Comment and Reply on "Evaporites and strata-bound tungsten mineralization". COMMENT. Alfonso G. Trudu, Department of Earth Sciences, Monash University,.
Comment and Reply on "Evaporites and strata-bound tungsten mineralization" COMMENT Alfonso G. Trudu, Department of Earth Sciences, Monash University, Clayton, Victoria 3168, Australia The interpretation of syngenetic W mineralization is controversial. Since Maucher (1965) proposed the syngenetic-exhalative model for strata-bound Sb-Hg-W deposits, no economic discovery, except for the Felbertal deposit (Austria), has been made by following such a model. However, Briegleb et al. (1985) and Trudu and Clark (1986) argued that this deposit is genetically related to a nearby granitoid complex. Ririe (1989) presented a variation on Maucher's theme of strata-bound W deposits by proposing an evaporitic model for some small W occurrences in the Kimberleys of Western Australia. His model consists of (1) an evaporitic stage, during which W presumably precipitates in colloidal form and is trapped in muds similar to those of Searles Lake in California; and (2) a metamorphic stage, during which the muds are metamorphosed and W is remobilized into lenses. Although Ririe's model is stimulating, the W occurrences he described lack the main features that would support his model. His paper raises two separate questions: (1) What type of W mineralization occurs near Halls Creek in Western Australia? and (2) If a metamorphosed equivalent of the Searles Lake W resource can theoretically occur, where could we find it and what would it look like? From Ririe's (1989) description of the strata-bound W mineralization in the east Kimberleys, it is evident that the host rocks to the mineralization lack many features typical of a metamorphosed evaporitic sequence, such as textures indicative of a chemical origin (cf. Strelley Pool Chert in Western Australia, described by Lowe, 1983). These host rocks also do not contain scapolite, tourmaline, lazulite, Na-plagioclase, Na-amphibole, and CI- or Li-bearing micas, all minerals which Ririe himself quoted (1989, p. 142) as characteristic of metamorphosed evaporites. Gypsumbearing cavities in^he uppermost part of the W-bearing quartzite and thin carbonate horizons provide very limited evidence of a possible evaporitic environment. However, Ririe contradicted himself further by inferring a detrital origin for the W-bearing quartzite. The photographic evidence (Fig. 3 in Ririe, 1989) for the textural relation between scheelite and gypsum-lined cavities is unconvincing, because it cannot be established 188
whether scheelite replaced former gypsum crystals in the quartzite or grew in an open space within a quartz vein. Ririe (1989) cited the presence of high-salinity fluids as evidence for an evaporific origin to the W mineralization in the Halls Creek Group. However, his preliminary data are difficult to interpret because of the secondary nature of the inclusions, which, in any case, could not have trapped solutions from which the hosting mineral precipitated, because this contradicts the definition of secondary fluid inclusions. Furthermore, if quartz and scheelite recrystallized at temperatures approaching 500 °C, it is very unlikely that premetamorphic fluid inclusions could have been preserved in these minerals. Likewise, the isotopic data used to estimate metamorphic temperatures do not provide any evidence with respect to the evaporitic origin of the W mineralization, because the isotopes reequilibrated during greenschist facies metamorphism. Although Ririe's model is interesting, the W occurrences hosted by the Hall Creek Group may be just as easily explained by a more conventional model. The regional geology of Dow and Gemuts (1969) showed that fewer than 5 km to the west of the W occurrences, the Sophie Downs granite is in intrusive contact with the Halls Creek Group. Therefore, it is possible that a granitic body may underlie the scheelite mineralization and may have been the source of the W-bearing solutions. If this hypothesis is correct, the mineralizing fluids may have exploited the most porous medium in the stratigraphy, i.e., the part of the quartzite which is the richest in cavities. If these cavities were lined with Ca-bearing sulfates, as Ririe suggested, the quartzite could also have provided a chemical trap for scheelite deposition, as shown by the following reaction: W02- + CaS0 4 = C a W 0 4 + S O f .
(1)
Regardless of the origin of the W occurrences in the Halls Creek Group, the question is, Where could we find a metamorphosed equivalent of the Searles Lake W resource, and what would it look like? The nonmarine sabkha-type environment (a closed system from which the heavy metals cannot escape) surrounded by major W districts and associated with W-bearing hot springs is a unique geologic feature of the Searles Lake setting. The small size of nonmarine sabkha-type environments and their GEOLOGY, February 1990
uncertain preservation throughout the geologic record further decrease the chances of finding an ancient Searles Lake style of W mineralization. If found, the metamorphosed equivalent could appear as either strata-bound and disseminated mineralization, as veins, or as a combination of the two styles, depending on the extent of W remobilization during burial and metamorphism. Strata-bound and disseminated W mineralization would be the result of a metamorphic environment characterized by limited circulation of fluids, so that in situ recrystallization of W-bearing muds would produce a pelitic horizon with disseminated scheelite. In a brittle metamorphic regime characterized by high fluid flow, W may be remobilized from the muds into fractures to produce veins hosted by Ca-rich units. In this case, remobilization could have occurred to the extent that it is difficult to recognize the important genetic relation with the paleomuds. Whether any of these processes has ever taken place in nature on an economically significant scale will only be proven when the first W deposit of such type is found.
REPLY G. Todd Ririe, Unocal Science and Technology Division, P.O. Box 76, Brea, California 92621 Trudu raises two questions addressed in my article (Ririe, 1989) concerning the possible link between continental evaporite sequences and strata-bound tungsten mineralization. The first concerns the type of tungsten occurrences I described from near Halls Creek in Western Australia, and the second is where a metamorphosed equivalent of a continental evaporite sequence might occur and what it would look like. The strata-bound tungsten mineralization I described from near Halls Creek is enigmatic in the sense that it occurs discontinuously along strike for approximately 20 km within quartzites. Because of the discontinuous nature of the mineralization, our exploration program was ended before there was an opportunity to adequately test my model. Thus, samples of the interpreted evaporitic section were not collected to determine the presence of minerals that might be expected from metamorphosed evaporite sequences. However, the stratigraphic section consisting of quartzites with crystal cavities after gypsum, overlain by a sequence of pelites, carbonates, and volcanic ash, must have formed in a restricted subaqueous environment. These characteristics are consistent with my interpretation that the rocks formed in a continental evaporite environment. Trudu argues that because the sequence lacks rocks such as chert, and contains detrital rocks, it did not form in an evaporitic setting. This reasoning is faulty, because most continental evaporite settings contain predominantly clastic rocks, and only when the proper combination of hydrologic, chemical, and climatic conditions exist will siliceous chemical sediments form. For example, an examination of my Figure 3 (Ririe, 1989) demonstrates that the stratigraphy in Searles Lake, a continental evaporite setting, is dominated by clastics (muds, sands, and gravels) and that no cherts are present. Trudu suggests that a buried intrusion may underlie the scheelite mineralization and thus be the source of the tungsten-bearing solutions. Although there may be a buried intrusion beneath the tungsten mineralization near Halls Creek, just as there is one beneath the sediments at Searles Lake (Fig. 3 in Ririe, 1989), there is no good evidence that an intrusion was responsible for the introduction of tungsten-bearing fluids. For example, there is a lack of any systematic changes in the oxygen isotope data as a function of proximity to mineralization. Regional metamorphism does not result in the homogenization of the oxygen isotopes unless there is an associated large hydrothermal fluid phase. More commonly, systematic changes are preserved in the isotopic record through metamorphism. For example, work by Larson (1984) has shown that significant changes in oxygen isotope data are preserved around the metamorphosed Bruce massive sulfide deposit in Arizona. I agree with Trudu GEOLOGY, February 1990
that the high-salinity fluid inclusions in scheelite do not prove an evaporitic environment but, as I have suggested (Ririe, 1989), are consistent with the formation of scheelite from saline brines either before or during regional metamorphism. Thus, the fluid-inclusion data are necessary but not sufficient evidence for an evaporitic environment. The geometry of the tungsten occurrences (scattered along strike for 20 km) also differs significantly from deposits formed from intrusion-related hydrothermal activity. No evidence exists for alteration zones spatially related to the tungsten mineralization in the Kimberleys. Regardless of the origin of the mineralizing fluids, the fact that the most permeable horizon in the stratigraphic column hosts the tungsten mineralization is consistent with almost any model whereby tungsten is precipitated out of solution. With regard to Trudu's second major question, where we might find a metamorphosed evaporite sequence and what it might look like, I cited several examples (Moine et al., 1981; Schreyer and Abraham, 1976; Serdyuchenko, 1975; Walker et al., 1977) that give a clue as to what a metamorphosed evaporite sequence might look like. As to where we might find one with tungsten, I emphasize that Searles Lake is not unique with regard to its geologic setting. There are many modern continental evaporite sequences in Nevada (Papke, 1975) that drain highland areas with tungsten skarns (Kerr, 1946) and receive waters from thermal springs enriched in tungsten (Wollenberg et al., 1977). The relatively small size of continental evaporite deposits that are now forming cannot be used as a quantitative indicator of their relative size or importance in the geologic past. I believe there are other stratabound deposits that formed in metamorphosed evaporitic sequences (e.g., the Proterozoic Starra gold deposit in Queensland) that await recognition. Although a variety of isotopic, fluid inclusion, stratigraphic, and geochemical trends can be used to infer an evaporitic setting, the most conclusive evidence is the presence of molds, casts, or pseudomorphs of evaporitic minerals such as I described from the Kimberleys. COMBINED REFERENCES CITED Briegleb, D., Finger, F., Kraiger, H., Pestal, G., and Steyrer, H.P., 1985, The Kl-Gneiss from the scheelite mine Felbertal (Hohe Tauern/Austria) [abs.]: Fortschritte der Mineralogie, v. 63, suppl. 1, p. 33. Dow, D.B., and Gemuts, I., 1969, Geology of the Kimberley region, Western Australia: The East Kimberley: Australia Bureau of Mineral Resources, Geology and Geophysics Bulletin 106, 135 p. Larson, P.B., 1984, Geochemistry of the alteration pipe at the Bruce Cu-Zn volcanogenic massive sulfide deposit, Arizona: Economic Geology, v. 79, p. 1880-1896. Lowe, D.R., 1983, Restricted shallow-water sedimentation of Early Archean stromatolites and evaporitic strata of the Strelley Pool Chert, Pilbara Block, Western Australia: Precambrian Research, v. 19, p. 239-283. Maucher, A., 1965, Die Antimon-Wolfram-Quecksilber-Formation und ihre Beziehungen zu Magmatismus und Geotektonik: Freiberger Forschungshefte ser. C, v. 186, p. 173-188. Moine, B., Sauvan, P., and Jarousse, J., 1981, Geochemistry of evaporite-bearing series: A tentative guide for the identification of metaevaporites: Contributions to Mineralogy and Petrology, v. 76, p. 401-412. Papke, K.G., 1976, Evaporites and brines in Nevada playas: Nevada Bureau of Mines and Geology Bulletin 87, 35 p. Ririe, G.T., 1989, Evaporites and strata-bound tungsten mineralization: Geology, v. 17, p. 139-143. Schreyer, W., and Abraham, K., 1976, Three-stage metamorphic history of a whiteschist from Sar e Sang, Afghanistan, as part of a former evaporite deposit: Contributions to Mineralogy and Petrology, v. 59, p. 111-130. Serdyuchenko, D.P., 1975, Some Precambrian scapolite-bearing rocks evolved from evaporites: Lithos, v. 8, p. 1-7. Trudu, A.G., and Clark, A.H., 1986, The Felbertal (Mittersill) scheelite deposit, Austria: A W-Mo-Be vein system related to felsic plutonism, not a submarineexhalative deposit [abs.]: Australia Bureau of Mineral Resources Record 1986/10, p. 73-74. Walker, R.N., Muir, M.D., Diver, W.L., Williams, N., and Wilkins, N., 1977, Evidence of major sulphate evaporite deposits in the Proterozoic McArthur Group, Northern Territory, Australia: Nature, v. 265, p. 526-529. Wollenberg, H., Bowman, H., and Asaro, F., 1977, Geochemical studies at four northern Nevada hot-spring areas: University of California, Lawrence Berkeley Laboratory Report 6808, 70 p. 189
