Fluorite Specimen Mining Frosterley, Weardale, England
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Everything we use is either harvested, farmed or mined. Naturally occurring chemical compounds termed minerals, are mined to provide all the metals and chemicals we require, be these for mobile phones, pharmaceuticals, jewellery or whatever! Sometimes conditions within the Earth allow minerals to develop into crystals of stunning colour and beauty and the Weardale valley in County Durham, England, is one such area.
Weardale Fluorite is famous for its bright emerald green, vivid purple, lilac and sometimes yellow crystals; their sharp cubic geometric form; wonderful interpenetrate crystal twins and, due to the presence of rare earth elements, its fluorescence in natural sun light. The specimen picture below shows the natural green colour (left) and fluorescing bright purple when in daylight (right).
Lead mining in Weardale dates back many hundreds of years and it’s likely Roman mining engineers exploited the rich lead veins during their occupation of the British Isles.
Although quite a few types of minerals can be found in Weardale, the most beautiful without doubt is Fluorite. Formerly known as Fluorspar, it is a compound of the elements fluorine and calcium which combine to form calcium fluoride or CaF2. Common reasons to mine Fluorite are for fluoride in toothpaste or as a flux in steel making. By the serendipity of nature, Weardale is fortunate to hold some of the most remarkably coloured and crystallised examples of Fluorite in the world. Specimens from its many mines are to be found in most museums and private mineral collections around the world. When perfectly transparent, crystals can be used to make faceted gemstones.
The Weardale valley is approximately 31 km (19 miles) in length, coursing between the villages of Cowshill to the west and Tow Law at the east. Within its bounds lie many towns and villages such as Blackdene, St. John’s Chapel, Daddry Shield, Westgate, Eastgate, Rookhope, Boltsburn, Stanhope and Frosterley. Mention any of these names to mineral collectors the world over and they will picture the Fluorite and other mineralogical treasures they evoke!
UK Mining Ventures is currently mining Fluorite specimens and associated minerals from the Rogerley and Diana Maria mines between Frosterley and Stanhope, more specific details of which can be found in the separate links to the specific mines. This article describes the minerals, mining and geology of the Weardale Valley in general, but with particular focus on the two operating specimen mines just west of the village of Frosterley; the Rogerley and Diana Maria mines.
The name Weardale is derived from the River Wear and the northern English name dale for a river valley. The River Wear rises in the Pennine hills close to the village of Cowshill and flows due east through Weardale and the city of Durham, finally entering the North Sea at Sunderland. Weardale is approximately 31 km (19 miles) in length, coursing between the villages of Cowshill and Tow Law. Within its bounds lie many towns and villages with names evocative of so many superb fluorite localities, for example Ireshopeburn, Blackdene, St. John's Chapel, Daddry Shield, Westgate, Eastgate, Stanhope and Frosterley. The Rogerley and Diana Maria mines are located just west of Frosterley (Figure 1), on the southern slope of Fatherley Hill and almost opposite to where Rogerley Hall once stood, from where the name originates. Main access along the Weardale Valley is by the A689 road (Figure 2), along which County Durham's magnificent scenery can be enjoyed (Figure 3).
Weardale lies within the Alston block; a horst of Carboniferous sediments bounded by the Stublick and Ninety Fathom faults (north), the Pennine Fault (west) and the Stainmore Trough. Underlying the area is the North Pennine Batholith, also termed the Weardale granite. The granite batholith has five plutons, the Weardale pluton being the largest and whose buoyancy in the crust is sufficient to uplift and maintain the horst above the adjacent areas.
Past deformation produced a dense grid of normal faulting in the Carboniferous limestones, sandstones and shales (mudstones) and it is these that were later mineralised to create the Northern Pennine Orefield (NPO) (Bevins et al., 2010; Dunham, 1990; Pattrick and Polya, 1993; Stone et al., 2010 & Symes and Young, 2008).
The main stratigraphic unit at Rogerley is the Great Limestone, in which the quarry was developed for the extraction of limestone. The Great Limestone is a Carboniferous bioclastic packstone of the Pendleian Substage (326 Ma) whose reference section is in the nearby Eastgate quarry and in this area, sit at the top of the Alston Formation (Bevins et al., 2010 & Dunham, 1990). The limestone, rich in crinoid debris, brachiopods and corals, is thickly bedded with interbeds of shaly mudstone partings following bedding (Figure 7).
The origin of mineralisation in and adjacent to the Weardale district has long been the subject of discussion and research, and work in this field continues today (Bevins et al., 2010 & Stone et al., 2017). The type and distribution of mineralisation is partly analogous to both Mississippi Valley Type (MVT) (Edwards and Atkinson, 1986; Fisher and Greenbank, 2003 & Park and MacDiarmid, 1970) and hydrothermal zonation like that in Cornwall and Devon (Stone et al., 2017).
The similarities to concentrically zoned mineralisation in Cornwall gave rise to the theory of an underlying Carboniferous (Variscan) granite mass, as proposed by Dunham in the early 1960's. Following a detailed gravity survey which appeared to confirm this, the Rookhope Borehole (Figure 8) was drilled in 1960-61 in which the Weardale granite was encountered at 390.5 metres. Initially this was regarded as the source of the mineralising fluids, but dating proved the Weardale granite to be Devonian, predating the Carboniferous host rock by around 100 million years. This, and a weathering surface capping the granite boss, disproved the granite as the metallogenic source.
The carbonate-hosted (limestone) lead and zinc mineralisation of the Weardale mines has many similarities to those of MVT deposits, particularly those of a high fluorine presence such as the Illinois-Kentucky ore fields in the USA.
Current knowledge (Benham et al., 2004; Bevins et al., 2010; Cooper, 1996 & 4. Dunham, 1990) suggests the sources of metals and fluorine were from several different sources which included:
Fluid pumping and circulation of the metal-rich brines through the host rocks was provided by various mechanisms including:
Permeability within the intervening country rock allowed these warm, metal-rich, hydrothermal brines to enter the Weardale sedimentary system, that was already pervaded by a network of faults and fractures, following uplift.
In such conditions, certain sets of critical faults and fractures remain open to fluid flow, given the favourable local orientation of horizontal stress azimuths. These can then act as highly permeable conduits within rocks of significantly lower permeability, with the capacity to circulate vast quantities of mineralising fluids.
Changes in temperature, pressure, flow rate and flow direction, together with chemical reactivity with wall rock and rubble, have resulted in the precipitation and growth of fluorite crystals. The vuggy porosity provided within brecciated faults and open fractures, provided the accommodation space in which crystals of fluorite and galena could fully develop. It is these faults and fractures that form the steeply dipping mineralised veins throughout Weardale.
The three known veins, Greenbank, Sutcliffe and the provisionally named "River Catcher", in and around Rogerley quarry are shown in Figure 9. The Sutcliffe and "River Catcher" veins strike normal (90°) to the Greenbank vein and likely intersect somewhere under Fatherley Hill. The Diana Maria mine currently works a section of the Sutcliffe vein on the north-eastern benches of the old Rogerley quarry.
The Weardale and Alston mining areas of Northern England are famous for their "flats", the term given by miners to the near-horizontal rich ore deposits which extend out from the veins to distances of up to 200m and at any vertical depth in the veins that is favourable.
Outward migration of ore-bearing fluid from the original, highly permeable, fault systems occurs through at least three processes:
Cartoons illustrating generalised vein and flat-style mineralisation processes can be seen in the Rogerley mine and Diana Maria mine mining geology sections.
Weardale fluorite is renowned for its bright bluish-white fluorescence (Fisher, 2003) in longwave ultraviolet (LWUV) and natural daylight. The term fluorescence derives its name from fluorspar, the old name for fluorite. In artificial LWUV, Rogerley and Diana Maria fluorite emits a bright bluish-white fluorescence, similar to many other Weardale localities. What makes Rogerley and Diana Maria fluorite remarkable is its ability to intensely fluoresce in daylight, termed daylight fluorescence. This phenomenon is observed in some other Weardale fluorites, primarily those from the Heights mine and Eastgate (Blue Circle Cement) quarry, but much less intense. Best observed in diffuse daylight rather than direct, emerald-green fluorite crystals from Rogerley glow an intense violet-blue. A vein specimen of fluorite, one of the earliest collected from Rogerley quarry, is shown in Figure 10, photographed in artificial light (left) and in daylight (right) for comparison.
Much of the fluorite from both the veins and the flats displays this intense daylight fluorescence which surpasses that of most other known fluorite localities, both Weardale and world-wide. Figures 11 and 12 show the same specimen from the Rat Tail pocket, collected in 2010; Figure 11 is as found and in daylight and Figure 12 in artificial light after cleaning.
The high rare earth element (REE) content of Weardale fluorite has been known for many decades and this is, most likely, the primary cause of its daylight and LWUV fluorescence. Fisher (2003) and Ixer (2003) describe the differences in REE content between early, high temperature, fluorite mineralisation in the Northern Pennine Orefield and that of the late stage, low temperature, as found at Rogerley. The REEs present in the Weardale ore field are listed in Table 3.
| Rare Earth Element | Symbol | Weardale Fluorite | Rogerley Fluorite |
|---|---|---|---|
| Cerium | Ce | ✓ | ✓ |
| Dysprosium | Dy | ✓ | ✓ |
| Erbium | Er | ✓ | ✓ |
| Europium | Eu | ✓ | ✗ |
| Gadolinium | Gd | ✓ | ✓ |
| Lanthanum | La | ✓ | ✓ |
| Neodymium | Nd | ✓ | ✓ |
| Praseodymium | Pr | ✓ | ✗ |
| Samarium | Sm | ✓ | ✓ |
| Ytterbium | Yb | ✓ | ✗ |
| Yttrium | Y | ✓ | ✓ |
| ✓ indicates relatively high wt% concentrations, all > 1% wt/wt of sample | |||
| ✗ indicates very low (>>1% wt/wt), below detection level or not present | |||
Early phase fluorite, above the cupolas of the Weardale Granite batholith, contains REEs in the form of discrete mineral inclusions, as set out in Table 4.
| Mineral Species | Chemical Formula | Remarks |
|---|---|---|
| Monazite-(Ce) | (CeLaNdPrY)P O4 | High wt% of five REEs occurs in monazite |
| Synchysite-(Ce) | Ca(CeLaNd)(CO3)2F | |
| Xenotime-(Y) | YPO4 | Containing : Dy, Er, Yb, Gd, Sm, Ce, La, Pr & U |
Analysis by direct-coupled plasma spectroscopy (Fisher and Greenbank, 2003) of Rogerley mine specimens shows elevated REE levels in all colours of fluorite (green, purple and yellow) from both hydrothermal veins and metasomatic flats. Interestingly, whereas europium was previously considered to be the main activator, levels of this element are low at Rogerley. REE enrichment is primarily by lanthanum (La), cerium (Ce) and yttrium (Y); other REEs identified in the Rogerley fluorite are neodymium (Nd), samarium (Sm), gadolinium (Gd), dysprosium (Dy) and erbium (Er) (Fisher, 2003 & Ixer, 2003).
The information provided on this website relating to mining in Weardale, Rogerley Mine and Diana Maria Mine is drawn from many sources. All referenced literature is listed at the end of each article, together with some additional useful references so providing a more general bibliography.
The current mining company, UK Mining Ventures Limited took over from the U.S.A. based UK Mining Ventures LLC, operated by Cal and Kerith Graeber and Jessie and Joan Fisher. Throughout this period Jessie Fisher methodically documented every aspect of mining activity, from mine development to specimen finds and some of his excellent articles are cited in the References. However, special thanks are made to Jessie for the vast amounts of detailed information he has supplied to UKMV Ltd., specifically to Phil Taylor who has been responsible for the website content. Jessie generously provided many specimen photos; pocket discovery information; detailed diary records and copies of the various published articles he has made over the years. As this website evolves, we hope to add more from Jessie’s archives as they provide an important addition to the history of mining in Weardale.
All employees of both UK Mining Ventures Ltd. and Crystal Classics Fine Minerals who have contributed their own photos and or information are thanked. Bryan Swoboda of Blue Cap Productions has filmed at several the UKMV mining operations and his excellent photos and videos are scattered throughout these webpages.
Jolyon Ralph of mindat.org is thanked for creating the original draft of this website, from which it has since evolved as new concepts and information have come forward.
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