178

the southeastern extremity of the deposit, with relations suggesting that
they have cut through the magnetite.

Limestone does not outcrop in the vicinity of the deposit. In the
tunnel, however, many angular fragments of crystalline limestone are seen
entirely enclosed by magnetite, indicating that they are unreplaced blocks
of what was originally a large lenticular mass or inclusion of limestone
in the quartz diorite porphyrite. Xenoliths of altered limestone also occur
in the intrusive.

OCCURRENCE OF THE MAGNETITE

The magnetite occurs as a massive deposit almost completely sur-
rounded by quartz diorite porphyrite. It has a maximum extension in a
northwesterly direction of 125 feet, a maximum width in a northeasterly
direction of 90 feet, and a proved depth of 50 feet to the level of the tunnel.
The attitude of the deposit is in doubt on account of the paucity of struc-
tural data. At the mouth of the tunnel, and at a point along the lower
contact about 60 feet west of the portal, quartz diorite porphyrite is in
contact with the magnetite along a surface dipping 60 degrees to the north
and northeast. The other observed contacts are more irregular than this
one and neither substantiate nor disprove this structure. The upper edge
of the lode along the line of section could not be found in exposed contact
with the intrusive, but from the areal distribution shown on Figure 29,
it can be seen that this boundary would not likely be very far to the
southwest of the indicated contact of the magnetite with the drift. In
addition, the face of the tunnel is without much doubt a distance of 19
feet within the main mass of the intrusive; and a line joining the innermost
edge of magnetite in the tunnel with a point on the surface a few feet
southwest of the margin of the exposed magnetite, would show a dip of
about 70 degrees to the northeast. The conformability of this dip on the
southwest margin of the lode with the 60 degrees dip of the northeast
edge lends weight to the suggestion of a general northeasterly dip for the
entire deposit.

The section through the tunnel shows, as mentioned above, a 10-foot
band of quartz diorite porphyrite that appears to cut through the magne-
tite. The contacts of this intrusive are not well-defined and there appear
to be gradations between it and the magnetite on either side. There is
nothing to prove that this is a dyke of later date than the magnetite.
On the other hand, the data point to the inference that the quartz diorite
porphyrite band is a dyke that penetrated the pre-existent limestone mass,
and that subsequent mineralization by magnetite both within the limestone
and the dyke obscured the actual contact. No body of intrusive that
might represent the continuation of this dyke in the tunnel was found
within the mass of magnetite on the surface. It is quite possible, however
that surface mapping failed to reveal the occurrence of such a dyke, as
the magnetite is in places covered over with moss, light shrubbery, and
some drift. ;

It seems quite clear that the Sirdar magnetite is an almost complete
replacement of a thick, short, lenticular mass of crystalline limestone
caught up in the main intrusive body. It differs, therefore, from the Con-
queror deposit in that it is not a mineralization of limestone along one
intrusive contact of a fairly large roof pendant, but a replacement of a
smaller inclusion of limestone from both opposite intrusive contacts.