TECTONIC IMPLICATIONS OF 50 MILLION YEARS OF GOLD-FORMING EVENTS IN THE SIERRA FOOTHILLS AND KLAMATH MOUNTAINS, CALIFORNIA

Lode Au deposits formed episodically in the Klamath Mountains and the Sierra Nevada during 50 myr of tectonic events in central and northern California in the Late Jurassic through Early Cretaceous. After terrane accretion, sinistral-oblique convergence and major changes in plate motion accompanied early movement on the western foothills faults at ~160 Ma. Orogenic Au deposits in N-S veins first formed at this time in Grass Valley under a compressional stress regime within the formerly contiguous Klamath-Sierra arc. Change to a more transcurrent sinistral regime with continued fault movement led to a second Au event with E-W veins in Grass Valley at ~152 Ma. Additional Au deposition within the Klamath occurred during this period, prior to the ~135-130 Ma westward dislocation from the arc. At this time, the KM became tectonically and magmatically quiescent, whereas major events continued to shape the Sierra.

Sinistral movement along the terrane-bounding faults of the Sierra foothills continued until approximately 125 Ma, at which point major plate reorganization in the Pacific basin led to a reversal of strike-slip motion and change in the absolute motion of the North American plate favoring dextral-oblique convergence. All but one (dated at ~135 Ma) of the Au deposits in the ~190 km long Mother Lode belt cluster between ~130-125 Ma and can be related to the switch to dextral motion on regional faults. A few precise ages suggest even younger Au events in the Alleghany and Bagby districts, in the northernmost and southernmost Sierra, respectively, with these districts forming at the same time as changes in plate motion at ~115 Ma.

Both local geology and regional tectonics are essential in the formation of orogenic Au deposits, and the timing of these hydrothermal events is a critical aspect in the reconstruction of the Klamath-Sierra orogeny. The timing of lode Au deposition in California has helped further constrain the timing of fault movements and the dissection of the Klamath-Sierra arc.