Mineralparagenesis, structure, and "ore sh00t" geometry atthe U.S.Treasury mine, Chloride district, New Mexico byRichard W. Harrison, New Mexico Bureau of Minesand Resources, Socono, NM 87801

Introduction TABLE l-Metal content vs. elevation for U.S. Treasury stage 1 minerali- zation. Data is tabulated "ore lower-grade f\9 U.S. Treasurymine is locatedin the southeasternportion from shoot" occurrences; miner- of alization (although showing the same metal ratios) is not included. the Chloride min_ingdistrict, SierraCounty, New Mexico(Fig. 1). A se-gmentof the St. Cloud-U.S. Treasury epithermal vein s stem, Ag Au which contains copper, lead, zinc, silver, and mineralization, Elevation (ft) CuTo PbTo ZnTo ozlton ozlton was exploitedby mining activity.The entire vein systemis approx- imately 7,700ft in strike length. Near the U.S. Treasurymine, the 6,800-7,050 0.80 0.05 0.05 J.I 0.01 vein generally trends N45'W, dips 65'SW, and is hosted by debris- 6,600-6,800 0.60 1.00 1.80 4.9 0.03 flow and other volcaniclastic deposits of the Eoiene-Oli- 6,300-6,600 0.80 1.20 2.r0 6.0 0.03 6,000-6,300 0.40 2.80 3.60 4.8 0.04 B!!ene, lower Rubio PeakFormation. A K-Ar radiometricage date of 26.5 -r 1.1 m.y. (late Oligocene)was obtainedfrom vein adularia in the St. Cloud portion of the vein system (Mike Bauman, First 'IABLE MississippiCorp., pers. comm. 1984). 2-Metal content vs. elevation for U.S. Treasury stage 2 minerali- The U.S. Treasurydeposit was discoveredand initially worked in zation; onlv mineralization from "ore shoot" is tabulated. the 1880'sand early 1890's;additional development and mining were done just before World War L Production from this early aitivity Ag Au was estimatedat 920,000by Harley (1934).In the early iSAO'sSi. Elevation (ft) Ct% PbVo ZnVo ozlton ozlton C_loud,MiningCompany (Goldfield Co.p. ) produced approximately 6,850-7,050 0.02 0.01 0.01 5.00 0.13 235,000ounces of silver and 4,000ounces of gold from about 60,000 6,700-6,850 0.52 0.14 0.37 8.80 0.08 tons of ore and developmentrock. -Previousgeologic reports on the vein systeminclude descriptions of the old U.S. Treasurymine by Harley (1934)and detailsof iecent mining activity and geologyat the St. Cloud mine by Freemanand amounts of , hematite, and magnetite. Gold mineralization Harrison (1984).General of the Chloride mining district was occurssporadically in both stages. described by Harrison (1986).Behr (1987)analyzed the-geochemical Mineralization stagesL and 2 are both contained within the U.S. aspectsof ore deposition for both St. Cloud and U.S. Treasuryde- Treasurydeposit, with two distinguishablesubstages of stage1 and posits;initial resultswere reported by Behr and Norman (1986). an additional late stage(stage 3) consistingof supergenesecondary .In parageneticorder, stagemineralogy present at the U.S. Mineral paragenesis Treasurymine are 1a)medium- to coarse-grainedgalena, sphalerite, Two pulses of sulfide mineralizationare recognizedat the U.S. and chalcopyrite as -fill and replacement of silicified wall Treasury mine: an early Pb, Zn, Cu (Ag, Au) pulse (stage1) and a ; 1b) medium-grainedchalcopyrite (galena, sphalerite, molyb- later Cu, Ag (Zn, Pb, Au) pulse (stage2; Harrisbn, 1986).These two denite) disseminationsin white , and pale-greenamorphous stages are notably lacking in iron mineralization, with only minor silica; very fine grained to medium-grainedbornite, stromeyerite (AgCuS), jalpaite (Cuo.*A9,.*S;sphalerite, galena) occurring as bands and disseminations;and late, secondarynative silver, copper,and I ltter gold with cuprite and copper carbonatesin localized areas.Gangue mineralogy for all stages is dominantly quartz with some calcite. A very late, massive, fracture-filling calcite cuts all other minerali- zation. Stage 1 mineralizationof the U.S. Treasurymine shows a large ,N variation in metal content with depth, but the same simple miner- alogy is found throughout. Table 1 illustrates the overall change in metal content, particularly displayed by base metals, for stage 1 o s ro 's )b-'i . mineralizationat the U.S. Treasury mine. Silver in stage1 miner- # alization occursboth as minor substitutionsfor copper in chalco- O 5lO 20 30km pyrite and as spotty occurrencesof light ruby silver (proustite, Ag.AsS.). Iron content in sphalerite from stage 1 mineralization was determinedbv x-rav analvsis;it is about 4-5%. Stage2 min"eralizitioni3 the main silver-bearingphase at the U.S. 56' Treasury mine as well as in other vein systemsin the southern portion of the Chloride mining district. Unfortunately, sampling of stage2 mineralization during mining activity or drill core exploration has taken placeonly above6,700 ft in elevationat the U.S. Treasury mine. From this depth to the surface,a moderate,vertical zonation =?," - in metal content is found (Table2). Z I'H:l'd [ai ".""" Stage 2 mineralizationin the southern portion of the Chloride .'G 5 Choride mining district is intriguing becausesilver occurs primarily as sub- "l stitutions for copper in the lattice of copper-bearingminerals and .*-'no'o'"?'o *"'*u in solid solution Cu-Ag sulfides.Chemical analysis of bornite from s.o 1;

February 1988 Nar MexicoGeology Skinner's maximum Ag value, it is possible for chalcociteto contain Footwoll almost 600 ozlton Ag. This explains the occurrence of pockets of high-grade silver mineralization at the U.S. Treasury mine and St. Disseminotions Cloud deposits that contain no primary silver sulfides. Skinner (1966) B slringers Grodofionol also indicated that a solid solution seriesexists from CurS(chalcocite) confocl SiogelAwithquortz to CunqAgr*S below 93.3'C. falpaite (CunouAg,uuS)was tentatively Strioted identified by Robert North (pers. comm. 1981)and probably formed SfogelBwithquortz foulfslip by breakdown of Cuo at 94.4"Cto jalpaite and Cun as breccioclosfs ,S *Ag, *S of lA described further by Skinner"Ag, (1966). Strioted Bondsof stoge2, Mineralizationtextures vary greatlythroughout the U.S. Treasury quortz,colcite holoing foullslip deposit. Thesetextures include wide, open-space,fissure-filling quartz breccioclosts and calciteveins with minor sulfides; thin, massive-sulfidestringers; of lAond lB Foulf-gouge banded and crustiform fine-grained sulfides alternating with quartz Crushedquortz I zone and calcite; and sulfide disseminations in wall rock as much as 100 stogesII 2 ft away from the main vein structure. In places, post-mineralization faulting has produced a crushed texture of sugary quartz and sul- Striofed fides. A generalized cross section through the vein structure on the Disseminotions foult-gouge Iower level of the U.S. Treasury mine (Fig. 2) shows the various B sfringers Hongingwoll mineralization textures, as well as relationshipsbetween paragenetic level stagesand development. FIGURE 2-Generalized cross section through vein structure on lower of U.S. Treasury mine showing various mineralization textures. Note the "Ore grade" mineralization is a difficult term to define becauseof overall development of fault structure from footwall to hanging wall depicted changingmineral economics and uncertaintiesin costfactors in min- by paragenetic sequence. ing. Therefore,rather than showing locationsof "ore," the plan map of the U.S. Treasurymine (Fig. 3) highlightsareas of mineralization >5 ozlton Ag equivalent(.015 ozlton Au equalsI ozltonAg). Note that such mineralization occurs not only within the massive quartz Continued on page 15

LEGEND

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FIGURE 3-Plan map of U.S. Treasury mine. Note that areasof anomalous mineralization occur within massivequartz vein, as well as within both the hanging wall and footwall. Note ilso, location of north-south fractures and prominent north-south trend of massive quartz on the lower level of the mine. Crosssection of Figure 2 is locatedjust eastof the shaft on the lower level. P, pillars.

Nao Mexico Ceology February 1988 Continued trom page 11 It was during this regime that the veins of the Chloride mining vein, but also as disseminations and stringers in both the hanging district formed, in coniunction with a thermal event related to re- wall and footwall. gional rhyolitic magmatism (Harrison, 7986). Qluartz veins and sil- Alteration accompanies mineralization at the U.S. Treasury mine icified structures in the area of the U.S. Treasury mine formed along and throughout the Chloride mining district. In general, rocks in dominant northwest-trending dip slip structures. The north-south the district are propylitically altered, particularly those rocks of in- fractures and vein segment acted as fundamental controls on min- termediate cornposition. This alteration assemblage is typically ep- eralization, as discussed later, and probably reflect buried basement idote, calcite, chlorite, and pyrite. In more felsic rocks, silicification fabric. The few north-northeast-trending structures in the area are and kaolinization are the dominant alteration products with epidote lower Tertiary strike-slip faults (Harrison, 1986) and, because they occurring as coatings on joints and other fractures. In the vicinity presented an'unfavorable direction for either wrenching or normal of the U.S. Treasurv deposit, some of the most intense alteration of iaulting during the period of late Oligocene mineralization, they are the entire district is forina. Within the U.S. Treasury vern sysrem, ^poorly mineralized. si-licification, chloritization, and potassic alteration are associated The domain of dominantly northwest-trending structures found with sulfide mineralization. X-ray analysis of chlorite from a hanging in the U.S. Treasury area is anomalous for the Chloride mining wall "ore" zone indicates that gonyerite [(Mn,Mg,Fe)oSioO,n(OH),]is district as a whole. Throughout the rest of the district north-south, the dominant chlorite species. This chlorite is strongly associated northeast, and east-west vein structures of similar age are dominant with high-grade silver and gold mineralization (select samples com- (Harrison, 1985). It is quite possible that the area of northwest- monly reach 50 ozlton Ag and 1.0 ozlton Au). trending structures around the U.S. Treasury mine overlies a buried pluton or cupola that has, in effect, cancelled the majority of pre- Strucfure existing basement fabric as discussed by Chapin in Aldrich et al. new U.S. Treasury fault structure shows pre-, syn-, and post-miner- (1986). The inferred pluton thus allows the development of the (i.e., alization movement. A progressive fault development from footwall structural fabric in iesponse to tectonic stresses northwest- directed to hanging wall is apparent for syn-mineralization movement (Fig. trending normal faults in resPonse to northeast-southwest rest 2). Offset of stratigraphic horizons indicates that the overall fault Ieast principal ). Correspondingly, veins throughout the structure is normal (hanging wall down) with approximately 150- of the Chloride mining district would probably represent reactiva- exists. 200 ft of dip separation. Overall strike direction of the U.S. Treasury tion of basement fabrics where no buried igneous body quartz vein is N46"W with an average dip of 65" to the southwest. Above the 300-ft level of the mine, sinuations from N30'W to N60'W "Ore shoot" geometry occur in the quartz vein. Below this level, a segment of the vein The U.S. Treasurv mineralized sYstem consists of a wide zone of bends to a nearly north-south direction (see Fig. 3). On the lower dominantly sub-economic, sulfide mineralization. Mineralized in- level of the mine, this north-south dogleg extends for about 100 ft tervals commonly exceed 60 ft of true horizontal thickness. Miner- along strike and appears to be increasing in length with depth. This alized rock was intercepted for 62 ft (horizontal) during deep diamond observed north-south trend coincides with the intersection of the drilling, 500 ft below the lowest mine level. It was as-sayedto contain vein and a set of fractures along the same trend present at the surface 0.7I7o Cu,0.107" Pb,0.37% 2n,0.92 ozltonAg, and 0.01 ozlton Au. and in the shallow levels of the mine. Within the mine, these frac- Occurring within this broad zone of low-grade mineralization are tures are observed mainly in the vein's footwall and are commonly areas of higher-grade material: "ore shoots." "Ore shoots" exist in filled with quartz and/or epidote. They are vertical to steeply dip- virtually all epithermal vein deposits (Buchanan, 1981) and are largely ping, hairline to 0.S-inch wide, and cover a zone approximately 50 structuially Controlled phenomena. In theory, "ore shoots" rePresent ft in width. There is no indication of movement along their surfaces. areas of high permeability tnat accommodate the flow of _deeply Figure 4 is a compass-rose diagram of surface fracture attitudes in circulating hydrothermal fluids, and thus, are the sites of major the footwall of the U.S. Treasury structure. mineral deposition. A component of left-lateral oblique movement is suggested by "Ore shoot" geometry at the U.S. Treasury deposit reflects the numerous surfaces occurring across the width of the paragenetic sequence of mineral stages in combination with struc- structure. Striations on arJabundant and are domi- tural controls on location of mineralization. Figure 5 shows the geo- nantly low-angle oblique, from 6-20'to the southeast in the plane metric orientation of "ore shoots" for the U.S. Treasury deposit in of the vein. Some horizontally striated, flat-lying slickensides and a longitudinal section. This "ore shoot" geometry closely approximates few higher-angle oblique slickensides, about 55" to the southwest, that of closed-cell convection systems described by Berger and Eimon have been observed along hanging wall areas in the structure. Most (1982). In their closed-cell model, no permeability barriers exist to of the slickenside surfaces found in the U.S. Treasury fault are def- restrict the upward movement of hydrothermal fluids and, therefore, initely post-mineralization in origin; however, the higher-angle striated near-vertical "ore shoots" develop. surfaces appear to be remnants of syn-mineralization movement. In Figure 5, the first "ore shoots" to form were the stage l_-footwall During the late Oligocene, northwest-trending extensional struc- shoot centered on the lower end of the U.S. Treasury shaft and the tures of the southern Rio Grande developed in response to re- stage l-hanging wall quartz vein. The hanging wall quartz. vein gional northeast-southwest least principal stress direction (Seager partially overlies the footwall shoot in longitudinal section' Struc- et a1.,1984; Lipman, 1981; Eaton, 1.979;Chapin and Seager, 1975). iural controls for the stage 1-footwall shoot consist of a near-vertical, concave-to-hanging wall flexure in the upper portion of the shoot and a structure that parallels the north-south fracture intersection in the lower portion of the shoot. Remnants of the stage 1-footwall shoot are found above the line of north-south intersection as clasts haloed by stage 2-quartz vein mineralization. The stage 1-hanging wall quartz vein is a structure that bifurcates near the western margin of the main U.S. Treasury vein on the lower level of the mine (see Fig. 3). It contains an "ore shoot" that sub- parallels the north-south fracture intersection between elevations 6,450 and 6,600 ft and merges upward into a near-vertical stage 2- hanging wall stringer "ore shoot" that is superimposed upon it Along the western margin of the U.S. Treasury vein is a. narrow "ore shoot" that parallels the line of north-south fracture intersec- 270" 900 tion. This shoot iontains footwall disseminations of stage 1 miner- FIGURE 4-Compass rose diagram of surface fractures in footwall of U.S. alization and is anomalously high in gold content (in excess of 0.15 Treasury structure. ozlton). This shoot is cut by development drifting, but its full extent

Nm MexicoGeology February 1988 along the southwestern margin of the rift). Second, the U.S. Treasury deposit consists of multiple pulses of mineralization in a structurally complex setting. And finally, structural features control epithermal min- eral deposition, and once recognized, they can be used to predict trends in "ore shoot" geometry.At the U.S. Treasurydeposit, in- tersection and relative movement between the northwest-trending fault structure and north-south fractures are the primary con- trols on locationof mineralization.In the 2,300 ft betweenthe U.S. Treasurydeposit and the St. Cloud depositon the samevein structure, a minimum of three relatively unexplored structural intersections occur, all of which show anomalously high surfacevalues of sil- ver and gold. AcrNowlrocurxrs-This paper is dedi- catedto Ed James,Sr., his early co-workers, and the many miners who toiled at the U.S. Treasury mine. Thanks to W. R. Seagerand G. R. Osburn for reviewing this manuscript and to St. Cloud Mining Co. for permission to publish this work about their property. The views and interpretations presented are those of the author and do not necessarily representthose of the St. Cloud Mining Co. References Aldrich, M. 1., Jr., Chapin, C. E., and Laughlin,A. W., 1986, Stress history and tectonic development of the Rio Grande rift, New Mexico:Journal of Geophysical Research,v.97, pp. 6199-6211.. Behr, C. 8., 1987,Geochemical analyses of epithermal ore fluid-St. Cloud-U.S. Treasuryvein system, Siena County, New Mexrco: unpublished M.S. thesis, New Mexico Institute of Mining and Technology,Socorro. Behr, C. B., and Norman, D. 1., 1987,Geochemical anal- yses of epithermal ore fluids-St. Cloud-U.S. Treasury vein system, Chloride district, Siena County, New Mexico (abs.):Geological Society of America, Abstracts with Programs,v 18, no. 5, p. 538. Berger,B. R., and Eimon, P I., 1982,Comparative models of epithermal silver-gold deposits: Society of Mining Engineers,AIME Preprint 82-13, 14 pp. Buchanan,L. J.,1987,Precious metal deposits associated with volcanic environments in the southwest:Arizona FIGURE S-Longitudinal section of U.S. Treasury vein showing relationships between "ore shoots" GeologicalSociety Digest, v.74, pp.237-262. and structural features. "Ore shoot" geometry above 6,700 ft elevation is based on mine sampling and Chapin, C. E., and Seager,W. R.,1975,Evolution of the mapping; below 6,700 ft, geometry is interpreted from drill core data. Rio Grande rift in the Socorro and Las Cruces areas: New Mexico Geological Society, Guidebook to 26th Field is unknown becausethe overlying surfaceis fractures)is necessaryto open spacealong Conference, pp. 297-32'1.. poorly exposed,and the shoot is not pene- these vertically oriented flexures. Eaton, G. P., 1.979,A plate-tectonicmodel for late Ce- nozoic crustal spreading in the western United States; trated by exploration drilling. Total vertical extent of the stage 2 quartz ln Riecker, R. E. (ed.), Rio Grande rift- and By far, the bulk of past mining activity at vein "ore shoot" is unknown due to lack of magmatism: American Geophysical Union, Washing- the U.S. Treasurymine hasoccurred on "ore drill core data along its trend to depth. At ton, D.C., pp.7-32. shoots" of stage 2 mineralization. Geometry the nearby St. Cloud deposit, stage2 min- Freeman, P S., and Hanison, R. W., 1984,Geology and development of the St. Cloud silver deposit, Siena of these shoots is strongly controlled by the eralizationextends to a depth of 6,100ft el- Coun$, New Mexico:Arizona GeologicaiSociety Di- intersection of north-south fractures as seen evation. Becauseepithermal systems tend to gest,v. 15, pp.237-233. in Fig. 5. Not only does the stage 2-juartz maintain upper and lower levelsto "ore ho- Harley, G. T., 1934,The geologyand ore depositsof Siena vein shoot lie adjacent to and parallel to the rizons" within individual hydrologic areas, County, New Mexico: New Mexico Bureau of Mines intersection, and Mineral Resources,Bulletin 10, 220 pp. but stage2 "ore shoots"in the it is reasonable to expect a similar bottom Hanison, R. W, 1985,General geology of Chloride min- upper portions of the U.S. Treasurydeposit depth at the U.S. Tr-easurydeposit along ing district, Sierra and Catron Counties, New Mexico: display numerous margins perpendicular to structurallycontrolled "ore shoots." New Mexico Geological Society,Guidebook to 37th Field the north-south intersection. Such margins Conference, pp. 255-272. are interpreted Summary Lipman, P. W., 7987,Volcano-tectonic setting of Tertiary to indicate control on open- ore deposits, southern RockyMountains: Arizona Geo- spacedevelopment by movement parallel to There are three main points in this paper logical Digest, v. 14, pp. 199-213. the north-south fractures. To some degree, that are worth re-emphasizing. First, signif- Neal, W S., and Larson,P. 8.,1985, Mineral and fluid small near-vertical flexures appear to control icant base-and precious-metalresources are geochemistryof the Hoosier vein, Chloride mining dis- stage2 "ore shoot" trict, New Mexico (abs.):Geological Sooery of America, geometryaround the old related to tectonicprocesses associated with Abstractswith Programs,v 18, no. 6, p.703. U.S. Treasury100-ft level (seeFig. 5). In par- the Rio Grande rift in southwestern New Seager,W. R., Shafiqullah,M., Hawley,j. H., and Mar- ticular, concave-to-hangingwall flexures Mexico.The U.S. Treasurvdeposit and other vin, R. F., 1984,New K-Ar dates from basaltsand the contain higher-grade mineralization than epithermalmineral depoiits in the Chloride evolution of the southern Rio Grande rift: Geoloeical convex-to-hanging wall flexures. A compo- mining district represent Societyof America Bulletin,v. 95, pp. 87-99. some of these re- Skinner, B. 1., 1966, The system Cu-Ag-S: Economic nent of oblique-slip (along the north-south sources(as do perhapsother similardeposits Geology, v. 67, pp. 1.26. tr

February 7988 Nao MexicoGeology