148 PROCEEDINGS OF SECTION C. 



14.— AN INVESTIGATION OF THE RELATIONSHIP BETWEEN THE- 

 ORE BODIES OF THE HEEMSKIRK-COMSTOCK-ZEEHAN 

 REGION AND THE ASSOCIATED IGNEOUS ROCK. 



By L. KEITH WARD, B.A., B.E. 



[Plate XII.] 



CONTENTS. 

 I. — Introduction. 



II. — Summary of the Geology of the Region. 

 III. — ^The Igneous Rocks of Devonian Age and the Relationship 



between them. 

 IV. — ^The Metallic Ores and the Relationship between them, 

 V. — ^The Geological Relations of the Ores. 

 VI. — ^The Relationship of the Ore-Bodies to the Igneous Rocks — 

 [a) The Source of the Metallic Ores. 

 {b) The Mode of Derivation of the Ores from the Magmatic 



Hearth, 

 (c) The Precipitation of the Metallic Ores. 

 VII. — Conclusion. 



I. — Introduction. 



Consequent upon the detailed study of a large number of different 

 varieties of ore-bodies in many parts of the world there has arisen 

 a fixed belief, on the part of the great majority of mining geologists, 

 in the doctrine of the genetic relationship between igneous rocks 

 and certain metallic ores. 



The exact meaning of this consanguinity is but seldom stated 

 at all ; and in many Ccises very different views appear to be held 

 by geologists with regard to the nature of the relationship. 



It appears to the author of this paper that there is a need for 

 the detailed statement of the case, if only for the benefit of those 

 to whom is entrusted the work of exploiting the ore deposits. When 

 engaged upon the geological investigation of mining fields the geolo- 

 gist is brought into touch with mine managers by whom the doc- 

 trine of the inter-relationship between ore deposits and igneous 

 rocks is accepted. But to many of these the actual genetic 

 relationship is not apparent. The association has been noted and 

 perhaps compared in a general way with other similar associations 

 in the same or in a neighbouring mineral field. 



In order that accidental characteristics of similarity or 

 dissimilarity may be distinguished from essential ones, and thus 

 that sound comparisons may be drawn, a much fuller appreciation 

 of the significance of association is necessary. The more 

 detailed investigation assumes a directly practical aspect when 

 the attention is attracted towards alterations in the character of 

 the primary ores. These alterations are found to take place in 

 all ore-bodies, and those to which most attention is naturally 

 given are characterised by an impoverishment of the lodes in the 

 particular minerals which are the immediate object of mining 

 operations. Confronted with such phenomena many mine 

 managers find that their conceptions of the genesis of the ore-bodies 



