| dc.description.abstract | Zn-Pb-Ag mineralization at Nanisivik, northwest
Baffin Island, is hosted by Proterozoic, laminated dolostone
of the Society Cliffs Formation. Mineralization of the Main
Orebody is highly variable in terms of texture and mineralogy
exhibiting both replacement and open space filling textures.
Sulfides are generally coarsely crystalline and banding,
consisting of interlayered pyrite, sphalerite, galena and
sparry dolomite, is common along the margins of the Upper
Lens of the Main Orebody. The eastern and central portions
of the Upper Lens are characterized by laterally extensive
mine units, which are distinguished on the basis of texture
and mineralogy. Contacts between units are generally sharp.
The physical and chemical parameters responsible for
the textural and mineralogical variations have been evaluated
through a study of fluid inclusions, sulfur isotopes and ore
mineralogy. Fluid inclusion homogenization temperatures from
simple, two-phase primary and pseudo-secondary inclusions in
sphalerite and sparry dolomite gangue indicate initial
temperatures of ore formation from 150—210 degrees C in the eastern
Upper Lens when the estimated pressure of ore formation is
taken into consideration- The temperature of ore formation
decreased to 100-150 degrees C in the western portion of the Upper
Lens. Freezing studies indicate that the ore-forming fluid was a brine containing 20-37 equivalent weight percent CaCl[subscript] 2,
The sulfur isotopic compositions of late and main stage
pyrite crystals range from [symbol]S = +27.4%. to + 28.0%., suggesting
relatively constant temperature, fluid source and
dominant sulfur species in the ore fluid during ore formation,
providing there has been no subsequent re-equilibriation of sulfur isotopes. The iron content of sphalerite
varies from 14 mole % to 0 mole % from crystal centers to
rims respectively, corresponding to well developed colour
zonation. Sphalerite iron contents constrain the oxygen
activity of the ore fluid from 10[superscript -46] to 10[superscript -41] at 200°C during sphalerite precipitation. The best developed zoning and,
thus, the highest oxygen activities occur within sphalerite adjacent to carbonate wall rock. Under high oxygen
activities, conditions were favorable for the generation of
sulfanes considered necessary for precipitation of marcasite.
X-ray diffraction studies indicate that primary marcasite has
inverted completely to pyrite. The stability of the simplest
sulfane, H[subscript 2] S[subscript 2] constrains the maximum allowable pH of the ore fluid at the time of marcasite precipitation to 5.0. The
presence of interbanded marcasite pseudomorphs and sparry
dolomite indicate that the ore fluid fluctuated around pH = 5.0.
Comparison of solid organics extracted from the
Society Cliffs dolostone to bitumen associated with mineralization suggests that organics within the host formation have
played a role in sulfate reduction. The model of ore formation
therefore proposed involves the in situ reduction of a
hot, saline, metal-bearing ore fluid by hydrocarbons
liberated by the replacement and dissolution of wall rock.
Sulfate reduction was probably concentrated at the wall rock
orebody interface along a replacement front that migrated
away from the orebody. Banding was likely the result of
repetitive sulfate reduction, metal precipitation and wall
rock dissolution in response to the pulsatory influx of ore fluid. Gross textural and mineraiogical variations are
probably a result of slight variations in the oxidation state
of the ore fluid, the availability of H[subscript 2] S and, to a lesser
extent, temperature. | |
