This sample is a rather fine-grained yet strongly porphyritic granite with cm-scale feldspar phenocrysts, cut by fine-grained, granular veinlets tinted bluish-black by abundant tourmaline. The individual tourmaline prisms are hard to see with the naked eye. The quartz-tourmaline veins enclose angular fragments of both feldspar phenocrysts and their host granite. The Wheal Remfry china clay pit is located near the western margin of the St. Austell granite Sample SWE165, a loose fragment from the top of level 1 on the west side of the pit, collected circa 1978. An "incipient breccia".

"Rock of the Month #101, posted November 2009" ---

Here is a sample of a tourmaline-rich hydrothermal breccia developed in granite of the St. Austell mass, at the Wheal Remfry clay pit in southern Cornwall. The geology, mineral deposits and mineralogy of Cornwall are well- documented (see, e.g., Selwood et al., 1998; Dines, 1969, and Embrey and Symes, 1987, respectively), as befits one of the classic historical mining districts of western Europe.

The rise of volatile-rich melts to shallow crustal levels can lead to brittle failure and brecciation, whether or not the system vents to surface, perhaps forming a tuff ring and associated maar- type lake. The breccia bodies may be of pipe-like or diatreme form, and be mineralized in base- and/or precious-metals. Examples occur in Queensland, Colorado and elsewhere (Baker et al., 1986).

The sample is a granite composed largely of quartz, orthoclase, albite and tourmaline with phenocrysts of grey quartz and larger phenocrysts of orthoclase, some showing Carlsbad twinning and faint perthitic exsolution. There are also accessory topaz and a trace of sulphide (chalcopyrite). Thin sections reveal a variety of pleochroic schemes and habit (from stubby prisms to felted, fibrous masses) in the tourmaline. Ion microprobe (SIMS) analyses suggest that the tourmaline became enriched in lithium with time, the youngest fibrous material with inky-blue colour containing up to some 0.3 wt.% Li (Wilson, 1980, pp.131-133).

The tourmaline-rich rocks in the well-exposed breccia at Wheal Remfry are a well-known, even a classic occurrence (Embrey and Symes, 1987, pp.7,9; Wilson, 1996; London, 2008). Tourmaline, the most abundant mineral of boron, presents technical challenges to both classical and modern instrumental analysis. It is however widespread, distinctive, and has been studied in detail (e.g., Wilson, 1980; London and Manning, 1995; Grew and Anovitz, 1996). The host mass, the St. Austell granite, crystallized at ~300 Ma. Hydrothermal alteration in the cooling granite body is nowadays thought to have formed the economic china clay deposits in the area (Bray and Spooner, 1983), which for many years had been ascribed a supergene, weathering origin. Granite magmatism, alteration and mineralization persisted to ~275 Ma or beyond. Chesley et al. (1993) reviewed the geochronology of the Cornubian granites and the sequence of vein deposits emplaced during this protracted cooling history. In a E.N.E.-W.S.W. direction, the granites of Devon and Cornwall include Dartmoor, Bodmin Moor, St. Austell, Carnmenellis, Tregonning-Godolphin and Land's End.

References

Baker,EM, Kirwin,DJ and Taylor,RG (1986) Hydrothermal Breccia Pipes. James Cook University of North Queensland, Economic Geology Research Unit, EGRU Contribution 12, 45pp.

Bray,CJ and Spooner,ETC (1983) Sheeted vein Sn-W mineralization and greisenization associated with economic kaolinization, Goonbarrow china clay pit, St. Austell, Cornwall, England: geologic relationships and geochronology. Econ.Geol. 78, 1064-1089.

Chesley,JT, Halliday,AN, Snee,LW, Mezger,K, Shepherd,TJ and Scrivener,RC (1993) Thermochronology of the Cornubian batholith in southwest England: implications for pluton emplacement and protracted hydrothermal mineralization. GCA 57, 1817-1835.

Dines,HG (1969) The Metalliferous Mining Region of South-West England. IGS, 2 vols, 795pp., published in 1956 and amended.

Embrey,PG and Symes,RF (1987) Minerals of Cornwall and Devon. British Museum (Natural History) / Mineralogical Record Inc., 154pp.

Grew,ES and Anovitz,LM (editors) (1996) Boron: Mineralogy, Petrology and Geochemistry. Min.Soc.Amer. Reviews in Mineralogy 33, 862pp.

London,D (2008) Pegmatites. Canadian Mineralogist Spec.Publ. 10, 347pp. + CD-ROM.

London,D and Manning,DAC (1995) Chemical variation and significance of tourmaline from southwest England. Econ.Geol. 90, 495-519.

Selwood,EB, Durrance,EM and Bristow,CM (editors) (1998) The Geology of Cornwall and the Isles of Scilly. University of Exeter Press, 298pp.

Wilson,GC (1980) Ion Microprobe Techniques, with Applications to Analysis of Lithium in Cornish Granites. PhD Thesis, University of Cambridge, 245pp.

Wilson,GC (1996) Tourmaline, intrusion-related. In `Atlas of Alteration: a Field and Petrographic Guide to Hydrothermal Alteration Minerals' (Thompson,AJB and Thompson,JFH editors), GAC Mineral Deposits Division, 119pp., 102-103.