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The Alston block is an area of geomorphologically high ground in the North Pennines. It is an area of surrounded by two troughs, the Stainmore and Northumberland, the boundaries being the Stublick and Lunedale faults. Essentially the area is a horst Link
In the early Devonian period (around 399ma) a series of granite plutons were emplaced, now collectively known as the Weardale batholith. As granite is less dense it provides sufficient buoyancy to maintain this piece of the upper crust as an area of raised relief. The area has gone through several extensional and compressional stages in the Carboniferous due to the formation of Pangea. This created many normal faults; later on these would provide the right environment for mineral development. Basement lower Palaeozoic volcanics and sediments were covered in Dinantian limestones and coal measures.
The area is also known as the North Pennine Orefield due to the large amounts of economic minerals present. Over 4 million tonnes (mts) of lead, 0.3mts of zinc, 2.1mts of flourite, 1.5mts of barytes and 1mts of Witherite as well 1.7mts of iron ore have been extracted.
Mineralisation occurred in three stages; first was the dolomitisation and ankerisation of the limestones, this created pore spaces. Secondly rain and sea water (from the Zechstein sea) filters down through the rocks, the sea water which has a high salt content picks up minerals and heat from the granite and shales. This created most of the veins of lead/zinc etc. Thirdly a barium deposition stage was at lower temperatures. The veins are usually large in size, many are over 3 meters long and many miles in length. Some show brecchiation, the minerals have filled in the fault lines. An interesting feature is that barium and the fluorite are not usually found together like in the South Pennine Orefield due to the different stages of mineralisation. Around 20% of the mineralisation is strata bound replacement of the host rock, like the siderite at Slitt vein (West Rigg opencut).

The minerals have been extensively mined and extracted, many of these places can be visited, both over and underground.
The source of this description was a BGS paper "Strata bound Pb–Zn–Ba–F mineralisation in the Alston Block of the North Pennine Orefield (England) — origins
and emplacement" by J E Bouch1, J Naden, T J Shepherd, B Young, A J Benham,J A McKervey and H J Sloane.

Created: Tue, 2 Aug 2011, Updated: Mon, 28 Nov 2011