Extensional quartz veins

- examples from county Kerry and county Cork, southwest Ireland

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Figs. 1-2: Quartz veins in outcrop and in building stones, small and accessible examples of features in structural geology with diverse uses. Though these veins seem to be composed of fibrous to prismatic milky quartz, such veins may also contain economic amounts of gold, silver and other metals. The examples on this page are all from southern Ireland, photographed in Sept. 2019. Left: quartz veins on the shore of a lake in Killarney national park, south of Killarney, county Kerry. The veins, cutting host slates, strike roughly NE-SW (235E, uncorrected). Right (and Figs. 3-6): detail at the James fort, Kinsale, county Cork. Be sure to "see through" the round blobs in Figure 2 - those are lichen!

"Rock of the Month # 225, posted for March 2020" ---

The great majority of quartz veins are not mineralised: they are generally composed in the main of white (gas-bubble-bearing) "bull quartz" or milky quartz, sometimes plus calcite, tourmaline, chlorite, white micas and other "gangue" (nonmetallic, "waste") minerals. But in some districts they are rich in gold, silver and base metals in varied proportions, as in New Zealand (Brathwaite et al., 2001), Quebec in Canada (Daigneault, 1991) and many other lands.

The emplacement and structures of veins are dealt with in a range of textbooks of physical and structural geology (e.g., Holmes, 1965; Hobbs et al., 1976; Wilson, 1982; Ramsay and Huber, 1983). Vein geometry and inter-vein spacing bear directly on mineral deposit resource estimation (e.g., Van der Pluijm, 1984; Pilote et al., 1990; Johnston, 1992; Neumayer et al., 2000).

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Figs. 3-4: Quartz veins in building stones, at the James fort, Kinsale, county Cork. These views show side (interior, "looking out from the centre") and end-on views, respectively perpendicular and parallel to vein length. Some ironstaining (limonite) is evident in Fig. 3. Since the rust stain is ubiquitous it probably reflects water flowing through the vuggy (permeable) vein and depositing traces of iron, rather than the decay of discrete crystals of iron-rich minerals such as pyrite, in situ. The principal wall rock to the veins is a pale green volcanic rock.

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Figs. 5-6: Views of the western wall of the James fort, showing a gate and the wall where Figures 2-4 were photographed. The fort was built to defend the strategic harbour and town of Kinsale, which had been occupied briefly by Spanish forces in 1601. The fort was built in 1602-1604. Its importance declined after 1678, when the massive Charles fort was built on the opposite shore. Both these formidable structures are examples of "star forts", named for their 5-sided design, best seen in the better-preserved and more touristy Charles fort. The frame of the gate is constructed of massive cut blocks of a fine-grained, pale grey igneous rock ("felsite"). This is probably an intrusive rock, possibly a dyke or sill, in contrast to the volcanic rocks used in the majority of the construction.


Brathwaite,RL, Cargill,HJ, Christie,AB and Swain,A (2001) Lithological and spatial controls on the distribution of quartz veins in andesite- and rhyolite-hosted epithermal Au-Ag deposits of the Hauraki goldfield, New Zealand. Mineralium Deposita 36, 1-12.

Daigneault,R (1991) Déformation et cisaillement, concepts et applications. MERQ DV 89-16, 49pp. (in Fr.).

Hobbs,BE, Means,WD and Williams,PF (1976) An Outline of Structural Geology. Wiley, 571pp.

Holmes,A (1965) Principles of Physical Geology. Thomas Nelson and Sons Ltd, London, 2nd edition, 1288pp.

Johnston,JD (1992) The fractal geometry of vein systems: the potential for ore reserve calculation. In `The Irish Minerals Industry, 1980-1990' (Bowden,AA, Earls,G, O'Connor,PG and Pyne,JF editors), Irish Association for Economic Geology, Dublin, 436pp., 105-117.

Neumayr,P, Hagemann,SG and Couture,J-F (2000) Structural setting, textures, and timing of hydrothermal vein systems in the Val d'Or camp, Abitibi, Canada: implications for the evolution of transcrustal, second- and third-order fault zones and gold mineralization. CJES 37, 95-114.

Pilote,P, Guha,J, Daigneault,R, Robert,F and Golightly,JP (1990) The relationships between structural evolution and a gold mineralizing system: the Casa-Berardi (Golden Pond) deposits, Quebec, Canada. Extended abstracts volume, Third International Archaean Symposium, Perth, 487pp., 351-353.

Ramsay,JG and Huber,MI (1983) The Techniques of Modern Structural Geology, Volume 1: Strain Analysis. Academic Press, 307pp.

Van der Pluijm, BA (1984) An unusual `crack-seal' vein geometry. J.Struct.Geol. 6, 593-597.

Wilson,G (1982) Introduction to Small-Scale Geological Structures. George,Allen and Unwin, 128pp.

Graham Wilson, 03,06,17 October, 2019.

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