Chromite deposits form by a process of concentration by crystallization in magmas. But not just any magmas. Chromites are almost exclusively restricted to ultramafics, magmas rich in magnesium and iron. Chromite deposits occur in two varieties known as stratiform and podiform. As the terms imply, stratiform means layers of concentrated chromite within the ultramafic rocks, while podiform chromite occurs as lenses or pods of concentrated chromite within the ultramafics. The Sukinda chromites are stratiform. During the late Archean- early Proterozoic period around 2.5 to 2 billion years ago geologists estimate, massive amounts of ultramafic magmas originating in the mantle were injected into the crust in the area that is now Orissa. But why is chromium so concentrated in ultramafic magmas? Chromium is a refractory metal i.e. a metal with a melting point higher than iron and cobalt. It takes a large amount of melting in the mantle to release significant amounts of chromium in to the liquid phase. But large amounts of melting also make the magmas rich in magnesium and iron. So the connection of chromium and chromite deposits with ultramafic rocks.
Stratiform and podiform chromite deposits occur in distinctive tectonic settings. Stratiform deposits occur mostly within ultramafic intrusions in stable continental crust, while most podiform deposits originated in oceanic settings, either within ultramafic rocks associated with mid-oceanic spreading centres or in subduction zone associated back-arc spreading centres. Later, plate tectonic movements have obducted or thrust up these chromite bearing ultramafics to form sections of spectacular mountain belts. The best example in India are the podiform chromites in Ladakh (Karmalkar N.R. et al. 2000), which originated in the oceanic crust between the Indian and Asian continents in the Mesozoic period, and were subsequently thrust up to form the early Himalayan mountains as the Indian plate collided with the Asian plate.
Chromite deposits show an interesting age distribution indicative of the thermal and tectonic evolution of the earth.
Image Source : Stowe (1994). Stratiform deposits occur almost exclusively in the late Archean-early Proterozoic period (2.8 - 1.8 billion years ago), while podiform deposits occur almost exclusively in the Mesozoic and Tertiary periods beginning around 200 million years ago (there are some podiform chromites in late Proterozoic orogenic belts around 800 million years old). Anyone familiar with the geological time scale with readily appreciate that biological evolution has imparted a temporal uniqueness to the sedimentary rock record, enabling geologists to subdivide geologic history into different periods. What is less appreciated is that the thermal and tectonic evolution of the earth has also produced a distinctive rock record (a good idea for a post series). Chromite deposits are one good example. The mid late Archean-early Proterozoic was a period of intense continental crust building. Owing to the high geothermal gradients in the Archean (the interior of the earth was hotter then), there were periods of copious amounts of melting in the upper mantle and lower parts of earlier formed crust. So, were formed large ultramafic complexes and associated chromites. Over time, the earth cooled and large scale melting of the mantle became localized to plate boundaries. The breakup of supercontinent Pangea beginning in the early Mesozoic led to significant worldwide development of subduction zones and associated back-arc spreading centres and the origin of ultramafic hosted chromite. This activity culminated in intense amounts of mountain building activity in the mid-Mesozoic and early Cenozoic. So, podiform chromites occur mainly within tectonically emplaced slices of oceanic rock sequences in this age group. The gap between around 800 million years to 200 million years is one of geology's unresolved problems.Chromium from Sukinda mines is present as air-borne dust and is also being leached into the groundwater and nearby streams in the form of hexavalent chromium (+6 oxidation state).
In image, the Sukinda syncline is clearly seen. In the core of the syncline the arrow points to the open pit Sukinda chromite mines. The rust color is due to oxidation of the ore. When ingested either through air-borne dust or through water, hexavalent chromium is reduced in our cells to pentavalent and trivalent chromium. This leads to a variety of health problems, including increased risk of cancer. An estimated two hundred and seventy thousand people around Sukinda are at risk or are suffering from chromium related poisoning. The Orissa Pollution Control Board have pleaded impotence, saying “It is unique, it is gigantic and it is beyond the means and purview of the (Orissa Pollution Control) Board to solve the problem,”. In effect they have questioned the rationale for their own existence. The board chairman L.N. Patnaik has predictably rubbished the report, saying that any mining activity will lead to some air pollution, apparently forgetting that chromium is getting into the water supply in massive quantities.
Hexavalent chromium gained recognition because of the movie Erin Brockovic, in which the character played by Julia Roberts campaigned successfully against a polluting industry. In India, maybe this lady on the left can help :-)References:
Karmalkar N.R., A. G. Dessai and R. A. Duraiswami: Alteration of Chromite from the Dunites of Indus Ophiolite Belt, Ladakh 27-34, Himalaya, India, Gondwana Geological Magazine; V. 15 (1)- June 2000
Stowe, C. W. 1994. Compositions and tectonic settings of chromite deposits through time. Econ. Geol. 89:528 – 546
it represents a clear picture about chromite emplacements in India and also raise a question about environmental degradation..... helpful especially for a geologist.
ReplyDeleteI had my Msc 1st industrial training in sukinda , only I know how I survived the searing heat, the red dust , the characteristics smell of ore ,& rice
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