SEDEX Mineral Deposits

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Sedimentary Exhalative (SEDEX) deposits are a type of sediment-hosted ore deposits that are rich sources of base metals such as zinc, lead, and sometimes silver. These deposits form through the discharge of metal-bearing hydrothermal fluids into a water body, typically an ocean or a sea, leading to the precipitation of sulphide minerals. 

Representational picture of a geologist extracting minerals from Himalayas, India
(Generated using DALLE)

Genesis of SEDEX Deposits

The formation of SEDEX deposits is closely linked to the circulation of hydrothermal fluids through sedimentary basins. These fluids, which are often derived from the dehydration of oceanic crust or the leaching of metals from volcanic and sedimentary rocks, become heated and metal-enriched. When these hydrothermal fluids reach the seafloor or the bottom of a sedimentary basin, they mix with seawater, causing the metals to precipitate as sulfide minerals due to changes in temperature, pH, and redox conditions (Large, R.R., et al., 2005). The process is often associated with rift basins or back-arc basins, where extensional tectonics facilitate the circulation of fluids.

Classification of SEDEX Deposits

Two main types are recognized:

Syngenetic SEDEX Deposits: These are formed contemporaneously with the surrounding sedimentary rocks. They are typically found in rift-related basins and are characterized by laminated ore bodies that reflect the depositional environment.

Epigenetic SEDEX Deposits: These form after the sedimentation of the host rocks and are often associated with fault or fracture zones that serve as conduits for the hydrothermal fluids. The ore bodies may cross-cut bedding, indicating a post-depositional emplacement.

Figure showing the similarities between the SEDEX and VMS deposits.

Indian Examples:

India hosts several SEDEX deposits, with the most notable examples being found in the Rajasthan region:

Rampura Agucha Mine: Located in Rajasthan, it is one of the largest zinc mines in the world. The deposit is hosted within Precambrian metasedimentary rocks and is characterized by a high-grade zinc and lead ore body (Deb, M., & Goodfellow, W.D., 2004).

Zawar Mining Complex: Also in Rajasthan, this complex is one of the oldest mining areas in the world, with a history of mining going back over 1,000 years. The Zawar deposits are rich in zinc and lead and are hosted in Precambrian dolomites.

Global Examples

Globally, SEDEX deposits are found in various geological settings, including:

Red Dog Mine, Alaska, USA: One of the world's largest zinc mines, the Red Dog Mine's deposits are of the Mississippian age and are hosted in carbonate rocks. The deposit is notable for its high-grade zinc, lead, and silver ores (Kelley, K.D., et al., 2004).

Mount Isa, Queensland, Australia: This mining complex contains one of the most significant SEDEX deposits in the world, with substantial quantities of zinc, lead, and silver. The deposits are hosted in Proterozoic sedimentary rocks and have been mined for several decades (Perkins, W.G., 1995).

Sullivan Mine, British Columbia, Canada: This was one of the largest SEDEX deposits ever mined, with over 100 years of operation before its closure in 2001. The deposit was hosted in Purcell Supergroup sediments and was rich in lead, zinc, and silver (Lydon, J.W., 1983).

Figure showing the differences between SEDEX and VMS Deposits.

Classification based on the tectonic setting, age, and the dominant metal present in SEDEX deposits:

1. Tectonic Setting

SEDEX deposits are primarily associated with specific tectonic settings that influence their formation and the characteristics of the mineralization:

Intracratonic Rift Basins: These deposits form in rift settings within continental crust, where extensional tectonics create basins that are favourable for the accumulation of sediments and the circulation of hydrothermal fluids. Examples include the Red Dog deposit in Alaska, USA.

Continental Margin Basins: These are formed at the edges of continental plates where sedimentation occurs in deep marine environments adjacent to continental shelves. The tectonic activity associated with these settings can promote the formation of SEDEX deposits.

Back-Arc Basins: Located behind volcanic arc systems, these basins are formed by the extensional tectonics associated with subduction zones. The hydrothermal activity in these settings can lead to the formation of SEDEX deposits, often with complex mineralogy.

2. Geological Age

SEDEX deposits span a wide range of geological ages, reflecting the long-term tectonic and sedimentary processes that have occurred throughout Earth's history:

Proterozoic: Many of the world's largest SEDEX deposits, including those in the Mount Isa region of Australia, are of Proterozoic age. These deposits are often found in stable cratonic settings or on the margins of ancient continental plates.

Phanerozoic: SEDEX deposits of Phanerozoic age are less common but can be found in various settings, including the Cretaceous deposits in the Selwyn Basin of Canada. These younger deposits often reflect the complex tectonic environments associated with active plate margins.

3. Dominant Metal

The classification based on dominant metals is crucial for economic evaluation and exploration strategies:

Zinc-Lead Dominated: The majority of SEDEX deposits are zinc-lead dominated, with these metals occurring in varying ratios but typically with zinc being more abundant. The Red Dog Mine in Alaska and the Mount Isa deposits in Australia are prime examples.

Zinc-Lead-Silver: Some SEDEX deposits also contain significant amounts of silver, either as a byproduct or as a significant economic component. The Sullivan deposit in British Columbia, Canada, is a notable example, where silver adds considerably to the deposit's value.

Barite: While not a metal, barite (barium sulfate) is a significant component of some SEDEX deposits, either associated with base metal sulfides or as the primary mineralization. Deposits with significant barite content are important for the chemical and petroleum industries.

Note:

Dear learner, kindly note that the classification of SEDEX deposits into these categories is not always straightforward, as individual deposits can exhibit characteristics that span multiple classes, reflecting the complex nature of their formation. Understanding the tectonic setting, age, and dominant metals of SEDEX deposits is crucial for geologists and mining companies in the exploration and development of these economically significant resources. This classification aids in the predictive modelling of deposit locations, the understanding of ore-forming processes, and the assessment of potential environmental impacts of mining activities.

References

1. Deb, M., & Goodfellow, W.D. (2004). Sediment-hosted lead-zinc sulphide deposits: attributes and models of some major deposits in India, Australia and Canada. Indian Journal of Geology, 76(3), 233-254.
2. Kelley, K.D., et al. (2004). The giant Red Dog SEDEX zinc-lead-silver deposit, western Brooks Range, Alaska. Economic Geology, 99(3), 421-450.
3. Large, R.R., et al. (2005). Secular distribution of highly metal-enriched black shales corresponds with peaks in past atmosphere oxygenation. Mineralium Deposita, 40(5), 526-543.
4. Lydon, J.W. (1983). Geological features of some massive sulfide deposits of the Canadian Appalachians. Canadian Institute of Mining and Metallurgy Bulletin, 76, 66-78.
5. Perkins, W.G. (1995). Mount Isa Silica Dolomite and Copper Orebodies; the Early Proterozoic Mount Isa Sequence, Northwest Queensland, Australia. Economic Geology, 90(6), 1602-1624.

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