As to placer mining in the Slacks Canyon area, I don't know. However, in Stone Canyon, over the ridge to the east, was the Stone Canyon Coal mine. It was worked off and on apprx. from 1900 to 1930's, with not much production. They did run a railroad from the mine to across the Salinas River to meet the Southern Pacific RR.
As I research local history, are you related to any of the early families of the area? Maybe we could have info to share.