Summary:
The Manh Choh Project contains two deposits, the Manh Choh South (MCS) and Manh Choh North (MCN). Both deposits have appreciable quantities of gold (Au), silver (Ag), and copper (Cu) associated with pyrrhotitechalcopyrite-arsenopyrite dominant stratabound replacement bodies interlayered with calcium-iron (Ca-Fe) amphibole which replace the calcareous portions of the interlayered calcareous to argillaceous schist unit. High angle discordant pyrrhotite-chalcopyritearsenopyrite-spalerite-galena-boulangerite-pyrite-amphibole-calcite-quartz veins show open space textures and are proposed to represent the D2 hydrothermal fluid conduits connecting from the source pluton to the chemically responsive host rock trap. The highest gold and silver grades are associated with the junctions of the discordant veining and the calcareous schist with precious metal grade rapidly decreasing down dip, and gently tapering up dip. Two major discordant vein orientations, one generally striking E-W and dipping steeply N, and a second one striking NW-SE and dipping steeply N control the shape of the mineralized body, which is elongate in the E-W direction with a NW oriented tail.
Gold, silver, and copper ore from the MCS and MCN deposits and is associated with sulfide mineralization largely within amphibole skarns in a complex folded geometry. Ore minerals include native gold, electrum, chalcopyrite, pyrargyrite (Ag3SbS3), acanthite (Ag2S), argentopyrite (AgFe2S3), and hessite (Ag2Te). Additional metals with anomalous values include bismuth (Bi), tellurium (Te), arsenic (As), antimony (Sb), zinc (Zn), cadmium (Cd), nickel (Ni), cobalt (Co), and tungsten (W), which contrasts with the known Au-Bi-Te ± As metal associations of intrusive related deposits in Interior Alaska such as Fort Knox and Pogo mines (Flanigan et al., 2000). Metallurgical testing has been conducted with respect to the recovery of gold and silver by cyanidation and results suggest that the mineralization is amenable to recovery.
MANH CHOH SOUTH DEPOSIT
MCS is a largely unoxidized distal skarn hosted in recumbent folded calcareous schist and marble interbedded with amphibolite grade argillaceous schist and quartzite. A penetrative foliation/axial planer cleavage characterizes the Chief Danny prospect, this cleavage is striking 150° and dipping 20°-30° SW. The numerous recumbent isoclinal folds measure 0.1 cm to 2.0 cm across the axis and form a composite overturned isoclinal fold shape with its axis sub parallel to the strike of foliation and opening to the NE. This larger composite fold body of calcareous schist, also opening to the NE, crops out at the surface, measures 200 m vertically and 300 m horizontally in cross section. In long section, the fold measures just over 500 m. The calcareous schist body is dissected by three to four high angle NE-SW trending normal faults which display offsets of 2 m to 10 m, appear to be post-mineralization in age, and have a periodicity of 100 m along the strike of the folded calcareous schist body. The eastern edge of the composite fold is proposed to be truncated by the B1 fault, a N-NE striking, moderately east dipping (45°-50°) reverse fault, also of D2 age. At least one NW striking, sub-vertical fault, thought to be a D2 feeder zone, can be traced along the long axis of the mineralization. This fault is characterized by +100 g/t Ag values associated with highly anomalous Pb, Sb, and Zn with the highest-grade concentration of these metals in the extreme SE portion of the MCS deposit.
Gold, silver, and copper mineralization is associated with pyrrhotite-chalcopyrite-arsenopyrite dominant strata bound replacement bodies interlayered with Ca-Fe amphibole dominant gangue which has replaced the calcareous portions of the interlayered calc-schist units. High angle discordant pyrrhotite-chalcopyrite-arsenopyrite-spalerite-galena-boulangerite-pyriteamphibole-calcite-quartz veins show open space textures and are thought to represent the hydrothermal fluid conduits connecting the source plutonic system to the chemically responsive host rock. The highest Au and Ag grades are associated with the junctions of the discordant veining and the calcareous schist with precious metal grade rapidly decreasing down dip, and gently tapering up dip. Two major discordant vein orientations have been identified at MCS one generally striking E-W dipping steeply to the north (the 275 fault), the other striking NW-SE and dipping steeply north (the 305 fault). Combined, these two feeder systems control the shape of the mineralized body, which is elongate in the E-W direction with a NW oriented tail. The intersection of these major discordant veins and secondary N-E to N trending faults creates east dipping shoots which have localized and accentuated hydrothermal fluid flow. As is true at the MCN resource, the eastern portion of these feeders returned significantly higher Ag grades (>100 g/t) associated with higher Pb, Sb and Zn, possibly indicating a higher temperature and fluid flow regime on the extreme SE end of the MCS resource.
MANH CHOH NORTH DEPOSIT
MCN is a largely oxidized distal skarn hosted in recumbent folded calcareous schist and marble interbedded with amphibolite grade argillaceous schist and quartzite. A significant portion of the MCN resource area is oxidized to depths in excess of 50 m below surface, resulting in widespread Fe, Cu, and As oxides. This strong, pervasive oxidation destroyed the magnetic and conductive pyrrhotite-arsenopyrite-chalcopyrite skarn mineralization, resulting in geophysical signatures unlike those over the unoxidized MCS zone.
A penetrative foliation/axial planer cleavage characterizes the Chief Danny prospect, striking 150° and dipping 20° to 30° to the SW. The numerous recumbent isoclinal folds measure 0.1 m to 2.0 m across the axis and form a larger composite overturned isoclinal fold shape with its axis subparallel to the strike of foliation and opening to the SW. This composite fold body of calcareous schist measures 150 m vertically and 100 m horizontally in cross section. In long section, the fold measures just over 270 m over the NW portion of the resource and is proposed to have had the upper limb and hinge eroded in the SE portion of the resource. The calcareous schist body is dissected by four to five high angle NE-SW trending normal D3 faults which display offsets of 2 m to 10 m, appear to be post-mineralization in age, and have a periodicity of 100 m along the strike of the folded calcareous schist body. The eastern edge of the resource area is proposed to be truncated by the B1 fault, a N-NE striking, moderately east dipping (45° to 50°) reverse D3 fault.
Gold, silver, and copper mineralization is associated with pyrrhotite-chalcopyrite-arsenopyrite dominant strata bound replacement bodies interlayered with Ca-Fe amphibole dominant gangue which has replaced the calcareous portions of the interlayered calc-schist units in the NW half of the resource. The weathering product of this mineralization, dominated by hematite, limonite, goethite, and scorodite-rich clays, make up a significant portion of the SE resource area. High angle discordant pyrrhotite-chalcopyrite-arsenopyrite-spalerite-galenaboulangerite-pyrite-amphibole-calcite-quartz veins show open space textures and are thought to represent the D2 hydrothermal fluid conduits connecting the source plutonic system to the chemically responsive host rock. The highest Au and Ag grades are associated with the junctions of the discordant veining and the calcareous schist with precious metal grade rapidly decreasing down dip, and gently tapering up dip. At least two steeply dipping, NW striking D2 feeder faults have been identified within the MCN zone, the 125 and 110 faults. As in the MCS zone, the eastern portion of these feeders returned significantly higher Ag grades (>100 g/t) associated with higher Pb, Sb and Zn.