One important issue we have to understand about the Au geochemical anomalies in the Greenstone Belt rocks of Quebec is that they always show a GRADUALTY even in non-economic zones. When we see thousands of gold assays of a zone it seems to observe a chaotic distribution but that is just a first impression. The gradual pattern is observed not only in the hanging wall or footwall of economic ore bodies but also in all the veins at decimetric scale.
To visualise this idea, we should imagine a single quartz-carbonate vein with economic Au values inside. Now we add two veinlets following local fractures starting from the mentioned economic vein like branches, with anomalous Au values (0.1-0.8 g/t Au) and some nugget values (more than 50 g/t Au) like clogs in the vein. Then, at the end of the economic vein in the top we add two veinlets with anomalous values following the same direction, it means the same fault or fracture. Consider now a proportional alteration halo for the economic vein and all the veinlets mentioned, containing anomalous values or 30-100 ppb Au sub-anomalous values. Finally, multiply this single vein system by 50 or 100 depending of the deposit. The result is an apparently complex stockwork system with a high probability of having a single vein system overprinting another one. A mixture of dozens of single vein systems with little branches and fingers.
The picture presented in the paragraph above is similar to the stockwork systems observed in modern porphyry or Au-disseminated deposits. The difference is that in these kinds of modern deposits we can extract the whole stockwork system in a comfortable open pit. In the Greenstone Belt of Quebec, the concentration of gold in the whole stockwork system is about 0.4 g/t Au in average, so in most of the cases it is not economic to extract everything. However, the possibility to have an economic open pit is always present, and it has been demonstrated in mines like Malartic Au mine (Val d’Or) or Troilus Au-Cu mine (Chibougamau).
Thus, the key factor to interpret Au geochemical values in the Greenstone Belt of Quebec is to identify a single vein system and its gradual anomalies. It is easy to identify a single vein system because they always follow the longitudinal alignments (normally SW-NE) by kilometers or tensional faults (SE-NW) by dozens of meters. And we can simplify the interpretation grouping some single vein systems in an alteration corridor. If we follow these steps, we can clearly observe the gradual process of the hydrothermal fluid and we can forecast new targets and make a correct economic evaluation of our deposit.
Fernando Alvarez V., P. Geo, MBA
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