Potassic Metasomatism of Slate Wall Rock and Polymetallic Mineralisation Associated with the Intrusion of a Quartz–Feldspar Porphyry Dyke in the Tregonning Hill Area of Southwest England

Granites of the Cornubian batholith of SW England and their host rocks are variably cross-cut by quartz–feldspar porphyry (QFP) and microgranite sheet intrusions locally referred to as elvan dykes. These are usually relatively potassic, have a porphyritic texture, and are often spatially and temporally associated with mineralisation. The processes by which they became so K-rich, their interaction with wall rocks, and their role in mineralisation remain poorly understood. Based on studies of a mineralised QFP dyke in the Tregonning Hill area of SW England, we present micro-textural and whole-rock geochemical evidence for potassic and then sericitic metasomatism of the dyke and its slate wall rocks, the latter to a rock strongly resembling granite, which could confound the identification of xenoliths and mapping of granite contacts. The metasomatism was caused by the through-flow of magmatic–hydrothermal fluids via inter-crystal pathways within a fluid–crystal mush, as evidenced by the presence of a network of vermiform micro-quartz veinlets that feed polymetallic quartz veins. QFP dykes acting as fluid–mush conduits, probably tapping larger underlying fluid-enriched mush reservoirs, is consistent with their association with metasomatism and mineralisation.