The systematic sampling of core rocks frequently done in drill hole programs allows us to identify the geochemical anomalies present in deposits. As we know, geochemical anomalies guide us to find new economic zones and build the geological model. In this article I give some ideas for interpreting Au anomalies for Au deposits hosted in the Greenstone Belt rocks of Quebec.
Au hydrothermal fluids passing through the Greenstone Belt produce three well-defined zones: 1) the economic zone, so obvious and easily recognizable especially when there is visible gold, 2) the alteration corridor zone, which is the zone containing most of the gold anomalous values, and 3) the veinlet zone, where hydrothermal fluids have formed a non-mineralised stockwork system with some anomalous values. Given this context, it is necessary to describe these three zones a little more.
The economic zone can be composed, depending of the deposit, by veins, or orebodies, or small irregular shapes or Au-disseminated zones, or a combination of any of them. It is important to remember that the economic zone will ALWAYS BE INSIDE the alteration corridor zone. Even in fracture-filling vein deposits where the fluid has been well channelized it is possible to observe the alteration halo around the ore. This simple concept will allow us to follow the economic zones by hundred of meters, even by kilometers. Au economic values go from 0.8 g/t Au to spectacular nugget values of more than 3000 g/t Au, depending of the deposit. In fact, only a feasibility study of the mining resources will determine what it is economic and the consequent cut-off value.
The alteration corridor zone basically is the alteration halo of the Au hydrothermal fluid. At regional scale, it is a tabular zone parallel to the direction of feeders in a section view and parallel to the regional direction of the Greenstone Belt in a longitudinal view (see article about hydrothermal fluid). At local scale, the corridor zone normally follows the contacts of the hypabyssal dykes and stocks or a predominant fault. The alteration zone contains most of the anomalous values of the property, ranging from 0.1 g/t Au to 0.8 g/t Au, and also includes some isolated nugget values. Corridors are 2-60 m width, defining a clear Au geochemical anomaly.
The veinlet zone is composed by a stockwork system of quartz-carbonate veins containing some extra Au anomalous values. This zone is located immediately around the alteration corridor zone and it contains mainly Au geochemical values from 5 ppb Au to 100 ppb Au plus the mentioned extra values. Occasional nugget values can also be obtained. The veinlet zone is not important for the economic evaluation of a deposit, but it is very useful in regional surface exploration: if a veinlet zone is recognized in an outcrop it could be possible to infer the alteration corridor zones and the lovely economic zones. Thus, a veinlet zone helps us to identify new target zones.
In conclusion, it is possible to know how far or close we are from the ore zones if we follow these guides to interpret Au geochemical anomalies in the Greenstone Belt of Quebec. We will share more ideas about in the next article.
Fernando Alvarez V., P. Geo, MBA
This article and their versions in French and Spanish are available at www.larocheverte.ca, in Blog section.