Most of the Au hydrothermal fluid observed over the Greenstone Belt in Quebec are strongly related to hypabyssal dykes and stocks formed by the same magmatism that it produced the basalts, andesites and rhyolites of the Greenstone Belt (2.8 – 2.7 billion years). Meanwhile the submarine volcanism was forming layers of lavas some meters deeper the hypabyssal intrusions occurred. Both events carried out hydrothermal fluids associated, however they were a little different: hydrothermal fluids associated to polymetallic deposits occurred with the volcanism event and, at the same time, Au hydrothermal fluids were related to the hypabyssal intrusions.
Since the volcanic and hypabyssal events have the same magmatic origin both types of hydrothermal fluids have similar patterns. Thus, in some places Au fluids can contain some Cu in economic values and Zn-Pb in anomalous values, or a polymetallic fluid can have economic values of Au. In fact, if we observed the hydrothermal event in a regional point of view, we can define a unique geochemical pattern for the whole magmatism. It means that we could follow the geochemical path of Au, Ag, Cu, Zn and Pb, establishing strong associations to forecast new targets.
Considering this context, the general characteristics of the Au hydrothermal fluid hosted in the Greenstone Belt rocks are:
- The dominant direction of the Au hydrothermal fluids in Quebec is parallel to the volcanic event, that means SW-NE direction, which can change drastically if a portion of the volcanic rocks is folded. This dominant direction probably was the volcanic-arc alignment formed between an ancient continent (Kenorland?) and an oceanic plate.
- As a consequence, hypabyssal dykes and stocks associated to Au hydrothermal fluids also have the same direction (SW-NE) however in a local view these intrusions have irregular shapes. A good follow-up of dykes and stocks in a 3D software can effectively show their parallelism to the volcanic arc.
- Au hydrothermal fluid mainly fills longitudinal faults and fractures (SW-NE) close to the contact of hypabyssal dykes-stocks and volcanic rocks. Some tensional faults and fractures (SE-NW) can also be filled locally. Besides, the hydrothermal fluid can be emplaced over contacts between lavas, like branches starting from the feeder fault.
- The intensity of the structural setting is not strong, just moderate. Thus, the Au hydrothermal fluid has a crackle-type appearance, not being regular like a typical Au hydrothermal vein. This condition implies that Au concentrations vary frequently, producing Nugget Effect at different degrees.
- Hydrothermal alteration associated to Au mineralisation can mainly be quartz-tourmaline, quartz-sericite, quartz-hematite or quartz-chlorite. Any of these alterations can be prevalent in a particular deposit, and the other will be secondaries or absents. In some deposits a biotite alteration can be observed. Carbonates (calcite, ankerite, siderite or occasional dolomite) are commonly present with the quartz fluid.
- The most common sulphide observed with the Au mineralisation is pyrite, in concentrations going from 1% to 12% Py and occasionally more than 12% Py. Chalcopyrite is the second one in importance, with a persistent anomalous dissemination (0.1% - 0.5% Cpy average) that can have economic values in some deposits. Sphalerite and galena are present in traces representing local anomalies. Finally, scarce molybdenite, pyrrhotite, arsenopyrite and Ag sulfosalts could be also observed.
- Every time we find the hydrothermal fluid associated to hypabyssal dykes and stocks an alteration halo will be present at both sides of the mineralisation. However, since dykes are abundant and can be overlapped the alteration halos accumulate themselves in a bigger alteration zone that can be called “corridor”. Alteration corridors are parallel to the dominant direction NW-SE and they are 2-60 m width. All the Au mineralisation will always be surrounded for an alteration corridor. Finally, if there is a bigger accumulation of alteration corridors an Au-disseminated deposit could be formed.
Fernando Alvarez V., P. Geo, MBA
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