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Schaft Creek, Laird |
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British Columbia, Canada |
| Main commodities:
Cu Mo Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Schaft Creek or Laird porphyry copper-molybdenum-gold deposit is located 72 km south of Telegraph Creek in the Stikine area of north-western British Columbia, Canada (#Location: 57° 21' N, 131° 0' W).
Published reserves and production are as follows:
910 Mt @ 0.3% Cu, 0.025% Mo, 0.1 to 0.3 g/t Au (Geol. Res.1984, Dawson, et al. 1991).
330 Mt @ 0.4% Cu, 0.36% Mo (Res. 1976, Fox, et al., 1976)
971.5 Mt @ 0.298% Cu, 0.02% Mo, 0.14 g/t Au 1.2 g/t Ag (Open pit resource, Spilsbury, 1995)
332 Mt @ 0.39% Cu, 0.026% Mo, 0.267 g/t Au (Measured and Indicated resource 2005)
The Schaft Creek (also known as the Laird) deposit is some 40 km to the north-east of the alkalic Stikine/Galore Creek Cu-Au deposit. The mineralisation is predominantly within upper Triassic volcanic rocks Takla Group which comprise mostly grey to green andesites, with the chief hosts being altered andesite lava, epiclastic volcanic conglomerate and breccia. These are cut by mineralised dyke and lens-like quartz-feldspar porphyry and by post mineral basic dyke swarms. The quartz-feldspar dykes comprise more than 10% of the ore host. They are typically fien to medium grained, white pink or grey, leucocratic quartz-monzonites, which are similar in composition and texture to the marginal phases of the nearby Hickman Batholith. The dykes comprise 60% plagioclase, 10% quartz and 10% or less mafic minerals set in a fine grained matrix of feldspar, quartz, chlorite, sericite and iron oxides. Virtually all dykes within the main ore zone are mineralised. The Hickman Batholith to the west of the ore zone is a hypidiomorphic-granular, white to pink, medium grained, biotite-hornblende quartz-monzonite (adamellite) to granodiorite (Fox, etal., 1976).
The ore deposit lies within a few hundred metres to the north and east of the Hickman Batholith. The ore is found in two major zones, the first of which is a steeply dipping irregular zone of intrusive and tectonic breccia, 30 to 200 m wide and more than 1000 m long and extending to at least 300 m in depth, called the Breccia-Zone. A second zone up to 500 m thick and 1000 m long called the Main Zone, adjoins the Breccia Zone to the east. The Breccia Zone is an irregular linear feature comprising fault gouge, mylonite and intrusive breccia enclosed by volcanic rocks lying sub-parallel to the eastern contact of the Hickman Batholith. The breccia consists of angular to sub-angular fragments of mixed andesitic volcanic rock and felsic porphyry identical to the enclosing host rocks. Fragments are enclosed by a dark matrix of chlorite, quartz, tourmaline, specular hematite and sulphides. The fragments range from 25 to 200 mm and comprise about 80% of the breccia. The Breccia Zone contacts are locally sharply defined, but are more commonly bounded by sheared country rock and crackle breccia. Andesite fragments are often largely chloritised and silicified, particularly near the walls (Fox, et al., 1976).
Breccia-Zone mineralisation includes bornite, chalcopyrite, molybdenite and pyrite as disseminations and veinlets in both the breccia matrix and fragments, while the Main Zone sulphides occur as bornite, chalcopyrite and molybdenite veinlets, fracture coatings and disseminations in altered volcanics and felsic porphyry. The Cu and Mo tenor appear to be proportional to the density of veinlets and fractures. The centre of the zones are richest in bornite with erratic pyrite, while the outer and lower parts are rich in chalcopyrite and contain small amounts of pyrite. Most rocks below and east of the Main Zone, and west of the Breccia Zone are weakly mineralised with pyrite and erratic chalcopyrite (Fox, et al., 1976).
Hydrothermal alteration includes secondary brown biotite, chlorite, carbonate, quartz, K-feldspar, actinolite, epidote, gypsum and anhydrite. The north-central segment of the Main Zone contains a weakly mineralised, generally low grade quartz-vein stockwork that appears to diminish downwards and to the east, west and south. It thickens to the north and plunges downwards. Numerous mineralised and altered faults and fractures, some associated with porphyry dykes and lenses occur beneath and appear to merge upwards into the Main Zone. Generally the deposit appears to be marked by a well developed K-silicta zone, containing brown biotite and some K-feldspar, which is bounded in turn by a broad propylitic zone. It is possible that the most of the secondary biotite is related to a hornfelsing event and not to the introduction if potash. The fine grained chlorite and carbonate appear to be retrograde alteration superimposed on the K-silicate phase. Phyllic alteration is confined to the swarms of felsic porphyry dykes in which sericite is abundant. The quartz vein stockwork with K-feldspar also contains biotite, and as such is a sub-phase of the K-silicate zone. This latter zone of veining generally contains the highest copper, whereas the propylitic zone carries only small amounts of pyrite and is copper poor to barren (Fox, et al., 1976).
Biotite-chlorite mixtures, disseminated granules of carbonate and small amounts of sericite are found in abundance in the volcanic rocks that host the high grade ore of the Main Zone. Epidote, actinolite, coarse grained chlorite and rare carbonate veinlets occur in weakly mineralised and barren rocks that underlie and fringe the Main Zone to the south and east. The same alteration minerals, along with tourmaline, form the matrix of the breccia zone. Secondary biotite originally formed a broad K-silicate zone, coincident with the Main Zone, which was bordered outwards and downwards by a propylitic epidote rich aureole. Late chlorite is superimposed on the biotite zone (Fox, et al., 1976).
Numerous altered and mineralised fault and fracture zones, some associated with porphyry dykes and lenses, occur beneath and appear to merge upwards into the Main Zone (Fox, et al., 1976).
The most recent source geological information used to prepare this decription was dated: 1996.
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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Fox P E, Grove E W, Seraphim R H and Sutherland Brown A, 1976 - Schaft Creek: in Sutherland Brown A (Ed.), 1976 Porphyry Deposits of the Canadian Cordillera, Canadian Institute of Mining and Metallurgy, Special Volume 15, pp 219-226
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Logan, J.M. and Mihalynuk, M.G., 2014 - Tectonic Controls on Early Mesozoic Paired Alkaline Porphyry Deposit Belts (Cu-Au ± Ag-Pt-Pd-Mo) Within the Canadian Cordillera : in Econ. Geol. v.109, pp. 827-858.
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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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