We are working on a mineralized hydrothermal system (Iron-oxide-copper gold i.e. IOCG) and want to perform some thermodynamic calculations (e.g., Biotite thermometry, halogen fugacity ratios of fluid). For that, we need to understand if the hydrothermal fluid was in equilibrium with the hydrothermal minerals (e.g., biotite). Biotite in our system have replaced earlier phases (e.g., scapolite, amphibole) and present in veins along with other phases (e.g., tourmaline, sulfide) and as disseminated grains in the host. These three textural types of biotite show similar composition. Biotite do not have visible zoning (in optical microscope) and appears to be homogeneous in terms of composition (Scanning Electron Microscopic analysis). Biotites most likely crystallized in high fluid/rock ratio. Are these observations good enough to consider that the fluid was in equilibrium with biotite? If not what type of evidence should we consider to understand if fluid-biotite equilibrium was attained?
Dear Abu, Step 1) Verify textural equilibrium among the phases that you have assumed to have crystalized from the hydrothermal fluid. Step 2) Corroborate this with stable isotope measurements of the individual mineral populations to determine if there is stable isotope equilibrium among the phases (perhaps tourmaline in your case, and quartz, if present). That is, that the minerals have crystallized from a fluid with a common overall stable isotope oxygen and hydrogen composition. You have then supported your assumption of mineral-fluid equilibrium. Hope this helps.
Dear Abu, in addition to textural and isotopic evidences I would try to compare halogene ratios in the micas.
Thank you, Dr. Fedorowich for your answer. The biotite veins are monomineralic mostly. In places, it is associated with tourmaline and/or apatite but it seems that these phases predate (slightly) the biotites. Therefore, H and O isotope equilibrium between biotite and other phases may not be suitable in our case. This is why I was thinking if the similar composition of different textural varieties of biotite (i.e., those present as disseminated grain in the host, those present in veins, and those have replaced earlier phases such as scapolite and amphibole) along with the fact that biotite crystallized in high fluid/rock ratio can be used as a criterion for fluid-mineral equilibrium.
Thank you, Dr. Walter. Could you please be more specific? We analyzed the Cl and F composition of these biotites. F is always below the detection limit of EPMA and Cl concentration varies between 0.2 to 3.2 wt.%. We have calculated IV(Cl) i.e. chlorine intercept value and halogen fugacity ratios of the fluids (i.e., fH2O/fHCl of fluid). For the later parameter, the biotite-fluid equilibrium was considered. Do you mean any of these parameters?
Dear Abu, I guess you are in principle fine with the textural argumentation. I thought about a combustion IC approach. See recent work of the petrology group in Tübingen. With a combustion IC you can determine very precise F, Cl, Br, I and sulfur contents in mineral separates. I would expect similar halogene ratios of the three mica popluations if they are related to one event (and hence to one fluid). EMPA analyses are probably not precise enough. I would be cautious with interpreting one population if Cl-contents varies significantly. Hope this helps...
Dear Abu, I am not sure, but assumptions about fluid/rock ratios for the various biotite populations may lead to circular reasoning. In particular, do the cross-cutting relationships support the assumption that the biotite populations formed contemporaneously? As Benjamin Walter notes, an independent method on the biotite (and other minerals) could help to provide stronger evidence. Best regards.
- Badges
- Science topic
- Similar topics
- Engineering
- Modeling