UNUSUAL GOLD DEPOSITS AT MALARTIC, QUEBEC

UNUSUAL GOLD DEPOSITS AT MALARTIC, QUEBEC A. Syngenetic, volcanic chert horizon within the Pontiac sediments B. Exhalative basalt-top  iron  formations,  locally  called  “diorite  ores” C. Structurally  deformed  “porphyry  swarm”  intrusions/extrusions in ultramafic lavas D. Poprhyry-type gold in intrusions/extrusions

PREVIOUS WORK The 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 backfilled caused caving problems in the underground mines.

SLADEN FAULT EXPOSED Pictures 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 oregrade 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.

Fig. 10 Further to the east the fault is buried under overburden

Fig. 11 About 3 m of overburden has been removed to be able to drill the ore-grade greywacke for mining. The width of the ore extends to 25 m in places.

Fig. 12 Part of the ultramafic lava (dark-coloured) that has been preserved north of the fault. The lighter rock above and to the sides of the crosscut is greywacke, south of the fault.

Fig. 13 Dark ultramafic lava with vertical foliation. The fault is near the left corner of the picture with lighter-colored greywacke across it.

Fig. 14 Light-colored ore-grade greywacke to the south of the vertical fault. It has been stripped for mining.

Fig. 15 The width of the overburden varies from zero on the right to over 3 m towards the left.

Fig. 16 The smoothness of the fault plane is prominent during the development.

Fig. 17 A large block of greywacke broke off during the development and slid down the smooth wall of the fault

Fig. 18 Several shades of grey exposed along fault plane. They may be result of faulting, foliation or lithology in the original sediments

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 clayrich 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”.

Dark ultramafic lava with the characteristic 55 degrees plunge towards the east (to the right) with a light-colored porphyry lens (mining hat). The whole outcrop is marked as ore-grade  (“or”   for French-speaking miners). The porphyry introduced the gold into this area. Photograph taken during the mining of zone 2-10 (East Malartic).

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

Fine layering in chert. Besides fine chert there is considerable sericite, calcite and up to 10 % pyrite 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) Section --------47 + 50 E 48 + 00 48 + 50 49 + 00 49 + 50 50 + 00 50 + 50 51 + 00 51 + 00 51 + 50 52 + 00 52 + 50 53 + 00 53 + 50 54 + 00 55 + 00 55 + 00 56 + 00 56 + 00 57 + 00

chert zone in hole -----------------chert next to Cadillac Fault 15  ‘ 55  ‘ 50  ‘ 45  ‘ 25  ‘ 25  ‘ 25  ‘ 20  ‘ + 40 degrees up 8' 20  ‘ X-cut 20  ‘ 20  ‘ 12.5  ‘ 10  ‘ 12.5  ‘ + 30 degrees up 12 ' 10  ‘ 15  ‘ east wall 20  ‘ West wall 15  ‘

grade in oz / ton ---------------------0.166 22-49 stope 0.104 “ 0.139 “ 0.093 “ 0.116 with VG “ 0.138 “ 0.090 “ 0.170 not developed 0.110 0.137 further east 0.170 0.160 0.118 0.070 0.158 with VG 0.160 0.090 0.140 0.110 0.125

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 Fe2O3 MgO CaO K2O Na2O P2O5 Au Cr Ni Sr Ba Zn Cu Rb Y Zr Nb

12.55 4.5 2.32 3.47 3.91 3.95 0.13 ppm 12.88 157 66 656 605 37 20 67 12 117 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”:

All pictures are in the quartz-rich portion of the porphyry. Visible gold has been marked on the wall with a circle.

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 TiO2 Al2O3 Fe2O3 MgO CaO K2O Na2O

% 65.83 0.5 15.16 3.17 1.16 2.91 1.41 7 .07

P2O3

0.2

Au Cr Ni Sr Ba Zn Cu Rb Y Zr Nb

ppm 0.16 29 15 1108 1372 42 12 29 8 209 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.