“Dry” (or barren) silica-carbonate alteration

“Dry” (or barren) silica-carbonate alteration

Image by Pete Tillman
Northern SLO County, for a short time in the mid 19th century, was among the leaders in mercury (quicksilver) production in California. Mercury was then used to recover fine gold from placer "clean-ups" — and still is, in small-scale placer mining worldwide.

There’s a LOT of quartz veining and stockworks in the rocks exposed along the northern SLO coast. (There’s plenty more in the backwoods, but it’s harder to spot, and generally off-limits to geo-tourists). So this is an example of barren quartz mineralization, that the early-day prospector could (and did) reject.