The key for success exploring Au deposits in the Val d’Or - Chibougamau trend is the integral understanding of the regional geology correlating with the specific characteristics of the deposit studied. It is necessary to define the importance of all the parameters observed, explaining why they happened and why they can change. A good comprehension of the geological setting will guide us TO BE IN CONTEXT about our Au research. After this, all the interpretations made by geologists should be in context. It means that any new idea should fit with the regional geological setting.
And what is the context for the Greenstone-Belt-Hosted-Au deposits in the Val d’Or – Chibougamau trend? Let’s see that.
- Greenstone Belt, 2.7 billion (+/-0.1) years: It is not a simple lithology nor a simple age. The Greenstone Belt 2.7 represents a huge event of volcanism and plutonism which originated highly-enriched Au hydrothermal fluids in different sub-environments. Since the Au-hydrothermal sub-event was part of the Greenstone Belt magmatic activity, most of the lithologies we can find (basalts, andesites, rhyolites, felsic dykes, etc.) are marked by the Au-hydrothermal print with variable intensities. Thus, rocks of Greenstone Belt have a unique geochemical model from a regional point of view that can be identified at any sample we send for assays.
- Au mineralisation events are parallel to Greenstone Belt strike: On the Val d’Or – Chibougamau trend the strike is about N60E, but it can change in zones of regional folding. The strike of Greenstone Belt is the result of the volcanic arc alignment from 2.7 billion years ago. Intrusive dykes and stocks mostly followed the same alignment and, therefore, Au hydrothermal fluids followed that direction too. The concept of parallelism of events is a key guide to follow Au mineralisation across the region.
- Intrusion-related Au deposits: All the Au-hydrothermal event in the Val d’Or-Chibougamau is strongly associated to intrusive dykes, sills and stocks, so deposits can be classified in the category of “Intrusion-related Au deposits”. It means that texts about this type of deposit can help to understand the context. However, sometimes hydrothermal fluids can take advantage of an adequate structural setting, so they can form Au hydrothermal veins. If the highly-enriched hydrothermal fluid was not trapped it can produce an Au-disseminated deposit. And it is possible to observe in some places a “porphyry behavior”, when Au and Cu disseminate over an intrusive stock. In all the cases the most important is to understand the strong relationship with intrusive dykes-sills-stocks, which normally are felsic or intermediate.
- Nugget effect: Au mineralisation in the Greenstone Belt has ALWAYS as a characteristic the nugget effect, which can be in different degrees that it should be determined by geostatistical analysis. Nugget effect is like any other engineering factor, so it should not represent a problem for the projects. The problem can appear if companies try to ignore or minimize the nugget effect. The good thing about the nugget effect is sometimes is positive, because in a zone you expected a low-grade you can find a very high-grade mineralisation. Remember that, statistically, nugget effect is present in both directions: expected high-grade could become low-grade and vice versa.
Half of the good understanding of Greenstone-Belt-Hosted-Au deposits in the Val d’Or – Chibougamau trend is the consideration of the geological context. Being in context makes exploration easier and faster. Having integral ideas is the key to open many Au mines in the Val d’Or – Chibougamau trend. If you work there, stay in the context.
Fernando Alvarez V., P. Geo, MBA
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