The particles of metal (usually it was gold everyone was searching for) were too tiny to be seen by the naked eye, and too distributed (read "low grade") to assay with field methods. This was the primary reason these enormous deposits still existed, unexploited—legions of gold prospectors had passed them by on their way to more-obvious nugget and vein gold deposits.
Teams of field techs went out looking for clues that had nothing to do with outcrops of shining golden metal: They collected water samples that might show elevated copper, mercury or arsenic, other metals than gold that went into solution in the geothermal waters, and were then leached from the deposit by groundwater. They mapped permeable formations, and collected samples of sand and soil.
But the strangest thing they collected was plants. Imagine hunting for gold by snipping the growth tips from sage, jimson, and timothy grass, carefully mapping the collection sites, and returning them to the lab to be reduced to ash so the ash could be assayed. Other plants (such as Astragalus or "loco weed") are themselves bio-indicators of metal in the soil, and could be logged without sampling.
The resulting data from field logs and water, soil, and plant assays, collected by dozens of field techs, was sent to a central office to be mapped by the single mapping geologist. Me.
One of those field tech reports prompted my entry for the Carrot Ranch Flash Fiction Challenge this week.
March 18, 2015 prompt: In 99 words (no more, no less) write a story to reveal a character's symptoms. It can be something the character is oblivious to, or terrified about. It can be a character concerned for a pet or a motorcycle. The symptoms can be what ails society. Go where the prompt leads. Or sleep on it, and see what a dream brings to you!
I'm happy to share the slightly-fictionalized account with you. __________________________________
Bob dumped his field pack by the campfire, gently so the water vials in it wouldn't break, and rotated his paper log away from any sparks.
"George," he observed, "I don't know what's got into you! You been dragging and jerking around all day!" George rolled an eye at his partner, and stood mute.
"Better believe I'm logging your antics. We'll see what the home office thinks about your gold-bricking!"
Bob's notes about his horse George's symptoms would provide vital clues to the location of loco weed plants. Loco weeds fix arsenic, an indicator element for distributed gold deposits.