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Endako |
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British Columbia, Canada |
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Mo
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Super Porphyry Cu and Au
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The Endako molybdenum deposit lies within the composite Jurassic Topley Intrusions which include the older Endako Quartz Monzonite, which is bounded on the south by the Francois Granite, and on the north by the Casey Alaskite and Glenannan Granite. The Endako Quartz Monzonite is elongated in a NW-SE direction and is 3 to 5 km wide in the Endako area (Kimura, etal., 1976).
Reserves+production to 1990 - 280 Mt @ 0.08% Mo (Dawson, 1991)
Production, from 1965-1982 - 139 Mt (Dawson, 1991).
Reserve 1998 - 85 Mt @ 0.136% MoS2 at a cutoff of 0.07% MoS2 (Selby et al., 1999)
Reserve 2005 (probable + proven) - 74 Mt @ 0.063% Mo (Blue Pearl Mining web site, 2006)
Resource 2005 (indicated) - 51.8 @ 0.070% Mo (Blue Pearl Mining web site, 2006)
Geology
The main rock types are as follows,
* Endako Quartz-Monzonite (adamellite), dated at 142 Ma - the host to ore, is generally equi-granular (3 to 4 mm), with some K-feldspar crystals in places up to 7 mm across. The rock consists of 30% quartz, 35% pale pink to orange tinged K-feldspar, 30% white to green tinged plagioclase, and 5 to 10% partially chloritised black biotite. The rock is characterised by the pink to bright orange-pink K-feldspar and the alteration in the mine area which varies from pale greenish-grey to dark green to bleached creamy white for highly altered variants;
* Casey Alaskite, dated at 138 Ma - which is normally a fine to medium grained (1 to 3 mm), sugary textured, leucocratic rock composed of 40% quartz, 45% pale pink K-feldspar, 5 to 10% white plagioclase, 2 to 5% partially chloritised biotite and 1% accessory pyrite and hematite;
* Glenannan Granite, dated at 138 Ma - which is a coarse grained granite to quartz-monzonite (adamellite), comprising 20 to 25% quartz, 40 to 45% pink K-feldspar, 15 to 30% white plagioclase and 5 to 10% biotite. Texturally the rock ranges from equigranular (5 to 6 mm) to porphyritic with K-feldspar and plagioclase crystals up to 2 cm. Pegmatite phases with crystals ranging up to 1 to 2 cm's are also noted;
* Francois Granite, dated at 137 Ma - a distinctive red, equigranular (3 to 4 mm) rock composed of 35% quartz, 45% perthitic K-feldspar, 15% white plagioclase with greenish cores, 3 to 5% chloritised biotite and up to 1% of accessory magnetite, pyrite and sphene;
* Wheeler Quartz-Monzonite (adamellite), relative age unknown - which occurs as a small mass some 4 km to the south of the orebody. It is normally a coarse grained leucocratic rock with local porphyritic varieties with scattered pink K-feldspar phenocrysts which average 2 cm in length. The modal composition is 30% quartz, 45% pink K-feldspar, 20 to 25% plagioclase and 1 to 3% biotite (Kimura, etal., 1976).
The intruded Takla Group volcanics are the oldest rocks in the area, and consist of dark green to purple lavas, tuffs and flow rocks which locally contain small white quartz, feldspar and biotite phenocrysts in a hard aphanitic matrix (Kimura, etal., 1976).
In the vicinity of the orebody the Endako Quartz Monzonite is intruded by pre-mineral dykes in the following chronological order, 1). aplite (6 mm to 1.2 m wide), 2). andesite (generally 0.3 to 0.5 m wide), 3). porphyritic granite (1.2 to 25 m wide) and 4). quartz-feldspar porphyry (5 cm to 50 m wide), and by post-mineral basalt dykes. In plan the orebody is an irregular elongated body that strikes at 120°, dips 40 to 60° to the south and measures 3360 x 370 m. A major crosscutting fault offsets the orebody in a northerly direction, and also forms a divisional boundary between the broader westerly plunging eastern half and the shallowly lenticular western half (Kimura, etal., 1976).
Mineralisation & Alteration
The most abundant primary minerals are molybdenite, pyrite and magnetite, with minor amounts of chalcopyrite and trace quantities of bornite, bismuthinite, scheelite and specularite. All are intimately associated with quartz veins. Calcite and chalcedony occur in late veins and fractures. Ore minerals occur in two forms, as large quartz-molybdenite veins, and in fine fracture fillings and veinlets in the form of a stockwork. The major quartz-molybdenite veins, 15 cm to 1 m thick, occur as a series of sub-parallel and complementary sets within the ore zone. Molybdenite typically occurs as thin (1 to 6 mm) closely spaced laminae that give the milky white quartz veins their characteristic ribboned appearance. Molybdenite also occurs as scattered to concentrated finely divided grains in quartz, so as to produce blue and black quartz veins. Magnetite, pyrite and minor chalcopyrite are intimately associated with molybdenite. Brecciation within veins with subsequent healing by quartz and molybdenite is common. The veins swell, pinch and horse-tail, and control the distribution of the stockwork veinlets (Kimura, etal., 1976).
Quartz and molybdenite and associated ore minerals occur within randomly oriented fractures forming a stockwork adjacent to and surrounding the major quartz-molybdenite veins. Veinlets range from minute wisps of molybdenite to 5 cm wide quartz-molybdenite veins. Veins within the economic stockwork are spaced from 1 to 2 cm to several metres apart. The economic limits of the mineralised stockwork range from 6 to 60 m in width. A pyrite zone bounds the orebody to the south, and consists of fine quartz and pyrite, minor magnetite and rare molybdenite as fracture fillings in a poorly developed stockwork. The pyrite content is estimated to be less than 1%. The transition from pyrite to molybdenite mineralisation is gradational . A pyrite zone has been noted in other peripheral sections of the orebody (Kimura, etal., 1976).
Hydrothermal alteration has locally produced three alteration phases, namely K-feldspar envelopes on veins and fractures, quartz-sericite-pyrite envelopes on veins and pervasively kaolinisation of the host rocks. The different styles are as follows (from Kimura, etal., 1976):
* K-feldspar bearing envelopes, present in three forms, namely, a). K-feldspar envelopes from 3 to 50 mm wide, developed as bright orange-pink borders adjacent to either quartz or quart-molybdenite veins. They comprise either wholly K -feldspar, or have up to 5% quartz also; b). K feldspar-biotite envelopes which contain up to 10% biotite and minor quantities of quartz. This assemblage forms bands or lenses commonly from 5 to 60 cm wide, but may be up to 5 m thick. Envelopes are more common than lenses; c). K feldspar-quartz envelopes which are distinctly different in that they contain 60% or more K-feldspar, 30% quartz, up to 5% biotite and up to 5% altered plagioclase. Hydrothermal K-feldspar is megascopically identical to the bright orange-pink primary K-feldspar of the host quartz-monzonite (adamellite).
* Sericite bearing envelopes, occur as grey, megascopically sharp borders to quartz-molybdenite, magnetite and quartz-pyrite veins, in widths from 3 to 50 mm. These envelopes are composed of 50 to 60% quartz, 30 to 35% very fine grained sericite and 1 to 5% finely disseminated pyrite. Within the envelope the original K-feldspar, plagioclase and biotite have been replaced by sericite and quartz, while iron from the breakdown of biotite has been sulphidised to form pyrite. Occasionally magnetite and more rarely chalcopyrite occur as disseminated grains in the envelope.
* Pervasive kaolinisation, of the Endako Quartz-Monzonite (adamellite) is widespread, and varies from a slight development of kaolinite in plagioclase in otherwise un-altered rock, to complete alteration of both plagioclase and K-feldspar to a soft creamy-white or green clay.
A paragenetic sequence of veining and alteration envelopes has been determined as follows, from youngest to oldest (from Kimura, etal., 1976):
Stage 1 - a). quartz with K-feldspar; b). quartz-molybdenite with K-feldspar and biotite; and c). quartz-magnetite±pyrite with K-feldspar and quartz.
Stage 2 - a). quartz-molybdenite and minor K-feldspar with quartz-sericite-pyrite; b). quartz-magnetite with quartz-sericite-pyrite; c). quartz-molybdenite with quartz-sericite-pyrite; d). quartz-magnetite-molybdenite with quartz-sericite-pyrite; all with ± pyrite, chalcopyrite and bornite, and e). quartz-pyrite±magnetite and molybdenite with quartz-sericite-pyrite of the pyrite zone.
Stage 3 - all with no envelopes, and ±pyrite and chalcopyrite a). quartz; b). quartz-molybdenite with coarse molybdenite; c). quartz-magnetite-molybdenite.
Stage 4- - again with no envelope, but occasional bleached haloes, a). quartz; b). quartz-pyrite.
Stage 5 - again with no envelopes, a). sphalerite with minor quartz; b). calcite; c). chalcedony.
Stage 6 - late unfilled fractures.
The Endako orebody is visualised as a restricted stockwork that has formed on an elongated easterly trending dome by uplift, intrusion and shearing, and localised at or near the intersection of regional north-westerly and easterly structures.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1999.
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.
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Selected References:
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Selby D, Creaser R A 2001 - Re-Os geochronology and systematics in Molybdenite from the Endako Porphyry Molybdenum deposit, British Columbia, Canada: in Econ. Geol. v96 pp 197-204
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Selby D, Nesbitt B E, Muehlenbachs K 2000 - Hydrothermal alteration and fulid chemistry of the Endako Porphyry Molybdenum deposit, British Columbia: in Econ. Geol. v95 pp 183-202
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Villeneuve M, Whalen J B 2001 - The Endako Batholith: episodic plutonism culminating in formation of the Endako Porphyry Molybdenite deposit, north-central British Columbia: in Econ. Geol. v96 pp 171-196
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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, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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