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Techniques are presented for the separation of alunite [KAl3(SO4)2(OH)<s] from commonly associated minerals and subsequent stable isotope analyses for 8D, 8 OoH» 8 18Oso4» and 834S. These analyses can be performed successfully on most natural samples containing at least 100 mg of alunite. Associated minerals (typically quartz, kaolinite, and pyrite) are first minimized by hand picking or microdrilling. Ultrasonic suspension followed by elutriation is usually sufficient to remove clays. Other silicates and quartz are removed by HF digestion. Pyrite is removed by heavy liquid separation or by CrCl2-HCl digestion. For 8^8OsQ4 and S^S analyses, alunite is selectively dissolved in a heated solution of 0.5N NaOH. The filtered solution is titrated with HC1; sulfate is precipitated as BaSO4 which is subsequently analyzed for 834 S and 8 18Oso4 by conventional procedures. 8D analyses are performed on water derived from alunite by inductively coupled stepwise thermal decomposition that minimizes SO2 and FfeSCU production and formation of K±Na oxides. 8 18OoH analyses are performed using a total fluorination method modified from that of Pickthorn and O'Neil (1985). Bulk oxygen produced by fluorination of alunite (818OfirF5), consists of 50±1% of the O2 from the SO4 site and 100% from the OH site. A large fractionation during fluorination of sulfates (A 18Oso4(GR-Fl)) is constant over the entire natural range of 8 18OsO4 values and is dependent on the ratio of K+Na to total oxygen. A 18OsO4(GR-Fl)alunite (10.5%o in this laboratory) was determined by comparison of 8 18Oso4 values obtained by conventional graphite-reduction (GR) with those by fluorination (Fl) of reagent mixtures. 818OoH is calculated from a mass balance equation which includes terms for 818OfirF5, 818Oso4» (measured by graphite-reduction), A 18Oso4(GR-Fl)alunite» and the molar ratios of liberated OH and SO* oxygen. The calculated 818OoH is insensitive to uncertainties in O2 yield and generally reflects the total analytical uncertainty of the fluorination and sulfate graphitereduction technique. INTRODUCTION Alunite is a characteristic mineral of acid sulfate mineral assemblages formed by several diverse processes (Rye, et al., 1992). It contains four stable isotope sites, more than any other common mineral except crystallographically similar jarosite. The isotopic composition each of the various sites provides different information about the formation of the mineral. Complete stable isotope analyses (5D, 818Oso4» 818Opn» and 834S) of all sites, especially when accompanied by stable isotope analyses of coexisting minerals, can provide information on origins of components, processes (including rates), physicalchemical environments, and temperatures of formation (Bethke, 1984, Rye et al., 1989; 1991; 1992; Alpers et al., 1992). The occurrence of oxygen in both SO4 and OH sites is of particular interest because of possibilities for a single mineral isotope geothermometer (Pickthorn and O'Neil, 1985). The separation of alunite from a typical acid-sulfate alteration assemblage and subsequent complete stable isotope analyses is a complex, time-consuming task. It involves up to four mineralogical and chemical separation procedures, extraction of three elements in four different gas extraction lines, and measurement of isotopic ratios using three different mass spectrometer configurations. We have completed over 500 analyses on over 200 alunite bearing samples from examples of each of the major alunite forming environments. The results, interpretations, and geologic significance of these analyses have been presented elsewhere (Rye et al., 1992). In addition, the experimental hydrogen isotope alunite-water and oxygen isotope alunite SO4-water and OH-water fractionation factors have been determined in this laboratory and presented elsewhere (Stoffregen et al., in press). The purpose of this report is to present a compilation of the laboratory methods and individual analytical techniques that we have used to separate and perform complete stable isotopic analyses of alunite and its commonly associated minerals. This report is organized around a flow chart that traces the sampling scheme and the various analytical procedures (Fig. 1). Many of the techniques have long been standard in mineral separation or stable isotope analyses and these are indicated by appropriate references. Our principal contributions have been: 1) the development of a NaOH dissolution technique for 8^Oso4 and 8^4S analyses of alunite and, 2) significant modifications of the BrFs technique of Pickthorn and O'Neil (1985) for S18O0H analysis of alunite. Both of these analytical techniques are discussed in detail. In addition, some refinements of standard techniques are presented for 8D analysis of alunite. The elutriation-centrifugation technique for the purification of kaolinite (or other clay minerals) or its removal from alunite is that of Jackson (1985) and is detailed here for the convenience of the reader. INITIAL SAMPLING Alunite formed in magmatic-steam and super gene (Rye, et al., 1992) environments is frequently found as veins and fracture fillings of almost pure alunite. Such samples need minimal preparation prior to isotopic analysis. However, the typical acid-sulfate alteration assemblage contains variable amounts of fine-grained alunite, quartz, kaolinite (dickite, halloysite or nacrite), and pyrite. Careful sampling using appropriate handpicking and microdrilling techniques often greatly reduces subsequent effort and time. Characterization of the sample, or sample splits, by optical microscopy, X-ray diffraction, or scanning electron microscopy (SEM), is critical in understanding the nature of each sample and how to treat it as well as to determine the amount starting material to be used in the various techniques. MINERALOGICAL SEPARATIONS Of the typical acid-sulfate assemblage, the most troublesome impurity is kaolinite, which must be physically removed (to where it constitutes <2% of the sample) for SD analysis of alunite and reduced to acceptable levels (5-10%) for the NaOH or HF dissolution techniques. Ultrasonic suspension followed by elutriation and centrifugation is usually sufficient to remove kaolinite and other clays from alunite. A detailed description of the procedure used in this study is presented in Appendix A. This technique, however, does not work well when both alunite and kaolinite are very fine grained as in some supergene samples. In such samples, kaolinite can be removed from alunite by dissolution in HF or NaOH, but large amounts of kaolinite relative to alunite can easily overwhelm the chemistry of these methods. For the other commonly associated minerals, quartz can be recovered during the NaOH dissolution of alunite and removed by HF dissolution, and pyrite can be removed gravimetrically, selectively extracted by CrCl2-HCl digestion (Zhabina and Volkov, 1978), and recovered during NaOH dissolution. CHEMICAL SEPARATION AND PURIFICATION OF ALUNITE The analyses of 818Oso4 and 8^4S of alunite can be performed on relatively impure samples (10-15% contaminants) by selectively dissolving alunite in a heated solution of 0.5N NaOH. In contrast, 8D and S18OoH analyses of alunite require a mineral separate with less than 2% quartz and/or kaolinite. Although in some cases the mineralogical separation methods noted above will provide a sufficiently pure alunite separate, most natural alunite-bearing samples require additional chemical treatment to reach this level of purity. For these samples, controlled dissolution in HF will produce a silicate-free, isotopically unaltered alunite separate providing they contain a sufficient amount of alunite. Alunite dissolution in NaOH The procedure to prepare alunite for 518Oso4 and 534S analyses is normally performed on samples containing 40-100 mg of alunite; however, the procedure has been used successfully on samples containing as little as 15 mg of alunite when only a 8 18Oso4 analysis is required. The method consists of dissolving alunite in NaOH and precipitating the sulfate as BaSO4. (A detailed step by step description of the procedure is included as Appendix B). Alunite is selectively dissolved in a solution of 0.5N NaOH that is stirred and heated to 80°C for several hours. After the alunite has completely dissolved, the solution is filtered to remove insoluble minerals. The filtrate is warmed, acidified with HC1, and BaSO4 is precipitated by addition of BaCl2. The HC1 acidification step is critical to prevent the coprecipitation of A1(OH)3 or alunite with the BaSQ*. Figure 2 shows the solubility of Al^+ as a function of pH for dissolution of alunite under the conditions described above and the HC1 tritation path for subsequent BaSO4 precipitation. The procedure involves rapid titration with concentrated HC1, lowering the pH of the filtrate to below 2.8 within a few seconds, followed by back tritation with 0.5N NaOH to a pH of 2.8-3.0. In this way the A1(OH)3 and alunite stability fields are crossed rapidly enough to prevent or minimize precipitation of these phases. Small amounts of A1(OH)3 or alunite that may form during this step are redissolved when the pH has stabilized in the desired range. Once the pH has been adjusted in this manner, BaSO4 is immediately precipitated by the addition of BaCl2. The precipitate is aged for 3 hours at 80° C to increase grain size, acidified, and cooled to room temperature. The NaOH dissolution technique works best with an alunite mixture mostly free of kaolinite. If the assemblage contains relatively large amounts of kaolinite and/or Kfeldspar, the A13+ concentration may become so high during NaOH dissolution that A1(OH)3 formation cannot be prevented by rapid HC1 tritation to pH <2.8. This problem can be solved by precipitating A1(OH)3 intentionally by tritating with HC1 to pH approximately 6 at 80°C. The hot solution is centrifuged to separate the A1(OH)3 gel, and the liquid fraction i

Methods for separation and total stable isotope analysis of alunite