The archaeological provenancing of metals (matching metal to its ore/s) is difficult and certainly contentious. It is usually attempted by employing trace element or stable isotope geochemistry.

However is it crucial that only the appropriate ores or potential ores are sampled rather than 'uneconomic' mineralization.

Petrographical descriptions of samples in reflected light are the most effective method for distinguishing between ore and non-ore as grade, mineralogy, grain size and the nature of the mineral intergrowths can all be assessed.

In addition, from the mineralogy and petrography it is possible to predict what trace elements will be present alongside the major metals after the ores have been subjected to different degrees of beneficiation.

Two polished blocks and one polished thin section were investigated in reflected light, using x8 air and x16 and x40 oil immersion lenses. The polished thin section was briefly examined in transmitted light.

The sample comprises a dark grey-green phyllitic rock with a strong foliation. Pale, white mica-rich bands alternate with darker chlorite-rich bands and with lensoidal, white quartz veins carrying euhedral pyrite, trace amounts of base metal sulphides and patches of limonite. Fine-grained arsenopyrite is disseminated throughout the lithic bands but is less abundant in the quartz segregations.

Arsenopyrite together with lesser amounts of pyrite and TiO₂ minerals replacing original FeTi oxide phases are the main opaque minerals accompanied by rare to minor amounts of chalcopyrite, sphalerite, galena and pyrrhotite. Native gold is present; and covelline and limonite are secondary minerals.

There is a suggestion that the original FeTi oxide minerals were concentrated in layers although no primary FeTi oxides remain. Since zircon and haematite are absent these layers are unlikely to be heavy mineral bands but represent a primary igneous feature. However, TiO₂ pseudomorphs after FeTi oxides are very abundant and, based on their shapes, include pseudomorphs after original ilmenite and 'titanomagnetite' (here used to mean thin, crystallographically orientated oxidation-exsolution ilmenite lamellae in a magnetite host). The grain size of the original FeTi oxides suggests that the primary lithology was a lava or fine-grained plutonic.

Equant, 20 - 200µm but mainly 60 -120µm diameter, ex-titanomagnetite grains now comprise pale-coloured, very fine-grained, poorly polished, TiO₂ replacing magnetite enclosing 2 - 5mm wide TiO₂ laths after ilmenite lamellae. Although many grains are present in the host rock, others are enclosed and extensively replaced by arsenopyrite and, to a lesser extent, pyrite. Since fine-grained TiO₂ is preferentially replaced many arsenopyrite crystals show thin, relict TiO₂ lamellae after ilmenite enclosed in a 5 -10µm, pitted arsenopyrite that represents totally replaced fine-grained TiO₂. Tabular, pale yellow to orange-brown to brown coloured TiO₂, up to 120µm in length, has replaced tabular ilmenite and many grains lie with their long axes along the foliation of the rock. Elsewhere TiO₂ forms aggregates up to 200µm in diameter of brown coloured crystals in silicates or up to 80µm long laths that are enclosed in arsenopyrite and pyrite. Sector and polysynthetic twinning, suggest much of this TiO₂ is rutile. Discrete, fine-grained, authigenic, 2 - 5µm diameter TiO₂ is widespread and much fine-grained, lensoidal/streaky TiO₂ shows disruption by the growth of arsenopyrite. Leucoxene, up to 200µm in diameter, is uncommon.

Discrete, subhedral to euhedral, broadly zoned or unzoned arsenopyrite crystals are 20 - 200µm but mainly 40 -120µm in length. Smaller arsenopyrite crystals combine to form 200µm diameter aggregates. Much arsenopyrite is fractured and long crystals show pull-apart textures. Arsenopyrite is mainly inclusion-free but larger crystals enclose small, 10 - 30µm long, earlier arsenopyrite crystals, rare, 2 -10mm diameter sphalerite, chalcopyrite, pyrrhotite or mixed chalcopyrite-pyrrhotite blebs and 5 - 30µm, euhedral pyrite. Arsenopyrite replaces ex-FeTi oxide minerals and small, 2 - 5µm diameter authigenic TiO₂ crystals.

Pyrite is present in a number of different generations. Trace amounts occur as 5 -20mm diameter crystals in the cores of altered FeTi oxides or as 5 -30µm diameter, anhedral to euhedral crystals often enclosed in a thin chalcopyrite rim in arsenopyrite. Much pyrite, as crystals up to 150µm in diameter, overgrows and encloses 40-200µm diameter arsenopyrite and 20-80µm diameter TiO₂ pseudomorphs after FeTi oxide. Large, euhedral pyrite crystals, up to 0.5mm in diameter, present in thin quartz veins/lensoidal segregations carry numerous inclusions and are gold-bearing. Commonly poorly polished pyrite cores are surrounded by well-polished margins. This generation of pyrite encloses rare, 5 -30µm diameter, dark sphalerite, galena and 2 -10mm diameter, mixed pyrrhotite-chalcopyrite. More commonly 2 -30µm diameter chalcopyrite and 2 - 20mm but up to 60µm diameter pyrrhotite inclusions are present, here adjacent pyrrhotite inclusions in areas, up to 200µm in diameter, are in optical continuity. Many of these sulphide inclusions are concentrated at the junction between the well-polished outer margins and poorly polished pyrite cores.

Minor amounts of base metal sulphides often intergrown with each other are present as discrete masses in quartz or infill void spaces in pyrite or more rarely arsenopyrite. These include 30µm diameter sphalerite with chalcopyrite disease; 50 - 200µm chalcopyrite some intergrown with galena and trace amounts of mackinawite or 100 x 10µm long pyrrhotite laths or partially altered to covelline; 200µm diameter covelline and rare, hexagonal pyrrhotite some altered to zwischen-product or to 80µm diameter pyrite-marcasite pseudomorphs. Large limonite patches after ?carbonate are also present in quartz.

Gold is present in trace amounts as a single 1µm diameter grain of possible gold next to TiO₂ in arsenopyrite and as a 4 x 1µm gold 'fibre' in pyrite associated with 80 - 200µm diameter areas of 2 - 5µm diameter pyrrhotite and 1 -2µm diameter, irregular, pale blue phase that may be arsenopyrite or marcasite.

Two polished thin sections of siderite nodules taken from mudstones above the Thick Coal (Westphalian B, Late Carboniferous c. 310 Ma) were petrographically described in transmitted and reflected light using X8 air and x16 oil immersion lenses.

The intense limonite-replacement/staining of the carbonate rendered the sections almost opaque in transmitted light. However, late-stage masses of fine-grained kaolinite and pale-to-purple-coloured sphalerite were recognised.

Most of the sections comprise a fine-grained, equigranular, mosaic of siderite crystals that have been extensively replaced by limonite. Trace amounts of up to 20µm long graphite, 10 - 50µm diameter organic matter (including ?spores), rare, 5 -25µm diameter, framboidal pyrite and 5 - 20µm, discrete, euhedral pyrite crystals are present in the carbonate mosaic. The pyrite shows little alteration to limonite.

The mineralogy and petrography are characteristic of siderite nodules although iron-nickel sulphides, present in many nodules, are absent.

Clients: Palaeobiology Research Group, Lapworth Museum of Geology