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University of California Publications in Geology [Vol. 10 



the several important deposits cited above before knowledge of ore 

 genesis approaches completeness. In the abstract there are two 

 possible explanations; (1) either the sulphides form synchronously 

 at a lower temperature than the lime-silicates and thus occur near the 

 outer margin of metamorphism. or (2) the sulphide-rich solutions are 

 delivered to the contact zone later than those characteristically forming 

 lime-silicates, and replace the limestone in preference to the meta- 

 morphic rock earlier developed. Probably most students of the subject 

 will favor the latter view and in stong support of it is the occurrence 

 of apparent feeders at Seven Devils which connect the ore downward 

 across the garnet zone with the quartz diorite. Also in support of 

 this alternative is the broad observation based on mineral paragenesis 

 "that emanation of sulphides and magnetite continues after the con- 

 tact action, resulting in silicate rock, is completed." 18 



But the alternative explanation must not be too lightly considered. 

 If the sulphides form at lower temperatures than the lime-silicates, 

 their zone of deposition may be expected gradually to draw in toward 

 the source of energy as cooling of the intrusive mass progresses. This 

 implies a superposition of minerals of lower temperature range upon 

 those earlier formed at higher temperatures, providing that their 

 source of supply is not earlier cut off or exhausted. AVould not this 

 give the same relations of sulphides to silicates a.s would a sequential 

 supply of solutions of different type? 



It seems to the writer that the explanation of the phenomena 

 requires broader considerations than the paragenetic relations of 

 minerals in the hand specimen, although these have their value. It 

 is generally recognized that the sulphides in part, perhaps in large 

 part, develop contemporaneously with lime-silicates and that most of 

 them occur in small amounts as constituents of igneous rocks. Thus, 

 we would not expect to find a temperature control sufficiently marked 

 to afford the relations observed in the deposits herein cited. The fact 

 that the silicate minerals in most places are more widely distributed 

 along the contact than the ore minerals is also probably significant. 20 

 But the silicate minerals themselves are localized along the contact 

 in many places and this has been considered as indicating the escape 

 of the solutions through fissures in an outer magma shell.- 1 The par- 



i9 Lindgren, W., The origin of the "Garnet Zone" and associated ore 

 deposits, Econ. Geol., vol. 9, pp. 283-292, 1914. 

 =o Lindgren, W., loc. cit., pp. 285-286. 



2i Butler, B. S., Geology and ore deposits of the San Francisco and adjacent 

 districts, Utah, Prof. Paper, U. S. Geol. Survey, no. 80, p. 12, 1913. 



