Mineralparagenesis, structure, and "ore sh00t" geometry atthe U.S.Treasury mine, Chloride mining district, New Mexico byRichard W. Harrison, New Mexico Bureau of Minesand Mineral 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 gold 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 breccias 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 pyrite, hematite, and magnetite. Gold mineralization Harrison (1984).General geology 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 minerals.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 fracture-fill and replacement of silicified wall Treasury mine: an early Pb, Zn, Cu (Ag, Au) pulse (stage1) and a rock; 1b) medium-grainedchalcopyrite (galena, sphalerite, molyb- later Cu, Ag (Zn, Pb, Au) pulse (stage2; Harrisbn, 1986).These two denite) disseminationsin white quartz, 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;<t333i" the St. Cloud deposit revealsabout 1.4 wtVo Ag. Neal and Larson (1986)reported 0.7 wtVaAg in bornite and even more in chalcocite from the nearby Hoosiervein system.Skinner (1966) found that up FIGURE 1-Location maps of the Chloride mining district to 3.4 wt % of sllver can substitute in chalcociteat 63.3"C.Using 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 fault 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 FAULT SLIC(ENSIE SWM WITH qP ,f SMIATION OIECTId WIff MGE BEW 9 HORIZONTAL , MASSTVE,FTSSURE-F|LL|NG QUNIZ VE|N tM AREA OF GREAER IHAN 5orlrd Ae ptsoz/ronAu. to2/16Ag AREA OF GREffER THAN loor^n A9 W Otaoz/bntu-lo2/64 OD UST SHAf, (6@) AFrcXIMAT€ €LEVANON OF FLM o 50 tod =---- 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.