16
ECONOMIC GEOLOGY
STAKE, ALASKA, AND Porcurine—In some goldfields the
solutions, in the absence of fissures, percolated through
irregular pores and spaces, and have thus produced impreg-
nations of gold ore, usually with pyrite. These ores are
usually low grade and have to be treated in bulk. Such
impregnations occur in Rhodesia (Tv. 1.M.E., 1906, XXXI,
P- 87) as mineralized schists along faults in the Gaika Mine,
in diorite dykes in the Ayrshire Mine, in crushed granite,
and in a complex of quartz-veins in a shattered mass of
Banded Ironstones at the Wanderer Mine. These impregna-
tions vary from rock with a sprinkling of auriferous pyrite
to ore-bodies which have completely replaced the country
rock.
The Homestake Mine at Lead in the Black Hills of South
Dakota is famous for its vast bodies of low-grade ore in
pre-Paleozoic rocks that were laid down as sandstones,
dolomitic limestones, and clays, and have been altered by
regional metamorphism into gneiss, garnetiferous-mica-
schists, cummingtonite {an amphibole) and chloritic schists,
quartzite, and crystalline limestone, The schists were bent
into crowded and overturned folds. In the Keewenawan
Period the area was intruded by amphibolite dykes, and worn
down to a plain which was covered unconformably by Cam-
brian quartzites. The field was invaded by Eocene rhyolites
and phonolites ; a dyke of rhyolite was forced along the crest
of the Homestake anticline, with minor dykes in all directions,
Later the ground was again disturbed by earth-movements
which fractured these dykes and formed along them layers of
pug. The ore-bodies occur in the Homestake Formation,
which consisted originally of magnesian limestone. The
schists near the Eocene rhyolites are traversed by many
“verticals,” or thin seams of pyrites, pyrrhotite, and quartz.
According to J..D, Irving, the ores are pre-Cambrian,
and provided the placer gold in the neighbouring Cambrian
conglomerate. According to Hosted and Wright (Eng. and
Min. Fourn. Press, cxv, 1923, pp. 793 and 842) the ores are
of Eocene age, and the gold in the Cambrian conglomerates
is not alluvial, but was introduced by infiltration, The
pre-Cambrian age of the Homestake ore is maintained by
S. Paige (U.S.G.S. Bull., 765, 1924, p. 42), who supports
Irving's view and considers that the Kainozoic gold. some of
