ORE DEPOSITS FORMED BY SURFACE PROCESSES
Surface (surficial) processes are the physical and chemical phenomena, which cause concentration of ore material within the regolith, generally by the action of the environment. Certain conditions required for the concentration of these ore deposits may include:
- Vigorous deep chemical weathering
- Lack of subsequent erosion
- Gentle topography (well drained but not steep)
IRON ORE IN ORE DEPOSITS
Iron ore deposits are known to occur in sedimentary, hydrothermal and magmatic environment, but production today is almost entirely from three types of deposit: Deposits related to Precambrian Banded Iron formation (BIF), that provide about 90% of all iron ore mined and the remainder is derived from metasomatic skarn and magnetite deposits.
Banded Iron Formation IN ORE DEPOSITS
Banded iron formation (BIF) are finely bedded chemical sedimentary rocks composed of interlaminated quartz (chert) and iron-bearing minerals with an iron content of about 30 weight percent.
The Banded Iron formations often called the Precambrian Banded Iron Stone formation because there are of the Precambrian Age are known on all the continents.
They contain the largest and richest iron deposits in the world. Notable examples include the Animike series of the Lake Superior region in North America, the Hammmersley basin of Western Australia, the Transvaal basin of South Africa e.t.c. (Beukes and Klein 1992).
The Precambrian banded ironstone are remarkably similar wherever they occur in the world. They all formed between 1900-2500 million years ago. This is shortly before the arrival of algae changes the atmosphere of the earth from reducing to oxic conditions. The deposits are found in laterally extensive sequences with the following succession from base to top: dolomite, quartzite, red shale, black ferruginous shale, banded iron formation and black shale.
Volcanic always occur somewhere in the sequence. The ore bodies themselves are sometimes several hundred meters thick and consist of rhythmically interbedded chert and iron ore.
There are two theories to explain their formation; the clastic – sedimentary and volcanogenic sedimentary. The clastic-sedimentary theory advocates that the iron was transported into the basic from a deeply weathered lateritic hinterland.
Deposition was then by normal sedimentation in a shallow aqueous environment. The rhythmic nature of the deposition may be seasonal or in response to longer alternation of oxidizing and reducing conditions.
The volcanogenic – sedimentary theory believes that the source for the iron and silica was the volcanic rocks that are commonly associated with the deposits. The first theory seems to explain the lake superior type of banded iron ore, which is developed across extensive continental shelves.
The volvanogenic theory seems more appropriate for the Algoma type of banded iron ore that occurs in subductive zones with extensive igneous activity and has generally undergone subsequent metamorphism.