METALLOGENY OF THE EAST MALARTIC GOLD DEPOSITS, QUEBEC
BUNTEP, Brandon University, Manitoba R7A 6A9
Abstract
The East Malartic mine, by itself a “world-class” gold camp based on production and reserves prior to closing in1979, was a rare geological museum exhibiting unique examples of Archean structural and ore deposits geology observable over kilometres of openings in rock formations that seldom outcrop on the surface.
The detailed studies done during the last 3 years before closure demonstrated the existence of 5 separate types of gold deposits at this mine. Most are structurally controlled and lie along the Sladen Fault, southern contact of the Malartic Tectonic zone, part of the regional Cadillac “Break”. One deposit, however, is clearly a stratiform chert horizon within the greywacke sediments and could have been the source of the gold to some of the other deposits. It is possible to reconstruct the sequence of events that created the gold deposits. It appears that it consisted of 3 stages. The first is accumulation of lavas in a submarine platform, probably along a rift zone where top-flow gold-rich exhalites also formed. This was followed by subduction of the ocean platform under a continent forcing the lavas to adopt an almost vertical posture while dextral faulting brought a sedimentary continent next to the lavas. Numerous subvolcanic Quartz Feldspar Porphyries (QFPs) intruded near the faulted lava / sediment boundary and their hydrothermal solutions deposited gold, especially within brittle formations. Some of the gold was leached out of the surrounding rocks, and some came from the stratiform chert. Finally, further movement between a massive QFP at depth and the sediment block formed a contact deposit at the same time increasing the width of the chert deposit near the faulted contact.
INTRODUCTION
Having produced over 100 tons of gold, the Malartic Gold belt is a “world-class’ gold camp (Kerrich et al, 1990). The authors classify the deposits at Malartic as Archean orogenic lode gold deposits which are defined as structurally controlled and epigenetic with respect to the host rocks. This group of deposits with examples worldwide are associated with regionally metamorphosed terranes of all ages and are linked to subduction-related processes (Kerrich et al, 1990). The East Malartic mine produced about 40 % of the gold in the Malartic Gold Camp.
PREVIOUS WORK
The nature of the gold mineralization at East Malartic / Barnat has been the subject of numerous papers since the 1940’s. Most authors stress the structural control and epigenetic nature of the gold mineralization. This is generally true, but some gold zones preserve evidence of a stratabound form.
The geology of the East Malartic mine was described in detail by Eakins (1962). In the late 1970’s extensive metallogenic studies were carried out by the author (company report, 1977). Numerous mining problems were encountered during this time, such as caving and rock bursting in the lower levels, thus several areas of the mine were being abandoned and were closed to the exploration efforts. However, some successes were also realized such as the development of a “forgotten” crown pillar on the East zone and the discovery of a stratigraphic mineralized horizon on the 22nd level. The former led to the development of the first known (1977) open pit operation on an underground gold mine in Quebec. Rare exposures of the Sladen Fault were also observed there (Fig.6 & 7). Subsequently, crown pillar exploitation was also carried out in other mines of the area such as the Barnat & Canadian mines in the Malartic area and the Macassa mine in Kirkland Lake. The discovery of what appeared to be a rich stratabound gold horizon took the mining company by surprise and delayed development. Eventually, only partial exploitation of the vein-type deposit was carried out in a mine that was accustomed to the long-hole, large-tonnage method of mining.
The studies also demonstrated that the gold at East Malartic did not occupy a single mineralized horizon, but was emplaced in five distinct deposits. On the adjacent Barnat property a sixth type of gold deposit comparable to a porphyry type has been described (Issigonis, 1980, and Robert, 2001).
Several authors published their investigations sponsored by the Quebec Government in the late 1980’s, but the mine had shut down a long time before and access to the underground workings was no longer available.
MINE GEOLOGY
The gold deposits of the Malartic Belt have formed within and around the so-called Malartic Tectonic Zone (MTZ) (Sansfacon & Hubert, 1990). This is a 600 –900 m wide belt of komatiite / basaltic flows – the Piche group - located between two sedimentary domains of different ages: the older Malartic Group to the north (part of the Cadillac Group) and the Pontiac Group of mostly greywacke sediments to the south.
The contacts on either side of the Piche Group are marked by wide vertical to sub-vertical faults. The north contact is the Malartic Fault, while the Sladen Fault (seen on exposures in Fig.6 & 7) follows the southern contact and to the west of East Malartic, it crosses into the Pontiac sediments in the adjacent Sladen and Canadian Malartic Mines.
Numerous intrusions of early diorite-gabbros and late monzonitic Quartz-Feldspar Porphyries (QFPs) cut across the Piche Group. Movement along these faults was instrumental in the formation of the gold deposits. The movement created the openings in the brittle intrusions and greywackes, and to a lesser extent in the komatiite flows, altering the host rocks and depositing quartz, biotite, carbonate and pyrite with microscopic gold. In the QFPs potassium feldspar is the most common alteration mineral.
Sansfacon & Hubert (1990) have recognized two stages of deformation. The first stage is associated with the intrusions, which were subjected to the second deformation. They postulate the alteration and mineralization took place prior to the second stage.
The East Malartic gold deposits that were mined between 1935 and 1979 belong to four different ore horizons as set out on Table 1. The fifth type was discovered on the 22nd level just before the mine closed.
Table 1 : Gold Production at East Malartic
per Deposit Type
and new reserves outlined on the 22nd level (1,050 m)
Location Ore Zones Tons mined grade grade tonnes Au% of Au
3.North of Sladen Porphyry Swarm 990,000 0.114 3.9 3.6 4.5
4. Along Sladen F. East Porphyry 4,720,000 0.117 4.0 17.6 22.0
TOTAL PRODUCTION 16,700,000 0.151 5.0 80.2 100.0
RESERVES ESTABLISHED ON THE 22ND LEVEL IN 1977-8, partly exploited
Ore Zones Reserves grade grade tonnes of Au
Table 1 : Gold Production at East Malartic per Ore Horizon
Table 1 : Gold Production at East Malarticper Deposit Type
and new reserves outlined on the 22nd level (1,050 m)
Location Ore Zones Tons mined grade grade tonnes Au% of Au
----------- ------------- ----------- oz/t g/t ----- ---------
1. Along Sladen Fault Main 9,820,000 0.173 5.9 54.3 67.7
2. Along Sladen Fault East 990,000 0.150 5.1 4.7 5.8
3.North of Sladen Porphyry Swarm 990,000 0.114 3.9 3.6 4.5
4. Along Sladen F. East Porphyry 4,720,000 0.117 4.0 17.6 22.0
TOTAL PRODUCTION 16,700,000 0.151 5.0 80.2 100.0
RESERVES ESTABLISHED ON THE 22ND LEVEL IN 1977-8, partly exploited
Ore Zones Reserves grade grade tonnes of Au
----------------- ------------ oz/t g/t --------------
3. North of Sladen Porphyry Swarm 816,750 0.304 10.4 8
(high assays cut to 68 g )
5. South of Sladen Chert & Wedge of 2,612,250 0.118 4.0 10
Altered Greywacke (high assays cut to 34 g )
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and new reserves outlined on the 22nd level (1,050 m)
Location Ore Zones Tons mined grade grade tonnes of Au % of Au
----------- ------------- --------------- oz/t g/t --------------- ---------
1. Along Sladen Fault Main 9,820,000 0.173 5.9 54.3 67.3
2. Along Sladen Fault East 990,000 0.150 5.1 4.7 5.8
3.North of Sladen Porphyry Swarm 990,000 0.114 3.9 3.6 4.5
4. Along Sladen Fault East Porphyry 4,720,000 0.117 4.0 17.6 22.0
TOTAL PRODUCTION 16,700,000 0.151 5.0 88.8 100.0
RESERVES ESTABLISHED ON THE 22ND LEVEL IN 1977-8, partly exploited
Ore Zones Reserves grade grade tonnes of Au
----------------- ------------ oz/t g/t -----------------
3. North of Sladen Porphyry Swarm 816,750 0.304 10.4 8
(high assays cut to 68 g )
5. South of Sladen Chert & Wedge of 2,612,250 0.118 4.0 10
Altered Greywacke (high assays cut to 34 g )
Most of the gold deposits were located along the Sladen Fault that continued further west into the Sladen Malartic and Canadian Malartic Mines and plunged some 50 degrees to the east. The easterly plunge of these ore zones is obvious in the three – mine longitudinal section shown in Fig.1.
Fig. 1 Longitudinal section of the East Malartic & adjoining Sladen and Canadian Malartic mines looking north
The Main zone is located at the intersection of the Sladen and South Malartic faults. The zone decreased in thickness with depth for unknown reasons. The absence of significant mineralization at depth at the same fault intersection suggests that either the fault movement was reduced, or that favourable host rocks such as diorite and monzonite (QFP) were not found at depth in great abundance.
The East zone was rather shallow and terminated at a depth of about 300 m. At that depth, the East Porphyry intrusion begins to fill the Piche / Pontiac contact area and this gives rise further down to the East Porphyry zone. Both the Main and East zones could be part of the same deposit.
The East Porphyry zone was controlled by the mere existence of the massive East Porphyry intrusion along the Sladen Fault separating the Piche lavas from the Pontiac sediments. The movement along this fault created the openings for the fluids to alter and mineralize the affected rocks. The deposit consisted of variable widths of silicified intrusion and greywacke. The QFP does not intrude the greywackes. Fig.2 shows a typical outline of this deposit on the 22nd level plan while in section (Fig.1) the deposit is unlike the other zones and extends vertically.
Fig.2 Plan of the 22nd level, East Malartic mine showing mined deposits (Main & East Porphyry) and new outlined deposits
(Chert, Wedge & Porphyry Swarm)
Some alteration and mineralization was intersected in narrow zones across the massive QFP away from the East Porphyry deposit. The northern contact was also altered and mineralized, but of low grade (1.7 g/t over 7.5 m). The intrusion extends some 1,270 m vertically, but the mineralization along the Sladen Fault extends only for 820 m. The grade diminishes with depth suggesting that maybe there was less movement at depth.
Deposits of the Porphyry Swarm Zone are located about 150 m north of the Sladen fault and are parallel to it. They are named after the several mineralized QFPs that comprise the deposit plus intervening lavas. These deposits are unique and difficult to outline, but are generally of much higher grade than any of the others at this camp. Throughout the Malartic area QFP bodies are numerous and occur in many stratigraphic horizons within the Piche group, but they are more abundant at East Malartic.
An example of their distribution is shown in Fig.3 from the 11th level (529 m) mine plan. Apparently, the deposit stretches across two different QFPs, and is clearly epigenetic.
Fig.3 Part of this deposit was mined
Since this zone was difficult to define by drilling, it was not pursued on some levels. This zone was only mined down to the 16th level (757 m), although it was encountered by drilling on the 31st level (1,477 m). In 1977, the zone was discovered by drilling on the 22nd level (1,050 m) (Fig.2), the only level accessible for exploration at that time.
The grade on the 22nd level appears to be much higher than elsewhere on this zone. This may be caused by the general mine procedure of cutting high assays down to 34 g ton. However, the Porphyry Swarm zone is much different than other zones in that high assays are not uncommon, therefore, a higher cut grade of 64 g was justifiable to use for the calculation of the new reserves on Table 1.
Large QFPs were emplaced within the MTZ in the East Malartic mine and immediately adjacent mines. They are small and uncommon further away to the east and west of Malartic. The abundance of komatiite flows with spinifex effects suggests an ocean environment into which the QFPs were intruded with well - developed contact envelopes.
The gold deposits that formed at East Malartic were confined to the contact areas of the QFPs rather than throughout the intrusions as was the case at the Barnat mine (Issigonis, 1980). The mineralization is structurally controlled and sometimes extended beyond their contacts. The surrounding lavas are altered only near the intrusions. Visible gold is very common at the contacts of the QFPs especially if they are near major faults such as the Sladen Fault. Examples from the 22nd level are in Fig.4.
Numerous apophyses or tongues of the intrusions extend to the east and west of the mineralized QFPs such as the East Porphyry and these are usually high grade, commonly with visible gold such as those in Fig.4. The east-west direction of these apophyses coincides with the direction of movement along the Sladen Fault. This implies that emplacement was contemporaneous with movement and mineralization.
Fig.4 Quartz-rich portions of QFP showing abundant quartz (white) in veins. Visible
gold is common along the quartz veins, while larger concentrations are marked
with yellow circles on the walls of the mine opening.
gold is common along the quartz veins, while larger concentrations are marked
with yellow circles on the walls of the mine opening.
Eastward projecting arm of the East Porphyry, 22nd level
Exploration by drifting on these porphyry tongues provided some of the most exotic & memorable places an exploration geologist can hope for in an otherwise low- grade gold mine.
Some rich intersections of contact areas of QFPs encountered during exploration on the 22nd level were:
- 553 g/t over 0.6 m or 227 g/t over 1.5 m
- 84 g/t over 2.1 m
- 20.4 g/t over 9 m
- 10.7 g/t over 18 m
Thin wedges of altered lavas between adjacent QFPs are commonly well mineralized.
The northern contacts of the intrusions are quartz-rich, while the southern contacts were mafic-rich. Mineralization, however, could be in either contact, irrespective of mineralogy. Thus, it would appear that the southern part of the intrusion cooled earlier forming a mafic-rich portion with a more felsic portion towards the north, or higher up in the stratigraphic sequence.
The new type of deposit was discovered in 1977 that appears to be parallel to the bedding in the greywacke sediments and is located some 100 m away from the Sladen Fault. It is remarkably uniform in thickness and grade along its 180 m of strike length that was explored by drilling. It had been exposed along the main haulage drift and a cross cut drift long before, but was only revealed after the walls were washed for observation during exploration efforts. A thick Wedge of Altered Greywacke around the chert horizon developed near its intersection with the Sladen Fault. The Wedge is about 150 m long on the 22nd level, where it was first outlined. The thickness averages 14.5 m and the grade 3.9 g/ton.
The discovery of stratiform chert on the 22nd level within the Pontiac Group south of the Sladen Fault was a completely new environment mineralized with gold. On the surface maps altered greywacke zones south of the Sladen were developed before, but they were shear zones (Sansfacon & Hubert 1990) with bands of silicified greywacke.
The apparent continuity of the mineralized chert on the 22nd level laterally and vertically showing more or less constant widths and grades led to some drifting and limited mining prior to the mine closure. Large-scale exploration was not possible at this time due to lack of access. However, data was compiled from previous exploration on other levels and reserves were calculated. In composition, the Chert zone contains fine muscovite and calcite with up to 5 % pyrite. Its ultra-fine layering is prominent (Fig. 5) as well as the fragments of volcaniclastic material that can make up to 90% of the rock (Kerrich, 1983). The Chert zone averages 4 m thick while its cut (down to 34g) grade averages 4.6 g/ton, and includes some spectacularly rich sections over 64 g ton. Minimum strike length on the 22nd level is 180 m, but its true extent is unknown. If it extends all the way to the surface this chert horizon would be exposed between mine sections 8,000 and 9,000 East.
Fig.5 Chert zone looking east, 22nd level Chert zone looking west, 22nd level
The yellow line marks the footwall with Yellow line with arrows is the hanging
the greenish altered greywacke wall of the chert
Until the discovery of the Chert zone the source of the gold in mineralized greywacke was uncertain. Kerrich (1983) describes the chert horizon as exhalite based on petrographic, isotopic and geochemical investigations. This could be the source of some of the gold along the Sladen Fault especially the East Porphyry zone.
PROPOSED EVOLUTION OF THE ORE ZONES WITH TIME
The important stages in the development of the five gold deposits at East Malartic appear to have formed in three stages.
Stage I
The development of a rift zone in this area led to the extrusion of komatiite flows followed by basaltic flows. The first investigators had described this rift as a major peridotite sill. Subsequently, exploration drilling & drifting encountered well preserved spinifex bands throughout the mine. Some of the earlier basaltic flows are topped with syngenetic gold-rich cherts, which would have been portions of mid-ocean rifts. A mixture of pyrite, quartz, amphibole and calcite mark the top of such flows and have been mined as ‘diorite’ deposits in the Barnat, Sladen and surrounding mines. However, they have not been found at the East Malartic mine that developed at lower (further south today) stratigraphic horizons entirely within the komatiite lavas.
At approximately the same time but further to the east, greywacke sediments were being deposited on a submarine slope. The greywackes are made up of quartz, feldspar and biotite, probably submarine turbidites. When the area was close to a mid-ocean rift a gold-rich chert was deposited. The presence of volcaniclastic fragments suggests temporary subaerial conditions (Kerrich, 1983).
Stage II
At a later time the area was located close to a subduction front possibly in front of a continental mass with a developing mountain chain. The sequence of komatiite / basalt flows tipped towards the north of today, probably towards a continental mass. Movement created openings and melting took place. Thus, the volcanic sequence was pushed to acquire an almost vertical posture (preserved till now) and was infused by numerous intrusions and some batholiths. The subduction front coincided with the extent of the 3 adjoining mines today, about 6 km from east to west. There are numerous intrusions in the Barnat mine (higher up in the stratigraphic sequence), but a small proportion of them are mineralized due to lack of movement away from major faults. The hydrothermal fluids that accompanied the intrusions leached gold from the surrounding formations (lavas, sediments) and deposited it as low-grade concentrations within the intrusives. Comparatively higher-grade gold was deposited at the contacts of intrusions lower in the sequence and some were of economic grade. The northern contacts, or upper parts, became quartz-rich while the southern contacts mafic-rich. Investigators have variably described the intrusions as monzonites, syenites, or just QFPs. The magma intruded from the base of the ultramafic sequence towards the top, or towards the north of today.
East-west movement associated with the development of the Cadillac Fault was taking place at this time. This sheared the northern and southern edges of the Piche Group. The southern Fault, the Sladen, crosses over into the Pontiac Group west of the mine boundary. The overall lateral movement appears to be dextral with an estimated horizontal displacement of 12 km. As the plunge of the fault is 50 degrees eastwards, this would give a true displacement of less than 10 km. The attitude of the fault is visible on the surface (Fig. 6 & 7).
East-west movement with a 50 degrees plunge eastwards created the Main Zone, East Zone, Porphyry Swarm Zone as well as the deposits of the adjacent Sladen and Canadian Malartic mines. The porphyry intrusions were contemporaneous with the movement. Their fluids were responsible for depositing gold within pyrite crystals. The sulfur was provided by the magma, and the iron from the alteration of iron-bearing minerals such as amphibole. Other intrusions further away from the faults have only low-grade mineralization and this is confined to their contacts.
One notable oddity is the #4 porphyry deposit of the Barnat mine that was located at the intersection of the North and South Malartic faults right in the middle of the Piche Group and surrounded by altered volcanics. Thus, the porphyry was emplaced during movement at this fault intersection and plunges westward. Not much has been written about this unique porphyry deposit that was mined in the Barnat mine. For geologists with a Malartic experience the #4 Porphyry was a mystery deposit, a geological puzzle that few tried to explain.
Porphyries, whether ore bearing or barren extend east- west with an easterly plunge of about 50 degrees. In other words, some are genetically associated with the mineralization. On the other hand, ‘diorite’ and porphyry deposits on the Barnat mine were vertical in outline, something like the East Porphyry zone, so they must have been emplaced either before (like the diorite zones) or after the main intrusion event.
Fig.6 The vertical Sladen Fault looking south. Metasediments of the Pontiac Group showing scratch marks from the movement of the
Piche Group downwards and towards the left of the picture. The softer, Piche Group lavas in front of the vertical plane of the
fault have long since weathered away
Fig.7 The vertical position of the Sladen Fault is obvious from this picture looking west.
Almost vertical scratch marks on the surface reveal the direction of movement along
this fault. The softer Piche Group of ultramafic lavas in front of the fault eroded away
prior to mining.
Almost vertical scratch marks on the surface reveal the direction of movement along
this fault. The softer Piche Group of ultramafic lavas in front of the fault eroded away
prior to mining.
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Table 2 SUMMARY OF EVENTS
STAGE ROCKS EVENTS GOLD ZONES PLUNGE
I lavas (Piche) rift system Chert 50 ‘ west
Sediments (Pontiac)
II intrusions subduction, Main, East, 50 ‘ east
faulting, deformation Porphyry Swarm
III faulting Altered Greywacke 50 ‘ west
Wedge
East Porphyry 90 ‘
Stage III
Lateral movement from plate reorganizations brought the gold-rich chert horizon, which originally must have formed further to the east, close to the intersection of the Sladen and South Malartic faults. The lateral movement between the Piche and Pontiac created eventually a 10 to 70 m thick zone of alteration within the softer Piche Group. The alteration mineralogy in this contact is variable. Within the Piche it is biotite, magnetite, talc and actinolite, while in the Pontiac it consists of biotite with silicification and carbonatization. In the QFPs it is K-feldspar with biotite or chlorite.
East-west movement removed some gold from the Chert zone and deposited it along the Sladen fault in contact with the East Porphyry. This movement resulted in the formation of the East Porphyry zone (entire contact of East Porphyry with the Pontiac Group, as well as the Altered Greywacke Wedge. The East Porphyry zone extended vertically and the grade diminished at depth away from the Chert zone / Sladen intersection. The East Porphyry zone formed only where the brittle porphyry moved against the hard sediments of the Pontiac Group unaffected by the soft flows of the Piche Group. The Chert zone and the Altered Greywacke Wedge zone appear, in the light of limited data, to plunge west at about 50 degrees.
The Chert and East Porphyry zones do not come close to any of the other zones to evaluate their age relationships. It is apparent that both the Chert and East Porphyry zones have different attitude (strike, dip and plunge) than all the others, and therefore had a different origin (see Table 2).
CONCLUSIONS
Gold mineralization at East Malartic does not occupy a single horizon, but developed in five separate deposits that formed in different stratigraphic / tectonic environments at different times. From the studies and exploration efforts in the late 1970’s prior to the mine closure and while the underground access was still available it is possible to unravel the sequence of events that led to the deposition of gold.
The occurrence of gold in this area within a sequence of ultramafic / mafic lavas attests to a rift environment. Subsequent crustal movements brought two sedimentary regimes of different ages across faulted contacts with the lavas. The numerous subvolcanic porphyritic intrusions, that are characteristic of the Malartic gold camp and form the host rocks to many of the gold deposits, indicate a subduction environment. It is during this stage that most of the gold was deposited in brittle formations such as altered intrusions, greywackes and some in the lavas.
A gold-rich exhalite zone was deposited within a sequence of greywacke sediments. When one end of this zone moved against the lava sequence some of the gold was remobilized to form a new mineralized horizon along a nearby intrusion/ greywacke contact area.
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