June 2003

New gold targets in the Chibougamau area
A porphyry-epithermal continuum

Patrick Houle
Géologie Québec

It has been shown that the discovery potential for porphyry gold and/or copper-gold deposits, and for lode-type or epithermal zinc-gold deposits is excellent in the Chibougamau area, since porphyry-type and epithermal deposits form a continuum derived from the same geological phenomenon (Groves et al., 1998).

Porphyry Cu-Au-Mo deposits

The Chibougamau mining camp hosts a few sub-economic to economic porphyry deposits (Île Merrill, Devlin, GrandRoy, Baie Hématite, Queylus, R2), which are spatially and genetically associated with the Chibougamau Pluton. Since these porphyry deposits were emplaced near surface, they were probably preserved as a result of tilting prior to erosion of the former geological system. The Chapais-Chibougamau area is interpreted as having been tilted early on (Chown et al., 1992), thus confirming the excellent preservation potential for this type of deposit.

The presence of porphyry Cu-Mo-Py occurrences is known southwest of the Chibougamau Pluton (near the Chapais-Chibougamau airport and Queylus); these occurrences may be related to NW-SE synvolcanic structures in the central part of the western Chibougamau Pluton.

Epithermal Zn-Au deposits

The potential of the Chibougamau area for epicrustal gold-rich polymetallic deposits is poorly understood and little explored. Recent investigations in the Chibougamau area outline a Zn-Au connection in certain areas where networks of locally brecciated quartz-carbonate-sphalerite veins and veinlets are found. This type of epithermal mineralization was identified at the Lac Berrigan deposit (PDF Format, 63,3 kb), namely polymetallic veins with Au-Ag-Cu-Pb-Zn-As (Pilote and Guha, 1998). The emplacement of these veins in ultramafic sills was ideally controlled by synvolcanic faults or associated subsidiary faults characterized by limited lateral movement. The latter faults may have served as conduits for mineralizing fluids or for the emplacement of intrusions. There exists a continuum, oriented NW-SE, between the Île Merrill porphyry deposits (9.4 Mt at 1.73% Cu, 0.72 g/t Au and 13.37 g/t Ag) at depth, and the Berrigan epithermal deposit (1.5 Mt at 3.31% Zn, 14 to 380 g/t Ag and 1.9 g/t Au), located in the upper part of the stratigraphic sequence.

In the western part of the Chibougamau Pluton, the Pointe showing (PDF Format, 84 kb) exhibits barite veins with traces of chalcopyrite. Fragments of quartz veins are present in the barite. These observations suggest that early quartz veins were emplaced and later brecciated, probably by shearing, then infilled with barite. Cavity infilling, to which quartz, calcite and barite are associated, may constitute the last hydrothermal phase in the formation of an epithermal zinc-gold deposit. The search for these epithermal-type veins (Au/Ag ratio < 1) in the western part of the Chibougamau Pluton may therefore represent another first-class exploration target.

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