its original composition. Thin sections consist very largely of
plagioclase and quartz, coloured constituents being small in quantity.
The plagioclase is twinned according to albite, pericline, and carlsbad
laws, and in one case a baveno twin was observed; optically it behaves
as almost pure albite and is decomposed to sericite instead of epidote.
A poikilitic arrangement is more or less distinct; the feldspar is also
micrographically intergrown with quartz. No orthoclase could be
identified in any of the specimens. Quartz does not appear to be much
more abundant than in the diabase. The only recognizable ferro-magnesian
mineral is a strongly pleochroic reddish brown biotite, which occurs
sparingly in small ragged flakes, partly altered to chlorite. Leucoxene
representing ilmenite is surprisingly abundant considering the scarcity
of iron bearing silicates. Apatite is an abundant accessory mineral,
forming small prisms, while zircon crystals are rare. The rock in the
specimens examined is remarkably rich in calcite, which in amount ranks
next to the plagioclase.
Comparison of sections from specimens intermediate between the aplite
and the ordinary diabase brings out some features of the differentiating
process. The governing changes are in the proportion of pyroxene, and in
the composition of the plagioclase. A specimen somewhat paler in colour
than the typical diabase was found to contain considerably less augite
and more abundant plagioclase, the other constituents remaining fairly
constant. The plagioclase was an acid andesine of the composition (Ab 65
An 35). A still lighter coloured type possessed oligoclase (Ab 72 An 28)
and a small amount of blue green hornblende, representing the wholly
altered pyroxene. Orthoclase could not be found in any of the sections,
so that none of these rocks can be properly called syenite or granite.
_Local description._—Diabase is most abundantly exposed in the area
between Gowganda lake and the portage route connecting Firth and Elkhorn
lakes, where it forms three parallel tongues of approximately equal
dimensions. Coarse grained types prevail in which the differentiation of
acid phases is pronounced. Aplite dikes up to 10 feet in width, fairly
coarse grained and sometimes tolerably rich in dark constituents, are
abundant. The rugged surface of this part of the country is largely due
to the presence of the diabase, the peculiar arrangement of which has
produced a constant system of north and south ridges. The contacts with
the Huronian occupy the bottoms of gullies, with the main body of
diabase forming high ground. This highly relieved and consequently well
exposed surface has greatly facilitated the exploitation of this area,
as indeed is the case for the whole district. Terminally each of the
three bodies tapers out or forks, but these details were not closely
mapped. In the case of the most easterly one the portions reaching the
east side of Firth lake are darker and more decomposed than the ordinary
diabase, and during the field operations were considered of other
character. Laboratory study of the specimens shows them, however, to be
basic diabase in which hornblende has completely replaced the pyroxene.
In consequence of this misconception the connexions of the exposures on
Firth lake were not well worked out, but they almost certainly lie as
shown in the map and may form a continuous connexion with the large
diabase body farther north. The two areas north-east of Firth lake
present much the same appearance as those of the Gowganda district.
The most extensive body is that which coincides in direction with Duncan
lake, resembling a great hook, the shank of which is traceable for
twelve miles, the crooked end lying about midway between the West branch
and Firth lake. Neither termination was found, on account of the swampy
or sandy character of the country, so that the present representation
may not be complete. Just east of L’Africain lake there are some
diabases which may be continuations of it, but owing to their altered
appearance they could not be distinguished with certainty in the field
from similar Keewatin forms. Along Duncan lake it appears to represent
the edge of a sill about 300 feet thick lying in the bedding plane of
the Huronian sediments which lie both above and beneath it. From the
attitude of the latter the sill is believed to dip eastward at an angle
of 30 or 40 degrees. The exposed width is usually less than in the
Gowganda area and its composition is more homogeneous, but at the wider
portions the same intermingling of basic and acid phases and aplite
dikes obtains, as for example where it crosses Wapus creek. The full
extent of that portion which extends north-westward from Mosher lake is
not known. The formation at that point is obscured by gravelly soil; it
appears, however, to be unusually wide and well differentiated. An
apparently large body of diabase lies between the north part of Duncan
lake and the West branch. The mapping of this was left incomplete, so
that its total southern extent cannot be given. Its northern part
presents nothing unusual, but about Vipond lake a syenitic phase is
developed, probably indicating a mingling of differentiated rock
varieties as at Gowganda lake or Wapus creek.
The remaining diabase bodies are dikes. Probably these are very numerous
and widespread, but this can only be determined by work of much more
widespread nature than the economic possibilities of such an
investigation warrant at present. They are most frequently seen in the
Laurentian, probably not so much because they are more numerous there,
but because they are more conspicuous in the granites than in the
Keewatin schists and Huronian, from which they differ less in colour.
They attain widths of 60, 100 or more feet, in which cases the texture
is like that of the large masses, but differentiated types and aplite
dikes do not seem to accompany them. The smaller dikes, diminishing to a
width of 1 foot, are compact black rocks of microcrystalline habit, but
like the gabbroid varieties mineralogically.
_Age._—The time of intrusion of these rocks can only be defined as
post-Huronian or post Middle Huronian, there being no younger formations
with which to correlate them. They are identical with the diabase in
other parts of the Montreal River region and of Cobalt. As already noted
the larger masses have produced very limited metamorphic changes in the
adjoining rocks, developing incipient adinole zones in the Huronian
greywackes, besides hardening and fracturing them. Usually the contact
edge of the diabase has been deeply eroded along with the adjoining rock
to form narrow ravines, but wherever it persists a slight chilling is
perceptible. The fine grained edges are much more apparent in dikes
where the mass of hot material being much less was more susceptible to
the influence of cold surroundings. The intrusive nature is also
demonstrated by angular blocks of Huronian materials enclosed within the
diabase, instances of which may be seen on Wapus creek.
_Olivine Diabase._
At the first rapid above Kenisheong lake the Huronian is cut by a broad
dike of different character from those belonging to the quartz-diabase
intrusion. It is a remarkably fresh compact rock of dark grey colour and
medium texture in which an ophitic structure is prominent, acicular
prisms of glassy feldspar penetrating the dark main mass. Its density is
2·991. Under the microscope it is holocrystalline and the constituent
minerals occur in only one generation. The constituents as determined by
linear measurements were found to be plagioclase, 67·5 per cent;
olivine, 14 per cent; augite, 1·5 per cent; iron ore, probably ilmenite,
5 per cent; apatite, 1·8 per cent; biotite, 1·3 per cent; and zircon,
0·03 per cent. The rock is remarkably fresh, even the olivine showing no
signs of decomposition. The plagioclase was determined by optical
methods to be a labradorite of the composition Ab 1 An 1. It forms
laths, twinned according to albite, pericline and carlsbad laws, which
penetrate the ferromagnesian minerals. A few large crystals show fine
zonary lamellation. Olivine is in idiomorphic or rounded grains,
occasionally bordered by a little rim of biotite, possibly a product of
reaction during the period of crystallization. The augite is reddish
brown in colour and later than either olivine or plagioclase, filling
the interstices between the feldspar laths. Irregular masses of black
iron ore, probably ilmenite, are scattered throughout the section, in
some cases showing good crystal forms. A deep brown strongly pleochroic
biotite with a very small optical angle occurs accessorily in small
shreds. Apatite is mostly in slender but sometimes stout hexagonal
prisms. Minute crystals of zircon are rare.
The rock may be designated an olivine diabase of markedly fresh aspect.
It penetrates the Huronian, but judging by its unusually fresh condition
it must be comparatively young.
PLEISTOCENE.
The present glaciated pre-Cambrian surface is scantily covered by
unconsolidated glacial sands and gravel which are being collected by
natural agencies from the hills and more elevated parts into the
depressions. As a consequence the hills are bare, while the valleys and
ravines are soil-filled and support a strong forest growth. Neither sand
nor gravel show signs of stratified arrangement. They play an
unimportant part in the topographical appearance of the country; the
only elevation composed of such materials being a small gravel hill to
the east of Porphyry lake. Between that point and the hills near Mosher
lake is a considerable extent of flat sandy country, the gentle contour
of which contrasts with that of the surrounding hills.
A rather peculiar low apron of sand occupies the very end of the
peninsula between Duncan lake and the West branch. It is of small
extent, and is probably due to river deposition, the West branch at this
point being sluggish and the surface of the sand showing indications of
shifting stream beds.
An extensive area of sand lies at the end of Duncan lake, from whence it
extends northward for several miles, also westward and south-westward
toward the 550 foot hill shown on the map. Seen from any neighbouring
hill this plain appears flat in comparison with the ordinary surface of
the country. It consists of a fine, yellowish, unstratified sand. The
several brooks crossing it are distinguished from those of other parts
of the country by their tortuous courses which are constantly changing,
and which render them unfit for canoe travel. The shallow character and
sandy shores at the north end of Duncan lake have been caused largely by
the transportation of sand into it by these brooks.
Glacial boulders are scattered over the whole district.
ECONOMIC GEOLOGY.
SILVER.
DISTRIBUTION.
With the knowledge acquired from exploitation of James township, and
other of the more recently discovered silver-cobalt camps, prospectors
in the Montreal River district gave exclusive attention to the diabase
formation, recognizing it to be closely connected with mineralizations
of this kind. Some work was done in 1907 and more in the following
season, with the result that on August 4, the first native silver
discoveries were made, almost simultaneously and at short distances
apart, by Messrs. Mann and Dobie, in the diabase just west of Gowganda
lake. The remarkably rich surface showings at once attracted the
attention of the whole prospecting body in the Elk Lake country, and an
activity began which, since the spreading of information to outside
points, has developed into a ‘rush’ of large dimensions.
The known silver bearing area is restricted as yet to about ten square
miles lying between Gowganda lake and the portage route from Elkhorn to
Firth lakes, and is commonly known as Gowganda. Extensive prospecting
only commenced in September, about the close of the field season, so
that only the earlier discoveries are known to the writer, and a
knowledge of the surface details could only be derived by examination of
the few beginnings of patient and continued exploration by claim owners.
The present account must accordingly be accepted as incomplete and by no
means representing the present status of the Gowganda camp.
SURFACE INDICATIONS.
Conditions in the region are such as to demand exploration of the
closest and most intensive order, for the indications of mineralization
are negative rather than positive in character. The Gowganda area was
entirely forested at the beginning of 1908, and a carpet of moss and
vegetable mould covered most of the rock surface. Glacial materials are
also fairly abundant, and sometimes thick enough to render surface
exploration arduous and expensive. Added to this the veins are eroded
more deeply than the country rock, and are represented at the surface by
crevices filled with soil, and thereby rendered inconspicuous. Were the
country a flat one the difficulties in the way of successful prospecting
would be very serious, but fortunately it is rugged, especially near the
diabase. Steep ridges of this material are a regular topographical
feature. The sides of these ridges are bare or readily exposed and offer
fine opportunities for examination. It is significant that the first
silver discoveries were made in the sides of such rock walls. The pink
bloom found at the surfaces of the veins and the adjacent country rock
is also an indicator whose value is fully understood by those working in
the region.
STRUCTURE OF VEINS.
The deposits are in the form of well-defined veins occupying fissures in
the diabase. The amount of surface work done in September was not enough
to throw much light on the continuity of the veins, but a few had been
traced for distances of 300 or 400 feet, and in one case across several
contiguous mining claims, so that they may be said to occupy persistent
fissures. They vary in width from 1ʺ up to 20ʺ. Little could be learned
concerning their attitudes except where they traverse hillsides; in such
cases they are approximately vertical. It is not yet known whether any
regularity exists in their arrangement, but some extend east and west
while others are north and south. The diabase shows no signs of
extensive deformation, all geological evidence indicating that since its
solidification its history has been uneventful, yet the cracks which the
veins occupy appear too persistent to be the result of contraction by
cooling. Besides the strong veins there are others of the gash type, but
the latter are small, not very continuous and poorly or not at all
mineralized.
COMPOSITION OF VEINS.
From comparison of veins at Gowganda, Duncan lake, and Wapus creek a
general uniformity of structure, though not of mineralization, is found
to obtain. The whole mineral association is not found in any one vein,
nor are the relative proportions either of ore or gangue at all
constant. The gangue minerals are quartz and calcite, always mutually
arranged in definite manner. The sides of the veins are composed of
white quartz, which may form only an insignificant coating on the walls
or may occupy nearly the whole space, but in all cases there remains a
central cavity into which the pointed ends of quartz crystals project
freely. At the surface this central portion is empty owing to
weathering, but farther down it is filled by calcite. Veins with
predominant quartz filling seem especially abundant in the Huronian
adjacent to the diabase. The rich veins near Gowganda, so far as
ascertainable, are poor in quartz.
Practically all of them carry chalcopyrite either as diffuse grains or
in considerable amounts. Pyrite is equally abundant but less constant.
Galena is not uncommon. All these occur with the quartz; their presence
in the calcite is not certainly known. Many of the veins show diffuse
stains of reddish pink colour due to cobalt bloom, which though not in
itself of value is important as a sign of the existence of smaltite from
which it is formed by oxidation. The minerals enumerated thus far are
widespread, but economically insignificant; the silver-cobalt
association is present in some cases, however. Little opportunity
existed in 1908 for favourable study of these minerals, so that only a
list of those found at the surface can be given. There native silver,
argentite, smaltite and cobalt bloom have been found, and a few feet
down small lumps of native bismuth. Because they occur either in calcite
or in loose decomposition materials filling the space which the calcite
formerly occupied they are believed to be associated with that gangue.
Infrequently gangue minerals are almost absent and the vein filled by
massive ore.
LOCAL DISTRIBUTION.
Valuable argentiferous veins were known in 1908 only in the Gowganda
district, and, so far as yet known, discoveries have been confined to
the diabase west of that lake. Most of them occur in the southern
portion of the central diabase strip which lies a short distance from
the shore and extends northward for about seven miles from Elkhorn lake.
On one of the Mann claims (T.R. 1966), now owned by Messrs. Foster, an
east and west vein averaging 4ʺ or 5ʺ in width had been traced for 300
feet, the original discovery being made in the exposed face of a low
diabase wall. At its surface the vein material had been weathered out
for a depth of about 15ʺ and the cavity filled by a brownish mass of the
decomposed matter mixed with vegetable mould and sand. Nuggets of mossy
or arborescent silver were scattered richly through this dirt, and a
fairly continuous spine of the same metal, sometimes an inch thick,
extended along the middle of the crevice. A test pit sunk about 8 feet,
but barricaded at the time of the writer’s visit, had exposed, according
to Mr. Mann, silver and smaltite in a calcite vein. On the adjoining
claim (T.R. 1982), a vein of massive smaltite about 1ʺ wide was seen; a
little silver had been found at its surface and streaks of argentite and
disseminated grains of smaltite were seen in the wall rock. Aplite dikes
on another claim were found to be stained by cobalt bloom, and full of
disseminated chalcopyrite.
The properties owned by Messrs. Crawford and Dobie about half a mile
farther south were not visited, but were generally reported to be of
about the same richness as that in T.R. 1966. Immediately north of
Hanging-stone lake Mr. F. A. McIntosh was conducting active prospecting
in a coarse gabbroid, locally syenitic, form of the diabase, intersected
by aplite dikes. A discovery of native silver has been made since then
and the property sold to Messrs. F. R. Bartlett & Co., of Toronto,
together with other claims located between the north-east and north-west
arms. Other discoveries are reported just south-east, also a short
distance north of Milne lake.
No silver had been obtained in the eastern diabase strip, although the
geological conditions appear identical and calcite veins are abundant.
An exceptionally large vein, about 18ʺ wide, and traceable across two
adjoining claims was seen on the property of Messrs. McLaughlin and
McIntosh, about half a mile north-east of the north-west arm.
Mineralization in it near the surface was very slight. Several veins
carrying small amounts of chalcopyrite, pyrite, bloom and smaltite were
seen on the properties of Messrs. Elstone and Reilly (T.R. 1961, 1962
and 1903). In one of them small amounts of bismuth are present; another
contains an unusually heavy black substance which proved to be calcite
filled with minute crystals of magnetite.
In the western strip less exploration had been performed and little
could be learned about the ore deposits. Loose pieces of native silver
had been found by W. H. Margueratt in narrow fissures on M.R. 1798, but
the vein material was not exposed.
Outside the Gowganda area systematic prospecting was in progress at only
one point—Wapus creek. Under the management of Mr. Robert Lett a group
of nine claims was being stripped and trenched, with the result that
numerous calcite veins had been traced through a diabase showing the
same complex intermingling of basic and acid phases and aplite dikes as
at Gowganda. Chalcopyrite and cobalt bloom were abundant, and smaltite
had been found as disseminations in the wall rock. Lumps of native
bismuth weighing several ounces had been taken from a fissure in an
aplite dike, analyses of which showed it to contain silver.
Tentative exploration was being conducted along Duncan lake and east of
Firth lake, but not with the closeness and persistence which the
topography of these veins require. In general the veins seen on Duncan
lake are exceptionally rich in quartz, and gash veins are common.
Chalcopyrite, pyrite and galena are the most noticeable metalliferous
minerals, but cobalt bloom stains are frequently observable. So far as
known no attention has been given to the large diabase body between
Duncan lake and the West branch, although its size and varied
composition are thought to make it a desirable prospecting ground.
FUTURE POSSIBILITIES.
The present knowledge of the Gowganda area indicates it to be highly
mineralized, at least in so far as number of veins and surface showings
are concerned. The number of discoveries within its area of ten square
miles is steadily increasing. The area, character of the mineral
association and the richness of the surface showings are comparable with
those of Cobalt. There is a general similarity in the geological
conditions. The mineralized veins in Gowganda occur in the diabase as do
some of the good Cobalt veins. There are, it is true, local differences,
but the resemblances are more pronounced than the differences and lead
to the hope that exploitation will reveal similar underground
conditions. The well mineralized veins are sufficiently long and uniform
on the surface to suggest similarly persistent vertical dimensions.
Further geological work may reveal something definite concerning the
character and size of the diabase bodies which form the country rock,
and thus afford a basis for predicting their subterranean distribution.
While there are grounds for hoping that the veins will persist in depth,
this has not yet been proved, nor, if this is the case, that the
mineralization and values are also persistent.
The relative importance of Gowganda is therefore a matter of uncertainty
as yet, but it may be confidently affirmed that for its state of
development the outlook is very favourable, and the number of veins,
area of mineralization and rich surface showings afford good grounds for
hoping that some at least of the veins will be found to be commercially
important.
The details of igneous intrusion, differentiation and mineralization may
never be sufficiently understood to allow of accurate prediction
regarding the location of silver deposits, but a general conception of
the sequence of events culminating in their formation does permit of the
formulation of certain criteria useful in the search for ores. Evidence
is accumulating to show that the silver-cobalt mineralizations in the
Timiskaming region are connected with a late stage of differentiation in
the magma which supplied the quartz diabase and aplite. It seems
reasonable, therefore, to anticipate ore deposits in or near such
bodies, especially if they are of large size and have undergone
important chemical differentiation, that is, if they contain a varied
and extensive association of basic and acid phases of the diabase.
Pre-existing channels to receive the mineralizers are also necessary and
their distribution a matter of vital importance, but in this region they
appear to have been everywhere abundant.
These conditions appear to exist quite as fully at several other
localities besides Gowganda. At Wapus creek they seem identical and,
indeed, results obtained thus far indicate that some mineralization of
the silver-cobalt type exists. Between Duncan lake and the West branch
the conditions require further study, but, as now known, are not
discouraging.
COPPER.
The chalcopyrite, which seems a much more constant constituent of the
veins associated with the quartz diabase, is sometimes aggregated into
bunches which yield ore specimens of such excellent appearance as to
arouse interest. The ore is, however, confined to veins a few inches in
width and so scattered as to render them valueless. Occurrences of this
kind characterize most of the great diabase bodies, examples of which
occur on Mosher lake, between the North-east and North-west arms of
Gowganda lake and elsewhere. The chalcopyrite is sometimes superficially
altered to malachite and azurite.
IRON ORE.