The petrography of the wallrocks and open-void fluorite mineralization
at Rogerley Mine, Stanhope, North Pennines.

R. A. Ixer

Abstract.

Locally, the clast-supported, dolomitised, brachiopod-foram-rich, bioclastic, Great Limestone has suffered thin, 0.5cm wide, wallrock alteration adjacent to open void mineralisation. The wallrock alteration consists of dolomitisation/ankeritisation and silicification. Void-infilling mineralisation is mineralogically simple, comprising a thin chalcedony rim overgrown by major amounts of quartz, green fluorite, galena and minor amounts of oxidised marcasite. Synchysite inclusions are absent from the fluorite and the galena is free of any sulphosalts, hence the ore belongs to the later lead-zinc-fluorite-baryte mineralisation of the North Pennine Orefield.

Introduction.

The North Pennine Orefield with its low temperature, Mississippi Valley-style lead-zinc-fluorite-baryte mineralisation is divided into two structural blocks, the northerly Alston Block and the Askrigg Block to the south (Dunham 1990; Ixer and Vaughan 1993). Within the Alston Block, the approximately 20 metre thick Great Limestone forms the base of the Namurian Millstone Grit and is well-known for the amount of mineralisation that it carries, including the attractive green fluorite mined from Rogerley Mine.

Typical specimens of the fluorite-quartz-galena mineralisation and its adjacent wallrocks were provided and some were selected for petrographical analysis.

Four thin sections of altered limestone and associated vugh infilling mineralization (ROG 1-4) were investigated in transmitted light; three polished blocks (R I-III) were investigated in reflected light. All opaque phases greater than 2 microns in diameter were noted.

Petrographical Results.

Altered Wallrocks
The least altered wallrocks comprise abundant sparite bioclasts set within a uniform, fine to medium-grained dolomite/ankerite matrix. This limestone/dolomitised limestone carries 40 - 120Ám long, euhedral dolomite/ankerite crystals and many have been partially or totally pseudomorphed by limonite and/or manganese oxide minerals. More heavily dolomitised specimens show numerous generations of dolomite/ankerite each with its own grain size and degree of limonite alteration. A late, coarse-grained, unzoned, euhedral dolomite/ankerite forms rims to small voids that are infilled by coarse-grained quartz and late-stage calcite next to local dedolomitisation. Trace amounts of 10-60Ám long, low reflectance, unzoned carbonaceous matter and up to 20Ám long, pale-coloured TiO2 minerals are detrital. Original micrite and detrital quartz are absent.

Trace amounts of oxidised pyrite showing orange to red internal reflections are present in the carbonate as isolated, 2- 10Ám but up to 40Ám diameter framboids or discrete cubes and pentagonal dodecahedral crystals up to 20Ám across. Locally, patches, up to 200Ám in size, of framboidal and euhedral pyrite are associated with sparite and fossil clasts, especially brachiopod shells.

Extremely rare, 5-10Ám long, twinned marcasite is associated with pyrite.

Metasomatic margins
When in contact with the mineralisation the dolomitised/ankeritised limestone is heavily silicified forming a fine-grained 'chert' up to 0.5cm in thickness. Close to the 'chert'-limestone junction, fossil fragments are preserved as relict sparite but in areas of more advanced silicification, they have been pseudomorphed by medium-grained quartz crystals or by the main, fine-grained 'chert' matrix. Locally, subhedral to euhedral fluorite and galena, both associated with coarse-grained quartz, are enclosed within the 'chert'.

Void infilling mineralisation
Quartz, locally preceded by a thin chalcedony rim, is the first mineral to crystallize into the void spaces forming euhedral, radiating crystals. It is probable that the same silica-rich fluids were responsible for the intense silicification of the limestone/dolostone, although there is very little chalcedony in the replaced wallrocks away from the mineralisation.

Galena forms subhedral to euhedral crystals within silicified limestone or is intergrown with green fluorite. Much appears to be slightly earlier than fluorite. Other than very rare, white or yellow-brown, 1-5Ám diameter pyrite and euhedral quartz crystals, galena is free of primary inclusions, including silver or antimony sulphosalts. Alteration rims about galena are up to 60Ám wide and show a pronounced caries texture. Galena alters initially to fine-grained, banded anglesite and subsequently to coarser, pale-coloured cerussite; both phases carry very fine <1Ám ?relict galena. Elsewhere, 20 -150Ám diameter, single cerussite crystals are present in silicified limestone or replace the cores of galena crystals.

Coarse-grained, inclusion-free, cubic fluorite overgrows quartz or locally, coarse-grained, cubic and octahedral crystals of galena. Some specimens show a crystallisation sequence of fluorite overgrown by small, oxidised marcasite crystals followed by coarse-grained quartz. Rare, small pyrite inclusions are present in fluorite but base metal sulphides and rare earth minerals (notably synchysite) were not recognised.

Later oxidation.
Locally, pale yellow limonite forms thin, 2-5mm wide rims about dolomite/ankerite crystals or totally replaces them. Where limonite is intergrown with manganese minerals, it is the first phase to form and displays the lowest reflectance.

At least three manganese oxide/hydroxide minerals are present in the wallrocks forming irregular patches, pseudomorphs after dolomite/ankerite or infilling void spaces especially those between quartz crystals.

Very minor amounts of a low reflectance, brown-grey, isotropic manganese mineral in association with limonite forms irregular areas with an open, spongy texture. Thin, acicular to tabular crystals, 20-60Ám in length with 'moderate' anisotropy are more widespread; these form 100Ám diameter aggregates infilling void spaces and overgrowing limonite. Small, 20-40Ám diameter aggregates of highly anisotropic pyrolusite are uncommon. Pyrolusite is the last manganese mineral to form, infilling void spaces within dolomite/ankerite rhombs or surrounding quartz crystals.

Discussion

Thin section petrography of the samples clearly demonstrates that, close to the mineralisation, the altered Great Limestone has suffered extensive wallrock alteration. This comprises dolomitisation/ankeritisation followed by very strong silicification. Later oxidation has produced abundant limonite and so many specimens are limonitically stained. These observations are in agreement with the macroscopic descriptions of Fisher and Greenbank (2000) and indeed are part of the characteristic wallrock alteration of the North Pennine Orefield (Dunham 1990; Ixer and Vaughan 1993).

The mineralisation has a simple mineralogy and paragenesis consisting of an early silica-rich phase (chalcedony and quartz) followed by fluorite, galena and minor amounts of pyrite and marcasite. Neither galena nor fluorite carry 'high temperature, exotic' mineral inclusions namely silver- or antimony-rich sulphosalts in galena and synchysite (Ca(CeLa)(CO3)2F) in fluorite. The absence of these minor phases and the general lack of copper- or iron-nickel-bearing phases suggests that the fluorite-galena-quartz mineralisation is not part of the earliest mineralisation concentrated above the cupolas of the Weardale granite (Ixer et al 1996). Instead it belongs to the later, lower temperature, main phase of mineralisation responsible for the majority of the ore from the North Pennines.

The lack of synchysite or other discrete REE minerals enclosed in fluorite, as seen elsewhere in the Alston block (Ixer et al 1996), would tend to confirm the suggestion of Falster et al (2000) that the rare earth elements (REE) in fluorite from Rogerley Mine are present within the lattice of the fluorite rather than as separate phases.

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