Geology of the western Malartic Group and correlations across the Abitibi Belt (2 of 3)
Pierre Pilote (MRNF), Vicki McNicoll (GSC), James Moorhead (MRNF) and Réal Daigneault (UQAC)

Our objectives for the second year of this three-year geological update and compilation project are as follows:

1. extend to the west the stratigraphic scheme of the Malartic Group recently developed by the MRNF in the Val-d’Or area;
2. examine at a larger scale possible correlations between lithologies in the Kidd-Munro Assemblage (KMA) of Ontario that host the renowned Kidd Creek VMS ore deposit, and certain areas in the Québec Abitibi with particular potential, namely the Destor area, the Lanaudière Formation (located along Rivière Kinojévis) and the lower part of the Malartic Group (MG). The two latter units were examined this year, in areas comprising the western half of NTS sheet 32D08 (Lac Preissac) and the eastern half of map sheet 32D07.

The Lanaudière Formation is composed of ultramafic, mafic and felsic lavas as well as numerous tonalitic to monzonitic intrusions. One of these rhyolitic complexes, located near Lac Matissard, defines a tholeiitic suite and was dated at 2716 +/- 1 Ma. This age, along with the close spatial association between ultramafic lavas and rhyolitic complexes, brings to mind significant analogies with the KMA. Ag-Au showings were also recently discovered in this location. The formation is bounded by the Aiguebelle and Manneville North faults (shallowly dipping to the north) that represent possible subsidiaries of the regional Destor-Porcupine Fault recognized further west.

The MG is composed, in stratigraphic order, of the La Motte-Vassan (2714 +/- 2 Ma), Dubuisson and Jacola (2704 +/- 2 Ma) formations. Further east in the Val-d’Or area, these lithologies are overlain by the Val-d’Or (2705 to 2704 +/- 2 Ma) and Héva formations. In the map area, the La Motte-Vassan Formation is mainly composed of ultramafic flows and sills, with basalt flows and volcaniclastic rocks. The Dubuisson Formation consists of equal proportions of ultramafic and epidotized basalt flows, with a variety of tuffs. Rhyolitic complexes occur in this formation near Ruisseau Dunn and Lac Patris (Py-Po-Au showings). The Jacola Formation is composed of mafic volcanic rocks with a few thick layers of volcaniclastic rocks. Volcanic units in the MG form a steeply dipping, south-facing, broadly homoclinal sequence. These rocks are intruded by the synvolcanic La Pause pluton, dated at 2710 Ma. The latter, comprising a diorite-tonalite suite, appears to have been emplaced as stocks and multiple sills and is responsible for an early anticlinal structural ridge, where facing directions are reversed along the northern limb.

The MG is in faulted contact to the south (La Pause fault) with sedimentary units of the Kewagama Group (< 2687 Ma). Along the contact, abundant tonalitic to monzonitic dykes are observed over several kilometres. Monzonitic dykes host many Au and Mo showings; they are most likely associated with the Preissac intrusive Complex (2681 to 2660 Ma). To the north, the Malartic Group is bounded by a zone of complex faulting (e.g. Manneville South), trending E-W to WNW-ESE and steeply north-dipping. These faults juxtapose sedimentary rocks of the Lac Caste Group (< 2691 +/- 8 Ma), volcanic units of the Lanaudière Formation and those of the MG. The regional S₁ schistosity trends WNW-ESE, and is reworked by an S₂ cleavage trending E-W, to produce Z folds at various scales. The overall metamorphic grade is the upper greenschist facies, but locally reaches the amphibolite facies near the Preissac-La Corne batholith (2691 to 2640 Ma).