If not then possibly a cold seep or basaltic weathering (plagioclase?). Does anyone have any idea of the Cu level?
I've not seen any of the details of the chemistry, but this will likely be a hot topic at the next LPSC.
The top of Tisdale which was APXS'd is either a weathered zone and/or has a inherent different chemistry or is a bedding plane/fracture which has a zone of Zn-rich material deposited in it (cold-water hydrothermal). That "weathered" unit has a distinctive light-ochre color is evident in other exposures along Shoemaker Ridge and should be examined further on the traverse to the "summit crater". I intuitively think of this yellowish colour as generally "limonitic", which could include almost anything, actually.
Remember, the original, deep crust here is from not only a warmer and wetter Mars, but a Mars with a hotter amd more mobile/active crust/mantle. The mantle of that era had more hotspots and plumes and there was extensive tectonic activity.
Meridiani Planum is on the edge of the Tharsis uplift, a major feature by any measure.
This primeval crust (Ur-crust) has since been pounded, mixed and blended into an impact breccia by countless impacts, and it has, among other things, has a much higher surface:volume ratio than rock and can be much more reactive to chemical weathering, weathering in the presence of water.
In a way, this is a glimpse at what the Earth may have been like in Hadean times, with tectonic cycles starting and bio-processes just beginning.
Exciting times, then and now.