Location and Geography

The mineralized deposits of the North Pennines orefield are located primarily in the Weardale and Teesdale regions of County Durham, extending westward into the Alston Moor and Escarpment regions of Cumbria, northward into the Allendale and Hexham regions of Northumberland and southward into portions of North Yorkshire. The region is mostly rural and was at one time heavily forested. Much of this was cleared centuries ago and the landscape is now predominantly open moorland, divided by stone walls and the occasional stone cottage. During the 18th and 19th centuries Weardale was the center of a large mining and quarrying industry in the North Pennines, and evidence of past mining activities may still seen in the numerous quarries, pits, and mine dumps, mostly long abandoned, that cover the hills surrounding the valley.

A view of Weardale looking southward from the hillside north of Eastgate. The large quarry created on the Great Limestone by the Blue Circle Cement Works 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

Structurally, the North Pennine Orefield is made up of two fault-bounded crustal blocks: the Alston Block to the north, in the counties of Durham, Cumbria and Northumberland, and the Askrigg Block to the south, in North Yorkshire. These blocks represent areas of crustal uplift and are adjacent to areas of crustal subsidence; the Northumberland Basin to the north, the Stainmore Trough between the two blocks, and the Craven Basin to the south. The rocks making up the two blocks are, for the most part, a cyclothemic sequence of sediments (limestones, sandstones, shales, and the occasional coal seam) deposited during repeated marine transgressions during the lower and middle Carboniferous Period. These were lain down over an earlier Paleozoic assemblage of slates and granitic batholiths.

The Carboniferous sedimentary sequence that hosts the ore-bearing deposits rests unconformably on the Weardale Granite which has a K/Ar age of 362 ± 6 m.y. (Dunham 1990). Nowhere is this granite exposed at the surface, and its existence was first proposed because of a negative gravity anomaly in the region. Its presence was later confirmed by a borehole near the site of the Boltsburn mine in the village of Rookhope. Though the Weardale Granite had been intruded, exposed by erosion, and reburied prior to deposition of the ore-hosting Carboniferous sediments, the fact that its location is coincident with the later fluorite/galena deposits suggests that its presence may have exerted a structural control on the emplacement of ore bodies (King 1982). Dunham (1990) describes the Weardale Granite as having an anomalously high heat flow, and it is hypothesized that this high heat flow drove a convecting system of hydrothermal fluids that leached the ore-forming elements, including F, from the granite and surrounding sediments and deposited them in concentric fashion as the waters rose, spread laterally, and cooled. Mineralization in the Weardale fluorite zone has been shown to correspond with the highest temperatures of deposition (around 180 OC) in the region (Sawkins 1966), and it is from mines in this zone that the fluorite specimens the area is known for have come. As mineralizing fluids spread laterally the temperature drop appears to have resulted in a fairly abrupt change from fluorite to barite (and to a lesser degree witherite) deposition.

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 “Hazel” are, in fact, sandstones, the names of which predate modern stratigraphic nomenclature. “Sill” refers to coal bearing units, aside from the Whin Sill, which is a dolorite (diabase) intrusive. Illustration by Bill Besse, adapted from King 1982.

Ore Deposits

The ore deposits of the North Pennines Orefield are classified as a fluorine-enriched variant of the Mississippi Valley-Type (MVT) deposits. In particular, they bear a close resemblance to the fluorite-bearing deposits of Southern Illinois (Fisher, et al, 2013). These deposits are typically eipgenetic, carbonate-hosted deposits, which have been emplaced at relatively low temperatures. Mineralization in the North Pennines orefield is of two types: (1) near vertical fracture-filling veins of hydrothermal origin and (2) horizontal, stratiform metasomatic replacement deposits locally referred to as “flats.” Regional doming during the late Paleozoic created a wide-spread network of fractures through the Alston Block, which subsequently allowed mineralizing hydrothermal solutions thought to be driven by high heat flow from the buried Weardale granite to invade the region. The main series of fractures trends east-northeast while a secondary set trends west-northwest. Vertically, mineralization occurs preferentially within the more competent stratigraphic units, usually limestones and hard sandstones, forming ribbon-like deposits. In less competent units such as shales, the ore-bearing veins usually break up into small, poorly mineralized stringers.

Map of North Pennines orefield showing mineralized veins, mineral zonation, and location of major specimen-producing mines. Of note is the well-defined boundary between the inner fluorite-rich zone centered on Weardale and the surrounding barium-rich zone. Adapted from Dunham, 1990.

Though less common than vein deposits, mineralized flats are an important source of crystallized mineral specimens because of their cavity-rich nature. Although flats have been found in nine different limestone units within the Alston Block, the majority of them occur within the Great Limestone, a thick unit that forms the base of the Upper Carboniferous series. These can be further divided into three separate levels within the Great Limestone - the Low, Middle, and High Flats horizons, of which the High flats is usually the best developed. Because the metasomatic replacement of limestone by the hydrothermal minerals results in a net loss of volume, cavities are common in the flats and are the source of much of the well-formed mineral specimens from the region.

Cross-section of the Boltsburn mine perpendicular to the Boltsburn East vein, showing distribution of the flats. The Boltsburn flats were the most highly developed of any encountered in the North Pennines, with mineralization frequently extending between all three mineralized horizons within the Great Limestone. The “Tumbler Beds” at the top of the Great Limestone were so-named by local miners because the limestone here is usually fractured into large blocks that, if left unsupported will often tumble down into mine workings developed along the High Flats horizon. Adapted from Symes and Young, 2008.

The distribution of various minerals within the orefield is strongly zoned, particularly within the Alston Block. The main fluorite-bearing deposits are centered about Weardale, Alston Moor and Allendale, with the barium-containing ores segregated to the north, south, and west. While galena is common through the orefield, Dunham (1937) observed that there is a very sharp boundary dividing fluorite and barite zones, and the two minerals rarely overlap in distribution. Mineral zonation is less well-defined within the Askrigg Block. As in the Alston Block, galena is the primary ore mineral, with lesser amounts of fluorite and barium minerals. Sphalerite, iron carbonates and quartz are notably less abundant (Symes and Young, 2008). Dunham (1952) identifies fluorite concentrations in the regions around Pateley Bridge and Grassington (Wharfedale), and smaller concentrations in Wenesleydale and Swaledale, further to the north. Unlike the Alston Block, fluorite and barite are commonly found together in deposits of the Askrigg Block. Metasomatic flats are occasionally present accompanying mineralized veins, but are less common than in the Alston Block deposits.

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, calcite, barite and witherite 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 most mines in the North Pennines until the collapse of the world lead market in late 19th century. While the silver content of galena from the region is generally low (averaging 4-8 oz/t per Dunham, 1990), some deposits had a high enough silver content to make its recovery economical. Local concentrations of sphalerite have been mined for zinc, and where sufficiently concentrated (such as in the deposits around Nenthead in Alston Moor), allowed some mines to survive the lead market crash by switching their primary output to zinc.

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, which is plentiful in many of the Weardale mines 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.

Galena and Fluorite from the Blackdene Mine, Weardale. While extraction of lead ore was traditionally the dominant mining activity in the North Pennines, the recovery of fluorspar became more important during 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 rarely 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.

The iron carbonates ankerite and siderite occur widely throughout the orefield, particularly within metasomatic flats. Many of these flats have been heavily oxidized, producing massive iron-rich gossens, known locally as "ironstone." For a period during the mid to late 19th century many of these ironstone deposits were mined commercially as a source of iron ore, and a once thriving foundry business centered around the town of Consett developed to process the output of these mines.

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).

Characteristics of North Pennine Fluorite

The habit of North Pennines fluorite is almost always cubic, and crystals under approximately 3 cm in size are often penetration twins on [111], where-as larger crystals are often untwined and opaque. The one notable exception to this is the Boltsburn mine, which produced twinned fluorite crystals of good clarity in excess of 10 cm on edge. Growth hillocks with four vicinal faces are common on the cube faces of twinned crystals and appear to emanate from the point where the twin penetrates the cube face. Tabular and elongate crystals. giving the appearance of tetragonal symmetry are occasionally found, particularly from the Blackdene and Boltsburn mines. And while rare, crystals with hexoctahedral modifications to the corner of the cube are sometimes seen.

A penetration-twinned fluorite crystal showing well-developed vicinal faces. From the Rogerley Mine, Weardale. Crystal is 2.5 cm on edge.

A 12 cm cluster of twinned fluorite crystals from the Blackdene Mine. Perched in the center is a larger, untwinned crystal of distinctly tabluar habit.

A twinned fluorite crystal from the Heights Quarry showing hexocathedral modification to the corner of the cube.

The most common colors of fluorite from the Alston Block deposits are various shades of purple and lavender, followed by green and occasionally yellow. Internal color zoning, though often present, is fairly subdued, particularly when compared to that seen in fluorite recovered from the well-known southern Illinois deposits in the United States. Transparent crystals of purple fluorite will show discrete layers of varying color intensity where-as crystals of green fluorite sometimes have pale purple cores. Anomalous color zonning occasionally occurs in fluorite specimens from some mines. Green fluorite from the Rogerley mine will sometimes contain asymmetrical zones of pale yellow, and one cavity found in the Blackdene mine during the 1960s yielded yellow fluorite crystals with bright green cores. The green color of fluorite from several Weardale localities, including Rogerley, Heights, and the Cement Quarry is unstable and will usually fade over time if left exposed to direct sunlight. Other colors including purple and yellow appear to be stable. Fluorite from the mines in North Yorkshire is typically less colorful than that from around Weardale, usually being colorless to pale yellow. A thin skin of dark blue is typical of some of the deposits around Pateley Bridge and Grassington, and some deeper colored yellow/amber fluorite has come from some of the Wensleydale and Wharfedale mines.

A cluster of fluorite crystals from the Elbolton Mine, near Grassington, North Yorkshire showing a dark blue skin over colorless fluorite. 7x5x4 cm overall size.

North Pennines fluorite crystals sometimes display an internal cloudiness known to local collectors as “white centers”. As the name implies, these crystals are white and opaque in the center while having gemmy transparent edges and corners. It is not unusual to find specimens where one or several larger transparent fluorite crystals are surrounded by numerous smaller crystals, all with white centers. This internal cloudiness appears to be caused by numerous microscopic void spaces within the crystal, but it is unclear whether these are a growth feature or the result of a later partial dissolution or etching of the fluorite.

A cluster of amber-yellow fluorite crystals from the Hilton Mine, Scordale, Cumbria showing a "white centers" in many of the smaller fluorites. 8x6x4 cm overall size.

Fluorite from the Alston Block deposits is highly fluorescent and often exhibits a notable color change between artificial and daylight illumination, an effect known as daylight fluorescence. Although the effect seems the least notable in yellow fluorite, purple or green crystals usually show pronounced purple to blue overtones in direct or even indirect sunlight. The ultraviolet (UV) fluorescence of North Pennines fluorite is exceptionally strong, particularly under long-wave UV, and it is said that studies of this material by Sir George Stokes in 1852 were the original source of the term fluorescence (Stokes 1852). Internal color zonation within fluorite crystals is often mimicked by variations in the intensity of its fluorescence.

A 15 cm cluster of penetration-twinned fluorite crystals from the Rogerley Mine, photographed in natural sunlight.

A 15 cm cluster of penetration-twinned fluorite crystals from the Rogerley Mine, photographed in artificial light.

A 15 cm cluster of penetration-twinned fluorite crystals from the Rogerley Mine, photographed in long wave ultravoliet light.

REE Content of North Pennines Fluorite

It has been known for some time that fluorite from this region contains elevated levels of a number of rare-earth elements (REEs), (Bill et al 1967), and it was initially speculated that some of these elements were acting as chromophores, particularly in the green varieties. As the green color of Weardale fluorite is generally unstable and will fade with prolonged exposure to direct sunlight, it is likely that this color is related to structural defects or “color centers,” which can be annealed by exposure to strong uv-containing light. Recent analyses involving fluorite from the Rogerley Mine have confirmed the presence of elevated levels of a number of REEs, including yttrium, lanthanum, cerium, neodymium, samarium and europium (Watkins, pers. com., 2014). Substitution of these elements for calcium within the fluorite structure may be causing distortions, which in turn, result in the coloration. Additionally, they are likely to be the cause of the intense UV fluorescence. Fluorite from the Askrigg Block in North Yorkshire is generally non-fluorescent, and Dunham & Wilson (1985) report that the REE content of fluorite from this region is negligible.

  • Yttrium (Y) - 173 ppm
  • Lanthanium (La) - 99.6 ppm
  • Cerium (Ce) - 203 ppm
  • Praseodymium (Pr) - 29.1 ppm
  • Neodymium (Nd) - 132 ppm
  • Samarium (Sm) - 53.1 ppm
  • Europium (Eu) - 55.3 ppm
  • Gadolinium (Gd) - 72.0 ppm
  • Terbium (Tb) - 11.4 ppm
  • Dysprosium (Dy) - 54.6 ppm
  • Holmium (Ho) - 8.00 ppm
  • Erbium (Er) - 15.2 ppm
  • Thulium (Tm) - 1.55 ppm
  • Ytterbium (Yb) - 6.97 ppm
  • Lutetium (Lu) - 0.86 ppm

Rare-Earth Element (REE) concentrations in green fluorite from the Rogerley Mine flats. Analysis by laser ablation ICP/MS courtesy of Prof. John Watkinson, Washington State University, Spokane.

A color-zoned fluorite crystal from the Heights Quarry showing complex layering in both visible and ultra violet light. Variations in the concentration of REEs within the fluorite may be responsible for the zoning.