Martin,
Try reading Robert Hunt, British Mining - 1884 pp. 685/686.
Geoff Treseder is quite right, Cornish ore values (£ or tons per fathom) were estimated based on
measurements of a 'square fathom' along the strike of the lode, coupled with a 1 fathom deep slice
of the lode proper. This would give you the cubic footage of lode available, and a sample across the lode
would give a good indication of its contents, enabling this to be related to the cubic feet of ore, to give
the total approximate value.
Tony Clarke
> Date: Mon, 18 Jun 2012 16:25:52 +0100
> From: [log in to unmask]
> Subject: Re: Ore grades - tons per fathom
> To: [log in to unmask]
>
> Thanks to Helen and Geoff for their replies.
>
> I can see why the mines might have quoted tons per a square fathom as this would give an idea of yield/value for a 1 fathom long by 1 fathom high (6x6 ft) stope across the full width of a vein. It would probably also be easy to evaluate when driving a level and makes it relatively easy to calculate tonnage of blocks between levels and shafts from just measuring the area on a longitudinal section of a mine. The measurement of course doesn't give an estimate of grade unless you know the width of the vein. But I guess with veins of say 4 foot wide or less then the width doesn't effect the economics of mining too much as there is a certain minimum width which has to be stoped. A 1 ton per [square] fathom on a 1 foot wide vein would probably given the same return as 1 ton per [square] fathom on a 4 foot wide vein as the width of the working is 4 foot in both cases. Only if the vein is very wide would mineralisation become less economic for the same yield per fathom (because more width of rock has to be removed).
>
> Martin
>
> Dr. Martin Critchley, ERA-Maptec Ltd., 40 Lower O'Connell Street, Dublin 1, Ireland
>
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