Rhyolite Domes Research Articles

Exploration in a volcano-plutonic center at Guanajuato, Mexico

In the northern part of the Guanajuato district, a Cretaceous volcano-sedimentary sequence has tectonically been overridden by thrust sheets that contain basic and ultrabasic rocks of ophiolite affinities and tonalite of a continental magmatic arc, all of Mesozoic age. Subsequently, the volcano-sedimentary sequence was intruded by an early Tertiary granitoid batholith. Subeconomic base metal and polymetallic veins and two examples of stratiform sulfides have been emplaced in the mixed sequence and deformed by tectonic events of Laramide age.Later, following erosion and preserved in the central and southern part of the district, the continental Guanajuato Conglomerate was overlain by a sequence of Oligocene volcanic rocks, related to a volcano-plutonic center which exhibits several characteristics of a caldera, formed near the intersection of regional northeast and northwest extensional fracture systems of mid-Oligocene age. Caldera fill includes a base surge deposit and up to 360 m of rhyolitic fill, megabreccia, and peripheral rhyolite domes. The northern and central parts of the volcanic center and beyond have been cut by three major, but local, northwest-striking, silver-rich, fissure vein systems accompanied by proximal silicification and widespread pro-pylitization of host rocks and less extensive crosscutting phyllic and argillic alteration facies.Previous investigations have shown that three mineralization stages occurred, with gold to silver ratios which vary from 1/72 to 1/214 in ores that have recently been exploited. In the economically important Lower zone, four overlapping, vertically stacked ore assemblages occur, in the lower three of which adularia is common. There is a deep base metal-rich assemblage below the 1,800-m elevation associated with a monzonite intrusion, followed upward by the main silver-bearing assemblage between the 1,800- to 2,350-m elevation. Recognition of this vertical zonal arrangement led to a major discovery on the southeastern part of the Veta Madre in 1968 that includes silver-rich, fissure-filling, and stockwork mineralization. The uppermost gold-rich, silver sulfosalt, and selenium assemblage is located in transverse, northeast-striking veins between the 2,250- and 2,550-m elevation. In the past, fluid inclusion data suggested that precious metals were deposited above a boiling level attended by repeated flashing, deposition, and sealing. But more recent investigations conclude that the boiling level was above the eroded surface that contains the Lower zone except for the gold-rich Cubo mine whose upper workings are above the reconstructed boiling level.Of major economic significance in the district was the discovery during 1978-1983 of the important, uppermost northeast-striking, gold-rich ores in the San Nicolas and adjacent veins, perpendicular to the previously known northwest-striking Sierra vein system in the Cubo mine. These veins cut the Peregrina rhyolite dome located in the wall of the collapse structure. Additionally, several parallel gold-rich veins in the eastern part of the district parallel or occupy northeast-trending fractures that control a latite dike swarm. The San Nicolas vein has an average gold to silver ratio of approximately 1/18 and the ore occupies the highest stratigraphic, structural, and topographic part of the district. All four ore assemblages of the Lower zone were emplaced by hydrothermal fluids of mid-Oligocene age.In contrast, Intermediate zone subeconomic mineralization discovered in 1988 occurs in northeast- and northwest-striking fractures and breccias in the northwestern part of the district and beyond the caldera limits. It is characterized by proximal silicification and advanced argillic alteration and widespread propylitization. A suite of trace elements in anomalous amounts includes Au, Ag, Cu, Hg, As, and Sb, occurs in a 200-m interval above the Lower zone, and is probably mid- to late Oligocene in age. The Upper zone, associated with a range front fault of northwest orientation, consists of sinter deposits of Recent age and active hot springs, characterized by H 2 S in escaping fluids that have a water temperature of 96.4 degrees C. These later and apparently unrelated geothermal systems occupy the highest part of the epithermal system, and with the Intermediate zone deposits, represent a shift of mineralizing activity from within the volcanic center and central part of the district, northwestward and peripheral to the economic orebodies.In the past, the source of metals in the Guanajuato ores has variously been attributed to processes related to granitic intrusions or leaching of Tertiary volcanic rocks or the Mesozoic sedimentary strata. More recently, oxygen, carbon, and lead isotope studies suggest that ore deposition is related in part to fluids of magmatic composition which mixed with isotopically exchanged meteoric water that circulated through the volcanic pile. This interpretation is supported by the recognition of volcano-plutonic activity in the form of a partially eroded and structurally deformed caldera that overlaps the ore-forming epoch. Spatially, the deep base metal-rich ore and the gold-rich assemblage of the Lower zone are related to plutonic phases adjacent to and within the collapse feature, respectively. Overall, the mineralizing epoch at Guanajuato is contemporaneous with widespread mid-Tertiary magmatic activity and formation of fissure-vein metallic deposits in northern Mexico, attributable to subduction processes.

The magmatic hydrothermal system at Julcani, Peru; evidence from fluid inclusions and hydrogen and oxygen isotopes

The Julcani district consists of at least six mineralized centers in a 16-km 2 late Miocene, dacitic to rhyolitic dome complex that is built on a approximately 3,000-m-thick Paleozoic-Mesozoic clastic sedimentary section. Mineralization occurred within 0.5 m.y. of the youngest dome formation and was prior to emplacement of an anhydrite-bearing dacite dike. Hydrothermal events began with formation of fracture-controlled acid sulfate alteration zones having vuggy silica cores and successive quartz-alunite and quartz-kaolinite envelopes. The acid sulfate alteration in the center of the district was overlapped and followed by the emplacement of radiating swarms of tourmaline-pyrite breccia dikelets cemented by quartz-containing saline fluid inclusions. Several centers of mineralization developed 0.5 to 2 km apart along reactivated regional structures after the emplacement of a dacite dike most likely related to an underlying intrusion. The ores consist of Ag-Cu-Pb-W-Bi-Au-bearing mineral assemblages that occur as fracture fillings within the domes and underlying tuffs. Mineralization reached within 200 to 300 m of the paleosurface. The ores are strongly zoned with a general succession of quartz + pyrite + wolfamite-, enargite-, tetrahedrite-, galena-, and barite-dominant assemblages grading outward east and west from the central zone. Late-stage siderite and/or botryoidal pyrite overprint all zones, even at the deepest levels.Filling temperatures and salinities for inclusion fluids in main-stage minerals range from 250 degrees to 325 degrees C and 8 and 19 wt percent NaCl equiv, respectively. Filling temperatures for inclusions in late-stage siderite range from 170 degrees to 225 degrees C and salinities average about 7.5 wt percent NaCl equiv. Temperatures tend to correlate with salinities and both tend to decrease in the fluids of successively younger and more distal minerals from wolframite to enargite to siderite.The calculated delta D (sub H 2 O) and delta 18 O (sub H 2 O) values of the preore alunite-forming fluids are about -46 + or - 5 and 7 + or - 1 per mil, respectively. The delta D (sub H 2 O) and delta 18 O (sub H 2 O) values of inclusion fluids in later main-stage wolframite, enargite, tetrahedrite, and galena and late-stage siderite show a linear trend, ranging from -60 to -130 and 4 to -18 per mil, respectively. Based on the composition of biotite phenocyrsts in glassy volcanic rocks that bracket ore deposition, primary magmatic fluids had delta D (sub H 2 O) and delta 18 O (sub H 2 O) values of about -70 + or - 20 and 9.7 + or - 0.6 per mil, respectively.The fluids responsible for main-stage mineralization as well as preore acid sulfate and tourmaline breccia fluids were distinctly magmatic. These ore fluids, however, were highly exchanged (similar in isotopic composition to alunite-forming fluids) and were not derived directly from the magma but from the low water-rock environment between the fluid-rich carapace of the magma and the brittle-ductile transition of the overlying rocks. Ore deposition occurred when the exchanged magmatic fluids mixed with meteoric water at higher levels.

Stratigraphy, paleomagnetism, geochronology and structure of silicic volcanic rocks, Waiotapu/Paeroa range area, New Zealand

The altered ignimbrite sequence underlying the Waiotapu Geothermal Field has long been correlated with unaltered ignimbrite sequences exposed in the Ngapouri and Paeroa fault-scarps to the west. Recent age-dating (fission-track and K-Ar) and magnetic polarity studies have indicated this correlation is invalid. The Paeroa Scarp section comprises three major ignimbrites (Paeroa, Te Weta and Te Kopia) erupted between 0.34 and 0.38 Ma ago from local caldera sources. The Caldera Boundary Fault (CBF) marking the eastern boundary of the Paeroa Caldera crosses the northern Paeroa Range between the Waiotapu and Waikite thermal areas. East of the CBF, the Ngapouri Scarp section comprises the Waiotapu Ignimbrite (0.58 ± 0.03 Ma) unconformably overlying a reversely magnetised sequence of late Matuyama age ignimbrite (Akatarewa A) and epiclastic tuff (Unit X), intruded by rhyolite (Ngapouri Rhyolite). Episodic caldera collapse has spalled exotic blocks of Waiotapu Ignimbrite and Ngapouri Rhyolite into the caldera, where they form a discontinuous layer between two sheets of Paeroa Ignimbrite. In the Waiotapu Geothermal Field - separated from the Ngapouri Ridge by a younger rhyolite dome - the Waiotapu Ignimbrite is underlain by Akatarewa ignimbrites (A,B) separated by epiclastic tuffs (Unit Y) overlying a truncated andesite cone (Ngakoro Andesite) and an older unnamed Ignimbrite C; all are of late Matuyama age, possibly extending back to the Jaramillo sub-chron. Younger sheets of Rangitaiki (0.35 ± 0.03 Ma), Matahina (0.28 ± 0.04 Ma), Ohakuri (0.27 ± 0.03 Ma) and Kaingaroa (0.22 ± 0.04 Ma) ignimbrite were emplaced across the area, the latter from the nearby Reporoa Basin (caldera). The progressive uplift of the Paeroa and Ngapouri tilted fault-blocks and further subsidence of the Reporoa Basin allowed the accumulation of thick lake beds (Huka Group) which capped geothermal reservoirs, originally established and later rejuvenated by episodic caldera formation. Present-day upflow zones are largely concentrated by active faults and fault intersections breaching the cap-rocks. Hydrothermal eruptions from self-sealed shallow reservoirs throughout the Waiotapu-Waikite area were commonly initiated by volcanic and/or tectonic events on major faults cutting and linking major calderas.

Physical volcanology and emplacement history of the Ben Lomond rhyolite lava flow, Taupo Volcanic Centre, New Zealand

Abstract Currently, there are two contrasting models concerning the state in which rhyolite lava reaches the surface: (1) the permeable foam model, where the lava reaches the surface as an expanded foam and collapses into dense obsidian during lava flow; and (2) the traditional model, where decreased load pressure leads to vesiculation of the upper surface of the dome. In order to test these models, textural parameters were documented on outcrops of the Ben Lomond rhyolite dome, which has two flow lobes comprising an eroded dome (c. 100 ka old) cut by the Whangamata Fault. A composite stratigraphy of the upper 60 m of the flow, assembled from three sections (roadcut, airstrip and fault scarp), comprises, irom the flow top: finely vesicular pumice, black aphyric obsidian, and a spherulitic transition zone above a central crystalline rhyolite core. An explosion breccia occurs as a pod at a depth of c. 35 m and cross‐cuts the upper two units us an inverted cone‐shaped deposit—an infilled explosion pit. Eight...

40Ar/39Ar age constraints for the Jaramillo Normal Subchron and the Matuyama‐Brunhes geomagnetic boundary

Our mid‐Pleistocene 40Ar/39 Ar age recalibration of the geomagnetic polarity timescale is nearly in accord with the oxygen isotope, climate record calibration of the astronomical timescale proposed by Johnson (1982) and Shackleton et al. (1990). 40Ar/39Ar ages of a normally magnetized rhyolite dome in the Valles caldera, northern Mexico, yielded a weighted‐mean age of 1.004±0.019 Ma. A K‐Ar age of 0.909±0.019 Ma for this rock by Doell and Dalrymple (1966) was the linchpin for the recognition and calibration of the Jaramillo Normal Subchron (JNS). Other 40Ar/39Ar ages from the Valles caldera and 40Ar/39Ar ages of Ivory Coast tektites indicate that the JNS began at about 1.11 Ma and ended before 0.92 Ma, probably near 0.97 Ma. The Matuyama‐Brunhes boundary occurred between 0.79 Ma and 0.76 Ma on the basis of 40Ar/39Ar sanidine ages from (1) three reversely magnetized rhyolite domes of the Valles caldera (0.793±0.018 Ma, 0.794±0.007 Ma, and 0.812±0.023 Ma) and pumice (0.789±0.006 Ma) from the reversely magnetized Oldest Toba Tuff of Sumatra and (2) pumice (0.764±0.005 Ma and 0.757±0.009 Ma) from the lower and upper units of the normally magnetized Bishop Tuff. The age of the boundary may be close to 0.77 Ma as deduced from rates of sedimentation in ancient Lake Bonneville, Utah.

40Ar/39Ar chronology and volcanology of silicic volcanism in the Davis Mountains, Trans-Pecos Texas

Field studies and 40 Ar/ 39 Ar dating reveal that silicic volcanism in the Davis Mountains part of the Trans-Pecos Texas volcanic field occurred in six episodes at 0.3 m.y. intervals between 36.8 and 35.3 Ma. Additionally, two groups of silicic intrusions were emplaced at 34.6 and 32.8 Ma. This episodicity is similar to that determined for volcanic fields dominated by ash-flow tuffs, yet voluminous, extensive silicic lavas are considerably more abundant than tuffs in the Davis Mountains, by number of flows and by volume. The preponderance of voluminous silicic lavas over tuffs most likely reflects low water contents and high temperatures of the alkalic and commonly peralkaline Davis Mountains magmas. The earliest episode, at 36.8 Ma, included a widespread and voluminous (possibly >1,000 km 3 ) suite of rhyolite and quartz trachyte lavas, several rhyolite domes, and a strongly rheomorphic, peralkaline ash-flow tuff erupted from a caldera in the northern Davis Mountains. The lava suite extends well beyond the Davis Mountains. Silicic lavas of all episodes probably erupted from widespread, fissure vents. The 36.5 Ma episode consisted of rhyolite to quartz trachyte lavas, also extensive and voluminous (∼200 km 3 ). The 36.3 Ma episode consisted of rhyolite to trachyte tuffs and lavas erupted from a central vent volcano in the southern Davis Mountains. The 35.9 Ma episode consisted of a single, moderately large (∼50 km 3 ) rhyolite lava and a small-volume ash-flow tuff erupted from a caldera in the western Davis Mountains. Rocks emplaced during the 35.6 Ma episode were also rhyolites, including an enigmatic rock that may be strongly rheomorphic ash-flow tuff or a combination of tuff and lava, followed by definite lavas. The 35.3 Ma episode consisted of two ash-flow tuffs, one of which is strongly rheomorphic, and additional voluminous rhyolite lavas (∼120 km 3 ). The rheomorphic tuff erupted from a caldera in the southwestern Davis Mountains. The source of the other tuff is probably in the western Davis Mountains. Intermediate and mafic rocks are minor, except around the southeastern flank of the Davis Mountains, where basalt is abundant. Mafic lavas erupted only during gaps in the silicic activity and on the flanks of the Davis Mountains. Nevertheless, basaltic magma probably drove the silicic magmatism, either by differentiation or by crustal melting, and was present throughout the time of Davis Mountains activity but could not penetrate the low-density silicic magma chambers until they cooled and solidified. The time required for cooling and solidification appears to be 0.1-0.2 m.y.

Flux of volatiles and ore-forming metals from the magmatic-hydrothermal system of Satsuma Iwojima volcano

Research Article| July 01, 1994 Flux of volatiles and ore-forming metals from the magmatic-hydrothermal system of Satsuma Iwojima volcano Jeffrey W. Hedenquist; Jeffrey W. Hedenquist 1Geological Survey of Japan, 1-1-3 Higashi, Tsukuba 305, Japan Search for other works by this author on: GSW Google Scholar Masahiro Aoki; Masahiro Aoki 1Geological Survey of Japan, 1-1-3 Higashi, Tsukuba 305, Japan Search for other works by this author on: GSW Google Scholar Hiroshi Shinohara Hiroshi Shinohara 1Geological Survey of Japan, 1-1-3 Higashi, Tsukuba 305, Japan Search for other works by this author on: GSW Google Scholar Author and Article Information Jeffrey W. Hedenquist 1Geological Survey of Japan, 1-1-3 Higashi, Tsukuba 305, Japan Masahiro Aoki 1Geological Survey of Japan, 1-1-3 Higashi, Tsukuba 305, Japan Hiroshi Shinohara 1Geological Survey of Japan, 1-1-3 Higashi, Tsukuba 305, Japan Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1994) 22 (7): 585–588. https://doi.org/10.1130/0091-7613(1994)022<0585:FOVAOF>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jeffrey W. Hedenquist, Masahiro Aoki, Hiroshi Shinohara; Flux of volatiles and ore-forming metals from the magmatic-hydrothermal system of Satsuma Iwojima volcano. Geology 1994;; 22 (7): 585–588. doi: https://doi.org/10.1130/0091-7613(1994)022<0585:FOVAOF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Satsuma Iwojima volcano, southwest Japan, degasses 5 x 106 t/yr H2O, 9 x 10 4 t/yr S, and 6 x 104 t/yr Cl from high-temperature (≤880 °C) fumaroles atop a now-altered rhyolite dome that erupted 1200 yr ago. Acidic hot springs (pH 1.1-1.8) discharge ∼20 x 106 t/yr H2O to the sea; this water is composed of ∼1 part magmatic vapor absorbed by 6 parts meteoric ground water. The Cl and SO4 in solution originate from the vapor, whereas cation components are derived largely by dissolution of the rhyolite. The flux of Pb, Zn, Cu, and Mo in the vapor and acidic springs ranges from 0.1 to 10 t/yr each, whereas the Au flux is 10-5 and 10-3 t/yr, respectively. The low concentrations of NaCl and metals in the vapor are due to the condensation of a hypersaline liquid from the vapor during ascent and depressurization, meaning that the atmospheric-pressure vapor does not reflect the composition of the fluid exsolving from the magma. Neither this low-pressure vapor nor the acidic waters can account for high-sulfidation Cu-Au ore deposits deduced to have formed in this environment; such mineralization requires the subsequent ascent of a metal-rich fluid. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Relationship between low-grade gold disseminations and high-grade veins in the Quartz Mountain district, Oregon

Gold and mercury mineralization related to epithermal hot springs occurs spatially associated with rhyolite dome complexes in the Quartz Mountain district, Lake County, Oregon. Drilling of over 800 holes from 1983 to 1991, mostly by Quartz Mountain Gold Corp., has resulted in a mineral inventory of at least 100,000,000 tons, averaging 0.0255 ounce gold per ton in two near-surface, disseminated gold ore bodies, Quartz Butte and Crone Hill. Extensive gold disseminations are underlain by and include rare, higher-grade veins that could be feeders for the near-surface, quasi-horizontal, pervasively mineralized gold dissemination. The veins contain about 2,000,000 tons of ore, aver-aging 0.29 ounce gold per ton. Approximately 750,000 tons of this underlie Quartz Butte, and the remainder is included in the Crone Hill low-grade dissemination. With a mineral inventory of about 3,000,000 ounces, Quartz Mountain is currently the largest known gold deposit in Oregon.

Preliminary geologic studies of Sierra El Aguajito (Baja California, Mexico): a resurgent-type caldera

Geologic field studies conducted in the Quaternary volcanic field of Tres Virgenes (State of Baja California Sur, Mexico) revealed the existence of a resurgent caldera. The caldera's eruptive products, the Aguajito sequence, overlie the products of the nearby Reforma caldera (Reforma sequence) whose youngest products have already been dated as Quaternary. The rim of the Aguajito caldera is inferred by the existence of an arcuate alignment of rhyolitic domes. The mean diameter of this subcircular feature is 10 km. The volume of its mapped acidic products is a minimum of 10 km 3. Several horizons within the sequence contain shells. K/Ar dates of the ignimbrites and domes of El Aguajito formation confirm that the unit are Pleistocene. The detailed stratigraphy also shows the evolution of a marine regression partly related to the caldera.

40Ar/39Ar Geochronology of Post‐Valles Caldera Rhyolites, Jemez Volcanic Field, New Mexico

The40Ar/39Ar dating of postcollapse rhyolites in the Valles Caldera (1140 kyr) defines five intervals during which rhyolite domes, flows, and pyroclastic rocks were emplaced from ring fracture vents. Previously, ages were based on limited K‐Ar dating and stratigraphic relations. The 40Ar/39Ar dates are concordant with the previous dates for about half of the samples; others have ages significantly older (7–25%) despite evidence for small amounts of xenocrystic material. Eruptive activity occurred during relatively brief periods at 1133, 973‐915, 800‐787, 557‐521, and ∼300‐170 kyr separated by quiescent intervals of ∼115–300 kyr. Isochrons defined by single‐crystal sanidine fusions indicate trapped 40Ar/39Ar ranging from ∼282 to 326. Sanidine analyses yield reliable and consistent results, whereas biotite and glass data may be ambiguous. Biotite may have low radiogenic yields, contain variable amounts of extraneous argon, or define apparent isochrons discordant with cogenetic sanidine. Unaltered, unhydrated glass in one case yields an isochron concordant with coexisting sanidine, whereas in another is undateable due to large amounts of non‐radiogenic argon. The revised chronology has implications for the evolution of the Jemez magmatic system as well as for the geomagnetic polarity timescale. The discrete periods of volcanism, relations among rhyolites erupted within these intervals, and the duration of the intervening quiescent intervals are consistent with the generation, evolution, and eruption of several distinct magmas following caldera collapse. In addition, the 40Ar/39Ar data suggest revision of the Brunhes ‐ Matuyama polarity reversal boundary from the currently accepted 730 kyr to 780 kyr.

Sedimentation in Volcanic Settings

The craters associated with the 1886 AD phreatomagmatic Rotomahana eruption, Okataina Volcanic Centre, New Zealand, and the near-vent geology are now hidden beneath Lake Rotomahana and its post-eruptive sediment fill. Lithic clasts from the near-vent lithic lapilli ash deposits of the Rotomahana Pyroclastics are used in this study to trace geological and geothermal conditions before the eruption, as well as vent excavation dynamics. Near-vent deposit characteristics were described in the field, representative lithic clasts were documented petrographically, and unaltered clasts were analysed for major and trace element compositions. The majority of the lithic clasts were rhyolites with subordinate ignimbrites and hydrothermally altered clasts, and trace siltstone and silicified clasts. The rhyolites were classified into four petrographic groups according to phenocryst content and assemblage and were more diverse with respect to geochemical compositions. Most of the rhyolite lithics in the Rotomahana Pyroclastics did not match the rhyolite domes exposed subaerially around the lake, but did have affinities with the pre-Matahina caldera Wairua Rhyolite, and potentially other older non-exposed domes. Ignimbrites most likely correlated either to the Matahina ignimbrite or older non-exposed units. Hydrothermally altered rhyolite and ignimbrite lithic clasts are common and suggest that there has been a long-lived hydrothermal system in this sector, possibly dating back to early activity of the Okataina Volcanic Centre. The diversity in lithic types indicate a spatial variation in country rock lithology and strength, which probably contributed to the vent position and morphology along the Rotomahana fissure.

Time-stratigraphic framework for the Eocene-Oligocene Mogollon-Datil volcanic field, southwest New Mexico

A time-stratigraphic framework for discontinuously exposed regional ignimbrites in the Eocene-Oligocene Mogollon-Datil volcanic field has been established using correlations aided by 40Ar/39Ar age determinations and paleomagnetic analyses. 40Ar/39Ar age spectra from sanidine separates (25 regional ignimbrites, 85 samples, 97 spectra) yield well-defined plateau ages that are precise (within-sample and within-unit 1σ < ± 0.5%) and agree closely with independently established stratigraphic order. Paleomagnetic remanence directions (404 sites) from individual ignimbrite outflow sheets are generally vertically and horizontally uniform throughout facies ranging from thick (100-500 m), densely welded, proximal ignimbrites to thin (1.5-30 m), unwelded, distal fringes. Between-unit differences in paleomagnetic directions provide useful correlation criteria, particularly for units having ages too close to be resolved using 40Ar/39Ar dating. The Mogollon-Datil time-stratigraphic framework clarifies ignimbrite history and provides improved age control for intercalated lavas and sedimentary rocks. Ignimbrite activity was strongly episodic; outflow sheets were primarily erupted in four discrete pulses representing synchronized activity of two separate cauldron complexes. Activity in the southern complex began at 36.2 Ma near Las Cruces, New Mexico, and subsequently migrated 220 km northwest, culminating in the 28.0 Ma Bursum cauldron. Activity in the northern complex, located west of Socorro, New Mexico, underwent a less defined and more modest 40-km westward migration over its 32.0 to 24.3 Ma life span. The four pulses of ignimbrite activity were (1) 36.2-24.3 Ma, 12 major units, >1,500 km3 total volume; (2) 32.0-31.4 Ma, three major units, >1,500 km3 volume; (3) 29.1-27.4 Ma, nine major units, >6,000 km3; and (4) 24.3 Ma, one major unit. The third and largest ignimbrite pulse was accompanied by extensive rhyolitic dome and flow eruptions in the area between the two main cauldron complexes.

Fluid flow in the rotorua geothermal field derived from isotopic and chemical data

A wide variety of isotopic and chemical measurements on geothermal fluids from shallow wells at Rotorua have given the following interpretations: 1. 1) The Rotorua field comprises one geothermal system. 2. 2) A primary upflow of (outgassed) alkali chloride water extends from northeast Whakarewarewa to Ngapuna and under Lake Rotorua (east side of the system). At the southern end a secondary upflow discharges dilute alkali chloride water. 3. 3) A second major upflow at Kuirau-Ohinemutu discharges chloride-bicarbonate waters formed by dilution of the primary water and reaction with rock. 4. 4) Boiling primary water flows from the eastern upflow zone under confining sediments into aquifers in Rotorua Rhyolite containing chloride-bicarbonate waters in the central region. 5. 5) Tritium-bearing groundwater penetrates from overlying aquifers in the sediment into the saddle area between the rhyolite domes or along the crest of the southern rhyolite dome and flows northeast into the northern dome.

Gravity anomalies, caldera structure, and subsurface geology in the Rotorua area, New Zealand

A re-examination of gravity data indicates that Rotorua Caldera does not have the circular, negative gravity anomaly typical of other rhyolitic calderas. New gravity measurements and residual gravity anomalies in Rotorua City are consistent with numerous rhyolite domes and ignimbrite sheets, interbedded with a thick sequence of poorly-compacted sediments. Within the city a gravity high extends from the shore of Lake Rotorua south to Whakarewarewa and is associated with a buried ridge, formed by the coalescing of two rhyolite domes. A gravity low centred near Linton Park suggests that rhyolites are thin or absent in this area and sediments extend to a depth of about 1 km. A quantitative analysis of the residual gravity anomalies was limited by insufficient information about the density, extent, and thickness of the material underlying the rhyolites, and the uncertainty in the distribution and density of silicification within the sediments.

Electrical resistivity and magnetic investigations of the geothermal systems in the Rotorua area, New Zealand

Electrical and magnetic data are used in an investigation of a 450 km 2 region in order to delineate the Rotorua City Geothermal system and determine its relationship with other geothermal systems in the region. Electrical resistivity mapping of the region has been carried out using Schlumberger array measurements with nominal current array (half) spacings of 500 m (586 measurements) and 1000 m (410 measurements). In addition continuous profiling using an equilateral dipole array provides close spaced data along 6.5 km of profile in Lake Rotorua. Modern draped aeromagnetic data (200 m line spacing, 60 m ground clearance) has been made over about 100 km 2 covering two areas. Where aeromagnetic data could not be obtained, these data have been augmented with previously measured ground magnetic (vertical force) and waterborne (total field) data. Three distinct regions of low (< 30 Ωm) apparent resistivity are delineated. The southern of these outlines the Rotorua City Geothermal System which has an area of about 18 km 2, with the northern third covered by Lake Rotorua. The boundary of the system is characterised by a rapid lateral change in apparent resistivity which can be modelled as a single, near vertical zone in which the distance between hot and cold water is very narrow. Magnetic properties also change in the vicinity of the discontinuity in some areas, consistent with hydrothermal alteration having destroyed the magnetite in the rocks of the geothermal system. Hot water is believed to be rising, driven by buoyancy forces across the whole of the low resistivity region. There is some indication, particularly in the south, that the boundary between hot and cold fluids dips away from the field. A second low resistivity zone (the East Lake Rotorua anomaly) with an area of about 8 km 2, is believed to outline a second independent geothermal system, with surface manifestations on Mokoia Island, and on the eastern shore of the lake. High heat flow in lake bottom sediments, and a reduction in magnetic signature over this region supports this conclusion. A third resistivity low under the west of Lake Rotorua has no associated thermal features and is believed to be a fossil hydrothermal system. There is no apparent relationship between the location of the geothermal systems and the Rotorua caldera. The aeromagnetic measurements have delineated several highly magnetic bodies which cannot be linked with surface geology. These are believed to be caused by buried rhyolite dome complexes at shallow depth.

Geological setting and styles of mineralization, north arm of Sulawesi, Indonesia

The north arm of Sulawesi consists of a Neogene island arc (North Sulawesi Arc) built upon Paleogene volcanic-sedimentary basement and underlain by oceanic crust, which is followed to the west by an arcuate, highly deformed terrane (neck of Sulawesi) characterized by metamorphic rocks and felsic granitoids belonging to the Sundaland continental margin. These two terranes have been contiguous during the Tertiary. The evolution of the North Sulawesi Arc is divided into two stages separated by collision of the north arm with the Sula Platform microcontinent in mid-Miocene time. During the Early Miocene a calc-alkaline andesitic arc developed in relation to west-directed subduction. Arc-continent collision resulted in back-arc thrusting, clockwise rotation of the north arm, and inception of subduction along the North Sulawesi Trench. Post-collisional magmatism in the North Sulawesi Arc produced felsic to mafic volcanic suites that are thought to be related primarily to rifting of the former arc rather than directly to subduction. In the neck of Sulawesi,LILE and LREE-element enriched, potassic granites (Dondo suite) of continental affinity (Sr 87/Sr 86 0.71) were generated. The north arm of Sulawesi is comparatively well mineralized. Porphyry CuAu and porphyry Mo mineralization is approximately 2–4 Ma old based on preliminary KAr dating and geological relationships. The former developed in an oceanic terrane following collision-related arc reversal, whereas the latter developed in a continental terrane that underwent lower crustal melting during extension following the same collision. Porphyry CuAu mineralization in the North Sulawesi Arc is associatedwith clusters of small hornblende diorite or quartz diorite stocks, but the specific volcanic environment above these intrusions and their magmatic affinity remain unknown. Mineralization related to porphyry CuAu districts includes CuAu skarn, polymetallic vein and high sulphidation epithermal Au (+enargite). Porphyry Mo mineralization in the neck of Sulawesi is related to small stocks or roof apophyses of the Dondo batholith. Cogenetic granites are exposed over 5000 km 2 and are intruded in an arcuate belt, more than 400 km long, parallel to the Sula Platform collision zone. Unimodal felsic volcanics of Plio-Pleistocene age, which occur locally along the inner side of the north arm and further to the south, are probably the extrusive equivalents of the Dondo granites. Epithermal Au mineralization (typically quartz-calcite veins ± adularia) is associated mainly with the Early Miocene andesitic arc but there are important exceptions, and in general the age of mineralization is poorly known. A district-wide correlation between epithermal Au mineralization and porphyry CuAu mineralization is not apparent. The most important epithermal Au district (Kotamobagu) is inferred to be associated with a long-lived volcanic center represented by the Moat caldera. Epithermal gold systems in the North Sulawesi Arc are generally eroded deeply, typically with chlorite-epidote wallrock alteration, and little is known about their associated volcanic environment. In addition to high sulphidation Au mineralization related to porphyry systems, other styles and settings of Au mineralization in the north arm are exemplified by the Gunung Pani rhyolitic dome, the Patung diatreme breccia, the Mesel jasperiod, hosted partly by carbonate rocks, and sedimentary replacement Au-base metal mineralization at Doup. Diversity of mineralization in the north arm is further illustrated by VMS mineralization in volcanic basement to the Neogene arc, and by metamorphogenic quartz vein Au mineralization in the neck of Sulawesi.

Volatile contents of obsidian clasts in tephra from the Taupo Volcanic Zone, New Zealand: Implications to eruptive processes

Abstract Obsidian clasts in 6500–20,000-year-old rhyolitic tephra deposits from the Taupo Volcanic Zone, New Zealand, contain variable H2O (0.2–2.5 wt.%) and CI (0.12–0.18 wt.%) contents. These are elevated compared to degassed obsidian from rhyolitic domes (∼-0.1 wt.% H2O2, ∼0.1 wt.% Cl) which quenched at the surface. Major- and trace-element compositions of obsidian from tephra deposits suggest that they are co-genetic with associated pumice. The clear, glassy appearance of the obsidian, their high H2O release temperatures, and δ18O of +7‰ show that the water is magmatic, and was not absorbed into the obsidian after tephra deposition. Based on analysis of melt inclusions, recent primary Taupo Volcanic Zone rhyolitic magmas contain 4.3 wt.% H2O and 0.18 wt.% Cl. The obsidian in the tephra deposits, therefore, represents partially degassed, quenched primary magma. Water and Cl contents correlate in the obsidian, suggesting that the magmatic Cl partitioned into a H2O-rich fluid phase during eruptive degassing. The partitioning of Cl between the water-rich vapor phase and the residual magma can be modelled as open-system Rayleigh fractionation. Using a pre-eruptive H2O content of 4.3 wt.%, and Burnham's (1979) water solubility model, the pressure and depth of initial vesiculation of the Taupo magma is calculated to occur at about 0.9 kbar or ∼3.5 km depth. Initial fragmentation (assumed to occur at a vapor:melt ratio of 3:1) began at 0.1 kbar, or ∼500 m depth. Obsidian in most tephra deposits contains less than 1 wt.% H2O, and are calculated to have formed at The 2000-year-old Hatepe phreatoplinian tephra contains many obsidian fragments with > 2 wt.% H2O. The meteoric water which caused this eruption to be phreatomagmatic may have caused obsidian to quench at unusually great depths, resulting in higher H2O contents.

Isotopic and chemical constraints on the petrogenesis of Blackburn Hills volcanic field, western Alaska

The Blackburn Hills volcanic field is one of several Late Cretaceous and early Tertiary (75–50 Ma) volcanic fields in western Alaska that comprise a vast magmatic province extending from the Arctic Circle to Bristol Bay. It consists of andesite flows, rhyolite domes, a central granodiorite to quartz monzonite pluton, and small intrusive rhyolite porphyries, overlain by basalt and alkali-rhyolites. Most of the field consists of andesite flows which can be divided into two groups on the basis of elemental and isotopic composition: a group having lower (87Sr86Sr)i, higher (143Nd144Nd)i, and moderate LREE and HREE contents (group 1), and a group having higher (87Sr86Sr)i, lower (143Sr144Sr)i, and lower HREE contents. Basalts are restricted to the top of the stratigraphic section, comprise the most primitive part of group 1 [(87Sr86Sr)i = 0.7033; (143Nd144Nd)i = 0.5129], and have trace-element ratios that are similar to those of oceanic island basalts (OIBs). In contrast to the basalts, group 1 andesites have higher (87Sr86Sr)i and lower (143Nd144Nd)i, and represent interaction of mantle-derived magmas with the lower crust of Koyukuk terrane. Group 2 andesites have (87Sr86Sr)i and (143Nd144Nd)i that are near bulk-earth values and probably formed by partial melting of the lower crust of Koyukuk terrane. The central pluton and rhyolite porphyries are isotopically uniform (87Sr86Sr)i ≈ 0.704, (143Nd144Nd)i ≈ 0.51275, and are interpreted to have formed by melting of young mafic to intermediate crustal rocks or by fractionation of group 1 andesites. The rhyolite domes have an isotopic range similar to that of the basalts and andesites [(87Sr86Sr)i = 0.70355-0.70499; (143Nd144Nd)i = 0.51263–0.51292], which suggests they formed by fractionation of the and site and basalt magmas. Although some workers have suggested that the volcanic field is underlain by old continental crust, none of the data require the presence of Paleozoic or Precambrian continental middle or upper crust under this part of the volcanic field. However, the ultimate source of some of the rocks in the Yukon-Koyukuk province that have high 87Sr86Sr and low 143Nd144Nd ratios may be old sub-continental mantle and/or lower crust, which was previously subducted beneath the Yukon-Koyukuk province during Early Cretaceous arc-continent collision.

Oxygen isotope evidence for past and present hydrothermal regimes of Long Valley caldera, California

Whole-rock oxygen isotope compositions of cores and cuttings from Long Valley exploration wells show that the Bishop Tuff has been an important reservoir for both fossil and active geothermal systems within the caldera. The deep Clay Pit-1 and Mammoth-1 wells on the resurgent dome penetrate mildly to strongly altered Bishop Tuff with δ 18O WR values as low as −2.6% (vs V-SMOW). The idfu 44-16 well intercepts a thinner Bishop Tuff section with δ 18O WR values of +0.4 to +2.3%. in the western caldera moat, where milder and more sporadic 18O depletions occur in Tertiary volcanic rocks of the western caldera floor ( δ 18O WR = +2.2 to +6.4‰ ). Bishop Tuff samples from deeper parts of the 715 m rdo-8 (Shady Rest) well in the SW moat are also strongly depleted in 18O ( δ 18O WR = −1.5 to +0.6‰ ). Four shallow thermal gradient wells (469–715 m td drilled in the western moat did not penetrate Bishop Tuff, but Early Rhyolites from two of these holes are depleted in 18O ( δ 18O WR = −1.2 to +6.0‰ in plv-1 +4.6 to +5.3%. in mlgrap-1 ), compared to lithologic equivalents from the other two holes ( δ 18O WR = +6.3 to +8.0‰ in plv-2 and mlgrap-2 ). Whole-rock oxygen isotope profiles for the resurgent dome wells are unlike profiles calculated assuming alkali feldspar-H 2O fractionation behavior and total O-isotopic equilibration with −14.3‰ fluids at measured temperatures. The sense of this divergence implies an earlier hydrothermal episode within the central caldera driven by one or more shallow intrusions. Geochemical similarities between an intrusive granophyre at the bottom of the Clay Pit-1 well and a nearby Moat Rhyolite dome with a K/Ar cooling age of ∼0.5 Ma suggest that vigorous hydrothermal activity beneath the central resurgent dome may have occurred as much as ∼0.5 m.y. ago. Calculated and measured O-isotope profiles are similar for deep wells that penetrate the western moat of the caldera, where steep temperature gradients and low δ 18 O WR values in Early Rhyolites from plv-1 are attributed to an active hydrothermal aquifer that has descended slightly from earlier, shallower elevations. Similarly, severe 18O depletions in Bishop Tuff samples from the idfu 44-16 and rdo-8 wells reflect active convection beneath the western moat, whereas milder 18O depletions in Early Rhyolites from mlgrap-1 were apparently caused by hydrothermal alteration at lower temperatures. The O-isotope profiles imply that surface discharge within and around the resurgent dome results from shallow, eastward-directed outflow from a zone of higher enthalpy hydrothermal upflow beneath the western caldera moat. Intrusive magmatic heat source(s) are inferred to exist beneath the western moat, perhaps beneath Mammoth Mountain.

Boiling and dilution in the shallow portion of the Waiotapu geothermal system, New Zealand

The Waiotapu geothermal system is the largest in terms of surface extent and heat flow (17 km 2 and 600 MW, respectively) of the 20 major geothermal systems in the Taupo Volcanic Zone, New Zealand. Two extinct dacite cones and a rhyolite dome rise about 500 m above the Waiotapu valley. The majority of thermal features occurs in the valley over a relief of only 100 m and lies within coincident apparent resistivity and 30 m depth thermal anomalies. Thermal features comprise near-neutral pH chloride springs (often boiling), acid sulfate springs and mud pools, and mixed waters, the latter including both bicarbonate-chloride and acid sulfate-chloride springs. The majority of the sulfate discharges to the surface in the northern, higher altitude area, while most chloride that reaches the surface does so in the southern portion of the system. Comparison of measured and calculated surface chloride flux (the latter from measured heat flow and reservoir chloride concentrations) indicates that at least half of the deep chloride does not discharge at the surface, but flows laterally to the south at shallow depths, mixing with groundwater. The composition of well discharges indicates a shallow (400 m) reservoir of chloride fluid (about 1350 mg Cl/kg at 230°C) is diluted by a CO 2-rich steam-heated water (nil chloride at about 160°C), the latter located on the margins of the system. This same dilution trend is apparent from the compositional variation and K-Na and K-Mg geothermometry of hot spring waters. However, there is chemical and stable isotope evidence from hot springs that cold groundwater also acts as a diluent at shallow levels, and very near the surface steam-heated acid sulfate waters mix with the upflow. Boiling of the deep chloride fluids prior to dilution results in the stabilization of K-feldspar. In contrast, dilution at shallower depths by the steam-heated waters (both CO 2-rich and acid sulfate) leads to the water compositions initially shifting to K-mica (and interstratified clay) stability, and to kaolinite stability with further dilution and temperature decrease. The composition and distribution of these fluids are consistent with alteration mineralogy and zonation observed in drill core. Although boiling occurs at deeper levels and up to the surface in the upflow zone at Waiotapu, mixing is also common and is the dominant process on the margin and in the near-surface portion of the system. Boiling of deep chloride fluid is very common in the upper 1–2 km of the upflow zone of most geothermal systems, though it may be quenched by mixing with marginal waters, either cold groundwater in high relief terrane, or steam-heated waters in areas of lower relief. CO 2-rich steam-heated waters are common on the margins of systems and will generally underlie the near-surface, acid sulfate steam-heated waters. Interaction with CO 2-rich waters will form K-mica and interstratified clays, while acid sulfate waters cause kaolinite and even alunite to form. The former assemblage is commonly associated with typical low sulfidation epithermal gold mineralization as wallrock alteration, where gold-bearing veins comprise adularia and illite. In contrast, kaolinite is rare, and alunite absent, from a direct spatial and genetic association with gold mineralization in this environment. This implies that mixing of the deep, neutral pH chloride fluid with surficial acid sulfate waters does not commonly lead to ore-grade gold deposition. Mixing with (and cooling by) CO 2-rich, steam-heated waters leads to clay stability, accounting for the large halos of clay alteration and low-grade metal anomalies which often surround veins of high gold grade. In contrast, mixing with surficial acid sulfate waters is unlikely to cause precious metal mineralization because the chloride fluids have already lost the majority of their gold through deeper boiling and dilution.