I’ll bet it’s mostly chemical brine. Been through DV at least a dozen times and the flats there are loaded with all sorts of minerals...remember Death Valley Days and the Borax mines? I’ve even got a huge satellite poster of it in my living room much like one of the pictures in the article.
I remember the late 1960s or early 1970s ad from Gulf Atomic, showing kids drinking and playing Death Valley water that had undergone reverse osmosis from "ROGA units" (reverse osmosis Gulf Atomic).