ENNIS, D.J.; DUNBAR, N.W.; CAMPBELL, A.R.; CHAPIN, C.E., New Mexico Institute of Mining and Technology, Socorro, NM 87801
Silicic Ignimbrites present within the Socorro, New Mexico area have undergone potassic alteration resulting in the enrichment of potassium and depletion of sodium and calcium over an estimated area of more that 1800 km2. The alteration had a direct affect on the mineral assemblage; the K-poor minerals were altered while K-rich minerals were relatively unaffected.
We have examined the relationship between the mineral assemblages that are present and the chemical variations that occur as a result of the potassium alteration. A total of 12 samples were collected from the Hell's Mesa and Upper Lemitar tuffs, which are both plagioclase-bearing ignimbrite sheets that extend across the potassium anomaly. The samples collected all show various degrees of alteration ranging from nearly pristine to highly altered. The plagioclase crystals in both the Hell's Mesa and Upper Lemitar tuff samples are replaced by fine-grained, chalky material due to the alteration. This material was hand-picked and analyzed by x-ray diffraction (XRD) to determine the alteration assemblage. A range of trace elements, plus major element Na, were determined in this separated material, as well as unaltered plagioclase crystals, by neutron activation analysis (NAA). X-ray fluorescence (XRF) and NAA were used to determine whole rock compositions of pristine and altered samples.
The alteration mineral assemblage is a combination of potassium feldspar (adularia) + quartz clay, as determined by XRD. The relative proportion of phases varies widely in the 12 samples analyzed, although all contain quartz. The mineralogy of some samples (exclusive of quartz) consists of plagioclase + adularia + clay, some adularia + clay, some of only adularia and a single sample contains only clay. Quartz/adularia ratios range from 1:1 up to 4:1 with one sample having a ratio of 1:2. Clay/quartz ratios as high as 1:5 and 3:1 exist, although clay minerals are not present in all samples. The clay minerals present include kaolinite and smectite group minerals, while illite group minerals appear to be absent, even though alkaline conditions and an abundance of potassium are present, thus making a suitable environment for illite formation. It seems possible that the calcium derived from the alteration of plagioclase promoted formation of smectite, or this mineral may have been formed by a less K-rich fluid.
Chemical trends seen by Chapin and Lindley (1986) are recognized in this study. All 12 samples analyzed show an increase in K2O content, with ranges of 6.28-10.93 wt. %, while unaltered values for the Hell's Mesa and Upper Lemitar tuffs are typically around 5 wt. %. Although all samples are enriched in K2O, the samples containing adularia as the main mineral display greater amounts of K2O enrichment. Na2O is depleted in the 12 samples with respect to the unaltered samples, with samples containing remnant plagioclase having the largest values for Na2O; unaltered values for Na2O are approximately 4 wt. %, while altered samples display .99- 2.95 wt. %. The presence of Ba within the samples appears to be quite variable, although a weak correlation with As is apparent, especially in the samples containing adularia as the main mineral constituent. The high Ba and As samples were collected near a zone of Mn mineralization, and may represent hydrothermal overprinting.
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