604 MINERALS AND VEINS OF jEGIRINE-SYENITE, 



Modern treatises on ore deposits* describe classes of lodes, quite 

 apart from the well-known stanniferous pegmatites and like 

 formations, closely connected with volcanic intrusions and 

 evidently of related origin with pegmatites. 



The best known cases in Australia are those described by E. 

 C. Andrews! genetically connected with the great granitic intru- 

 sions of the New England District of New South Wales. 



Here in South Australia, where pegmatisation is evidenced on 

 an excessive scale, there is no end to examples illustrating its 

 connection with ore-bearing reefs, both oxide and sulphide. Such 

 usually contain complex ores, and in addition the ore-bodies are 

 generally very irregularly distributed ; mining development, 

 consequently, seldom leads to profitable results. 



The aplites are, therefore, at one extreme of a series of related 

 rock forms differing progressively in abundance of mineralisers 

 (water chiefly) contained in the original magma or solution. It 

 is contended that the origin of aplite is to be ascribed to ruptur- 

 ing of the recently solidified cooling magma, due perhaps chiefly 

 to contraction forces, % followed by consequent inflow of yet liquid 

 material, partly by sweating from the sides, but largely augmented 

 by outwellings from the interior extension of the fissures where 

 they approach the unconsolidated magma. Infillings originating 

 in this latter manner would likely show flow-structure and be of 

 the nature of dykes within the parent mass; at their source, 

 passing from the condition of schlieren, they would blend as one 

 with their surroundings. Hydrous conditions being more in 

 force at the margins of intrusive masses, such locations would 

 be characterised by the occurrence of pegmatites in contraction 

 fissures, whilst aplites would be more abundant interiorly. 



* See especially Dr. E. Beck's, " The Nature of Ore Deposits," Trans, by 

 W. H. Weed, 1905. 



+ Records Geol. Survey N. S. Wales, 1905, Vol. viii., Pt.l, p.22. 



J Plain-sided, uniform-textured aplite veins showing little deviation in 

 direction, and continuing for long distances, most likely occupy fault fissures 

 developed during solidification. 



