It has been a very long time since I walked that bit of coast from Portreath. If I remember rightly, the rock offshore (in the background of the pic) has holes in it, which look like workings.
I think between Bassets Cove and Ralph's Locker (the cove in from that rock) there is a bit of quickly eroding cliff which I recall having some man made holes in it.
The shafty thing is similar to a manway/small whim shaft in size and format. Probably 6x4ft ish. I wondered if used to be a full shaft and the cliff dropped and this is probably a more likely explanation. However, there is a cave at the bottom with no adit and there are no signs of mineralisation or drives which I can see.
I've looked along the whole stretch of coast for workings and the only thing significant I found was that at Fishing Cove.
Above the cliff path, and slightly below where the photo was taken is a flat piece of ground which I am pretty certain was something mine related. I'm thinking Horse Whim and capped shaft. Although it isn't on any of the maps. There is a concrete slab above it.
I'll make an effort to have a poke around down there and get a couple of photos.
The cove adjacent to the tehidy entrance, now on the OS as Basset's Cove used to be called Spratting Cove.
Talking of works north of Portreath, there is a shaft which appears on the 1909 map and is not on the 1888, not mentioned by Ham Jenks and is clearly a whopper of an engine shaft....think double skip road, ladderway and pumps which is here:-
http://www.flashearth.com/?lat=50.268194&lon=-5.283355&z=17.7&r=0&src=msa It has the most bombproof grille I have ever seen on any shaft. It's made of huge bars placed at 2" intervals. Obviously any adit is a boat job. Again, the experts I know don't know anything about it either. Which means it was probably something to do with the weapons factory! đŸ˜‰