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Following the 2002 M 7.9 Denali fault earthquake, clear changes in geyser activity and a series of local earthquake swarms were observed in the Yellowstone National Park area, despite the large distance of 3100 km from the epicenter. Several geysers altered their eruption frequency within hours after the arrival of large-amplitude surface waves from the Denali fault earthquake. In addition, earthquake swarms occurred close to major geyser basins. These swarms were unusual compared to past seismicity in that they occurred simultaneously at different geyser basins. We interpret these observations as being induced by dynamic stresses associated with the arrival of large-amplitude surface waves. We suggest that in a hydrothermal system dynamic stresses can locally alter permeability by unclogging existing fractures, thereby changing geyser activity. Furthermore, we suggest that earthquakes were triggered by the redistribution of hydrothermal fluids and locally increased pore pressures. Although changes in geyser activity and earthquake triggering have been documented elsewhere, here we present evidence for changes in a hydrothermal system induced by a large-magnitude event at a great distance, and evidence for the important role hydrothermal systems play in remotely triggering seismicity. Keywords: Yellowstone National Park, geysers, hydrothermal processes, earthquake swarms.

Quartz veins in porphyry copper deposits record the physiochemical evolution of fluids in subvolcanic magmatic-hydrothermal systems. We have combined cathodoluminescence (CL) petrography with fluid-inclusion microthermometry to unravel the growth history of individual quartz veins and to link this history to copper ore formation at Bingham, Utah. Early barren quartz veins with K-feldspar + biotite (potassic) alteration selvages occur throughout the 2 km vertical exposure of quartz monzonite porphyry stock. At depths of 500 m to at least 1350 m below the orebody, fluid inclusions in these barren veins trapped a single-phase C[O.sub.2]-bearing fluid containing ~2-12 wt% Na[Cl.sub.equiv]. Within and to depths of 500 m below the orebody, early quartz veins contain abundant hypersaline liquid (38-50 wt% Na[Cl.sub.equiv]) and vapor-rich inclusions trapped together at temperatures of 560-350[degrees]C and pressures of 550-140 bar, consistent with fluctuations between lithostatic and hydrostatic pressure at paleodepths of 1.4 to 2.1 km. CL petrography shows that bornite and chalcopyrite were deposited together with a later generation of quartz and K-feldspar in microscopic fractures and dissolution vugs in early barren quartz veins and wall rock. This late quartz contains hypersaline liquid (36-46 wt% Na[Cl.sub.equiv]) and vapor-rich inclusions trapped at 380-330[degrees]C and at 160-120 bar hydrostatic pressure. We conclude that a single-phase magmatic-hydrothermal fluid underwent phase separation to hypersaline liquid (or brine) and vapor ~500 m below the base of the orebody at a paleodepth of ~2.5 km. Brine and vapor continued to ascend and formed multiple generations of barren quartz veins with potassic selvages. Thermal decline to temperatures below 400[degrees]C was the main driving force for copper-iron sulfide deposition, given the lack of evidence of mixing of brines with low-salinity waters, the lack of correspondence of the ore zone with the initiation of phase separation, and no change in wall-rock alteration style. Keywords: Bingham Canyon, porphyry copper, fluid inclusions, cathodoluminescence.

The chemistry of brine, vapor, and low-salinity fluid inclusions measured by proton-induced X-ray emission from the Bismark skarn deposit, Mexico, is consistent with an evolving magmatic-hydrothermal system with no evidence for external fluid inputs. The results support a model that invokes early phase separation of magmatic fluids into brine and vapor (two-phase field) at high temperature and lithostatic pressure, followed by the entrapment of a low-salinity magmatic fluid (one-phase field) at lower temperature and hydrostatic pressure. The early brine and vapor inclusions contain high Pb and Zn concentrations and low Cu; however, the vapor contains significantly more Cu than the brine and was likely transported as a sulfur complex. The fluid phase changes observed and behavior of Cu are comparable to those of porphyry Cu systems. The later, low-salinity fluid at Bismark represents a distinct pulse of magmatic fluid with a high K/Ca ratio in which base metals, including Cu, were transported by chloride complexes. Paragenetic relationships and variations in the relative concentrations of Cu, Pb, and Zn suggest that this fluid was primarily responsible for ore deposition. The relative Cu, Pb, and Zn concentrations in fluid inclusions at Bismark are consistent with those measured from base-metal ores in high-temperature carbonate-replacement deposits throughout Mexico, and suggest that the bulk-metal budget of these deposits is primarily controlled by magmatic-hydrothermal fluids deficient in Cu. Keywords: Mexico, skarn, fluid inclusions, proton-induced X-ray emission, base metals.

We analyzed 25,675 traveltime residuals from a three-dimensional seismic tomographic inversion to investigate crack-induced seismic anisotropy in the upper oceanic crust. The study covered two regions with contrasting levels of magmatic activity on the western limb of the 9[degrees]N overlapping spreading center on the East Pacific Rise. The level of anisotropy gradually decreases with depth in the magmatically and hydrothermally active ridge region. In contrast, we observed a highly variable anisotropic structure in the magmatically and hydrothermally less active tip region at the end of the dying ridge segment: a weakly anisotropic layer beneath strongly anisotropic extrusive volcanic rocks is likely to be the result of relatively shallow cracks closed by hydrothermal precipitation. Strongly anisotropic dikes with inferred narrow and water-saturated cracks provide important along-axis pathways for the circulation of hydrothermal fluids beneath the shallow cracks in the less magmatically active regions. Furthermore, a significant clockwise rotation (20[degrees]-30[degrees]) of fast directions occurs in both regions with increasing depth. Such a rotation provides evidence that the geometry of the underlying crack structure of the western limb is significantly different from that defined by the bathymetric ridge crest. Keywords: dikes, East Pacific Rise, hydrothermal processes, mid-ocean ridges, ophiolite, paleotectonics.

The collision of a seamount or microcontinental block with an arc may produce complex strain distributions and changes in the tectono-magmatic setting. Slab rollback and associated upwelling of asthenosphere occurred ca. 440 Ma near the margin of East Gondwana in response to subduction zone lock-up following arc collision ca. 455 Ma. Worldclass porphyry copper gold deposits (e.g., Cadia) formed in the backarc during extension and magmatism associated with slab roUback. Upwelling of the asthenosphere also provided the heat engine for generation of hydrothermal fluids. The result was elevated fluid pressures and compressional brittle deformation associated with orogenic gold mineralization (e.g., Bendigo) inboard of the East Gondwana margin. Keywords: East Gondwana, Lachlan orogen, gold, slab rollback.

Ocean Drilling Program (ODP) Leg 193 investigated two sites of hydrothermal activity along the crest of the Pual Ridge in the eastern Manus Basin. A site of low-temperature diffuse venting, Snowcap (Site 1188), and a high-temperature black smoker site, Roman Ruins (Site 1189), were drilled to depths of 386 and 206 m below seafloor (mbsf), respectively. Although the two sites are Keywords: hydrothermal vents, sulfides, anhydrite, isotopes.

The effect of gold scavenging by bismuth melts is investigated using equilibrium thermodynamic modeling of an aqueous solution-mineral-melt system. The calculations for the Au-Bi-Na-Cl-S-H-O system, performed at temperatures between 300 and 450 °C, demonstrate that Au concentrations in the melt are several orders of magnitude higher than in the coexisting fluid, indicating the possible formation of economic gold deposits from undersaturated aqueous fluids, in which mineralization would not be expected in the absence of a bismuth melt. The model applies to any deposit where a Bi melt is stable and coexists with a hydrothermal fluid; examples of such deposits are known from skarn, intrusion-related, orogenic, and volcanogenic massive sulfi de (VMS) gold systems. In sulfur-poor systems the partitioning curves presented here can be used directly to correlate the gold concentration in the fluid and the Au grade in the ore (e.g., Escanaba Trough VMS deposit). These results also illustrate important principles generally applicable to understanding magmatic-hydrothermal and metamorphic deposits that may have contained significant volumes of more complex polymetallic melts. [ABSTRACT FROM AUTHOR]

We present a new method to detect and reconstruct fluid pathways in fossil hydrothermal systems that is based on systematic study of white micas in hydrothermally altered rock, using near-infrared spectroscopy. This method, developed in an Early Archean volcanic sequence in the Panorama district in Western Australia, uses new near-infrared spectroscopic data and published geological and geochemical data of fossil submarine hydrothermal systems in the area. Analysis of new near-infrared spectroscopic data revealed that the abundance of white mica and its Al content in altered volcanic rock vary systematically along fossil fluid pathways, from zones of low-temperature recharge to zones of high-temperature discharge, as a function of hydrothermal fluid chemistry, temperature of alteration, coexisting minerals, and composition of volcanic host rock. The abundance of white mica relative to that of chlorite and the Al content of white micas can be used to discriminate among the hydrothermal alteration facies along these fluid pathways, which permits detection and reconstruction of fossil fluid pathways. [ABSTRACT FROM AUTHOR]