What the Lead Miners Threw Away

The lead miners of the Yorkshire Dales worked their ground from Roman times — the period of the lead pigs of Domitian, Trajan and Hadrian — until lead prices collapsed in the 1880s. The southern half of the Northern Pennine Orefield has yielded something like a million tonnes of galena, the lead sulphide that was the ore, along with much smaller quantities of copper and zinc.

Alongside it came gangue: the non-ore minerals that filled the fissure beside the metal, and that ended on the spoil heaps. Baryte — barium sulphate, BaSO₄, also spelled barite or barytes — is the widespread gangue mineral of this orefield, and it ranges more broadly through it than fluorite does.

The pattern in the gangue

Around each mineralising centre the gangue is arranged in rough concentric zones. Innermost is a fluorite zone, the mineral the memoir also calls fluorspar. Outboard of it sits the baryte zone. Peripheral to both is a calcite zone — calcite being the carbonate of the group — which yielded mostly small and unprofitable trials; the mineral occurs in veins of every zone, but predominates only here.

The arrangement was mapped in the memoir's Figure 21, and it is the single most useful guide to where the richest ore lay. Most of the largest lead producers sat inside the fluorite zone, within about a mile of its margin.

Depth followed the same pattern. Across the field as a whole, oreshoots — the workable concentrations of ore within a vein — rarely exceeded 70 feet (21 m) of vertical range. Only in the fluorite zone did that change: more than 750 feet (230 m) in the North Swaledale belt, more than 500 feet (152 m) at Greenhow.

Unlike the Alston Block to the north, very few veins in this field carry fluorite without barium minerals at all, so no pure inner fluorite zone is developed here. The mapped fluorite zone almost everywhere carries baryte, or witherite, the barium carbonate, intergrown with the fluorite — which makes it equivalent only to Alston's intermediate zone rather than to its core.

The barium carbonate

Witherite is BaCO₃: the same metal as baryte, held in a carbonate rather than a sulphate. It is a relatively uncommon mineral worldwide.

The memoir singles out its abundance here — "surprisingly common", it says, "considering its rarity elsewhere in the world", and particularly so in the North Swaledale belt and in South Swaledale and Wensleydale. It was worked in a small way at Barrass.

Witherite also frequently appears as baryte-after-witherite. That is a pseudomorph — the later mineral replaces the earlier one and inherits its outward shape, so the crystal has one mineral's form and another mineral's substance.

The order the vein filled

Where fluorite and baryte occur together, fluorite crystallised first, and the evidence for it is geometric: the fluorite presents fully formed crystal faces toward the baryte.

In the replacement deposits, where the fluid replaced limestone with new minerals, baryte comes later still. At Summer Lodge and bodies like it, baryte post-dates dolomite — the carbonate that is ubiquitous in them — and only rarely replaces the limestone directly.

Calcite and aragonite are, everywhere in the field, the last of the primary minerals to form.

The sequence is not a tidy one-way run. The memoir records many surges of mineralising fluid, and the surges repeated parts of the order.

Where the fluid came from

The deposits are epigenetic: introduced into open fissures, and into the limestone they replaced, after the wall rocks had already hardened, rather than laid down with the sediments. John Phillips made that argument in 1836, stressing "the consolidation of the walls, and the openness of the fissure".

The favoured mechanism is low-temperature deposition from heated saline groundwater: deep formation brines moving through the fractured block.

Which brines, and from where, is not agreed. Rogers proposed in 1978 that they were expelled into the block from the Craven Basin to the south and the Stainmore Trough to the north. Small, the same year, explained the zoning in the northern part of the block by connate brines — water trapped in the rock since its deposition. A dissenting note from stable-isotope data, by Mitchell and Krouse in 1971, favoured a fluid of igneous origin at Greenhow and Skyreholme.

Temperatures come mainly from fluorite. About 3,000 fluid inclusions — droplets of the original fluid sealed inside the crystals as they grew — gave, with pressure corrections applied, a range of roughly 92 to 164 °C. Baryte has one determination in the memoir, from Haw Bank, Carperby: about 132 °C, consistent with baryte and fluorite having crystallised together.

Within the exposed fluorite zone, no systematic temperature gradient was discernible.

Timing rests on where the veins stop. Lead veins scarcely cut Permian strata, and grains of fluorspar and barytes eroded out of the veins are found in the Saxonian Yellow Sands at the base of the Magnesian Limestone — so the veins were there, and already being eroded, before that sand was laid down. The memoir's preferred date for the main mineralising surge is the Stephanian–Thuringian, latest Carboniferous into Permian, with possible later resurgences if basin brines were the agent.

This is the authors' 1985 synthesis. The brine mechanism and the temperature range are well supported. The precise timing, and the source of the fluid, remain interpretive.

The heaps

Mining in the southern field is effectively inactive. Most of the ground now lies inside the Yorkshire Dales National Park, constituted in 1954, and the lifetime output of the field was valued at £200 million or more at 1980 prices.

The memoir's chief economic interest in baryte, at the time it was written, was not in the veins. It was in the residues: substantial recoverable baryte, with or without subordinate fluorite, left in the spoil of the whole working life of the field, and a target for retreatment.

The Dales were mined for lead, not for baryte. In the memoir's reckoning, the baryte is what remains.