104

ECONOMIC GEOLOGY

view has been supported by the discovery (G. M. Butler,
Econ. Geol., vii, 1912, p. 318) of the Louemma Vein (Fig. 33),
along a fault which passes down from the limestone and porphyry
 through quartzite into the underlying granite; it
is a primary ore containing 8 oz. per ton of gold, 27 oz. per
ton of silver, 11-1 per cent. of zinc, and 29-5 per cent. of lead.
The Leadville ores have been followed to depths of 1500
feet and have been traced far from the outcrop. Emmons
and Irving (U.S.G.S., Bull. No.
320, 1907)! showed that the
ores extend beneath the Devonian
 quartzite, where they
are away from the porphyry.
Nevertheless the ore-bodies in
general follow the distribution
of the porphyry. Those isolated
 in the Ordovician limestone
 may be near some undiscovered
 intrusion or due to
solutions which rose along the
course of the porphyry. The
Leadville ores do not contain
the lime silicates found in contact
 deposits and the country
hasbeen affected by the hydrothermal
 changes that happen
near dykes and sills.
Deposits analogous to those
of Leadville occur in the Magdalena
 field in New Mexico
(US.G.S., Prof. Pap. 68, 1910,
pp. 51-6), and are associated with quartz-porphyry and
granite-porphyry. Lead ores beside quartz-porphyry intrusions
 also occur in Kelantan in the Malay Peninsula.
BroxeN HiLr—The Broken Hill mining field in the arid
plains of Western New South Wales is notable for its special
geological features, its effect on metal prices and on metallurgy
and the many mineral species first found in its gossan. The
main lode is 3 miles long, in places 200 feet in width, and has

Gr

1 For results from later mining, cf. G. F. Loughlin, U.S.G.S., Bull
770, 1926.