1922] Hudson: Geology of the Cuyamaca Region of California 235 



Theories op Epigenetic Origin, Involving Replacement 



Previous to the work of Bell, Barlow, and Vogt, the deposits of 

 nickeliferous pyrrhotite were thought by all investigators to have been 

 formed after the formation of the inclosing rocks by pneumatolytic or 

 hydrothermal agencies. 



Thus in 1888 Collins advanced the theory that the Sudbury de- 

 posits were concentrations of the copper that was originally dissemi- 

 nated through the clastic or fragmental beds, this concentration taking 

 place after the intrusion of the diorite. He was impressed by evidence 

 of veining action and faulting and failed to note deposits enclosed 

 entirely within the igneous rock. 57 



Posepny, referring to the theories of Bell and von Foullon, stated 

 that "these surprising statements assume a chemical impossibility, 

 namely, the presence of metallic sulphides in the magma of the molten 

 eruptive rock .... on the strength of metallurgical analogies." 38 



This objection has not been put forward since Posepny 's time. 

 There seems to be no theoretical basis for it, and, as a matter of fact, 

 sulphides have been collected in samples of molten volcanic rock. 



Dickson from the results of a careful petrographic study of Sud- 

 bury ores concluded that they were of epigenetic origin, formed by 

 replacement processes along crushed and faulted zones. 50 His obser- 

 vations pointing to this conclusion are : 



(1) Brecciation, faulting, and shearing are everywhere characteiistic. (2) 

 The main brecciation and shearing was anterior to formation of the ore bodies 

 proper. (3) Abrupt contacts of ore and barren rock and angular nature of 

 rock fragments in the ore seem irreconcilable with magmatic theory. (4) Ural- 

 itization and ehloritization of the rocks is widespread and where fresh pyroxene 

 remains it is brecciated. (5) This alteration is most marked near the ore bodies. 

 (6) In general, the more complete the alteration of the rock the more complete 

 has been its replacement by sulphides. (7) In all cases the sulphides show a 

 tendency to occur along lines of weakness and in connection with fibrous min- 

 erals. (8) Secondary quartz and calcite are often present in the ore in appreci- 

 able amount while they are insignificant or lacking at a little distance. (9) 

 Sulphides are practically lacking in the rock a short distance from the ore. The 

 rock fragments included in the ore are also comparatively free from ore, except 

 in veinlets. 60 



Weinschenk studied the nickel deposits of St. Blasien and ob- 

 served phenomena much like those noted by Dickson in the Sudbury 

 ores. The rocks here are thoroughly altered to a mixture of uralite 

 and saussurite. The ore occurs invariably in the most altered rock 



67 Collins, J. H., Quart. Jour. Geol. Soc. London, vol. 44 (1888), pp. 836-837. 



Posepny, F., Am. Inst. Min. Engin., Trans., vol. 23 (1893), p. 330. 

 so Dickson, C. W., A. I. M. E., Trans., vol. 34 (1903), p. 63. 

 wlbid., pp. 59, 60, 61. 



