If your property allows for it and you are up for an adventure, I would suggest making hydroform soil-cement pavers. The hydroform paver presses are made in Johannesburg, and lime for the cement is mined there as well. It's possible you could even rent one for your project. The soil would be from your site. You can't get much more local than that, and local means green!
If that isn't practical for you, I'd vote for the locally made metal tile, as most metals (you didn't mention which type) are recyclable and often have recycled content; plus which, again, it is a local material.
I tend to avoid slate because the quarried material leaves such a scar on the land. By that token one could argue the cons of lime and metal too, but in this case, I think they would win out over slate.