Hi Koan,
Tables are screwed up, but this paper presented at the last PDAC gives a flavor of what's happening at Lac des Iles.
Distinguishing the Ore Types at the Lac des Iles PGE-Gold-Copper-Nickel
Mine,Ontario:Implications for Resource Modeling, Mining and Processing
Michael J. Michaud
(SRK Consulting)
Maurice J. Lavigne
(North American Palladium Ltd.)
Abstract
A cornerstone of any successful mining project is an understanding of the mineral assemblage and metal distribution
comprising the deposit. Knowledge of the precise mineral/ metal zoning enables an operation to utilize different
mining and processing methods optimized for different parts of the deposit. The Lac des Iles mine, located in
northwestern Ontario, is one operation that depends on a good understanding of these issues. Current resources for
the Roby Zone total 94.1 million tonnes grading 1.66 g/t Pd, 0.18 g/t Pt, 0.14 g/t Au, 0.062% Cu, and 0.053% Ni.
The Roby Zone occurs within the gabbroic portion of the Lac des Iles intrusive complex and consists of three
compositionally and texturally different ore types with variable PGE, base and precious metal contents. PGEs occur
as sulphides and bismuthotellurides in net-textured sulfides, coarse sulfide blebs and as fine-grained inclusions within
secondary silicates.
Knowledge of the precise mineral/metal zoning has directly and indirectly improved several operational aspects,
including resource modeling, mining, and processing. For example, metallurgical recoveries have been increased by
up to 3-5% and concentrate grades have increased by up to 100%.
INTRODUCTION
Lac des Iles Mines Ltd., a wholly owned subsidiary of North American Palladium Ltd., owns and operates a 3,000
tonnes per day open pit mine and concentrate facility, located 100 kilometers north of Thunder Bay in Northwestern
Ontario (Figure 1). The Lac des Iles PGE-gold-copper-nickel mine has been in commercial production since 1993,
and in 1997 produced 59,477 ounces of palladium, 4,083 ounces of platinum, 3,675 ounces of gold, 963,530 pounds
of copper and 777,684 pounds of nickel (North American Palladium Ltd., Annual Report, 1998).
Under the Industrial Research Assistance Program (IRAP), a two year study of the mineralogy of the Roby Zone was
undertaken with a goal of improving the profitability of the mine. Geological mapping, petrography and geochemistry
of the Roby Zone were completed during this study. Utilization of the detailed mineral/metal zoning identified as part
of this study was implemented at the mine and shown to positively impact many operational aspects including mining
and processing.
REGIONAL GEOLOGY
The Archean Lac des Iles mafic-ultramafic complex forms part of an east-northeast trending linear zone of mafic
plutons extending from Atikokan to Lake Nipigon (Sutcliffe, 1986). The Lac des Iles Complex is the largest of a
series of mafic to ultramafic intrusions defining a circular outcrop pattern that is approximately 30 kilometers in
diameter (Sutcliffe, 1986).
The Lac des Iles complex, measuring 20 kilometers north-south and 8 kilometers wide, is divided into an ultramafic
portion and a gabbroic portion. The ultramafic rocks of the Complex consist of dunite, wehrlite, olivine
clinopyroxenite, westerite and gabbronorite. The gabbroic rocks consist of gabbro, norite, gabbronorite, leucogabbro
and a lesser amount of ultramafic rocks. The gabbroic rocks are host to several zones of significant platinum group
element (PGE) mineralization, including the Roby Zone.
GEOLOGY OF THE ROBY ZONE
The Roby Zone extends approximately 600 meters north-northwesterly and is up to 350 meters wide (Michaud,
1998). The northern portion of the Roby Zone consists of a layered sequence including leucogabbro, gabbro,
clinopyroxenite and gabbronorite (Figure 1). The layers strike north-south and dip 70 degrees to the east. Significant
PGE mineralization occurs within the gabbronorite layers, or reef zone, which typically range in thickness from 1.0 to
2.5 meters (Figure 2a,b).
The southern portion of the Roby Zone consists of a lithologically and texturally complex unit, the breccia zone. The
breccia zone consists of a predominantly medium-grained gabbroic to melagabbroic matrix with pegmatitic gabbro
segregations as patches and discordant dikes (Figure 2e,f). The gabbroic matrix is host to numerous angular to sub-rounded
fragments ranging from several centimeters to several meters in diameter. The fragments are compositionally
and texturally similar to layers in the northern layered portion of the deposit. Textural and compositional variability,
observed in hand sample and in outcrop, decrease gradually to the southwest.
The breccia zone is flanked to the east by a northwest trending, vertically-dipping, 15-meter wide shear zone, defined
by a well-developed mylonitic foliation. The rocks of the shear zone are very fissile and composed almost entirely of
talc, actinolite and chlorite (Figure 2 c,d).
Alteration
Alteration has effected lithologies to varying degrees throughout the Roby Zone and is strongest in the shear and
breccia zones. Within the breccia zone, alteration varies with the host lithology and is commonly strongest along
narrow fractures. Throughout the deposit orthopyroxenes have been altered to talc, anthophyllite and serpentine;
clinopyroxenes have been altered to actinolite, chlorite, tremolite and hornblende; plagioclase has been altered to
zoisite, epidote and sericite. Amphibole pseudomorphs of the original clinopyroxene grains are common in the
breccia and shear zones.
Sulfide Mineralization
Sulfide mineralization varies considerably in habit and concentration throughout the deposit (Table 1). Sulfides rarely
exceed 5 vol.% of the rock and consist of intergrown pyrrhotite, pentlandite, chalcopyrite and minor pyrite; however,
the relative proportion of sulfide minerals varies by rock type. Additional sulfide minerals, including millerite,
violarite, sphalerite and galena, typically occur in trace amounts (Dunning, 1979).
PGE Mineralization
Platinum group minerals are commonly associated with sulfide mineralization, within sulfide blebs and along sulfide-silicate
boundaries (Sweeny, 1989). Twelve platinum group minerals have been identified in the Roby Zone,
including braggite (Pt,Pd)S, kotulskite Pd(Te,Bi)2, isometrieite Pd11(Sb,Te)2As2, merenskyite PdTe2, moncheite
PtTe2, palladoarsenide Pd2As, sperrylite PtAs2, stibiopalladinite Pd5Sb2, stillwaterite Pd8As3, vysotskite Pd,S,
unnamed Ag4Pd3Te4 and Pd5As2 (Watkinson, 1975; Dunning, 1979; Cabri and Lafamme, 1979; Sweeny, 1989).
Palladium also occurs in solid solution in melonite, gold, and pentlandite (Dunning, 1979). The relative order of
abundance of the PGEs varies considerably across the deposit.
Although Pt and Pd are enriched relative to the other PGEs (Pt+Pd/Os+Ir+Rh >1,000), Pd is enriched by an order of
magnitude relative to Pt. The Pd:Pt ratio varies considerably across the deposit, averaging 13:1 in the breccia zone,
15:1 in the shear zone and 19:1 in the reef zone. The variable Pd:Pt ratio indicates that primary zoning of the
magmatic mineralization has been modified during secondary alteration.
TABLE 1: Summary of petrography of ore types comprising the Roby Zone
Ore Type Mineralogy Sulfide Mineralization PGE Mineralization
Reef Zone Unaltered cumulate
hypersthene, labrodorite and
intercumulate diopside and
augite
Fine grained (<100?)
disseminated and net-textured
po, pn and cpy, 2 vol%
Braggite and vysotskite, <50?
in diameter, erratic
distribution, sharp grade
contacts
Breccia Zone Highly variable based on
fragments, matrix comprised
of cumulate labradorite and
diopside, strong, pervasive
alteration produced talc,
anthophyllite, sericite, chlorite
and epidote
Medium grained disseminated
and within blebs up to 1cm in
diameter, po, pn and cpy, 5
vol%, primary mineralization
visible within unaltered
fragments
Kotulskite, merenskyite are
dominant as inclusions within
secondary silicates, sulphide
blebs and as wispy veinlets,
200-500?, uniform
distribution, gradational
contacts
Shear Zone Composed of talc, actinolite,
chlorite and anthophyllite,
amphibole pseudomorphs of
original clinopyroxene grains
Fine grained (<100?)
disseminated and as inclusions
within secondary silicates, po,
pn and cpy, 2%
Kotulskite, merenskyite are
dominant as inclusions within
secondary silicates, <100?
Note: pyrrhotite (po), pentlandite (pn), chalcopyrite (cpy).
IMPLICATIONS OF UNDERSTANDING THE MINERAL/METAL DISTRIBUTION
Resource Estimation
The mineral/metal zoning and an understanding of the controls on mineralization within the Roby Zone were used to
construct a robust resource model that delineated the various ore types/domains according to their physical
characteristics and metal distribution. This provided constraints during grade interpolation, assignment of metal
recoveries, densities, processing and mining costs. All these factors ultimately impact the NSR value of the ore and
need to be predicted as accurately as possible for a successful operation.
The three ore types/domains comprising the Roby Zone were defined based on the mineralogy and metal habit and
distribution, which are ultimately related to the style of mineralization. For example, in the breccia zone, PGE
mineralization is uniformly distributed compared to the more erratic, ?nuggety? mineralization within the reef and
shear zones. An appreciation of the type of domain boundaries, whether sharp, as for the shear and reef zones, or
gradational as for the breccia zone, as well as the local variability of grade, influenced the parameters used during
grade interpolation. A high local variability of grade within the shear and reef zones dictated a greater degree of
averaging during grade interpolation compared to the less variable breccia zone. Domain boundaries ensured that
excessive grade averaging was controlled so as to correspond to the degree of selectivity predicted during mining.
This provided an opportunity to estimate the amount of ?recoverable reserves? within the deposit.
In addition, the mineral/metal zoning and continuity of grade were considered for resource classification.
Variographic analysis indicated that a drill spacing of 30 meters is sufficient to outline indicated resources for the
breccia zone, while a drill spacing of 15 meters is required to classify resources as indicated for the reef and shear
zones.
Mining
Consideration of the mineral/metal zoning provided the mine an opportunity to identify ?a deposit within a deposit?,
i.e. either a smaller tonnage/higher grade deposit or a low grade/large tonnage deposit based on the estimation of
?recoverable reserves?. Mining dilution and recovery were determined based on the grade distribution and the type of
domain boundary. Mining dilution experienced in the open pit has been typically high for the reef zone because of the
high local variability of grade and moderate dip, while dilution and mining recovery are moderate in the shear zone.
In contrast, mining dilution is low and mining recovery is high in the breccia zone due to the uniform distribution of
grades and gradational contacts relative to the other two ore types.
As a result of the good continuity of grade and gradational contacts within the breccia zone, the mining costs are
considerably lower than for the reef and shear zones since blasts within the breccia zone are in the order of 15,000 ?
20,000 tonnes, while blasts in the breccia zone are often only 5,000 ? 10,000 tonnes. In addition, a variety of grade
control techniques including predefinition drilling of the bench prior to mining, lower bench heights and mining
longitudinally (i.e. mining waste first adjacent to ore and then blasting ore along strike into open cut), are essential in
the reef and shear zones to minimize mining dilution and maximize ore recovery. Good grade control is essential in
these areas due to the sharp contacts between ore grade rock and barren country rock.
An understanding of the proportion and spatial distribution of the various ore types has been used for mine planning
to ensure that a supply of known ore-types is maintained at the processing plant.
Metallurgy
Although many of the processing plant variables are commonly determined by experimentation, an understanding of
the mineralogy and metal habit and distribution within each ore type has helped to focus analysis of the processing
procedures and to explain the results. The initial benefit of understanding the mineral/metal zoning has provided
representative samples of the various ore types for mineralogical/metallurgical analysis and testwork.
A number of improvements have been realized in the processing plant upon consideration of the mineral/metal
zoning. One of the most important improvements has been an increase in the concentrate grade, particularly in the
reef zone ore. Average concentrate grades were 12-13 ounces per tonne palladium for the three ore types (palladium
typically comprises 50-60% of the total NSR value of the ore). A microscopic examination of the concentrate
identified that a number of fine grained (<50 ?) PGE minerals and associated sulfide grains were only partially
liberated, commonly referred to as ?middling particles?. Although sufficient amounts of sulfide grains were liberated
and recovered in the flotation circuit, excess silicates were also captured, diluting the concentrate grade. Since
concentrate shipment and custom smelting are based on weight, any increase in concentrate grade positively impacts
the project economics. It was based on this mineralogical discovery that the regrind mill was installed and maintained
to better separate the PGE and silicate minerals prior to the final cleaning/flotation process. After installation of the
regrind mill, concentrate grades increased from 12-13 ounces per tonne palladium to 30-35 for reef zone ore, 15-16
for breccia zone ore and 18-20 ounces per tonne palladium for shear zone ore.
Since the mineralogy of the various ore types is different, stockpiling of the ore has proven to be essential for either
ore blending or campaign/batch processing. Campaign processing is currently the most favourable alternative as it is
possible to identify a ?recipe? for processing the different ores to optimize recovery and concentrate grade, while
reducing reagent costs (Table 2). For example, the feed rate is reduced for the shear zone ore in order to achieve
better liberation and recovery, while the feed rate for the sulfide-rich, (3-5%vol. sulphide) breccia zone is reduced to
allow sufficient retention time in the flotation circuit. This eventually prompted the installation of additional flotation
cells. As a result, metal recoveries have been increased significantly. For example, palladium recoveries increased by
up to 3-5% during 1995.
Direct economic benefits were realized in higher mineral recoveries, improved concentrate grade and quality, and
lower processing costs. This enabled a more definite determination of the economic cutoff grade for ore:waste
selection and allowed for a stockpile strategy to be established.
TABLE 2: Summary of 1995 metallurgical results for various ore types comprising the Roby Zone
Rock Type Pd Recovery
(%)
Concentration Ratio Processing Cost
CDN$/tonne
Reef Zone 81-83 150 $8.00
Shear Zone 75-76 110 $9.75
Breccia Zone 76-78 80 $9.00
SUMMARY
The Lac des Iles mine has utilized an understanding of the mineral/ metal zoning at the Roby Zone to identify and
optimize different mining and processing techniques best suited to different parts of the deposit. The optimization of
these techniques has positively benefited the resource modeling, mining and processing aspects of the mine thereby
improving the overall economics of the operation.
REFERENCES
CABRI, L.J. and LAFLAMME, J.H.G. 1979.
Mineralogy of samples from the Lac des Iles area, Ontario. Canadian Institute of Mining and Metallurgy. Energy,
Mines and Resources, Canada, Report 79-27, 20p.
DUNNING, G.R. 1979.
The geology and platinum-group mineralization of the Roby Zone, Lac des Iles complex, northwestern Ontario.
Unpublished M.Sc. Thesis, Carleton University, Ottawa, Ontario, 129p.
MICHAUD, M.J., 1998.
The Geology, Petrology, Geochemistry and Platinum-Group-Element-Gold-Copper Nickel Ore Assemblage of the
Roby Zone, Lac Des Iles Mafic-Ultramafic Complex, Northwestern Ontario. Unpublished Masters Thesis, Lakehead
University. 196p.
NORTH AMERICAN PALLADIUM LTD., 1998.
Annual Report. 32p.
SUTCLIFFE, R.H. 1986.
Regional geology of the Lac des Iles area, District of Thunder Bay; p.70-75 in Summary of Field Work and Other
Activities 1986 by the Ontario Geological Survey. Ontario Geological Survey Miscellaneous Paper 132, 435p.
SWEENY, J.M., 1989.
The geochemistry and origin of platinum group element mineralization of the hybrid zone, Lac des Iles Complex,
Northwestern Ontario. Unpublished M.Sc. Thesis, University of Western Ontario, London, Ontario, 185p.
WATKINSON, D.H. and DUNNING, G.R. 1979.
Geology and Platinum-group mineralization, Lac des Iles complex, northwestern Ontario. Canadian Mineralogist.
Vol. 17, pp. 453-462. |
