Economic Geology Vol. 84, 1989, pp. 162-166 ANALYSIS OF TRACE ELEMENTS INCLUDING RARE EARTH ELEMENTS IN FLUID INCLUSION LIQUIDS DAVID I. NORMAN, PHILIP R. KYLE, AND CHARLES BARON Department of Geoscience, New Mexico Institute of Miningand Technology, Socorro, New Mexico 87801 Introduction The mobility of most elements in hydrothermal so- lutions is not well known.Analyses of fluid inclusion liquids canyield the composition of fluids once pres- ent in a rock;however, micron dimensions of typical fluid inclusions and the low ratio of inclusion fluid to mineral have presentedsevere analyticalproblems and limited most analyses of inclusion fluids to the major anions and cations (Roedder, 1984). It ishighly desirable to obtainmore completeanalyses of inclu- sion liquids than has been possible because the only direct means of analyzing fluids associated with most geologic-hydrothermal processes, including meta- morphism,is by fluid inclusionmicroanalysis. The position of minorelements in ore-depositing fluids is of special interest to students of mineraldeposits be- cause someof the minor elementsare deposited as ore minerals andthe minor elements maybe used as a tracer of ore. A methodhas been developed to an- alyze minor elements in fluid inclusion liquids,in- cludingthe rare earth elements, with a precision of 5 to 50 percent;the methodcaneven measure con- centrations of some elements as low as tens of ppb. Herein isreported the analysis of 28 elements in fluid inclusion fluids from a Cu-Mo porphyry and an as- sociated precious metal vein deposit. The CopperFlat deposit waschosen for study be- cause it contains a breccia on which economic min- eralization is centered that has coarse-grained, near- pegmatiticminerals (Dunn, 1982) and because the fluid inclusions in these minerals have been little dis- turbed by postdepositional events.Minerals in the brecciamatrixare quartz, biotite, K-feldspar, pyrite, chalcopyrite, magnetite,molybdenite, fluorite, cal- cite, and apatite. The porphyry deposit is ringedby Au-Ag-Cu-mineralized quartz veins with variable amounts of pyrite,galena, and sphalerite, which occur principallyalongthe margins of dikes(Fig. 1); they are postulated to be associated with the Copper Flat quartz monzonite(Harley, 1934). The district pro- duced 6.9 million dollarsin Au-Ag-Cu oresprior to 1931 from at least18 lodeandplacer operations, and Au has been produced sporadically from placer de- posits since then. The CopperFlat porphyrydeposit wasdeveloped in 1980, but it soon ceased operations because of low copperprices. Inclusion watershave been analyzed in seven sam- plesof quartzfromthe CopperFlat breccia bodyand three samples of quartzfromthe Wicksvein.The fluid inclusions are >95 percent primaryandgenerally less than 0.01 mm in diameter.There are three types of inclusions: type i has liquid, vapor,and one or more daughter minerals including halite;type 2 has liquid anda small vapor bubble;andtype 3 has vapor-dom- inant,liquid-vapor inclusions with variable vapor/liq- uid ratios that indicate boilingduringmineralization. Salinityand homogenization temperature(Th) mea- surements ontype i and2 inclusions indicate salinities andtemperatures of depositing fluids for the Copper Flat breccia of 34.4 to 7.7 equivwt percentNaC1 and 359 ø to about 179øC; and for the veins, 33.7 to 5.7 equiv wt percent NaC1 and388øto 226øC.Secondary inclusions have salinities and homogenization tem- peratures of about 6.6 equiv wt percent NaC1and 217øto 133øC,respectively. Arithmetic averages and the range of values for each sample, each representing about 20 measurements, are included in Table 1. The analytical method was modified from thatgiven by Czamanske et al. (1963). Quartz was crushed to 2- to 4-mm-diameter grains, handpicked, thenground to 0.25 to 0.09 mm,subjected to magnetic separation, andhandpicked again. Cleaning was by organic sol- vents,boiling in aquaregia for 8 h while changing the solution every 2 h, boiling in distilled-deminer- alizedwater for 24 h, andboilingin electrolytic cells for more than a week, changing the water daily. Ap- proximately 60 g of sample was thermally decrepi- tated in a quartz flask over a bunsenburner, then poured into a teflon beaker containing 30 ccof super- pure i percentHNOa and allowedto sit for several hours. The leach solution was filtered, reduced to 2 cc by heating, and evaporated to dryness in a 5-mm- diameter, high-purity silica (Suprasil) vial. This yielded i to 14 mg of salts. Examination of decrepi- tated quartz indicated that virtually all observed in- clusions were emptied. Salts were analyzedquanti- tatively by instrumental neutronactivation methods following irradiation for 72 h at a flux of 2 X 1013 n/ cm2/sec and using various rock standards. Blanks were prepared by duplicating the inclusion waterextraction process using an equalamount of leachate; analyses were corrected for amounts in the blank which was typicallyless than 5 percent of the elementpresent in the extractedsalts. The amountof water present in quartzsamples was measured by thermallydecrep- itating a split ofthe sample in vacuum at 600øC(which opened virtually all inclusions), then condensing the 0361-0128/89/904/162-552.50 162