PREVIOUS WORKThe Malartic gold deposits may not be typical of Archean gold mining camps, because they have numerous types in them. Most investigators have been stressing the structural control of the deposits, but have ignored the uniqueness of some of the deposits that appear to be stratiform and probably syngenetic in origin. Their observations in government reports and journal articles that I am aware of, were NOT made in situ, but were taken from description of others or restricted to the open pits.The author worked and studied the above underground while the mining company at the time undertook a comprehensive study of the area in order to outline additional reserves. With the gold price around $100 an ounce it was a desperate effort to survive. A couple of academics visited some of the new developments underground, during the narrow window between their discovery and initial exploitation and the results of their study provided additional evidence of their uniqueness.Most of the gold deposits at Malartic were deposited along the southern Cadillac Fault, locally known as the Sladen Fault. The above 4 categories of gold deposits were different and provided a challenge to miners and managerial staff alike during their exploitation.The bulk tonnage formations such as the porphyry–type and Sladen Fault ores that were not back- filled caused caving problems in the underground mines.SLADEN FAULT EXPOSEDPictures were taken along the fault during open pit operations on ore zone 2-10 (East Malartic). These exposures were destroyed during exploitation.The apparent movement seems to be almost vertical with the Lavas between the two Cadillac faults moving downwards with a plunge of 65 degrees towards the east against the Pontiac sediments.
Fig 1 The near vertical surface is the fault plane looking east. The rock is Pontian greywacke. The platform is set for drilling. The first level opening of the underground mine is visible near the bottom of the picture. When mining took place the tunnel was completely underground. After mining ceased the ground north of the fault caved in exposing the first level tunnel (called a drift). At this location the ore is about 12 m thick.
Fig 2 Looking east along the fault plane which strikes east-west. You are looking at the fault surface. The dip of the fault varies from 65 degrees north to almost vertical. Originally, lavas would occupy the area north of the fault, but the rock is much softer than the greywacke and was at a lower level to start with and later caved in due to the mining activities.
Fig.3 Looking at the fault surface towards the east. A prominent white quartz vein within ore- grade metamorphosed greywacke. The top of this outcrop and partly down the slope has been shaped by glaciers. The near vertical part of the outcrop has been smoothened by the movement along the Slade Fault itself. Some of the “lines” visible on this surface are intersections of foliation with the current surface or striations from fault movement.
Fig.4 Looking south at the Pontian greywacke. The smooth vertical surface is the fault itself. At this location the bedrock outcrops (there is no overburden or glacial till).
Fig. 5 Looking east along the strike of the Sladen Fault with exposed Pontian greywacke.
Fig. 6 Looking west along the strike of the fault. The fault is almost vertical and the strike takes a swing towards the south towards the observer.
Fig. 7 Looking south at the Pontian greywacke. The smooth vertical surface is the Sladen Fault. The tunnel is the first level of the underground miners and was originally completely underground. So was the hole in upper right, which was a crosscut. All the rock north of the fault has caved in a long time after mining had ended in this part of the mine.
Fig. 8 Vertical “layering” is due to the fault movement. The roof support rods along the drift were
positioned by the underground miners during development in the 1950’s.
Fig. 9 Notice the smoothness of the fault surface. Towards the right side notice several vertical bands of different colors which are not constant in width. They may represent various lithological units within the metamosphosed greywacke.
TYPICAL GOLD ORE ALONG THE SLADEN FAULT
Alteration fluids during this movement created the ore zones when competent formations (like intrusions) within the Lava block moved against the sediments creating numerous fractures that were filled with alteration minerals plus gold-bearing pyrite. The gold was leached out of the lavas and sulfur was provided by volcanic activity that deposited chert and the lavas.
The lava/sediment contact did not create any gold deposits and was typically sheared into a clay- rich band (impervious to circulating solutions). This material was almost impossible to drift
through underground unless you cement it and build a concrete bridge. If encountered in diamond
drilling it resulted in “rods stuck” and “hole abandoned”.
ORE ZONES THAT WERE UNUSUAL
A. Stratiform chert with gold-bearing pyrite
This horizon was intersected in a drift, but was ignored despite its high gold content. The only reason: It was not along the Sladen Fault.
Exploration drilling and drifting outlined new ore zones that were partly mined along a horizontal distance of 100 m out of an explored distance of about 300 m plus. The limits of the ore horizon were left unexplored due to mine closure. The horizon may extend more than a 1 km in length and has been intersected previously in other levels for 400 m in vertical extent. Therefore, its vertical extent is probably more than 500 m.
The characteristics of this ore zone were the uniform continuity in thickness and gold grade along strike and dip. Visible gold was extremely rare in drilling and not encountered during mining, so it must be very fine.
Painted lines mark the chert contacts (below white line in top picture & above yellow line in second picture). The very fine layering / banding is clearly visible in places.
The fine layering includes a white quartz pod
The results of studies done at the University by Western Ontario were compiled and published by Kerrich.
Armed with the knowledge about the syngenetic nature of this deposit, exploration geologists from the same company were sent to re-examine previously explored gold prospects throughout Canada. At the same time, their exploration branch came upon the Bousquet gold deposits in the Cadillac area which were similar in nature, but of different age. The two above cases of apparently syngenetic deposits led these geologists to target the Hemlo area of Ontario as another example of the Malartic–type syngenetic deposit. The Hemlo showings were known for quite awhile, but were
left unexplored due to the enigmatic nature of the mineralization. The discovery of the Hemlo deposit caused a sensation in Canadian gold exploration.
CHERT ZONE underground Drill intersections (widths in feet)
Strike length : 950 ft. (285 m)
Ore grade : 469 samples with average grade of 0.306
However, channels & mucks (miners take sample while scooping up broken rock) averaged 0.095. What was the ore grade after mining is difficult to say. There was no time to find out as the mine was shutting down.
Data supplied by academic investigators:
Average chemical composition of 22 samples (Kerrich, 1983)
% SiO2 64.87
TiO2 0.4
Al2O3 12.55
Fe2O3 4.5
MgO 2.32
CaO 3.47
K2O 3.91
Na2O 3.95
P2O5 0.13
ppm Au 12.88
Cr 157
Ni 66
Sr 656
Ba 605
Zn 37
Cu 20
Rb 67
Y 12
Zr 117
Nb 17
B. Stratiform iron formations (gold-bearing pyrite) on basaltic lava tops
(“diorite ores”)
These were relatively high grade, chimney-like deposits consisting of pyrite, calcite, quartz with limited extent horizontally/vertically. They were called after the mafic hosts (actually basalt) that were closely associated with.
Their vertical extent (original horizontal) is longer than their horizontal extent. Those that were observed on surface were located on the stratigraphic top of basalt and were overlain by ultramafic lavas.
The nature of the gold is probably very fine, because no visible gold was marked on mine plans and sections.
The following block diagram was obtained in the paper by Eakins (1935):
C. Bands of folded narrow porphyry intrusions/extrusions, usually referred to as
“porphyry swarm” zones
The location of these deposits must be related to their proximity to the nearby massive “East Porphyry” intrusion. The intrusive bodies maybe lateral offshoots into the surrounding lavas. Similar such offshoots were encountered on the east side of the massive “East Porphyry”, but not on the northern edge.
The group of porphyries are arranged en echelon on mine plans, but are folded and easy to see that aspect in cross-sections. Each porphyry is strongly altered, is quartz-rich on one side and quartz-poor on the other side. Gold values are erratic, but can be spectacular with lots of visible gold that was observed in drillcore and driftwalls. They were frequented by staff and visitors alike, but the mine had no policy on access restrictions as the mine had been historically a low-grade operation.
The attractiveness of mining these high-grade bodies is, however, minimized by pockets of soft, clay-rich altered lavas in between them that made exploration drilling extremely difficult. At the same time, the milling people often complained about the clay content of the ore. Apparently, gold recovery decreases with increasing clay content.
Spectacular intersections encountered in the limited exploration efforts:
Intersections with over 1 oz of gold
16. 13 / 2 feet 2.45 / 7 ‘ 6.64 / 5 ‘ 1.28 / 9 ‘ 1.25 / 5 ‘ 1.26 / 15 ‘ 1.11 / 3 ‘ 2.58 / 5.5 ‘ 1.50 / 2.5 ‘ 2.10 / 2.5 ‘ 1.74 / 2.5 ‘ 1.17 / 2.5 ‘ 4.03 / 3 ‘
Some pictures from the limited exposure of these “porphyry swarm zones”:
The yellow line separates porphyry from lava. Notice the quartz veins stop at the contact (they
don’t continue into the softer lava)
Average chemical composition of 4 samples (Kerrich, 1983)
%
SiO2 65.83
TiO2 0.5
Al2O3 15.16
Fe2O3 3.17
MgO 1.16
CaO 2.91
K2O 1.41
Na2O 7 .07
P2O3 0.2
ppm
Au 0.16
Cr 29
Ni 15
Sr 1108
Ba 1372
Zn 42
Cu 12
Rb 29
Y 8
Zr 209
Nb 13
D. Porphyry-type disseminated ore throughout porphyry intrusions/extrusions
Historically, the # 4 porphyry deposit in the old Barnat mine was an oddity and could not be explained. Miners just came up to it during their mining of “diorite ores”. As mine plans show gold values were disseminated throughout the intrusion. Unfortunately, the #4 porphyry deposit was not back-filled and created a large opening underground. Eventually, pressure from the surroundings gave in and the ground collapsed from the surface one Sunday morning when no miners were at work. The pressure on the underground door structures was tremendous and smashed all of them to smithereens. On the surface it created a new moon-like “crater” not far from the edge of Malartic town. No fatalities were reported.
During the 1970’s Nos 6 # 7 porphyries were mined partially, but their grade (0.1 oz/ton) was nowhere near that of the #4 (0.15 or better). The stratigraphic location of these porphyries within the sequence of ultramafic/mafic lavas was also a puzzle. Exploration efforts in the area discovered numerous large intrusive/extrusive bodies, but without mineralization. Therefore, this type of porphyry-type gold deposit is not a common occurrence within the Lava Block.
In the following section through porphyry # 6 deposit, it is clear the mineralized section extends almost throughout the porphyry intrusion/extrusion. The alteration / mineralization events took place at the same time. A narrow barren intrusion cuts across the ore zone. These features are also observed in porphyry copper deposits. Besides gold and some silver the only other metal that was found to be present in this deposit was minor tungsten.
URANIUM below the Martin Formation, N. Sask., Canada
MARTIN FORMATION: CONGLOMERATE, ARKOSE, SILTSTONE deposited in a basin 20 km long by 3 - 6 km wide. Small uranium deposits are dotted all around the basin. Some were under the basin before erosion exposed them.
Main characteristic: hematite alteration “tinting” all minerals (quartz, feldspars, calcite, etc)
The conglomerate is made up of rounded stones of all types and sizes. Rivers draining from the surrounding mountains deposited these stones in a basin probably much bigger than the remains of it today. Part of the conglomerate is traversed with uranium-bearing veins and stringers cutting across altered zones of hydrothermal hematite, quartz and calcite with minor pyrite – chalcopyrite.
Age of sediment deposits between about 1,750 and 1,550 m. y ago.
MARTIN FORMATION: CONGLOMERATE, ARKOSE, SILTSTONE deposited in a basin 20 km long by 3 - 6 km wide. Small uranium deposits are dotted all around the basin. Some were under the basin before erosion exposed them.
Main characteristic: hematite alteration “tinting” all minerals (quartz, feldspars, calcite, etc)
The conglomerate is made up of rounded stones of all types and sizes. Rivers draining from the surrounding mountains deposited these stones in a basin probably much bigger than the remains of it today. Part of the conglomerate is traversed with uranium-bearing veins and stringers cutting across altered zones of hydrothermal hematite, quartz and calcite with minor pyrite – chalcopyrite.
Age of sediment deposits between about 1,750 and 1,550 m. y ago.
ALTERATION IN MARTIN CONGLOMERATE AND TAZIN GNEISS BELOW THE UNCONFORMITY
First stage of alteration
A mylonite development with hematite alteration in Martin rocks and a separate episode of mylonite development with chlorite alteration in Tazin rocks.
Additionally, the unconformity is a chlorite-rich shear zone about 0.5m thick that produced a valley on the glaciated surface.
Examples of mylonite with hematite alteration in conglomerate as seen in core: they are darker areas with small rounded fragments in a very fine matrix, mostly of hematite. Notice hematite “coloring” of fragments and minerals like feldspar. In the first sample, there is also a later calcite vein with additional hematite.
Second stage of alteration
Affects both Tazin gneiss and Martin conglomerate. Pervasive siicification with hematite, fracturing with veins of calcite, comb quartz and patches of pitchblende, pyrite and chalcopyrite. Epidote in the veins is found probably outside (peripheral to) those with uranium.
Fracture surface on the right has patches of pyrite (silvery)
Some of the calcite has been dissolved and comb quartz seen in the cavity
Pyrite veins with minor hematite
Pervasive calcite alteration (plus minor quartz) is rare
Comb quartz crystals lining a vein wall from surface exposures
Veinlet of massive pitchblende (black) with secondary uranium oxides (yellow and green)
Rocks under the Martin
Tazin gneiss is also affected by hematite alteration and silicification with some uranium in stringers with calcite and quartz.
In the core it is very hard to differentiate between Tazin gneiss and Martin conglomerate when both are altered by silica and hematite.
Patches of clay alteration and graphite in open fractures (remobilized?) in gneiss (without uranium, so far) appear to be peripheral to uranium stringers.
It is possible that there are pockets of rich uranium concentrations under the Martin similar to uranium occurrences under the Athabasca Formation further south in Saskatchewan.
The uranium in veins within the Martin conglomerate may originate from pockets of uranium under the Martin. Such mineralization would also alter Tazin rocks into clay and remobilize graphite in fractures outside the mineralized area.
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