IA the Tamar Valley (Booker 1967)
States that " These works originally consisted of a large percipitating
and arsenic refining plant built in 1875 to treat ores from Holmbush,
Kelly Bray and Wheal newton. Lack of capital forced them to close down
shortly afterwards but they restarted in 1901. Arsenial Mundic was then
brought from Wheal Crebor and other mines, treated, packed inro casks made
on the premises and shipped from Calstock quays until the 1930's"
Acton & Brown In Vol 4 of their exploring Cornish Mines Books states that
the closure date was uncertain, 1923 or 1930's. They give a good reference
that may be worth following up:
P.H.G Richardson " The Greenhill Works Gunnislake" in the Plymouth Mineral
& Mining club Journal Vol 25, no 3 Feb 1996. A detailed account of its
layout and history.