Many of the Au deposits in the Val d’Or – Chibougamau alignment are Intrusion-Related deposits. And most of the intrusions observed are from hypabyssal origin. Even when a deposit cannot be classified like “Intrusion-Related” the presence of hypabyssal dykes is indirectly associated to Au mineralisation. It means that we have to be prepared for finding a hypabyssal environment and for understanding the particular characteristics of this type of rocks. If we do that, the complex relationship between volcanic and intrusive rocks including hydrothermal fluids will be easier to be focused. And being well-focused, Au mineralisation will be adequately evaluated and calculated.
A quick refresh: hypabyssal rocks are formed at intermediate depths, with a minimum volume of magma in comparison with plutonic intrusions. They come from digitations of a big magmatic chamber or from a small magma sub-chamber. Consequently, they form dykes, sills, irregular intrusions and stocks, all of them different of a typical plutonic batholith. Another name for hypabyssal rocks is “subvolcanic rocks”: something that it did not arrive to the surface, but it was close to do it, something that is similar to volcanic, but it is intrusive.
Hypabyssal dykes and stocks were the feeder for Au hydrothermal fluids over the Greenstone Belt from Val d’Or to Chibougamau, and maybe over the whole Greenstone Belt. Given the hypabyssal importance, we describe their characteristics observed in the region:
- Porphyritic texture: the typical texture for hypabyssal rocks is clearly observed on the dykes and stocks of the region, so it represents the main characteristic.
- Granular texture: sometimes hypabyssal dykes have a medium-grained granular texture similar to aphanitic texture, so they become very similar to a volcanic rock.
- Quick change of dyke composition: since it is possible to have some differences in depths and temperatures of formation in hypabyssal dykes because they are not formed exactly at the same moment and place, lithology composition can change frequently. In general, dykes on the region can pass from felsic to intermediate composition. Mafic hypabyssal dykes are not frequent, but they can occur.
- Distribution of hydrothermal fluids: Au mineralisation associated to hypabyssal rocks will produce a different distribution of anomalies and economic zones in comparison with plutonic and volcanic feeders. Shape of Au mineralisation in the studied zone indicates a strong association to hypabyssal dykes and small stocks.
- Hybrid rocks: hypabyssal dykes are weak intrusions because of their minimum volume, so it sometimes occurs they cannot replace the previous volcanic rock totally, producing a “contact metasomatic” mix (0.1 – 3 m width) where it is not possible to say if the rock is volcanic or intrusive. To put things more complex, hydrothermal fluids are also present, so the resulting mix becomes a non-classified product that we can call “hybrid rocks”. Hybrid rocks cannot be ignored because they usually have good Au mineralisation, and they occur everywhere chaotically. A next post about hybrid rocks will be necessary to explain their importance.
Fernando Alvarez V., P. Geo, MBA
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