Timescales for fluid storage and release in porphyry Cu-Mo systems—Timescales for felsic magma storage and volcanic eruptions H. STEIN 1,2 , R. MARKEY 1 1 AIRIE Program, Department of Geosciences, Colorado State University, USA (hstein@warnercnr.colostate.edu) 2 Geological Survey of Norway, 7491 Trondheim, Norway In today’s world, society demands scientific data to quantify geologic processes and predict geologic events that impact daily life. Primary among these are volcanic eruptions and all the surfi- cial chaos associated with these colossal releases of energy. To investigate the timeframe and cyclicity for release of magmatic- hydrothermal fluids at sub-volcanic crustal levels, we use Re–Os dating of molybdenite from very young porphyry-style Cu–Mo deposits in Chile. Using a double Os spike to correct for isotopic fractionation and any common Os, we define the timing for suc- cessive bursts of ore-forming fluids carrying sufficient energy to rupture overlying rocks. The porphyry Cu–Mo environment is ideal for this exercise, as fluid bursts are instantaneously captured as veins filling fractured rock rather than released to the atmo- sphere as happens with eruptions. We obtained well-documented vein samples representative of distinct hydrothermal stages linked to the evolution of a porphyry Cu–Mo stockwork deposit. In some cases we dated molybdenite from two cross-cutting quartz veins in the same hand sample, obtaining the expected age rela- tionship. For magmatic-hydrothermal systems whose age is less than 10 Ma, hydrothermal bursts can be delineated on a time- frame of 10,000 to 20,000 years. The collective magmatic-hydro- thermal activity for a large porphyry-style Cu–Mo deposit may span several million years, but sets of B-stage (mainstage) planar stockwork veins form on the scale of thousands of years. The win- dow for superposition of intense economic veining to build a giant porphyry-style deposit is 100,000 to 400,000 years. We compare data for hydrothermal bursts to estimates for arrival and sequestering of felsic magma prior to its release through eruption. The development of a stockwork vein system and its alteration halo (with or without economic mineralization) provide episodic crack and seal events that repeatedly but only temporarily relieve upward pressure. As such, the development of a sub-volcanic Cu–Mo porphyry system may have the ability to forestall eruption in silicic magma chambers provided the peri- odic fluid release events and associated alteration strengthen (e.g. silicification) rather than weaken the overlying rocks. The relative rates of uplift and erosion also influence the ability of a porphyry system to breach the surface. doi:10.1016/j.gca.2006.06.1136 Experimental study of Nd and Ho hydrolysis at elevated temperatures by spectrophotometric method S.A. STEPANCHIKOVA, G.R. KOLONIN Institute of Geology, Geophysics and Mineralogy Novosibirsk (step@uiggm.nsc.ru; kolon@uiggm.nsc.ru) The geochemical fractionation of REE elements in hydrother- mal processes depends on difference in the stability of their aque- ous species. A comparison of temperature influence of Nd and Ho chloride complexes as representatives of the first and third triades of REE has been demonstrated at one of the previous Goldsch- midt Conferences 1 and later in the extended paper 2. The first spectrophotometric data about the stability of both Nd and Ho hydroxide complexes at 25 °C as well a temperature influence on their speciation can be presented now. The special spectrophotometric method using 2-naphtol as pH- indicator was used to compare Nd and Ho complexing at ionic strength no more then 0.0004. As follows from results of experiment, the hydrolysis of Nd pass from Nd 3+ to Nd(OH) 2 + through Nd(OH) 2+ , as well as hydrolysis of Ho pass from Ho 3+ to Ho(OH) 4 À through Ho(OH) 2+ , Ho(OH) 2 + and Ho(OH) 3 0 . References Kolonin, G.R., Morgunov, K.G., Stepanchikova, S.A., . Goldschmidt Conf. 62A, 803–804. Stepanchikova, S.A., Kolonin, G.R., . Rus. J. Coord. Chem. 3, 207–217. doi:10.1016/j.gca.2006.06.1137 Goldschmidt Conference Abstracts 2006 A613