Western San Gabriel Mountains, CA

Stop #2:  Placer Gold deposits of Placerita Canyon & the San Gabriel Fault Zone

Placer Gold

Most Californians are aware of the 1848 gold rush in California’s Sierra Nevada started with the discovery of gold along the American River by James Marshall. However, there was an earlier gold rush in California that was much smaller and not as well-known that occurred in southern California. In 1842, Francisco Lopez, a cattle herder for a local ranch, was taking a noon break with two companions at an oak tree in Placerita Canyon, when he decided to look for gold in a nearby grove of Sycamore trees. He dug around the roots a bit and, indeed, found gold. His discovery began a short gold rush that netted approximately 125 pounds of gold (worth about $3,400,000 at today’s prices) in 6 years. About 2,000 prospectors were involved in the gold rush.


The mineral gold mostly forms within fractures in bedrock where hydrothermal fluids (hot water with dissolved atoms) are flowing. The gold deposits are formed 1 atom at a time as the gold precipitates from the water. Gold that is found in fractures within bedrock is known as “mother lode” gold.

In some places, the bedrock gold has been exposed at the surface, eroded by water or ice, and carried to another location where it is deposited (“placed”) in alluvium, which is sediment deposited at the Earth’s surface. This gold deposit type is known as “placer” gold. The gold that Francisco Lopez found in 1842 was the placer gold and this is how Placerita Canyon derived its name. It is not known where the mother lode gold from which the Placerita Canyon gold originated is located, but the source may have been completely removed by erosion such that there is no longer mother lode gold at the surface.

 

 

San Gabriel Fault Zone – Honor Ranch Segment

The San Gabriel Fault zone is a complex network of strike-slip faults that extend northwest through the San Gabriel Mountains, roughly paralleling and pre-dating the San Andreas Fault Zone (Matti, Morton, & Cox, 1992). Numerous roughly east-west trending valleys extend through the San Gabriel Mountains as a result of this fault zone.  The Honor Ranch Segment is a part of the South Branch of the San Gabriel Fault Zone, and travels down the axis of Placerita Canyon.  The fault in this location is obscured by the alluvium covering the canyon floor, which makes locating a fault with accuracy difficult.

 

Geology

The rocks that comprise Placerita Canyon State Park includes the following (Battle, 1939) (Seward, 1974) (Yerkes & Campbell, 2005):

  • Saugus formation (late Pliocene to early Pleistocene):  Marine, brackish, to non-marine cobble to pebbly sandstones. (Ballard, 2018)
  • Pico formation (Pliocene):  Marine clayey siltstone and sandy siltstone
  • Placerita formation (Paleozoic):  225 million-year-old granitic gneisses.