238 University of California Publications in Geology [Vol. 13 



Applicability op the Various Theories to the Friday Mine Deposit 



Syngenetic theories. — Three theories have been advanced to ex- 

 plain the differentiation process by which syngenetic deposits of sul- 

 phides are formed in igneous rocks. The failure of these theories to 

 fully explain the origin of the Friday Mine deposit is shown by the 

 following observations : 



(a) Segregation according to Soret's principle. While in many 

 districts the pyrrhotite bodies occur at the edge of the irruptive masses, 

 in other localities, as for instance the Friday Mine, the ore bodies are 

 found well within the igneous mass. It might be thought that the 

 schist body of the Friday Mine acted as the cool surface toward which 

 the sulphides migrated. Again, certain outcrops of gossan that are 

 found along the norite contact north of the Friday Mine are thought 

 to represent oxidized pyrrhotite. If such is the case, we have here 

 examples of deposits along the contact, and as the contact is nearly 

 vertical Soret's principle might be urged as against the idea of gravi- 

 tative settling. 



Many ore bodies, however, belonging without doubt to the mag- 

 matic class, occur within igneous rocks at some distance from boun- 

 daries and with no relation to included bodies of older rocks. For 

 instance, of numerous chromite deposits studied by the writer in the 

 Coast Ranges of California, south of San Francisco, not one is located 

 at a contact. Of some 200 chrome deposits examined by Mr. N. L. 

 Taliaferro in the Sierra Nevada only one was directly on a contact 

 of the basic intrusive against older rock, and, taking into consider- 

 ation the width of the various igneous bodies, the remainder of the 

 deposits can be said to be well within the igneous rock. The writer 

 has seen one chromite deposit, at the Daisy Prospect, west of Jolon, 

 in Monterey County, where the ore bodies occur along the median 

 plane of a narrow body of serpentinized peridotite, and is informed 

 that similar deposits occur in Montana. 73 



From the above observations it seems very doubtful if Soret's 

 principle is the law under which magmatic ore deposits accumulate. 



(b) Gravitative settling. Evidently gravitative settling will not 

 explain the Friday Mine deposit, and it appears also inadequate to 

 account for the supposed ore bodies along the steep north contact of 

 the norite. 



73 Mr. Geo. White, oral communication. 



