I worked at Geevor during the period of much greater production from 
lodes in killas (clay slates). Although these lodes tended to have a 
higher sulphide content as well as minerals like garnet I am not aware 
of any impact on the quality of high grade tin concentrates. However 
perhaps of more relevance in comparing with earlier practice may be the 
overall content of all the modern product, medium and low grade tin 
concentrates as well as the amount of sulphides produced. I guess Levant 
would have predominantly gone for a single tin product. David Wright at 
Geevor should be able to provide information on any significant changes 
during this period. It is likely someone at Geevor is watching this list 
and can pose the question to him, if not I'll speak to him soon.

I am aware of 2 events in my time at Geevor that did show how a plant 
can be upset by a sudden occurrence of something unexpected. When old 
dumps were being brought in from Wheal Cock the presence of scheelite 
had a major impact on the high grade tin. Being of similar colour to 
ground cassiterite and slightly lower density it ended up lowering the 
grade significantly resulting in the dump being abandoned. On another 
occasion a development heading on Coronation lode there was an 
occurrence of very fine cassiterite intimately mixed with silica. As a 
development heading only about 100 tons of rock were being added to the 
feed to the mill in a weekly throughput of about 3,000 ton yet the 
slightly lower density made it much more difficult to get a clean cut on 
the high grade tables resulting in a significant drop in tin content. 
Fortunately this didn't last long.
-- 
Geoff Treseder