Structural and metamorphic evolution of the Ormsby Zone and relative timing of gold mineralization: a newly defined Archean orogenic gold prospect hosted on the Discovery Property, Yellowknife greenstone belt, Slave Province, Canada

• Main Author: Whitty, William H. R.
• Language: en
• Published: University of British Columbia 2008
• Subjects: Northwest Territories; geology
• Online Access: http://hdl.handle.net/2429/255

The Yellowknife greenstone belt has produced approximately 14 million ounces of gold, 13 of which came from the well explored southern half of the belt. The northern half of the belt, where the Discovery Property (1 million ounce past producer) is located, is not as well explored, but geologically very similar to the gold producing southern half. Gold on the Discovery Property, Northwest Territories, has been historically mined from a quartz vein formed adjacent to Archean metabasaltic bodies enclosed in metasedimentary rocks. Some of the metasedimentary rocks are correlated to the Archean Banting Group, based on a U-Pb age of 2.66 Ga on zircon (SHRIMP-RG). The metabasalt is correlated to the Banting Group based on its deposition relationship with the adjacent metasedimentary rocks. The Banting Group is present in the Yellowknife greenstone belt, located near the well-known Con and Giant Mines near Yellowknife, 90 km to the south. The two gold deposits forming the Discovery Property contains almost 2 million ounces in past production and current resources. The historical Discovery Mine was developed in quartz veins hosted in the metasedimentary rocks folded around the northern tip of one of the metabasalt bodies. In contrast, the newly defined Ormsby Zone consists of gold in silicified and sulphidized domains within metamorphosed pillowed and brecciated mafic volcanic rocks, and shares many characteristics with other Archean Orogenic gold deposits. Pyrrhotite ± arsenopyrite is directly correlated with the gold-bearing zones. Gold is also associated with pyrite and retrograde phases of chlorite, sericite and carbonate. Through detailed outcrop mapping and petrography the history of structural and metamorphic events are sequenced and the relative time of gold mineralization placed into the geological framework. Peak-metamorphic mineral assemblages on the property record upper greenschist to amphibolite facies conditions. Rocks on the Discovery Property have undergone at least four foliation-forming, deformational events. Three generations of folding are described, as well as a vertical lineation across the property. Gold mineralization occurred post-ductile fabric forming deformation, post-peak metamorphism, and syn-retrograde metamorphism. Although no visible evidence was observed mapping, metamorphic data present the possibility of a regional scale fault separating two metamorphic domains on the property.