The Effects of K-Metasomatism on the Mineralogy and Geochemistry of Silicic Ignimbrites Near Socorro, New Mexico

Chemical Geology 167Ž. 2000 285±312 www.elsevier.comrlocaterchemgeo

The effects of K-metasomatism on the mineralogy and geochemistry of silicic ignimbrites near Socorro, New Mexico

D.J. Ennis a,b, N.W. Dunbar a,c,), A.R. Campbell a, C.E. Chapin c a Department of Earth and EnÕironmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA b RAMCO EnÕironmental 2065-G Sperry AÕe., Ventura, CA 93003, USA c New Mexico Bureau of Mines and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA Received 8 December 1998; accepted 9 November 1999

Abstract

K-metasomatism of the upper Lemitar and Hells Mesa silicic ignimbrites near Socorro, New Mexico is thought to be the result of downward percolation of alkaline, saline brines in a hydrologically closed basinw Chapin, C.E., Lindley, J.I., 1986. Potassium metasomatism of igneous and sedimentary rocks in detachment terranes and other sedimentary basins: economic implications. Arizona Geological Society Digest, XVI, 118±126.x . During the chemical changes associated with metasomatism, Na-rich phases, primarily plagioclase, are replaced by secondary mineral phases. Adularia, a low-temperature K-feldspar, is the dominant mineral formed during K-metasomatic alteration, but mixed-layer IrS, discrete smectite, and kaolinite can also be present in the assemblage. The formation of discrete smectite within the assemblage during K-metasomatism may have occurred during periods of low cationrHq ratios in the alkaline, saline brine. Upon an increase in the cationrHq ratio, such as could occur during evaporation of the alkaline lake, the solution may have become sufficiently concentrated to cause illitization of smectite resulting in the formation of mixed-layer IrS within the assemblage. Distribution of phases in the alteration assemblage strongly suggests a dissolution±precipitation reaction for K-metasomatism in the Socorro area as indicated by the presence of dissolution embayments in plagioclase crystals, the presence of euhedral adularia, and the common occurrence of authigenic clay minerals in the assemblage. K-metasomatism causes significant chemical modification of the silicic ignimbrite, particularly increases in K2 O and Rb and depletion of Na2 O and Sr with increasing adularia abundance. The correlation between Rb and K2 O suggests that Rb is enriched during alteration due to substitution for K in adularia. The effect of hydrothermal activity, either prior to, or following metasomatism, is also observed in some samples, as shown by high concentrations of elements such as Ba, As, Sb, Pb and Cs. Enrichment of middle and HREE in the upper Lemitar Tuff samples and depletion of middle and HREE in Hells Mesa Tuff samples suggests attempted re-equilibration between the secondary alteration assemblage and the metasomatizing fluid. Preliminary data indicate clay minerals within the secondary assemblage may have played an important role in the incorporation of REE during redistribution. q 2000 Elsevier Science B.V. All rights reserved.

1. Introduction

) Corresponding author. A number of areas of the western United States, E-mail address: [email protected]Ž. N.W. Dunbar . particularly areas of basin and range extension, are

0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0009-2541Ž. 99 00223-5 286 D.J. Ennis et al.rChemical Geology 167() 2000 285±312 characterized by rocks that have been secondarily to youngest, of five silicic ignimbrites interbedded enriched in potassiumŽ.Ž K Duval, 1990 . . Many of with mafic lava flows: Hells Mesa, La Jencia, Vicks these areas, including the Socorro, New Mexico Peak, Lemitar, and South Canyon Tuffs. Together, area, are hypothesized to be caused by K-metasoma- the stratigraphic thickness of the tuffs is approxi- tismŽ. Chapin and Lindley, 1986 , a secondary alter- mately 500 m in the Socorro area, and the ages range ation process caused by interaction between a K-rich from Oligocene to early MioceneŽ Chapin and Lind- fluid and the host rock. During K-metasomatism ley, 1986. . Both the upper Lemitar and Hells Mesa some elements, such as K, become concentrated in Tuffs are present both within and outside the zone of the altered rock, whereas other elements become K-metasomatic alteration thereby allowing for the depleted. In some cases, the altered rocks are signifi- sampling and examination of both altered and unal- cantly enriched in elements of economic interest tered rocks. The volcanic stratigraphy in the Socorro Ž.Chapin and Lindley, 1986; Dunbar et al., 1994 . area is extensively faulted and offset. As a result of Therefore, it is important to understand the process this structural complexity, the Hells Mesa and Lemi- by which K-metasomatism occurs, and the origin of tar Tuffs are present at a variety of elevations, and the chemical and mineralogical alteration. The fluids during an alteration event would have been exposed that cause K-metasomatism may be derived from a to a range of fluid compositions. Variable degrees of number of sources including; magmaticŽ Shafiqual- alteration are also present within each of the other lah et al., 1976; Rehrig et al., 1980. ; hydrogen ignimbrite units. metasomatized basement rocks from deep within a Based on a number of factors, Chapin and Lind- metamorphic complexŽ. Glazner, 1988 ; or from leyŽ. 1986 interpret the Socorro K-anomaly to be the low-temperature alkaline, saline aqueous systems result of widespread alteration by alkaline, saline within a hydrologically closed basinsŽ Chapin and water derived from a large playa lake. First, the large Lindley, 1986; Turner and Fishman, 1991; Leising et aerial extent affected by K-alteration is difficult to al., 1995. . explain as a result of hydrothermal alteration. Al- The aim of this study is to investigate the miner- though localized areas of hydrothermal activity are alogical and chemical alteration within two rhyolitic known to exist in the Socorro K-anomaly, a mag- ignimbrite sheets, the upper Lemitar and Hells Mesa matic system of the magnitude necessary to cause Tuffs, as a function of intermediate to advanced such a large alteration is not known to have existed degrees of K-metasomatism. Specifically, the objec- in the time frame when K-metasomatism was thought tive is to study the mobility of major and trace to be active. Based on 40Arr39Ar radiometric dating, elements with respect to the alteration mineral as- K-metasomatism is confirmed to have been active semblage formed during K-metasomatism. between 8.7 and 7.4 Ma, but possibly started as early as 15 MaŽ. Dunbar et al., 1994, 1996 . In addition, 1.1. Background the K-metasomatized rocks in the Socorro area are Many areas that have undergone regional crustal enriched in d18 OŽ Chapin and Lindley, 1986; Dunbar extension show evidence of K-metasomatism of vol- et al., 1994. , which is not indicative of fluids associ- canic and sedimentary rocks along regional, low-an- ated with a high-temperature magmatic system. gle normal faults or `detachment faults'Ž Davis et al., Lastly, the presence of thick playa deposits in the 1986. . Evidence for K-metasomatism associated with Socorro area suggest the occurrence of a long-lived regional extension is reported by BrooksŽ. 1986 at evaporative lacustrine environment, possibly of Picacho Peak, Roddy et al.Ž. 1988 in the Harcuvar evolving alkaline±saline waters. Once the closed Mountains in Arizona, and by GlaznerŽ. 1988 in the lacustrine environment was established, the metaso- Sleeping Beauty area, Central Mojave Desert. matizing fluids may have been transported by verti- The K-anomaly near Socorro, New Mexico is cal advection caused by salinity-induced density located in the northeast corner of the Mogollon±Datil stratificationŽ. Leising et al., 1995 . A similar mecha- volcanic field and is approximately L-shaped with an nism was invoked for K-alteration of tuffaceous area of roughly 40±50 km on a side and 20 km in rocks in the Green River Formation of Wyoming widthŽ. Fig. 1 . The area contains a sequence, oldest Ž.Surdam and Parker, 1972 . D.J. Ennis et al.rChemical Geology 167() 2000 285±312 287

Fig. 1. Map of the Socorro K-metasomatized area after LindleyŽ. 1979 . Samples numbered with a hyphen are inclusive, i.e., 112 through 121. Stippled portions of the map represent mountains, while the white portions represent alluvial basins with limited outcrop.

The alteration process involves the selective re- Lindley, 1986. . Increases in Rb and Ba along with placement of plagioclase and other K-poor phases to decreases in Sr, MnO and MgO are also recognized produce mineral assemblages of K-feldspar Ž.Chapin and Lindley, 1986 . Ž.adularia qhematiteqquartz"clay minerals Ž Lin- Several localized hydrothermal systems overprint dley, 1985. . Rocks affected by K-metasomatism the Socorro K-metasomatic alteration including the commonly have a characteristic red±brown color due Luis Lopez manganese districtŽ Northern Chupadera to hematite from the oxidation of groundmass mag- mountains. , Socorro Peak district, and the Lemitar netiteŽ. Lindley, 1985 . K2 O contents are as high as and Magdalena Mountains districts. Psilomelane is 13.5 wt.%, whereas Na2 O, CaO and MgO are typi- the primary manganese phase in the Luis Lopez cally each depleted to 1 wt.% or lessŽ Chapin and district and is typically associated with gangue min- 288 D.J. Ennis et al.rChemical Geology 167() 2000 285±312 erals of calcite, hematite, and quartz with minor al., 1978. consisting of two members. The upper barite and fluoriteŽ. Norman et al., 1983 . Structural member of the Lemitar Tuff is particularly crystal- evidence suggests the manganese was deposited be- rich and contains abundant plagioclase, a phase that tween 7 and 3 MaŽ Chamberlin, 1980; Eggleston et is strongly altered during metasomatism. The upper al., 1983. as open-space fillings in fault breccias and Lemitar Tuff contains 30±35% phenocrysts and pla- fault openingsŽ. Norman et al., 1983 . The hydrother- gioclase averages between 10% and 15% of the total mal alteration associated with the Luis Lopez man- phenocrysts presentŽ. Osburn, 1978 . The basal por- ganese district is younger than the K-metasomatic tion of the upper Lemitar contains between 5% to event and is superimposed on the regional K-anomaly 10% plagioclase phenocrysts. Quartz and biotite Ž.Eggleston et al., 1983; Norman et al., 1983 . The along with minor amounts of magnetite, clinopyrox- host rocks in the northern portion of the Luis Lopez ene, sphene, and zircon are also found within the manganese district are the upper and lower members upper memberŽ. D'Andrea, 1981 . of the Lemitar Tuff, while the Hells Mesa Tuff is the The Hells Mesa Tuff was emplaced at 32.06" host rock in the southern portion of the district 0.10 MaŽ. McIntosh et al., 1990 . It is a crystal-rich, Ž.Eggleston et al., 1983 . densely welded, simple cooling unit containing 35% to 45% phenocrysts. Sanidine represents 10% to 30% 1.1.1. Lemitar and Hells Mesa Tuffs of the total phenocryst percentage. Plagioclase aver- The Lemitar Tuff was emplaced at 28"0.08 Ma ages between 8% and 30% of the total phenocrysts Ž.McIntosh et al., 1990 and is a multiple-flow, com- presentŽ. Osburn, 1978 . Sub-angular quartz along positionally zoned, rhyolitic ignimbriteŽ Chapin et with minor to trace quantities of biotite, clinopyrox-

Table 1 Relative percentages of minerals in the alteration assemblage Percentages are estimated using ratios of peak area from XRD patterns. Smssmectite, Mxsmixed layer illite±smectite, Isillite, Kskaolinite, NrAsnot applicable due to absence of clay. `Not determined' indicates that clay is present in the alteration assemblage however, semi-quantitative analysis was not able to be performed on that sample due to flocculation. Sample a % % % ClayŽ. parts per 10 Sample a % % % Clay Ž. parts per 10 Adularia Plagioclase Clay Adularia Plagioclase Clay KM-36a 67 0 33 Mxs6, Sms3, Is1 KM-113 50 0 50 Ks10 KM-41 78 11 11 Ks10 KM-114 75 0 25 Ks10 KM-46a 33 67 0 NrA KM-115 67 0 33 Ks10 KM-51a 50 0 50 Sms10 KM-116 50 25 25 Ks9, Mxs1 KM-54 50 0 50 Mxs8, Sms1, Ks1 KM-127 86 0 14 Ks10 KM-55 89 0 11 Mxs9, Is1 KM-128 67 0 33 Ks7, Mxs2, Sms1 KM-56 91 0 9 Mxs10 KM-129 50 50 0 NrA KM-58 67 0 33 Ks9, Mxs1 KM-131 25 50 25 not determined KM-59 50 0 50 Ks10 KM-144 89 0 11 Mxs7, Ks2, Sms1 KM-60 29 14 57 Ks9, Is1 KM-148 20 80 0 NrA KM-61a 25 25 50 Ks7, Sms3 KM-149 33 67 0 NrA KM-86 75 0 25 Ks9, Mxs1 KM-150 25 75 0 NrA KM-90 83 0 17 not determined KM-153 100 0 0 NrA KM-91 29 14 57 not determined KM-154 75 0 25 Mxs7, Is3 KM-94 33 0 67 Mxs7, Is2, Sms1 KM-155 33 0 67 Is5, Mxs4, Ks1 KM-95 86 0 14 Ks7, Mxs3 KM-156 100 0 0 NrA KM-96a 50 0 50 Mxs6, Is4 KM-157 12 38 50 Ks6, Sms4 KM-102 67 0 33 Mxs7, Is3 KM-158 57 29 14 Ks6, Sms4 KM-107 80 20 0 NrA KM-159 83 0 17 Is5, Mxs5 KM-111a 55 27 18 Sms9, Mxs1 KM-160 75 0 25 Is5, Mxs5 KM-112 75 0 25 Ks10

a Indicates samples examined by SEM. D.J. Ennis et al.rChemical Geology 167() 2000 285±312 289 ene, sphene, and lithic fragments are also present gree of alteration from nearly pristine to highly within the Hells Mesa TuffŽ. D'Andrea, 1981 . metasomatized. When feasible, altered plagioclase Although the geochemical composition of neither was carefully hand-picked or drilled out from the tuff has been investigated in detail, both appears to interior of former plagioclase phenocrysts using a exhibit some compositional zonation. The Hells Mesa Foredom hand drill in order to directly evaluate the tuff ranges from rhyolitic to quartz latitic composi- mineralogy produced during K-metasomatism. Four tion and is reversibly zoned with a more mafic, unmetasomatized samples, two from each tuff unit, quartz-poor base grading into a more silicic, quartz- were examined in thin section in order to assess the rich upper portionŽ. Chapin et al., 1978 . There also degree to which plagioclase has been altered through appears to be some compositional zonation within processes other than K-metasomatism, such as the upper member of the Lemitar Tuff, resulting in a weathering and diagenesis. Altered plagioclase sepa- distinct chemical gradation, particularly within the rates were obtained from 41 whole rock samples. major elements. As will be discussed later in the The removed, altered plagioclase material is here- paper, the major element composition of the two after referred to as `separated samples'. This material tuffs are very similar, although the trace elements, was finely ground with an agate mortar and pestle particular the rare earth elements, are distinctly dif- for X-ray diffraction analysis. Whole rock samples ferent. were crushed in a jawcrusher to approximately peb- ble sized grains then milled in a TEMA ring and 1.2. Sampling and analytical methods puck-type grinder with a tungsten±carbide inner con- tainer to -200 mesh. 1.2.1. Sampling and sample preparation The Hells Mesa and upper Lemitar Tuffs were sampled at a number of locations across the K- 1.2.2. Mineralogy anomalyŽ. Fig. 1 . Some areas within the K-anomaly, Mineralogy of separated samples was determined however, could not be sampled due to basin fill. on a Rigaku DMAX-I X-ray diffractometer with Sixty-three samples were collected, ranging in de- Ni-filtered CuKa Ž.s1.5418 AÊ radiation and JADE

Fig. 2. SEM photo of partially dissolved plagioclase with adularia and quartz. XRD pattern shows the same assemblage; plagioclaseŽ. P at ;21.98 2u, adulariaŽ. A at ;13.88 and ;15.18 2u, and quartz Ž. Q at ;20.88 2u. Overlap of plagioclase and adularia at ;23.58 2u. 290 D.J. Ennis et al.rChemical Geology 167() 2000 285±312 pattern-processing software. The receiving and scat- sent, the area of the ;22.58 adularia peak is then tering slits of the Rigaku XRD are, respectively, multiplied by the average area factor, resulting in an 1r28 and 48. An initial 2u angle of 28 and a final estimated relative abundance of adularia present in angle of 708 was used with a goniometer step size of the sample. The separated samples were ranked for 0.058 2u and count time of 1 s per step. Operating adularia content based on the calculated or measured conditions were set to an accelerating voltage of 40 area of adularia. Although this method is only semi- kV and a sample current of 25 mA. For XRD quantitative, it appears to provide a reasonable first analysis, altered plagioclase was placed in an alu- approximation to the relative proportions of phases minum sample holder. present within the separated samples. XRD analysis was used to estimate the relative Where applicable, semi-quantitative clay mineral proportions of mineral phases in separated samples, analysis was performed on separated samples using particularly adularia abundance. For each sample three scans for each sample: air-dried, saturated with examined, the same volume of separated plagioclase ethylene glycol, and heated at 3758C for 30 min. The material was used, thus allowing for comparison of method utilized for semi-quantitative clay mineral the peak areas measured by JADE pattern-processing analysis allows for the determination of clay miner- software. If plagioclase is absent in the separated als to parts in ten of the clay fractionŽ G. Austin, sample, the amount of adularia is estimated based on personal communication, 1992; Austin and Leininger, the area of the peak at ;23.58 2u. For samples in 1976. . which plagioclase is present in the assemblage, an Morphology of the alteration products was exam- average area factor for adularia is first calculated. ined using scanning electron microscopyŽ. SEM The average area factor is calculated by averaging analysis of six carbon-coated sample chips. The SEM the ratios of the areas under the ;23.58 peak used was a Hitachi S450 equipped with an energy Ž.adularia " plagioclase and the ; 22.58 peak dispersive spectrometerŽ. EDS . Ž.adularia only for each separated sample XRD scan that does not contain plagioclase within the sample 1.2.3. Geochemistry set. An average area factor of 4.48 was calculated for Two carbon-coated polished thin sections were this study. For samples in which plagioclase is pre- examined using a JEOL 733 electron microprobe

Fig. 3. SEM photo of adularia and kaolinite books. XRD pattern shows kaolinite at characteristic 2u angle of 12.48;Kskaolinite, Csclay 021 band, Qsquartz, Asadularia. D.J. Ennis et al.rChemical Geology 167() 2000 285±312 291

Fig. 4. SEM photo of mixed-layer IrS and euhedral adularia. XRD pattern shows the same assemblage; Mxsmixed-layer IrS, Asadularia, Csgeneral clay peak, Qsquartz.

Ž.EMP equipped with five wavelength dispersive a count time of approximately 45 s. Element distribu- spectrometersŽ. WDS , one EDS, and a back-scattered tion maps were also obtained. electron detector at the University of New Mexico. X-ray fluorescenceŽ. XRF analysis was performed Operating conditions for the EMP during analyses following the procedure of Norrish and Chappell were an accelerating voltage of 15 kV, a beam Ž.1977 using a Rigaku 3062 instrument at both the diameter of 1±10 mm, a beam current of 20 nA and New Mexico Institute of Mining and Technology

Fig. 5. SEM photo of euhedral adularia and smectite. XRD pattern shows a distinctive smectite peak at approximately 6.08 2u; Smssmectite, Cssmectite 021 band, Qsquartz, Asadularia. 292 D.J. Ennis et al.rChemical Geology 167() 2000 285±312 xtures within the main Ž S with element maps for silica, potassium, and calcium higher brightness indicates greater r mixed-layer I q . embayment as well as the presence of clay in direct association with plagioclase. concentrations . Plucked areas are due to sample preparation. Image at left is an enlargement of the plagioclase embayment. Note small dissolution te Fig. 6. Electron microprobe back-scattered image for adularia D.J. Ennis et al.rChemical Geology 167() 2000 285±312 293

X-ray Fluorescence Facility and the University of tron activation analysisŽ. INAA of whole rock sam- New Mexico. XRF was used to analyze major ele- ples was performed at X-ray Assay Laboratories. ments, along with trace elements Rb, Sr, Y, Zr, Nb, INAA was performed on separated samples at New Pb, Th, and U. Operating conditions were 40 kV and Mexico Institute of Mining and Technology after 25 mA at both locations, and data reduction was having undergone irradiation at the University of accomplished following the method of Norrish and Missouri Research Reactor. Irradiation took place for ChappellŽ. 1977 . Incident X-rays were generated approximately 40 h at a flux of 2.5=1013 NPcmy2 using a rhodium end-window tube. Samples were Psy1. Analytical errors, based on replicate analysis fused into disks for major element analysis using a of samples, are as follows for major elementsŽ in " " lithium tetraborate flux, and were prepared as pressed wt.%. : SiO22230.16 wt.%, TiO 0.01 wt.%, Al O " " " pellets for trace element analysis. Instrumental neu- 0.03 wt.%, Fe23 O 0.01 wt.%, MnO 0.04

Table 2 Unaltered upper Lemitar and Hells Mesa geochemistry nrsNot reported, oxides in wt.%, elements in ppm. Hells Mesa Tuff samples Upper Lemitar Tuff samples Sample H.M. 78-7-75 84-10-28 19-D LJ4 Lemitar 78-6-48 78-6-61 78-6-62 78-6-63 84-5-4 540B TS-23 Tuff

SiO2 75.46 69.50 73.90 69.84 69.34 71.34 65.98 69.07 67.60 68.67 73.60 65.3 71.2 TiO2 0.21 0.33 0.25 0.37 0.22 0.50 0.49 0.40 0.56 0.42 0.27 0.65 0.05

Al23 O nr 14.83 13.50 15.12 14.75 nr 16.23 15.55 16.36 15.38 14.08 16.2 14.2 Fe23 O 1.44 1.56 1.52 2.26 2.45 2.52 1.93 1.74 2.40 1.95 1.42 3.14 2.55 MnO 0.03 0.06 0.05 0.04 0.03 0.05 0.03 0.07 0.07 0.06 0.02 0.12 0.07 MgO 0.44 0.33 0.43 0.72 0.65 0.06 0.37 0.40 0.53 0.55 0.11 0.76 0.54 CaO nr 0.83 0.90 1.84 1.63 nr 1.17 1.09 1.66 1.31 0.55 1.88 1.62

Na2 O 3.47 4.24 3.73 3.90 3.82 3.93 4.25 4.51 4.75 4.30 3.58 4.33 4.2 K2 O 4.76 5.54 4.56 4.47 4.40 4.80 5.58 5.58 5.38 5.11 5.54 5.05 4.62 Zn 24 nr 30.7 nr nr 75 nr nr nr nr nr As 2 nr 1.4 nr nr 1 nr nr nr nr nr Rb 195 169 193 144 169 157 124 74 77 93 nr 206 159 Sr 180 178 235 384 249 248 247 nr nr nr nr 361 239 Y18711127348041786080nr7671 Zr 137 286 142 219 219 363 350 352 483 325 nr Nb 23 30 23 26 nr 27 18 29 14 29 nr 24 27 Ba 524 nr 593 1028 1063 1245 nr nr nr 1299 nr 2290 1430 Pb18nr5nrnr31nrnrnrnrnr1414 Th 39.6 20 24 18 16.80 22 14 50 14 nr nr 15 17 U 4 6 4 3 3.6 4 9 20 6 0 nr 4.9 4.3 Co nr nr 41.0 nr 3.80 nr nr nr nr nr nr Br 0.3 nr 1 nr nr 0 nr nr nr nr nr Sb 0.30 nr 0.40 nr 0.50 0.1 nr nr nr nr nr Cs 4.0 nr 3.0 nr 2.00 1.8 nr nr nr nr nr La 42.4 nr 38.1 nr 42.60 70.2 nr nr nr nr nr 130 71 Ce 71 nr 76 nr 67 136 nr nr nr nr nr 210 150 Nd 25 nr 18 nr 36.00 61 nr nr nr nr nr 107 69.5 Sm 3.0 nr 2.7 nr 5.00 12.4 nr nr nr nr nr 15.7 12.7 Eu 0.55 nr 0.6 nr 1.22 1.9 nr nr nr nr nr 2.98 1.93 Tb 0.32 nr 0.30 nr 0.52 1.9 nr nr nr nr nr 1.9 1.8 Yb 2.0 nr 1.6 nr 2.24 7.0 nr nr nr nr nr 7.7 7.5 Lu 0.36 nr 0.26 nr 0.38 0.97 nr nr nr nr nr 1.13 1.02 Hf 4.7 nr 4.6 nr 5.50 10.9 nr nr nr nr nr 14 11 Ta 2.2 nr 2.0 nr 1.30 2.2 nr nr nr nr nr 1 2 W nr nr 340 nr nr nr nr nr nr nr nr 120 220 294 D.J. Ennis et al.rChemical Geology 167() 2000 285±312 .2 10.0 - .2 1.10 - .2 - 2 2 2 23 23 2 Table 3 Upper Lemitar whole rockOxides geochemistry reported in wt.%, elements reportedSample in ppm. KM-31 KM-36SiO KM-41TiO KM-46 KM-51Al 77.38 O KM-55Fe O 0.19 KM-56 74.79MnO 11.81 KM-86 67.25MgO KM-90 1.13 0.20 13.04 KM-91CaO 0.05 70.60 15.47 KM-92 0.57Na 0.17 1.69 O KM-93 62.48K 0.06 13.37 0.17 O KM-94 2.94 0.43Cr 0.34 65.77 1.59 KM-95 0.04 17.36Zn KM-106 0.31 0.40 0.63 66.03 7.81 2.04 KM-107As 15.99 171 1.48 0.04 KM-108 0.42 67.49Rb KM-109 0.56 15.68 3.11 0.55 40 KM-110 1.87 8.32 71.21Sr 0.03 KM-111 49 1.32 10.00 KM-122 62 15.43Y 318 0.56 3.23 1.07 1814175869737075756567616866596862691442659 2.95 68.67 14.67 0.07Zr 55 105 0.83 685 6.64 0.58 65.15Ba 2.90 22 0.42 296 2.34 14.94 0.10Pb 29 0.40 175 0.33 68.26 9.50 111 16.26 2.83 401 0.48 39Sb 1.85 478 0.06 3 66.43 10.93 0.37 1.51 14.84 0.54 149 68 17 32 0.39 2348 0.03 1.86 255 66.18 10.87 41 15.35 0.48 5.2 0.63 122 2.81 0.12 435 3 64.97 146 2.14 0.03 16.05 384 9.06 0.27 81 33 387 0.56 2.20 2.65 0.30 69.18 377 1288 0.9 15.52 8.50 0.02 142 0.44 376 21 5 74 0.59 2.86 71.36 85 1582 0.42 2.25 14.59 0.3 550 0.04 9.20 0.37 376 93 67.11 0.60 1493 2.88 13.10 19 77 0.22 1.26 12.21 84 0.04 454 0.2 0.44 9 66.86 1474 394 15.60 0.51 10.35 2.98 91 1.59 0.33 0.02 68.91 23 91 444 0.43 56 10.29 15.01 12 2678 329 0.2 2.19 0.50 1.83 0.30 144 0.05 15.01 68.47 10.02 0.43 464 595 25 12.0 66 438 2.41 0.42 9 2.41 0.85 21 55 0.04 15.79 304 3.58 1.77 1786 8.4 4.66 535 2.14 0.52 34 64 34 0.39 0.07 14 2186 151 9.15 0.51 426 2.29 2.54 0.41 486 0.4 0.37 1845 36 21 0.04 0.74 8.57 9 514 21 83 0.36 1.92 1.80 1.9 2584 456 0.42 10.13 21 0.05 0.34 38 443 127 2.44 1991 19 2.17 0.49 16.0 7 455 0.05 0.69 4.73 1.30 20 459 2247 132 43 0.41 3.99 2.32 6.82 526 2.1 1.20 21 0.04 9 1462 489 3.01 159 12 31 0.22 402 5.4 9.57 0.27 1110 34 15 455 391 2.24 18 54 395 1564 1.7 15 419 10 90 1319 10.0 18 422 53 337 3 17 967 77 1.1 241 22 35 429 2298 305 10 0.2 3 89 20 401 52 379 31 22 67 197 19 18 12 234 16 397 13 40 101 128 26 5 32 15 3 15 26 26 CsLaCe 3.2Nd 34.7Sm 78Eu 5.9 32.2 69.3Yb 140 8.4 38.3Lu 5.0 57.1 0.5 12.0 52 19 5.5 54.9 4.8 0.8 2.3 2.7 82.2 121 5.6 52.9 9.5 0.5 0.7 10.3 69.2 165 2.1 69.4 3.0 0.4 13.5 67.8 1.6 151 61.7 5.7 12.9 82.8 3.5 0.8 2.5 59.9 141 86.7 6.4 12.4 6.0 67 0.9 2.3 148 74.8 11.7 6.8 3.0 158 69 0.9 12.7 2.2 78.6 6.5 8.0 134 10.4 77.2 0.9 57 2.1 5.7 5.0 146 10.3 69.4 1.5 0.9 59 5.6 10.2 74.6 137 4.0 0.8 2.4 63 90.2 5.0 126 9.7 5.0 0.8 2.4 68.2 54 10.0 4.9 138 4.0 0.8 1.9 60.3 10.0 153 54 4.7 88.0 0.7 2.6 77.2 9.6 128 5.3 61 5.0 114.0 0.76 1.9 9.1 112 50.1 4.8 3.0 0.76 50 1.6 10.6 122.0 142 0.63 4.2 2.0 48 2.0 12.9 201 0.80 5.0 77.0 4.4 62 1.7 0.84 83 8.0 5.7 13.7 0.83 2.1 78 209 6.0 5.7 0.79 28 1.9 0.41 5.3 1.2 80 2.7 0.78 2.7 5.1 D.J. Ennis et al.rChemical Geology 167() 2000 285±312 295 M-16 .2 11.0 1.3 0.5 2.7 3.0 - .2 0.4 0.5 - .2 - .2 - .2 - .2 0.5 0.4 130 - 0.261.50 0.520.04 0.330.07 2.560.22 0.04 1.71 0.552.42 0.22 0.037.24 0.39 0.11 0.62 2.76 2.23 0.29 0.05 9.38 0.54 2.94 3.18 0.48 8.29 0.08 0.60 0.42 2.81 0.37 2.59 0.06 0.42 8.87 2.21 0.52 0.38 2.24 0.06 10.30 0.61 0.32 0.43 2.67 2.21 9.21 0.58 0.07 0.59 2.89 2.32 0.35 7.01 0.06 0.85 0.51 3.08 0.36 3.20 7.26 0.08 0.44 0.51 2.36 0.67 2.27 7.39 0.03 0.57 0.39 2.68 0.30 11.43 2.02 0.05 0.36 2.05 10.72 0.44 0.49 2.85 0.06 0.40 9.97 0.41 0.43 2.30 1.84 0.73 0.05 5.88 0.53 3.28 2.26 0.51 0.07 1.11 4.79 0.50 2.74 0.39 3.65 0.03 0.44 7.82 0.52 2.69 0.39 2.73 0.04 0.49 9.22 0.55 2.81 0.40 2.57 0.02 0.51 8.15 0.49 2.98 0.58 2.72 0.05 0.30 8.90 0.54 2.70 0.38 1.03 0.06 0.33 9.89 0.57 2.85 0.47 2.11 0.03 0.48 7.33 3.02 0.29 2.41 10.10 0.07 0.30 0.46 1.53 9.26 1.19 1.87 4.0 4.0 3.00.75.70.79 5.0 3.0 5.0 0.72 3.0 1.5 6.2 0.93 4.0 2.8 0.95 6.6 2.0 2.3 0.73 4.8 3.0 0.82 1.8 5.6 0.94 4.0 1.3 6.2 0.83 2.0 1.6 0.75 5.7 4.0 1.1 0.73 5.2 3.0 2.3 0.74 5.1 3.0 0.73 2.6 4.8 1.03 1.0 1.5 0.87 4.5 3.0 1.5 0.94 6.8 3.0 1.3 0.81 5.7 2.0 0.89 1.4 6.4 0.67 8.0 2.4 5.5 0.84 3.0 1.5 0.81 6.2 6.0 2.1 0.71 4.2 3.0 0.81 1.7 5.6 7.0 1.4 5.2 1.3 4.8 2.1 5.4 3327 4848 3772 24 177 78 6418 47 5310.0 15 75 23 168 47 8.3 10 78 26 133 5.9 46 127 9 66 42 21 1.2 122 18 71 23 0.4 51 181 79 130 13 20 43 96 18 70 18 54 118 24 67 17 146 46 19 70 18 76 44 121 19 76 47 24 206 8 59 25 49 114 84 17 3 38 56 74 17 19 44 113 83 19 41 3 65 117 20 40 26 127 79 60 106 45 3 57 111 25 57 44 71 184 13 59 11 68 25 67 3 63 22 42 70 25 18 74.67 68.1312.72 71.34 15.89 66.86 14.50 65.96 15.59 68.66 15.98 72.57 14.93 70.04 13.43 72.61 14.70 63.83 12.95 64.10 16.79 69.06 17.50 72.90 14.53 72.05 13.50 71.77 14.00 68.47 14.21 69.42 15.29 70.07 14.75 68.15 14.21 69.80 15.35 69.16 14.61 66.57 14.60 15.43 70.1 121.05811.5 93.1 79 13.7 83.2 13.4 70 69.3 11.1 66.2 58 59.8 9.7 53 64.7 10.0 10.1 61.2 55 72.9 51 9.7 105.0 9.0 49 65.2 10.2 57.5 48 11.8 62.1 57 9.2 66.7 10.1 70 66.9 60.2 9.9 52 65.8 10.5 49 70.0 9.6 51 59.8 10.0 62.6 51 8.9 71.2 51 10.1 8.9 47 8.6 49 9.8 55 50 50 52 241 86285 149 367235 104633 319 450 100 2687 364 293 579 140 453 442 142 2285 384 2222 484 101 278 1718 414 158 989 282 309 68 1500 313 390 1055 504 43 338 1878 402 116 1981 493 118 378 1458 468 226 739 98 416 1093 168 278 101 1060 309 327 97 1458 136 1100 330 90 320 1605 418 115 1822 386 380 108 1278 490 417 2683 120 342 2728 421 167 452 382 112 397 419 433 KM-123 KM-124 KM-125 KM-126 KM-127 KM-128 KM-129 KM-130 KM-131 KM-138 KM-143 KM-144 KM-147 KM-148 KM-149 KM-150 KM-151 KM-154 KM-156 KM-157 KM-159 K 134 205 165 152 127 117 117 124 111 129 188 118 112 118 121 124 114 121 128 111 116 131 296 D.J. Ennis et al.rChemical Geology 167() 2000 285±312

" " wt.%, MgO 0.04 wt.%, CaO 0.11 wt.%; Na2 O tially associated with adulariaŽ. Fig. 3 . Mixed-layer " " " r 0.04 wt.%, K225 O 0.03 wt.%, P O 0.01 wt.%, I S is also a major clay component and is found in and for trace elementsŽ. in ppm V"4, Cr"12, quantities G5 parts per 10 in 11 of the 19 separated Ni"1, Cu"1, Zn"1, Ga"0.3, As"0.7, Rb" samples. When present, mixed-layer IrS appears to 0.8, Sr"0.7, Y"1, Zr"8, Nb"0.1, Mo"0.1, be either metastable or near equilibrium with adu- Ba"10, Pb"0.2, Th"1, U"0.5, Sc"0.04, Co laria, for both minerals show no apparent indication "0.13, Br"0.07, Sb"0.1, Cs"1.4, La"1.8, Ce of dissolution when in the same assemblageŽ. Fig. 4 . "1.4, La"1.8, Ce"1.4, Nd"0.7, Sm"0.07, Eu In contrast to the frequent occurrence of kaolinite "0.14, Tb"0.02, Yb"0.07, Lu"0.01, Hf"0.12, and mixed-layer IrS, significant abundances of dis- Ta"0.04, W"0.05. crete smectite and discrete illite are uncommon in the alteration assemblage. Large quantities, G5 parts per 10 of the clay fraction, of discrete smectite are 2. Results present in only two of the 41 separated samples with 2.1. Mineralogy trace quantities found in 12 samples. Smectite tends to be subhedral and typically coats euhedral adularia 2.1.1. Mineral abundances, morphology and element crystalsŽ. Fig. 5 . Discrete illite is also scarce with mapping only three samples containing G5 parts per 10 illite Unmetasomatized upper Lemitar and Hells Mesa and trace amounts in nine of the 41 samples. Tuff samples examined in thin section show no Calcite is present in at least four of the separated significant signs of plagioclase alteration, however, samples from the Socorro K-metasomatized area, one fresh Hells Mesa Tuff sample does show a which has not been documented previously. When minor amount of plagioclase replacement by clay or present, calcite is extremely fine-grained and found possibly calcite. The unaltered nature of plagioclase in exceedingly small amounts. Barite is also found in fresh upper Lemitar and Hells Mesa Tuff samples within a few samples collected in the vicinity of the outside of the K-metasomatism area indicates the Luis Lopez manganese district. mineralogy found in the separated samples is at- The back-scattered electron microprobe image of tributable to an alteration process other than weather- a polished thin sectionŽ. sample KM-48 from the ing. The mineral assemblage of the separated sam- Hells Mesa Tuff shows a sharp boundary between ples consists of variable combinations of adularia, the minerals in the alteration assemblage and resid- quartz, kaolinite, mixed-layer illite±smectiteŽ. IrS, ual plagioclaseŽ. Fig. 6 . An enlargement of the pla- discrete smectite, discrete illite, and remnant plagio- gioclase embayment shows additional, smaller em- clase with minor calcite and barite. bayments at the margin between plagioclase and the Adularia, a low-temperature K-feldspar, is present alteration phasesŽ. Fig. 6 inset . The dominant K-rich in all 41 of the separated samplesŽ. Table 1 and phase in the alteration assemblage appears to be consistently displays euhedral habit. Fine-grained adularia while the clay matrix is somewhat K-rich, quartz is also present in all 41 separated samples and suggesting mixed-layer IrS. This is supported by tends to display an amorphous, globular habit. Resid- semi-quantitative clay analysis which resolved ual plagioclase was found in 17 of the 41 separated mixed-layer IrS to be the dominant clay mineral samples and consistently appears partially dissolved present in this sample. The Ca-map indicates the in SEM imagesŽ. Table 1; Fig. 2 suggesting chemical presence of extremely fine-grained calcite within the instability during the conditions imposed during alteration assemblage. potassic alteration. 2.2. Geochemistry The most common clay types found in the alteration mineral assemblage are kaolinite and mixed- 2.2.1. Upper Lemitar and Hells Mesa Tuffs layer IrS with each being present in 19 of the separated samples. Kaolinite is typically a major clay 2.2.1.1. Whole rock major element concentrations. constituent, 16 of the 19 samples have G5 parts per Despite overall similarity between the upper Lemitar 10 kaolinite in the clay fraction, and is found spa- and Hells Mesa Tuffs, there are distinct chemical D.J. Ennis et al.rChemical Geology 167() 2000 285±312 297 5 - 55.7 - 56.2 - .5 0.8 0.3 0.5 0.6 0.5 - 5 5 6 7.7 7.3 10.2 - 5 10.8 - 555.38.259.56.4 - 56.76.3 3 8.3 9.1 7.4 7.2 8.7 8.3 10.5 9.7 9.1 10.7 10.7 8.4 6.9 - - 5 3 - - 2 2 2 23 23 2 25 Table 4 Hells Mesa whole rockOxides geochemistry reported in wt.%, elementsSample reported in KM-54 ppm. KM-58 KM-59 KM-60SiO KM-61 KM-96 KM-102TiO KM-112 KM-113 72.79 KM-114Al KM-115 KM-116 O 72.80 0.27 KM-117 KM-118Fe KM-119 74.96 O 13.26 KM-120 0.30 KM-121MnO KM-153 72.36 14.47 2.22 KM-155 KM-158 0.24MgO 0.08 75.90 14.03 2.18CaO 0.37 13.53 0.26 0.02 74.52 1.77 13.30Na 0.79 0.64 O 0.22 0.02 73.03K 1.98 1.42 12.92 O 0.36 0.41 73.38 0.09P O 1.85 0.99 13.34 7.83 0.24 0.28 0.48LOI 0.06 71.18 13.25 0.05 1.26 8.39Vnanananana222835423027356030313334262426 1.79 0.35 0.25 0.15 1.61 71.81Cr 1.67 0.05 14.11 7.27 0.01 0.61 0.26 0.18 73.33Ni 1.62 2.53 0.23 0.04 13.77 7.18 0.01 73.51 0.18 562 1.99 0.17 0.05 0.29 13.13 6.28 0.21 na 0.05 0.33 268 0.60 71.65 13.07 0.04 0.22 0.33 0.28 0.06 2.11 73.95 na 510 8.52 0.71 0.05 0.27 13.41 0.23 74.11 0.06 0.29 2.01 1.12 461 1.06 8.86 12.91 na 0.04 0.24 73.47 0.46 0.06 0.28 1.55 7.60 13.00 641 1.21 1.63 0.05 74.37 na 0.07 0.43 13.78 1.85 0.41 1.73 0.27 77.27 0.05 26 7.69 1.16 12.87 na 0.20 0.25 78.47 0.22 0.08 2.81 11.33 1.59 0.80 8.79 0.04 0.44 19 75.54 0.22 11.88 0.08 0.45 1.65 2.00 1.78 8.49 0.07 144 13.00 0.86 0.25 0.44 0.06 1.65 6.79 1.56 2.29 0.04 0.69 0.07 165 0.21 1.36 0.30 1.77 2.85 6.23 0.04 0.16 0.50 134 0.23 1.59 0.13 1.47 2.18 5.82 0.03 0.15 0.90 1.09 0.30 0.07 137 0.03 0.87 3.14 6.37 0.21 0.64 0.08 1.13 151 0.06 1.43 0.01 2.66 5.70 0.10 0.34 1.50 0.83 0.07 0.84 0.03 171 5.62 0.04 0.39 0.11 0.06 1.23 8.74 0.53 182 0.02 0.66 2.61 5.83 190 2.45 0.03 5.10 148 1.32 0.05 177 227 144 129 ZnGa 33As naRb 74 9.2Sr na 424Y6061202117222521282628222718222120641819 16.1 52Zr 120 310 na 13.0Nb 16 30 341Mo 444 6.2 na 23 335 77 152 45 na 10.3 28 na 305 157 92 104 na 4 335 138 31 16 117 na 52 134 311 11.1 26 88 15 na 17.6 134 69 372 29 16 151 na 98 7.8 328 56 23 140 75 375 16 8.1 59 23 175 144 10.4 419 16 22 44 158 340 4.5 111 16 136 39 22 353 74 7.8 135 16 110 291 45 11.6 22 159 17 10.5 330 214 32 25 127 261 8.2 15 151 23 35 129 297 147 5.5 16 155 28 306 19 9 230 16 126 277 31 24 171 155 4.8 16 251 18 27 23 108 3 14 84 40 119 24 17 24 118 24 15 28 22 24 Cu na na na na na 12.6 10.7 8.8 8.7 6.1 10 419 856547665556685 BaPb 794Th 26U451099 998 19Sb 34 600Cs 27 0.54La 511 19 8.3Ce 0.55 30 39.5 502Nd21.426.622.219.617.4212320242420192017162319262015 3.0 20.0 17 69Sm 41.4 858 26 10.5Eu 0.48 42.7 29 3.3 77 1187 10.3 0.57 29 39.1 0.7 4.6 773 12.3 182 76 36.7 0.8 3.2 25 3.4 1622 327 67 4.0 45.9 0.6 3.3 1.4 1586 22 39 54.4 66 4.0 0.7 1911 2.6 0.9 42.1 21.0 25 0.5 69 636 28 45.3 0.4 2.8 6.0 1742 22 74 0.6 29 45.1 3.2 0.5 1044 8.0 71 20 45.6 0.8 36 2073 3.0 14.0 0.8 45.3 0.8 2274 30 16 89 9.0 0.7 3.9 46.6 1362 27 0.8 22.0 75 275 43.3 33 3.8 1.0 0.7 11.0 44.0 21 598 70 2.9 0.7 58 10.0 49.2 0.7 473 2.9 71 29 0.4 24 44.7 5.0 0.5 32.4 28 4.3 0.5 73 18.0 29 1.0 47.0 3.0 2.2 1.0 26 65 25 41.4 0.6 7.0 8.0 2.5 29 17 65 1.7 4.0 3.1 22 278 76 0.3 2.5 29 19 69 5.4 30 62 2.8 69 1.9 59 YbLu 2.1 0.34 2.4 0.35 2.6 0.42 0.41 2.5 0.37 2.3 0.35 2.4 0.36 0.40 2.5 2.4 0.43 0.44 2.6 0.46 2.7 0.44 2.5 0.45 2.7 0.34 0.39 2.7 0.43 1.9 0.35 0.74 2.2 0.33 2.5 0.32 2.0 5.0 1.9 2.0 298 D.J. Ennis et al.rChemical Geology 167() 2000 285±312 D.J. Ennis et al.rChemical Geology 167() 2000 285±312 299

differences between the two units. The SiO2 content whole rock samples are as high as 90.6 ppm, and Sb of fresh upper Lemitar Tuff is ;69 wt.% and that of was found to have concentrations up to 130 ppm. ; the Hells Mesa Tuff is 72 wt.%, with K2 O and Many samples containing elevated concentrations of ; Na2 O values for the two units averaging 5 wt.% As and Sb were collected near areas of recognized ; r K2222 O and 4 wt.% Na OŽ. Table 2 . K O Na O areas of hydrothermal activity, which may account ratios for unaltered samples utilized in this study are for the extremely high concentration of these ele- between 1.13 and 1.55, which is typical for unaltered mentsŽ. discussed later . rhyolite samplesŽ. Nockolds et al., 1978 . Although some trace elements show consistent K-metasomatized upper Lemitar ignimbrite sam- enrichment or depletion within K-metasomatized ) ples generally contain 66 wt.% SiO2 , 3.6±12.2 samples, the majority appear to be unaffected by the wt.% K22 O, and 1.0±4.7 wt.% Na OŽ. Table 3 , while alteration. The trace elements V, Ga, Y, Zr, Nb, Pb, ) the Hells Mesa Tuff contains 71 wt.% SiO2 , 5.1 Th, Sc, Nd, Hf, and Ta show little variation on r to 8.8 wt.% K22 O and 0.6 to 3.1 wt.% Na OŽ Table enrichment depletion diagramsŽ. Fig. 7 , indicating r 4.K. 22 O Na O ratios for whole rock samples within that these elements are unaffected by K-metasoma- the K-anomaly range from 0.77 to 9.69 for the upper tism. Lemitar Tuff and 1.8±14.2 for the Hells Mesa Tuff Most elements appear to behave similarly be- and are characterized by both K2 O enrichment and tween the upper Lemitar and Hells Mesa Tuffs dur- Na2 O depletionŽ. Ennis, 1996 . ing K-metasomatism, however, several trace ele- Enrichmentrdepletion diagrams comparing fresh ments within whole rock samples of the tuff units to altered samples from the Lemitar and Hells Mesa show distinctly different trends upon K-metasomatic tuff indicate a significant amount of K2 O, CaO, and alteration. The elements Zn, U, La, Ce, Sm, Eu, Tb, Na2 O mobility during K-metasomatismŽ. Fig. 7 . CaO Yb, Lu, and U all display depletion in the upper content of altered samples for the two units ranges Lemitar Tuff, but enrichment in the Hells Mesa Tuff from 0.1 to 1.8 wt.%; CaO of unaltered samples are Ž.Fig. 7 . Although the magnitude of enrichment or around are ;1.15 wt.% for both units. Other slightly depletion of these elements is relatively minor, the variable major elements include MgO and P25 OŽ Fig. trend is striking, especially within the heavy rare 7. . The remaining major oxides, SiO2223 , TiO , Al O , earth elementsŽ. HREE . The REE Sm, Eu, and Tb Fe23 O , and MnO, show neither significant enrich- display variable concentrations when compared to ment nor depletion as a result of K-metasomatism. unaltered samples, yet show net depletion in the upper Lemitar Tuff and enrichment in the Hells 2.2.1.2. Whole rock trace element concentrations.A Mesa Tuff whole rock samples. number of trace elements in altered upper Lemitar and Hells Mesa whole rock samples show consistent 2.2.1.3. Neutron actiÕation analysis of separated enrichment or depletion as a result of K-metasoma- samples. The separated samples, obtained from tism. When compared to unaltered samples, metaso- hand-picked relict plagioclase crystals, analyzed in matized whole rock upper Lemitar and Hells Mesa this study allow the clearest and strongest picture of Tuff samples show significant enrichment of Rb and the chemical processes that occur during K- depletion of SrŽ. Fig. 7 resulting in RbrSr ratios of metasomatism because they provide a chemical sig- 0.82 to 6.8 for the upper Lemitar Tuff and 1.1 to nature of the alteration process that is undiluted by 19.4 for the Hells Mesa Tuff. unaltered phases in the rock sample. Geochemical Other trace elements that are consistently enriched investigation of separated samples allows a detailed within altered samples include Ba, As, Sb and Cs chemical examination of the alteration assemblage. Ž.Fig. 7 . Values for As within upper Lemitar Tuff For this study, two unaltered separated samples, one

Fig. 7. Enrichmentrdepletion diagrams for major and trace elements in whole rock samples. The enrichmentrdepletion factors are based on averages of eight unaltered upper Lemitar Tuff samples and eight unaltered Hells Mesa Tuff samples. 300 D.J. Ennis et al.rChemical Geology 167() 2000 285±312 1 1.01 3.9 1.17 - 0.5 1.24 5.86 1.49 3.10 0.82 6.61 108.0 69.0 40.1 32.1 2.95 8.9 5.55 108.6 5.0 11.9 8.0 3.9 2.2 1.1 3.9 - 2 ThU 4.2 0.2 0.78 0.5 2.4 1.0 5.2 1.0 11.3 6.2 1.9 13.8 1.1 10.6 1.5 9.6 1.2 4.19 1.5 5.17 0.65 5.11 1.11 2.9 0.76 2.7 0.78 5.4 0.96 2.7 1.1 2.9 1.0 2.3 0.76 0.81 4.9 1.65 7.2 1.95 6.7 0.93 6.7 1.11 Table 5 Upper lemitar and hellsOxides mesa reported separated in sample wt.%, geochemistry elements reportedSample in ppm. KJH-26 KM-31Na O KM-36FeO KM-41 6.86 KM-46Sc KM-51 0.27Co KM-54 4.70 KM-55Zn 0.23 0.18 KM-56As 0.27 0.14 KM-58 KM-59Br 0.84 0.1 5.9 KM-60 1.45 0.11Rb KM-61 0.94 KM-86Sr 11.0 0.26 3.70 nr KM-90 3.48Sb 1 KM-91 1.17 2.0 KM-94 1.0Cs 0.10 5.25 na KM-95 165 0.59Ba 0.01 KM-96 0.40 3.9 6.05 KM-102La 110 0.13 0.08 0.80 KM-107 35.2 naCe 0.10 664 KM-111 0.35 2.17 0.55 510 5.7Nd 0.22 0.89 22.3 30.2 0.32 1.17Sm na 32.4 8.49 112 0.60 23.9 512 0.92 70.1 4.8Eu 3.5 9.7 0.19 2.25 3.33 0.11Tb 4.9 1.15 3600 8.7 0.64 na 187 79.2 0.10Yb 9.7 16.1 1.73 0.21 nd 2.6 289 0.10 0.23Lu 0.62 44.1 0.11 215 4.7 211 0.06 12.5 na 0.10Hf 0.74 5.3 0.45 0.85 1259 3.9 2.2 18 0.51 31.8Ta 0.05 204 0.12 0.08 382 42.2 13.6 0.22 0.80 262 2.75 0.33W na 2.7 0.67 1.31 76.0 1.6 1.4 0.55 9.74 7.4 98.5 11.4 0.04 25.4 519 0.08 0.10 0.34 3.07 1.34 747 0.89 21.9 0.57 2.3 0.23 na 24.7 3.77 4.3 114.6 12.9 0.16 12.4 1.7 0.08 26.78 1.11 483 0.06 0.95 0.57 0.87 3771 32.7 1.75 103.7 0.48 0.26 7.72 16.1 4.5 70.0 0.15 8.7 0.06 0.44 1.5 na 0.17 2.1 411 1.86 1.16 25 1.0 116.6 051 95.7 2.09 0.23 0.74 8.2 1.4 48.8 0.28 257 0.34 0.65 4.5 17.9 0.32 87 1.35 0.30 2.0 294 2.0 93.5 na 4.05 0.80 0.32 0.10 15.3 0.22 0.72 44.3 1.00 1.7 4386 17.2 0.64 1.40 0.36 8.5 21.3 223 184 5.27 6.2 0.99 0.76 3.6 0.19 13.5 0.20 na 179 1.93 38.5 1.4 14.6 1.36 2.50 0.16 1.3 24.5 2.85 1.34 309 34.1 1.5 167 0.16 8.7 69.4 0.37 3.5 0.78 7.12 440 50.1 0.66 5.6 0.19 0.27 na 33.0 4.47 0.32 1.31 10.3 420 2.21 141 15.1 0.80 0.15 3.2 2.9 1.21 14.8 0.71 3.7 2416 0.29 65.7 0.78 5.6 1.65 0.89 11.6 na 5.01 0.84 27.0 662 0.10 0.25 87 1.13 641 90.3 0.94 0.68 42.8 1.5 3.2 73.6 1.6 1.2 23.4 3.0 0.72 4.53 55.1 0.26 0.81 617 0.46 6.8 91.7 0.53 na 1.31 1074 8.3 248 140.2 0.69 3.4 3.9 4.2 2.5 1.9 31.8 0.64 0.26 2.6 4526 25.1 552 0.31 84.8 0.68 1.72 120 26.7 1.39 141 13.98 2.0 0.88 5.1 1.3 4.3 861 8.6 0.52 24.4 0.22 32.3 1.4 517 5.9 1.26 56.0 1.40 17.91 nd 27 2.38 0.68 0.31 13.2 0.20 651 47.7 2.23 1.07 11.51 508 1.7 71.4 2.5 29.0 1.3 13.8 0.91 0.71 0.74 nd 24.09 0.62 16.3 0.18 22.2 5.15 1566 31 0.14 15.7 421 5.9 12.2 2.71 0.66 0.49 3.5 0.99 27.18 0.18 0.34 39.3 1.4 3.1 1787 nd 17.3 3.4 0.09 0.84 3.2 0.57 475 1.9 27.59 0.19 0.27 77.2 1.48 6.1 709 0.24 36 5.0 0.29 2.1 4.0 0.20 0.20 205.0 4.1 1.52 49.6 9.9 0.45 0.33 0.21 0.19 35 2.6 1.2 1.41 nd 1.87 0.17 0.27 1.02 27.3 1.2 2.5 2.3 49 3.4 0.33 1.32 0.51 0.53 23.2 1.5 2.5 6.7 6.7 nd 0.68 0.24 1.8 1.65 4.4 0.37 586 3.3 2.7 D.J. Ennis et al.rChemical Geology 167() 2000 285±312 301 M-160 0.5 10.90 9.20 7.40 14.20 8.50 16.78 26.00 7.3 1.8 4.9 7.7 - KM-112 KM-113 KM-114 KM-1150.24 KM-116 KM-1270.59 KM-128 KM-129 0.361.2 KM-131 KM-144 0.6813.4 KM-148 KM-149 0.70145 KM-150 1.8 KM-152 0.58 8.612.2 KM-153 0.13 KM-154 199 KM-155 1.4 KM-156 0.62 6.1 KM-157 9.74 2.37 KM-158 KM-159 132 0.81 K 1.0 14.47 9.11 0.20 126 1.16 44.8 26.57 7.3 0.27 0.92 158 37.3 6.77 3.60 9.7 1.17 10.77 3.70 61 2.72 6.9 1.657 0.99 3.20 0.27 143 7.17 1.18 2.0 20.80 4.86 42.6 3.95 7.50 1.44 12.2 3.77 113.4 2.02 8.91 1.26 15.8 121 2.83 2.67 1.38 2.9 4.95 35.2 2.86 0.84 1.65 29 2.7 3.94 0.67 0.35 1.8 3.7 47.8 1.65 0.43 150 4.07 5.1 1.46 0.82 2.53 23.6 0.96 0.8 1.69 1.94 68.8 0.84 6.2 5.36 1.53 167 0.63 0.59 1.8 1.4 1.0 61.2 0.60 4.7 133.4 7.4 1.1 42 10.9 5.8 2.2 202 231 7.2 6.0 0.94346 0.67450.26 219 0.7910.9 108 0.291336 303 0.90 6.89.02 211 6402 0.1828.0 284 0.84 7.934.3 5.5 8510 0.20 416 46.71.2 0.73 295 10.0 380.29 899 3.9 11.1 0.29 217 23.90.22 0.87 991 1.1 467 9.8 0.280.88 20.8 0.36 8.1 20.2 nd 0.17 nd0.14 950 469 1.4 0.44 11.9 0.62 19.11.4 0.19 31.8 0.30 3.8 0.100.20 86 935 2.50 374 0.48 15.1 1.3 1.6 22.8 0.331.73 57.9 1.4 0.63 0.19 0.22 5.83.7 0.65 481 0.51 20.9 330 157 2.1 0.31 2.40 2.3 1.40.85 63.7 0.32 0.30 1.7 0.18 0.52 0.82 17.5 2.2 32.2 926 854 0.35 465 nd 0.59 91.0 20.1 1.91 0.54 3.7 0.29 0.23 2.09 0.67 0.81 20.5 26.1 0.06 594 3.3 141 53.8 0.77 68 146 1.29 0.53 nd 0.16 0.24 4.2 2.7 0.34 1.32 33.5 23.6 0.40 704 67.8 1.6 315 3.4 2.2 1.28 1.16 0.18 449 0.13 1.16 nd 7.4 1.7 22.0 27.5 1.6 0.39 103.4 1068 359 3.87 0.72 2.4 0.57 4.7 2.5 0.18 382 0.61 24.6 33.4 78.2 0.24 4.7 0.78 nd 2.2 894 2.2 0.69 300 0.30 1.16 0.59 0.44 2.1 599 18.6 28.4 27.7 65.2 2.6 1.6 5.3 829 1.4 4.8 2.9 0.21 0.40 384 1.60 0.61 1.55 8.3 27.1 55.8 417 103.5 2.3 7.7 2.0 1044 0.74 7.6 0.22 5.0 1.0 2.6 2.5 0.5 461 0.77 83.9 24.6 23.2 2.1 99 14.5 800 3.1 0.47 1.7 1.5 5.9 0.61 34.7 3.3 410 0.8 49.9 0.84 16.0 2.6 35.8 3.5 16.0 73 1256 0.34 0.58 2.6 nd 66.4 0.60 nd 0.80 39.7 2.5 5.3 704 2.5 1.7 6.5 13.3 1859 4.8 0.54 0.43 118 53.0 0.86 25.09 3.2 0.96 263 0.89 11.8 10.8 15.2 5.9 415 4.0 1.6 3.0 0.72 17.3 27.1 0.23 59.9 0.23 106 1.06 0.95 2.0 16.1 7.1 16.9 93 674 67.8 0.71 5.6 1.5 3.3 9.2 19.7 1.8 31.2 0.42 166 0.62 45.0 20.4 1.0 4.4 13.9 0.18 2622 662 20.6 5.1 0.39 55.8 0.45 3.7 6.5 2.9 1.12 491 2.2 21.6 1385 23.8 0.27 1.5 3.5 37.8 588 0.59 94.1 1.09 1.2 1.2 1.8 3.0 45 7.4 6.4 48.2 0.31 0.79 0.97 0.39 4.5 1.9 0.68 3.3 5.5 22.5 0.15 168 3.6 0.18 1.1 0.39 5.3 1.0 2.2 34.5 0.56 5.9 2.4 1.0 4.0 0.39 4.8 1.3 1.7 0.53 1.0 17.6 4.8 0.30 1.1 3.6 1.0 5.0 3.7 0.33 3.9 3.0 1.9 1.9 7.8 4.0 3.6 2.4 0.90 1.6 9.8 4.9 2.7 1.3 4.3 2.7 2.7 1.2 3.6 302 D.J. Ennis et al.rChemical Geology 167() 2000 285±312 from each unit, were compared to altered samples the separated samples show consistently elevated from their respective eruptive units. Sample KJH-26 concentrations including Sc, Co, Zn, As, Br, Rb, Sb, consists of separated plagioclase crystals from an Cs, and BaŽ. Fig. 8 . This figure does not reflect As unaltered Hells Mesa Tuff sample. For the upper enrichment or depletion because this element was Lemitar Tuff, sample KM-148 is used as the unal- not detected Ž.-0.5 ppm in either KJH-26 or KM- tered separated sample. Although KM-148 has un- 148. However, As is interpreted to be enriched upon dergone a small degree of alteration, we feel the very alteration as indicated by the relatively high values slight alteration that has taken place has no material found in the separated samples. The concentration of effect on the conclusions presented in this paper. Ba was found to vary dramatically between 112 to Geochemical analytical results for the separated sam- 38,900 ppm for the two units. ples are shown in Table 5. Elements showing significant depletion include

When compared to the chemistry of the separated Na2 O, Sr, and Eu; all elements compatible in plagio- samples KM-148 and KJH-26, several elements in claseŽ. McBirney, 1993; Fig. 8 . Na2 O within the

Fig. 8. Enrichmentrdepletion diagrams for elements within the separated samples of the upper Lemitar and Hells Mesa Tuffs. Unaltered samples used to calculate the enrichmentrdepletion factors were KM-148Ž.Ž. upper Lemitar Tuff and KJH-26 Hells Mesa Tuff . D.J. Ennis et al.rChemical Geology 167() 2000 285±312 303

separated samples of both units ranges from 0.10 to In contrast, Na2 O and Sr are depleted as a result of 5.36 wt.% with KJH-26 containing 6.86 wt.% and plagioclase instability and are removed from the

KM-148 having 4.86 wt.% Na2 O. Values for Sr in system upon alteration. The observation that the the separated samples range from 35 to 599 ppm. chemical changes reflect the alteration mineral as- KJH-26 was not analyzed for Sr, therefore an enrich- semblage is supported by the greater relative in- ment or depletion factor was not calculated for Hells crease in the enrichmentrdepletion factors calculated Mesa Tuff separated samples on Fig. 8. KM-148 for the separated samples compared to whole rock contains 449 ppm Sr and reflects overall depletion of samples. Sr within the upper Lemitar Tuff. Several elements, including As, Sb, Ba, and Pb, As with whole rock samples, several elements display anomalous enrichments that are geographi- show markedly different trends between the two cally associated with known areas of hydrothermal tuffs. For instance, the elements Fe, La, Ce, Sm, Tb, alteration and mineralization. For instance, when the Yb, Lu, Hf, Ta, Th, and U, are depleted within upper concentration of As in over 200 K-metasomatized Lemitar Tuff separated samples when compared to samples is contoured, there is a broad As enrichment KM-148, but are enriched within Hells Mesa Tuff of approximately 5 ppm over the entire K-meta- separated samples when compared to KJH-26Ž Fig. somatized area, but superimposed upon this are sharp 8. . The enrichment of these elements within the concentration spikes centered on areas where high- upper Lemitar Tuff separated samples and the deple- temperature hydrothermal activity has been recog- tion within the Hells Mesa Tuff separated samples is nizedŽ. Dunbar et al., 1995 . These anomalous values similar to the trends found in whole rock analyses contribute significantly to the enrichment factors cal- Ž.compare Figs. 7 and 8 . culated for As and Sb in Fig. 7, especially for the upper Lemitar Tuff. It is also likely that samples containing high concentrations of Ba represent sam- 3. Discussion ples affected by hydrothermal alteration as many Ba-enriched samples are found either associated with 3.1. Major and trace elements the Luis Lopez manganese district, or with Mn- mineralized areas on the east side of the Magdalena K-metasomatic alteration resulted in significant MountainsŽ. Fig. 1 . A small number of Hells Mesa deviation from the primary chemistry of the upper Tuff samples collected from Socorro Peak, an area of Lemitar and Hells Mesa ignimbrites. Notably, sam- fairly intense hydrothermal alteration resulting in ples from this study are significantly enriched in galena mineralization with extensive secondary barite

K2 O, Rb, As, Ba, Pb, Sb, and Cs and depleted in Ž.Lasky, 1932 , have considerably elevated Pb and Ba Na2 O, CaO, and SrŽ. Figs. 7 and 8 . The enrichment concentrations, suggesting the enrichment factors for or depletion of many of these elements has been these elements calculated for Fig. 7 are not a result recognized by Chapin and LindleyŽ. 1986 and Dun- of K-metasomatism. Low level increases of As and bar et al.Ž. 1994 . The lack of significant deviation for possibly Ba and Sb enrichment appear to be related

TiO223 and Al O indicates no significant mass loss or to K-metasomatism, however, high levels appear gain within the units, suggesting the trends seen in attributable to hydrothermal alteration. other elements represent true rather than apparent chemical changes in the rock. As discussed previ- 3.2. Rare earth elements ously, Na22 O depletion and K O enrichment is a result of plagioclase dissolution and adularia forma- Depletion of REE in the upper Lemitar Tuff and tion. MgO depletion is noted by Dunbar et al.Ž. 1994 enrichment in the Hells Mesa Tuff observed in this in silicic ignimbrites, however, only the Hells Mesa study is unique, for most studies of K-metasomatism Tuff shows minor depletion in this study. report little to no variation in REE distribution as a

Elements such as K2 O and Rb appear to be result of alterationŽ i.e., Glazner, 1988; Roddy et al., controlled primarily by the stability of the alteration 1988; Hollocher et al., 1994. . Some researchers be- assemblage produced upon plagioclase dissolution. lieve that movement of fluids during diagenesis has 304 D.J. Ennis et al.rChemical Geology 167() 2000 285±312 the ability to change REE distributions and ratios middle and HREEŽ. Sm to Lu, excluding Eu content Ž.i.e., Alderton et al., 1980; Condie et al., 1995 , compared to plagioclase from the Hells Mesa Tuff. however, others believe diagenetic and low-grade This substantial difference between the middle and metamorphic effects on REE are minimalŽ i.e., HREE content of plagioclase from the two units may Chaudhaui and Cullers, 1979; Elderfield and explain why enrichment or depletion of middle and Sholkovitz, 1987; Sholkovitz, 1988. , especially when HREE is particularly striking while LREE show the fluid temperature is -2308CŽ. Michard, 1988 . variable concentrations. Data indicate a convergence In this study, the separated sample REE trends re- toward similar REE contents upon alteration, sug- flect those seen in whole rock samples, but are gesting an equilibrium fluid REE concentration ap- distinctly more pronounced suggesting that variation proximately intermediate to the REE content of up- in REE content may be a function of alteration per Lemitar and Hells Mesa Tuff plagioclaseŽ Fig. mineralogy produced by the metasomatic process. 9. . Because the difference between the LREE con- A possible mechanism through which REE may tent of plagioclase from the two units is relatively have been depleted in the upper Lemitar Tuff and minor, a fluid of an intermediate composition would enriched in the Hells Mesa Tuff is through re-equi- not likely cause a significant change in the LREE libration between the mineral assemblage and the content upon re-equilibration. Initial results from this altering fluid. Trace element analyses indicates that study, however, appear to indicate that re-equilibra- unaltered plagioclase from the upper Lemitar Tuff is tion between the fluid and alteration mineral assem- approximately 1.2 to 3.0 times greater in LREEŽ La, blage may be a viable mechanism to explain the Ce, and Nd. content and 6.6 to 11 times greater in REE contents of the separated samples. Preliminary

Fig. 9.Ž. A and B Heavy rare earth element and trace element trends in separated samples compared to unaltered plagioclaseŽ KM-148 and KJH-26. . D.J. Ennis et al.rChemical Geology 167() 2000 285±312 305 data from the Socorro K-anomaly also suggest that rating REE and may provide the mechanism for REE clay minerals may play a significant role in incorpo- redistribution during K-metasomatismŽ. Ennis, 1996 .

Fig. 10.Ž. A±F Increasing adularia peak area Ž adularia abundance . within separated samples vs. whole rock Na22 O Ž A and B . , Na O in separated samplesŽ. C and D , and Sr in separated samples Ž. E and F for the upper Lemitar and Hells Mesa Tuffs.)) Indicates samples where peak area was estimated based on similar XRD patterns. 306 D.J. Ennis et al.rChemical Geology 167() 2000 285±312

3.3. Mineralogy and geochemistry adularia abundance should vary inversely with the amount of residual plagioclase contained within a

3.3.1. Residual mineral phases sample. The amplified Na2 O depletion in separated samples is due to the dilution effect caused by the

3.3.1.1. Plagioclase. Plagioclase phenocrysts in the presence of other Na2 O bearing phases within whole upper Lemitar and Hells Mesa Tuffs were replaced rock samples which are less easily altered than pla- by a mineral assemblage dominantly composed of gioclase. The Sr content of both whole rock and adularia, quartz, and a variety of clay minerals. separated samples correlates with Na2 O content due Although clay minerals such as smectite and kaolin- to Sr compatibility in plagioclaseŽ. Fig. 10e and f . ite commonly replace plagioclase during diagenesis and weathering, unmetasomatized upper Lemitar and 3.3.2. Mineral phases related to K-metasomatism Hells Mesa Tuff samples examined in this section contain unaltered plagioclase with no significant signs 3.3.2.1. Adularia. The dominant phase produced by of replacement. metasomatic alteration is adularia. Adularia has been During K-metasomatism, plagioclase is chemi- observed in a number of other tuffaceous rock units cally unstable but remains in the mineral assemblage that have undergone alkaline±saline brine alteration, as a result of incomplete dissolution during alter- such as in Jurassic lake T'oo'dichiŽ Turner and ation. Plagioclase dissolution during K-metasoma- Fishman, 1991. the Green River Formation in tism caused a marked decrease in Na2 O and Sr WyomingŽ. Surdam and Parker, 1972 , and in tuffs in content in both whole rock and separated samples. A ArizonaŽ. Brooks, 1986 . Based on SEM observa- negative correlation is observed between adularia tions, adularia is produced through the alteration of abundance in altered whole rock samples and Na2 O plagioclase, and is characteristically euhedral. Based contentŽ. Fig. 10a and b . Na2 O depletion in the on the chemical composition of the samples, the separated samples is also evident and elucidates the transformation from plagioclase to adularia appears process by which Na2 O depletion takes place during to be relatively simple exchange of K for Na. This alterationŽ. Fig. 10c and d . Within the separated reaction assumes an albite endmember composition samples, Na2 O content decreases dramatically with for plagioclase, which is reasonable based on elec- increasing adularia abundance, presumably as a re- tron microprobe data from the two volcanic units sult of plagioclase dissolution. As such, increasing Ž.Ennis, 1996 . Assuming that the plagioclase con-

Fig. 11. mol Kqq vs. mol Na for upper Lemitar and Hells Mesa Tuff whole rock samples. Upper Lemitar samples show two trendsŽ UL1 and UL2. due to compositional zonation, while the Hells Mesa Tuff only shows one. Closed symbols indicate unaltered samples collected outside of the K-metasomatized anomaly. D.J. Ennis et al.rChemical Geology 167() 2000 285±312 307 tains little Ca, the K:Na ratio this type of reaction When samples are ranked based on increasing should approximate 1:1, which is confirmed by geo- adularia abundance, several chemical trends are ap- chemical results from this studyŽ. Fig. 11 . This parent. In general, an increase in the adularia content figure shows trends for the Hells Mesa and Lower of the upper Lemitar and Hells Mesa ignimbrites

Lemitar Tuffs, showing evidence of close to a 1:1 corresponds to an increase in whole rock K2 O and exchange of Na for K. The reason for the three Rb contentŽ. Fig. 12a and b . The increase in Rb distinct trends shown on the figure is that the initial concentration is likely due to substitution for K in Na:K ratios for the Hells Mesa, and two different adulariaŽ. Fig. 12c and d; Deer et al., 1992 . Whole sample groups of the Upper Lemitar are different rock Hells Mesa Tuff samples show a 2-fold increase Ž.see the unaltered compositions on the figure , giving in Rb content while separated samples show a strik- different starting points to the regression lines. Based ing 10-fold increase when compared to unaltered on dissolution textures observed in partially altered material. The dramatic Rb increase of the Hells Mesa plagioclase crystalsŽ. Fig. 6 , as well as euhedral Tuff separated samples when compared to whole adularia crystals observed on a plagioclase substrate rock samples suggests that minerals in the secondary Ž.Fig. 2 , the adularia appears to form by a dissolu- alteration assemblage, particularly adularia, strongly tion±precipitation reaction. influence the concentration of Rb.

Fig. 12.Ž. A±D Increasing adularia peak area Ž adularia abundance . within separated samples vs. whole rock K2 O Ž A and B . and Rb Ž C and D. content for the upper Lemitar and Hells Mesa Tuffs.)) Indicates samples where peak area was estimated based on similar XRD patterns. 308 D.J. Ennis et al.rChemical Geology 167() 2000 285±312

3.3.2.2. Illite, smectite, and mixed-layer I r S. anism for formation of mixed layer IrS is by break- Mixed-layer IrS is one of the most common clay down of K-feldspar. This process is frequently called minerals present within the Socorro K-metasoma- upon to form illite andror mixed-layer IrS in diage- tized area, whereas discrete smectite and illite are netic settingsŽ i.e., Boles and Franks, 1979; Altaner, rare in the alteration assemblage. The smectite ob- 1986; Wintsch and Kvale, 1994. . However, in the served is though to be a product of the early stages case of the Socorro K-metasomatism, mixed layer of metasomatism, when the KqrHq of the metaso- IrS is associated with plagioclase, rather than K- matic fluid is low. Smectite is known to form in feldspar. alkaline, saline environmentsŽ i.e., Jones and Weir, 1983; Hay and Guldman, 1987; de Pablo-Galan, 1990. , but smectite cannot form in the presence of a 3.3.3. Mineral phases related to hydrothermal alter- high K fluidŽ. Deer et al., 1992 . However, if the ation metasomatizing fluid had a relatively low cationrHq activity ratio, precipitation of smectite associated 3.3.3.1. Kaolinite. Kaolinite is an extremely common with plagioclase dissolution may occurŽ Duffin et al., clay mineral in the alteration assemblage of samples 1989; Harrison and Tempel, 1993. . As the fluid collected within the K-anomaly. SEM photographs increased in KqrHq ratio andror temperature, due clearly demonstrate the euhedral morphology of to progressive evaporation of the lake brine, the kaolinite, indicating an authigenic origin, however, formation of smectite would cease. At this point, the mechanism through which kaolinite formed in formation of mixed-layer IrS could occur as a result these samples is somewhat difficult to ascertain. of illitization involving addition of K to smectite. Within the Socorro K-anomaly, presence of kaoli- The reaction that causes illitization of smectite re- nite in the alteration assemblage tends to coincide leases silicaŽ. Boles and Franks, 1979 , which may with areas of hydrothermal alteration, suggesting a also explain some of the fine-grained quartz within hydrothermal source for kaolinite within the assem- the alteration assemblage. A fluid-mediated reaction blage. For example, Hells Mesa Tuff samples KM- such as illitization may be governed by three pro- 112 through KM-116 were collected near the Luis cesses: dissolution, solute mass transfer, and precipi- Lopez manganese district and contain kaolinite as tation or crystallizationŽ. Eberl and Srodon, 1988 . the dominant clay mineral in the secondary assem- The fine-grained nature of the primary smectite and blageŽ. Table 1 . Kaolinite is recognized in many the sheet-like morphology of the grains significantly types of hydrothermal alteration such as argillic and increase the rate of dissolution, allowing extensive advanced argillic due to wall±rock interaction. In cationic substitution within the smectiteŽ May et al., these regimes, kaolinite formation is restricted to 1986; Whitney, 1990. . K-metasomatism in the So- lower temperatures Ž.-2008C , indicating its forma- corro area was presumably fluid-dominated with an tion during the late stages of hydrothermal activity abundance of ions as indicated by the extensive Ž.Guilbert and Park, 1986 . Hydrothermal kaolinite addition of K throughout the anomaly. The high K typically exhibits a characteristic texture of tightly concentration in the fluid would cause smectite to packed small flakes which tend to occur as single become illitized by K fixation between the smectite sheets, sheaves, or thin packets rather than expansive layersŽ. Jones and Weir, 1983; Awwiller, 1993 . The booksŽ. Keller, 1976 . This is similar to the kaolinite temperature required for transformation of smectite morphology observed through SEM analysis in this to mixed-layer IrS occurs at temperatures of 1008C study. Additionally, kaolinite formation during dia- or lessŽ. e.g., Aoyagi and Kazama, 1980 , although genesis occurs under dilute, acidic conditions while illitization has been reported at lower temperatures basic and concentrated solutions result in kaolinite Ž.Jones and Weir, 1983; Srodon and Eberl, 1984 . instabilityŽ. Dunoyer de Segonzac, 1970 . These temperature estimates are similar to the tem- Conditions for K-metasomatism in the Socorro peratures at which K-metasomatism is thought to area are believed to have been alkaline,Ž Chapin and have occurred in the Socorro areaŽ Chapin and Lind- Lindley, 1986. , again implying kaolinite was formed ley, 1986; Dunbar et al., 1996. . An alternative mech- as a product of hydrothermal events superimposed D.J. Ennis et al.rChemical Geology 167() 2000 285±312 309 on the Socorro K-anomaly. The presence of barite in ther supporting a hydrothermal origin for this min- the separated sample of KM-115, as well as elevated eral. Calcite, which is extremely rare in the alteration Ba content in several other samples, further suggests assemblage, is fine-grained and likely formed as a some amount of hydrothermal overprinting. result of plagioclase dissolution. Barite is present in the assemblage due to localized hydrothermal events in the Socorro area. The detailed study of altered plagioclase reveals 3.4. Summary the presence of a clay mineral, kaolinite that should not have formed during an alkaline±saline alteration K-metasomatism near Socorro, New Mexico has event. The presence of this mineral helped unravel affected the geochemical and mineralogical composi- the complex chemical changes that took place due to tion of Cenozoic rocks including two silicic ign- several episodes of hydrothermal alteration. We also imbrites, the upper Lemitar and Hells Mesa, which interpret that the enrichment of some elements in were investigated in this study. K-metasomatism altered rocks is related to the hydrothermal events strongly affected Na-rich minerals, particularly pla- that produced kaolinite. Without the recognition of gioclase, while K-rich minerals are basically unaf- the origin of kaolinite, enrichment of some elements fected during alteration. Using XRD and semi- in altered rocks could have been incorrectly at- quantitative clay mineral analysis, the alteration as- tributed to K-metasomatism. semblage was found to consist of various amounts of The chemical composition of altered rocks shows r adularia, quartz, mixed-layer I S, kaolinite, discrete overall enrichment of K2 O, Rb, Ba, As, Sb, Cs, and smectite and illite, and minor calcite and barite. Pb along with marked depletion of Na2 O, CaO, and Some residual plagioclase remains in the assemblage Sr when compared to unmetasomatized samples. En- as a result of incomplete dissolution during K- richment of K2 O and Rb in whole rock samples metasomatism. Kaolinite and mixed-layer IrS are correlates positively with the abundance of adularia the most common clay minerals present within the in the separated, altered plagioclase samples. A strong altered plagioclase. Discrete smectite and illite occur negative correlation is also shown for Na2 O and Sr infrequently in the alteration assemblage. SEM and content with increasing adularia content in the sepa- electron microprobe data from this study suggest the rated samples due to plagioclase dissolution during alteration assemblage formed through a dissolution± the alteration process. The separated samples typi- precipitation reaction as indicated by the presence of cally show the biggest chemical effects, which tend dissolution textures within plagioclase phenocrysts to be diluted in whole rock samples. and mixed-layer clay in the assemblage. Enrichment of Ba, As, Sb, Cs, and Pb in the two It is probable that mixed-layer IrS, discrete smec- ignimbrites is likely a result of input from a hy- tite and illite were formed during K-metasomatism drothermal source as demonstrated by a correlation as a result of changes in the fluid chemistry. Smec- between high concentrations of these elements in tite may have formed during stages of metasomatism samples collected near areas of hydrothermal over- when the fluid was relatively dilute and had a low printing. Low levels of As and possibly Ba and Sb cationrHq ratio. As the fluid became progressively enrichment, however, appear to be related to K- more concentrated in Kq, it is likely that smectite metasomatism. illitization occurred resulting in the formation of Other elements showing enrichmentrdepletion in- mixed-layer clays. Kaolinite is most likely the result clude the middle and HREE. Enrichment of middle of hydrothermal overprinting as indicated by a strong and HREE in the upper Lemitar Tuff and depletion correlation between areas of known hydrothermal in the Hells Mesa Tuff suggests attempted re-equi- activity and large abundances of kaolinite in the libration between the secondary alteration assem- alteration assemblage. Kaolinite formation is also blage and the alteration fluid. Evidence suggests the promoted by acidic conditions suggesting it is not fluid REE content was intermediate to the REE likely to have formed under the alkaline conditions concentrations found within upper Lemitar and Hells imposed during K-metasomatic alteration, thus, fur- Mesa Tuff plagioclase, which would account for 310 D.J. Ennis et al.rChemical Geology 167() 2000 285±312 both enrichment and depletion within the two units. Altaner, S., 1986. Comparison of rates of smectite illitization with Preliminary data suggest clay minerals may have rates of K-feldspar dissolution. Clays and Clay Minerals 5, played an important role during redistribution and 608±611. Aoyagi, K., Kazama, T., 1980. Transformational changes of clay may have provided the necessary binding sites for minerals, zeolites, and silica minerals during diagenesis. Sedi- REE. mentology 27, 179±188. This study documents enrichments and depletions Austin, G.S., Leininger, R.K., 1976. Effects of heat-treating of many elements in rocks altered by K-metasoma- mixed-layer illite±smectite as related to quantitative clay min- tism. Calculations suggest that 6.4=1010 tons of K eral determinations. Journal of Sedimentary Petrology 46, 206±215. have been added to the Socorro K-metasomatized Awwiller, D.N., 1993. Illitersmectite formation and potassium areaŽ. Chapin and Lindley, 1986 , and many tons of mass transfer during burial diagenesis of mudrocks: a study other elements have been depleted. The K that has from the Texas Gulf Coast Paleocene±Eocene. Journal of been added to the rocks could be leached from rocks Sedimentary Petrology 63, 501±512. in a very wide geographic area, concentrated in playa Boles, J.R., Franks, S.G., 1979. Clay diagenesis in Wilcox Sand- stones of Southwest Texas: implications of smectite diagenesis fluids by evaporation, and then added to the Socorro on sandstone cementation. Journal of Sedimentary Petrology area rocks. Hoch et al.Ž. 1999 describe how high K 49, 55±70. waters can be produced from weathering of freshly Brooks, W.E., 1986. Distribution of anomalously high K2 O vol- exposed surfaces on ash flow tuffs, and a similar canic rocks in Arizona: metasomatism at the Picacho Peak mechanism may produce K-rich fluids in the Socorro detachment fault. Geology 14, 339±342. Chamberlin, R.M., 1980. Cenozoic stratigraphy and structure of area. If the K-rich fluids were produced by this the Socorro Peak volcanic center, central New Mexico. PhD mechanism, no distinct K-poor source area could be Thesis, Colorado School Mines, Golden. recognized in the Socorro area, and none is. Chapin, C.E., Chamberlin, R.M., Osburn, G.R., White, D.W., 1978. Exploration framework of the Socorro geothermal area. New Mexico Geological Society Special Publication 7, 115± 130. Acknowledgements Chapin, C.E., Lindley, J.I., 1986. Potassium metasomatism of igneous and sedimentary rocks in detachment terranes and Funding for this project was provided by the other sedimentary basins: economic implications. Arizona Ge- National Science FoundationŽ EAR-9219758 to ological Society Digest XVI, 118±126. Chaudhaui, S., Cullers, R.L., 1979. The distribution of REE in A.R.C. and C.E.C.. , the New Mexico Bureau of deeply buried Gulf Coast sediments. Chemical Geology 24, Mines and Mineral Resources, and the New Mexico 327±338. Geological Society. We would like to thank Philip Condie, K.C., Dengate, J., Cullers, R.L., 1995. Behavior of rare Kyle, Peter Mozley, Richard Chamberlin, George earth elements in a paleoweathering profile on granodiorite in Austin, Chris McKee and Mike Spilde for discus- the Front Range, Colorado, USA. Geochimica Cosmochimica Acta 59, 279±294. sion, support, and constructive suggestions. We D'Andrea, J.F., 1981. The geochemistry of the Socorro anomaly, would like to thank J.I. Drever, K. Hollocher, and an New Mexico, Thesis, Florida State University, 243 pp. anonymous reviewer for insightful review of the Davis, G.A., Lister, G.S., Reynolds, S.J., 1986. Structural evolu- manuscript. XRF analysis was performed at the New tion of the whipple and south mountains shear zones, South- Mexico Institute of Mining and Technology X-ray western United States. Geology 14, 7±10. Deer, W.A., Howie, R.A., Zussman, J., 1992. In: An Introduction Fluorescence Facility for which partial funding was to the Rock Forming Minerals, 1. Longman Group, London, p. provided by National Science Foundation Grant 528. EAR93-16467. []JD de Pablo-Galan, L., 1990. Diagenesis of oligocene±miocene vitric tuffs to montmorillonite and K-feldspar deposits, Durango, Mexico. Clays and Clay Minerals 38, 426±436. 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