The Following Was Taken From A Field Trip Guidebook
(8th IAGOD Symposium)
On Geology and Ore Deposits of the Timmins District, Ontario.
Geological Survey Of Canada
(open file 2161, field trip 6)
Table 2: Base metal production from the Timmins Resident Geologist's area, Abitibi
greenstone belt, Ontario, Canada (Luhta et al., 1990).
Mine
Township
Date
0re Milled Tonnes
Cu
%
Zn
%
Ni
%
Ag
(g/t)
Au
(g/t)
KOMATIITE-ASSOCIATED NICKEL SULPHIDE
         
Alexo(1)
Dundonald
1912-19 

1943-44

51,529
0.07
 
3.93
   
Langmuir #1 (2)
Langmuir
1973-77
220,000
   
1.5
(open down rake)
   
Langmuir #2 (2)
Langmuir
1973-77
320,000
   
1.3
(open below 400m)
   
Sothman 

(3) 

Sothman
 
231,000
440,000
   
1.3
0.9
   
McWatters (2)
Langmuir
 
181,500
   
1.92
0.73
   
Dundonald (3)
Dundonald
 
Small tonnage, high grade
         
Redstone (4)
Eldorado
 
746,120
0.09
 
2.29
(to a depth of 300m)
   
Hart (2)
Eldorado
 
770,000
   
0.9
(to a depth of 300m)
   
VOLCANIC-ASSOCIATED MASSIVE BASE METAL SULPHIDE
         
Canadian Jamieson (3)
Godfrey
1966-71
800,600 (5)
2.39
4.05
     
Jameland (10)
Jamieson
1969-72
461,805
0.99
0.88
 
3.50
0.05
Kam Kotia(10)
Robb
1943-44
1961-72
6,007,194
1.09
1.03
 
3.50
0.05
Kidd Creek (6)
Kidd
1965-
81,000,000
2.06
7.18
 
106.0
 
United Obalski (7)
Godfrey
1965-66
254
         
Potter (8)
Munro
1967-72
477,572
1.63
1.5
 
3-15
traces
Potter-doal (8)
Munro
1925-30
1,758
15.22
4.15
 
92.5
1.54
PORPHYRY-HOSTED CHALCOPYRITE-M0LYBDEN1TE          
McIntyre (9)
Tisdale
1963-81
10,162,640
0.62
   
0.09
0.023
(1) Noranda

(2) Timmins Nickel

(3) Falconbridge

(4) Timmins Nickel (51%) - BHP-Utah (49%)

(5) Middleton (1975), approximate tonnage

(6) grades supplied by D. Brisbin, Geologist, Falconbridge Limited, Kidd Creek Mine

(7) Concentrate produced, P. Binney, Geologist, Falconbridge Exploration

(8) Coad, 1976.

(9) Giant Yellowknife Mines Limited

(10) Crown Held in Abeyance, pending issuance of exploratory license of occupation (as of May 11, 1990)

4.3- Kidd-Munro Assemblage

This assemblage consists of ultramafic, pyroxenitic, and basaltic komatiite, tholeiitic picrite, magnesium-rich tholeiite, high-alumina basalt, iron-rich tholeiite, icelandite (andesite), and thin units of high-silica rhyolite (Basaltic Volcanism Study Project 1981, Chapter 1.2; see Section 7.1.2.2 for discussion or high-silica rhyolite). Variolitic, massive, and pillowed basalt units also occur (e.g., Leahy and Ginn 1961a,b; Arndt and Nesbitt 1982). Associated with these metavolcanic sequences are layered, tholeiitic (e.g., Munro Lake Complex, Centre Hill Complex, McCool Hill Complex; MacRae 1969) and ultramafic intrusions. Lithologies generally strike east to southeasterly and dip steeply. In the eastern part of the assemblage, units face both to the north and south, which is attributed, in part, to the presence of several east-striking folds and several east- to southeast-striking shear zones. Facing and dip direction of units in the western part of the belt are poorly constrained.

The most comprehensive descriptions of mafic and ultramafic flows are for those which crop out in Munro Township (Arndt and Nesbitt 1982), the location of the classic ultramafic komatiites of Pyke Hill (Pyke et al. 1973). Based on geochemical criteria several types of basalts are recognized (Arndt and Nesbitt 1982): Type 1: LREE-depleted, flat HREE, low abundances of incompatible elements; Type 2: flat REE, enriched in incompatible elements; Type 3: Theo's flow basalts, strongly enriched in incompatible elements, slightly enriched LREE pattern, fractionated HREE; 4) Warden Township basalts, having trace element characteristics similar in some respects to those of "Type 2" basalts (enriched incompatible elements, inferred flat REE patterns). " Type 1" basalts could have been derived from a depleted mantle source (Arndt and Nesbitt 1982). All other magmas could have come from a second source which had roughly chondritic trace element ratios (Arndt and Nesbitt 1982). The extent to which these detailed petrochemical relationships, described from Munro Township, can be applied through the Kidd-Munro assemblage remains to be confirmed.

Rhyolites and high-silica 2715 Ma (Barrie 1990) rhyolites in the western part of the assemblage, near the Kidd Creek Mine, have relatively flat REE patterns ([La/Yb] = 1-4), pronounced negative Eu anomalies (Eu/Eu = 0.20 to 0.61), low Zr/Y (2 to 6), high abundances of high field strength elements, and low abundances of Sc and Sr (Lesher et al, 1986). Felsic metavolcanic rocks from Beatty Township, in the eastern part of the Kidd-Munro assemblage, have an age of 2714 +/ - 2 Ma (Corfu et al. 1989). This dated rhyolite is chemically similar to rhyolites in the western part of Munro Township, and provides the probable age for the komatiites in Munro Township. Rocks and associated base metal mineralization in Munro Township are discussed in more detail In Section 6.3.

4.3.1. Contact Relationships

The northern contact of this assemblage with the Duff-Coulson-Rand assemblage is not exposed, but locally appears to be coincident with a shear zone having a dextral horizontal slip component (see discussion above). The Pipestone shear zone defines the contact with the Hoyle assemblage to the south (e.g., Leahy and Ginn 1961a). No kinematic analysis of this shear zone has been carried out. This western contact of the assemblage is quite irregular and it is not known if this reflects a stratigraphic or structural interleaving of lithologies. It is not known if this assemblage extends to the west of the Mattagami River fault (Fig. 4).

MINERAL DEPOSITS

  1. Fyon and A.H. Green 2
 The deposit-scale characteristics of deposits of several types, that are represented in the Timmins area, are described in the following sections. These descriptions provide the visitor with an overview of the geological characteristics of the geological settings and related deposit types. These descriptions form the basis for depositional models. Accompanying each descriptive component, a tour guide is included. For some mines, the guides are very brief, and of a philosophical nature, because active mining prohibits the definition of a firm tour far in advance. 5.1. Time and Space Relationships In this section, the mineral deposits are considered within the framework of the proposed regional geological setting (Section 4.) and the available geochronological constraints. Relevant gold and base metal production data for mines in the Timmins area are summarized in Tables 1 and 2.

5.1.1. Volcanic-associated, Massive, Base Metal Sulphide Mineralization

In the southwestern part of the Abitibi greenstone belt, polymetallic, massive, base-metal sulphide mineralization occurs in the Kidd-Munro and Kamiskotia assemblages. At the western end of the Kidd-Munro assemblage, the giant (Table 2), stratiform, polymetallic Kidd Creek deposit occurs associated with a sequence of massive to autobrecciated flows and pyroclastic felsic metavolcanic rock and effusive and intrusive mafic and ultramafic igneous rocks (see Section 7.2.). In the eastern part of the Kidd-Munro assemblage, copper-zinc mineralization at the Potter Mine is hosted by hyaloclastite units, within a sequence of tholeiitic olivine basalts and komatiites (Coad 1976; see Section 6.3.3.). Felsic metavolcanic rocks within the Kidd-Munro assemblage have ages of 2714 +/-2 (Beatty Township, immediately west of Munro Township; Corfu et al. 1989) and approximately 2717 Ma (Kidd Creek rhyolite; Barrie and Davis 1990). These dates provide a constraint on the probable age of the Kidd Creek and Potter Mine polymetallic, massive sulphide mineralization in this assemblage.

Polymetallic, massive, sulphide mineralization also occurs within the Kamiskotia meta-igneous assemblage, associated with felsic and mafic metavolcanic, rock within the Kamiskotia metavolcanic complex (see Section 7.1.). Zircon ages for the Kamiskotia gabbroic complex and the Kamiskotia rhyolite are 2707+/-2 Ma and 2705 +/-2 Ma, respectively (Barrie and Davis 1990). Hence, the polymetallic massive sulphide deposits in the Kamiskotia area appear to have formed approximately 10 Ma after those in the Kidd-Munro assemblage.

Polymetallic, massive, base-metal sulphide mineralization occurs elsewhere in the southern Abitibi greenstone belt in older (e.g., Normetal deposit in 2730 +/-1.5 Ma felsic volcanic rocks; Mortensen l987) and younger (e.g., Noranda deposits in 2700 Ma felsic metavolcanic rock; Mortensen 1987) metavolcanic assemblages.

KOMATIITE-ASSOCIATED NI-CU-PGE MINERALIZATION
A. Green1 and D. MacEachern2

The most important association of nickel mineralization in the Timmins area is the komatiite-hosted one, and the area represents one of the best examples of this type outside of the type example at Kambalda in the Yilgarn block of Western Australia.

The purpose of this chapter is to provide basic information on nickel deposits of this association in the Timmins area, and on the komatiites and other rocks with which they are closely spatially associated. It is the intention to provide information of a nature that will be helpful in the development and refinement of ore deposit models for these deposits. Some specific descriptions of field trip stops are given but, as with other sections of the guide, it is not possible to guarantee access to mines at the time of writing this guide.

6.1. Geological Setting The komatiite suite of rocks, from basalt, pyroxenitic basalt, peridotite and dunite (Arndt and Nisbet 1982) are well represented in the southern half of the Abitibi Belt, as shown on figure 1. These rocks are most readily identified by the distinctive spinifex texture, which is relatively common in the upper parts of the flows of basaltic to pyroxenitic to peridotitic composition. In areas of poor outcrop, the full extent of komatiites is difficult to determine, hence the map is only an indication of their minimum extent.

Sufficient evidence from precise U-Pb dating shows that the komatiites are not all of the same age (see Sections 4.3, 4.6., 4.11.). However, there are insufficient data to confirm the probably different ages of the komatiites hosting nickel deposits in the Alexo-Dundonald and Shaw-Bartlett Dome areas.

6.2 Nickel Deposits of the Timmins Area Known deposits in the Abitibi belt are clustered around the Timmins, Ontario and Malartic, Quebec areas (Fig. 1). The only known dunite-hosted deposit of the Mt. Keith association is the Dumont in Quebec (Duke 1986). All other known deposits are associated with peridotitic komatiites, variously interpreted as flows and sills. These are most commonly known as the Kambalda type of deposits. Of the nine nickel deposits in the Timmins area (Table 2) eight are hosted by komatiites. The ninth, the Montcalm deposit is hosted by a tholeiitic intrusion. It has been described in detail by Barrie and Naldrett (1989). The deposits are listed, with estimates of their sizes and grades, in Table 2. 6.3. Pyke Hill, Munro Township The Komatiites exposed on Pyke Hill in the centre of Munro Township, Ontario (Fig. Nl1, Nl2), have become accepted as a type example of ultramafic flows. Many of these flows are layered with spinifex-textured upper parts and olivine cumulate lower parts (Pyke et al. 1973). There is a regular variation in the size and orientation of skeletal olivine grains in the spinifex textured lava, from fine, randomly-oriented tablets at the tops of the flows, to larger, parallel plates lower down. The presence of spinifex texture has become recognized as a diagnostic feature of komatiite flows (Nesbitt 1971; Donaldson 1982; Arndt and Nisbet 1982), and the distribution and variation in size of olivine grains in spinifex textures are commonly used to delineate flow units and to determine facing directions in sequences of flows (Pyke et al. 1973; Barnes et al. 1974; Pyke 1982).

The Pyke Hill outcrop is unique and has already suffered damage from visitors. Please do not use hammers or dislodge samples here. This outcrop lies 300 m east of the abandoned Potter Mine, at the edge of the tailings pond. The outcrop is comprised of unusually well exposed and little altered komatiitic lava flows described by Pyke et al. (1973) and Arndt and Naldrett (1987). The spectacular spinifex bearing flows are an end member of a continuum from spinifex rich flows to uniform ones in which spinifex is absent. A map of the outcrop is shown in figure Nl3. Typical sections through the end member flows are shown in figure Nl4. The following description is taken from Arndt and Naldrett (1989).

The spinifex-textured flows consist of:

Al zone - sparse olivine phenocrysts in a matrix of altered glass. This is the flow top and displays joints (cooling cracks) breaking it up into polyhedra;

 A2 zone - skeletal olivine blades become progressively coarser, and are randomly oriented in altered glass with augite needles;

A3 zone - olivine blades develop as randomly oriented books that begin to align at right angles to the flow;

B1 zone - thin zone of tabular skeletal olivine grains;

B2 zone - polyhedral and equant olivine occupies up to 80 percent of the rock with interstitial augite needles and altered glass;

B3 zone - knobby weathering of clots of augite rich matrix is distinctive here, but not common In komatiites outside Munro township;

B4 zone - olivine gives way to altered glass in this basal chill zone of the flow.

More than half of the olivine porphyritic flows on Pyke Hill have no spinifex texture. They are pervaded by polygonal jointing, which is more intense near the flow margins. In the southwest portion of the outcrop lava toes are well exposed. They have massive interiors but their margins are defined by zones of intense polygonal jointing. 6.3.1. Komatiite Lava Lake Arndt (1986) has described a komatiite lava lake in the centre of Munro Township (Fig. Nl5 and Nl6) and the following Is a summary of his findings. The lava flows face to the north, and the oldest rocks, in the southwest, are normal spinifex-textured and massive komatiites. The lava lake immediately overlies these flows, and is in turn overlain by a thick sequence of mainly mafic pyroclastic rocks. A small deposit in the upper part of the sequence, the Potter Mine, is found in the upper part of the pyroclastic sequence of komatiite flows. These are on strike with the Pyke Hill komatiite flows some 300 metres to the east. On the east side of the lava lake, the volcanic rocks are intruded by, or are in faulted contact with, gabbros from the upper portion of a large layered mafic-ultramafic intrusion called the Centre Hill complex (MacRae 1969). This contact will be described below.

The flows at the top of the sequence are normal spinifex-textured and massive komatiites. They range in thickness from one to thirteen metres and have textures and mineralogies just like those of Pyke Hill, described above. The flow sequence underlying the lava lake is made up of komatiites and pyroxene spinifex-textured basalts.

The lava lake is about 120 metres thick (Fig. Nl5). The lower two-thirds is composed of massive, medium-grained dunite, now largely but not completely serpentinized, and upper third is composed of fine-grained olivine porphyry. The upper 30 to 40 metres of the unit are cut by veins with unusual swirling tabular olivines, and in the uppermost 10 to 20 metres numerous spinifex-textured veins appear.

6.3.1.1. Dunite The dunite is an olivine accumulate. Before serpentinization most samples contained between 70 and 80 percent olivine and minor chromite. Where the olivine grains are not intergrown and do not interfere with one another, most of them are euhedral or distinctly rounded, but a small fraction are more elongate and tabular. Grain size varies from about one millimetre in the lower 20 metres to a maximum of about two millimetres halfway up through the unit. The olivines have relatively uniform compositions, with cores of Fo89 and margins of Fo88. Other cumulus phases are chromite, which occurs as relatively large (0.2 millimetre) euhedral grains, and altered orthopyroxene. 6.3.2. Centre Hill Complex The Munro Lake sill, according to MacRae (1969), is a grossly layered intrusion exposed in three principal areas: Centre Hill, McCool Hill, and a broad area at the boundary between Warden and Munro townships. The body is indicated to be continuous by aeromagnetic maps, and the stratigraphic sequences or rock layers in the three principal sections are strikingly similar. The sill is approximately 11 kilometres in length and 500 to 1000 metres thick. It is folded along a major east-west syncline having almost vertical limbs, and is complexly broken by both longitudinal and cross faults. Minor folds are common on both limbs, especially between Centre Hill and the Warden-Munro areas.

The Centre Hill outcrop area is the only one showing a complete stratigraphic section of the Munro Lake sill between its lower and upper contacts. Within the exposure at Centre Hill, the intrusion is essentially vertical. It generally has an east strike, but it is folded sharply southward at its western end where MacRae (1969) interpreted it to be dragged against and cut off by the Centre fault (Fig. Nl2). Both the northern and southern (or upper and lower) contacts of the sill with volcanic rock are exposed. On the north it has intruded a mafic fragmental rock and on the south, basalt. The contacts are sharp and at places there is a slight intertonguing of the intrusive and volcanic rocks. Immediately adjacent to the lower contact, the intrusive rock may have been slightly chilled, but an exact interpretation is difficult because of the subsequent metamorphism and alteration of the rock.

The following description of the Complex is provided by Coad (1976). "The complex is not a simple layered body, but is composed of many alternating layers of ultramafic and mafic rocks. MacRae (1969) recognizes seven cyclic units. The lower five consist of successive layers of peridotite and clinopyroxenite, the sixth is composed of clinopyroxenite and gabbro and the upper most seventh unit consists of a layer of melanocratic gabbro overlain by normal gabbro. The ultramafic layers range in thickness from one to fifty metres and lie in sharp contact with each other. The uppermost gabbroic unit is over 200 metres thick. At the lower contact there is a unit of hornblendite ten to thirty metres thick."

The uppermost gabbro unit is overlain by a fragmental rock which is best described as a pillow breccia. This particular rock unit was previously mapped as a rhyolite agglomerate (Satterly 1951); however, It has a composition intermediate between a tholeiitic olivine basalt and picritic basalt. The contact between the pillow breccia and gabbro appears to be faulted where examined in aerial photographs and is seen to be sheared where exposed. The pillow breccia outcrops over a stratigraphic thickness of approximately 60 metres. Drill hole information, together with extrapolation of rook unit thicknesses along strike, indicate that this thick unit of fragmental rock is approximately 75 metres thick in the east, but becomes progressively thinner towards the west, along strike. The eastern extension of the pillow breccia is not exposed because of extensive drift cover and major north south faulting along the eastern edge of the Centre Hill complex. Pillow breccia is stratigraphically overlain by komatiitic lava. To the west, outcrop exposure of the pillow breccia is not continuous along the top of the Centre Hill complex, but drill hole data, together with outcrop exposure further to the west, indicates that this rock unit grades laterally along strike into hyaloclastite (Coad 1976).

6.3.3. Potter Mine The Potter mine is located in the central portion of Munro township immediately north and west of the Centre Hill complex (Fig. Nl1). Arndt (1975) suggests that this complex, a large differentiated peridotite/gabbro body, lies on the southern limb of the major west plunging synclinal structure referred to previously, and that it may be correlated with Theo's flow on the northern limb of the same structure. The Centre Hill complex compares in some respects with Theo's flow. However, the hyaloclastite which forms, a cap to the Centre Hill body grades westward along strike into pillow breccia. Hyaloclastite is not restricted to the top of the complex but intertongues with komatiitic lavas forming three discrete horizons further to the west. The aphanitic pyroxenite member of Theo's flow is absent from the Centre Hill complex; instead a massive, ropy, brecciated, quench-textured tholeiite commonly occurs between the hyaloclastites and the gabbro forming the top of the underlying complex. A further difference is that the complex consists of a number of cyclic mafic and ultramafic units in contrast to the continuous progression of Theo's flow (Coad 1976).

It seems a curious coincidence that the mine overlies the junction of the Centre Hill complex to the east and the Komatiite lava lake to the west. The zone of shearing between these was always assumed to be a fault by MacRae (1969) and others. However the excellent mapping, drill core logging and petrography described by Coad (1976), which is referenced extensively in this section, permits another interpretation. He shows that the hyaloclastite and the mineralized horizons of the Potter Mine lie at the same levels without offset either side of the shear zone. Although some faulting undoubtedly did occur, part of it may have been synvolcanic, and the zone is an excellent candidate for a feeder zone for the mineralization. It is quite possible that the lava lake and the Centre Hill sill or flow lie close to their original positions and represent a facies change.

6.3.3.1. Mine History and Geology The copper at the Potter Mine was first investigated in the 1920's. In 1952 the Centre Hill Mines Ltd. commenced exploration that included 12,190 metres of drilling in 1956. In 1957 a shaft was sunk to 125 metres. The deposit came under the control of Zenmac Metal Mines Limited in 1959 and in 1964 underground exploration resumed. In 1967 production commenced from a shaft deepened to 295 metres, using an on site mill rated at 710 tonnes per day. The mine was sold to the Patrick Harrison Company Limited in 1969 and closed in 1972. Between 1967 and 1972, 485,210 tonnes of ore were milled at an average grade of 1.63 percent Cu, 1.5 percent Zn, 3.1 to 15.5 g Ag/tonne and traces of gold.

The following descriptions are directly taken from Coad (1976). "The mine geology is characterized by two distinct volcanic series, namely komatiitic lavas and tholeiitic olivine basalts. The komatiitic lavas consist predominantly of picritic flows and intercalated with these flows are massive peridotitic komatiites, characterized by an MgO content greater than 30 percent. The massive peridotitic komatiites are not characterized by associated flow tops marked by spinifex texture. The tholeiitic and olivine basalts are chemically distinct from the komatiitic lavas and consist of three different rock types: 1) hyaloclastite; 2) quench-textured tholeiite; and 3) pillow breccia. Although volumetrically insignificant, thin beds of volcanic ash occur at the top of the hyaloclastite horizon and actually represent a third distinct rock sequence, having a dacitic composition."

The predominant rock type in the central portion of the map area is hyaloclastite, a term used to describe a fragmental consisting of pea-sized fragments of pillow breccia and glass (Coad 1976). The hyaloclastite is commonly stratigraphically underlain by quench-textured tholeiitic lava which outcrops over a stratigraphic thickness of approximately 12 metres. The hyaloclastite matrix can consist of ash and fine grained carbonate, quartz, plagioclase and up to 19 percent disseminated graphite (Coad 1976). Volcanic ash occurs at the top of the hyaloclastite units. This ash is composed of grains of quartz and broken plagioclase crystals and it locally is banded and shows load-cast structures (Coad 1976). Thin layers (<0.5 metres) of chert also occur within the ash unit (Coad 1976). The hyaloclastite and quench-textured tholeiite form a wedge-shaped mass at the western end of the Center Hill complex which have been drag-folded and down-faulted stratigraphically into place, or, may have formed approximately in their current position (Coad 1976).

Coad (1976) provides the following description of the pyroclastic rocks in this area. "The pyroclastic sequence contains a lower portion of crudely-bedded well-sorted basaltic scoria and an upper portion of coarser agglomerates and welded spatter, also with basaltic composition and also crudely bedded or massive. Fragments in the scoria average 0.5 to 1 millimetres across. Most are approximately equidimensional, with shapes varying from subangular to rounded, to amoeboid or ribbon-like. Some smaller grains in the matrix have shard shapes. Textures vary considerably, from originally completely glassy (now altered to chlorite), to fine-grained porphyritic with small, altered olivine and clinopyroxene phenocrysts, and in rare cases microspinifex. Most fragments are not amygdaloidal or contain only sparse amygdules (1 to 2 percent), but some fragments have 10 to 15 percent. Amygdules are less than 0.5 millimetres across. One or two fragments in each thin section have a wispy, patchy, ribbon-like form and may be squashed pumice. Each thin section also contains a few, usually larger, angular fragments of exotic rock types such as feldspar-porphyritic felsic volcanics, pale cream relatively pure cherts, and dark-coloured graphite- rich (?) cherts. The matrix is cryptocrystalline cherty quartz, feldspar and chlorite, or carbonate."

"The agglomerates higher in the unit have fragment sizes between one and fifty centimetres, usually around two to five centimetres. Fragments are rounded or have complex amoeboid shapes. They are commonly deformed and moulded against adjacent fragments. They also have basaltic composition and fine grained aphanitic or olivine clinopyroxene porphyritic texture."

6.3.3.2. Mineralization

Massive and matrix sulphide mineralization at the Potter Mine consists predominantly of pyrrhotite, equal proportions of sphalerite and chalcopyrite, and minor pyrite (Coad 1976). Economic concentrations of sulphide mineralization are restricted to the hyaloclastite horizon, but sulphides were remobilized along shears into the adjacent picritic basalt flows (Coad 1976). Iron-rich sphalerite occurs throughout the hyaloclastite horizon, associated with chalcopyrite and pyrrhotite. Iron-poor sphalerite occurs at the top of the hyaloclastite horizon (Coad 1976).

Matrix sulphide is the more common habit, whereby sulphides occupy the interstices between fragments of glass, pillow breccia of the hyaloclastite (Coad 1976). The matrix or disseminated sulphide grades into massive sulphide lenses. Massive ore lenses occur within the upper hyaloclastite horizon, although some massive ore may also have occurred in the lower hyaloclastite (Coad 1976). These lenses, ranging from 15 to 30 metres in length, lie directly on top of the hyaloclastite horizon. The massive sulphide lenses average one metre in thickness, but are continuous in the vertical dimension to a depth of 365 metres (Coad 1976). The massive sulphide lenses were commonly banded with respect to chalcopyrite, sphalerite, and pyrrhotite (Coad 1976).

The massive sulphide is intimately related to volcanic ash which occurs throughout the hyaloclastite unit, but which is more abundant at the top of the unit (Coad 1976). The upper surface of the massive sulphide lense lies in sharp contact with the chilled base of the overlying picritic (komatiitic) flows (Coad 1976). A thin concentration of iron-poor sphalerite may occur along this upper contact (Coad 1976).

Coad (1976) did not observe stringer mineralization. However, stringer mineralization, associated with chlorite alteration, was reported to occur in an area of the mine that was disrupted by shearing (Coad 1976, p.144). Chlorite also occurs as a matrix mineral where the hyaloclastite is sheared or mineralized with sulphides (Coad 1976).

6.3.3.3. Metal Zoning

Despite the abundance of komatiites in this area, the sulphide mineralization did not contain significant nickel, and averaged only 0.03 weight percent (Coad 1976, p. 172). The copper/zinc ratio was close to or less than one (Coad 1976), although the massive ore was zoned from a zinc-rich top to a copper-rich base (Coad 1976). Zinc grades were much less in the vicinity of the strong chlorite alteration zone, supporting the premise that the chlorite alteration represented a feeder-pipe (Coad 1976). Nickel/cobalt ratios of the ore were less than one (Coad 1976).

6.3.3.4. Genesis

According to Coad (1976), the source of metals concentrated in that mine is probably the mafic-ultramafic magma represented by the Centre Hill Complex, in particular the gabbro member. The metals were probably transported either by primary hydrothermal fluids escaping during magmatic differentiation or by secondary leaching involving the convection of sea water. Whether or not sea water penetrated the gabbro, a combination of solutions was probably instrumental in the transportation of metals from the gabbro member of the Centre Hill complex through the overlying hyaloclastite units. The sulphur may have been derived from both sea water and the magma (Coad 1976).