Abstract

Oxygen isotopes can potentially provide information on 1) temperature of mineral formation, 2) source of the aqueous fluid, 3) water/rock ratios and 4) extent of chemical equilibrium. This presentation focuses on the origin and interpretation of oxygen isotope anomalies (positive and negative) and the application of this material to mineral exploration.

Background

Temperature information can be extracted from isotopic analysis of mineral pairs. The oxygen isotopic composition difference, D¹⁸O, between two coexisting (coprecipitated) minerals at equilibrium is only a function of temperature (Fig. 1). The general form of the calculation is:

The isotopic composition and hence the identity or source of hydrothermal fluids may be obtained by applying mineral-water fractionation factors to isotopic data on individual minerals (Fig. 2). This procedure is just an extension of the previous concept except that temperature must be known independently (ex. from fluid inclusion, mineral equilibria, isotopic composition of equilibrium mineral pairs, or assumed) and one of the minerals is water. The temperature equation above is simply re-written to solve for M1 where M1 represents water.

Numerical modeling of water-rock interaction allows us to estimate water/rock ratios and the extent of equilibrium. Lets begin with the mass balance expression:

  (eq. 2)

and solve this expression for W/R:

for simplicity let A = d¹⁸Oⁱ_W ; B = d¹⁸Oⁱ_R ; C = d¹⁸O^f_W ; D = d¹⁸O^f_R

so we now have:

W*A + R*B = W*C + R*D W*A - W*C = R*D - R*B W(A - C) = R(D - B) W/R = (D - B) / (A - C)

Note that we know each term except for d¹⁸O^f_W (it's long gone or diluted) so we compute it from d¹⁸O^f_R using the appropriate fractionation factor; i.e. we substitute (d¹⁸O^f_R - D_(rock-water)) for d¹⁸O^f_W where D_(rock-water)) = 2.68*10⁶*T^-2 - 3.53. In this case we have used the constants 2.68*10⁶ and 3.53 which correspond to water - plagioclase fractionation to simulate a whole rock.

C = D - D_(rock-water) and D_(rock-water) = 2.68*10⁶*T^-2 - 3.53

So, W/R = (D - B) / (A - D + D_(rock-water))       (eq. 3)

At this point you can substitute in the original terms for A, B, C, and D defined above. However, this format is particularly handy for doing the modeling calculations in a spreadsheet (Fig. 7).

Application to Exploration

Recent case studies

Until recently the relatively high cost of isotopic analyses has prevented the application of this tool to mineral exploration. Initial district studies have focused on locating negative anomalies. In the Yankee Fork District (Fig. 3) country rock oxygen isotope values are above 6 permill. The producing mines are located where the country rock values have been lowered to 2 to 4 permill. This is a negative anomaly because the values are lower than (more negative than) the surrounding counrtry rocks. A reconstruction along the Custer faults shows that an approximately circular anomaly is present.

In the Noranda District typical counrty rocks have oxygen isotopic values between 6 and 9 permill (Fig. 4). Figure 4 is difficult to read because it is a 3 dimentional projection on 2 dimensions. The sites of VMS mineralization are closely associated with negative anomalies (ie. d¹⁸O < 6 permill). The zones of negative anomalies coincide with feeder zones and semi-conformable alteration zones (sub horizontal aquifers).

Franklin et al., 1981, show that not all the feeder zones (Fig. 5) of VMS deposits have negative anomalies. In fact, one of the largest deposits, the Kidd Creek deposit has a positive anomaly. Does this mean that giant deposits are characterized by positive anomalies? This question is examined in the next section.

With the relative drop in in the cost of isotope analyses and faster turnaround time more districts are being evaluated with this tool.

Interpretation of Anomalies

How can we interpret the anomalies in terms of geological processes? The Tonopah districthas been studied in some detail (Fig. 6). A typical concentric negative anomaly appears to be present. The country rocks have values near +7 permill and all the known mines are located within the 0 permill contour. What is the significance of the values of the contours?

The oxygen isotope values can be interpreted as W/R contours or temperature contours. If we assume a constant temperature, say 285°C, we can solve eq. 3 for W/R. The spreadsheet facilitates this calculation and shows the following correspondence:

If we assume a constant water/rock ratio we can re-lable the contours as temperature contours (solve eq. 3 for T) and get the following results:

It is apparent that the zones of mineralization are located in zones of either hotter temperatures or higher water pass areas. How can we determine which is the case? Independent field data would be needed. For example, the presence of abundant fractures and veins towards the center of the district would indicate higher water/rock ratios. Fluid inclusion and alteration studies may provide temperature information. The geometry of the anomaly may be used to infer fluid movement paths.

Application to the Geco VMS deposit

The Geco VMS depsosit was studied by Petersen and DePangher. Negative oxygen isotope values (relative to country rock background values) are found imediately below the deposit in semi- conformable alteration zones. Positive oxygen isotope values are found stratigraphically above the deposit.

Using the modeling spreadsheet software we can infer that the positive anomaly above the doposit was formed at low temperature (~90°C) and high water/rock ratios as the mineralizing system was expiring. The negative anomaly below the deposit was formed at high temperature (~300 to 400°C) and high water/rock ratios as the mineralizing system was waxing. The isotope values could also be used to map thermal gradients around the deposit. Alternatively, in combination with independent paleotemperature determinations we could map fluid flow paths.

This technique is very powerful because it can be used to see through the metamorphism to which this deposit has been subjected (upper amphibolite facies). Thus, if mineralogy is difficult to interpret or only weakly affected by alteration, oxygen isotopes may be used to identify drill targets.

Figures

• Fig. 1 Mineral-mineral fractionation (Faure, 1977, pdf)
• Fig. 2 Mineral-water fractionation (Faure, 1977, pdf)
• Fig. 3 Yankee Fork District (Nesbitt, 1996, pdf)
• Fig. 4 Noranda Mining District (Nesbitt, 1996, pdf)
• Fig. 5 VMS deposits (Franklin et al., 1981, pdf)
• Fig. 6 Tonopah isotope map (jpg).
• Fig. 7 Sample spreadsheet output (jpg)

References

Nesbitt, B.E., 1996, Applications of Oxygen and Hydrogen Isotopes to Exploration for Hydrothermal Mineralization, SEG Newsletter, No. 27, 1-13.

Campbell, A.R. & Larson, P.B., 1998, Introduction to stable isotope applications in hyddrothermal systems. in J.P. Richards & P.B. Larson, Eds., Techniques in Hydrothermal Ore Deposits Geology. Reviews in Economic Geology, Volume 10, 173 - 194.

Faure, G., 1977, Principles of Isotope Geochemistry. John Wiley & Sons, New York, 464 p.

Franklin, J., Sangster, D.M., & Lydon, J.W., 1981, Volcanogenic-Associated Massive Sulfide Deposits. in 75th Anniversary Volume, Society of Economic Geologists. 485-627.

Petersen, E.U. & DePangher, M., 1987, Regional footwall alteration of mafic and felsic volcanics in the Manitouwadge massive sulfide district, Ontario, Canada: Geological Society of America Abstracts, 19, 7, 804.

Simulation Spreadsheet Software

This spread sheet allows the user to calculte wate/rock ratios for any temperature and two chemical systems (igneous rocks, limestones). The program can be easlyly modified to do calculations in other systems by simply changing the fractionation factors as indicated. Note: The program can be downloaded as a zip file that needs to be unzipped. PC Version

Petersen, E.U., 1999, Oxygen isotope simulation spreadsheet (EXCEL 5.0), The University of Utah.

Acknowledgements

I wish to thank the National Science Foundation (NSF) for the grant that made the oxygen isotope analyses at Geco possible.