The Diana Maria Mine
Frosterley, Weardale, County Durham, England
Philip G. Taylor, Crystal Classics Fine Minerals Ltd., No. 1, The Old Coach Yard,
East Coker, Yeovil, BA22 9HY, United Kingdom
History and Location
For the past 45 years, the Rogerley mine near the northern England village of Frosterley in Weardale, County Durham, has been worked for its magnificent green fluorite, famous for its intense daylight fluorescence (see Lapis Jg. 35, Nr. 1, January 2010, pp.13-26).
The Weardale valley 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 classic fluorite localities, for example Ireshopeburn, Blackdene, St. John’s Chapel, Daddry Shield, Westgate, Eastgate, Rookhope, Boltsburn, Stanhope and Frosterley. The Rogerley quarry and the recently named Diana Maria mine are located just west of Frosterley, on the north side of the A689 road to Stanhope.
The Rogerley specimen recovery project has now been run by several teams and in spring 2017 ownership passed to Ian Bruce under the company of UK Mining Ventures Limited (UKMV Ltd.).
Since the old limestone quarry at Rogerley was initially investigated in 1972 by Lindsay Greenbank and Michael “Mick” Sutcliffe, two mineralised veins have been known, named in their honour, the Greenbank and Sutcliffe veins. The former was named by Sir Kingsley Dunham, then Professor Emeritus of Geology at the University of Durham and director of the British Geological Survey.
Preparations by UKMV Ltd. for progressing specimen mining operations commenced at Rogerley in mid-spring 2017. This included detailed geological mapping and modelling of the Rogerley mine, including surface and underground geological mapping, 3D scanning and a 3D geological model of the fluorite and galena veins. During the site investigation, a previously unknown mineralised vein was discovered running in parallel, and close to, the Sutcliffe vein. Although the Sutcliffe vein has been known for many years, it was never exploited, with all mining effort being put into development of the Greenbank vein, which now forms the Rogerley mine.
A few hundred metres north-west of the Rogerley mine portal, the Sutcliffe vein crops out high in the old quarry face, the original site of where specimen-grade fluorite and etched galena crystals were first found by mineral collectors on the quarry floor. As this mineralisation is within about six to eight metres of the surface, it was decided that the first project would be to strip back this overburden and explore the potential of this vein. Within less than two weeks a collapsed “flat” was unearthed, covering a 10 x 6 m area, containing six exceptionally mineralised and differing pockets of specimen-grade fluorite. This new mining operation has been named the Diana Maria mine, for Diana Maria Bruce, life-long mineral collector and from a family of highly respected mineral dealers and collectors in the ancient Saxony mining region of Germany.
Geology and Mineralisation
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 fractures and normal faulting in the Carboniferous limestones, sandstones and shales (mudstones) and it is these that were later mineralised to create the Northern Pennine Orefield. The main stratigraphic unit at the Diana Maria mine is the Great Limestone, in which the Rogerley quarry was originally developed for the extraction of limestone.
The carbonate-hosted (limestone) lead and zinc mineralisation of the Weardale mines has many similarities to Mississippi Valley Type (MVT) deposits, particularly those with a high fluorine presence such as the Illinois–Kentucky ore fields in the USA. Current knowledge suggests the supply of metals and fluorine were from several different sources which included:
- The Weardale granite (a fluorine-rich boss of the underlying North Pennine Batholith).
- The Whin Sill (a late Carboniferous dolerite intrusion, dated at 295 million years).
- Metal-rich brines expelled from ongoing dewatering and lithification of adjacent sedimentary basins.
- Metals dissolved from adjacent country rocks by the passage of flowing brine.
Fluid pumping and circulation of the metal-rich brines through the host rocks was provided by various mechanisms including:
- Convection above the “high-heat retaining” Weardale batholith.
- Heat from the, then, recently injected Whin Sill.
- Natural overpressure within adjacent sedimentary basins from dewatering and diagenesis.
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 the accompanying sketch. 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.
Metasomatic Replacement and Flat-style Ore Emplacement
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 200 m and at any vertical depth in the veins that is favourable.
Outward migration of hydrothermal ore-bearing fluids, from the steeply dipping, high permeability fracture and fault systems, occurs through at least four processes:
- Lateral extensions of faults through mechanical breakage of the adjacent limestones, in the form of brecciation, fracture development and anastomosing (criss-crossing) fracture swarms.
- Fracture development along bedding planes and as joint sets, due to flexure during tectonic deformation. Tension fractures, parallel and normal to bedding, create accommodation space for the deposition of minerals. Limestones which tend to be brittle, are more prone to this.
- Natural permeability within the rock fabric; i.e. fluid flow between intergranular pore space.
- Infiltration into very low permeability rock, driven by gravity and capillary pressure.
Once the hydrothermal fluid has entered, and is in contact with, the limestone beds, chemical reactions result and so begin to alter the limestone. Limestone is essentially calcium carbonate with varying quantities of clastic sediment impurities; clay, silica, etc., and this readily reacts with the warm metal and halide-rich brine. Chemical alteration within the flats is also driven by differences in acidity-alkalinity disparity (pH); redox potential (Eh); temperature; pressure and flow rate. By piecemeal alteration, the limestone is gradually replaced through processes of dissolution and deposition to become both chemically and mineralogically different. Throughout this process the rock mass remains solid and intact, so existing accommodation space is not destroyed allowing well-developed crystals to grow. The alteration assemblages so far exposed at the Diana Maria mine include very fine green and purple fluorite, sometimes coated with drusy snow-white aragonite or over-growing glassy mounds of quartz crystals. The matrix varies between the typical earthy ironstone and a chalcedony or “jasperised” ironstone, rich in silica and iron.
The cartoon shows some of the structural and mineralisation processes present in the Diana Maria mine. The Sutcliffe vein is shown to steeply dip at around 86° (field measurements vary between 78° and 88°). Prior to mineralisation, such steeply dipping fractures/faults were created through flexure of the lithified sediments during horst uplift by the Weardale granite. Fractures and faults were generated by the competency contrasts between brittle limestones and ductile mudstones.
Near-horizontal fractures associated with both the vein system and limestone deformation, pervade the limestone and allow mineralising fluids to extend horizontally through the country rock. Other processes are noted in the diagram, for example, permeability controls to fluid flow and colour distribution in fluorite.
Pockets and Specimens
Stripping back the overburden above the exposed Sutcliffe vein commenced in early July 2017. Explosives were not needed as the near-surface shales and limestones were already heavily fractured through weathering and deformation. Having excavated down to vein level, and only some two weeks into the operation, a large 6 x 10 m collapsed metasomatic flat was encountered, extending laterally from the Sutcliffe vein. Almost all the cavity was found to be richly mineralised with fluorite and has since yielded many hundreds of specimens. Only a limited amount, however, is of collector-grade, perhaps less than 5%, which is quite typical for such a deposit. Un-mineralised cavities within the flat were filled with sticky, viscous clay and mud, remnants of material deposited during the last Ice Age.
While excavating the flat, five distinctly different mineralised pocket zones were recognised. Two pockets associated with the Sutcliffe vein, the Snowstorm Pocket and Pavel’s Pocket, contained purple and purplish-green fluorite, reminiscent of that found in the Greenbank vein. The remaining three pockets (Graeber Jones, Emerald Peaks and Green Hill) all contained green fluorite, but each with their own unique characteristics. Many of the crystallised green fluorites from throughout the flat sit on a thin layer of massive pale yellow-mauve fluorite, which coats the altered limestone matrix.
Graeber Jones Pocket
Named for Charles Clavert “Cal” Graeber Jr., well-respected USA collector/dealer and one of the previous owners of Rogerley mine, and for British collector/dealer Ian Jones, the Graeber Jones Pocket was the first pocket discovered at the Diana Maria mine during June 2017. Initially it contained green, interpenetrant twinned fluorite crystals usually growing on equant, stubby quartz crystals, milky in appearance and tinged slightly yellow due to iron. The outer faces of the fluorite crystals tend to be a gemmy sea-green, with many of the centres slightly cloudy, in paler, mossy shades of green. This small zone quickly gave way to beautiful deep leaf-green to emerald-green twinned fluorites, often with distinct colour zoning. Zoned crystals often have a gemmy intense blackberry-purple centre surrounded with gemmy green faces. It is observed that these fluorites always grow on stubby quartz crystals, often of stalactitic form.
The evocatively named Snowstorm Pocket occurred at the contact between the Sutcliffe vein and mineralised flat. This contained gemmy green fluorite twins coated with snow-white aragonite. The fluorite occurs mainly as green gemmy twins on quartz overcoated with snow-white aragonite. Occasionally, crystals can be a grassy to sea-green and, rarely, a purplish-green.
Named for Pavel Škácha, discoverer of the pocket, Pavel’s Pocket occurred on the south side of the Sutcliffe vein in a very hard jasperised limestone. The deep purplish-green fluorite is often prettily crazed with snowy-white centres and are typically directionally coated in drusy snow-white aragonite with one or two fluorite faces still visible. A frequent feature of this pocket is that of a larger gemmy purple twin emerging from the matrix. This observation of intense purple fluorite from the vein corroborates in-situ observations at the Greenbank vein. In daylight, the fluorite fluoresces an intense blackberry-purple.
Emerald Peaks Pocket
The Emerald Peaks Pocket, named for the fluorite’s colour and crystal form in this zone, is characterised by small green untwinned fluorite crystals coating an ironstone matrix, over which much larger, gemmy emerald-green twinned crystals grow. Specimens show remarkable artificial UV and daylight fluorescence.
Green Hill Pocket
Named for lush green slopes of Fatherley Hill, on the side of which the Diana Maria mine is situated, the Green Hill Pocket are most distinct with large intergrown single fluorite crystals, sometimes on a quartz matrix. The fluorites have beautiful gemmy corners with frosted and crazed green-mossy interiors, making spectacular combination. In daylight, the corners fluoresce intense blackberry-purple while their centres remain translucent, leaf-green mossy interiors, giving a stunning contrast.