Geology topics
Publications and source records attributed to J.C. Laul.
Three fossil fumaroles in rhyolite-rich ash-flow tuff from the lower Valley of Ten Thousand Smokes (VTTS) have been studied to ascertain chemical losses and gains and mineralogical evolution during fumarolic alteration and subsequent cooling and weathering processes. Major oxide and trace element data were interpreted using the isocon method of Grant (1986).
Samples from a fossil fumarole originating in the 1912 ash-flow tuffin the Valley of Ten Thousand Smokes have been analyzed to ascertain chemical changes resulting from high-temperature fumarolic alteration and subsequent cooling and weathering of the protolith. Samples of the underlying, dominantly leached, dacite-rich portion of the ash-flow tuff adjacent to the fumarolic conduit and samples of encrusted fallout from the shallow part of the fossil fumarole were interpreted using the isocon method of Grant (1986). The results show that, relative to unaltered l9l2 dacite, chosen as a standard composition for the protolith in this fossil fumarole, mass was conserved during the alteration reactions for most of the system, but mass gains of l4–2D% were determined for three samples in the leached ash-flow tuff Relative to unaltered dacite protolith, significant enrichments occurred in SO 3 , LOI (~H 2 O), Cl, F, Zn, Pb, Cu, Sn, Cr, Ni, As, Sb, Au, Br in various parts of the fossil fumarole. Some of these were during the high-temperature part of the alteration, and some were during cooling processes when acid alteration becomes prominent. The REEs indicate some depletion in highly altered samples relative to dacite protolith and differential mobility of Eu 2+ relative to trivalent REEs. This is manifested by positive Eu anomalies in REE patterns normalized against REE in the dacite protolith. Mineral phases introduced in the alteration assemblages include alunite reflecting high SO 3 , activity, hydrated aluminum hydroxy-fluoride (a ralstonite-like phase) and fluorite reflecting high F activity, smectite, magnetite, hematite, and goethite reflecting oxidation and hydration reactions. Opal and a portion of the α-cristobalite reflect SiO 2 , mobility; however, the abundance of α-cristobalite is formed from pumice leached during high-temperature vapor-phase processes and devitrification of the altered glass.
Interaction between country rock and fluids derived from the Tip Top pegmatite has resulted in a series of boron enriched assemblages. Between unaltered quartz-mica schist to the pegmatite contact is a succession of four mineral assemblages: 1. (1) Quartz-Biotite-Potassium Feldspar assemblage (Q-B-K), which consists essentially of the original metamorphic silicate assemblage plus anomalously high amounts of modal tourmaline 2. (2) Quartz-Biotite-Tourmaline assemblage (Q-B-T) 3. (3) Tourmaline-Quartz-Muscovite assemblage (T-Q-M) 4. (4) Tourmaline-Quartz assemblage (T-Q). Alkali elements (Cs, Rb, K, Li), SiO 2 , and Ba show a decrease from the Q-B-K assemblage to the T-Q assemblage. A1 2 O 3 , Ga, B, total Fe and Zn increase moderately from the Q-B-K assemblage to the T-Q assemblage. The mineral chemistries also change considerably. The Mg/(Mg + Fe 2+ ) ratios in biotites range from 0.54 to 0.50 in samples from the Q-B-K assemblage to 0.39 in the (Q-B-T) assemblage. The range in tourmaline end-member components from the Q-B-K assemblage to the T-Q assemblage is as follows: Q-B-K: Dravite .63 Schorl .23 Elbaite .05 Buergerite .09 T-Q: Dravite .23 Schorl .37 Elbaite .17 Buergerite .23 . Observed variations in mineral assemblage and whole rock chemistry within the alteration zone appear to a first approximation to be a function of μB 2 O 3 (boron metasomatism) and μK 2 O (alkali leaching). The breakdown of feldspar and biotite may be approximated by reactions: 2HCl + 2(K, Na)AlSi 3 O 8 /ai 2(K, Na)Cl + Al 2 SiO 5 + 5SiO 2 + H 2 O and 2 Annite + SiO 2 + 5Al 2 SiO 5 + 2NaCl + 6H 3 BO 3 /ai 2 Tourmaline + 2KCl + 7H 2 O. The alteration zone may represent either a single episode (B-, Cs-, Li-, Rb-enriched fluid) or multiple episodes (B, Zn, Mn fluid and Cs, Li, Rb fluid) of pegmatite fluid-schist interactions. In both situations, B in the aqueous fluid from the pegmatite reacts with the schist breaking down sheet silicate “traps” for Cs, Rb, Li, and K and forming tourmaline-rich assemblages.