Arm Waving here:

RE: the regional extent of the Opportunity Formation, (even though I can't officially use the term formation until it is proven to be the same rock in other places), some quick thoughts and obs.

Hot springs occur in groups, typically along fault lines, and can be fairly extensive. I'll see if I can't find some dimensions somewhere. Check out the Norris Basin in Yellowstone, it's big, but not regional.

However, Hydrothermal Cells, the convective ground water cycle that creates hot springs can be pretty extensive. Some deposits occur below the surface, by changing the rock (alteration) which can lead to complete replacement of the original bedrock, and by deposits that occur in lateral transport zones.

Something to remember. A hydrothermal cell needs a long lived heat source. The typical source is a deep magma chamber, again, think Yellowstone, or faults deep enough to get some internal heat. But they also form in thick lava flows, either mafic or felsic.

Also, I don't think anything I've seen rules out a volcanic origin. Imagine this: there is a big, regional ash fall or pyroclastic flow. It's pretty thick so it takes a long time for the center to cool. If there is a subsurface fluid about (note I said fluid) then that will create a hydrothermal cell, which then alters the rock on a regional scale, which also vents at the surface creating localized areas of hot springs deposits.

Oh, another obs. that doesn't need a new post. Are those long etched areas in the microscope images on the top of a bed and not in a x-section view? I may have to compare the time of the images and compare them to the hazcam images to see exactly what we be look'n at.

Barite (BaSO4) is a very interesting sulfate, by the way, with tabular crystals that often incorperate small grains in their structure. Google "barite roses" for some examples. Barite occurs in both hydrothermal and evaporitic enviroments.