Give the lithostratigraphic classification and distribution of Delhi Supergroup. Also comment on the economic significance of this supergroup.

Lithostratigraphic Classification of the Delhi Supergroup

The Delhi Supergroup is broadly divided into two major components based on geographic location, age, and degree of metamorphism: the North Delhi Fold Belt (NDFB) and the South Delhi Fold Belt (SDFB). While some suggestions propose the division is arbitrary, the recognized groups within these belts provide a framework for classification.

North Delhi Fold Belt (NDFB)

The NDFB, primarily exposed in areas like the Alwar-Bayana and Khetri basins, is typically divided into three major groups from oldest to youngest:

• Raialo Group (or Rayanhala Group): This basal unit primarily comprises limestones, dolomites, and associated mafic volcanic rocks. It reflects an early rifting stage with shallow marine deposition, including well-rounded conglomerates suggesting a beach-face environment. Carbonate sedimentation is also noted in parts like Dogetha, often metamorphosed into marbles.
• Alwar Group: Overlying the Raialo Group, the Alwar Group is dominantly arenaceous, characterized by thick successions of quartzites interbedded with schists and volcanic flows. These rocks represent a high-energy clastic environment, indicating braided fluvial to tidal deposition. Formations like the Pratapgarh formation with ortho-quartzite and sericite quartzite are characteristic.
• Ajabgarh Group: The youngest group in the NDFB, the Ajabgarh Group, is predominantly calcareous and argillaceous, with significant occurrences of phyllites, mica schists, limestones, and impure marbles. Transition beds between the Alwar and Ajabgarh Groups, such as the Kushalgarh Limestone in the Alwar area, are known for their coarse, crystalline character due to high-grade metamorphism. This group also exhibits phosphorites interspersed with carbonates and volcanics.

South Delhi Fold Belt (SDFB)

The SDFB, with younger geological history compared to NDFB, is exposed in regions like Beawar-Rupnagar-Babra and is often considered to consist of different lithounits:

• Gogunda Group: This group primarily consists of an arenaceous sequence of quartzite, interbedded schist, and metabasic rocks.
• Kumbhalgarh Group: Characterized by pelitic schist, calc-gneiss, and impure marble, suggesting a different depositional setting than the NDFB.
• Sirohi Group: In some classifications, a distinct Sirohi Group is recognized as a lithostratigraphic unit overlying the Delhi Supergroup, with an age dating back to approximately 900-850 Ma.

Distribution of the Delhi Supergroup

The Delhi Supergroup rocks form a prominent NE-SW trending linear belt, often referred to as the Delhi Fold Belt, which is approximately 450 km long. This belt forms the backbone of the Aravalli Mountains and extends across several Indian states:

• Rajasthan: This is the primary state where the Delhi Supergroup is extensively exposed. It stretches from northeastern Rajasthan through central Rajasthan (e.g., Alwar, Khetri, Ajmer, Pali, Udaipur districts) and continues southwestwards. Major exposures are found in the North Delhi Fold Belt (like the Alwar basin, Khetri basin, Lalsot-Bayana basin) and parts of the South Delhi Fold Belt (e.g., Barr-Sendra and Phulad-Deogarh regions).
• Haryana: The Supergroup extends into southern Haryana, where it forms significant geological features.
• Delhi: Isolated hills and ridges composed of Delhi Supergroup rocks are found in Delhi, representing the northeasternmost extension of the Aravalli range.
• Gujarat: The belt continues into the northeastern part of Gujarat in its southwesternmost extension.

The overall outcrop pattern is often described as fan-shaped, being broadest in the north, narrowing in the middle, and flaring out in the south.

Economic Significance of the Delhi Supergroup

The Delhi Supergroup is an important host for a variety of mineral resources, contributing significantly to the economic geology of Rajasthan and surrounding regions. Its rocks have undergone significant metamorphism and deformation, creating favorable conditions for mineral deposition.

The economic significance can be broadly categorized into:

Metallic Minerals:

• Copper: The Delhi Supergroup is famously known for its copper mineralization, particularly in the Alwar and Ajabgarh Groups. The Khetri Copper Belt in Rajasthan is a prime example, renowned for its chalcopyrite, pyrrhotite, and arsenopyrite occurrences. The Thanaghazi formation of the Ajabgarh Group in the Khera North block (Alwar district) also shows significant poly-metallic deposits.
• Lead-Zinc: Minor lead-zinc occurrences are reported, especially in association with carbonate and argillaceous rocks of the Ajabgarh Group. For instance, the Dokan-Dariba copper belt, though primarily copper-focused, also shows lead occurrences.
• Gold: Arsenopyrite, often found in association with copper mineralization within the Delhi Supergroup rocks (e.g., Khera North block), is considered a pathfinder mineral for gold, indicating potential gold mineralization in the area.
• Iron Ore: Localized occurrences of iron ore may be associated with some metamorphosed sedimentary sequences, although not as prominent as other metallic minerals.

Industrial Minerals and Building Materials:

• Limestone and Marble: The limestones and dolomites of the Raialo Group and the marbles/limestones of the Ajabgarh Group are widely quarried. These are extensively used as building stones and for cement manufacturing. The Makrana marble, though primarily from the Aravalli Supergroup, highlights the regional significance of Proterozoic carbonates.
• Quartzite: The thick quartzite successions of the Alwar Group are excellent building materials, widely used for construction and road aggregates due to their hardness and durability.
• Feldspar and Mica: Pegmatites and schists associated with the Delhi Supergroup's metamorphic terrains can host economically viable deposits of feldspar and mica, used in ceramics, glass, and electronics industries.
• Phosphorite: The Ajabgarh Group is known to contain phosphorite deposits, which are crucial for the fertilizer industry.
• Other Industrial Minerals: Localized occurrences of minor industrial minerals, such as talc, soapstone, and asbestos, are also reported from the highly metamorphosed zones.

The Delhi Supergroup's complex geological history, including multiple phases of deformation and magmatic intrusions (e.g., Erinpura granites), has played a critical role in the concentration and remobilization of these economic minerals, making it a highly prospective region for mineral exploration.