The Keys To The Past website got its information from me, so you can discount that as a source straight away. The tunnel isn't straight at all - it just follows the seam. I'll go deeper into it next year (with overalls on this time). Although the mine is reported as having been reworked in the industrial period it's wrong to assume there was a continuous history - the Romans missed it, for instance, yet were probably involved in lead and silver extraction just a few miles away. I'm not sure what you mean about artefacts - there's a carved stone mould, for a spearhead, found two miles away (it's in Tullie House now) and that's a good sign of metalworking activity, I would think. The valley that the mine is in is stacked full of mounds, cairns, carved stones, stone rows, possible small circles, remains of roundhouses and, perhaps, a whacking great cultivation terrace (but these aren't artefacts.
You'll either have to wait for my Linecast2.mp3 (it could be quite a wait) or hop in your wheeled machine, with a torch, some overalls and an iron will. The roof's sound beyond where I went this summer (but I'm getting older) and I recall the tunnel forks beyond where I turned back this year. The righthand side runs into a mud slump - as scary as hell - I've not explored the left. After returning to camp, in June, the keepr came and trashed my stuff - so there must be illegal traps set near the mine - I can give you the landowner's name and phone number (he can't say no).
Billy O'Brien's Bronze Age Copper Mining is the standard work - that's where I picked up the shape of the hammerstone marks from - and I actually have a piece of the roundwood that was used (unbelievably, I know) to roast the work face. I guess the same wood was used to smelt the ore - which I presume was Malachite. (There's none left where I went !) Children must have been used in the mine and getting so close to the workings is very unsettling. I recommend it - failing that have a listen to Linecast2 (when it comes out).
And if you do go near there don't miss Holymire - the stone circle - that's the best artefact of all !