32

ECONOMIC GEOLOGY

gold so thoroughly that it was the finest that has been found
and contained 998 parts of gold in 1000. The microscopic
structure of the quartz gives no support to the geyser theory.
The mine (Fig. 16) is on an ancient volcano, as observed
by the author in 1909, as the quartzite proved to be quartzporphyry
 and the slates to be volcanic tuffs and altered
spherulitic basalt with crusts of variolite. The formation
of the mine began during the Paleozoic (Newman and Campbell
 Brown, Tr. Austral. I. M.E., xv, 1911, p. 443) with the
injection of a complex series of dykes, followed by the intrusion
 of diorite which so weakened the crust that the
igneous rock reached the surface and established a volcano.
Its base was intruded by a second series of dykes and after
the volcano became extinct, the rocks were saturated by

FO.

WW,

F

Fig. 16.—SECTION ACROSS THE MounT Morgan MINE, QUEENSLAND,
The ore-body (O.B.) traverses mainly variolitic diabase (V) which is associated
 with a band of limestone and quartz-porphyry (dotted), To
the west is an area of old granite covered by Desert Sandstone with
placer gold (X) indicating the pre-Cretaceous age of the ore-deposits.
0.C. is the open cut; black lines, diabase dykes. F.O. former outline
of the hill.

waters which deposited sulphides of iron and copper rich
in gold. The plutonic origin of these waters is indicated
by the presence of tin oxide, cassiterite. A large mass of
pyrites was deposited by replacement beside the volcanic
pipe. The country was worn down to a peneplane on which
was deposited the Upper Cretaceous Desert Sandstone, which
contains placer gold from the mine. As the country was
lowered by denudation, surface waters dissolved silica, gold,
and iron oxide and re-deposited them as the rich gossan.
The upper part contained 3 oz. of gold to the ton; the lower
part of the open-cut was enriched to 6 oz. to the ton, below
which the grade fell to 14 oz. Pyrite was deposited below
the enriched oxidized belt, but the copper was carried lower
and deposited in the underlying pyritic ore-body beside the