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TitleA new lode gold discovery at Otter Creek: another source for Atlin placers
AuthorMihalynuk, M G; Zagorevski, AORCID logo; Devine, F A M; Humphrey, E
SourceBritish Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper no. 2017-1, 2016 p. 179-193 Open Access logo Open Access
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20160313
PublisherBritish Columbia Ministry of Energy and Mines
Mediapaper; on-line; digital
File formatpdf
ProvinceBritish Columbia
Lat/Long WENS-133.5000 -133.1667 60.0000 59.7500
Subjectsplacer deposits; gold; erosion; placer deposits, stream; cassiterite; thorite; batholiths; Surprise Lake batholith; Fourth of July batholith; Kedahda Formation; Cache Creek terrane; Otter Creek; Slonski
Illustrationslocation maps; geological sketch maps; photographs; graphs; photomicrographs
ProgramGEM2: Geo-mapping for Energy and Minerals Western Cordillera, Redefinition of crustal blocks
AbstractPrimary exploration targets for lode gold near Atlin, northwestern British Columbia have historically been quartz-carbonate-mariposite altered (¿listwanite¿) ultramafic and mafic bedrocks. These have long been considered the source of nearby placer deposits, and locally do contain fine visible gold. However, despite more than a century of searching, coarse gold such as found in the placer gravels has never been discovered. This has prompted the belief that the "listwanites" are mere erosional remnants of bedrock sources of the coarse placer gold, and that these sources have been almost entirely lost to erosion. An alternative hypothesis argues that, although gold is found with listwanite, it was not the only bedrock source. Streams with placer deposits are distributed around the evolved Surprise Lake batholith (Sn-U-Th-Mo-W-F rich) and rare gold nuggets contain Sn- and Th-rich mineral intergrowths (cassiterite and thorite), demonstrating that at least some of the placer gold may be related to the batholith. Discovery of coarse lode gold (>5 mm) in graphitic and quartz-rich phyllitic bedrock beneath placer deposits along Otter Creek in 2016 confi rms that listwanite-altered mafic and ultramafic rocks are not required for lode gold mineralization. At the discovery site, native gold is intergrown with quartz-albite veins and occurs as open space fillings. Rutile in quartz veins, and perhaps altered titanite in the adjacent phyllite may have grown along with gold deposition. Geochemical analysis of pyrite-rich phyllite adjacent to the gold veins yields no appreciable gold, but is slightly elevated in Cu (11-54 ppm), Pb (~33 ppm) and zinc (60-70 ppm). These results are consistent with petrographic observations that show abundant chalcopyrite inclusions but no gold inclusions in pyrite. Discovery of coarse gold in quartz veins cutting graphitic and quartz-rich phyllitic country rocks significantly expands the lode gold exploration target in the Atlin placer camp.
Summary(Plain Language Summary, not published)
This report presents preliminary results and models developed through GEM2 program on volcanic and plutonic rocks in British Columbia. It details the age and characteristics of magmatic rocks and associated mineralization on the basis of field and laboratory analyses. The results presented herein help progress GEM2 model to ultimately redefine the regional tectonic models of the Cordillera.

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