Chemical Controls on Ferrierite Crystallization During Diagenesis Of
Total Page：16
File Type：pdf, Size：1020Kb
American Mineralogist, Volume 77, pages 314-328, 1992 Chemical controls on ferrierite crystallization during diagenesisof silicic pyroclastic rocks near Lovelock, Nevada SrBpnBN B. Rrcn* Exxon Researchand Engineeringcompany, Annandale, New Jersey08801, U.S.A. Knrrn G. PIPrB 2180 SchroederWay, Sparks,Nevada 89431'U.S.A. Devro E. W. Ylucn.lN Exxon Researchand Engineeringcompany, Annandale, New Jersey08801, u.s.A. Ansrru'cr The zeolites ferrierite, mordenite, and clinoptilolite have formed with smectite through hydration reactions in a sequenceof rhyolitic pyroclastic rocks near Lovelock, Nevada. Fiigh activities of silica and magnesia were required simultaneously for the large-scale crystallization of ferrierite in thesediagenetically altered rocks. Post-depositionaladdition of Mg coincided with ferrierite and smectiteformation. The sourceof the Mg is not known, but it could have originated either from a brackish, saline lake or from nearby leached basalts.Thus, origins of the Lovelock ferrierite deposit may have involved both lacustrine deposition and an open ground water system for the transformation of vitric tuffs into zeolites. Ferrierite crystallized early, mainly in the pore space of the tuffs. Clinoptilolite and cristobalite grew later by replacementof glass shards. Orthoclase and mordenite crystal- lized somewhat later, orthoclase usually in intimate associationwith ferrierite in K-rich zonesand mordenite alone in open pore spaces.Ferrierite crystallizationproceeded through a mechanism of dissolution and reprecipitation on glass and clay surfaces' Significant variation in chemical composition of individual ferrierite crystalsoccurs on the scaleof a few cubic millimeters. near INrnonucrroN Since its discovery in an outcrop ofolivine basalt the shore of Kamloops Lake in British Columbia (Gra- Ferrierite is a medium-pore zeolite that has interesting ham, l9l8), the zeolite ferrierite has been observed in potential applications in petrochemical processing. nearly 30 localities. With the exception of a very few Klowledge of the conditions leading to its formation may occurrencesin altered volcanogenic sediments,these all provide insight into creating a simple, inexpensive syn- appear to be hydrothermal vug occurrences.In the de- thesis method. The Lovelock, Nevada, deposit, in which posit near Lovelock, discoveredin 1965 by P' E' Galli, ferrierite occurs in minable proportions, provides a nat- ferrierite formed through hydration reactions in a se- ural laboratory for studying the processesof zeolitization quence of rhyolitic pyroclastic rocks. A plzzle at Love- and the conditions under which ferrierite crystallizes. lock is the dominance of ferrierite over mordenite and What were the special conditions responsiblefor the un- clinoptilolite, two zeolites that are very common ln slm- usually large quantity offerrierite at Lovelock? From the ilar tuffaceoussettings in the western United States. In results of a study of ferrierite synthesis,Cormier and Sand fact, severalzeolites, including erionite, mordenite, chab- (1976) concludedthat metastability may be the key. Their azite, clinoptilolite, and analcime, are very common in conclusion, although soundly based on experiment and such deposits, whereasferrierite is only known to occur intuitively pleasingwhen applied to the Lovelock depos- in the Lovelock deposit and in smaller amounts in the it, nevertheless requires further examination through Stillwater Range near Fallon, Nevada. A Korean occur- careful study of the mineralogical and chemical relation- rence was interpreted by Noh and Kim (1986) to result ships in the rocks themselves.In this study we have com- from diagenesisof silicic vitric tuffs. bined electron petrographic characterization with bulk Reports describing the Lovelock zeolite deposit con- chemical and phaseassemblage data to interpret the or- tain widely divergent opinions about the genesisof the igins offerrierite in altered pyroclastic rocks. zeolites.Sand and Regis (1968) suggesteda hydrothermal origin for the ferrierite, but no supporting evidence for or sourc€of the fluids was given. * Presentaddress: Exxon Production ResearchCompany, P.O. estimatesof temperature Box 2189. Houston.Texas 77252-2189, U.S.A. Regis(1970) describedthe Lovelock and Stillwater Range 0003-004x/92l0304-03I 4$02.00 314 RICE ET AL.: FERRIERITE CRYSTALLIZATION 315 \ ll a foE-l I ptioceneto OUATERNARY Sedimentaryand rclcanic rocks Hotocene I I Ferrieriterichtulf LOVELOCKZEOLITE DEPOSIT I T$ I mZ ptircene i and Z Mordenit+richtuff Tutfand sedimentaryrocks Basaltand andesite Miocene TERTIARY I tg zeolitictuff GTn;l -ll6mts 1 Mioceneto N ctass Votcanicrocks Oligocene J I Diatomite tl:KE,ll I Cretaceousto CRETACEOUSto CoreHole Locations plutonic n weldedrhyolite tufl a Metamorphicand rocks J Triassic TRIASSIC E Alluvium a FerrieriteDeposit Fig.2. Geologicmap of the Lovelockdeposit. Fig. 1. L,ocationmap for theLovelock zeolite deposit (Shep- pardet al.,1983). material. This sequenceagrees with the order of appear- occurrencesof central Nevada, giving compositions of ance of these zeolites deduced from the experiments of the ferrierite at each locality, and he also reported zona- Cormier and Sand (1976). The cores themselves,how- tion for ferrierite, mordenite, and clinoptilolite basedon ever, are more difficult to correlate. The discontinuities outcrop sampling. Low alkalinity conditions were sug- betweencores 2. 3. and 4 could be fault-bounded discon- gested.In a guidebook accompanyingthe zeolite fieldtrip tinuities or unconformities related to variations in the Zeo-Trip'83, Sheppardet al. (1983)described the Love- sedimentary environment such as erosional surfacesor lock occurrencein somewhat$eater detail and concluded pinch-outs. that the original pyroclastic material was deposited in a lake but that little was known about the fluids responsible Cores and outcrop samples for zeolitization. An examination of the chemical con- Four vertical core holes (Fig. 2), each 5 cm (2 in.) in trols on zeolite crystallization in the Lovelock deposit diameter, were drilled under contract for W. R. Grace that account for this unusual occurrenceis the subject of and Company in the early 1970s. Portions of the cores the present study. B.q.crcnouNn Geologic setting of the Lovelock deposit The Lovelock zeolitedeposit occursin a group ofun- named sedimentary and volcanic rocks of Tertiary age MordenileHill (Miocene or Pliocene)in a small valley within the Trinity Range(T. 28 N., R. 30 E.) in PershingCounty, northwest T Nevada (Fig. l). Reconnaissancemapping placed the Lovelock deposit in the Upper Tertiary volcanic group JUm A welded ftyolite tutl (Tuv) of Stewart and Carlson (1978). The ferrierite de- D Finegla$ + smectite posit of the Stillwater Range also lies within the Upper E coaR gla$ + Eolile N Ferrlerhe+ smecllte group. The observablesurface features of the deposit and I Benlonile the core locations are shown in Figure 2. Z Mordenite+ lerrierite+ orthoclag VedicalEraggeEtion I Ferrierite+ orthoclase The stratigraphy based on the bulk chemistry, petrog- -10x I Oinoptilollre+ crlstoballte raphy, and phase-assemblagedata is shown in Figure 3. There is a zonation characteizedby mordenite at the top, Fig. 3. Stratigraphy ofthe Lovelock zeolite deposit. Layers grading downward into less mordenite and increasing A, B, and C are defined on the basis of composition (as in Fig. amounts of ferrierite, and finally a large amount of glassy I 4) and assemblage. 316 RICE ET AL.: FERRIERITE CRYSTALLIZATION diffractometer with CuKa radiation. Scanswere run al2 per min from 2 to 60 20, at 40 kV and 30 mA. Phase identification, particularly for zeolites, was enhancedby a computer-based matching progam with a library of natural and synthetic zeolites as well as other silicates. The scanning electron microscope used was a JEOL 35C equipped with an LaB6 gun, a four-crystal wave- length spectrometer (WDX), adjunct energy-dispersive X-ray detector (EDX), and PGT SystemIV analysis sys- tem. Both normal secondaryelectron imaging (SEI) and backscatteredelectron imaging (BEI) were used. Electron microprobe analysis was performed by a method com- bining EDX and WDX, using orthoclase from Benson Mines, St. Lawrence County, New York (K, Al, Si), lab- radorite plagioclasefrom l,ake County, Oregon (Ca, Al, Si), and hornblende from Kakanui, New Zealand (Mg, Ti, Fe, Al), as standards.Alkali difusion during analysis of zeolites and glasswas corrected by monitoring counts for 30 s and extrapolating to zero time. Clay and zeolite analysestotaled less than l00o/oprimarily becauseof the presenceof HrO, both adsorbedand structural. Ferrierite typically has l0-l5o/o HrO by weight.Matrix corrections were calculated by the NBS Frame C program. The sec- ondary ion massspectrometer (SIMS) usedwas a Cameca 3f with a high-gain camera(20 million ASA) and a Crys- tal image processor(kta, 1985). Becausethe charging problems inherent with the imaging SIMS method are more severethan with SEM, SIMS samplesrequired em- bedding in a conductive mount. A Philips 420ST transmission electron microscope (TEM) with EDX and adjunct PGT System IV analysis Fig. 4. SEM micrographshowing (a) a fracturesection of systemwas usedfor characterizationofglassy and zeolitic glassin sampleSR75 and (b) the morphologyof smectiteon the tuffs and for identification ofzeolites by electron diffrac- glasssurface. tion. Chemical compositions of ferrierite crystals were determined using the Cliff-Lorimer thin-film method (Goldstein et al., 1977). are stored at the Nevada