yO 
ECONOMIC GEOLOGY 
G. W. Card’s microscopic examination of the ores, and 
R. J. Frecheville showed (1898) that the lodes had been 
formed along crushed zones. Some bands were still regarded 
as slates of sedimentary origin until 
shown by C. O. G. Larcombe (Geol. 
Kalgoorlie, 1913, pp. 77-82) to be 
sheared fine-grained varieties of the 
country, which is mainly quartz-andesite 
and granophyric dacite. Some altered 
tuffs show that the rocks were in part 
volcanic. 
The lodes are of three types. In the 
N.E. of the field the Oroya-Brownhill 
lode is a curved sheet of quartz, which 
has been called a saddle-lode. The 
Associated Northern Mine is due to 
impregnation where dacite (quartz 
andesite) is faulted against tuffs (Fig. 
14). The third type, as in the Great 
Boulder Proprietary (Fig. 15) and Lake 
View Consols mines, consists of branch- 
ing quartz-veins and sheaves of ore- 
lenticles in sheared country, which is 
slate-like aphanite and quartz-andesite. 
The sheared bands have been altered 
by hot water into quartz-sericite-car- 
bonate rocks, with epidote and chlorite. 
The carbonates were formed by de- 
scending meteoric waters and were 
followed by silicification and shearing 
with the formation of secondary plagio- 
clase; later the felspars and ferro- 
magnesian minerals were converted to 
an aggregate of quartz, sericite, epidote, 
and chlorite. That the gold was prob- 
ably introduced by deep-seated waters 
before these changes is shown by the 
abundance of telluride. 
Further N. in West Australia are gold mines of a simpler 
character, associated with quartz-veins and banded iron- 
stones, and connected with granitic and basic intrusions.