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Home , Christite, Frankdicksonite, Getchellite, Lorandite, Stibnite

American Mineralogist, Volume 64, pages 701-707, 1979

Ellisite, TlrAsSro- a new mineral from the Carlin deposit,, and associatedsulfide and sulfosalt minerals

FRANK W. DIcrcsoN

Department of Geology, Stanford University St anfo r d, Califu rnia 94 3 05

ARTHUR S. Reorre AND JocELYN A. PETERSoN U.S. Geological SurveY Menlo Park, 94025

Abstract

Ellisite, TlrAsSr, occurs with , , , , and native as dispersed grains, and with realgar, lorandite, christite, and getchellite in small discontinuous patches, in mineralized laminated carbonaceous dolomite beds in the Carlin gold deposit, Nevada. The mineral is named for Dr. A. J. Ellis of the Chemistry Division, D.S.I.R.' New Zealand. Ellisite is opaque, and the color is dark gray, is light brown with a tinge of orange, and luster on fresh surfaces is metallic but less bright than that of . The min- : eral is trigonal, hexagonal parameters a : 12.324(2), c : 9.6 7Q)A; cell volume '7 1269.0(qA3, Z : . The d values (A) of the strongest X-ray diffraction lines and their relative intensities arc 2.669 (100), 3.214 (53), 5.333 (3'7),2.327(28), and 3.559 (20). Ellisite shows ex- cellent to good rhombohedral and hackly . Vickers hardnpss measured with a 50-g load varied from 36.4-44.0 and averaged 39.3 kg mm-2, and with a 25-g load varied from 41.0-51.5 and averaged 45.9 kg mm-2 (Mohs hardness,about 2). Density of synthetic Tl.AsS, is 7.10(5) (meas) and 7.18 g cm-3 (calc). In reflected light the mheral is light gray with a distinct purple tint (light purplish-gray), very weakly bireflectant, ranging from light purplish-gray to light pinkish-gray, and anisotropic, with polarization colors blue-purple, 1e6-purple, and brownish-orange; the mineral has a deep red to deep orange-red internal re- flection.Reflectances (R) in air are:650nm:28.3,589nm:28.9,546nm:29"1 ,470nm: 31.7. Electron microprobe analyses gave Tl 78.2, As 9.6, S 12.3' sum 100.1 weight percent. Most abundant trace elements, determined by emission spectrographic analysis' are Sb (500 ppm) and Fe (200 ppm). Disseminated assemblages include: the gold-- in the bodies; the - arsenic assemblage of ellisite-christite-lorandite-getchellite-realgar-native arsenic-quartz; the assemblage of galkhaite--quartz; and the carlinite-pyrite-hydro- carbon-quartz assemblage. Vein mineral assemblageswhich crosscut rocks, ' and dissem- inated zones include christite-lorandite*-realgar-barite-quartz, weissbergite-stib- nite-quartz, and frankdicksonite-quartz. Veins are distinct and separate from each other and postdate the ore and zones of disseminated . Gold ore bodies formed from solutions at temperatures between 160-180"C, but many minerals in late veins formed at temperatures above 200oC. Most of the disseminated and sulfosalt minerals probably formed dur- ing the late stagesof the more moderate temperature event and the vein minerals were depos- ited from horter fluids (200-300"C) in response to supersaturation caused by falling temper- atures and pressures of the fluids during ascent.

Introduction Eureka County, Nevada. Ellisite is named for Dr. The new mineral ellisite, of composition TlrAsSr, A. J. Ellis of the ChemistryDivision, D.S.I'R., New belongs to a suite of minerals containing thallium, ar- Zealand,in recognition of his many contributions in senic, and occurring in the Carlin gold deposit, the field of hydrothermal geochemistry'The mineral *93 -gMx / 79 /0708-070 I $02.00 701 702 DICKSON ET AL.: ELLISITE. NEW MINERAL

kaolinite and sericite formed in the matrix of the car- bonate host rock. The rocks also contain up to 6 weight percent organic carbon which was introduced by hydrothermal fluids. Both areas also contain veinlets, seams, and irregu- lar patches, normal and parallel to bedding planes in the laminated host rock, made up of ellisite, abun- dant realgar, small amounts of lorandite, christite, and getchellite. The veinlets and patches cut the min- eralized rocks, indicating that they were later than the main hydrothermal stage of gold mineralization (Radtke and Dickson, 1974;Dickson et al.,1978\.

Fig. l. Scanning electron microscope photograph of ellisite Physical grains showing rhombohedral cleavage fragments. and optical properties Most of the grains of ellisite were anhedral to ir- name and designation as a new mineral have been regular, and ranged from about 0.5 to 1.3 mm in approved by the Commission on New Minerals and length; however, several broken grains showed good Mineral Names of the International Mineralogical rhombohedral shapes (Fig. l). Rhombohedral forms Association. were also observed on crystals of TlrAsS, synthesized Type material in very limited amount is deposited from melts by slow cooling through the freezing tem- in the Epithermal Minerals Collection, Depanment perature (325"C) followed by holding at 3lO"C for of Geology, Stanford University, Stanford, Califor- three weeks. Small grains enclosed or intergrown ma. with the assemblage of lorandite, getchellite, christ- ite, and realgar are about 0.4 to I mm in length. Occurrence Grains of ellisite in the veinlets and patches, inter- Ellisite occurs in mineralized, laminated, argil- grown with other arsenic minerals, range from about laceous, carbonaceous dolomite beds of the Roberts 0.2 to 0.5 mm in longest dimension. Mountains Formation in the East ore zone of the Ellisite has hackly fracture and good to excellent Carlin gold deposit. Samples containing ellisite have rhombohedral cleavage. The color is dark gray, and been found in two places. The first is in an area at the luster on fresh surfaces is metallic and slightly less southwest end of the East ore zone near mine coordi- bright than that of stibnite. Streak is light brown with nates 22,900 N, 19,800E, benches6380 to 6400. The a tinge of orange. No natural material was found that second is in the central part of the East ore zone be- was suitable for density measurement. The density of tween mine coordinates 23,lOON, 20,300E, to 23,500 synthetic TlrAsSr, measured with a Berman balance. N, 20,700 E, between benches 6340 and 6350 is 7.10(5) g cn-', which is slightly lower than 7.18 g (Radtke, 1973). cm-' calculated from the cell volume. The Vickers In both areas dispersed grains of ellisite are spa- hardness of ellisite determined with a Leitz hardness tially associated with lorandite, getchellite, christite, indenter with a 50-g load ranged from 36.4 to 44.0 and realgar. Prior to this last stage of mineralization, and averaged 39.3 kg mm-2 (10 determinations); the hydrothermal quartz and pyrite replaced , and hardness of the mineral using a 25-g load ranged from 41.0 to 51.5 and averaged 45.9 kg mm-, (8 de- terminations). Both average values correspond to Table l. Reflectances of ellisite, in percent about 2 on the Mohs scale of hardness. Ellisite is opaque. In reflected light it is light gray l.lavelength (nn) with a distinct purple tint (light purplish-gray), very 470 weakly bireflectant with colors ranging from light purplish-gray to light pinkish-gray, and anisotropic. In air 3r.7 29 0 29.7 28.3 Polarization colors range from blue-purple to red- In o11 Ll.) t6.2 15.3 L4.9 purple to brownish-orange. Broken grains display a deep red to deep orange-red internal reflection. Re- 't03 DICKSON ET AL: ELLISITE, NEW MINERAL flectancesof ellisite in air and in immersion oil (n : Table 2 X-ray powder diffraction data for ellisite and synthetic " TI.AsS. 1.5150) (Table l) were measured with a Reichert Spectral Reflectometer using SiC as a standard. Al- SydEhetic T13A6S3 though a visible bireflectance exists with regard to Calculated* obEerved** ob6erved 0bselvedr' 0bselved color, there was no measurable change in percent re- hkl dhkL o dhkr A I dttrA r flectance. 100 tc,613 00r 9.647 t0I 7,r57 Crystallography Il0 6.t62 200 5.337 5,340 38

Ellisite has trigonal symmetry, but the lattice is I I I 5.193 powder diffraction data for ellisite 002 4,a24 hexagonal. X-ray 201 4.670 and synthetic Tl.AsS., using silicon as an internal ),o2 4.396 2I0 4.034 standard, are given in Table 2. The hexagonal unit r t2 3.798 cell constants, determined from X-ray powder dif- 2lr 3.722 202 3.578 fraction data, using the Appleman and Evans (1973) 300 3.556 3,559 20 3.558 25 30r 3.338 cell parameter least-squares prog,ram, atei a : : 003 3.216 3.214 3.218 50 12.324(2),c :9.647(2)A, and V: 1269.0(4)A';Z 2r2 3,095 : 220 3,08r 7[Tl'AsSl 2 I [TlAs.,S]. I03 3.079 The of ellisite has not been deter- 310 2.960 mined. However, it appears to be related to the crys- z2r 2.935 302 2.463 tal structure of carlinite, Tl2S, (Z : 27) which con- rr3 2,85r 2.854 2.853 13 3rr 2.830 sistsof sheetsof Tl atoms developed parallel to (00 I ), 203 2.154 2.757 2,755 t4 in which S atoms occupy octahedral interstices be- 400 2.668 2.669 2.661 r00 222 2,597 tween pairs of planes of Tl atoms (Ketelaar and Gor- 40r 2.572 3r2 2,523 ter, 1939).Removal of one of the planes of Tl atoms 2r3 2,515

from each pair, and the substitution of one small 320 2.449 (0.58A) for three large Tl* (1.47A) re- 004 2.4t2 As'* each 303 2.236 2.387 2,385 3 moved, as required for charge balance, would cause 321 2.373 I04 2.353 compaction along c from the original value for carl- 402 2.335 inite (18.175,4)to the c value for ellisite (9.6474, 410 2.329 2.327 2.326 4II 2,264 about one half of the original distance)without much I 14 2.246 223 2.225 disturbance of the c parameter from the value for 204 2.t98 carlinite (l2.l2{). 322 2.r83 2.l8r I ellisite 3t3 2.178 No identifiable forms were observed on 500 2.135 4r2 2.097 2.09E roughly equant rhombohedrons 2.090 6 grains, but euhedral, 501 2.084 2.Oa6 were observedon synthetic Tl.AsS.. 2r4 2.070 330 2.054 403 2.053 Chemical composition 420 2.017 331 2.009

Five grains of ellisite were analyzed by standard 5r0 1.917 r .916 3 I. 91E 3 electron microprobe techniques. Analyses of 4 to 5 I 15 1.84I I .839 2 1.838 3 600 1.779 1.780 1) t.776 t7 520 I .709 spots on each grain showed the grains to be uniform \ t-.707 I.707 5 423 I.709I in composition. Data from these analysesand the an- 602 I.669 1.670 1.566 6 alytical conditions used are given in Table 3. Because 006 1.608 I .608 1.507 6 530 1.525 1.524 t0 synthetic Tl.AsS. was used as a standard, no matrix oo7s33 r.378I r.371 406 r.371f 8 corrections were necessary. Results of the micro- 416 r.323 \ r.323 3 probe analysesshow the composition of ellisite to be 8or t.lzz f hand-picked grains and fragments rAlL calculated hklts :.r.ted for d9p7 )2.OOo A. Al1 observed h/(/rs Tl,AsSr. Small tot dhkl>l.3}o A are ladered. IndIiEs and d(cslc) frm the were combined and analyzed by the standard semi- leaerjsqrarea atrelysis of X-ray Powdel data uslnt th€ dlgltal coBputer protrd of Appl4an end Evan6 (1973); and e hexaSooel cell quantitative spectrographic technique of the U.S. cllh @ - l2.32rtand e' 9.647 L. **x-ray dlffracto[eter condltLons are: Nl-flltered cu radlatlon; Geological Survey (Table 4). The only additional Cu t

Table 3. Microprobe chemical analyses of ellisite ogical and chemical study. The presenceof the un- usual minerals was not suspected Weight Percent until christite was Grain first noted in 1974(Radtke et al., T1 As s Total 1974a;1974b), by which time many localities already had been de- r 78.4 9.8 t2.3 100.5 stroyed. One locality, in the central area in the East 2 77.9 9.5 L2.2 99.6 Pit, remains,but it is in a location inaccessibleat the 3 78.5 9.6 r2.r 100.2 time of this writing (fall, 1978). Consequently,de- 4 7a.2 9,6 :I2.3 loo.r tailed and systematicobservations and sampling have 5 77.8 r2.4 99.9 not been possible.However, a summary state- ment, integrating results of mapping 78.2 9.6 r2.3 100.1 and collecting by Radtke with subsequentgeochemical and miner- TI3AsS3 78.18 9.55 12.21 I00,00 alogical studies,is presentedin the following para-

Analytical graphs. conditions: (l) Tha1ltun, Mo characteristic line,

ADP The mineral localities are shown on the clystal, l5 t

Ellisite is the fifth new mineral discoveredat the [Sought but nor found: A1, Au, B, Ba, Be, Bi, Ca, Cd, Ce, Eu, ca, Ce, Carlin gold depositby Radtke and his co-workersin Hf, In, I(, la, Li, M8, Mo, Na, Nb, p, pd, pt, Re, Sc, 51, Sn, Sr, Ta, samplestaken from 1965to 1975for routine mineral_ Te, Th, U, V, W, Y, Yb, Zr. Analyst: A. S. Radrkel DICKSON ET AL.: ELLISITE. NEW MINERAL

500 semblage,Ga + Ci + Q; and the carlinite assem- t5?_ blage, Ca + Q + Py * hydrocarbons(HC), scattered through brecciated carbonate rock. Two types of N veins crosscutthe country rocks,ore and the dissemi- Toc E nated zones;these are near-vertical,barite (B) veins rorr,-12! . L 3rl with minor qtJartz,and quartz veins. Vein mineral 300 300 292' + B + W E"f associationsinclude: Ch + Lo + O + R Q; .\Z ."\,r'.. I + St + Q; and F + Q. The barite veins occur domi- Ca+E nantly in fractures in upper oxidized and leached E+ Lo Lo+O rocks, but they narrow downward and terminate in t50 the top of the underlying unoxidized rocks. The o2040 60 80 ro0 quzrtz veins tend to occur in the hanging wall at or I t Mole9o AsrS, t tl I I immediately abovethe disseminatedzones. The veins Tl2S TlrAsSa TlAs52 AsrS, are spatially separatefrom each other. Figure 5 Fig. 2. Reconnaissance phase diagram of the system TlrS- showsthe compositionalrelationships of the minerals As2S3 as a function of bulk composition and temperature, at the on a Tl-Hg-As-Sb diagram. vapor pressures of the assemblages, after Peterson (1976). Sym- With regard to the timing, the disseminatedzones : : : bols: Ca carlinite, Tl2S; E ellisite, Tl3AsS3; Lo lorandite, clearly formed earlier than the sulfosalt-and sulfide- TlAsS2; O : orpiment, As2S3; L : liquid, Tl2S-As2S3; SS : solid solutions. bearing veins. However, timing of the spatially asso- ciated minerals within each disseminatedzone can- rock as dispersedgrains, seams,patches, and irregu- not be deduceddirectly becausethe minerals rarely lar masses.The disseminatedzones trend more or occur in contact. The persistentassociation of setsof lessparallel to the stratiform ore bodies (Fig. 3 and minerals within common volumes of rock is consis- 4). Four disseminatedmineral assemblagesoccur: the tent with but does not prove simultaneity. Indeed, gold assemblage,gold + pyrite (Py) + quartz (Q) of phase-rule considerationssuggest that it is improb- the ore bodies; the thallium-arsenic assemblage, able that all the minerals of the disseminatedthal- E + Ch + Lo + Ge * R + A + Q; the mercury as- lium-arsenic assemblageformed simultaneously.In

./ ,/

W+St

E+Ch+L6+Ge+R+A ,/ \,%

5OO FEET *c+-

Fig. 3. Simplified geologic map of East ore zone of the Carlinv gold deposit at the 6,400 foot level showing spatial relations between sulfide and sulfosalt assernblagesand their relationships to prominent geologic features. Narrow zones with short bar pattern are pre-ore igneous dikes; heavy lines are faults; gray areas are gold ore bodies. Stippled areas contain dispersed thallium-arsenic minerals; lined pattern denotes areas containing galkhaite and cinnabar; cross-hatch pattern denotes areas containing weissbergite and stibnite, carlinite, or frankdicksonite. The occunenc€s of W + St, Ca and F in the hanging wall above ore have been projected down onto the plane of the map. 706 DICKSON ET AL: ELLISITE. NEW MINERAL

The temperatureand pressuresattending ore dep- osition are difficult to estabtsh becausemost of the hydrothermal ore minerals are exceedingly fine- grained and not suitablefor fluid inclusion studies,or Original are intergrown with other minerals, making separa- S u rface tions for isotopic analysis almost impossible. How- ever, fluid inclusions in quartz in some small seams containing pyrite and gold, collecteddeep in primary EAST PIT ore, had filling temperatures ranging from 160"- 180"C (A. S. Radtke, unpublisheddata). In contrast, fluid inclusions of later vein minerals show temper- aturesabove 200oC [realgar, 210"C to >230o;sphal- erite, 266"-299o; and qvattz, 235"-285" (A. S. Radtke, unpublished data)]. The partitioning of ,oS between barite and sulfide vein minerals indicates depositional temperaturesin the range 250"-290", possiblyas high as 305oC.We visualizethat at the beginning of the episodethe hydrothermal solutions 6300 'were cool, and initially their passagethrough the host Fig. 4. Cross section along line A-A, (Fig. 3) showing vertical rocks did little except dissolvesome calcite and de- positions of sulfide-sulfosalt assemblages and geologic features. posit small amounts of quartz. The ore assemblage Symbols correspond to those of Fig. 3. Zones containing carlinite formed later as the temperaturesincreased and the (C) and weissbergite (W) + stibnite (St) above zone of dissemi_ hotter fluids permeated the prepared horizons. The nated thallium-arsenic minerals are projected into plane of sec_ deposition of the disseminatedsulfide and sulfosalt tion. Geologic forniations are thin-bedded carbonate beds of the Roberts Mountains Formation (DSrm) overlain by thick_bedded minerals probably was during the late stagesof this carbonate beds of the Popovich Formation (Dp). moderate-temperatureevent, judging by the tendency of the zones containing the disseminated contrast,the vein minerals are intergrown in textures minerals to parallel or envelop the ore bodies. The typical of simultaneity, and no unlikely constraints lower-temperaturestage gave way abruptly to a are encounteredby applicationof the phaserule. higher-temperaturestage of vein formation. The flow The presenceof distinctly different assemblages of the fluid switched from slow lateral permeation between separate unconnected veins strongly sug- through large volumes of rock to rapid movement geststhat either the veins were formed at different times,or, if they were synchronous,that efficientflow of fluid was impaired in the fracture system.The re- tention of minerals such as carlinite, which oxidizes easily, and frankdicksonite, which is relatively sol- uble and reactiveto componentsavailable from wall rocks (calcium, carbonate,sulfate), sb testifiesto the ef- Hg ficiency of sealingof the veinsfrom ground-waterat- tack. The ore-forming fluids at Carlin were derived from meteoric waters, according to D/H ratios of vein o+R mineralsand alteredrocks, and they containedsulfur of marine affinity (Dickson et al., l97g\. The ore Fig. fluids also transportedhydrocarbon 5. Representationof compositionsof minerals in the sys- compounds,now tem Tl-Hg-Sb-As-S at Carlin. Heavy linesjoin found or encompassas- in intimate associationwith ore but also dis- sociatedminerals. Solid heavy lines join or enclosedisseminated persed in hydrothermally altered unmineralized assemblages,ellisite (E) + christite (Ch) + lorandite (Lo) + get- rocks. The ore and gangue componentscould have chellite (Ge) * realgar (R) + native arsenic (A), and galkhaite been leachedfrom deepersedimentary rock teranes (Ga) + cinnabar (Ci). Carlinite (Ca) not associatedwirh other sul- fidesis shown asa singledot. Broken heavy linesjoin by fluids set into circulation by heat released synchronous from mlnerals of late veins,christite (Ch) + lorandite (Lo) + orpiment Tertiary volcanic rocks (Dickson et al.,l97g\. (O) + realgar(R), and stibnite (St) + weissbergite(W). DICKSON ET AL.: ELLISITE. NEW MINERAL 707 along faults and near-vertical fractures. The event References could have been triggered by sudden breaking ofthe fluids through a near-surface impermeable barrier, Appleman, D. E. and H. T. Evans, Jr. (1973) Job 9214: Indexing allowing the fluids to vent at the water table or at the and least squares refinement of powder diffraction data. Natl. Virginia, earth's surface. The hypothesized sudden release of Tech. Inf. Serv., U.S. Dep. Commerce, Springfield, Document PB-2t6 188. pressure would promote rapid flow, fed from regions Canneri, G. and L. Fernandes (1925) Contributo allo studio di al- of higher temperatures at greater depths. The tem- cuni minerali cont€nento tallio. Analisi termica dei sistemi: peratures of the vein-forming fluids at the level of the Tl2S-As2S3; Tl2S-PbS. Atti Accad. Naz. Lincei, Rend. 6, Cl. Sci. deposit rose to a maximum before the rapid flow Fis.Mat. Nat. 1,671-676. Radtke (1977) The unique mineralogy of damped away and the deposition of vein minerals Dickson, F. W. and A. S. Hg-As-SFTI sulfides at the Carlin gold deposit, Nevada, and ceased. implications as to the origin of the deposit. Mineralogical So- The deposition in the veins probably was largely in ciety of America-Friends of Mineralogy, 3rd Joint Symposium, response to supersaturation induced by decreased Crystal Growth and Habit, Tucson, Ariz., 13-14. temperatures and pressures of the fluids during their - and - (1978) Weissbergite, TlSbS2, a new mineral gold deposit, Nevada. Am. Mineral., 63,72{u-. vertical ascent. Some deposition was caused by boil- from the Carlin 724. ing in the upper portions of the vein system. An in- B. G. Weissberg and C. Heropoulos (1975) Solid teresting possibility is that emplacement of high-tem- iolutions of , arsenic, and gold in stibnite (Sb2S3), or- perature fluids in fractures cutting mineralized rocks piment (As2S3),and realgar (As2S2).Econ. Geol',70,591-594. soaked with lower-temperature fluids set up increas- R. O. Rye and A. S. Radtke (1978) The Carlin gold de- posit as a product ofrock water interactions. Int. Assoc- Genesis ing temperature and decreasing pressure gradients in Ore Deposits (IAGOD) Fifth Symposium, Snowbird, Utah (in the rocks; these gradients were directed toward the press). fractures. Some components originally in the ore Hausen, D. M. and P. F. Kerr (1968) Fine gold occurrence at body and mineralized zones may have been leached Carlin, Nevada. In J. D. Ridge, Ed., Ore Deposits of the United by interstitial fluids. Delivery to the vein system States,1933-1967,p.908-94O. Arn. Inst. Mining, Metall. Petrol. Engineers, New York. could have happened in stagnant fluids by diffusion Ketelaar, J. A. A. and E. W. Gorter (1939) Die Kristallstruktur up the temperature gradient. More likely, fluids mi- von Thallosulfid (TlrS). Z. Kristallogr., 101A,367475. grated to the vein system by flow down the pressure Peterson, J. A. (1976) Phase Relations in lhe System Tl2S-As2Sj. gradient. The geometries of the vein systems and M. S. Thesis, Stanford University, Stanford, California. crushed zones are complicated. Fractures and open- Radtke, A. S. (1973) Preliminary geologic map of the Carlin gold mine, Eureka County, Nevada. U.S. Geol. Surv. Misc. Field ings were poorly interconnected in places. Special- Studies Map MF-537. ized processes took place locally in response to boil- - and G. E. Brown (1974) Frankdicksonite, BaF2, a new ing or to the delivery to the fractures of fluids of mineral from Nevada. Am. Mineral.,59, 885-888. unusual composition. Therefore, ellisite and the ore - and F. W. Dickson (1974) Genesis and vertical position of and exotic minerals at Carlin represent the integrated fine-grained disseminated replacement-type gold deposits in Nevada and Utah. Problems of Ore Deposition, Volcanic Ore effects of a hydrothermal cycle triggered by the intru- Deposils, I, Fourth IAGOD Symposium, Varna, Bulgaria,'l l-78. sion of hot igneous material into water-containing - and - (1975) Carlinite, Tl2S, a new mineral from Ne- rocks of the upper crust. v ada. Am. M ineral., 60, 559-565. - and J. F. Slack (1978) Occurrence and formation of avicennite, Tl2O3, a secondary mineral at the Carlin gold de- Acknowledgments posit, Nevada. J. Res U.S. Geol. Surv.,6,24l-246. The authors thank Dr. David Vaughan, Department of Geolog- thallium mineral from the Carlin gold deposit, Nevada' , rn. ical Sciences, University of Aston in Birmingham, Great Britain, Mineral.,62,421425. for measuring the reflectance of ellisite. The manuscript was re- - c. M. Taylor, F. W. Dickson and c. Heropoulos (1974a) viewed by Joan R. Clark, Terry E. C. Keith, and B. F. Leonard of Thallium-bearing orpiment, Carlin gold deposit, Nevada. the U.S Geological Survey, Professor James R. Craig, Virginia "/. Res. U.S. Geol. Surv.,2,J4l-342. Polytechnic Institute and State University, and Professor Gordon - R. C. Erd and F. W. Dickson (1974b) Occurrence E. Brown, Stanford University. Scanning electron microscope TlAsS2, at the Carlin gold deposit, Nevada. Ecoz. photographs of ellisite were taken by Robert Oscarson of the U.S. of lorandite, Geol.,69, l2l-124. Geological Survey, and Raymond W. Wittkopp at the University - and c. Heropoulos (1973) Antimony-bearing orpi- of California at Davis carried out electron microprobe analyses. ment, Carlin gold deposit, Nevada. J. Res. U.S. Geol. Surv., I, The support by the staff of the Carlin Gold Mining Company in 85-87. ongoing studies of the mineralogy and geochemistry of the deposit is greatly appreciated. Dr. Akira Kato, National Science Museum, Tokyo, , made valuable suggestions during the initial stages Manuscript received, December 6, 1978; ofthis study. acceptedfor publication, February 21, 1979.