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MINERALISATION IN DETAIL

The mineralisation of Alderley Edge has been a matter of speculation for centuries.  We do not know what ideas the Bronze Age or Roman miners had about the deposition and disposition of minerals but it is fair to assume that they would recognise the link between minerals and faults, even if they had no idea what a fault was.  Certainly the mediaeval miners would have recognised that there were veins at Alderley, resembling the veins in Derbyshire. In the nineteenth century, geology was developing as a science and geologists were drawing up ideas about the deposition at Alderley. Early theories, based on the malachite deposits being worked at the time, said that the ores had been deposited "syngenetically", in other words at the same time as the sandstone.  This theory was based on the way in which ore was found and extracted in "beds".  However, by the twentieth century, most geologists accepted that the ore was laid down in the faults, after the sandstone was deposited.  This theory, referred to as the "epigenetic" method, was much closer to the truth as we understand it now.

The current belief is detailed in a paper published in 1989 by Helen Naylor and others (see references).

In brief, the present idea is that the deposits are not hydrothermal (i.e. from a magma source) as the chemistry does not match that from hydrothermal sources.  The work by Naylor et al looked at sulphur isotopes and inclusions in the rock and deduced that the mineral had been deposited from cool solutions.  The process by which the ores reached Alderley as malachite and azurite dispersions is considered in four stages: (a) origin, (b) transport, (c) precipitation and (d) redeposition.  This process is illustrated diagrammatically on this page in the table below.

ORIGIN: The source of the copper, lead, zinc, barium and iron found at Alderley Edge is believed to be the black shales of the carboniferous rocks underlying the Cheshire Basin.  These would contain traces of the metals referred to above.  

TRANSPORT: Fluids from the Triassic beds are responsible for dissolving the metal ions from the shales and transporting them through the Cheshire Basin.  Such fluids would contain chloride and sulphate ions capable of making lead, copper, barium etc. soluble.  The chlorine and sulphur source is the evaporite (salt and gypsum) beds in Cheshire.

PRECIPITATION: In order for the mineral veins to form at Alderley, the solutions need firstly to be trapped and then to form insoluble sulphide compounds.  The entrapment would arise from the presence of faults and clay bands in the same manner as an oil-field or gas-field trap.  In the absence of organic matter, and by simple mixing and cooling, the barium sulphate would precipitate as barite.  It is believed that this event occurred separately from the other metal deposition explaining why barite veins are far more common.  With organic material present, the sulphate ions in the transport solutions could be reduced to sulphide leaving minerals such as galena, bornite and chalcopyrite.  Where did the organic materials come from?  The most likely source is methane from the subjacent coal measures; this is borne out by the discovery of methane in similar deposits elsewhere in the world.  The product of the reduction of the sulphates would be oxidation of the methane into weak organic acids which would be capable of oxidising adjacent iron rich beds, hence the decoloration of the sandstone in the proximity of veins.  A picture of the vein at Engine Vein is included on this page.

REDEPOSITION: The fourth stage to the process is the redeposition of copper and (to a smaller extent) lead as carbonate ores in the sandstone adjacent to the faults.  This process would arise from invasion of the faults by weak carbonic acid from the land surface; carbonic acid is formed from rainfall onto organic matter on the surface.  The location of these deposits is controlled by the porosity of the sandstone and the barriers formed by clay bands.

The illustrations below are reduced for speed of loading.  Please click on the images to see a full screen version and use your back button to return to this page.

Diagram of possible mineralisation routes to Alderley Edge

Summary diagram to show possible origins of mineralisation around the margins of the Cheshire Basin.  Potential sources of metals and sulphur exist in the underlying Carboniferous and in the basin itself where diagenesis of the Lower Triassic and Upper Permian sandstones may have resulted in the release of trace metals into solution. The traps around the basin margins are topographically higher than the rich Cl? and SO4? ? source in the form of the evaporites due to basin inversion during the Tertiary. (Source: Naylor et al)

Diagram of mineral formation paths

Flow diagram showing the possible fluid migration pathways in the Cheshire Basin. (Source: redrawn from Naylor et al)

Mineral vein in Engine Vein

The vein in Engine Vein. View looking east along the vein (dipping to the NE) which here comprises largely pink/white barite brecciated and infilled by fine grained galena and some chalcocite (both grey to black).  Malachite (green) is abundant on the margins of the vein especially as impregnations along the marly layers. The slickensides on the plane of the vein (which infills a NW-SE trending fault known as the Engine Vein Fault) suggest predominantly dip slip movement. The suggested paragenesis is barite then galena. (Source: Marcus J Tomkinson)

CREDITS:

I have drawn on the article by Naylor et al for this page.  I was prompted to write it by Marcus J Tomkinson on a visit by him to the Edge and the photograph and its caption were kindly provided by him.  I would appreciate any comments on the text of this page which is not my strong subject!

REFS:

Naylor H, Turner P, Vaughan D J, Boyce A J & Fallick A E, "Genetic studies of red bed mineralization in the Triassic of the Cheshire Basin, northwest England", Journal of the Geological Society, London, Vol. 146, 1989, pp 685-699

Rowe J. & Burley S.D., "Faulting and porosity modification in the Sherwood sandstone at Alderley Edge ?", Geol. Soc., Publn. No 124, 1997,  pp 325-352


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