In Colorado? Isn't that above the 36° 30' parallel? (Missouri Compromise)
Also there is the issue of white miners absolutely not tolerating that.
The dirty little secret of slavery in the United States is that most of the opposition to it came not from people being concerned about the welfare of the slave, they were mostly concerned about the slave taking jobs away from them.
Slaves picking cotton was a job white people didn't want, so they more or less tolerated it, but the one thing that would have caused an instant confrontation and bloodshed was slaves attempting to do jobs whites wanted.
The parts of the country that were heavily anti-slavery back in the 1850s are the same parts of the country that are heavily Unionized (as in labor unions) today. They saw slaves as "scabs", and they would absolutely not tolerate any slave taking money out of their own pockets.
So no, you would have had an explosion of rage if anyone had attempted to use slaves to do that kind of work. I bet the US government would have stepped in and put a stop to it.
It is interesting to read your post and think illegal immigrants today instead of slaves 150 years ago. Goes along with some of the Unions tilting towards Trump, or at least not endorsing anyone.
Diogenes, you are daft as ever. Arizona and New Mexico were sparsely settled and had a history of slavery going back to the Spaniards and the Indians. If pro-slavery Southerners got there first, they would have been able to impose slavery and use slaves in the mines and as domestic servants.
The point of getting slavery in the territories was to get the votes in Congress to protect slavery in the areas where slavery was profitable. Missouri was a microcosm of this. There were small areas where cotton-growing and slavery were profitable, but slavery was maintained in other parts of the state to produce proslavery votes in Congress and prevent the state from tipping into abolition.