I posted this a week or two ago but have clickable links now and I added some stuff.

The Opportunity Ledge reminded me of Hot Spring Deposts or evaporites from the first micro-image.

Given: the Outcrop contains alot of sulfur and no (?) silicates, carbonates, or hematite (at least the matrix).
The data has also been called "interesting". I understand from some of this reading that sulfate signatures on some of the instruments on the rovers are, well, interesting, and difficult to discern from some other rock types. Could explain some things.

Therefore I've been reveiwing sulfate mineralogy (anhydrite, gypsum, barite, etc.) and their occurance (evaporitic and volcanogenic).

The outcrop sure as hell looks like some sort of evaporite or sinter (AKA tuffa, exhalites, etc.) to me (seen plenty o' both) so I've been reviewing that stuff too. Thanks for the ever so cool stereo photos NASA, you can keep my tax cut if you send more rovers!

All kinds of interesting stuff is coming through (Gawd I wished I had the web back in grad school!).

Here's some stuff to chew on:

Some of these articles come from a Geological Society of America conference back in 2001. The program for this session can be found here:

Here are a couple that have insights to our little puzzle:

EVALUATION OF ALTERATION ZONES IN ARCHEAN IGNEOUS ROCKS AS POSSIBLE SITES FOR CARBONACEOUS FOSSIL MICROBES

EVAPORITE MINERALS IN MARTIAN SAMPLES: CLUES TO THE NEAR-SURFACE HISTORY OF MARS Evidence of sulfates on Mars from meterorites.

MARS REMOTE SENSING ANALOG STUDIES IN THE BADWATER BASIN, DEATH VALLEY: ANALYSIS OF GROUND TRUTH This is a good one for our "interesting" spectra problem.

Some things I turned up by searching the key words "Hot Spring" + "gypsum".

The Rye Patch Hot Spring a Pleistocene sinter containing gypsum, sulfur, mercury.

Yae-Kokonoe and Miharashi open-air baths which displays:

Temperature at origin: 68ºC (154.4ºF)
Temperature of bath: 45ºC(114ºF)
Characteristics:
This hydrogen sulfide spring has clear, colorless water with floating particles of metal sulfides and gypsum.

Looks like a nice place, eh?

Oh, I'm still reviewing sulfosalts and sulfarsenides, too. And evaporites. What would a hot spring look like if expressed within an evaporite deposit? I'll try to pull some things together to post.

Enjoy the reading.