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Geology and Ore Deposits of the Northern Pennines |
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Geology and Ore Deposits of the Northern Pennines |
Location and Geography
Weardale district of the Northern Pennine Orefield is located in the largely rural northwestern portion of County Durham, and is not far south of Hadrian’s Wall in the north of England. The Wear Valley runs east-west, following the course of the River Wear, beginning near the town of Wolsingham in the east and continuing westward for more than 20 miles. While the area was at one time heavily forested, much of this was cleared centuries ago and the region is now predominantly open moorland, divided by stone walls and the occasional stone cottage. Evidence of past mining activities abounds, and the hills surrounding the valley are covered with numerous long abandoned quarries, pits, and mine dumps, the names and histories of which are, for the most part lost in antiquity.
A view of Weardale looking south from near the Rogerley Mine. A large disused quarry is visible in the middle ground.
Inhabited towns and villages, for the most part, occupy the valley floor, and Stanhope is the center of commercial activity for the valley. A few miles to the north of Stanhope is the picturesque village of Rookhope which was formerly the center of much of the local mining activity. A number of the most productive mines in the Weardale district, including the Boltsburn, Stotsfieldburn, Groverake, and Frazer’s Hush were located at or near Rookhope, but with the closure of these mines, the village is now fairly quiet.
A map of Weardale and surrounding regions showing important specimen and ore-producing mines. Base map courtesy of MultiMap.com.
Geology of the Northern Pennines
The North Pennine Orefield is a fault-bounded block covering an area of approximately 550 square miles. A series of Paleozoic faults which were reactivated during the Tertiary form the southern and northwestern margins of the block, tilting it eastward with a maximum displacement along the fault margins of up to 3000 meters (Sawkins, 1966). The Pennine block can be further divided into northern and southern portions by the east-west trending Stainmore syncline. The northern portion containing the Weardale district is known as the Alston block, while the southern portion is known as the Askrigg block (Dunham, 1990). The ore-bearing deposits of the Pennine block are intruded into a series of lower to upper Carboniferous cyclothemic sediments - sandstones, limestones, and shales - deposited during repeated marine transgressions.
The Carboniferous sedimentary sequence rests unconformably on the Weardale granite which has a K/Ar age of 362 ± 6 m.y. (Dunham, 1990). This granite is nowhere exposed at surface, and its existence was first proposed because of a negative gravity anomaly in the region, and was later confirmed by borehole. Though the Weardale granite is not believed to be genetically related to mineralization in the northern Pennines, its location is coincident with the later fluorite/galena deposits and King (1982) suggests that its presence may have exerted a structural control on the emplacement of ore bodies. Dunham (1990) describes the Weardale Granite as having an anomolously high heat flow, and suggests a mantle source for the heat. Though the Weardale Granite had been intruded, exposed by erosion, and reburied prior to the emplacement of the Northern Pennine Orefield, it is probable that the granite acted as a localizing conduit for the heat driving the mineralizing process. Several igneous dikes and sills have been intruded into the Carboniferous sedimentary sequence, the largest of which is the Great Whin Sill, a quartz-orthopyroxene diabase (dolerite in British usage) of late Carboniferous age.
An idealized stratigraphic column for Carboniferous sediments and intrusives in the Weardale area. With the exception of the Great Whin Sill, which is a dolerite (diabase), units named “Sill” and “Hazel” are, in fact, sandstones, the names of which predate modern stratigraphic nomenclature (illustration by Bill Besse, from King 1982). |
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Ore Deposits
The ore deposits hosted by the Carboniferous sequence are of two types; near vertical veins of hydrothermal origin, and horizontal metasomatic flats. The ore-bearing veins have been intruded as open space fillings along a series of regional fractures which are believed to have been created by regional doming at the end of the Carboniferous (Sawkins, 1966). The main series of fractures trends ENE while a secondary set trends WNW. Ore minerals were preferentially deposited within the more competent stratigraphic units, usually limestones and hard sandstones. In less competent units such as shales the ore-bearing veins usually break up into small, poorly mineralized stringers (Dunham, 1990). While the veins were often rich sources of ore, crystallized mineral specimens found in vein cavities were, with a few exceptions, not of the quality found in the flat cavities (King, 1982).
A map of Weardale and part of East Allendale east of the Burtreeford monocline showing location of major ore-bearing veins, fluorspar mines, villages, and surface outcrop of the Great Limestone (illustration by Bill Besse, adapted from Dunham 1990).
The flats are sheet-like metasomatic mineral deposits which were emplaced along favorable horizons in limestones adjacent to veins. They appear to occur most frequently around the intersection of two or more veins (Dunham, 1990). While flats have been found in nine different limestone units, the majority occur within the Great Limestone, a thick unit which forms the base of the Upper Carboniferous series. From studies made at the Boltsburn Mine, Dunham (1990) further correlates the occurrence of flats in the Great Limestone with regions where the overlying Coal Sill sandstone is thin and replaced by shale. The metasomatic flats are frequently vuggy, and have been the source of many of the best crystallized mineral specimens from the district.
A fluorite-contain vug exposed in the flats of the Rogerley Mine, June 2002. |
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Mineralogy of Ore Deposits
The mineralogy of both the veins and flats in the Weardale region is relatively simple. Galena, fluorite, and quartz are common and widespread, and sphalerite, ankerite, siderite, and calcite may be locally abundant. Other sulphides including pyrite, marcasite, chalcopyrite, and pyrhhotite are occasionally found as well. Galena was the principal ore recovered from the Weardale mines until the late 19th century, and while the silver content of Weardale galena is generally low (averaging 4-8 oz/t per Dunham, 1990), some silver was recovered along with the lead. Local concentrations of sphalerite have been mined for zinc, and where sufficiently concentrated by oxidation processes, deposits of ankerite and siderite have proved economic as ores of iron. Fluorite was not an economic commodity until the advent of modern steel-making processes during the late 19th century created a demand for it as a fluxing agent. Prior to that, fluorite encountered in mining was considered waste (or "deads") and used as backfull or dumped. The rise in demand for fluorite coincided with a declining market for lead, and helped to extend the life of the mining district into the 20th century.
Barite and witherite also occur in economic concentrations in the northern Pennines, but the distribution of Ba-rich deposits is peripheral to the concentrations of fluorite and little, if any has been mined in the Weardale district proper. Dunham (1937) states that there is a very sharp boundary dividing fluorite and barite zones, and the two minerals do not overlap in distribution. Sawkins (1966) has shown that fluorite from the Northern Pennine Orefield formed at higher temperatures than the barite, suggesting that a temperature gradient, along with the mixing of hydrothermal solutions with Ba-rich connate waters present in areas surrounding Weardale resulted in this concentric pattern of mineral deposition.
Based on fluid inclusion studies, Sawkins (1966) determined that, for the most part, fluorite, quartz, galena, and sphalerite from the Weardale area were deposited at temperatures between approximately 200-100 °C. In addition, he determined that the Na/K ratios of the included fluids were low, suggesting that the minerals were deposited from hydrothermal solutions of meteoric rather than connate origin. The low temperature of deposition, presence of meteoric hydrothermal solutions, along with the geographic and temporal relationship of ore deposition to regional doming, and a lack of apparent igneous source indicates that these deposits are genetically similar to the Mississippi Valley-type Pb-Zn-fluorite deposits of the central United States. Moorbath (1962) reports a mean lead isotope age for northern Pennine galena of 280 ± 30 m.y., suggesting that regional mineralizatation occurred during the Permian and may be related to Hercynian orogenic activity (King, 1982).
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This text was adapted from an article by Jesse Fisher and Lindsay Greenbank, which appeared in the January/February 2000 issue of Rocks & Minerals. All photographs by Jesse Fisher unless otherwise noted.
