342 iV. 7^. Janner : 



Physical Alterations. 



Increase in density. 



With a view to deteiiuiniug the ehange in density due to the 

 alteration, the specitie giavities uf the Dry Creek and Diamond 

 Creek dykes were compared, with the following result : — Density of 

 little altered dyke at Dry Creek, equals 2.59. Density of the Dia- 

 mond Creek dyke from the 800 feet level of the mine, equals 2.72. 

 We see thus that a marked increase in specific gravity has occurred, 

 and this is largely exj^lained by the presence of pyrite in the one 

 and its absence in the other. The chemical and mineral changes 

 agree very well with those tabulated above, and the writer feels 

 quite justified in calling the alteration a " propylitic " one. In 

 the typical projiylitic alteration, chlorite is generally developed, 

 and the potash percentage is generally increased. The alteration 

 of the above dykes appears to differ from the typical propylitisa- 

 tion in these i-espects. The only previous cases of propylitisation 

 that have been described in Victoria are the Woods' Point dykes, ^ 

 and a proi^ylitised dacite^ from Macedon. The Dian]ond Creek 

 example differs from lioth of these in the absence of chlorite, and 

 also in the fact that the ground mass recrystallised in eutectio 

 proportions in both of the above cases, Avhile it has not done so in 

 the Diamond Creek dyke. 



(e) Byhc veins and slate reefs. 



Section A23. — Cold-bearing quartz vein in the dyke, Diamond 

 Creek mine. 



The vein is small, but very rich. The gold is seen without the 

 aid of a lens sticking out at points through the vein. A consider- 

 able amount of stibnite is present, giving the quartz a dark colour. 

 Under the microscope the gangue is seen to be practically entirely 

 quartz, and the metallic minerals are chiefly stibnite and gold. 

 Orains of a. translucent, highly refracting and apparently isotropic 

 mineral associated Avitli the stibnite appear to be the oxide senar- 

 montite. The section .shows that the gold occurs in anhcdral grains 

 and masses, frequently disseminated through the slilmite, and oc- 

 casionally intergrown Avith it. The intimate association of the 

 gold and the stibnite is well i(.'coguised l)y tlic minei-s. for they say 

 that wherever you find stibnite, gold is certain to be ]»rcsent. Lin- 

 colnS has examined twenty-eight sjxcimens of gold stibnite veins 



] I'lof. C!re!,'ory. Men. Geol. Siiiv. Vict., No. ::, l!)Or>, p. 34. 



'1 I'rof. Skeatsaiid H. S. Suniiuers, M.Sc. null. No. 24, Geol. Surv. Vii-t., 1012. 



;; V. C. Lincoln. Ceitain iiatunil associations of ;;ol<l. I'lconomic O.colojjf.v, \ol. vi., 1011, \\ 287. 



