58

ECONOMIC GEOLOGY

Transvaal; the ore is at two horizons in the pre-Palazoic
basic rocks of the Bushveld Complex. The lower horizon
is a thick band of silicified serpentine which, according to
P. A. Wagner and E. T. Mellor, is an altered dunite containing
 a variety of olivine, hortonolite. This dunite occurs
in veins and nodules and is traversed by veins of quartz,
chalcedonic silica, and magnesite, so that it has been subject
to hydrothermal action. A pegmatite—with black hornblende
 crystals 3 inches long, and diallage, phlogopite, and
magnetite—is associated with the hortonolite-dunite.
According to Stanley and Wagner (F. Chem. Soc. S. Afr.,
xxv, 1G25, pp. 254-9) the platinum is a primary constituent
of the dunite; but it frequently occurs as irregular interstitial
 grains that solidified after the other constituents.
The upper horizon of platinum is in a diallage-norite that
contains pyrite and chalcopyrite, which are presumably
secondary.
A third platinum occurrence in the Transvaal is in a quartzlode
 in the Waterberg Sandstone (pre-Cambrian). This lode
has been formed by solutions circulating along a fault,
which is post-Karroo and therefore post-Lower Jurassic.
Some of the platinum is a replacement after pyrite. The ore
has yielded about 24 dwt. to the ton.
Platinum has been found in Rhodesia with sulphides in a
long dyke-like band of diallage-norite at Makwiro. The
rock contains up to 3 dwt. of platinum to the ton, but the
particles are said to be so minute that they float on water
and cannot be recovered by washing. R. S. Lightbody
(Rep. 19, S. Rhod. G.S., 1926) remarks that the presence of
the platinum is not easily explained, as it is not in the heaviest
of the norite.
Brisa CoruMsia—Platinum has been found at Tulameen
in the Rocky Mountains of British Columbia in a Jurassic
magnetite-olivine-diallagite (koswite), which has been hydrothermally
 altered (Poitevin, Canada G.S., Summ. Rep.
1013, pp. 84-101). The koswite surrounds masses of dunite,
which also contains platinum.
Genesis OF PraTinum OrEs—As chromite, the mineral
most often associated with platinum, is an essential constituent
 of dunite, the view has been adopted that platinum
is a primary constituent of that rock, and is normally of