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This programme looks at hydrothermal mineralisation in parts of the Cornish ore field and its similarities with the case study on the Porphyry copper ores of El Salvador in Chile. In the field John... Wright describes the mineral greisen veins and wall rock alteration at Cligga Head and St. Michael;s Mount in Cornwall. Then in the studio Dr. Julian Pearce examines the structural controls and the orientation and distribution of mineral lodes. One question that is currently under investigation concerns the formation temperature at which the ores were deposited. Dr. Andrew Rankin shows how the fluid inclusions trapped within minerals can be studied to determine their respective formation temperatures.
Metadata describing this Open University video programme
Module code and title: S333, Earth science topics and methods
Item code: S333; 13
First transmission date: 21-08-1976
Published: 1976
Rights Statement:
Restrictions on use:
Duration: 00:25:00
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Producer: Peter Clark
Contributors: Julian Pearce; Andrew Rankin; John Wright
Publisher: BBC Open University
Keyword(s): Cligga Head; Fluid inclusions; Greisen veins; Hydrothermal mineralisation; Ore formation temperature; St. Michael's Mount; Wall rock alteration
Footage description: John Wright, at Cligga Head, North Cornwall, introduces the programme. Shots of old mine workings. Wright points out some of the mineral veins the miners were after. Wright examines an example of metasematic wall-rock alteration related to hydrothermal mineralisation - a central quartz bordered by an envelope of greisen. Wright next looks at granites between greisen veins in which the potash feldspar is in the process of being altered to kaolinite (china clay). Wright lists the pre-requisites for the above mineralisation and alteration. Wright at St. Michael's Mount, South Cornwall, a mineral rich area which has not been mined and where the hammering out of petrological samples is prohibited. He points out the greisen and seriatic wall-rock alteration and compares them with those at Cligg-Head. Julian Pearce, in the studio, uses a geological map of Cornwall to point out areas and types of mineralisation. Pearce speculates on the processes which led to fracturing of the granite. Pearce, using a graph which plots the stability fields for the fluids forming the greisen, speculates on the composition of the mineralisation fluids. Pearce examines some rock specimens (ore minerals) and relates these to a detailed geological map of Cligga Head which shows the distribution of minerals. Pearce shows that the nature of the fluids changed with time. John Wright, with a large quartz crystal containing a large fluid inclusion, explains how crystals with such fluid inclusions can be used to gain information on ancient hydrothermal solutions. Andrew Rankin explains and demonstrates a method for determining the minimum ore formation temperature of a host crystal based on a study of fluid inclusions trapped in minerals. Rankin shows the results of many such experiments plotted on a graph. He goes on to explain how true temperatures rather than minimum formation temperatures are obtained.
Master spool number: 6HT/72177
Production number: 00525_1219
Videofinder number: 1378
Available to public: no