Glauconite in New Zealand. New Zealand has greensands on both main islands and Chatham Is. They are restricted to the Upper Cretaceous and early Tertiary.
Glauconite is a mineral for our future. It is a green attractive mineral which in massive accumulations forms rocks called greensands. It is of interest to industry because it contains potassium, the K in the N-P-K of fertilizers and essential for all plant life. It occurs in sedimentary sequences, most commonly in upper Mesozoic and Lower Tertiary times when earth's upheavels slowed and sediment accumulation on seabeds slowed. Under such conditions and in shallow water depths glauconite forms from silica gels and potassium in the sea water, and scavenges iron giving it colour. It is often found infilling and replacing the shells of small organisms called forams and very often takes the form of faecal pellets that are on the sea bed. This gives rise to the oval shapes of most of the glauconite accumulations in the rocks.
Glauconite forms in layered strata that were once the original seabed. Often these beds are made up almost completely of glauconite pellets glued together by more glauconite, clays or calcium carbonate. It can contain up to about 8% potassium in the mineral structure, but typically it is around 5-6%. The overall grade of a glauconite bed depends upon two factors: the amount of glauconite in the sediment and the amount of potassium in the glauconite. If for instance the potassium content of the glauconite is 6%, and the greensand is 80% glauconite, then the rock grade will be 6% x 80% =4.8% potassium. In assays potassium is generally expressed as potash, the oxide, which is K2O. This is 85% potassium metal.
Mining glauconite is simple. It is usually mined in quarries by open cast methods, using excavators and then crushed and ground. If the greensand is lacking in concentration, then the mineral can be separated from the rock by magnetic separation as glauconite contains iron and is weakly magnetic. Glauconite is also stable, and in water does not give up its potassium which is bound with silicon, aluminium, and oxygen. In order for it to be a source of potash for industry, the potassium must be removed. This is done by breaking down the glauconite structure by either heat or acid or alkali attack.
Pre 1925 most potassium came from either evaporite deposits or glauconite. The main use was for fertilizer so the mining and treatment needed to be cheap. In the US the common method was acid attack, but once large available sources of evaporite (beds left behind after seas evaporate) were found, glauconite mining and K production stopped. In the 1980's India put a lot of money and energy into glauconite mining methods as they had few indigenous potassium deposits. Their work failed as the price of potash stayed low making Indian farmers reluctant to look for alternatives. The same can be said for Brazil who at the same time were looking into thermal treatment to produce direct spread fertilizers.
Since 2000, the price has risen, steeply in 2007-2008, falling in 2009 with the world recession. However the price stayed above past levels. This prompted reignition of effort in Brazil. Here a metamorphosed glauconite (enriched) was re-examined; In 2008-2010 the resource was outlined and concurrently work was commissioned on beneficiation methods. A patented process emerged and the operation is scheduled for production in 2013. In 2008 A West Australian company Potash West began evaluating the West Australian glauconite belt. They raised $6MA to test the areas and develop methods of extraction. This appears to have been successful.
Greensands occur in Northland, East coast of the north island down the Wairarapa into Marlborough, Canterbury and Otago. Unconsolidated greensands are on the Chatham Rise where glauconite acts as the matrix cement for phosphate nodules which litter the seabed. These are currently under investigation from Widespread Energy. Glauconite NZ began exploration in the East Coast North Island because of its proximity to farming activity. This work was supported by TechNZ early on, and led to the discovery of interesting deposits near Gisborne in 2009. The company is currently exploring the potential of several deposits.
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