The Mother Lode (Spanish " La Veta Madre") is an incredibly rich vein of gold stretching 120 miles from Mariposa’s Bear Valley to Auburn in Northern California.  It was originally discovered in 1848. 

There’s still gold in them ‘thar hills’ (in Mariposa, about 30 miles from Yosemite National Park.)

There are still several working gold mines in Mariposa County and as the price of gold increases I’m sure there will be more.
Mariposa Creek which runs right behind downtown has been a source of gold since the discovery and many people still "pan for gold there". 
In the 1960’s a Gold Nugget about the size of a baseball was discovered by a County Road crew right in front of the Mariposa Hotel Inn in downtown Mariposa.