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One crystal aggregate, consisting of about a dozen dodecahedrons
with hopper-shaped faces, having an arborescent arrangement and almost
entirely enclosed in a mass of limonite, was obtained by the authors. The
mass was about the size of a bean, and when found, only about three
minute corners of the gold penetrated the envelope of limonite.

Hypidiomorphie crystals of gold are common. Angular pieces or
fragments of quartz are frequently associated with the gold and much of
the quartz is iron-stained.

_ ‘Spongy masses with a high degree of porosity are not uncommon, and
many specimens consist of a fine-textured quartz-breccia, the fragments
of which are held together by a cement or fracture-filling of gold.

Leaf-like gold is very common, and appears under the microscope

as if the leaves or flakes had just been removed from between walls of
some other mineral the rough fractures of which are preserved in relief
on the sides of the leaves.

Very many of the pieces of gold have a fine, columnar, and wire stcuct-
ure, apparently due to incipient crystallization.t

ORIGIN OF THE VEIN GOLD

The outstanding characteristics of the vein gold are: (a) its occurrence
in the form of leaves or veinlets—acting as cement in fractured quartz;
(b) its occurrence as crystals; (c) its limitation to those parts of the veins
characterized by limonite.

These characteristics definitely correlate the free gold with oxidizing
conditions, and with deposition in those parts of the veins which are highly
fractured and cavernous. Well-developed crystals and crystal aggregates,
such as that of the group of dodecahedrons mentioned above, could only
develop in open spaces, or in spaces filled with the soft products of rock
decay, where they had the opportunity of taking on definite bounding
faces, without interference from adjacent hard minerals. Vein-like gold,
which is so common both by itself and as a cement in the quartz breccias,
could only have been formed in zones of minute shattering of the quartz.
Gold crystals and leaves or veinlets of gold may be formed in shear zones
in quartz either near the surface or at considerable depths; but the limita-
tion of the free gold to those parts of the veins in which limonite is char-
acteristic, confined the possibilities of such occurrences in this area to
fractured zones at or near the surface. The free gold, therefore, is genetic-
ally related to some set of processes which operated under conditions of
oxidation near the surface.

The hypothesis which is here advocated as an explanation of the
occurrence of the free gold is that the gold content of the unoxidized parts
of the vein was and still is very largely, if not entirely, confined to the
sulphides, mainly arsenopyrite and pyrite; that the oxidation of these
sulphides under near-surface conditions released the gold, which passed
into solution only to be precipitated in crystal and leaf form at short dis-
tances below in the belts of weathering and cementation.

1Maclaren, J. M.: ‘‘Gold, Its Geological Occurrence and Geographical Distribution’; London, 1908, p.L).
20285—15