Chemistry and Mineralogy of Wood-Tin, Black Range, New Mexico
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American Mineralogist, Volume 62, pages IN-106, 1977 Chemistryand mineralogy of wood-tin,Black Range,New Mexico JoHN L. LurxlN Department of Geological Sciences, The Uniuersity of Texas at Austin Austin. Texas 78712 Abstract Wood-tin occurs as placer accumulations at several localities in the Black Range, south- western New Mexico. Individual nuggets, 3 cm or less in diameter, commonly enclose fragments of porphyritic rhyolite, and contain cassiterite, hematite, cristobalite, and/or chalcedony. Color zoning is well developed in wood-tin and appears to have chemical significance;the iron content in cassiterite-richlayers, as much as 7.8 percent Fero3 by electronprobe analysis,generally increases through the color sequence:tan, yellow, red, dark brown. The addition of iron is correlated with a decreasein both the c cell parameter and the cell volume of cassiterite,suggesting that Fes+has substituted for Sno+in the crystal structure. Although wood-tin is rarely found as lode deposits,it probably was depositedoriginally as fracture fillings in the Tertiary Taylor Creek rhyolite with which it is closely associated.A low temperature,low pressureenvironment of depositionis reasonablefor the wood-tin. Introduction veinletsin the Taylor Creek rhyolite which is exposed Wood-tin, a term generally applied to sub- over an area of 177 kms'?.Almost half of the material microscopic to finely crystalline, concentrically-lay- studied has cores or inclusions of altered and re- ered cassiterite,occurs as placer accumulations in placed rhyolite, and the placer deposits are closely eluvial and alluvial gravels in the Black Range tin associatedspatially with outcrops of Taylor Creek district of southwesternNew Mexico. Rarely, it is rhyolite. At Squaw Creek, a small amount of wood- observed in situ as fracture fillings in the Taylor tin has been observedin place as fracture fillines in Creek rhyolite of Tertiary age (Fig. l) that forms this rhyolite. flow-domesexposed just west of the Continental Di- vide in Sierra and Catron Counties(Fries, 1940;Fries Mineralogy and Butler, 1943;Lufkin , 1972).Based on estimates Wood-tin nuggetsare rarely composedof pure cas- of the U.S. Bureau of Mines (Volin et al., 1947), siterite.Other mineral phasespresent include, in or- production of tin concentratesfrom the district dur- der of decreasingabundance, hematite, cristobalite, ing World War II was about l0 long tons, averaging and chalcedony. Cristobalite and chalcedony are 50 per cent tin. Since 1970,small mining operations much less abundant, and commonly absent from have shipped approximately 40 short tons of wood- many nuggets.The typical colloform texturesuggests tin concentrateaveraging about 6l percent tin from sequentialencrustations of theseminerals on walls of the vicinity of H ardcastleCreek (Frank Gitman, oral fractures.Many of the nuggetcores consist of altered communication). The following account is basedon porphyritic rhyolite with a groundmassthat is divitri- petrographic, X-ray, and electron probe studies of fied to aggregatesof fine-grained quartz and alkali more than twenty specimenscollected by the writer feldspar.These rhyolite inclusionsare typically elon- and ProfessorR. H. Jahns from eluvial gravelsand gate and as much as l8 mm long by 4 mm wide. The jig concentrates in Sawmill Canyon, Hardcastle cuspateor botryoidal depositionalpattern of wood- Creek, and Squaw Creek. tin adjacentto the rhyolite contact commonly reflects The Black Range wood-tin typically occurs in small irregularitiesin the planar to slightly convex Holocene gravel depositsas isolatednuggets, gener- rhyolite surface.In places,the rhyolite groundmassis ally less than 3 cm in diameter, although fist-sized replaced by fine-grained, disseminated to colloform specimenshave beenobserved. It seemshighly prob- cassiterite,with phenocrystsof quartz and sanidine able that the wood-tin was precipitatedoriginally as left intact. In one specimen, quartz, sanidine, and LUFKIN: WOOD-TIN FROM NEW MEXICO pm-*l f-r'rr-w* sonrcrc | i OSocoro / l-l lTruth or Conlcqucncas ""*PJ- Lj "".tr-// r / osompl€ tocotions ,/ \/ / L \ ( ""t I <l/ ""y q -L> wmill Conyo Loco lity c.--t ( ,-)'2 1 t' I T After Fig. l. MapshowingoutcropdistributionoftheTaylorCreekrhyoliteandsamplelocationsofwood-tinexaminedinthisstudy Fries and Butler (1943). topaz(?)were presentin the groundmassof the rhyo- hematite,cristobalite, and/ or chalcedony.Cassiterite to lite core, but not in the body of the surrounding varies in size from microcrystalline aggregates wood-tin nugget. elongate crystals, 2.2 mm long; much of the cassiterite Proceeding outward from the rhyolite core, the is presentas elongated,radial grains a few microns in body of the wood-tin nuggetsis composedprimarily length. There does not appear to be any systematic of microcrystallinecassiterite, with lesseramounts of variation in grain size,although cassiteritetends to be t02 LUFKIN'WOOD-TIN FROM NEW MEXICO coarsergrained in the rhyolite inclusionsand in pe- body of wood-tin it generally occurs in layers and ripheral shells of the nodules. Color variations in varies in shape from radial, needle-likecrystals to cassiteritehelp to define the colloform texture of more equant grains. Less commonly, it forms semi- wood-tin. In transmitted light, cassiteritedisplays circular massesof radiating crystals.The distribution shadesof tan, gray, yellow, orange, red, and brown, of hematite-rich layers is non-uniform; most of the or is opaque. Less commonly, it is almost colorless. hematite is depositedin areasadjacent to the nugget Where opaque, cassiteritegenerally contains abun- interiors, suggestiveof an early formation. However, dant inclusions of hematite. In reflectedlight, cas- some nuggetsalso featurehematite in their peripheral siterite appearsdull gray with low reflectivityin air. shellsas well. There is a generalcorrespondence between color and Where present, cristobalite and less commonly, crystal layering, but in coarser-grainedlayers, color chalcedony,form thin shells,0.4 mm or lessthick, on boundariesmay depart from crystal outlines, or ter- the wood-tin surfaces.Cristobalite occurs in equi- minations. Slight pleochroism is noted, but only in granular mosaicsor is fibrous, and the X-ray diffrac- the larger crystals. tion pattern of this material matches that of low Hematite rangesfrom minute particlesto platesof cristobalite. In one specimen,cristobalite occurs as specularite,2.8 mm wide. In the altered rhyolite in- linear, mosaic aggregratesinterlayered with cassite- clusionshematite is disseminated,but throughout the rite. Portions of these mosaic patchesare optically continuous, possibly suggestiveof inversion to the Table I Electronprobe analysesofwood-tin, Black Range,New mofe stable alpha quartz. Chalcedony is the least abundant constituentof wood-tin. It typically forms a thin, light-colored coating on the nodules. Specimen No. Sn02 Fe203* Co]or of cassiteritei Chemistry Nine wood-tin nodules were chemically analyzed 94.3 0.3 ran by electron probe. Traverseswere made acrossindi- 92.6 0.7 col orl ess vidual nuggetsfrom rim to core to determine their 86.2 3.? yellol-green brovln 84.6 4.4 yelIov brown purity and to seeifthe variouscolors ofthe cassiterite 86.8 5.9 orange brovn layers had chemical significance.Analyses for SnO, 86.5 5.3 yellovl brown 86.5 6.9 red brolvn and FerOg were determined for several differently colored layers of cassiteritein each specimen,ex- 90.5 'I1.6 ran 9l.5 .6 yellov cluding grains of other minerals.A beam diameterof 89.9 1.7 1ight brown 88.I 2.5 ye1I ol orange 2-10 microns was employed. Due to the generally 91.8 2.7 reo orange submicroscopictextures developed in wood-tin, grain 93.I 2.5 yellorv brown boundaries of cassiteritegenerally could not be re- 9t.6 3.7 ye11ow v l.o 4.2 orange red solved in polished sectionduring probe analysis. 90.3 4.5 yellow brovln Probe analysesof wood-tin are presentedin Table 89.5 5.3 dark brown I . With the exceptionof specimensl8 and 21, most of 96.1 0.8 f,an 91.6 2.5 ye1 1ow the analyzedcassiterite consists of aggregatesof tiny 9l .6 a.o brown 90.9 5.8 red crystals a few microns long. Specimens l8 and 21 86.8 5.8 dark brovn have largergrains, 2.2 mm or lessin length.The SnO, 90.7 5.0 brown orange and FerOsanalyses of the micron-sizecassiterite do 94.0 5.3 red orange 89.4 7.8 yel low orange not total 100 percent, whereas those of the coarser 'l0l grained material do. As an independentcheck, rela- .5 0.2 color]ess l0l.3 0.5 ye'l 1or orange tively pure wood-tin practicallyidentical to specimen 100.6 0.4 oran9e 2 was submitted to the Gulf Chemical and Metallur- 99.9 1.0 ye1 I our gical Company for wet chemicalanalysis. The partial 96.9 2.1 orange 96.8 2.8 red brovn analysisis: SnO2,97.17; FezOr,0.43; SiOr, 0.77. By comparison,the probe analysisfor SnO2in specimen xTotal Fe as Fe203. 2 is almost 3 percentless than the wet chemicalanal- ysis of similar material, while the iron content is tColor basedon observationsof wood-tin in thin section, transmitted light. about the same.The explanationfor the low totals of probe analyses,particular SnO2,probably is related LIJFKIN: WOOD-TIN FROM NEW MEXICO 103 to the fine-grainedtexture of wood-tinand micro-pore compositioninclude variationsin texture,porosity, spacesalong grain boundaries(which are observed in and mineralthickness from specimento specimen. thin-sectionof specimen2, but generallynot in other Much study has been devotedto color zoning in material).This suggestionis compatiblewith the re- coarse-grainedcassiterite of deep-seatedvein