County Geology Site Criteria for Cornwall and the Isles of Scilly

80/2. Bromley, A.V.1989. Field Guide to the Cornubian Orefield. Sixth International Symposium on Wayer-Rock Interaction, InternationalAssociation of Geochemistry and Cosmochemistry, Malvern. Camborne School of Mines.Site SignificanceScientific importance:Educational value:The site is considered to be of regional scientific importanceThe site is of educational valueDetails of Educational InterestRock types; cross-cutting relationshipsEducational Groups most likely to benefit are: Secondary, Adult & Continuing and Higher Ed.Historical importance: The site is not known to have any important historical associations.Aesthetic Appeal:Details of any Economic AssociationsThe site forms a key part of an attractive or evocative landscape.Full description of the site and its significance:The focus of interest is a breccia dyke, about 3m wide, intruded into killas. It lies on the east (footwall) side of an elvan dyke which strikesNE/SW and widens from 3-4m, at its seaward end to 15-20m near the low cliff. The elvan intersects the breccia and has a 0.5m selvedgecontaining breccia fragments along its footwall. Fragments in the breccia are of Mylor Slate Formation metasediment, greenstone and a smallamount of granite, set in a tourmaline matrix. There are many breccias of this type in Cornwall, some intrude granite eg. Wheal Remfrey, StAustell and most are associated with elvans and mineral veins. This suggests they are just post-granite in age, their constituent fragmentsinclude several rock types, the proportion of granite depending on the proximity to a granite intrusion, and vary both in size and angularity.The matrix consists of fine rock and mineral fragments in which quartz and tourmaline predominate and in which there has usually beenextensive tourmalinisation and chloritisation. Their origin lies in the build-up of pressure from volatiles (chiefly water and boron) beneath agranite carapace at the end of magmatic crystallisation. The release of pressure from jointing or fracturing was followed by the implosion ofsurrounding rock into the cavities, creating the breccia. The exposure of the hydrothermal breccia is mainly below the high water mark andcan be obscured by shingle. Also exposed is a cross-cutting NW – SE striking epithermal quartz vein, 2-3m in width, probably displacing theelvan dyke. At SW5280 3017, some 25m SE of the dyke, there is an excellent exposure of the same quartz vein exhibiting both a bandedcockscomb appearance. In the cliff, a raised beach of pea gravels is exposed, overlain by periglacial ‘head’.Site Management and Monitoring SummaryKey issues/potentially damaging activities:None identifiedManagement required:NoneDetails of management carried out:Date of last visit to the site: January 2007Surveyor: Simon CammDate site was last photographed : January 2007Cornwall RIGS Group County Geology Site Summary FormP/2 Venton Cove11 December 2007