Mid-1870s. The 'real' Deadwood was founded in 1876. The show opens weeks after Custer's last stand, which took place on 25 June, 1876. And the murder of J.B. Hickok, which took place on 2 August, 1876, is in (IIRC) the third episode. So it's safe to say the show opens between those two dates.
Both the Cornish and the Welsh were legendary miners because both tin and copper were abundant in Cornwall and Wales. That made them one-stop-shopping back in the Bronze Age (ca. 3300 - 1200 BC) because bronze is an alloy of copper and tin.
They Cornish have been using steam engines to "dewater" mines since at least 1714. Wales and Cornwall both are coastal regions and they had hand-dug mining tunnels that went down as much as 1000 feet and then ran horizontally as much as a mile offshore and beneath the sea. All dug with pick-axes, shovels, and sweat. And it took serious horsepower to pump water up from those depths, well more than what humans or draught animals could produce.
The Cornish pasty meat pie was made like it was so the miners could hold it by the braided crust as they ate it so as not to get the powdered tin covering their hands on what they were about to eat. Then they'd lose nothing but a bit of bread when they threw out the contaminated crust.
That's why they would have imported miners from the other side of the Atlantic. Because they long since were experts in digging and managing the mines. Even figured out how to eat down there without cleaning up before or killing themselves after.
Yes you’re right. Near the show’s opening they flash the date on the screen I must’ve got it mixed up with another show. It was 1876.