300 AUSTRALASIAN ANTARCTIC EXPEDITION. 



believes these rocks to be pegmatitic derivatives of the gabbro magma which are 

 intruded as a metasomatic replacement of a zone of country rock. Further, these 

 authors appear to regard the garnet as a primary igneous mineral. 



The comparison with the Broken Hill occurrence should therefore imply that the 

 garnet-magnetite rocks are derived from an igneous source. 



Another related group of rocks are the " Skarn " rocks of Norway and Sweden. 

 Skarn* is an old Swedish mining term for the sillicate gangue of certain iron ore and 

 sulphide deposits of Archaean age. Its use has been extended by Goldschmidtf to all 

 lime-iron-silicate rocks which arise through the contact metamorphism of limestone, 

 including andradite rocks, hedenbergite rocks and andradite-hedenbergite rocks. The 

 iron ore of the skarn rocks (including garnet-magnetite rocks) is arranged in layers 

 parallel to the schistosity or bedding. Goldschmidt is sceptical of the view that such 

 rocks are magmatic differentiation products a view adopted by Browne for the Broken 

 Hill type. Goldschmidt considers them to be pneumatolytic contact rocks determined 

 by three factors, (1) pneumatolytic action in depth, (2) presence of a limestone, and (3) 

 absorption and enrichment of the constituents of the magmatic gases. 



The skarn formation from limestone takes place through increase of silica and 

 iron. It is supposed that iron chloride is introduced in gaseous form along joints and 

 fissures with the result 



2 FeCl 3 + 3 CaC0 3 = Fe 2 3 + 3 CaCl 2 + 3 C0 2 . 



In this way the limestone collects the iron from the issuing magmatic gases. The 

 silicon is probably introduced as a halogen compound which reacts with the calcite to 

 form quartz. Quartz then combines with calcite forming wollastonite which, when 

 there is sufficient iron oxide, combines to form andradite. 



3 CaSi0 3 + Fe 2 3 = Ca 3 Fe 2 Si 3 12 . 



Wollastonile. Andratite. 



Manganese, which is a notable component of many skarn rocks, is introduced in 

 the same way as the iron. 



MnCl 2 + CaC0 3 = MnO + CaCl 2 + C0 2 . 



The MnO, however, generally enters the silicate. 



Goldschmidt thus considers that the skarn rocks are the result of metasomatic 

 pneumatolytic processes acting on limestones, and their unusual composition is 

 explained by the acquisition of material from the magmatic source. 



Somewhat similar rocks at Morenci, Arizona, are considered by LindgrenJ to 

 have been derived from limestones by the addition of large amounts of ferric oxide and 

 silica. 



* " Nomenclature of Petrology." A. Holmes. London, 1920, p. 211. 



t " Die Kontaktmetamorphose im Kristianiagebiet." V. M. Goldschmidt, Ohristiania, 1911, p. 211 et scj. 

 J " The Copper Deposits of the Clifton-Morenci District, Arizona." W. Lindgren, U.S. Geol. Surv., Prof. pap. 43. 

 1905, p. 160. 



