, can significantly modify existing mineralization. The passage of hot metamorphic fluids can lead to the redistribution and concentration of metals, potentially enhancing the economic viability of pre-existing deposits.

Importance of Hydrothermal Alteration

Hydrothermal alteration refers to the changes in host rock mineralogy and chemistry caused by interaction with hydrothermal fluids. It provides valuable clues to understanding the nature of the hydrothermal system. One can classify hydrothermal alteration, based on temperature, pressure, and fluid composition conditions. These alterations include:

• Silicification: Hydrothermal alteration in which the quartz, opal, chalcedony, jasper, or other form of amorphous silica content of the rock increases. The term often refers to cases where there is a net addition of silica in the altered rock.
• Albitization: Replacement of calcium-rich plagioclase with sodium-rich albite.
• Argillic Alteration: Formation of clay minerals, with variations depending on the intensity of acidity. Advanced argillic alteration yields alunite and quartz, while intermediate argillic alteration results in kaolinite or montmorillonite.
• Propylitization: Complex alteration producing chlorite, epidote, albite, and carbonates, often imparting a greenish colour to the rocks.