Geology and hydrothermal alteration of the Fab lake region, Northwest Territories

Detailed mapping of the Fab Lake magnetite-group iron oxide-copper-gold (IOCG) system has defined an alteration footprint extending almost 10 by 5 km, with the long axis trending in a southeastnorthwest direction. Within this area, field mapping identified seven alteration assemblages: 1) high temperature albite and albite + amphibole ± magnetite [HT Na and HT Na-Ca-Fe]; 2) amphibole + magnetite ± apatite and amphibole + magnetite + K-feldspar [HT Ca-Fe and HT Ca-Fe-K]; 3) high temperature K-feldspar + magnetite ± hematite [HT K-Fe]; 4) K-feldspar; 5) chlorite; 6) hematite; and 7) low temperature epidote + K-feldspar + quartz [LT Ca-Fe]. These alteration assemblages are expressed as incipient to pervasive alteration in the form of veins, hydrothermal breccias and replacement fronts in the host rocks exhibiting multiple crosscutting and overprinting relationships. All of the historic Fab U-Cu-Fe mineral showings occur within zones characterized by intense, texture-destructive alteration comprising multiple episodes of high temperature albite/albite + amphibole + magnetite (Na and Na-Ca-Fe) and amphibole + magnetite ± apatite/amphibole + magnetite + K-feldspar (Ca-Fe and Ca-Fe-K) overprinted by K-feldspar + magnetite ± biotite (K-Fe) assemblages. The conceptual alteration to brecciation and mineralization model is key to understanding the alteration assemblages documented in the Fab IOCG system. These assemblages record the build-up of a magnetite-group IOCG system: early high-temperature Na/Na-Ca-Fe and Ca-Fe/Ca-Fe-K alteration, overprinted by high temperature K-Fe alterations and incipient to well-developed hydrothermal breccias. The alteration assemblages themselves and the zoning observed in outcrop provide true vectors towards mineralization, which based on the nature of the hydrothermal system, can be applied elsewhere in the Great Bear magmatic zone and other prospective terranes. As field observations indicate that lower-temperature K-Fe alteration associated with hematite-group IOCG systems is only weakly developed, the Fab Lake region is most prospective for magnetite-group types of IOCG mineralization.