Garnet-PyroxeneAlteration Mapping in the LudwigSkarn (Yerington, Nevada) with GeoscanAirborne Multispectral Data*
Abstract tions be reevaluated. The Ludwig skarn in the Yerington dis- Geoscanairborne multispectral image data of the skam and trict, Nevada, was selected for such a study. The geology of calc-silicate-metamorphosedrocks near Ludwig (Yerington the area is well-characterized (Hanis and Einaudi, 1982; district, Nevada) have been analyzed and comparcd with Proffett and Dilles, 1984; Dilles and Einaudi, 1992), while published and unpublished maps. This study examines the extensive exposures and sparse vegetation cover provide spectral dffirentiation betweentwo stagesof gamet-pyrox- ideal conditions for remote sensing. This study employs ene alteration, ea* metamorphic skamoid and late metaso- Geoscan AMSS Mk II image data and follows an approach matic skarn. Both types of calc-silicate rock were similar to that of an exploration program: successfullydelineated using thermal infrared band dffir- I description of the setting and the target alteration types, ence images. Differentiation between the two alteration . selection of empirical imag€ treatments based on spectral styles was achieved using a (0.717-pm- 0.573-pm) difference characteristics of the dominant minerals (acquired from both image, based on the deeper 0.87-p,m Fe3' garnet absorption lab measurementsof reconnaissancefield samples and from in skarn. the literature), o irnage processingresults with geologic Compositional variations, grain size, and weathering comparison of the maps, and styles were examined as possiblesources of this spectal r identification of the sourcesof spectral differentiation to de- variation. While all three contribute, weathering characteris- termine whether the treatments can be exported to similar tics have the greatesteffect. Skarnoid contains finer-grained targets or are limited to the conditions present in the study and greater proportions of pyroxenes than skam, which readily weather to a limonitic coat that mutes the 0.87-pm absorption. lnstrumentationandMethods lntroduction The GeoscanAMSS Mk II scanner was first described by In their review paper on skarn deposits,Einaudi ef o/. (1981) Lyon and Honey (1990).The AMSS Mk II was developedas describeskarn as "coarse-grainedCa-Fe-Mg-Mn silicates a tool for mineral exploration by GeoscanPty. Ltd., a divi- formed by replacement of carbonate-bearingrocks accompa- sion of Carr-BoydMinerals basedin Perth,West Australia. It nying regional or contact metamorphism and metasoma- is an imaging spectrometerusing grating-dispersive optics tism." The majority of the worlds tungsten is produced from with three sets of linear aray detectors, one each in the visi- skarn deposits and this deposit type is an important source ble/near infrared (vMR), short-wave infrared (s!vIR),and ther- of copper, iron, molybdenum, and zinc. mal infrared (rn) (Table 1). The data set used in this study Despite their economic value, there is a distinct paucity was flown 27 lu.ly 1990 (1250 PDT)at an elevationof 4700 ft of studies applying remote sensing to skarn deposits. The above ground level (acr,), with a nominal pixel resolution of vast majority of remote sensing efforts in economic geology 3m. have been directed toward the identification of iron-staining Data acquisition by the Geoscansystem differs from or hydroxyl-bearing alteration products in porphyry copper most other scanning systemsin that the instrument gains and or epithermal districts, or in the determination of regional offsets are not fixed, but are adjusted for the conditions pres- structural trends. Skarns are a common feature in carbonate- ent along the flight line. The system is flown twice over the bearing strata intruded by porphyry copper plutons (Einaudi, target area. During the first flight, an operator adjusts the 1982)but, as Abrams and Brown (1985)comment in a study gains and offsets in each channel to maximize the surface of the Silver Bell district, Arizona, "detection of most tactite contrast into an B-bit dynamic range (0 to 255). The same (skarn)is unlikely becauseof the small size of outcrops and area is then flown a second time with the channels held con- the lack of spectrallydiagnostic minerals." stant at the new gains and offsets, By adjusting the offset val- The increasedspatial and spectralresolution of modern ues such that all channel means have a digital number (nN1 imaging spectrometers,however, suggests that such asser- of t27, the Geoscandata are effectively pre-corrected for the solar radiation curve (Rubin, 1991; 1993).This lessensthe necessity for calibrations such as the "flat field" and log-re- *Presented Ninth Thematic Remote at the Conferenceon Geologic (Green 1985;Roberts et aL.,1986)in order Sensing,Pasadena, California, 8-11 February1993. sidual and Craig, to compare image data with reflectance. This method does require corrections for variation in the offset and gain set- tings (Windeler, 1992);raw Geoscandata were transformed Photogrammetric Engineering & Remote Sensing, to apparent reflectance using the "two-point correction" Vol, 59, No. 8, August 1993,pp.7277-7286.
0099-11 1 2/93 15908-727 7$03.00/0 Donald S. Windeler, Jr. 01993 American Societyfor Photogrammetry Department of Applied Earth Sciences,Stanford University, and RemoteSensing Stanford,CA 94305.
PE&RS t277 Tretr 1, GeoscrttCrnnnet WAVELENGTHS eruoBaruoworxs (Ocroeen 1990 spessartinewere standards for Ca and Mn, respectively, ro PnesErur) while oxides were used for Fe, Mg, and Ti.
Central Band wavelength Bandwidth Geology Number (pm) (pm) Numerous workers have contributed to the understanding of 0.522 o.o42 the Yerington district, extendingback to Knopf (1918).Re- 2 0.583 0.067 gional studies have been conducted by Proffett (1977) and J 0.645 o.07L Proffett and Dilles (1984;in press).Information regardingthe V4 0.693 o.o24 petrology and alteration of the Yerington batholith are pro- N5 o.7L7 o.o24 vided by Dilles (1987)and Dilles and Einaudi (1992).De- I6 o.740 0.023 scriptions of skarn mineralogy and genesisin the district are o.o22 R7 0.830 summarizedfrom Einaudi (7977),Harris (1979),and Hanis o.o22 8 0.873 and Einaudi (1982). 0.915 o.o21 I volcaniclastic 10 0.955 0.020 Triassic and furassic sedimentary and rocks are exposed in the SingatseRange near Yerington' Ne- 11 2.044 0,044 vada [Figure 1). These Mesozoic rocks were intruded and al- o.o44 72 2.088 tered by a Jurassic quartz monzodioritic to granodioritic s 13 2.136 o.o44 batholith. Early stage alteration of the Mesozoic rocks was 2.776 o.o44 w 74 largely metamorphic, resulting in the formation of fine- 2.220 o.o44 I 15 grained, garnet-pyroxenehornfels (skarnoid) in argillite, vol- R 16 2.264 0.044 t7 2.308 0.044 caniclastic, and silty limestone units. Skarnoid is iron-poor, t8 2.352 0.o44 with intermediate grossular-andraditegarnets and diopsidic pJnoxenes.Early stagealteration also formed garnet-pyroxene 19 8.64 0.530 and plagioclase-idocrase-clinozoisiteendoskarn in apophyses 0.s30 T20 9.17 of quartz monzodiorite. Intrusion of granite porphyry dikes l2r 9.70 0.530 porphyry copper mineralization in the R22 70.22 0.533 was accompanied by 23 70.75 0.533 batholith and skarn formation in a massive limestone unit' 24 77.28 0.533 Late stageskarn is metasomatic and iron-rich; garnets com- monly approach pure andradite, and pyroxenes are salitic. Tertiary Basin and Rangefaulting has rotated the district -90o westward,resulting in the horizontal exposureof a cross-sectionthrough the district as it was in furassic time method of Lyon et al. (7s751.Examples of transformed visi- (Proffett,1977). ble/near-infrared data are provided in the section on Image The textural, compositional, and genetic features of Data and Treatments, No thermal infrared spectrometerwas hornfels and skarn in the study area are summarized and available for this study, however, and thus uncalibrated ther- compared in Table 2. From an exploration standpoint, the mal imagedata were used, distinction between these two mineralogically similar altera- The data set exhibited considerable distortion perpen- tion types can be critical; at Yerington, early hornfels altera- dicular to the east-westflight direction, even after applying a panoramic correction. This distortion resulted from the com- bination of a relatively low flight level with considerable topographic relief (400-m increase from west to east on the image).The image data were registeredto a 1:4800-scaletop- ographic map using drainage junctions or other distinct geo- graphic features as control points. The averageRMs euor between the registered image and the map is 7 to 8 pixels. Color composite images in this study are described with a shorthand notation. The three single bands or band differ- ences in an image are each given a subscript denoting their screen display color. For example, a "true color" treatment would be describedas (0,522 p,m)u(0.583p,m)s(0.645 F.mJs or, in Geoscanchannels, 18 2c 3R. Over 350 reflectance spectra were acquired from 202 samplescollected by the author and 37 samplesprovided by M.T. Einaudi. Spectrawere recordedfrom 0.4 to 2.5 pm using an IRIS Mark [V spectroradiometermanufactured bI Geofhysical EnvironmentalResearch, Inc. Halon was used as a reflectancestandard. Fault,showing direction I [W ../-r Garnet and/or pyroxene compositions were analyzed by ]|1i1il"113:"*' [1;or'oi"no'tn"n/ anddegree ot dip fEll Triassic.Jurassic t;;l Mesozoic souhern electron microprobe in 23 thin sections. These analyseswere @ volc&s€docks tJ balholith -J Axisol overturned \ anticline pOl, Superprobe733 and calculatedusing f= Mesozoic massive*-'" l;n uuaternaryn, alluvium performedon a E iil;;;;; [4 ZAF correctionson a Kevex Sesame'Deltasystem, Typical operatingconditions were 15-keVaccelerating voltage,. 20-nA Figure1. Generalizedgeolo$c map of the Ludwigarea, biam current, 10-pm spot size, and 3O-secondcount time. Yeringtondistrict, Nevada (after Harris and Einaudi, 1982). Albite was used as an AI and Si standard.Wollastonite and
t278 PE&RS TISLE2. CoupnntsottBEMEEN Sxenru lruo Honurels (Sxnntoto)ALreRrtror.r Types rN rxe LuowroSruov Anee, Hornfels Skarn o early, mostly metamorphic o late. metasomatic o extensively replaced silty limestone, calcareousargillite units; also o replaced massive limestone unit replacedmassive limestone and volcaniclasticunits closeto the ig- neouscontact o fine-grained (variable, most <0.5 mm) . coarse-grained(0.1 to > 10 mm) a iron poor . iron-rich garnets 40-60 mol % andradite garnets 65-100 mol % andradite pyroxenes < 15 mol % hedenbergite pyroxenes 15-45 mol % hedenbergite o barren, no associatedmineralization o mineralized, several small chalcopyrite orebodies
tion was barren, whereas skarn formation was locallv by the thermal differences treatment. Several common points accompaniedby chalcopyrite mineralization. of reference are indicated by numbers 1 to 5 on Plates 1a and 1b; other features of note or mismatch are indicated on the image with smaller numbers. All annotated features are lmageData and Treatments briefly described in Table 3. Epidote also appears dark on this treatment becauseof Calc-SilicateDiscrimination its long-wavelength absorptions between 10.5 and 11 pm The region of the electromagnetic spectrum between B and (Salisburyet o/., 1991).This is most apparentnear the south- 13 pm is of particular interestto geologistsbecause of the ern edge of the image (area 6), where several dark anomalies fundamental vibration of the Si-O tetrahedral bond. This vi not identified on the garnet-pyroxenemap result from expo- brational absorption is a key feature in the spectra of the sili- sures of andesite altered to massive epidote. cate rocks that make up much of the Earth's crust. The Bright responseson the (23-27)B(23-20)6(23-19)pimage location of this absorptionhas been shown to shift to pro- correspond to quartz- and feldspar-rich lithologies. The lone gressively longer wavelengths with depolymerization of SiOn bright yellow spot on the northwest edge is a quartzite (area tetrahedra and substitution of atomically heavier cations for 7), while the north-northeast-trending white area immedi- Si (Lyon, 1962; 1965).This absorptionshifts from g pm in ately west of the main exposures of garnet-pyroxenerock re- felsic rocks (quartz, K-and Na-feldspar-rich) to nearly 72 p,m sults from a felsite unit (area B). The bright area on the in the most mafic rocks (Ca, Fe, Mg-rich) and thus can be an eastern edge is a tightly folded rhyolite flow (area 9). important tool for rock type identification. Emittance specba for quartz, feldspar, epidote, and gros- sular garnetare presentedin Figure 2. The emittanceminima for quartz and garnet are separatedby more than 2 pm, a for- 19l20l21l22l23l24 tuitous occurrence that aids in discrimination between the two. In terms of Geoscanthermal bands, quartz and quartzo- feldspathicrocks will be relatively dark in the 8,64-pm and 9.17-pm channels(19 and 20), whereasvery mafic rocks will appearrelatively dark in the 10.22-pmand 10.75-pmchan- m i nels (22 and 23). In the caseof the skarn body, a relative gra- t dient betweenthese channels (determined by either I differenceor ratio) should thus indicate quartz-rich (pos- itive), calc-silicate(negative), or intermediate(flat) litholo- n gies. e In order to discriminate between garnet-pyroxene-altered rocks and other lithologies, a thermal band difference treat- ment has been applied to the Ludwig 3-m data set in Plate 1a.This imagedisplays (10.75-pm-9.70-pm)" (70.75- pm-9.17-pm)6 (10.75-pm-8.64-pm)p and has been 8910111213 smoothedusing a 3 by 3 low passfilter to eliminate excess wavelength(pm) noise.In this orientation,north is to the right and the top of ngure2. Emittancespectra of the imagecorresponds to exposurescloser to the paleosur- guarE,feldspar, epidote, and gros- face. Garnet-pJnoxenerock appears dark, while quartz and sulargarnet. The quarE, feldspar, feldspar-rich material is bright. This treatment also identifies andgrossular garnet spectra are di- the drainageswhich carry alluvial skarn and hornfels. Here- rectemittance measurements from after, this difference treatment will be referred to by its Geos- Lyonand Green (1975). The epidote can channels: (23-27)s(23-20)c(23-19)R. spectrumis calculatedfrom a reflec- A distribution map of garnet-pyroxenerock is presented tancemeasurement from Salisbury in Plate 1b. Comparisonbetween the (23-21)"(23-20)c(23-19)R et al. (1997;sample Epidote.l, frac- image and the geologic map shows an extraordinary correla- ture surface)using Kirchoffs Law tion. It is apparent that the overall distribution and morphol- (E:1-R). ogy of garnet-p)iroxeneskarn and hornfels are clearly defined
PE&RS TnaLe3. Covuolr RereRencrPorNrs FoR Pures h rNo 1a (Nos.1 ro 5) and interpretation of pyroxene spectra in their investigation eNoAoottrotnL AREAS oF lrurenesron DrsneneevenroN THE lrr.nce Dere (Nos, of the spectraleffects resulting from pyroxeneoxidation, as 6 ro 11). well as additional references.The following discussionhas Image-mapreference points been summarizedfrom the latter paper. Pyroxeneshave a structural formula of (M1)(M2)SirO". I DouglasHill Mine Metal cations occur in two crystallographicpositions, the M1 2 west and south outcropsof the CastingCopper mine and M2 sites.Pyroxenes in the Ludwig areaare calcic clinopyroxenesof the diopside (CaMgSi,Ou)-hedenbergite 3 GreenwoodBreccia (CaFeSirOu)series (Harris, 7579).In calcic clinopyroxenes, Garnet Hill Fe2*and Mg2* occupy the M1 position and Ca2*fills the M2 site. Spectraare dominatedby Fer* absorptionsnear 0.95 major drainagecarrying alluvial garnetite and 1.15 pm. While theseare weaker than absorptions causedby Fez+in the M2 site (e.g.,pyroxenes of the Other features of note enstatite-orthoferrosiliteseries, (Fe, Mg)SiOr), the greater massiveepidote alteration; dark on imagebecause crystal field stabilizationenergy imparted to the Fe2*ion in epidoteabsorption in ch. 22 & 23 is similar to clinopyroxenes results in absorptions that are considerably garnet(see Fig. 2.) stronger than the Fe3t bands present in garnets. Bidirectional reflectance spectm of three skarn samples quartzite; yellow on image becauseof quartz absorp- tion in ch. 19 & 20 and four hornfels samples are presented in Figures 3a and 3b, respectively.The three skarn samples(YER3-5, YER3-33, "felsite" (f.g. quartz-feldpar rock); white on image and YER4-4) display progressively increasing becauseof quartz and feldsparabsorptions in garnet:pyroxeneratios. The hornfels samples (Figure 3b) are channels19 through 21 also arranged in order of decreasingpJtroxene content from U rhyolite; samecomments as felsite YER6-19to MTE 502 and 514 to MTE 305. 10 gypsum;orange to red on imagebecause of gypsum absorptionin ch. 19 (120)
L1 alluvial garnetite deposit; may be erosional remnant r 2 31 4 s 6 t7 I 9 ro from Basin and Rangefaulting (M.T. Einaudi, pers.comment)
o YER4-4garnet skarn Gypsum dumps in the northwest (area10) appearred to F orangeon the (23-21)aQ3-20)c(23-19)Rtreatment. This was not initially expectedbut follows from the thermal spectrum i I of gypsum.The sulfate ion has intense absorptionsat 8.3 and o 8.7 pm which are narrow relative to the Si-O absorptionin E YER3-33 garnet-pyx skam quartz (Lyon, 1964).This absorptionwould appearin the o 8.64-pmand (to a lesserdegree) 9.17-pm channels (19 and -9 20), and is thus brighteston the (23-19)difference (red on o YER3-5pyx skam this treatment).
Skarn/HornlelsDiscrimination wavelength(um)
Vrsrnrdlllnan-Innnanrp Sprctnan Fnarunrs oF GARNET- '7 1 2 3' 4 5 6 I 9 10 Pvnoxrrur Sr,q,nwAND HoRNFELS Thermal difference images were effective at defining areas of calc-silicatealteration in the study area,but could not o E differentiate between skarn and hornfels. The vun laboratory o spectra of skarn and hornfels samples from Yerington were s compared in order to identify features that could be used for @ discrimination, Garnet and pyroxene comprise the dominant E mineralogyof thesealteration types and generallydetermine I the spectralfeatures. Garnets in the study areafall within the grossular(Ca.AlrSi"Orr) - andradite (Ca"Fe,Si.Orr)series o (Harris, 1979).With thesecompositions, the spectraof 6 Yerington garnetsare dominatedby effectsof Fe"* in the g octahedralsite. The reflectancespectra of andradite-grossular E garnetsshow two major Fe features:a strong drop in reflectancetoward the w due to chargetransfer and a crystal field reflectanceminimum at 0.87 pm (Hunt et aL.,7973'). wavelength(um) The visible and near-infraredspectral features of Figure3. Bidirectionalreflectance spectra of garnet-pyrox- pyroxeneshave been studied in much greaterdetail than ene (a) skarnand (b) hornfelssamples. Spectra have been those of the garnets,due to their probableimportance in the offsetfor clarity;actual reflectance at 1.3 pm is labeled. remote sensingof planetarysurfaces. A paper by Straub ef 41. Geoscanband centers are included. (1991)contains a concisediscussion of the theory behind
1280 PE&RS opaque inclusions as the remainder. Only the garnet band at 90 0.87 pm is presentin the r,ryn. The majority of the skarn and skarnoid in the study area 80 are garnetites containing lesser,varying quantities (0 to 20 percent) of pyroxene. In spite of the much stronger spectral z features of pyroxene, the spatial prevalence of garnet-rich o rocks results in most skarn and hornfels spectrabeing 3es G dominated by the ferric iron garnet signature. In Iaboratory o -Q 60 spectrameasured from reconnaissancefield samples,skarn o tended to show a deeper0.87-pm minimum than did hornfelses.This suggestedthat Geoscanchannel 8 (0,873 5U pm) might be used to discriminatethese alteration products. 45 In order to determinewhether this differencein absorption 40 depth was also presentin the Geoscandata, pixel spectrawere extractedfrom the image Iocationsof these same seven samples(Figures 4a and 4b). These spectraare not strictly 90 comparablewith those in Figures 3a and 3b, as these 85 laboratory spectrawere measuredfrom fresh surfacesand the ^80 r'--JYt""---/ ) image data representweathered material. Nevertheless,the z in Figure 4a show a deeperlocal minimum in .r,t MTE514 ^ ,/ skarn samples o in Figure 4b. 3zo ,/ Po channel B than the hornfels samples 6 t' * 6 2Y.Eaetss -/=- -Q 65 o t4-''*4 SxanNfiIonr.rrrls Irracr Tnnarrtrurs anp Rrsulrs 60 **. ltE'ot The treatmentthat proved most useful at differentiating /y'-,-, V " betweenskarn and hornfelsapplied a (O.777-p.m-0,S73-pm) f-/ 50 band difference(Geoscan channels 5-B). The bright areason ,tt b J (5-8) imageof the study area(Plate 45 the smoothed single band match well with the mapped distribution of skarn (Plate u_5 0.9 2a) 2b). Only a few nN separatethese anomalies from wavelength (pm) background,however; contrast stretching was used to figure 4. CorrectedGeoscan spectra extracted identify areasthat were obviously brighter than the norm. from imagefor samplesused in Figure3. Dataare A number of areasnot mapped as skarn appearbright on averagesof four pixels.Some curueshave been this treatment and are annotated on Plate 2a. Several are slightlyoffset (< 5 DN)for clarity. small garnetitedumps from the DouglasHill mine (1). Muscovite schists and pyritic hornfels in the andesite unit immediatelyeast of the main skarn body (2) are heavily stainedwith iron oxides;limonite (goethite?)is the dominant mineral, but hematiteis common and jarositelocally is Pyroxene-richsample YER3-5 shows a strong,broad present along fractures. The outcrop and dumps of the absorptioncentered near 1.1 pm, resulting from Fe2' in Ludwig silica-pyritebody (3) are similarly iron-stained,but pyroxeneM1 sites.The reflectancemaximum at 0.55 pm is hematiteis less common and no jarositewas observed.These not directly relatedto an electronictransition but is a iron minerals show a strong crystal field absorptionranging "window" betweentwo strong absorptions,the M1 crystal from 0.87 (hematite)to 0.95 pm (goethite),just as do field at 1.1 pm and the intenseFe-O chargetransfer in the trr,m garnetscontaining ferric iron (Townsend,1987). Strongly uv. SampleYER3-33 (garnet-pyroxene ratio 60:40)still iron-stainedrocks give a (5-B)value similar to that of garnet shows a weak pyroxeneband at 1.1 pm, but the spectrumis skarn,particularly if hematiteis present. now dominatedby ferric absorptionthrough 0.55 pm and at 0.87 pm. SampleYER4-4 is nearly pure andraditegarnet, and the spectrumshows a well-developedo.gz pm ferric band. PossibleSources of Skarn/HornfelsDiscrimination SampleYER6-19 (Figure 3b) is a pale green,fine-grained The observedspectral discrimination betweenthe garnet-py- diopsidehornfels with significant (20 to 30 percent)garnet. roxene skarn and garnet-pyroxenehornfelses could result The spectrumexhibits two Fe2* absorptions:a weak 0.95-pm from differencesin composition,grain size,weathering and an asymmetric1.1-pm feature.Sample MTE 514 consists styles,or any combination of the three. Garnetsand pyrox- of B0 to 90 percentisotropic garnet,with the remainderfine- enesin skarn are considerablymore iron-rich than those in grainedpyroxene needles; the r,ryIRspectrum shows the 0.87- skarnoid;given that the (0.717-pm-0.873-pm)treatment pm garnetabsorption and a weak 1.2-pm band of used abovetakes advantage of the 0.87-pm Fe3*absorption indeterminateorigin. SampleMTE 502 also consistsof B0 to in garnet, the higher (5-B)values in skarn may result from 90 percentisotropic garnet (0.2-to 1.O-mmdiameter) with the higher iron content. It is well establishedthat depth of intimately intergrown pyroxeneand epidote.The spectrumis an absorptionfeature increases with increasinggrain size nearly identical to that of MTE 514 but shows a slightly (Adamsand Filice, 1967;Clark and Roush, 1984),a factor stronger0.87-pm garnetabsorption and a steeperslope which would also favor a deeperabsorption feature in the toward the blue and try. This is probablv an effect of the coarser-grainedskarn. Weathering may affect skarn and horn- protolith, a dolomitic marble.Hornfelsei and skarn formed fels differently and should be considereda critical factor in in pure carbonatetend to contain fewer precursormineral any study concerned with spectral differentiation. inclusions and typically show stronger spectral features as a Laboratory spectra were averagedover Geoscanband- result. SampleMTE 305 formed in silty limestoneand passesfor comparison with the airborne data. When analyz- contains B0 to 85 percentanisotropic garnet with semi- ing laboratory spectra, ratios of reflectance are often
PE&RS t2al -Nffi. +Nffi-
(b) (b) Plate1. (a)Geoscan (23-21)B(23-2O)c(23-19)* color com- Plate2. (a)Geoscan (5-8) skarrVhornfelsdiscriminator treat- positecalc-silicate treatment. Garnet-pyroxene and epidote rocks ment.Bright areas are garnet-pyroxene skam and stron$y Fe- appearblack on thistreatment. (whiterVellow = QU?rtz and feld- stainedrocks. This image covers an area2,7 Rmfrom left to spar-richrocks, orange/red : rypsum).This image covers an right,with north to the right.(b) Distributionof skarnrock in the area2.t km from left to right,with north to the right.(b) Distribu-Ludwigarea. From Hanis (1979). tion of garnet-pyroxenerock in the Ludwigarea. (grey = skarnoid, black: skarn,black patterned : endoskarn.)From Harris (1979),Einaudi (unpub.).
employed to isolate the relative absorption strength (Clark ComposltlonalEffects and Roush, 1984). In this study, differences were used in or- The 0.87-pm absorption feature results from Fe3* within a der to duplicate the conditions of the image treatments. All mineral or in a weathering coat on the surface of a mineral. difference analyseswere checked with ratios, however, and Given the dichotomy in iron content between Fe-rich skarn in no caseswere there significant effects on the results. and Fe-poor hornfels, compositional variations must be con-
1282 effects. If garnetite exhibited a tendency to fracture along rims of a given composition, a cut surface would not yield a spectrum representativeof the "typical" garnetite hand sam- ple. No such tendencieshave been noted in this study area, o70 but the possibility should not be disregarded in similar stud- Eoo ies. More importantly, these are spectra of hand samples and Fso the effects of grain size have not been removed. This factor is :- 40 in the following section. While these caveatssug- o considered -30 gest that this relation may not be applicable to all skarn de- posits, presented that the 20 the data in Figure 5 indicate majority of the garnetiferous rocks at Yerington exhibit a t0 trend toward stronger 0.87-pm absorption with increasing 0 -10123456789101112 iron content. (5-8) value of convolved lab spectra (DN) Figure5. Plotof molarpercent Fe-endmember GrainSize/Textural Eff ects compositionof garnetsvs. (5-8) values.The In a study of the reflectance characteristics of olivines as a (5-8) valuesare calculated from laboratory function of composition,King and Ridley (1987)found sys- spectra(O to 100 percent)averaged over tematic spectral variations with Fe/lvlgratios and with Ni im- Geoscanbandpasses and multiplied by 2.55 to purities but cautioned: allowcomparison with the 8-bit(0 to 255) "The areaof an absorptionband, i,e., the areaunder a straight- Geoscandata. Bars represent one standard de- Iine continuum,is not simplyrelated to the FeMg ratio of the viation.Open triangles represent hornfels, sampleunless the grain sizeis severelyrestricted ... . The area closedsquares are skarn. of the l-pm absorptionband is positivelycorrelated with grain sizefractions of a givenolivine composition." (p. 11,468.) It is well established that the specbum of a particulate mate- rial is strongly affected by its grain size (Adams and Filice, sideredas a possiblesource for the observedspectral dis- 1967; Clark and Roush, 1984).As grain size decreases,over- crimination. If the depth of the absorption feature increases all brightnessincreases and spectralfeatures become less with Fe content, it would fit the observedpattern of larger pronounced. The skarns in the study area definitely tend to (5-B)values in skarn. be coarser grained than the skarnoid; an increase in the There is someempirical support for such a hypothesis. depth of the 0,87-pm featurewith grain size could result in Moore and White (1972)studied the absorptionspectra of the larger (5-B) values for skarn. nine garnets in the andradite-grossular-uvaroviteseries as a Unfortunately, a comparison between grain size (esti- function of their composition. They found a linear relation- mated from thin section) and (5-8) value for 22 samples ship between absorbanceand weight percent Fer0" for five proved inconclusive.The data suggestedthat the depth of absorptionbands between0.35 and 0.75 pm. However,sev- the 0.87-pm absorptionincreased with depth, but with a cor- eral factors prevent the direct application of their results to relation coefficient of only 0.26. The strongest evidence for this study. Moore and White (1972) did not examine the the importance of grain size effects on spectra lies in their 0.87-pm absorptionband of interest in the Yerington garnets. identification in numerous other studies (Adams and Fi]ice, They studied only shorter wavelength bands, none of which 1967;Clark and Roush, 1984;Crowley, 1986;King and Rid- could be resolved in reflectance spectra of garnets used in ley, 1987;Windeler and Lyon, 1991). this study. Their samples contained no garnets with interme- A common feature in the hornfelses is the occurrence of diate compositions similar to those found in the Yerington many fine-grained mineral inclusions in garnet; such sam- hornfels; their grossularitic samples had compositions rang- ples were excluded from the comparison becausethe pres- ing from ad" to ad"u,while their andraditic samples had ence of numerous inclusions would make the estimation of compositionsof adn, and adnn,Finally, they found absor- the "average" grain size difficult. However, this is a textural banceto be a linear function of FerO,conten! reflectance, effect that would effectively result in a finer grain size. Alter- however, does not vary directly with absorbanceand would natively, mineral inclusions can be opaque materials that ab- not necessarilyshow a linear relationship.Nonetheless, their sorb across all wavelengths, Mineral inclusions in garnet are study lends credenceto the hypothesisthat the 0.87-pm ab- much more common in the hornfelses than in skarn and are sorption featurewould deepenwith increasingFe. yet another effect that would favor deeper spectral absorp- To test this hypothesis, feflectance spectra averagedover tion features in skarn. Geoscanbandpasses were comparedto compositionfor 16 garnet-pyroxene skarn and hornfels samplesin the study WeatheringEffects area.All samplesconsist of >70 percentgarnet. Samples The rocks of the Yerington district are not deeply weathered. were selectedthat minimized spectral effects not related to Outcrops are plentiful and most hilltops and hillsides lack- garnetcomposition; all are free of opaquesor inclusions and ing outcrops are covered with a surficial layer of residual showedlittle or no alterationto epidote or other secondary rock fragments (desert pavement) reflecting the underlying minerals.Figure 5 shows the (5-B)values of convolvedlabo- lithology. ratory spectra versus the averagemolar percent Fe-endmem- Nonetheless,electromagnetic radiation in the visible and ber of garnetsas determinedby electronmicroprobe. short wave infrared regions is most strongly affected by the There is a definite trend toward an increasing (5-B)value top 50 p,m of a surface and is thus highly sensitive to weath- with mole percent andradite shown in Figure 5. Two com- ering (Buckingham and Sommer, 1983). Figure 6 illustrates ments with respectto the spectraused in this plot should be the importance of weathering on spectral features. The mean noted, however. These laboratory spectra were measured (5-B) values for laboratory spectra of skarn and hornfels sam- from cut surfaces,a precaution to reduce possible shadowing ples averagedover Geoscanbandpasses were compared. surfaces differ from fresh surfaces in the vtvlR in several ways (Figures Ba and 8b): (1) The averag€albedo decreases; z (2) The slope of the spectrum at wavelengths shorter than 0.7 9e pm may steepen;and o (3) The Fe3+absorption feature resulting from garnet is usually Er retained, but is often canted downward towards the shorter o wavelengths as a result of (2) While this results in an appar-
o ent shift of the band location, isolation of the absorption o feature by continuum removal proves this to be an illusory
P'" effect (Clark and Roush, t98e). 6 d-2 Whereas the garnet-rich rocks absorb strongly in the w and 6 thus also have relatively steep positive slopes into the visi- ble, fine-grained iron oiides ixtriUit a steeperslope because of their "trans-opaque" behavior. At the longer wavelengths, reflectance increaseswith decreasinggrain size (trans-), Figure6. Average(5-8) valuesfor convolvedlaboratory while at shorter wavelengths absorbanceincreases with de- spectraof skarnand hornfels, fresh and weathered. creasinggrain size (-opaque).The slopebreak betweenthese Horizontalticks are means, vertical bars represent one two regionsoccurs near 0.55 pm (Salisburyand Hunt, 1968), standarddeviation. The spectra of pyroxene-rich rocks (Figure 8c) change similarly on weathering, although they differ from those of the weathered garnets due to differences in the original sam- ple spectrum. Sample albedo decreaseswith weathering at wavelengths shorter than 0.6 pm but increasesat longer While the (5-8) values for skarn tend to be higher than those wavelengths, again due to the trans-opaquebehavior of the for hornfels, there is significant overlap. When the data are oxides. A study by Straub ef o/. (1991) determined that on subdividedinto spectraof fresh (brokensurfaces, thin sec- oxidation of Fez* in calcic clinopyroxenesto Fe3*,the Fez*- tion heels)and weatheredsamples, however, the setsof Fe3*charge transfer transition (0.75 pm) maskedthe Fe2* weathered skarn and hornfels are distinctly more separable crystal field absorption feature. They also found evidence for than the fresh. nanophase (particle diameters < 10 nm) hematite on oxidized In order to lessen possible sampling errors in this analy- surfaces.In this study, the strong,broad 0.95- and 1.1-pm sis, (5-8)values were comparedfor spectraof fresh and Fe2* absorption bands in the pyroxenes remain but are typi- weatheredsurfaces of 28 samples(Figure 7). Weatheringre- cally less well-defined than on fresh surfaces.An absorption duced the (5-B)value for all skarnoid samplesexamined, may develop near 0.9 pm but is typically broad and shallow. while for skarn samples weathering resulted in little or no Effects (2) and (3) thus give a possible justification for change.In five samples(denoted by a dotted connectingline) the drop in (5-S) values between fresh and weathered horn- weatheringactually increasedthe (5-B)value in the skarn. fels samples illustrated in Figures 6 and 7. Fine-grained py- The skarn samplewith the greatestdrop in (5-B)value, roxenes weather to iron oxides, and the spectral qualities of YERT-44,is weakly coatedwith desertvarnish and thus this oxide coat reduce the continuum reflectance in channel probably does not represent a typical frestVweatheredpair of 5. The strongest effects of the limonite coat are seen in the spectra. The spectral differentiation between skarn and horn- visible region; channel B is less affected or unchanged by fels clearly is due in part to differences in their weathering this coating and thus the (5-S)value decreases.As the inten- characteristics. The weathering characteristics of the garnet-pyroxene rocks in the study area vary with texture and mineralogy. As 11 pyroxenes weather much more readily than garnets, their sKarnsamples weathering properties would have a more immediate effect 7 than those of the garnets.Fine-grained rocks developsurface z o :u coatings more readily than coarse-grainedrocks. Garnet-py- .. ?ll o rll roxene hornfelses in the study area typically weather to a 3 t ln 6 t- brown or dark red-brown color suggestiveof goethite.Fine- 'I t- grained pyroxene skarn acquires a similar brown (goethitic) -1 r E 'l or reddish-brown(hematitic) coat, whereascoarse, bladed o salite is coated locally but tends to retain the grey-green e. o \f'r' color of the fresh pyroxene. Garnet-dominated skarn shows 6 -7 fewer surface weathering effects than the finer-grained skar- o noid; the weathered surfaces of garnet-pyroxeneskarn darken 6 t oo NOdNo6 -11 oooo+lAAirlN* IdcEcEccccEEd dE EEECCCEECdE EE A slightly, while garnet skarn exhibits little color change and UUUUUUUUUUUUUUU= the glassy luster of the fresh garnets usually is preserved. -13 Overall, the garnet-pyroxenehornfelses contain greater pro- ,15 portions of finer-grained pyroxene than skarn. The better-de- Figure7. Pairsof (5-8) valuesfor fresh and weathered veloped limonitic coats on skarnoid undoubtedly result from surfacesof 12 skarnand 16 hornfelssamples. Open this abundance of readily-weathered,fine-grained pyroxenes. symbolsrepresent fresh surfaces, closed are weathered. Most calc-silicatealtered rocks in the Ludwig areacon- Solidconnecting lines indicate a decreasein (5-8) on sist predominantly of garnet but contain significant pyrox- weathering,dotted lines indicate an increase. ene.For thesegarnet-rich rocks, spectraof weathered
t284 PE&RS goethite- and hematite-stained andesitesand for the goethitic
1 ? 3 | 4 5 6 t7 I 9 10 gossansof the Ludwig silica-pyrite body. Conclusions YERT-58 fresh surlace Thermal n difference images were successful in defining the in the c distribution of garnet-pyroxeneskarn and hornfels I Qao Ludwig study area,while a (O.777-p"m-A.873-p,m;5-S) dif- ference image provided a means of differentiating between f the two. Areas heavily stained by iron oxides were present as o 9er YERT-58 weatheredsurlace errors of inclusion on the (5-8) image, but the additional in- 6 formation gained through the thermal In differences could -9 o readily be included to distinguish thesefrom skarn, One 23 simple method would be a (23-20)s(5-B)c(23-19)pimage (Windeler, 1992).Alternatively, this combination of thermal and vwtR band differences could be used as a means of iden- tifying iron-stained, silicified rocks - common targets in min- wavelength(um) eral exploration. Compositional, grain size, and weathering variations were investigated as possible sources for the skarn/hornfels YERT-27fresh surface discrimination displayedby the (O.777 -p.m - O.873-g.m;5-8) difference image. All three factors contribute, but spectral variations due to weathering appear to be the most impor- tant. These results are summarized in Table 4,
I Skarn/hornfels discrimination in the Yerington contact YERT-27weathered sudac€ aureole thus appearsto result from several spectral variations s that fortuitously all contribute in the same direction. o Whether this treatment would be effective in other districts is uncertain, although higher iron contents and coarsergrain E size relative to early hornfels are common features of skarn districts (Einaudi et o/., 1981). The vtrIIRspectral effects of b manganesemay also play a role, particularly in Pb-Zn garnethorntels skarns. However, the use of multispectral thermal infrared data for detectionof garnetiferousrocks should be generally applicable to skarn exploration. Most skarns are associated wavelength (um) with felsic to intermediate intrusions, which would have emission specua distinct from the relatively long-wavelength minima of garnetites in the B- to 13-pm region. Discrimina- tion between different stagesof calc-silicate alteration may often depend on site-specific, empirically determined treat- ments, -9 I 944 Acknowledgments o s The author would like to thank his thesis advisors,Ron Lyon YER6-19fresh sudace and Marco Einaudi, for all the help and constructive criti- Pgo cism offered throughout the duration of this research.Credit g to Marco for unpub- t.tl for the alteration mapping also belongs E lished work done with the AnacondaCompany in the 1970s. c I would also like to thank Frank Honey and GeascanPty. pyroxenehornfels Ltd. for providing the imagery of the Ludwigskarn. Reviews by Stuart Marsh and Veronique Carrere helped to improve question at ERIM Roger Clark iden- wavelength(um) the manuscript and a by tified a blind spot that neededto be checkedout. Figure8. Reflectancespectra of fresh and weatheredsur- facesof (a) garnetskarn, (b) garnethornfets, and (c) py- roxenehornfels samples. Veftical ticks gve locationsof References (continuum) reflectanceminima after hull removal.Curve Abrams, M. f., and D. Brown, 1985. Si-IverBelI, Arizona, porphyry crossoversare circled.Geoscan band centers are included. copper test site report, The Joint NASNGeosatTest CaseProi- ecf, American Association of Petroleum Geologists,Tulsa, pp. 4.14.73. Adams, J. 8., and A. L. Filice, 1967. Spectralreflectance 0.4 to 2.0 mi- crons of silicate rock powders,/our. Geophys.nes., 72t57O5-5775. sity of the iron-oxide staining increases,however, the ferric Buckingham,W. F., and S. E. Somrner,1983. Mineralogical charac- iron absorptionnear 0.86 (hematite)or 0,93 pm (goethite) terization of rock surfacesformed by hydrothermal alteration deepenswith a concomitantincrease in the (5-B)value; this and weathering - applications to remote sensing,Economic Ge' would explain the (5-B) anomaly in Plate 2a for the heavily olog, 78(41t664-674. TneLE4. Suut',tanvoF FAgroRSCotlrntsLrtr.ro ro Sxenn/ HonruresDrscRrMrNAnoN av (0,7t7 ro 0.873-pr'a)Drrruneruce lurce. Possiblesource of discrimination Theorized effect on sDectra Why considered in this study Actual effect on sDectra Iron content Minerals with greater Fe-content may Skarn garnets are considerably more Some effect; garnets show increase in show stronger Fe-absorption iron-rich than hornfels depth of Fe3+minimum at 0.873 pm Grainsize Coarsergrain size results in deeper Skarn is generally coarser-grained Probably contributes, but by itself is absorption minima than hornfels only a minor factor Weatheringstyles Different weathering styles or prod- Skarn and hornfels contain different Major factor in discrimination; greater ucts may result in different spectra grain size and quantities of garnet & quantity of fine-grained pyroxenes in pyroxene hornfels weather easily and reduce brightness in channel 5 relative to channel 8.
Clark, R. N., and T. L. Roush,1984. Reflectance spectroscopy: quan- terrain in the mid-infrared (6-25 m) region, Inltared and Raman titative analysis techniques for remote sensing application, /our. Spectroscopyof Lunar and Tenestrial Minerals (C. Kan, Ir., edi- Geophys.fies., 89(B7):6329-6340. tor), AcademicPress, New York, pp. 165-195 Crowley, I. K., 1986. Visible and near-infrared spectra of carbonate Lyon, R, f. P., and F. R. Honey, 1990. Direct mineral identification rocks: reflectance variations related to petrographic texture and (DMI) with GeoscanMklI Advanced Multi-Spectral Scanner impurities, lour. Geophys..Res., 91 :5001-5012. (AMSS), Proc. Imaging Spectroscopy of the Tenestrial Enuiron- Dilles, J. H., 1987. Petrology of the Yerington batholith, Nevada: evi- ment, !6-77 February, pp. 50-61. dence for the evolution of porphyry copper ore fluids, Economic Lyon, R. I. P., F. R. Honey, and G. L Ballew, 1975,A comparisonof Geo lo gr, 82:175O-77 89. observedand model predicted atmospheric perturbations on tar- - Dilles, f. H., and M. T. Einaudi, 1992.Wall-rock alterationand hy- get radiances measuredby ERTS: Part 1 Observed data and drothermal flow paths around the Ann-Mason porphyry copper analysis, Proc. 6th Conferenceon Decisionand Control and leth deposit, Nevada - a 6-km vertical reconstruction, Economic Ge- Symposiumon Adaptiie Processes,10-12 December,Houston, olog. 87|L963-2OO7. Texas, pp. 244-249. Einaudi, M. T., 7977. Petrogenesisof the copper-bearingskarn at the Moore, R. K., and W. B. White, 1972. Electronic spectraof transition Mason Valley Mine, Yerington district, Nevada, Economic Geol- metal ions in silicate garnets, Canadian Mineimlogist,TTtTgt-l77. ogr,72:769-795. Proffett, l. M., 1977. Cenozoic geolory of the Yerington district, Ne- 1982. General features and origin of skarns associatedwith vada, and implications for the nature and origin ofBasin and porphyry copper plutons, Advances in the Geologr of the Por- Rangefaulting, G.E.A. Bulletin, 88:247-266. phyry Copper Deposits:Southwestern North America, (S. R. Ti- Proffett, f. M., and f. H. Dilles, 1984. GeologicMap of the Yerington tley, editor),University of Arizona Press,Tucson, pp, 185-210. District, Nevada, Nevada Bureau of Mines and GeologyMap 77, Einaudi, M. T., L. D. Meinert, and R. f. Newberry, 1981. Skam depos- Reno. Nevada. its, Economic Geologr 75th Annivenary Volume (B. f. Skinner, ed- in press. Early Mesozoic sedimentary and volcanic rocks of the itor), Econ. Geol. Publishing Co., EI Paso,Texas, pp. 317-391. Yerington region, Nevada, and their regional context, U.S. Geologi- Green, A. A., and M. D. Craig, 1985. Analysis of aircraft spectrome- cal Suwey CentetpieceProject on Mesozoic Geologr, Geophysics ter data with logarithmic residuals, Proc. TstAirborne Imaging and Ore Depositsof WestemNevada, U.S.G.S. Bulletin. SpectrometerData Analysis Workshop,S-10 April, fet Propul- Roberts, D. A., Y. Yamaguchi, and R. f. P. Lyon, 1986. Comparison of sion Laboratory, California Institute of Technology, Pasadena, various techniques for calibration of AIS data, Proc. 2nd Air- California,pp 111-119. bome Imaging SpectrometerData Analysis Worl
1286 PE&RS