How to produce fluids used for modeling from geochemical data obtained from geochemical analysis?

[Teng Deng]

Hello guys

As a GWB user to study the fluid rock interaction in hydrothermal ore deposit, one of the most important initial work is to obtain the composition of fluids. Generally, we use LA-ICP-MS analysis to get the chemical composition of ore fluids. But the data have to be processed with SpecE8 before used as the input file for React. However, the processing by SpecE8 is seldom detailed shown in the paper, confusing starters like me a lot.

Here I’d like to show a typical example, and wish to get some help.

According to microthermometry and LA-ICPMS analysis of fluid inclusions, we got the following conditions:

T=145 °C, CH₄=300 ppm, Cl^-=4.81 mol/kg, Ca²⁺ =0.5 mol/kg，Mg²⁺ =0.12 mol/kg, K⁺=0.1 mol/kg, Na⁺=3.5 mol/kg

By studying phase diagram, we got other key parameters:

pH=4.4, log fO₂=-57, H₂S= 2*10^-3 mol/kg

Since the deposit contain quartz, feldspar, pyrite, chalcopyrite, sphalerite and galena, we use these minerals to buffer the contents of SiO₂(aq), Al³⁺, Fe²⁺, Cu²⁺, Zn²⁺ and Pb²⁺ in fluids. In addition, I also use dolomite to balance Ca²⁺ in the fluids.

However, what I got is: Newton-Raphson did not converge after 999 iterations…

So, how can I solve such a problem?

Pb-Zn deposit-initial fluid.sp8

[Jia Wang]

Hello Teng,

Thank you for providing details to your problem and attaching an input file. However, the input file was not available when I try to download it.

The convergence error means that the program cannot calculate the equilibrium state of your system. This can mean that the constraints prescribe doesn't describe a model in equilibrium and the a numerical solution cannot be found. Or, the constraints might be entered it in a way that the program does not expect which does not lead to an easy solution. Since I cannot access the input file provided, I can make a couple of general suggestions for your simulation.

Start with a simple system and then build more complexity with it. If you start by putting everything you have in the GWB, it makes it difficult to troubleshoot. When you are setting up your initial system in the Basis pane, make sure the species that you have added (or swapped into) are reasonable. For example, I would suggest swapping out SO4-- with H2S(aq) given that your system is so reduced. Occasionally, you may need to swap an appropriate species in for the default species, to help the program converge.

If you are swapping in quartz, feldspar, pyrite, chalcopyrite, sphalerite and galena in for SiO2(aq), Al3+, Fe2+, Cu2+, Zn2+ and Pb2+, I would check if this is a reasonable assumption. Just because the minerals are present, it does not necessarily mean that the fluid is in equilibrium with it.

If you would like more help with troubleshooting, please reattach your input file so we can take a closer look.

Best regards,
Jia Wang

 

[Teng Deng]

4 hours ago, Jia Wang said:

If you would like more help with troubleshooting, please reattach your input file so we can take a closer look.

Hello Jia Wang

Thanks for your explanation. I have reattached the file.

Pb-Zn deposit-initial fluid.sp8

[Teng Deng]

Hello Jia Wang

In order to better illustrate my problem, I would like to give more details.

I am current reconstruct the modelling in Perry, (2016).  If you want to know more details, please see attached.

There are several models in this paper, and what I am showing is the first model (reduced-sufur). The fluid initial of this model used by the author is shown in Table 4.1 of the page 155.

The paragenesis sequence is shown in Fig.4.4 of the page 141. The minerals of the ore deposit is shown there. In addition, since the fluids have been through sandstones, so they have been buffer by quartz and feldspar.

 

Please note that due to the size limitation of the attached files, I have deleted most pages in the PDF. The remaining part is related to what we are talking about here.

 

Thanks for your patience and time.

Best wish

Teng Deng

Perry, 2016-sent-many pages deleted.pdf

[Jia Wang]

Hello Teng, 

Thanks for reattaching your input file and providing the additional details. In my post above, I mentioned that minerals present with the fluid does not necessarily mean that it is in equilibrium with the fluid. When you swap a mineral into your fluid sample, the program is trying to set the mineral in equilibrium with the fluid. If you do that with minerals that are not in equilibrium with your fluid, then the program cannot solve numerically for the equilibrium state. 

To troubleshoot your script, I recommend that you begin by setting up your initial fluid with the chemical analysis in Table 4.1 on your basis pane in SpecE8. Then run the calculation to see if SpecE8 is able to calculate an equilibrium state for your initial fluid.  For more information on swapping and setting up the initial fluid, please refer to section 7 Using SpecE8 in the GWB Essentials User Guide. 

Once you are able to use SpecE8 to calculate the initial equilibrium state, then use React to build a reaction model.  Start with the same initial fluid constraint used in SpecE8 to set up the Basis pane in React and add in reaction paths that you want to model (e.g. polythermal paths, titrations, mineral reactions, etc). To see examples on setting up a reaction model, please see section 2 Getting Started with React in the GWB Reaction Modeling User Guide. 

Hope this helps,
Jia

[Teng Deng]

49 minutes ago, Jia Wang said:

To troubleshoot your script, I recommend that you begin by setting up your initial fluid with the chemical analysis in Table 4.1 on your basis pane in SpecE8.

Hello Jia Wang

Of course I can balance the reaction with SpecE8 using the data in Table 4.1, since they are the result after processed from the raw data by geochemical analysis.

The geochemical results are what I show above.

T=145 °C, CH₄=300 ppm, Cl^-=4.81 mol/kg, Ca²⁺ =0.5 mol/kg，Mg²⁺ =0.12 mol/kg, K⁺=0.1 mol/kg, Na⁺=3.5 mol/kg

By studying phase diagram, we got other key parameters:

pH=4.4, log fO₂=-57, H₂S= 2*10^-3 mol/kg

In addition, there are also some minerals buffering the fluids.

 

My question is: How can I get the data in Table 4.1 with the chemical analysis?

 

Best wishes

Teng Deng

[Teng Deng]

9 hours ago, Jia Wang said:

Start with a simple system and then build more complexity with it.

Hello Jia Wang

Thanks for your time and patience. I believe your suggestion “Start with a simple system and then build more complexity with it” is quite useful.

 

Following your suggestions, I add the components one by one.

(1)  At last, I found that swap pyrite with Fe++ would cause the convergence error (see the SpecE8 file “Fig. 16A-V4”).

(2)  If I give a certain value like 30 ppm for Fe++ as shown in Table 4.1, the convergence error is gone. However, the Cu++ contents are incredibly low (7*e^-7 ppm), much lower than the one in Table 4.1. Consequently, I do not think that this model is correct. (see the SpecE8 file “Fig. 16A-V4-no pyrite”)

(3)  The only reasonable result is that I set 1 ppm and 30 ppm for Cu++ and Fe++ respectively. (see the SpecE8 file “Fig. 16A-V4-no pyrite and chacopyrite”). However, it does not make any sense, since we can not know Cu and Fe values from geochemical analysis in the H₂S-rich fluids of this paper.

 

So my questions is: How can I do the three thing at the same time: swap pyrite in, get reasonable results, kick out the convergence error?

Fig. 16A-V4.sp8 Fig. 16A-V4-no pyirte and chalcopyrite.sp8 Fig. 16A-V4-no pyirte.sp8

[Jia Wang]

Hello Teng,

Unfortunately, there's no way to know how the authors processed the data in SpecE8 if they don't specify what they did. Even if the parameters were changed so that the model is converging, there's no guarantee that the results would be comparable. If you are trying to replicate their results, then I suggest you email the corresponding author to see what constraints were set in SpecE8. 

Best,
Jia