215

(2) The auriferous constituents of the unoxidized parts of the veins
are the sulphides, arsenopyrite and pyrite. There is no evidence that the
eroded parts of the veins were richer in primary free gold than the remaining
parts. It is probable that they contained minerals similar to those of the
lower parts.

(83) Deep decomposition of the veins permitted oxidation of the sul-
phides and removal of the soluble constituents. Part of the fine gold thus
set free from the sulphides formed enrichments in the oxidized parts of
the quartz veins. Gold enrichment also took place by a process of
alternate solution and deposition of the free gold in the form of crystals,
crystal groups, plates or veinlets, and irregular masses, in cracks and cavities
in the veins and adjacent country rock near the base of the zone of oxida-
tion. The crystal groups, plates, and irregular masses thus formed, and
subsequently modified by the action of the streams, are the main source
of the nuggets in the gravels.

(4) The processes by which alternate solution and deposition of the
gold took place are not definitely known, but it is probable that removal
was effected chiefly by chloride solutions. Abundant precipitants in the
form of siderite, pyrite, and carbonaceous material are present.

(5) During late Cretaceous and (or) early Tertiary time the area was
being reduced to one of low relief—possibly under semi-arid conditions—
and, therefore, conditions were suitable for deep secular decay, for slow
removal of the products of decomposition, and for gold enrichment in the
oxidized parts of the quartz veins. In late Tertiary time, uplift of the
region caused rejuvenation of the streams. Deep valleys, which, for the
most part, coincided with the ancient valleys, were cut and in them the
gold, liberated in the zone of Tertiary weathering, was concentrated.

(6) In Pleistocene time ice-sheets covered the region, but were stagnant
and, therefore, accomplished little erosion; valley glaciers were not suffic-
iently extensive or long-lived to be effective agents of erosion except in
parts of the region, and the valley bottoms in places were protected from
erosion because of the narrowness of the valleys and the presence of out-
wash gravels overlying the Tertiary gravels. A small part of the Tertiary
placers, therefore, was preserved. Some of the placers are post-Glacial
and others interglacial and Glacial in age, and are the result of concentra-
tion by streams, of the gold in gravels, which were derived by ice-erosion
of the Teritary gravels, and thus became included in the glacial drift.
The Tertiary weathered zone of rocks was largely removed by ice-erosion,
but, in places, deeply weathered rocks occur beneath unweathered glacial
drift, and, as little post-Glacial weathering has taken place, it is probable
that the upper oxidized and highly fractured parts of the veins containing
free gold are, in part, remnants of the Tertiary belt of weathering.

EVIDENCE OF THE DERIVATION OF THE PLACER GOLD FROM
THE BELT OF AURIFEROUS QUARTZ VEINS

The placer gold occurs, along with concentrates of other heavy minerals
such as pyrite, galena, scheelite, barytes, most abundantly in poorly
assorted angular and partly rounded gravel mixed with yellow or blue