Diagenesis

Diagenesis

is the physical, chemical, and biological changes that happen to sediments after they are deposited but before they become metamorphic rock. It is basically the process that turns loose sediment into sedimentary rock (called lithification).

1.Compaction:

As sediments are buried, overlying pressure squeezes grains closer together, reducing pore space and expelling fluids.

2.Cementation:

Minerals (such as quartz or calcite) precipitate from groundwater into pore spaces, cementing grains together into solid rock.

3.Dissolution:

Groundwater dissolves unstable components, often creating secondary porosity.

4.Recrystallization:

Change in crystal structure or size (e.g., aragonite to calcite) without changing the chemical composition.

5.Authigenesis:

The formation of new minerals (e.g., clay minerals) within the sediment.

6.Replacement:

One mineral dissolves while another precipitates in its place.

7.Bioturbation:

Biological mixing of sediment by organisms.

DIAGENETIC PHASES

1. Early Diagenesis (Eogenesis)

2. Burial Diagenesis (Mesogenesis)

3. Late Diagenesis (Telogenesis)

1. Early Diagenesis (Eogenesis)

-Happens right after deposition (near surface) Low temperature and pressure

-Processes: compaction starts, early cementation, microbial activity

2. Burial Diagenesis (Mesogenesis)

-Occurs as sediments are buried deeper Higher temperature and pressure

-Processes: strong compaction, recrystallization, mineral replacement, fluid movement

3. Late Diagenesis (Telogenesis)

-Happens at deeper levels or during uplift Influenced by migrating fluids

-Processes: ore mineral formation (like galena, sphalerite), fracturing, alteration

ROLE OF DIAGENESIS IN ORE FORMATION

Diagenesis plays a key role in ore formation by controlling the transport, concentration, and deposition of metals within sedimentary basins. After sediment deposition, processes such as compaction, fluid movement, dissolution, and chemical reactions occur, allowing basin fluids to circulate and dissolve metals from surrounding rocks. As burial continues, compaction drives metal-rich fluids through sedimentary layers, concentrating metals in specific areas. Changes in temperature, pressure, pH, or chemical conditions cause dissolved metals to precipitate as ore minerals like galena and sphalerite. Processes such as cementation and replacement further enhance metal accumulation, eventually forming economically significant deposits. This process is commonly seen in sediment-hosted deposits such as MVT Pb–Zn and SEDEX deposits.

Mississippi Valley-Type (MVT) Deposits

are epigenetic ore systems hosted mainly in carbonate rocks such as limestone and dolostone. They typically occur in stable platform environments associated with passive continental margins

Formation Mississippi Valley-Type (MVT) Deposits

These deposits form when metal-bearing fluids migrate through sedimentary basins and precipitate ore minerals in favorable structural and chemical traps.

epigenetic

meaning mineralization occurs after the host rocks have already formed and undergone initial diagenesis.

DIAGENESIS IN MVT PB-ZN DEPOSITS

A. Fluid Migration and Source

B. Key Diagenetic Processes

C. Controls on Mineralization

A. Fluid Migration and Source

The ore-forming fluids are basinal brines derived from sedimentary basins, often associated with evaporites. These fluids migrate through faults, fractures, and permeable rock layers.

Dolomitization

- Limestone is often altered into dolostone, increasing porosity and permeability, which enhances fluid flow.

Fluid Flow and Replacement

- Metal-rich fluids move through the rock and replace carbonate minerals with sulfide minerals like galena and sphalerite.

Cementation and Recrystallization

- Minerals precipitate in pore spaces and fractures, forming ore bodies

C. Controls on Mineralization

-Structural features (faults and fractures) guide fluid pathways -Lithology (carbonate rocks are more reactive and favorable)

-MVT mineralization is therefore strongly linked to late-stage diagenesis and fluid migration, rather than initial sediment deposition.

Special Types of MVT Pb-Zn Deposits

-Irish-type deposits

-Alpine-type MVT

-Fluorite-rich MVT

Irish-type deposits

– occur in carbonate rocks with strong structural control (fault-hosted, e.g., Navan)

Alpine-type MVT

– associated with orogenic belts and deformation

Fluorite-rich MVT

– significant fluorite (CaF₂) with Pb-Zn (e.g., Illinois-Kentucky district

locations of MVT PB-ZN DEPOSITS

Mississippi Valley (USA), Canada, Ireland

ore mineral content MVT PB-ZN DEPOSITS

SPHALERITE (Zn,Fe)S, GALENA (PbS)

Economic Significance of MVT Pb-Zn Deposits

The diagenesis of MVT Pb–Zn deposits is economically important because it forms valuable lead and zinc ores within carbonate rocks through the movement of metal-rich basinal fluids during late-stage diagenesis. These fluids deposit minerals like galena and sphalerite through replacement and cementation processes. MVT deposits are often high-grade, widely distributed, and structurally controlled by faults and fractures, making them important exploration targets. They contribute significantly to global lead and zinc production for industries such as batteries, construction, and metal alloys.

SEDIMENTARY EXHALATIVE (SEDEX) Deposits

-deposits, also known as clastic-dominated Pb-Zn deposits, are typically hosted in fine-grained sedimentary rocks such as shale, siltstone, and sandstone . They form in marine basins, often in tectonically active settings such as rifts or passive margins.

-are generally stratiform and laminated, reflecting their close relationship with sedimentation processes.

syngenetic

meaning mineralization occurs at or shortly after sediment deposition.

Diagenesis in SEDEX Deposits

A. Hydrothermal Activity

B. Key Diagenetic Processes

C. Basin Controls

A. Hydrothermal Activity

Metal-rich brines are expelled from the basin and discharged onto the seafloor. When these fluids mix with seawater, changes in temperature and chemistry cause metals to precipitate.

Seafloor Precipitation

- Sulfide minerals precipitate directly on or just below the seafloor, forming layered (laminated) deposits.

Early Diagenetic Modification

- After deposition, sediments undergo compaction and minor chemical alteration.

Burial and Recrystallization

- With increasing depth, sulfide minerals may recrystallize and become more compact.

C. Basin Controls

-Tectonic setting (e.g., rifting) drives fluid flow

-Sedimentation rate affects preservation of ore layers

-Organic matter influences redox conditions SEDEX deposits thus reflect early diagenetic processes closely tied to sedimentation and basin evolution.

Special Types of SEDEX Deposits

-Clastic-dominated SEDEX

-Carbonate-hosted SEDEX

-Barite-rich SEDEX

Clastic-dominated SEDEX

– hosted in shale/siltstone (most common type)

Carbonate-hosted SEDEX

transitional with MVT features

Barite-rich SEDEX

– high BaSO₄ (barite) with sulfides

locations of SEDEX Deposits

Australia, Canada, Ireland

Ore Mineral Content of SEDEX Deposits

Sphalerite (ZnS), Galena (PbS), Pyrite (FeS2)

Economic Significance of SEDEX Deposits

SEDEX deposits are major sources of lead and zinc because of their large size and high tonnage. They form in marine sedimentary basins during syngenetic to early diagenetic stages, where hydrothermal fluids deposit metals near the seafloor. Diagenetic processes such as compaction and recrystallization help preserve extensive sulfide layers. Their large scale supports long-term mining operations and provides essential metals for industries, particularly zinc for galvanizing steel. Understanding SEDEX diagenesis helps geologists identify favorable environments and locate economically significant ore deposits.