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

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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 metasoma- tism, 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.
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