MAGNETITE GARNET ROCKS COULSON. 



IV. GENERAL DISCUSSION. 



As the specimens are morainic Ixmlders there is no field evidence bearing on the 

 question of their origin. The nature and character of the magnetite-schists Nos. 912 

 and 294, the quartz-magnetite-schist No. 296, the epidote-magnetite-schist No. 989, 

 the mica-magnetite-schiste Nos. 55 and 933, and possibly also the garnet-magnetite- 

 gneiss No. 827 indicate that they very more likely to be derivatives of sedimentary 

 rocks rather than igneous rocks. The probability is not so high in the case of the garnet- 

 magnetite rocks. The evidence of the chemical analyses of the three specimens in 

 Table V is not decisive. There is a considerable excess of alumina over the 1 to 1 ratio 

 with the lime and alkalies and this, combined with the mineral content, suggests 

 derivation from a sediment. On the other hand, CaO is in excess over MgO in two 

 analyses and NajO is in excess of K 2 O. Less weight is attached to these points and in 

 general it is believed that a sedimentay rather than an igneous origin is to be ascribed 

 to the whole class. Yet the chemical composition is clearly not that of a common 

 sediment. 



An interesting comparative occurrence is the quartz-garnet-magnetite-schist from 

 Broken Hill, New South Wales. It occurs* in the neighbourhood of the Broken Hill 

 lode in discontinuous lenses along bands which dip conformably with the associated 

 garnet-sillimanite-gneisses. A specimen from a locality, one mile south of Menindie-road, 

 4 miles south of Broken Hill, has been studied. It is a schistose rock composed of 

 magnetite, garnet, quartz, and apatite in the proportions given in Table I. It may be 

 noted that the proportions of iron ore to garnet are approximately the reverse of that in 

 No. 102, its nearest relative among the Cape Denison specimens. While No. 102, with 

 2 per cent, of apatite, is comparatively rich in phosphorus, this is greatly exceeded by 

 the Broken Hill specimen '-with 14-2 per cent, of apatite. 



The chemical analysis of the Broken Hill type is quoted in Table V, and the 

 negligible alkalies, the low magnesia, and high iron are strong pointe of resemblance. 

 The differences in silica percentages correspond with differences in the amount of quartz. 

 The percentages of lime and phosphorus are much higher than those in No. 102. corre- 

 sponding to the abundant apatite. The Broken Hill type contains 7-08 per cent, of 

 manganese and is comparable with the 8-23 per cent, in No. 348, where the garnets are 

 manganiferous. 



There are thus striking resemblances between the Broken Hill type and the 

 garnet-magnetite specimens from Cape Denison. The Broken Hill type is described by 

 Andrewsf as a lode formed from emanations given off, apparently at a depth, from 

 igneous material along a crush zone, and further formed by replacement. He pointe out 

 (p. 174) that the garnet- magnetite type is associated with garnet-sillimanite rocks, while 

 the quartz-magnetite types, without garnet, are associated with mica schists. Browne J 



" Geology of Broken Hill." E. C. Andrew* Mem. 8, Oeol. Surv. N.8.W.. 1923. p. 171. 



t " Geology of Broken Hill District." K. 0. Andrrws. Owl. Surv. .V.S.W., No. 8, 1922, p. 181. 



? Appendix I of " Geology of Broken Hill Ditri< t." W. K. Browne, p. 340. 



