175 m, or about halfway between the 0.14 and eruptions that formed the Toledo and Valles Abstracts 0.25 s reflectors. Based on this geophysical evi- calderasin the JemezMountains. Most of the dence, we believe that the gravity anomaly is eruptions over the past 5 m.y. havebeen smaller the result of brecciation along a fault passing (VEI:3-4) and mainly basaltic. Simple linear New Mexico GeologicalSociety through the 3,650-m/s layer and that shallow, regression of age vs. frequenry data seemsto The New Mexico GeologicalSociety an- lower velocity layers were draped passively over show that the eruption frequenry is increasing nual spring meeting was held at New deeper faults to form a small monocline along over time. There is an averageeruption prob- Mexico Institute of Mining and Technol- each side of the graben. Location of the fissure ability of 0.0002per year over the next million ogy (Socorro)on April 6, 1990.Following within the graben contradicts existing models years, equivalent to 200 eruptions. The poten- for the formation of fissures by concave-down- tial activity includes lava, cinder cone, maar, are abstractsthat were omitted inadvert- ward bending of thin alluvium layers over bed- dome, and plinian eruptions. All of thesehave ently from thosepublished in the August rock projections. These models predict that the capacity for minor to severeimpact on the and November 1990issues and the Feb- fissures should form by propagating down- state'seconomy, transportation, agriculhrre, and ruary 1991issue of Nno MexicoGeology. ward from the ground surface in areas of con- communications. cave-downward topography where the surface Geophysics, petrology, and is in tension. In contrast, the San Marcial fis- Lers Pnorenozorc ANDCauarunN-Onoovrcnru structural geology session sure is located in an area where topography, ALKAUCPLUTONISM INWEST TEXAS AND SOI,JTHERN seismic reflectors, and the gravity profile are Nrw Mgxtco: A REACTIvATEDsHEAR-zoNE (Continued from November 1990issue) all gently concave-upward to straight, where MoDEL,by M. H. Sandidge.Department of the ground surface should be in compression, GeologicalSciences, University of Texasat El GpopnysrceL coNsrutNTs oN A MEcHANTCAL and the fissure appears to have propagated up- Paso,El Paso,TX79924 MoDEL FoR THE oRtctN or rue Seru Menctal ward. Analysis of elastic stresses and displace- Two parallel, linear, alkalic igneous belts have EARTHFrssuRE/ by W. C. Haneberg, New Mex- ments developed in a two-dimensional gravity- been recognizedin west Texasand southwest- ico Bureau of Mines and Mineral Resources, loaded span, which we use as an analog for a ern New Mexico. The northernmost of them New Mexico Institute of Mining and Tech- soil layer lying over a bedrock graben, shows measures240 kms in length while the southem nology, Socorro, NM 87801; and C. B. Rey- that minimum principal stress (taking com- belt extends for some 100 kms. Zircon U-Pb nolds, Charles B. Reynolds and Associates, pressive stresses to be negative) will be tensile ages for the northern belt range from 970 Ma 4409 San Andres NE, Albuquerque, NM 87110 to neutral throughout the lower portion of the to 7MS Ma from east to west. However, ages The San Marcial earth fissure, which ap- span. Magnitude of the tension decreaseswith of 495-507Ma (Clemons, 1989)along the same peared during a heavy rainstorm in August 1981, layer thickness and will increase if outward- trend have been obtained. Ages for the south- crosses I-25 along the western edge of the Rio directed shear is applied to the base of the layer. ern belt range from 623 Ma to 1400Ma from Grande valley about 50 km south of Socorro For layers with length:thickness ratios greater east to west. Theseages and geochemicaldata The fissure was widened by erosion immedi- than about 10:1, which can be analyzed using point toward an anorogenic magmatic event ately upstream of an embankment for south- elementary beam theory, the absolute value of and/or events rather than an orogenic event or bound I-25, then under construction, suggesting fiber stress along the upper and lower bound- events.Three possibletectonic processes may that blockage of the arroyo may have contrib- aries will decrease with the cube of thickness. have been operating during the Proterozoic and uted to its growth. On a large scale the trace Tensile to neutral normal stress would decrease Cambrian-Ordovician; they are: 1) plutonism of the fissure is straight, following a small un- soil shear strength-as well as capacity to resist associatedwith pre-rift or pre-ridge extension, named arroyo, and is nearly 1,500 m long. Strike seepage forces-and we speculate that the San 2) plutonism associatedwith a mantle-derived of the trace, which is not affected by topogra- Marcial fissure must have originated in a zone plume, and 3) plutonismassociated with shear phy, is northwest-southeast. Closer examina- of tension at depth and grew upward by pip- zones. The fact that both belts exhibit varia- tion of 1:250 aerial photographs, taken one tng. As piping continued, layer thickness would tions in ageswould seem to rule out an exten- month after the fissure appeared, shows that have decreased continuously, thereby increas- sional process.Associated ring complexeswould the trace is composed of many short segments ing tensile stresses at shallower and shallower illustrate no disparity of ages.A plume model meters to tens of meters long. Some segments depths until the ground surface was broken also must be rejectedbecause the northern belt are crudely en echelon but with no consistent and surface runoff continued to enlarge the fis- exhibits an older-youngest4 km3) basalts at approximately the same time The late Miocene PeraltaTuff Member of the define layers with P-wave velocities, inferred (ca. I ka): the Carrizozo and McCarty's basalt. BearheadRhyolite, in the southeasternJemez by previous investigators using seismic refrac- Seismic and geodetic evidence suggest that volcanic field, contains crossbeddedtuffs of both tion surveys, of approximately 800 m/s from 0 magmatic activity of New Mexico has not ceased pyroclastic surge and eolian origins. Study of to 48 m in depth, 1,900 m/s from 48 to 67 m in Leveling measurements indicate an inflation rate textural characteristics,stratification, and flow depth, and 3,650 m/s below 67 m in depth. of approximately 1.8 mm/yr over the Socorro directions was undertaken to discriminate de- There is no evidence of recent fault scarps albng magma body (SMB, a sill at 19 km depth). This posits produced by these very different pro- either of the graben margins, and weak reflecl research assesses both the recent and potential cesses.Surge deposits examined in this study tors at two-way times less than 0 10 s do not volcanism of New Mexico in light of the pos- are part of a phreatomagmatic tuff-ring ex- appear to be broken by faults. The average Bou- sibility for volcanic activitv and a lack of infor- posed at the Tent Rocks; their characteristics guer gravity profile, calculated from five read- mation about future volcinism and its effects were also compared to others previously stud- ings taken over two days at each seismic here. While volcanic eruptions may not present ied by the senior author and to published tex- - shotpoint, shows a 1 mgal anomaly over the an immediate threat to the population of New tural data. Other crossbeddedtuffs are northeastern boundary of the graben, which Mexico, they clearly pose a risk that should be commonly found overlying pyroclastic-fall tuffs coincides with both an 0.04 s vertical offset (to clarified when planning long-term develop- in the Peralta Tuff. Strongest evidence for eolian 0.29 s) of the 0.25 s reflector and a topographic ment in the state. The lareest volume of ma- origin is measured paleocurrent directions in- depression within the arroyo. Half-width anal- terial was erupted between 1-2 Ma. The maiority dicating flow from the south and southwest. ysis of the anomaly gives a maximum depth of of that came from the large (VEI = 6-Z) rhyolitic All known BearheadRhyolite vents are located

May 1991 Nm Merico Geology to the north and the fluvial paleoslopewas in- of high accretion rates and fast-moving cur- tures include: sandwave, planat and massive clined to the southeast.Although theie is some rents and are rarely observedin the eolianites. bedforms previously describedby Wohletz and overlap in textural parameters, the eolianites Fine-grained surge deposits and sand-sheet Sheridan (7979, Geol. Soc. Amer., Sp. Paper are generally better sorted than the surge de- eolianites appear very similar in the field, and 180),as well as accretionarylapilli, impact sags posits, lack fine ash, and lack grains larger than great care must be taken to distinguish them. with plastic deformation due to ballistically 0.5 cm, which are abundant in the surge de- The ubiquitous occurrence of eolianites with emplacedblocks, and soft sediment structures. posits. The textural characteristicsof the fall deposits of similar grain size suggest that Each of thesebedforms and depositional struc- the falls were extensivelyreworked by wind to turescan be athibuted to differentwater/magma produce faciesthat could be mistaken easily as ratios. Hence, the stratigraphy of thesedepos- the product of surges. its mimics relative changesin the water/magma ratio during the eruption. Accretionary lapilli, SrnertcnepnyoF THEBAsE-suRGE DEpostrs AT THE and massive beds with associatedsoft sedi- sourHERNEND oF KrlaounNe HoI-r vaen, ment structures increase upward in the sec- sourHERNNEw MEXrco,by D. Bahar,Depart- tion, and thus reflect increasing amounts of ment of Earth Sciences,New Mexico State water in the surge cloud as the eruption pro- University, Las Cruces, NM 88003 gressed.Further evidencefor the progressively composed of low, undulating cross-stratifica- Kilbourne Hole, a late Pleistocenemaar in increasing amounts of water is preserved in tion typical of sand-sheet deposits. Laminae the Potrillo volcanic field, lies on the trace of comoositional variations within the section. The are less than 1 cm thick where not disturbed the Rio Grande rift-related Fitzgerald-Robledo base surge is predominantly composed of ac- by abundant burrows. The second type are dune fault system in southem New Mexico. It formed cidental material from the SantaFe Group and faciescomposed of wind-ripple and grain-flow as the result of a phreatomagmatic eruption the Afton basalt flow; the fraction of iuvenile strata0.2 to 1.2cm thick. Theselatter deoosits that broke through pre-existingSanta Fe Group basalt fragments increasesaway from the base were formed by migrating dunes that were I sedimentsand 0.1 Ma Afton basaltflow, and of the deposit. Thus, both bedform and com- to more than 4 m high. Surge deposits form deposited the exposed ejectarim. Subsequent positional variations in the stratigraphic se- low-angle, climbing cross-strata,generally more calderalike collaose and erosron are resDon- quence of the base-surgedeposits at Kilbourne than 1 cm thick, in setsaveraging 0.5 m thick, sible for the size of the modern crater.The pre- Hole illustrate the dynamic intelplay of magma with common preservationof stosssides of the served ejecta rim displays classicfeatures and water in the formation of base-surgede- migrating bedforms. Thesefeatures are typical associatedwith base-surgedeposits. These fea- posrts. !

limited S, representing changing oxygen- and Abstracts sulfur-fugacity conditions during at least two periods of mineralization.

New Mexico Mineral Symposium Krlaounre Holr uenn pERlDor,DoNe ANe The Eleventh Annual New Mexico Min- CouNry, NEw Mrxrco, by lohn R. Fuhrbach, eral Symposium was held November 10- 3133Fleetwood, Amarillo, TX 79lW (2) 11,7990,at New MexicoInstitute of Min- Peridot occurs in explosion debris from a ing and Technology, Socorro. Following 25,000-year-oldvolcano as small, but brilliant are abstractsfrom talks given at the sym- gems in a wide color range. Unlike peridot found elsewherein the Southwest, peridot in the posium that concern New Mexico. Kil- The bourne Hole maar is found in elliptical"xeno- numbers in parenthesesrefer to locations lith bombs" of volcanic origin ranging from 2 on the map. to approximately tlOcm long. In chemistry, color, density and hardness, optical properties and Panaceresrs oF BIsMUTHAND AssoclATED sILVER- PIXE (Proton Induced X-ray Emission) analy- BEARINGPHASES, PNOSALTOS DISTRICT, GRANT sis, the Kilboume peridot is similar to the San Courury, New Mexco, bv WillinmX. Chaoez, Carlos, Arizona material except for the greater Department /r., of Geological Engineering, color range in Kilbourne material and a char- New Mexico Institute of Mining and Tech- acteristic inclusion not heretofore describedin nology Socorro, r NM 87801(1) L. gemologicalliterature. The R.I.-S.G. color re- Base-metal(Cu-Zn) and precious-metal(Au- lationship is reviewed with regard to Mg:Fe Ag) mineralization of the Pinos Altos district ratio and the effect of heat treatment and ir- comprises distinct chalcopyrite- and sphaler- radiation. The future of this materialas a source ite-rich assemblageshosted by the Pennsyl- Ag and Bi minerals from thesequartzite-hosted of the seldom-seen greenish-yellow "chryso- vanian SyrenaFormation and by the overlying assemblages,comprising native bismuth, bis- lite" peridot is discussed. Comparisons made CretaceousBeartooth Quartzite (KB). Replace- muthinite (BirS3),stromeyerite (AgCuS), native with other documented worldwide sourcesadd ment of calcareousand siliceoushost rocksare silver, wittichenite (Cu3BiS.)and emplectite to the cumulative knowledge we have con- common; structurally controlled mineraliza- (CuBiS). Parageneticstudies suggest the fol- ceming peridot as a gem material. tion is locally important, most notably in the lowing sequenceof mineral deposition. Early Beartooth Quartzite. Petrographicstudies con- pyrite and pyrite-marcasite were followed by NEw pHospHATEoccuRRENcEs tN sourHWEsrERN ducted on samples from the quartzite-hosted initial copper deposition as chalcopyrite. Zinc New Mexrco, by Ronakl B. Gibbs,P.O. Box KB orebody indicate the presenceof locally sig- was introduced as sphalerite, succeedingchal- 448, Tyrone, NM 88065(4) nificant quantitiesof bidmuth and silver'miX- copyrite. Apparent reintroduction of copper,as Turquoise had been mined in southwestern eral phases,usually associatedwith high-grade cc+bn+cp, was accompaniedby silver (as New Mexico by Indians long before the first chalcopyrite+ bornite * chalcociteassemblages stromeyerite;? and native silver?)and, sub- settlersarrived. After the arrival of the settlers, or occurring in cp + bn + cc associationshaving sequently,by bismuth. Apparently, early base- many deposits were rnined commercially. Some hematite of hypogene(?) origin. Preliminary ore metal assemblageswere succeededby later in- of these became famous for the quality and pehography has identified relatively late stage troduction of Ag and Bi with attendant Cu and

Neu Mexico Geology May 7997 quantity of the turquoise, such as the Azure northern reaches of the Sierra Oscura moun- New or rare minerals, including rare-earth mine at Tyrone, the mines at Hatchita, and of tain range and has been known traditionally element (REE) arsenates, oxides, and vana- course, the Chino pit at Santa Rita. The intro- as the Mex-Tex mine. It is located about one dates, have been identified from lithophysal duction of open-pit mining brought many other rnile north and slightly east of the Blanchard cavities and veins with associated tin miner- species to light. Occurrences at the Tyrone rnine group. The second area, just north of the main alization in the Black Range tin district (BRTD). were reported at the 1986 symposium, but since Mex-Tex mine and across an arrovo, is better These minerals occur in sDarse to minute then several new occurrenies have been noted. known as the Royal Flush mine. Exiensive min- amounts but are distinctive because of their The copper phosphates, libethenite and pseu- eralization is found primarily between a shale color, habit, and mineral association. Details of domalachite, were found recently at the 85 rnine zone and a limestone formation, particularly the mineralogy of the BRTD are given in Foord near Lordsburg. Another occurrence of phos- along fault lines. The ore shoots are banded, et al. (1988), in which additional pertinent ref- phates was no[ed recently at Tyrone along the cmstiform, and quite vuggy. Large pockets with erences are listed. REE arsenates occur as three same trend that hosted an earlier suite of ca- coarse crystals are found throughout. The main principal separate minerals having two differ- coxenite, chalcosiderite, apatite, wavellite, and Mex-Tex group consists of many pits, adits, ent crystal structures. Chemovite-(Y) is the high- torbernite. This latest occurrence also contains shafts, tunnels, and stopes along this half-mile temperature form of YAsOn and has the tetra- cacoxenite along with strengite and leuco- stretch. Because of their shallow nature, exten- gonal xenotime (YPO4)structure, as well as being phosphite. Several interesting occurrences have sive weathering has taken place and cave-ins isostructural with zircon (Z$iOe\ and thorite been noted at the Santa Rita mine recently. A have occurred. ALL UNDERGROUND WORK- (ThSiO4). Gasparite-(Ce), CeAsOa, has the limited occurrence of beraunite, laubmannite, INGS ARE CONSIDERED QUITE DANGER- monoclinic monazite (CePO) structure and was and leucophosphite was discovered this sum- OUS AND HAZARDOUS. The bulk of the first described from the Italian Alps (Graeser mer in the Townsite Island area of the pit. This mineral specimens found consist of combina- and Schwandeg 7987). Both of these minerals may be the first reported occurrence of these tions of four minerals: fluorite, galena, barite, have been found at Souaw Creek and Para- species in New Mexico. In another area of the and quartz. Any of the following can also be mount Canyon. The Ce-dominant analogue of pit, South Pit, excellent specimens of libeth- Present on the four main minerals: linarite, chernovite has been found only at Paramount enite, apatite, and pseudomalachitewere found. plattnerite, murdochite, brochantite, spango- Canyon and is the third new species to be de- Apatite crystals also have been found in the lite, cerussite, anglesite, wulfenite, caledonite, scribed from the BRTD, the first two being Townsite Island area. chalcopyrite, hemimorphite, calcite, and sel- squawcreekite and maxwellite (Foord et al., in enite. Numerous striking/aesthetic plates, press). Solid solution exists between chernov- groups/ and clusters have been found up to 30 ite (As) and xenotime (P) (Graeser et al.,7973). X-ney Facnrry er NEw MEXrco BuREAUoF MNES inches in diameter. Fluorite from the Mex-Tex The range of solid solution for samples from ,qNo MrNenal RrsouRCES,bv Chris McKee,New group has proved the rnost desirable of the the USSR, Switzerland, and the BRTD is from Mexico Bureau of Mines and Mineral Re- minerals found. At the main Mex-lTex mine, it 63 to 99 mol7, YAsOa. Our studies indicate that sources, Socorro, NM 87801 is seldom found as simple cubes. Complex forms solid solution, approaching 50 mol% substi- The New Mexico Bureau of Mines and Min- prevail, with the most abundant being the hex- tution of P forAs, also exists between gasparite eral Resources X-ray Facility operates a wave- octahedral form. Even the cubic forms when (As) and monazite (P). Coupled Th-{a substi- length-dispersive, sequentiai x-iay fl uorescence found are highly modi-fied, with edges rounded tution for REE also is present in the REE ar- spectrometer and three powder x-ray diffrac- or other crystal faces showing. Colors include senates, as well as some Si substitution forAs. tometers. Both qualitative and quantitative the classic "Bingham blue," surface blue (typ- Chernovite-(Y) from Squaw Creek contains as analyses are performed. X-ray fluorescence ical of Naica, Mexico fluorite), green (various much as 5 wt% P2O., 3.5 wt%o ThO, and 0.4 spectrometry (XRF) is a rapid, nondestructive, shades), clear, and purple/maroon. Single fluo- wI% SiOz. Gasparite-(Ce) from Squaw Creek comparative method of geochemical analysis rite crystals in combination with other minerals contains as much as 6.0 wt% PrOt and 4.8 wt% requiring well-characterized standards. Sam- sometimes exceed 3 inches across. Quartz crys- ThOr. Small amounts of S substitute forAs and ples are commonly prepared as briquetted tals though small, seldom exceeding one inch P in both minerals. The fourth possible per- powders, fused-glass disks, or loose powders. in length, are quite beautiful and almost always mutation of the REE-As matrix, monoclinic XRF is a useful and versatile analytical method. present with other minerals as showy groups. YAsOo, has not been found but should occur Crystalline phases are identified by x-ray dif- In addition to clear quartz. both amethvstine in nature. The high-temperature form of fraction (XI{D) analysis. Loose po*d".r, b.i- and smoky are also found. The smoky color is REEAsO. is tetragonal and the low-tempera- quetted powders, solid pieces of metal and quite attractive, for it is almost always present ture form is monoclinic. Geologic, textural, and ceramic, and rock slabs are routinelv analvzed. on the crystal tips only and not within the body. mineralogical evidence indicates that the dep- The X-ray Facility supports basic reseaich at When the smoky color is present, fluorite as- ositional temperatures in the BRTD were high New Mexico Institute of Mining and Technol- sociation is most often the classic "Naica blue" (300-800'C), with those at Paramount Canyon ogy. The facility also accepts outside contract or a light-green color, a most unusual but strik- (tetragonal YAsOe and CeAsOn) being higher work and samples from the general public. ing combination. Galena is always found cov- than those at Squaw Creek (tetragonal YAsOa ered with other minerals. Crystils as large as and monoclinic CeAsO+). Associated with the 4 inches have been collected. Manv times pseu- REE arsenates at Paramount Canvon are sDe- MTNERATSoF THeMgx-Trx cnoup, BNGHAM, NEW domorphs of galena completely replaced by cies that probably are brabantite, Ca,Th(PO.),, Mrxco, by Tbm Mnssis,9313 Lagrima de Oro cerussite are found. Barite blades to 12 inches occurring as cores in some grains of gasparite- NE, Albuquerque, NM 87111 (4) have also been collected. Most recentlv, one or (Ce); the La-analogue of gasparite-(Ce) and cer- The Hansonburg mining district, especially more new forrns of wulfenite for the Bingham ianite, CeO2, occurring as inclusions and seg- the mines of the Bl-ancharJgroup locat'ed near area have been found at the Mex-Tex gioup. regations within Brains of gasparite-(Ce) that Bingham, New Mexico in Socoiro County, is Though individual crystals of the other min- show evidence of late-stage alteration; and well known to mineral collectors. Locited erals present at the Mex-Tex are small, they wakefieldite-(Y), YV04, occurring as discontin- nearby, in this district, is another series of mines can cover large areas on a group. Examples are uous coatings on grains of chernovite-(Y). not generally known to collectors called the murdochite on quartz and plattnerite on quartz, Identification of these latter four soecies is based Mex-Tex group that in the last three years has fluorite, and galena. Most if not all of the easy solely on microprobe data because the small produced a large suite of superb mineral spec- finds and pockets have been located. Future grain size (less than several microns) precludes imens rivaling those of the Blanchard group. success will involve considerable work but mav x-ray diffraction studies. Other REE-bearing The Mex-Tex group of mines consists of nu- be productive. The danger in the remaining minerals from the BRTD include chevkinite-(Ce) merous claims that were mined for barite until workings will limit the amount of future suc- (or perrierite-(Ce)) and titanite, both from Wil- about 1960. More than 967o of the barite oro- cess for rnineral specimens at the Mex-Tex mine low Spring Draw. The titanite is unusual be- duced in New Mexico has come from the Mex- and workings. cause of its high content (approximately 9 wt% Tex group. Mining activities have occurred REzO:) of REE. The unusual combination of sporadically since then with very little produc- Ranr-renrn ARSENATESAND orHER RAR-E-EARTH rare elements (e.g. Sb, Sn, As) and the absence tion. In 1987, after abandonment of thi claims MINERALSFROM THE BLACK RANGE TIN DISTRICT, or rarify of some more common ones (e.9. P) by Westem General Mhing, the Mex-Tex group Snnne elo Clrnoru Cour.nrss, Nrw Mrxco,* in the host rhyolites and later hydrothermal was claimed by mineral collectors for speiimen by Eugene E. Foord, MS 905, U.S. Geological fluids, combined with appropriate P-T condi- purposes. The Mex-Tex group consisti of two Survey, Box 25046, Federal Center, Denver, tions, resulted in the formation of several new distinct areas. The first or main area running CO 80225; and Paul Hlnzta, Div. 1822, Sandia or rare mineral species. It is likely that more north./south for approximately one-half rnile ii National Laboratories, Albuquerque, NM such minerals are present in the area, waiting located along the western escarpment of the 87185 for the diligent to discover them.

May 1991 NewMexico Geology References for the most part are replaced by quartz and In New Mexico, the Harding Pegmatite near Foord, E. E., Maxwell, C. H., and Hlava, P F., are intimately associated with light-green fluo- Dixon in Taos County and the Pidlite pegmatite 1988,Mineralogy of the Black Rangetin dis- rite octahedrons. The fluorite has been etched in the Rociada district, Mora County are the trict, Sierraand Catron Counties, New Mex- and the luster is dull, but crystals to 3 cm occur outstanding examples of deposits that contain ico: 9th New Mexico Mineral Symposium, here. Olive-green to dark-green mottramite a variety of colored Li-bearing micas, ranging Socorro, NM, pp. 23-27; New Mexico Geol- "crusts" often coat these specimens. Ramsdel- from reddish oink to lilac and violet. At the ogy, v. 17, no.2, pp. 39-40. lite has been confirmed from this mine in 0.5- Harding pegmitite mine, true lepidolite ranges Graeser, S., and Schwander, H., 7987, Gas- 1 mm crystals in association with purple fluo- from lilac to wine red and purple and typically parite-(Ce) and monazite-(Nd)-two new rite.The Confidence mine is on the west slope contains approximately 3.5-4.5 wt% Li2O and mineralsto the monazitegroup from theAlps: of the range about 2 km due west of the Dic- approximately 1wt% MnO. In contrast, lithian Schweiz. Mineral. Petrogr. Mltt., v. 67, pp. tator mine. Wulfenite and willemite sDecimens muscovite ranges from rose to lilac to gray and 273281. are common at this mine. Bright-yellow to or- pale green and may contain from about 0.2 to Graeser,S., Schwander,H., and Stalder,H. A., ange crystals of wulfenite in tabular, blocky, several weight percent Li2O, and typically only 1973,A solid solution series between xeno- and pvramidal habits to 0.8 mm are found'in a few tenths of a percent MnO. Some pink or time (YtPOa)and chemovite (YtAsOa):Min. vugjissociated with cerussite and willemite. purple, relatively hard, massive mineral ma- Mag., v. 39, pp. 145-151. Hexagonal prisms of willemite to 0.5 mm are terial is often mistakenly assumed by visitors *Part common/ but the luster is duII and the pinkish to the mine to be lepidolite. but it is actuallv of this work was supported by the U.S. color is not particularlv attractive. Hemimor- microcline feldspar sirowing an incipient alte;- Department of Energy under contract DE- phite and vanadinite microcrystals also be ation to lepidolite; the bulk of the material is AC04-76DPN789. can found here and, rarely, linarite and brochan- still feldspar, as evidenced by its cleavage and tite. hardness. Lithium-bearing micas in pegmatites Mrxenem oF THE CucFilLLo NEcRo DrsrRrcr, are not usually of primary magmatic origin, SrsnnaCouvry, NEw Mtxco, bv RamonS. Pwx aNo vIoLET MICAs---{oMpostrIoN, NoMEN- and hence are not found in "simple" pe9ma- DeMark,5509Dodd PlaceNE, Al'buquerque, CLATURE,AND OF AND LEP. GENESIS MUSCOVITE tites, but occur in zones that were subiect to NM 87110(6) IDOLITE MICAS FROM PEGMATITESAND hydrothermal alteration during crystallization METAMORPHICROCKS IN NORTHERNNgW MEXCO Recent mineral discoveries in the Cuchillo and cooling in "complex" zoned pegmatites. AND CENTRALCoLoRADo, by Peter Modreski, Negro mining district indicate a virtually un- l. The hydrothermal, postmagmatic formation of U.S. Geological Survey, 25046, Federal tapped collecting area for New Mexico mineral Box Li-micas is evidence of the increasing concen- Center, MS Denver, 80225 enthusiasts. The Cuchillo Negro district is in 905, CO tration and chemical activity of Li, F, Mn, and Sierra County southeast of the small town of Red-tinted, lithium-bearing micas are prom- other chemical components in the evolving Winston. The district was mined extensivelyin inent and readily visible mineral components hydrothermal fluids.Other occurrences of red- the early 1900'sfor lead, copper and zinc; how- of pegmatites in some of the pegmatite districts dish-colored micas in New Mexico include ever, most prospectswere small operationsand in Precambrian rocks of New Mexico and Colo- sparse reported occurrences of rose muscovite thus mine workings generally are not very large. rado. Although pink or red colors are often in the Petaca pegmatite district, Rio Arriba Becauselittle has been known about this dis- characteristic of lithium-bearing micas, the County. Pale-violet to lilac-colored muscovite trict, few collectorshave been to the area.Thus, Presence of the lithium ion itself (Li*) has no near Pilat Taos County, is associated with pie- the variety and quality of minerals will come direct effect on the color, which is due to man- montite or with piemontite+thulite+vesu- as somewhat of a surprise to seasonedNew Sanese as the weakly colored Mn2* ion or the vianite + grossular in manganese-rich layers in Meico collectors.The mines are situatedat the more intensely colored Mn3*. Aside from peg- schist of the Picuris Range. This metamorphic contact between Pennsylvanian Magdalena matites, certain manganese-rich metamor- mica contains approximately 0.2-0.8 wt% MnO limestone and a Tertiary monzonite porphyry. phosed sedimentary rocks also contain pink- (MnzOg) and little or no lithium.The most Shafts and tunnels intercept the conlact-meta- or violet-colored micas.The following table of prominent occurrence of lepidolite in Colorado morphic ore deposits. Five major groups of lithium-bearing and related rnica species shows is the Brown Derby pegmatite in Gunnison mines in the district were investigated. These the ideal chemical formulas written to distin- County. However, Iepidolite is also known from included the Dictator, Covington, Vindicator, guish the octahedrally coordinated ions (Li*, the Meyers pegmatite near Royal Gorge, Fre- Black Knife, and Confidence mines. Mineral Mg2*, Mn2*, Fe2*, Al3*) from the tetrahedrally mont County; the Chief Lithia pegmatite near assemblagesat each mine are distinctive. At coordinated ions (Al3*, Sia*). Muscovite, lep- Texas Creek, also in Fremont County; the Bald the Dictator mine. superb discoidal willemite idolite, biotite, and zinnwaldite occur in gr-a- Mountain pegmatite east of Mount Evans in crystals to 7 mm across were found on the nitic pegmatites and in granites (zinnwaldite Clear Creek County; and the Kings Kanyon dumps in recrystallizedlimestone. The crystals does not appear to have been reported from pegmatite in the Crystal Mountain district, Lar- are colorless to yellow, highly lustrous, and New Mexico). In contrast, the higher-lithium imer County. There are a few other minor or often hansparent. The willemite fluorescesgreen mica, polylithionite, is found in alkaline ig- unverified reports of lepidolite in Colorado. In under short wave while the calcite fluoresces neous rocks (such as the nepheline syenite of addition to the "pink" micas referred to in the crimson red. Willemite crystals also are found Mont St-Hilaire, Quebec, and the phonolite at title of this paper, the brown, lithium- and iron- inside the main tunnel of the mine but aremore Point of Rocks, Colfax County, New Mexico). bearing mica, zinnwaldite, is common in the typical of the species, being gray, hexagonal Taeniolite (again not known from New Mexico) miarolitic pegmatites of the Pikes Peak bath- prisms about 1-2 mm long. Lustrous brown is likewise a mineral occurring in alkali syenites olith. Zinnwaldite resembles biotite, but is a crystals of descloiziteare also found imbedded rather than granitic pegmatites. The rare, man- lighter, reddish-brown color, especially in the in calciteon the dumps. Additional minerals ganese-rich lithium mica, masutomilite, is in"terior of color-zoned crystals. n found aremottramite, vanadinite, smithsonite, known from only four localities worldwide (two cerussite, wulfenite, azurite, malachite, ga- in fapan, associated with topaz, schorl, and in lena, and sphalerite. No noteworthy mineial one case/ cassiterite, plus Czechoslovakia and specimenswere found at the Covington group Idaho); no micas fromNew Mexico pegmatites of mines about 0.5 km west of the Dictator are rich enough in manganese to be classed as mine. The Vindicator mine at the northern end masutomilite. The lithium-bearing micas com- of the SierraCuchillo range produced very aes- monly show high-fluorine contents, in contrast thetic fluorite specimens.Grass-green octahed- to the low-fluorine contents of common mus- rons to 2 cm, often coated with a thin crust of covite and biotite in most igneous and meta- white adularia crvstals, were collected from two morphic rocks. large pocketsin fhe main inclined shaft. Small, Ideal water-clear, scepteredquartz crystals along with Species IdealFormula wt% Li2O very small (0.5 mm) black crystals of mottram- ite occur in associationwith the fluorite. Calcite Muscovite KAl2[Alsi3]o1o(oH,F)' 0.00 scalenohedronsreplaced bv quartz and coated Lepidolite KLiAll.slAbssi3.sloro(4oH)z 3.77 with olive-green,earthy mbttramitealso occur Polylithionite KLirAllsi4lolo(F,oH), 7.65 here.The Black Knife mine Taeniolite KLiMgzISir]Oro(F,OH)z 3.69 about L km south + of the Vindicator Masutomilite KLiMn2 AlIAlSi3]O1o(F,OH)'3.42 mine and on the east side of + the rangeproduced unique specimensof calcite Zinnwaldite KLiFe2AllAlsi3lols(F,OH)z 3.47 Biotite + and fluorite. The scalenohedralcalcite crystals KFe2 dAlsi3lolo(oH,F)2 0.00

Naa Metico Geology May 199"1