Heptinstall and 4 co-authors, reported in early 2023 that copper, uranium, and rare earth element (REE) mineralization occurs in hydrocarbon-bearing Devonian continental sandstones in southwest Orkney, Scotland. The aeolian Yesnaby Sandstone Formation and fluvial Harra Ebb Sandstone Formation were mineralized following oil emplacement. The REE-bearing APS mineral florencite is particularly associated with bituminous nodules, many of which contain brannerite. Subsequently, hydrothermal copper and other sulphides, and barite, further mineralized the oil reservoir at a temperature of ~190 °C.
Oil was mobilized through mineralized fractures at this stage. Biodegradation of the oil occurred later, following the Carboniferous-Permian uplift. The occurrence confirms that Cu-APS mineralization is possible in relatively low-temperature regimes in sedimentary basins.
An exhumed oil reservoir occurs in Devonian fluvial-aeolian sandstones in southwest Orkney, Scotland [1–3]. The reservoir was previously known to host U-bearing minerals [4–6] and is shown here to additionally host copper-uranium-rare earth element (Cu-U-REE) mineralisation. Copper mineralisation occurs in exhumed oil reservoirs in several basins, e.g., [7–12]. However, the combination with REE-bearing APS (aluminum phosphate sulphate) minerals, mostly florencite in this case, is unusual in sedimentary rocks.
Aluminum phosphate sulphate (APS) minerals are common accessory products in a range of volcanic, hydrothermal, sedimentary, and metamorphic systems, occurring under a wide range of pH, redox and oxygen fugacity conditions [13]. APS minerals have been widely documented in Cu-mineralised systems [13], including porphyry-copper ores [14,15], epithermal deposits [15–17] and along uranium-mineralized unconformities [18,19].
REE-bearing APS minerals hosted in carbonaceous rocks are limited but have been documented in black shales [20]. Examples of a range of copper-mineralised systems which contain APS minerals are given in Table 1. They consistently formed at temperatures above 200 °C, and hence do not normally show any relationship with organic matter.
In this paper, the authors report an example of APS minerals with U and Cu mineralisation at lower (<200 °C) temperatures. This study examines petrographic relationships at Yesnaby in the Devonian Orcadian Basin, Orkney. The relationships are assessed in terms of:
(i) The spatial relationships between mineralization and hydrocarbons,
(ii) The paragenetic relationships between mineralization and hydrocarbons,
(iii) The timing of mineralization and hydrocarbon emplacement within the burial/uplift history.
The concluded that the mineralization in a Devonian sandstone-hosted oil reservoir at Yesnaby, Orkney (Yesnaby Sandstone Fm and Harra Ebb Fm), includes copper-barite veining, REE-bearing and uranium as brannerite. A petrographic study of the deposit shows several aspects of the relationship between the mineralization and oil residues:
- All mineralization is spatially associated with bitumen in the sandstone.
- All mineralization postdated oil charging of the reservoir, i.e., oil emplacement occurred first.
- The mineralization all occurred in the temperature range of 100 to ~190 °C
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