Thanks again, useful info.

As for economic geology, even if the outcrop were 100% pure heroin I don't think that'd cover transportation costs anytime soon, but it is an intriguing thought.

As for "fluids", hmm, I wonder if you can create a thermal fluid system based on liquid CO2 instead of water? The theory is that there are liquid nitrogen geysers on Triton so stranger things are possible. And if so, what kind of crazy mineral deposits would THAT give you?

On the other hand, I think Nick's big objection is to liquid water having had a major role in the surface erosion we see, or to there being near-surface aquifers in anything like present Mars conditions. If you have enough heat to drive an underground liquid CO2 thermal system, shouldn't that also be enough heat to drive a hydrothermal system? Sure, CO2 freezes at a lower temp than water, and that difference is very relevant to current Mars surface temperatures, but it doesn't amount to much relative to a near-surface magma chamber.