Ah "the stuff of volcanoes" - like H2SO4, HCl, HBr - the acids needed to form jarosite. Nice little salt lake NASA has there - acid and very saline past the point of halite precipitation if we are to get Br salts also precipitated. And all the while the waves are lapping on the shores forming the sediments, we are staying in the very narrow stability field of jarosite and not forming goethite. An example of all these process can be seen in Lake Tyrell in Australia and there are many other aspects of acid-saline water/ rock geochemistry which must be met to sustain the NASA hypothesis, not just the few tibbits they have released to date.