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The Hemlo district includes three main mines, Williams, David Bell and Golden Giant together exploit the Hemlo orebody. The district is located ~30 km east of Marathon, 350 km east of Thunder Bay and 330 km west of Timmins in southern-central Ontario, Canada (#Location= 48° 41' 47"N, 85° 54' 14"W).
The Hemlo deposit was discovered in 1982 after along history of exploration, and all three mines had commenced production by the end of 1985 (Muir et al., 1991).
The combined production of these mines in 2000 made Hemlo the largest gold producing deposit in Canada with an output of 28.65 t (0.921 Moz). Williams alone was the largest single gold mine in Canada yielding 12.9 t Au.
The Hemlo deposit occurs within a generally east-trending, mixed assemblage of Archaean metavolcanic and metasedimentary rocks sandwiched between the major Gowan Lake and Heron Bay granitoid plutons to the north and south respectively. These supracrustal rocks are part of the Schreiber-Hemlo greenstone belt of the Wawa Sub-province of the Superior Province. The Schreiber-Hemlo greenstone belt appears to comprise of a basal pre-2697 Ma suite of tholeiitic basalt with minor ultramafic to mafic intrusions and flows. This suite is overlain by 2695 to 2688 Ma, calc-alkaline flows and volcaniclastic material, locally intercalated with sedimentary rocks. Quartz porphyritic units were emplaced between 2698 and 2693 Ma and preceded deposition of the sedimentary rocks in that area. Wackes to the southeast are younger than 2693 Ma but older than 2688 Ma. These observations are consistent with a regional sedimentary dominated sequence overlying, and in part derived from, the volcanic units. The sedimentary dominated rocks are locally intercalated with intermediate to felsic calc-alkaline volcanic rocks (as at the Hemlo gold deposit) and some sedimentary rocks may be distal facies equivalents of more proximal fragmental volcanic units. There are not any sedimentary or volcanic rocks known to be coeval with the late stage 2675 Ma plutonic rocks that intrude the belt (after Jackson et al., 1998).
The rocks in the Hemlo district have been regionally metamorphosed to amphibolite grades. An early metamorphism (temperatures of 500 to 600°C and pressures of 5 to 6 kbars) was probably accompanied by the intrusion of the Cedar Lake pluton and produced a mineral assemblage containing hornblende, kyanite, staurolite, garnet and micas from metasedimentary protoliths. A second metamorphic event (temperatures of 300 to 400°C and pressures of 1 to 3 kbars) produced an assemblage of actinolite, fibrolite, andalusite, calcite, garnet, epidote, chlorite and muscovite (Kuhns et al. 1994).
The Hemlo deposit is localised within a domain of generally east-west striking (110 to 115°), tightly folded, generally north dipping (60 to 70°) clastic sedimentary rocks with an intercalated suite of intermediate to felsic volcanic rocks. It lies in the hanging-wall of the transition zone from dominantly volcanic rocks to the south to dominantly sedimentary rocks in the north. The deposit is also only a few hundred metres north of the regional Hemlo reverse shear zone that separates the sequence of volcanic and the sedimentary rocks to the north, from the southern domain of mafic volcanic and clastic sedimentary rocks (Jackson et al., 1998).
Within the mine area, the host sequence has been divided into 8 lithological (not necessarily stratigraphic) units that may be repeated within the sequence, but are generally from south (mining footwall) to north (mining hanging wall) after Kuhns et al. (1988; 1994):
• Unit 1, Lower hornblende-bearing schists and amphibolite - amphibolite to plagioclase-quartz-biotite-tremolitic-hornblende schist, which occurs as bands of 0.1 to >3 m thick interbedded with biotite schists of the footwall sequence. It has no appreciable alteration or mineralisation, except for limited epidote and pyrite with late fractures and brittle faults.
• Unit 2, Lower (southern) metasedimentary schists and granofels - defining the mining footwall sedimentary rocks, and are very similar to the sequence described under unit 7 below.
• Unit 3, Quartz porphyroclast muscovite schists - which contains porphyroclasts of quartz and feldspar, which represent deformed relicts of pre-existing phenocrysts. It occurs as several sub-types which are found over 50 to 80 m of the section between the structural footwall and hanging-wall schists in direct spatial association with mineralised zones. It locally hosts mineralisation, contains tourmaline and frequently pyrite, and locally contains biotite, V-Ti muscovite, microcrystalline garnet, barite, anhydrite, fluorite and rare scapolite. Anomalous gold is characteristic of the unit, with local ore grades. This unit, which is well developed at Golden Giant, thins down-dip and to the east in the David Bell mine, and is present as multiple zones to the west in the Williams mine.
• Unit 4, Ore zone-related biotite-hornblende-bearing schists - a strongly foliated and compositionally banded schist, subdivided on the basis of the presence or absence of lenticular fragments.
Subunit 4a contains 1-15% lenticular fragments of feldspar porphyry, 0 to 20%, 0.5 to 8 cm diameter microcrystalline grey quartz granofels, and 1 to 2%, 0.5 to 5 cm biotite schist, supported in a quartz-mica-feldspar-hornblende schist matrix.
Subunit 4b is a non-fragment-bearing quartz-mica-feldspar-hornblende schist. Biotite is dominant over muscovite in both sub-units, while green V-Ti muscovite is locally abundant. Minor calcite and rutile, and traces of garnet, scapolite, barite, and andalusite. In the upper part of the Golden Giant mine, unit 4 hosts gold mineralisation of up to ~8 ppm Au, although large portions contain little or no gold. The distribution of the unit has an overall antipathetic relationship with the bulk of the gold mineralisation and p,inches out rapidly with depth and to the east and west of the Golden Giant mine. Generally unit 4 is found structurally above unit 3, although the two units are locally interbanded.
• Unit 5, Ore zone schists and granofels - which has been sub-divided into three sub-units, namely,
Subunit 5a, a microcline-rich - exhibiting extreme textural heterogeneity, typically with a bluish-grey colour due to molybdenite, and fine-grained (0.05 to 0.6 mm), and a granoblastic mosaic-textured microcline-quartz granofels. It is tectonically fragmented and boudinaged, with molybdenite and native gold-rich granofels fragments supported in an Au-Mo poor granoblastic matrix of barite and/or pyrite, or transposed with the other subunits. Gold is locally intergrown with microcline.
Subunit 5b, a muscovite-rich - a white to yellowish-grey, foliated, microcline-muscovite-quartz schist. Molybdenite and native gold are characteristically foliated and intergrown with muscovite and green mica. This sub-unit, typically has flaser fabric and compositional banding (0.5 to >30 cm) defined by alternations with sub-units 5a and 5b.
Subunit 5c, a biotite-rich mineralised rock - composed of biotite-microcline-quartz schist containing molybdenite and native gold, which is texturally similar to subunit 5b. Gold and molybdenum are intergrown with biotite and green mica, and occur in both the fragments and the matrix. It seems to grade laterally into subunits 4a and 4b and typically has abrupt contacts with the muscovite dominated rock types such as some of the subunits of unit 7.
All three sub-units characteristically exhibit thin (1 to 20 mm) banding, small-scale S and Z folds, and lenticular fragments (boudins) of grey to blue-grey microcline-quartz-molybdenite-gold-bearing rock in a fine to coarse grained (0.5-5 mm) granular barite-quartz or pyrite-quartz granofels or schist matrix. Each is further divided on the basis of i). pyrite-rich, ii). barite-rich, and iii). quartz pod and vein-rich varieties of ore. Contacts between the mineralised subunits and between the ore and country rock are frequently gradational over several metres but are locally abrupt in areas of intense shearing.
• Unit 6, Fragment bearing hornblende-biotite schists - occurs as two heterolithic schist and granofels bands within the unit 7 schists. These bands of unit 6 rocks are each 20 to 40 m thick, 70 to 100 m apart, and 180 to 200 m structurally above the Main ore zone in the Golden Giant mine. Within these bands there are six lithological subunits:
Subunit 6a is a foliated, banded to massive plagioclase-biotite-quartz-tremolitic-hornblende schist which contains biotite-rich lenticular fragments (5 to 35%, 0.1 to 5 cm).
Subunit 6b is a fine-grained quartz-biotite-plagioclase-hornblende schist, similar to 6a, but devoid of lenticular fragments. It contains minor calcite, pyrite, and in some places contains 0 to 0.5%, 1 to 3 mm feldspar or quartz porphyroclasts.
Subunit 6c has a matrix composition similar to that of the previous subunits, but contains lenticular fragments of feldspar but it contains 5 to 40%, 0.5 to >10 cm lenticular fragments of feldspar porphyry and microcrystalline grey quartz granofels. In places the grey granofels contain visible molybdenite and rare visible native gold, and is similar in part to subunit 4a.
Subunit 6d occurs as thin 0.1 to 2 m thick bands of foliated muscovite-feldspar-quartz schist containing 5 to 10%, 2 to 4 mm, quartz porphyroclasts. It is locally weakly calcareous and contains trace green mica.
Subunit 6e is a pyritic (1 to 10% pyrite) equivalent of subunit 6d and is similar to pyritic subunits interlayered with biotite schists unit 7e. It locally contains anomalous gold and molybdenum and green mica.
Subunit 6f a feldspar-biotite-quartz schist containing 5 to 10%, 1 to 3 mm, feldspar 'eyes' or porphyroclasts. It locally contains lenticular fragments of biotite-rich schist, feldspar porphyry and microcrystalline grey quartz granofels.
• Unit 7, Upper (northern) metasedimentary schists and granofels - includes the bulk of the lower (southern or mining footwall) and upper (north, or mining hanging wall) schist rocks and is at least several kilometres thick. It comprises a diverse group of foliated and compositionally banded, fine- to medium-grained metasedimentary rocks. The hanging wall and footwall rocks are similar, but with some differences. Unit 7 is divided into 14 subunits, as follows:
Subunit 7al, Garnet porphyroblastic feldspar-biotite-quartz schist;
Subunit 7a2, Strongly foliated, anthophyllite porphyroblastic microcline-biotite-quartz schist;
Subunit 7b1, Strongly foliated, staurolite (garnet) porphyroblastic plagioclase-mica-quartz schist;
Subunit 7b2, similar to 7b 1, but chloritoid-rich, occurring as a chloritoid porphyroblastic plagioclase-mica-quartz schist;
Subunit 7b3, Banded kyanite-fibrolite porphyroblastic feldspar-mica-quartz schist - kyanite and sillimanite zones are almost exclusively found in close proximity to mineralised rocks (either the main and lower ore zones or the barren pyrite zones) and occasionally may contain up to 20% pyrite and 1 g/t Au;
Subunit 7c1, Strongly foliated, feldspar-muscovite-quartz schist - occurring throughout the Hemlo section but is most common in association with the ore zones and the barren pyrite zones where it can contains up to 6 g/t Au in close proximity to unit 5 and up to 1 ppm Au in the barren pyrite zones;
Subunit 7c2, Weakly foliated, muscovite-plagioclase-microcline-quartz schist - occurring as thin (0.5 to 5 cm), sometimes weakly gold mineralised, layers adjacent to the barren pyritic zones of subunits 7cl and 7e;
Subunit 7d, Lenses or pods and 1 to 10 cm thick bands of massive to weakly foliated biotite-microcline-quartz-hornblende schist-granofels that occurs throughout the sequence at Hemlo;
Subunit 7e, Laminated, locally calcareous, pyritic feldspar-muscovite-quartz schist (barren pyritic zone rocks), and anomalous in gold (up to 1 g/t);
Subunit 7f, The most abundant rock type in the Hemlo section, typified by laminated (1 to 10 mmm) and banded (1 to 5 cm) feldspar-biotite-quartz schist;
Subunit 7g, Similar to 7d but occurs as coarse grained granoblastic-textured, massive thick-bedded (1 to 15 m) microcline-biotite-tremolitic-hornblende-quartz schist - locally pyritic and gold bearing;
Subunit 7h, Hornblende-biotite-feldspar-quartz schist - an uncommon rock-type, interbedded with 7f;
Subunit 7i, Rounded feldspar-bearing biotite-plagioclase-quartz schist;
Subunit 7j, Muscovite-feldspar-calcite-quartz schist-granofels.
• Unit 8, Hornblende-bearing schists - subdivided into
Subunit 8a, Plagioclase-quartz-biotite-tremolitic hornblende±magnetite schist, after basaltic protoliths - the most abundant of the two unit 8 subunits, occurring as two 1 to 5 m thick principal bands in the hanging-wall of 7f and as thin horizons throughout the Hemlo section;
Subunit 8b, Fragment-bearing plagioclase-quartz-biotite-hornblende schist, also after basaltic protoliths.
The immediate hosts to the ore comprise greywacke, mudstone and minor conglomerate and fragmentals, and a quartz-eye porphyry unit of uncertain intrusive but more likely extrusive origin, but with which the mineralisation has a close spatial association. Post ore dykes of 2680 to 2690 Ma cut the sequence. All of the rock units strike sub-parallel to a strong SSE trending and north dipping S2 penetrative foliation which increases in intensity towards the Hemlo Shear.
The Main ore zone occurs as a 2.9 km long tabular body, concentrated on the northern margin of the porphyry, hosted both by the sheared porphyry of Unit 4 and adjacent fragmentals, extending to a depth of at least 1.35 km and 2.5 km down plunge. The thickness ranges from 2 m in David Bell to 50 m in Williams. A Lower ore zone is found over strike length of ~600 m at depth on the southern contact of the porphyry, ~100 m south of the Main Zone in the central section of the main mines area. This zone is now believed to be a folded repetition of the Main ore zone. Other lesser mineralised zones lie either totally within the porphyry or within adjacent sediments.
Mineralisation comprises both fracture controlled and disseminated pyrite and molybdenite, commonly with associated barite, roscoelite, tellurides and quartz-stibnite-cinnabar-realgar veinlets. It is associated with intense K-feldspar alteration, grading outwards into muscovite within the porphyry and to alumino-silicates in the sediments. Relationships suggest the ore pre-dates the penetrative S2 foliation and may have been emplaced early in the geological history of the district.
In 1997, David Bell & Williams were jointly owned by Barrick-Homestake and Teck Cominco and had respective reserves of:
David Bell: 3.66 Mt @ 10.35 g/t Au (+ resource of 3.75 Mt @ 11 g/t Au),
Williams: underground 17.35 Mt @ 5.7 g/t Au (+ resource of 20.21 Mt @ 6.24 g/t Au),
open-pit 5.40 Mt @ 2.12 g/t Au (+ resource of 4.70 Mt @ 2.37 g/t Au).
Golden Giant is owned by Newmont Canada and has reserves of 4.78 Mt @ 9 g/t Au.
In the longitudinal section above, the Teck Corona claim is worked by the David Bell mine, Hemlo Gold by Golden Giant and Williams by the mine of the same name. The Quarter Claim was shared.
These reserves and resources amounted to 370 t (11.9 Moz) Au (Muir et al., 1995). The estimated resource+production at the Hemlo mines in 1988 was 585 t (18.8 Moz) Au, although this figure had only grown to 597 t Au in 2000. The Golden Giant mine was acquired by Newmont in 2001, and was exhausted and closed in 2006, after which rehabilitation took place. During its life, Golden Giant produced over 185 t of gold. David Bell and Williams is wholly owned by Barrick Gold, following the purchase of Homestake in 2001 and purchase of Teck's share.
Published reserves and resources at 31 December, 2013 were:
David Bell underground mine and Williams underground and open pit mine
proved+probable reserve - 12.80 Mt @ 2.47 g/t Au for 31.7 t Au (Barrick Gold 2014), included within
measured+indicated resource - 52.84 Mt @ 1.13 g/t Au for 59.2 t Au (Barrick Gold 2014),
inferred resource - 1.52 Mt @ 4.9 g/t Au for 7.45 t Au (Barrick Gold 2014).
The most recent source geological information used to prepare this summary was dated: 2001.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Muir T L 2003 - Structural evolution of the Hemlo greenstone belt in the vicinity of the world-class Hemlo gold deposit: in Can. J. Earth Sci. v40 pp 395-430|
Muir T L 2002 - The Hemlo gold deposit, Ontario, Canada: principal deposit characteristics and constraints on mineralization: in Ore Geology Reviews v21 pp 1-66|
Pan Y, Fleet M E 1992 - Calc-silicate alteration in the Hemlo Gold deposit, Ontario: mineral assemblages, P-T-X constraints, and significance: in Econ. Geol. v87 pp 1104-1120|
Pan Yuanming and Fleet M E 1995 - The late Archean Hemlo gold deposit, Ontario, Canada: a review and synthesis : in Ore Geology Reviews v9 pp 455-488|
Smyk M C, Schnieders B R and Muir T L 1990 - Field guide to the Hemlo area: in 8th IAGOD Symposium, Field Trip Guidebook, Mineral Deposits in the western Superior Province, Ontario, Field Trip 9 Geol Surv Canada Open File 2164 pp 26-38|
Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.
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