1922 J Hudson: Geology of the Cuyamaca Region of California 239 



(c) Limited miscibility of silicate and sulphide melts. After 

 studying the textural relations between sulphides and silicates in the 

 norites, gabbros, and other basic rocks that carry disseminated particles 

 of ore, there can be little doubt that there was extremely limited misci- 

 bility between the silicate and sulphide portions of the magma, immedi- 

 ately prior to its consolidation. If this was the case with small particles 

 of sulphide, it seems likely that it also held for the large masses. 



The theory of limited miscibility by itself, however, does not ex- 

 plain why the sulphides aggregated into large, fairly pure masses. 



Intrusion of sulphide magma. — Howe and Bateman, while stating 

 that some of the Sudbury deposits are the result of magmatic differ- 

 entiation in situ, believe that the most of them were formed by the 

 intrusion into already solidified norite of a sulphide magma, which 

 formed by differentiation in the magma reservoir from which the 

 norite came. 



At the Friday Mine there is strong evidence for differentiation, 

 in place, of the sulphide mass from the norite magma. Not having 

 seen the Sudbury occurrences and having examined only a few speci- 

 mens from that locality, the writer is not qualified to pass judgment 

 on the applicability of the intrusive sulphide theory to those deposits. 

 It is thought, however, that one of Coleman's objections to this theory 

 is worth noting. He says : 



As mentioned before, pyrrhotite-norite is invariably found above the ore bodies 

 in the marginal mines, and the enormous volume of this rock, running into cubic 

 miles, is quite unaccountable if the ore was segregated before the norite reached 

 its present position. These completely enclosed blebs of sulphides are like shots 

 of matte in slag where cooling has advanced too rapidly to allow of complete gravi- 

 tational separation. The pyrrhotite-norite probably contains as much ore as all 

 the mines of the region, and if half the sulphides of the original magma are still 

 enclosed in the rock, is it probable that the other half lagged behind and came up 

 after the norite had cooled and solidified f 74 



In other words, with a competent source, the disseminated sul- 

 phides of the norite, at hand, why deny the possibility of differentia- 

 tion in the norite body now exposed to view, and seek the locality of 

 this action in some deeper magma chamber? 



Epigenetic theories involving replacement. — The theories for the 

 origin of nickeliferous pyrrhotite bodies advanced by Campbell, 

 Knight, and Tolman and Rogers are based to a large extent on the 

 observed textural relations between the various ore minerals and 

 between these minerals and the silicates. 



t* Coleman, A. P., letter to editor, Econ. Geol., vol. 10 (1915), p. 392. 



