Geoffrey Kahlil Abdullah MSc thesis abstract

Thesis Title: 
Ore Mineralogy of Mattabi Mine, Northwestern Ontario, Canada
Geoffrey Kahlil

The Mattabi Mine, a volcanogenic massive sulfide deposit located 60 km north of Ignace, Ontario, in the Archean Superior Province, produced 13.5 mt of ore with an average grade 7.5% Zn, 0.88% Cu, 0.77% Pb and 3.10 oz/t Ag in the period 1970-1983.  Preliminary studies indicated the presence of an especially varied mineral assemblage, which was studied in detail.  As well as chalcopyrite and sphalerite, the deposit contains abundant galena accounting for its unusual lead production.  Argentian Tetrahedrite, or frelbergite, is also abundant, and in the absence of native silver and acanthite, is clearly a major carrier of silver.  Other sulfide minerals include pyrite, pyrrhotite, arsenopyrite, bornite, mackinawite, pyragyrite, boulangerite, freleslebenite, stephanite and veenite.  Oxide minerals include magnetite, hematite, llmenite, rutile, jacobsite, cassiterite and gahnite.

The mineral zoning in the orebody is pronounced.  Sphalerite, tetrahedrite, galena and the sulfosalts all occur in the upper portion of the orebody, whereas pyrrhotite and chalcopyrite are concentrated in the lower portion.  The oxide minerals, except for jacobsite, occur in the ore zone as well as in the footwall alteration zone.

A Brillouin zone model for tetrahedrite in which 204 to 208 electrons are accommodated in the 51st Brillouin zone is verified in analytical data from 60 grains.  However, when three or more Ag atoms per unit formula are present, the number of valenic electrons increases from 204 to 208 as Ag atoms increase from three to six.  The presence of (Fe+Zn) in excess of two atoms per unit formula appears to stabilize the 52nd Brillouin Zone with >208 valence electrons per unit cell.

Analyses of sphalerite yielded a mean mole % FeS = 12.11 " 0.70, indicating a pressure of metamorphism of 7.53 kbar, using the sphalerite geobarometer.  This pressure is approximately 2 to 3.77 kbar too high in comparison with pressures suggested by silicate systems.  A possible explanation is that sphalerite-pyrite-pyrrhotite equilibrated at peak pressure, whereas silicate assemblages indicate conditions of peak temperature in the P-T-t history of metamorphism of the deposit.  Retrograde alteration variably affected sphalerite composition, producing some scatter in data, as well, as evidenced by variable occurrence of late-stage monoclinic pyrrhotite.

The low gold content, high lead content, diversity of sulfosalt species and abundance of silver in tetrahedrite are features more similar to those of kuroko-type deposits of the Phanerozoic than to Noranda or primitive-type massive sulfide deposits characteristic of the Archean.