Take a look at this article
" Putting The Geology Back Into Martian Science

Mars 2003 by Hubble
by Peter Ravenscroft
White Cliffs - Apr 16, 2004
If you find layered sediments, they do not have to be either water-transported or the outpourings of volcanoes. They may be aeolian, that is, wind-blown. On Mars, they may also be a bit strange. Those are simple facts that the Martian Wet Worlders seem to generally forget, or, more to the point, resolutely ignore. "
at http://www.spacedaily.com/news/mars-water-science-04c.html
An alternative dry model for Mars which addresses the same issues of dust from a geological view point.
The sulfate story is interesting. It is a ubiquitous element of the Martian regolith. Overtime all the gases from volcanics activity or meteorite/ cometary impact have remained on the surface or been lost back to space. My feeling the diricrust seen by Viking 1 and the sediments at Eagle Fram etc are of "recent" origin and not exhumed >3.6 Ga watery sediments as NASA would have ( see Squyres comments at http://www.spacedaily.com/news/mars-life-04d.html). What little NASA has shown of the trace element values suggest in-situ alteration of basalt by acids (SO4, Cl, Br) , as you suggest,without any diffrerential transport of different metals that is commonly seen with flowing acid saline waters.