200

beyond which limestone beds of varying thickness are interlayed with
tuffaccous members. The southern and western parts of the island are
underlain principally by the two intrusive rocks above mentioned. Striking
northwesterly from Clifton point into the heart of the island is a prominent
contact between diorite and andesitic tuffs; and it is along this contact
that the magnetite deposit is situated. The interpretation of the structure
in the vicinity of this contact is given in the idealized section, Figure 31,
where it is shown to be of an undulating character plunging to the north
with varying dip, lying on top of which are similarly dipping beds of
andesitic tuff. This structure is slightly complicated by apophyses of
the diorite cutting the sediments, and by dykes of granodiorite intruding
all the other rocks. No limestone was observed in the area immediately
about the magnetite deposit, and there were no evidences of its metamor-
phosed equivalents.

The diorite-tuff contact may be observed to dip 25 degrees to 30
degrees northeasterly on the Mountain claim at the summit of the island,
and the tuffs in the same locality have about the same dip. This contact
steepens to 50 degrees along the water’s edge, some little distance to the
north, thus giving a clean-cut picture of an intrusion of diorite that has
only recently been unroofed.

Replacement of the tuff by garnet, epidote, and cherty quartz with, in
large measure, the preservation of the original bedding structure, has
taken place close to the diorite contact, and shallow roof-pendants of replaced
and unreplaced tuff occur near the top of the island between small cupola-
like masses and dykes of diorite and granodiorite.

OCCURRENCE OF THE MAGNETITE

Magnetite occurs, along with garnet and epidote, as a replacement of
some of the beds of tuff close to their contact with diorite (See Figure 33).
In some cases the bedding of the tuff is preserved and is emphasized by the
occurrence of thin bands of magnetite (4 to 4 inches thick) in contact with
thin bands of brown garnet or of quartz and epidote. The individual
magnetite bands consist of a series of closely spaced, flattened, somewhat
irregular lenses up to 2 or 3 feet in length, having the general dip of the
tuffaceous rocks. Between the flattened lenses occur various mixtures of
the silicates, mostly andradite and grossularite. The structure of the
deposit, therefore, is controlled by, and is a function of, the attitude of
the dipping sedimentary series.

The tuff and its metamorphosed and mineralized products may be
subdivided into separate bodies of relatively pure magnetite, various
mixtures of garnet and magnetite, garnetized tuff, and unaltered tuff (See
Figures 33 and 34). The contacts are gradational. Very little solid mag-
netite is present, since most of it is not only interbedded, as described
above, with silicate layers, but is itself highly impregnated with garnet.
The principal bodies of nearly solid magnetite are developed along the
sides of three large open-cuts (See Figure 33), where a blanket-shaped
deposit of magnetite with a maximum thickness of 10 feet occupies a dip
slope, and can be seen to be a replacement of a thinly-bedded rock series.

The open-cuts and the tunnels in each case penetrated the foot-wall of
the magnetite zone, since they were driven in a direction opposed to the
dip. The sheeted dipping structure can be plainly seen on the sides of the
southernmost open-cut c.