Editing Northern Pennine Orefield: Weardale and Nenthead - an excursion

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== Geological background ==
 
== Geological background ==
  
The Carboniferous rocks of the Northern Pennines are cut by numerous '''mineral veins'''. These typically occupy normal '''faults''' of small vertical '''throw'''. A characteristic feature of the veins is their relationship to the wall-rocks. In hard rocks such as limestone and many sandstones the fault fissures are typically wide and nearly vertical; where they cut soft beds such as shale they are very narrow and inclined. Veins are therefore commonly wide and productive in hard beds and are usually barren in soft beds. Many veins are little overt m wide but in places widths of over 1 o m have been recorded. Adjacent to some veins the chemically reactive mineralizing fluids have altered the wall-rocks, particularly limestones, giving rise to wide horizontal replacement deposits commonly rich in ore and known locally as ''''flats''''.
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The Carboniferous rocks of the Northern Pennines are cut by numerous mineral veins. These typically occupy normal faults of small vertical throw. A characteristic feature of the veins is their relationship to the wall-rocks. In hard rocks such as limestone and many sandstones the fault fissures are typically wide and nearly vertical; where they cut soft beds such as shale they are very narrow and inclined. Veins are therefore commonly wide and productive in hard beds and are usually barren in soft beds. Many veins are little overt m wide but in places widths of over 1 o m have been recorded. Adjacent to some veins the chemically reactive mineralizing fluids have altered the wall-rocks, particularly limestones, giving rise to wide horizontal replacement deposits commonly rich in ore and known locally as 'flats'.
  
A marked zonation of the constituent minerals within the deposits, especially the non-metalliferous or '''gangue''' minerals, is a striking feature of this orefield ([[:File:YGS_NORTROCK_FIG_14_1.jpg|Figure 14.1]]). Deposits in the central zone carry abundant '''fluorite''', commonly with '''quartz'''. '''Chalcopyrite''' is locally common near the central parts of this zone. '''Galena''' is most abundant towards the outer parts of the zone, where in places, workable concentrations of ''sphalerite'' are also present. Surrounding the fluorite zone is a wider zone of deposits in which barium minerals including '''baryte''' and '''witherite''' are the characteristic gangue. Galena and other sulphide values are commonly low in this zone.
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A marked zonation of the constituent minerals within the deposits, especially the non-metalliferous or gangue minerals, is a striking feature of this orefield ([[:File:YGS_NORTROCK_FIG_14_1.jpg|Figure 14.1]]). Deposits in the central zone carry abundant fluorite, commonly with quartz. Chalcopyrite is locally common near the central parts of this zone. Galena is most abundant towards the outer parts of the zone, where in places, workable concentrations of sphalerite are also present. Surrounding the fluorite zone is a wider zone of deposits in which barium minerals including baryte and witherite are the characteristic gangue. Galena and other sulphide values are commonly low in this zone.
  
The mineral zonation is interpreted as reflecting progressively lower temperatures of crystallization from mineralizing fluids as they flowed outwards from central 'emanative centres' above high spots (cupolas) on the underlying Weardale '''Granite'''. The granite may have contributed some of the elements to the deposits but its principal role in their formation is likely to have been as a heat source.
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The mineral zonation is interpreted as reflecting progressively lower temperatures of crystallization from mineralizing fluids as they flowed outwards from central 'emanative centres' above high spots (cupolas) on the underlying Weardale Granite. The granite may have contributed some of the elements to the deposits but its principal role in their formation is likely to have been as a heat source.
  
 
== Excursion details ==
 
== Excursion details ==

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