162

all show a development of copper minerals considerably later than that
of magnetite. This is believed to be due to the fact that those parts of
the coast where they occur are underlain by two intrusives, an older
diorite and a younger granodiorite or granite; and the magnetite miner-
alization is believed to have been caused in large part by the influence
of the diorite, whereas the copper mineralization is ascribed to the younger
intrusive.

During the course of the investigation of the various contact deposits,
some observations were necessarily made regarding the method of intro-
duction of the magnetite from the intrusive into its host rock; but as the
prime object of the survey was an estimation of tonnage based on geological
facts and relationships, a systematic detailed study of these methods
could not be undertaken.

As stated or implied in many places throughout this report, the
magnetite is in all cases believed to have been introduced not as a magne-
tite magma by ore injection, but by a slow, gradual, progressive replace-
ment of limestone, tuff, andesite, or diorite by tenuous solutions of consid-
erable penetrability. This conclusion is stated here so as to give a back-
ground for the sections dealing with the ore genesis of each deposit; but
the evidence for it is not included in a report of this character. It is
the writer’s hope to present in a separate paper at a somewhat later date
his ideas regarding the replacements by these magnetite bodies.

Since their designation as contact metamorphic deposits will con-
stitute one of the principal criteria by which the extent and mineralogical
continuity of the individual deposits will be judged, it will be pertinent to
indicate at this stage the nature and chief characteristics of contact metam-
orphiec deposits in general. The following quotations from eminent
authorities will suffice for this purpose. Lindgren! states that:

“In many geological provinces, granular igneous rocks, such as granite,
diorite, and syenite, have broken through and invaded sedimentary rocks.
. . . Along these contacts, bodies of useful minerals are often found,
most commonly where the sedimentary rock is limestone, or, at least.
salcareous.

“All the world over, this group presents certain characteristics, the
more essential of which are the following:

“Form: The deposits generally follow the contact, but are extremely
irregular in detail, and almost always very bunchy. No regular law has
been recognized as governing the form of the ore-bodies, which are some-
times lenticular masses. :

“Constituent minerals: The gangue contains garnet, wollastonite,

epidote . . . . amphibole,pyroxene . . . . vesuvianite, quartz,
and calcite. . . . The ore minerals are specularite, magnetite, bornite,

chalcopyrite, pyrite, pyrrhotite, and more rarely galena and zine blende.
- _. . The characteristic feature is the association of the oxides of iron
with sulphides.”

Emmons? says that:

“Contact metamorphic deposits vary greatly in size. Many of the
sulphide deposits are merely small concentrations of ore minerals in a
gangue of heavy silicates, and the ore-body grades into the country rock

a a Li area W.; “The Character and Genesis of Certain Contact Deposits’; Trans. Am. Inst. Min. Eng., vol.

2 Emmons, W. H.: “The Principles of Economic Geology’’; New York, 1918, pp. 35-36,