1 917 ] Clark: Geology and Ore Deposits of the Leona Bhyolite 379 



SUMMARY 



The Leona Rhyolite occupies a narrow belt along the front of the 

 Berkeley Hills from Hamilton Gulch at Berkeley to a point about five 

 miles southeast of Haywards. 



It comprises a lava flow that probably followed an erosional valley 

 along the contact of the Knoxville shale and the Franciscan. The 

 rock is very much fractured and has undergone alteration by surface 

 agencies. The rhyolite contains pyrite as an original mineral, the 

 oxidation of which has colored the surface reddish brown. 



The rhyolite is probably Pliocene or older, and has been quite 

 extensively faulted in post-Pliocene time, some of the movements 

 occurring in very recent time. 



The petrographical study of the rhyolite shows that it may be 

 divided into four distinct facies, based principally on texture. It 

 shows a gradual gradation from glassy and spherulitic through por- 

 phyritic to felsitic facies. This supports the field evidence that the 

 rock probably came from a common source and represents a single flow. 



The chemical analyses show that the rock is a soda rhyolite. 



East of Oakland, near Leona Heights, there are considerable masses 

 of potato-shaped bodies of pyrite in the rhyolite. 



The metallographic microscope shows that there are two types of 

 pyrite : one soft and massive, and another highly silicified and very 

 hard. The ore contains chalcopyrite in very small quantities, which 

 was mostly deposited contemporaneously with the pyrite. Traces of 

 secondary chalcocite have also been detected. 



The ore deposit was formed as a metasomatic replacement in the 

 rhyolite by descending meteoric waters. The ore body is in a frac- 

 tured zone determined by a fault in the rhyolite which allowed free 

 passage of meteoric water. The reducing condition of the ground 

 water may have been induced in part by carbonaceous matter from 

 the Knoxville shale that probably underlies the ore body at Leona 

 Heights. 



