Geology of the Greenwood - Grand Forks Area, British Columbia
BCMEMPR Open File 1990-25
View Open File (PDF, 6.24 MB)
Open File 1990-25 portrays the geology of the Greenwood – Grand Forks area in southern British Columbia. The report gives details of the formations and contains eleven measured stratigraphic sections. It presents the geology and the structural style in a modern context and gives new information that will be important to mineral exploration in the well-known Boundary mining camp.
The Greenwood - Grand Forks area contains Late Paleozoic and Mesozoic volcanic and sedimentary rocks, which are intruded by Mesozoic plutons and unconformably overlain by Tertiary volcaniclastic and flow rocks. The pre Tertiary stratiform rocks are contained in a series of five, north-dipping thrust slices with bounding faults, which are marked at many places by layers and lenses of deformed serpentinite. These thrust slices lie above high-grade metamorphic complexes, which are exposed to the south in northern Washington and to the east beyond the Granby River fault.
This publication consists of a 1:50 000-scale geological map with cross sections, accompanied by a 19-page report covering the geology of the Rock Creek - Greenwood - Grand Forks area, in southern British Columbia.
The focus of the mapping was to provide a geological framework for the interpretation of mineral deposits in the area. The Paleozoic Knob Hill Group comprises most of the pre Tertiary rocks in the area. It is a thick assemblage of mainly greenstone and chert. The volcanics are dominantly pillow lavas and breccias, and agglomerates derived from them. Dioritic intrusions and altered serpentine or listwanite are included in the Knob Hill Group. Unconformably over the Knob Hill rocks are sediments of the Middle Triassic Brooklyn Formation. There is a lower unit of chert-pebble conglomerate and chert breccia, and an upper unit of limestone, siltstone, sandstone, limestone conglomerate and skarn. Above a disconformity, is an Early Jurassic fragmental greenstone. These rocks are intruded by the Jurassic and Cretaceous Nelson plutonic rocks. These comprise feldspar porphyry, granodiorite and quartz diorite. Unconformably overlying all the older rocks are rocks of the Tertiary Penticton Group. A lower package of feldspathic and lithic tuffaceous sandstone, conglomerate and siltstone is overlain by mainly flows of andesite, trachite and phonolite and intruded by dikes, sills and small plutons of alkaline syenite.
Tertiary faults dominate the structure of the area. They bound the Tertiary rocks and are often filled by Tertiary dikes. The most important are listric normal faults but later block faults predominate.
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