(a) Describe the formation of Banded Iron Formation (BIF) during Precambrian metallogenic epoch. Write a note on the Indian BIF…

(a) Formation of Banded Iron Formation and Indian BIF deposits

A Banded Iron Formation (BIF) is a chemically precipitated sediment of alternating iron-rich bands (magnetite, haematite, siderite) and silica bands (chert, jasper), each a few millimetres to centimetres thick, with 25–40% Fe. BIFs formed mainly between about 3.8 and 1.8 Ga, peaking at 2.7–2.4 Ga, with a minor return at about 0.7 Ga in glacial (Snowball Earth) times.

Formation.

(i) Source of iron. The deep ocean was anoxic and held dissolved Fe²⁺, supplied by hydrothermal vents at mid-ocean ridges.

(ii) Oxidation. Fe²⁺ rising to shallow oxygenated waters, made by cyanobacterial photosynthesis, was oxidised to Fe³⁺ and precipitated as ferric hydroxide. Photoferrotrophic bacteria, which oxidise Fe²⁺ by anoxygenic photosynthesis, and photo-oxidation by ultraviolet light are other proposed routes.

(iii) Banding. Silica was precipitated from silica-saturated seawater as a gel. The alternation of iron and silica bands may reflect seasonal or cyclic changes in upwelling, productivity or volcanic input. Diagenesis turned ferric hydroxide to haematite and magnetite.

(iv) End of BIF. After 1.8 Ga deep water became oxidised or sulphidic, so iron was no longer transported.

Types. Algoma type is small and lenticular, associated with volcanic rocks of greenstone belts. Superior type is large and laterally extensive, on stable shelves.

Indian BIF.

(i) Singhbhum–Odisha Iron Ore Group (Noamundi, Joda, Gorumahisani) has the most valuable haematite ores, formed by the upgrading of BIF by supergene and hydrothermal enrichment.

(ii) Bailadila in Bastar, Chhattisgarh, has high-grade haematite.

(iii) Dharwar craton: Bababudan, Kudremukh (magnetite quartzite) and Sandur–Hospet (Bellary) belts.

(iv) Goa and parts of Rajasthan.

The richest ore (above 62% Fe) in India is haematite from upgraded BIF, used for steel and export.

(b) Late magmatic ore-forming processes and their field characters

Late magmatic ores form from the residual melt of a crystallising mafic or ultramafic magma, after much of the silicate has crystallised, unlike early magmatic ores, which are the first crystals.

Processes.

(i) Residual liquid segregation and accumulation. Fe, Ti, V, P and volatiles are concentrated in the remaining liquid. It fills the space between silicate grains, giving net-textured or interstitial ore, such as chromite and titaniferous magnetite.

(ii) Filter-pressing and injection. The dense ore-rich liquid is squeezed from the crystal mush into fractures in the solidified host or in the country rock, forming dykes and veins.

(iii) Liquid immiscibility. Immiscible oxide or sulphide melt separates and settles or is injected.

Field characters.

(i) Ore bodies are lenses, pods and irregular dykes, cutting earlier rock, unlike the conformable layers of early ores.

(ii) Sharp, sometimes brecciated contacts, with inclusions of the host.

(iii) Ore minerals fill the spaces between earlier silicates, and have no chilled margin.

(iv) They occur within or on the margin of mafic and ultramafic bodies.

Indian examples include the vanadiferous titaniferous magnetite of Mayurbhanj, Odisha, and the chromite of Sukinda, Odisha, and abroad the Bushveld Complex of South Africa.

(c) Origin of porphyry copper deposits and an Indian example

Porphyry copper deposits are large, low-grade (0.3–1.5% Cu) bodies of disseminated and stockwork veinlet sulphides in and around a shallow porphyritic intrusion, mostly in subduction-related volcanic arcs. They are mined in bulk, often with Mo and Au.

Origin.

(i) Hydrous, oxidised calc-alkaline magma, made in the mantle wedge above a subducting plate, rises to 1–4 km depth and is stored in a cupola.

(ii) On crystallisation, it becomes saturated in water and releases a metal-rich, saline magmatic–hydrothermal fluid (second boiling). Cu, Mo, Au and S are carried by it.

(iii) The fluid pressure fractures the rock into a stockwork, and ore minerals deposit on cooling and mixing with meteoric water.

(iv) Alteration zoning outwards: potassic (K-feldspar, biotite, with ore), phyllic (quartz–sericite–pyrite), argillic and propylitic (chlorite–epidote).

Supergene enrichment can add a chalcocite blanket above.

Indian example: Malanjkhand, Madhya Pradesh. It is India's largest copper deposit, with about 1–1.5% Cu, in the Central Indian Tectonic Zone. The host is the Malanjkhand granitoid (Palaeoproterozoic, about 2.5 Ga), and the ore is chalcopyrite with pyrite, magnetite and molybdenite in quartz veins and stockwork around an arcuate quartz reef, with K-feldspar and sericite alteration. Its porphyry-type origin is supported by the alteration and by Re–Os dating.