Certain dyke-hosted W-Sn ore bodies (I am avoiding to mention the deposit) have their highest ore accumulation (at least locally) at the tips of the dykes, where flanking structures occur (pinnate veins for instance). Dykes lengthen parallel to the sigma 1 (at least the local sigma 1), while fluids are injected and precipitate due to the local change of P conditions.
Hence, do the higher accumulation of ore-bearing fluids at the dyke tips indicate different phases of fluid injection, or remobilisation of fluids as the structures grow, or anything else?
I understand that detailed textural analysis is needed, but I don´t have it.
Dear Pablo,
we should to understand, that local enricment of ore mineralisation don't depend on presence, or concentration, or source of fluids, but mostly depends on conditions where metal complexes within the fluid were decomposed causing ore minerals precepitation. In other words, we observe ore mineralization not in places where fluid was concentrated, but in places where its components were decomposed more effectively. In places where metal complexes (for example KSnF5) hadn't decomposed we don't found ore minerals despite fluid had penetrated these volumes of rock and went in farther portions of rock.
From this point of view, tips of your dike had conditions favourable for decomposition of fluid and precepitations of ore minerals, while central portions of the dike hadn't necessary conditions. So fluid came through these central portions of the dyke and left them still with complete ore load and didn't precepitate ore.
Thanks Pavel,
I fully agree with your assertation. What I wonder is if there is a way of traking or linking such favourable conditions for ore precipitation with the veins/dyke growth history.
What I was thinking is that, as dikes/veins growth by propagation of their tips and widening, the concentration and location of the ore minerelas migth indicate different phases of fluid injection (like first quarzt, and then quartz + the ore/s). This obviously deserves carefull textural and structural work. Fluid inclusion studies migth also be very useful.
Regards
Pablo
Dear Pablo,
Here is a tentative explanation to this repartition of ore at the tip of intrusions: it is possible that the particular mineral you are looking at requires some phase transition of the fluid in order to precipitate, like the liquid to vapor transition for example. Such phase transition is more likely to occur at the tip of intrusion because the fluid is not heated yet, whereas it is already in the vapor phase in contact with the remainder of the intrusion walls. If this explanation is correct, you should be able to detect a difference in the filling ratio and the nature of your fluid inclusions at the tip of dykes and elsewhere.
Hi Vincent, thanks.
I am sorry I do not fully understand your argument.
What do you mean with "...at the tip of intrusion .... the fluid is not heated yet, whereas it is already in the vapor phase in contact with the remainder of the intrusion walls".
Pablo
Dikes of viscous fluid that freezes (basalt) behave differently than veins of low-viscosity hydrous fluid. The opening of a viscous dike depends on the force balance over the surface of the dike. The fluid pressure in the dike needs to overcome the pressure from the rock (stress perpendicular to the dike). If the dike propagates the excess pressure from the dike above the rock needs to overcome the strength (stress intensity factor) of the rock at the dike tip. Dike tips are preserved when propagation stops; so then the excess pressure is low. The viscous fluid in the dike sometimes does not make it all the way into the tip. Hydrous vapor likely fills the tip while the dike material freezes. The channel in the plane of the dike along the tip can be path for ore forming fluids. The process is not relevant to low-viscosity vein fluids that don't freeze.
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