Presence of Hybrid Rocks over Greenstone Belt in Quebec: Their Importance

Presence of Hybrid Rocks over Greenstone Belt in Quebec: Their Importance

Hybrid rocks over Greenstone Belt in Quebec occur when a hypabyssal dyke affects the previous volcanic lava. It is a kind of “metasomatic gradual contact” between these intrusive and volcanic rocks. They are normally 0.1 – 3 m width, having  irregular shapes in any direction. However, hybrid rocks follow dykes’ direction at regional scale, as same as the Au hydrothermal fluids.

The typical characteristics of hybrid rocks over Greenstone Belt in Quebec are the following: a) undefined texture that can be described like a mix of porphyric and aphanitic texture at any possible combination, depending of the intensity of mixing. Sometimes it is possible to see two or three textures in the same hand sample: porphyric, aphanitic and granular, which is amazing, b) colours goes from greenish gray to whitish beige, or any colour resulting of mixing of green basalts or gray rhyolites and white felsic to intermediate dykes, c) mineralogical composition is really a hybrid: looking in the chart of igneous rocks a hybrid rock will be in the border of two or three type of rocks. At microscope you will see a spectacular disposition of crystals hard to classify (Volcanic? Intrusive? Alteration?). And when you use some XF tools you will see non-sense values of guide elements.

Hybrid rocks over Greenstone Belt in Quebec are important for these reasons: a) they represent another characteristic to understand the distribution of Au hydrothermal fluids because they are in the contacts of mineralizing hypabyssal dykes and volcanic rocks, b) they should be recognized in order to avoid a confusion that could produce an erroneous geological interpretation, c) they can give ideas for research about temperatures of formation, intensity of hypabyssal intrusions and depth of geological environments, d) they are a key factor to understand Intrusion-related deposits around the world.

Teams of geologists working in the Greenstone Belt of Quebec must have a good comprehension of hybrid rocks as a proof that they are in the good way to develop Au deposits in the zone. It will be a good signal if they can manage to have something very complicated in the geological model. Hybrid rocks are telling us many geological clues that will facilitate the opening of Au mines in the Greenstone Belt of Quebec. Let’s do our work with enthusiasm and intelligence.

Fernando Alvarez V., P. Geo, MBA

This article and their versions in French and Spanish are available at www.larocheverte.ca, in Blog section.

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