Message in a Bottle. Kathryn Reiss

When Julie visits Gold Moon Ranch, a farming commune in California's gold rush country, the back-to-the-land lifestyle seems idyllic at first. But peculiar problems keep cropping up, almost as if someone was trying to shut down the commune! Then, on a secret trip to an abandoned gold mine, Julie stumbles on a mysterious message that hints at long-lost treasure. Julie is desperate to find it, before danger closes in on Gold Moon Ranch.