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Gold mineralization:
New data from the Uran-Barry Belt

Pierre Rhéaume and Daniel Bandyayera
Direction de Géologie Québec

During the summer of 2005, Géologie Québec conducted field studies at selected sites in the Urban-Barry Belt (UBB), particularly in the Lac Windfall and Lac Rouleau sectors. Newly stripped zones and recent drill cores were examined to gain a better understanding of the geological context of the gold mineralization for which the sector is known. Preliminary examination of the collected data reveals several similarities between the Lac Windfall sector and epithermal precious metal deposits.

This work supports previous studies and highlights the development of several types of auriferous mineralization during the UBB’s geological history. In particular, the hypothesis of a large epithermal system in felsic and intermediate rocks for the Windfall and Rouleau members has significant consequences for exploration. The following features are considered particularly important:

  • Epithermal-type mineralization observed thus far is associated with recognizable alteration defined by silicification, carbonatization, potassium gain, sodium depletion and the presence of tourmaline. This signature, evident in the field, can serve as an exploration guide.
  • The effects of this epithermal system can be seen within a broad area which includes several rock types. This hydrothermal signature is notably (but not exclusively) expressed in the Lac Windfall-Lac Rouleau sector and the Lac aux Loutres sector. The exploration for epithermal precious metal deposits should take into account such a large volume of altered rocks.
  • The structural style in the UBBfeatures doubly plunging folds and structural imbrication from south to north which are favourable to the preservation of extensions of the main altered unitsat relatively shallow depths.

Location and geological context

The UBB is located 100 km south of Chapais (Figure 1). It consists dominantly of volcanic rocks and extends for 135 km from Lac Wilson in the west to the Grenville Front in the east. The UBB displays a prominent E-W structural trend marked by doubly plunging folds and E-W faults that produce thrusting to the north (Figure 2). Numerous felsic volcanic assemblages are present, including the Novellet (2,714 Ma), Freeman (2,701 Ma), Windfall (2,718 Ma), Rouleau, Chanceux (2,727 Ma) and Fecteau (2,791 Ma) members.

Figure 1
Location of the Urban-Barry Belt.

Figure 2
Simplified geology of the Urban-Barry Belt.

The 2005 work focused primarily on the Windfall and Rouleau members where a number of exploration companies, including Ressources Murgor Inc., Noront Resources Ltd. and Beaufield Consolidated Resources Ltd., have been active for several years. Stripping and drilling in the Windfall sector revealed pyritic stockworks crosscutting a complex of quartzo-feldspathic porphyry sills, andesites and felsic tuffs. The stockworks (Figure 3) are typically only weakly deformed and have yielded gold grades of more than 10 g/t Au. The Au/Ag ratio for the samples collected in 2005 is high, on the order of 1/1, and the host rock displays alteration that is often intense and characterized by silicification, carbonatization, potassium gain and sodium depletion. Pyrite also contains tourmaline needles and locally forms tourmaline-cemented breccia similar to that found in the Lac aux Loutres sector.

Figure 3a
Sample from a gold-bearing interval displaying a pyritic stockwork in an intensely silicified zone (drill hole NOT-4-27).
Figure 3b
Detail of the boxed area in A displaying brownish tourmaline crystals with triangular cross-sections in pyrite. Sample provided by Noront Resources Ltd.

Current information indicates that mineralization predates deformation and is syn-volcanic in origin. The early timing of the mineralization, the development of pyrite-gold stockworks in silicified zones with a potassic alteration halo, and the Au/Ag ratio of about 1 collectively suggest that these mineralizations belong to a large epithermal system or to an alteration pipe in an auriferous volcanogenic-exhalative system.

Considerable quantities of disseminated tourmaline (1-2%) and pyrite were also observed in intermediate tuffs west of Lac Rouleau. south of the Windfall sector (NAD 83–18 - 454587E - 5431846N). These tuffs coincide with a folded magnetic high that can be followed for several kilometres.

The Lac Rouleau sector has very little exposure and was mainly investigated by studying drill core. Core samples are currently being processed for lithogeochemical analysis that will help characterize this large unit. The rocks of the Rouleau Member observed in drill core were mostly lapilli or block tuffs of intermediate composition. We also observed significant carbonate alteration locally associated with shearing, as well as quartz veins and specks of visible gold. Earlier work reported reserves of 544,000 t grading 7.2 g/t Au. Preliminary evidence is compatible with vein-type mesothermal gold mineralization.