Not being intimate with Cornwall's mining area, I can only suggest from my own experience in the past in Derbyshire, that one visits the office of the Local Authority in whose area the consent for the planning application has been granted for this extraction, and ask to view the planning application and the planning permission granted. When applications for mineral extraction have been submitted, Local Authorities have to advertise such planning applications in local newspapers, and this is the time to visit the planning office, view the application and then send in an objection if one is against it. One has to have valid objections such as the demolition of an engine house (especially if it is a Listed Grade 1 and 2 building), the destruction of a footpath, destruction of flora and fauna etc., etc., one can't just object on the grounds that you don't want the extraction to take place because it's going to disturb your peace and quiet. Once planning consent has been approved it is too late.
I am not against mineral extraction, it is part of the life and economy of the area in which ever part of the UK it occurs, and is essential to providing work hopefully to keep the indigineous population in the area. Also having been a builder/developer myself after previously being a Local Govenment Officer for many years, I know only too well that without mining and quarrying there would be no materials for house, road building etc., all the things that modern civilisation in the UK take for granted without counting the cost of where it comes from.
It's complicated in Derbyshire, the County Council approve mineral applications (so perhaps an approach to Cornwall County Council Planning Department could also be made regarding this application), then part of Derbyshire is within the Peak Park Planning Authority who have jurisdiction over the area within the Peak Park. Not forgetting the smaller Local Authorities within the County lying outside the National Park boundaries. When viewing a planning application we used to have to visit Derbyshire County Council Minerals Section at Matlock, Derbyshire Dales District Council also at Matlock and the Peak District National Park Authority at Bakewell to gather all our facts together. It got complicated.
Trust this helps to locate the original planning application and consent.
EDIT:- Have had an afterthought, many Councils now show planning applications on their website. You could always search the appropriate Council's website to see if you could find it!
P.S. Good evening to you too Iclok.