The crater, with its depth would be a natural place for artesian flow. Even if the crater was filled with water, hot springs could deposit minerals subaqueously. However, these would take a form more like black smokers, I would guess. I have seen one place on mars, in the examples of exhumed craters on Malin's site, where it had every appearance of being a hot spring terrace. There were many holes, I thought to be karstic sinkholes, but maybe these were the actual throats of springs. Perhaps you know the image, Nick. I forgot where on Mars it is. I have only looked at these a few times, but they completely changed my opinion about past Mars.

In my useage of the term "salt," I meant more generally, including the Sulfate salts. I would guess that these are plastic like Halite too. Do you know anything about Nitrate at Mars, or Nitrogen content in the rocks? Earth only has Nitrate in Atacama, but on Mars, I would expect for it to be a common late stage evaporite mineral.