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Garnet-Pyroxene Alteration Mapping in the Ludwig Skarn

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Garnet-Pyroxene Alteration Mapping in the Ludwig Skarn 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Nevada) have been analyzed and comparcd with Proffett and Dilles, 1984; Dilles and Einaudi, 1992), while published and unpublished maps.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.
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