Geology of the Gondor volcanogenic massive sulphide deposit, Slave Province, N.W.T.

CIM Bulletin, Vol. 78, No. 876, 1985

D.S. BUBAR and J.B. HESLOP, Kidd Creek Mines Ltd., Toronto, Ontario

The Gondor Zn-Ag-Pb-Cu deposit is a stratabound massive sulphide lens occurring within Archean volcanidastic rocks of the Yellowknife Supergroup. The deposit is located in the west-central portion of the Slave Structural Province, approximately 350 km north of Yellowknife. Initial diamond drilling at Gondor has outlined a steeply-dipping, L-shaped, folded, massive sulphide body, which may be traced over a strike length of approximately 800 m, and to a depth of 350 m. True widths of up to 40 m have been intersected in the thickest part of the deposit. Preliminary estimates suggest that the deposit contains a minimum of 7.5 million tonnes grading 6% Zn, 50 g/tonne Ag and less than 1 % combined Cu and Pb. In addition, significant gold, grading up to 2.8 g/tonne, occurs in some parts of the deposit. The mineralization is open both down-dip and along strike. The Gondor deposit lies conformably within a predominantly east-west striking, north-facing sequence of felsic to intermediate volcanidastic rocks. Coarse fragmental rocks are abundant in the hanging wall, whereas ash and lapilli-tuffs predominate in thefootwall. The main structural feature in the area is a large-scale, steeply northeast-plunging, Z-shaped fold. The sulphide mineralization lies within the synformal portion of this fold, with the thickest part of the deposit coinciding with its hinge zone. Several episodes of faulting have also affected the local stratigraphy. The wallrocks of the Gondor deposit were hydrothermally altered prior to the onset of medium-grade regional meta-morphism. A halo of altered rocks surrounds the massive sulphide lens, with no apparent difference in the degree of alteration between the hanging wall and footwall strata. The alteration zone is characterized by the addition of MgO and K2O and by the depletion of Na2O and CaO. These changes are reflected in the mineralogy of the altered wallrocks by local enrichment in chlorite, biotite, muscovite and cordierite. The mineralized zone is medium- to coarse-grained, well-banded and is composed largely of pyrite, quartz, and sphalerite with subordinate galena and chalcopyrite. Preliminary studies of metal distribution indicate that a lateral zonation is present, with lead enrichment in the central part of the deposit, and copper enrichment toward both ends. Stringer-type copper mineralization has not yet been discovered in thefootwall of the Gondor deposit.

Mots Clés: Mineral exploration, Geology, Massive sulphide deposits, Gondor deposit, Slave Province, Drilling, Wallrock alteration, Stratigraphy.