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 branching
 quartz-veins and sheaves of orelenticles
 in sheared country, which is
slate-like aphanite and quartz-andesite.
The sheared bands have been altered
by hot water into quartz-sericite-carbonate
 rocks, with epidote and chlorite.
The carbonates were formed by descending
 meteoric waters and were
followed by silicification and shearing
with the formation of secondary plagioclase;
 later the felspars and ferromagnesian
 minerals were converted to
an aggregate of quartz, sericite, epidote,
and chlorite. That the gold was probably
 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 ironstones,
 and connected with granitic and basic intrusions.