Origin of carbonate hosted gold rich replacement deposits and related mineralization styles in the Ketza River deposit, Yukon Territory

• Main Author: Fonseca, Ana L.
• Format: Others
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/8406

The Ketza River deposit consists of gold-rich, base metal-poor oxidized and unoxidized manto style orebodies and Fe-silicate alteration zones hosted in Early Cambrian limestones, and quartzsulphide veins and stockwork in Late Proterozoic to Eady Cambrian metasedimentary rocks. Sulphide mineralogy in unoxidized mantos, Fe-silicate alteration zones and in veins and stockwork consists mainly of pyrrhotite, pyrite, arsenopyrite, marcasite, and chalcopyrite. Minor amounts of native bismuth are present in all ore types. Gangue minerals are mainly carbonates (calcite and ankerite), and quartz. Additionally, Fe-silicate alteration zones have locally abundant magnetite and Fe-amphibole. The area was affected by two episodes of folding, followed by two of faulting. All units exposed in the area are affected by the deformation. The two phases of folds have coincident, E-W-trending axes, and are distinguished on the basis of their vergence and nature of the axial planar foliation. First phase folds (FO have upright axial-planar foliation (SO, whereas second phase folds (F2) have moderately NE-dipping axial planar foliation (S2) defined by spaced crenulation cleavage. Folding was followed by NNE-directed thrusting, which was in turn followed by doming and extension that produced horsts and grabens. Arsenopyrite geothermometry was performed in ore samples from various locations in the deposit, and yielded 27.4 to 29.4 average atomic % arsenic, corresponding to temperatures below 330°C. ^Ar/^Ar analysis of white mica from a quartz-sulphide vein gives a mid-Cretaceous (108 +/- 0.3 Ma) age for the mineralization that coincides with the emplacement of the Cassiar plutonic suite. Lead isotopic ratios of samples from the different styles of mineralization in the Ketza River deposit plot in overlapping fields, suggesting a genetic relationship between the different ore types. Analyses from carbonate hosted mineralization have a wider spread, suggesting more complex rock-fluid interactions or less homogenization of Pb isotopes. The distribution of orebodies along normal faults and the coincidence of the age of mineralization with the emplacement of a plutonic suite in the region, make the Ketza River deposit a typical intrusion centered sedimentary rock hosted hydrothermal system, with different ore manifestations according to host rock type. However, the central part of the system (the intrusive source of heat, and possibly fluids, metals, and sulphur) is not exposed. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === Graduate