Abstract

The Zone 1 intrusion of the Callahan property, Val-d'Or, Quebec, is a relatively small (0.045 km2), pretectonic to syntectonic trondhjemitic mass that has been altered to varying degrees by progressive hydrothermal alteration. The entire intrusion is gold enriched, and the degree of mineralization is proportional to the degree of alteration. Examination of drill core reveals that visible colour variations are related to mineralogic and chemical alteration. Three degrees of alteration are identified: least, medium, and most-altered. The least-altered facies has a green–grey colour, whereas the medium-altered facies has a pale grey colour and the most-altered facies has a cream–white colour. As the intensity of alteration increases, the amount of albite, carbonate, and pyrite increases and the amount of phengite decreases. These progressive, mineralogic changes are correlated chemically to increases in Na2O, CO2, and S and to a decrease in K2O. The median gold values for least-, medium-, and most-altered trondhjemite are 42, 148, and 158 ppb, respectively, and the average gold values (excluding highly anomalous values) are 85, 172, and 188 ppb, respectively, indicating that gold enrichment is already advanced at the medium intensity of alteration. Despite its weak enrichment, tungsten is the most anomalous trace element of the typically gold-associated elements in lode deposits and is, therefore, of special interest. Gold, found principally in the medium- and most-altered facies, is most commonly observed as free grains in the gangue. It may also be associated with pyrite and, to a much lesser extent, with tellurides. The majority of the free gold grains have an estimated diameter between 20 and 60 μm, and their average composition is 95 wt.% Au and 5 wt.% Ag. This study and comparisons of the Zone 1 trondhjemitic intrusion with other gold-enriched, felsic intrusions in the Abitibi greenstone belt suggest that elevated sodium (i.e., albitization) and anomalous gold values, relative to barren felsic rocks, should be used to target ore-grade zones within pretectonic to syntectonic felsic intrusions.

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