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Bjorkdal |
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Sweden |
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Au
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Super Porphyry Cu and Au
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The Björkdal quartz vein-hosted gold deposit is located approximately 25 km northwest of Skellefte in northern Sweden (#Location: 64° 56' 1"N, 20° 35' 21"E).
The Björkdal deposit was originally discovered through examination of anomalous gold values in till samples in 1983 by Terra Mining AB. Terra commenced open pit mining operations in July, 1988. Open pit mining continued through 1999 under Terra and its successor William Resources, until closure due to low gold price. The open pit mine was reopened in 2001 under new ownership (International Gold Exploration), and continued operation under a succession of owners, Minmet, Gold-Ore, and Elgin. Gold-Ore added the underground mine to the operation and Elgin continued both the open pit and underground mining. Mandalay Resources Corp. acquired 100% interest in the Björkdal gold mine in September, 2014, when it purchased 100% of Elgin Mining.
The deposit is located at the contact between marble and volcanic rocks of a Paleoproterozoic volcanosedimentary sequence and an underlying 1905 to 1854 Ma granitoid of the Jörn G1 Phase intrusive suite. See the Skellefte record for the regional setting.
Gold mineralisation occurs within a conjugate set of steeply dipping, 0.1 to 1 m thick quartz veins, which predominantly strike at 30 to 50°, although some trend at 90°. The quartz veins have sharp contacts and are hosted by a medium-grained quartz monzodiorite to tonalite intrusion with an ENE striking penetrative foliation. Within the mine, the upper limit of the veins is controlled by a thrust plane separating the host from an overlying allocthonous marble unit.
The mineralised zone is developed over a 750 x 100 m zone, oriented east-west
The veins contain sulphides, predominantly pyrrhotite, pyrite, chalcopyrite, galena and tellurides, while tourmaline and scheelite are also commonly present, with tourmaline being more abundant in veins with high gold grades. The quartz veins are sandwiched by weak alteration halos which are generally <30 cm in thickness and are composed of sericite, chlorite, biotite and sulphides.
Gold is believed to have been precipitated in two main phases, namely: i). an early stage with scheelite, sulphides and tellurides, with visible gold observable in scheelite grains, ii). a late phase of sulphide- and telluride-bearing fractures.
Quartz, which is the dominant mineral within the veins, shows clear evidence of post-crystallisation deformation. Locally the quartz also has a ribbonlike appearance, with strained porphyroclasts of quartz surrounded by a fine aggregate of recrystallised grains and deeply sutured margins. Fluid inclusion data suggest that primary quartz and scheelite were deposited from a gold-bearing fluid at ~375°C that precipitated the tellurides and sulphides, and is similar in age and interpreted to be related top the host Jörn intrusive suite. A later, possibly metamorphic derived, 145 to 220°C brine is observed in fluid inclusions within crosscutting microfractures and is interpreted to have been responsible for some gold remobilisation in the younger vein set (Roberts, et al., 2006).
Age data suggests the VHMS mineralisation at Skellefte and the gold mineralisation at Björkdal are coeval within the limits of measurement (Roberts, et al., 2006).
The Bjorkdal mine commenced production in 1988 and operated until 1999 during which time it produced 24.8 tonnes of gold. Remaining resources are variously estimated at between 5.5 and 19.5 tonnes of gold (Weihed et al., 2003).
Total production from 1988 to 2015 was - 20.14 Mt @ 1.8 g/t Au (Hallberg et al., 2016)
Ore Reserve and Mineral Resource estimates as at 31 December 2019 (Mandalay Resources Reserve and Resources statement 2019) were:
Proved + Probable Reserves - 11.09 Mt @ 1.54 g/t Au;
Measured + Indicated Resources - 15.558 Mt @ 2.15 g/t Au;
Inferred Resources - 5.483 Mt @ 1.71 g/t Au.
Note: Resources are inclusive of reserves.
The most recent source geological information used to prepare this decription was dated: 2006.
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.
Bjorkdal
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Selected References:
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Roberts S, Palmer M R and Waller L, 2006 - Sm-Nd and REE Characteristics of Tourmaline and Scheelite from the Björkdal Gold Deposit, Northern Sweden: Evidence of an Intrusion-Related Gold Deposit?: in Econ. Geol. v101 pp 1415-1425
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Weihed P, Weihed J B, Sorjonen-Ward P, 2003 - Structural Evolution of the Bjorkdal Gold Deposit, Skellefte District, Northern Sweden: Implications for Early Proterozoic Mesothermal Gold in the Late Stage of the Svecofennian Orogen: in Econ. Geol. v98 pp 1291-1309
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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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