This discussion took place during September, 1999

 

Ulf Huenken

Larry Meinert

Bob Bodnar

Jacques Touret

Bob Bodnar

Michel Dubois

Ioan Pintea

 

Dear all:

During my investigations of a porphyry copper deposit in Papua New Guinea I have found three types of fluid inclusions: high-salinity multiphase (halite, sylvite, Ca-rich?) inclusions (HS), gas-rich inclusions with low to moderate salinity (GR) and low- to moderate-salinity liquid-rich inclusions (LR). This is of course no surprise but the HS inclusions show a strange behavior concerning their homogenization temperatures and salinities. There are two distinct groups of HS inclusions. The first group shows homogenization temperatures between 450 and 550 °C and contains about 40 to 50 wt.% salt. The second group shows homogenization temperatures between 350 and 400 °C with salinities of about 50 to 60 wt.%. Both groups are associated with GR inclusions (same temperature range of homogenization) indicating a boiling system. My problem is that the "low temperature" HS inclusions contain about 10 wt.% more salt than the "high temperature" HS inclusions. If there was a dilution of the HS fluid by meteoric waters (as reported from other deposits) the salinity should decrease with decreasing temperatures as shown by the GR inclusions in the same samples. Is there a possibility of salt fractionation between gas and liquid?. Has anyone an idea (references) concerning this pattern? Thanks a lot for help.

Cheers

Ulf

Dr. Ulf Huenken
Mineralogisches Institut der Universitaet
Am Hubland
97074 Wuerzburg
Germany
Tel.: ++49-(0)931-888-5432
Fax: ++49-(0)931-888-4620
Email: ulf.huenken@mail.uni-wuerzburg.de

****************************************************

I think that you are correct that this lower temperature set is not related to meteoric water. One possible explanation for this phenomenom is pressure. If the pressure changes, possibly due to partial transition from lithostatic to hydrostatic pressure, then upon phase separation there will be a different ratio of vapor to liquid. For a fluid with a given starting salinity, separation into 90% vapor versus 50% vapor will yield a liquid phase with higher salinity. Thus, to explain your data you could invoke a single parent fluid hitting the solvus at two different pressures and thus generating liquid phases with different salinities. Depending on the pressure path, this could occur at the same or different temperatures. I invoked a similar scenario to explain fluid inclusion observations in the Ertsberg district across the border in Irian Jaya (see Meinert, L.D., Hefton, K.K., Mayes, D., and Tasiran, I., 1997, Geology, zonation, and fluid evolution of the Big Gossan Cu-Au skarn deposit, Ertsberg district, Irian Jaya: Economic Geology, v. 92, p. 509-526.)

Larry Meinert

Department of Geology
Washington State University
Pullman, WA 99164-2812
Office: 509-335-2261
Sec:	509-335-3009
FAX:	509-335-7816
meinert@wsu.edu
****************************************************

Dear Dr. Huenken and others who may be interested:

I believe that you have made the mistake that many (even experienced) inclusionists make. That is, you have assumed that just because two (or more) types of fluid inclusions can be seen in the same field of view when looking down the microscope, that they must have been trapped at the same time. You state in your message that "Both groups are associated with GR inclusions (same temperature range of homogenization) indicating a boiling system." But, it is not possible for fluid inclusions that homogenize by halite disappearance (which should be your second group based on your reported Ths and salinities) to be in equilibrium with a vapor phase (unless the liquid- rich inclusions trap some solid halite along with liquid, which invalidates the use of these inclusions in any case.). Thus, your second group of high salinity flincs are probably NOT associated with vapor-rich inclusions in a genetic sense, they just happen to occupy the same portion of the quartz that you are looking at.

You should also be aware that it is unwise to accept the common model of decreasing salinity and temperature with time for the porphyry copper deposits. The actual temperature-salinity trends depend on many factors, one of the most important of which is the pressure (depth) of formation. The old model of dilution and cooling of magmatic fluids by low T low salinity meteoric fluids is now known to be a gross over-simplication of the actual process.

I suggest you look at the following references (especially Bodnar, 1994):

Bodnar, R.J. and R.E. Beane (1980) Temporal and spatial variations in hydrothermal fluid characteristics during vein filling in preore cover overlying deeply buried porphyry copper-type mineralization at Red Mountain, Arizona. Economic Geology, 75, 876-893.

Roedder, E. and R.J. Bodnar (1980) Geologic pressure determinations from fluid inclusion studies. Annual Review of Earth and Planetary Sciences, 8, 263-301.

Cline, J.S. and R.J. Bodnar (1991) Can economic porphyry copper mineralization be generated by a typical calc-alkaline melt? Journal of Geophysical Research, 96, 8113-8126.

Bodnar, R. J., (1992) Can we recognize magmatic fluid inclusions in fossil hydrothermal systems based on room temperature phase relations and microthermometric behavior?. Geological Survey of Japan, report No. 279, p. 26-30.

Cline, J. S. and Bodnar, R. J. (1994) Direct evolution of a brine from a crystallizing silicic melt at the Questa, New Mexico, molybdenum deposit. Economic Geology, 89, 1780-1802.

Bodnar, R. J. and M. O. Vityk (1994) Interpretation of microthermometric data for H2O-NaCl fluid inclusions. in Fluid Inclusions in Minerals, Methods and Applications, B. De Vivo and M. L. Frezzotti, eds., pub. by Virginia Tech, Blacksburg, VA, p. 117-130.

Bodnar, R. J. (1994) Synthetic fluid inclusions. XII. Experimental determination of the liquidus and isochores for a 40 wt.% H2O-NaCl solution. Geochimica et Cosmochimica Acta, 58, 1053-1063.

Sincerely, Bob Bodnar

*******************************************************************

Dr. Robert J. Bodnar
C.C. Garvin Professor of Geochemistry
Department of Geological Sciences
4044 Derring Hall
Virginia Tech
Blacksburg, VA 24061-0420
Tel: (540) 231-7455 (O)
 (540) 953-2448 (H)
 (540) 353-2448 (Cellular)
Fax: (540) 231-3386
e-mail: bubbles@vt.edu
http://www.geol.vt.edu/profs/rjb/rjb.html

********************************************************************

Dear all,

I follow with interest the ongoing discussion of salinity increase during a boiling porphyry system at lower temperature, and I agree with Bob that it might be quite difficult to visualize this as a single, continuous event, but, if this was the case, would it not simply indicate that you are dealing with a fixed amount of initial fluid (most probably magmatic) whose high-density (liquid) part will progressively become more salty when boiling proceeds (the same for my soup when I leave it on the fire for a too long time). Of course, pressure has to vary accordingly (in an overall cooling system), and the change from lithostatic to hydrostatic regime may indeed be of great help. I had thought that in general this would correspond to the invasion of meteoric fluids (Is this the too simple model, Bob?), but may be in the present case meteoric fluids were simply absent. Was Papua completely desertic at the time of the intrusion?

Cheers,

Jacques Touret

touj@geo.vu.nl

********************************

Larry:

I think you are wrong. His inclusions with Th (L-V) of 350-400 and salinities of 50-60 wt.% could NOT have been trapped on the solvus at ANY pressure, assuming the inclusions trapped a single, homogeneous liquid phase. Of course, they could have been trapped on the solvus IF they trapped mixtures of liquid and halite, but then the whole argument is moot because the inclusions have violated on of the basic assumptions upon which microthermometry is based.

Bob Bodnar

********************************

I have interleaved Michel’s questions and Ulf’ responses in the following section —phil

Dear Ulf and other inclusionists,

I think that your question requires more information about microthermometric behavior of inclusion and what you mean by "association". Thus, following you text, I ask the following questions:

>There are two distinct groups of HS inclusions

what does "groups" mean? two different groups of inclusions completely separated as a petrographical point of view (different host minerals or growth zones or fluid inclusion planes) ? or two groups in a halite dissolution temperature vs vapour disappearance temperature diagram?

Two groups on a salinity vs vapor disappearance diagram

>The first group shows homogenization temperatures between 450 and 550 C and contains about 40 to 50 wt.% salt.

40 to 50 wt% NaCl corresponds to halite dissolution temperatures of about 320 to 430 C (please check this point with your measurements), which means than homogenization occurs by vapour disappearance. Fluid was thus, in a general point of view, at 450 C at least

I will check it

>The second group shows homogenization temperatures between 350 and 400 C with salinities of about 50 to 60 wt.%.

50 to 60 wt% NaCl corresponds to halite dissolution temperatures of about 420 to 500 C (also check this point with your measurements). This means that 350-400 C is the range of vapour disapperance (not homogenization) temperatures and that bulk homogenization occurs by halite dissolution. It means that trapping conditions were at least 420 C.

I will check it too

Therefore, I deduce group 1 and group 2 inclusions can have been trapped in the same temperature conditions

>Both groups are associated with GR inclusions (same temperature range of homogenization) indicating a boiling system.

I agree with Bob. "Association" is greatly different to "contemporaneity", which is difficult to prove. Do you observe many "mixed" inclusions, with variable vapour and halite volumes, which is a strong argument to trapping of an heterogeneous fluid system?

Yes, I've found many "mixed" inclusions

I also agree with Bob: it is not possible to generate inclusions with halite dissolution higher than vapour disappearance temperatures. However, when you write "same temperature range of homogenization" of GR inclusions, is it with group 1 (450 to 550 C) or group 2 (350-400 C)? Depending of the answer (if the answer is 450 and 550 C), group 1 (but not group 2) can be contemporaneous with GR inclusions, and thus indicate boiling.

I've found GR inclusions that show homogenization temperatures in the range of 350 to 550 °C. These temperatures show a small gap between 400 and 450 °C like the HS inclusions.

Hope my remarks and questions can help you.

Michel

Michel.Dubois@univ-lille1.fr

***********************************

Dear Dr. Huenken,

Please let me come into your very interesting discussion. I believe that your HS inclusions were trapped in the same kind of process during the retreating downward crystallization of the host rock. In my work related to fluid inclusion microthermometry from porphyry copper from Metaliferi Mts. (western Romania) I have observed all time two types of high salinity fluid inclusions homogenization 1. by vapor bubble disappearance and 2. by halite dissolution, in both cases the salinities (i.e. the melting point of halite) are quite similar. These is the case when beside halite we can distinguish a lot of other daughter minerals, mainly chlorides, sulphates, oxides (hematite is a common phase) etc. Moreover you can search about the term: "halite trend solution" (e.g. Quan et al., 1987), and you can ask if is this solution a heterogeneous system generated by boiling, mainly because of the influx of the meteoric water? If is so, the salt rich liquid became mainly from silicate magmatic liquid by immiscibility in a early stage and if you are lucky, perhaps you can find some silicate melt inclusion together with HS and vapor rich inclusions in the same quartz grain. For this reason please have a look to the data published by Eastoe and Eadington about Panguna porphyry copper ore deposit.

In fact my data on HS from Metaliferi Mts suggests a continuous line of evolution for this salt rich liquid which was exsolved around 1000 - 1100 C from silicate melt and finishing in a dilution state around 200 C. Perhaps the different ratio between the solids + vapor and liquid in different fluid inclusions populations (mainly as trails) is directly related to the trapping conditions: temperature, pressure changes, wetting phenomena, some time halite and other salt could be trapped as solid particles etc

I hope that my observations could be helpful for your work.

Best regards,

Ioan

Ioan Pintea
Romanian Geological Institute
Cluj Napoca branch, Fluid Inclusion Research Group
P.O.Box 181, 3400 Cluj Napoca,1
ROMANIA
ipintea@gs.ucsd.edu