As a newbie lots goes through my mind when I look around here but the thing that's really got me puzzled is how did miners find payable ore. I'm really thinking about cornish tin mines here but I suppose it applies to any mining venture.

I found a picture on this site, which you all have probably seen:

http://projects.exeter.ac.uk/geomincentre/05West%20Penwith.pdf 

It's on page 5, I would have copied it here but thought I might be infringing someones copyright so I didn't. It's the one that shows the gated adit at the bottom of Savealls lode. This is pretty obvious, if you find ore visible in a cliff you can work it and follow it inland but given that most mines are well inland how do you know there's any point in digging a shaft. Given that your going to dig hundreds, possibly thousands of feet in hard rock this can't just be guesswork but there aren't any visible signs on the surface of the ore you want. I'm assuming there's some sort of method for knowing where to stick your spade.

Put simply how did they know where to dig.

Keeping this brief and sticking to the first part of your question. Going back a few centuries when alluvial 'mining' on the moors and streaming predominated speculation was rife on how the tin found it's way to the moors and streams.
The tinners worked out the direction from which the deposits had come and followed the trail back to the hills. Eventually they would trace the flow of the of the tin stone to a single line and by following this the outcrop of the parent lode was almost always found.
Once discovered, the usual way of working the lodes in early times was either by sinking pits upon them, by driving tunnels along their course from the outcrop, or else by exposing them in open trenches known as "coffins" or "goffens".

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Once you find a lode on surface and work out the strike and dip it's quite easy to calculate/guess where it *should* be and drive a tunnel/sink a shaft to hit it. They would then drive down the lode looking for good ground. Maybe a bit of pot luck in the very early days. As more knowledge was gathered over time, then they would know that you need a certain type of geology as well as the vein to give you payable ore. Factoring in the dip/strike of the geology into the dip/strike of the vein/lode gives you a target of likely ground to aim for.
These days you would probably prove the vein by core drilling first before committing resources to driving a tunnel to the target.

You wouldn't drive a tunnel/sink a shaft into barren ground just for the hell of it.

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1) surface geology.

2) local information, if any. Local inspection, pick-up sampling, review of

3) aerial photography, photogrammetry, satellite imagery

4) core drilling programme

5) repeat 2-4 as required

the new WUM website and recent thread on here, is a good example of the process. The local surface geology is a classic example of the sort of igneous intrusion with mineralisation at the contact zone. Local knowledge and historic records provide a fair amount of incomplete detail about what is believed to be there. Current activities include driving a designed haulage to where the extraction zone is believed to lie, to be followed by core drilling from within the new workings to establich in more detail where the actual paying ground might be.

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As an exploration geologist I have been doing this for 50 years. On surface use geochemistry and geophysics combined with geology to locate the best areas to focus on. When previous underground workings exist, examin in detail all previous stope localities and vein directions for lithological and/or structural explanations for ore location.

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Interesting and informative as ever. Thanks to you all.