Tertiary Volcanic Rocks Research Articles

Structure of the Michoacán-Guanajuato volcanic field, Mexico

The distribution and the alignment of volcanoes and fault scarps provide new insights into the structural framework of the Michoacán-Guanajuato volcanic field. A total of 1894 volcanoes occur over an area of 34,500 km 2. Contour maps of the density of volcanoes and average volcano height indicate a shift from polygenetic to monogenetic volcanism from north to south in the field. Volcano edifice height has a bimodal density distribution, which can be described by two populations with mean heights of 95 m and 510 m. Plotting the distribution of these two populations on separate contour maps reveals a N35E trend in monogenetic volcanoes and a N60W trend in polygenetic volcanoes. Fault scarps cut Tertiary volcanic rocks and older Quaternary volcanoes, but are overprinted by volcanoes and lava flows in the central part of the field. Fault scarps define two fault zones oriented N60E in the northeastern part of the area and N60W in the northwest part of the area. E-W trending faults are common and alignments of volcanoes often parallel these faults. Regional trends in volcano distribution appear to be related to plate convergence. The N60W trend of the polygenetic volcanoes is colinear to the western Trans-Mexican volcanic arc, parallel to the Middle America Trench, and parallel to a major fault zone, and represents an extinct volcanic arc. The N35E band of monogenetic volcanoes may reflect the segmentation of the subducted plate.

Probing the evolving Andean Lithosphere: Mid‐Late Tertiary magmatism in Chile (29°–30°30′S) over the modern zone of subhorizontal subduction

In the mid‐late Tertiary (27–6 m.y. ago), volcanism diminished, then ceased in the main Andean Cordillera in Chile between 29° and 30°30′S. Over the same time span the subduction angle shallowed under this region, which today lies in the modern shallowly dipping, nonvolcanic segment of the south central Andes (28°–33°S). The trace and major element chemistry and mineralogy of the volcanic rocks in this region record the detailed pattern of lithospheric cooling and crustal thickening that accompany this major tectonic transition. Volcanic changes include, in addition to a decreased rate of volcanism, a decrease in the range of compositions from basaltic andesites to rhyolites in the oldest rocks to only rhyolite in the youngest rocks, a change in source and fractionating mineralogy with an increased role for amphibole and garnet in the younger rocks, and an increase in crustal assimilation. These volcanic changes also correlate in time with deformational events that help to account for crustal thickening and the space‐time pattern of deformation and volcanism in the foreland region. Although the model stands alone, support for the tectonic setting and magmatic source region of these Tertiary volcanic rocks can be found in comparisons with recent volcanic rocks erupting over more steeply dipping modern Andean segments. The older Doña Ana Formation (27–18 m.y.) is similar to volcanic rocks in the Southern Volcanic Zone, while the younger Cerro de las Tortolas (14–10 m.y.) and Vallecito (6 m.y.) formations are similar to volcanic rocks over regions of thicker crust in the Central Volcanic Zone and the northern part of the Southern Volcanic Zone.

Geochemical studies in the Indian Pass and Picacho Peak Bureau of Land Management Wilderness study areas, imperial county, Southern California

The U.S. Geological Survey has conducted geochemical studies in the Indian Pass (CDCA-355), 124 km 2, and Picacho Peak (CDCA-355A), 23 km 2, Wilderness Study Areas (WSA's) as part of a program to evaluate the mineral resource potential of designated areas in the California Desert Conservation Area. These two WSA's are of particular interest because they lie within a region which has intermittently produced significant quantities of Au since the mid-1800's, and is currently the site of much exploration activity for additional Au resources. Within a 15-km radius of the WSA's, there is one actively producing gold mine, a major deposit which began production in 1986, and one recently announced discovery. In the reconnaissance geochemical surveys of the two WSA's - 177 μm (-80 mesh) stream sediments, heavy-mineral concentrates from stream sediments, and rocks were prepared and analyzed. Four areas of possible exploration interest were identified within the WSA's. The first area is characterized by anomalous W and Bi in nonmagnetic heavy-mineral concentrates, and is underlain primarily by the Mesozoic Orocopia Schist which has been intruded by monzogranite of Oligocene age. Alteration and mineralization appear to be localized near the intrusive contact. The mineralized rock at the surface contains secondary Cu and Fe minerals where the monzogranite intrudes the metabasite horizons of the Orocopia Schist and scheelite where the monzogranite intrudes marble within the Orocopia Schist. The second area is characterized by anomalous As, Sb, Ba, B, and Sr in nonmagnetic heavy-mineral concentrates and by anomalous As in - 177 μm stream sediments. Geologically, this area is underlain by metasedimentary and metavolcanic rocks of Jurassic(?) age; a biotite monzogranite of Jurassic(?) age; and Tertiary volcanic and hypabyssal rocks composed of flows, domes, and tuffs of intermediate to silicic composition. All these rock types are cut by a set of north-south-striking normal faults. The anomalies in the heavy-mineral concentrates are believed to be related to silica-clay alteration observed in the vicinity of some of these faults.

Geochemical variation with time in the Cenezoic volcanic rocks of southwest Hokkaido, Japan

Abundances of major and trace elements were determined for the Tertiary volcanic rocks from SW Hokkaido. The Late Miocene to Pliocene volcanic rocks of this region show geochemical features similar to those of the Quaternary rocks, that is, K/Si, Th/Si and LREE/HREE ratios increasing across the arc, east to west, from the Pacific to the Japan Sea side. In contrast, the Early Miocene volcanic rocks, which are geographically restricted to the Japan Sea coast, are distinct from all later volcanics and show “within-plate” characteristics — in particular, high concentrations of HFS elements. The Quaternary basalts have low Hf/Yb ratios and Hf contents, whereas the Early Miocene basalts are high in Hf/Yb and Hf, similar to Hawaiian alkali basalts. The compositional variation with time may result from the progressive depletion of incompatible HFS elements in the mantle source. Th/Yb ratios increase from Early Miocene to Quaternary, possibly reflecting increase in the LIL element contribution to the mantle source during that time.

Intertidal conglomerate south of Yanbu — An episodic clastic deposit in the eastern Red Sea

A thin sheet of clastic conglomerate has been located overlying the Pleistocene coral limestone in the intertidal zone, south of Yanbu on the west coast of Saudi Arabia. The conglomerate exhibits a polymodal grain-size distribution with gravel to cobble size clasts and sand-silt matrix. Grain and fabric characters of the conglomerate are consistent with a flash flood deposit rather than with a wave-processed nearshore sediment. Wadi Wasit draining the adjacent highlands and the coastal plain, covered with alluvial deposits derived from the Tertiary volcanic and metamorphic rocks, provided the coarse-grained sediments to the shelf. The carbonate cement in the conglomerate is composed of low Mg-calcite and appears to have been precipitated in non-marine conditions. The void filling low Mg-calcite exhibits a coarsening of crystal size away from the grains towards pore interiors. The 14C dating of the carbonate material gave an age of 3580±80 yr B.P.. All evidences suggest that the conglomerate was deposited on the emergent reef flat during an episodic flood stage of the Wadi during the Middle Holocene pluvial period on the west coast of Saudi Arabia.

Stable isotope (O, H, S) relationships in Tertiary basalts and their mantle xenoliths from the Northern Hessian Depression, W.-Germany

18O/16O, 34S/32S, and D/H ratios as well as vacuum-fusion H2O+ contents were measured for late Tertiary volcanic basaltic rocks ranging in composition from quartz tholeiites and alkali olivine basalts to melilite-bearing olivine nephelinites and for peridotite xenoliths from the Northern Hessian Depression of W.-Germany. Measured Oisotope ratios in both basalts and peridotites were corrected for variable degree of post-eruption, secondary alteration. The ranges and means of corrected δ18O values (‰ SMOW) for the North Hessian lavas and peridotites are: (i) 8 tholeiites: ca. +6.1 to +7.3 (¯x=+6.6), (ii) 21 alkali olivine basalts: ca. +5.4 to +7.6 (¯x=+6.5), (iii) 19 nepheline basanites, limburgites, and olivine nephelinites: ca. +5.3 to +8.0 (¯x=+6.6), and (iv) 23 peridotites: +5.1 to 7.0 (¯x+6.0). The δ34S values (‰ CDT) for the tholeiites range from −0.6 to +1.4 (¯x=−0.03) and for the alkali basalts range from +0.9 to +8.6 (¯x=+2.5). The approximate δD value (‰ SMOW) of the pristine basalts and peridotites is estimated to have been ca. −90‰

Comparative anatomy of volcanic-hosted epithermal deposits; acid-sulfate and adularia-sericite types

Two types of epithermal precious and base metal deposits hosted primarily by Tertiary volcanic rocks have been distinguished through a detailed examination of the characteristics of 16 well-documented epithermal districts. These characteristics include the ore, gangue, and alteration mineral assemblages, the spatial and temporal distributions of these assemblages, host-rock composition, age relations between ore deposition and emplacement of the host rock, size of the district, temperatures of mineral deposition, chemical composition of the fluids, paleodepth estimates, origin(s) of the fluids, and regional geologic setting. The relative value of each of these features for distinguishing epithermal precious and base metal deposits is estimated. The vein and alteration mineral assemblages are the most important factors, and age relations between ore deposition and emplacement of host rock also appear to be significant. The two major types are designated the acid-sulfate type (Goldfield, Nevada) and the adularia-sericite type (Creede, Colorado; Round Mountain, Nevada). Both types have gold-rich and silver-rich subsets. Base metal contents in the acid-sulfate-type deposit are relatively high and relatively copper rich; there is a wide range in the base metal contents in the adularia-sericite-type deposit, although most are relatively copper poor. Adularia-sericite-type deposits are considerably more abundant than acid-sulfate-type deposits.The two types of deposits appear to form under similar pressure-temperature conditions but in different geological and geochemical environments in ancient geothermal systems. The acid-sulfate-type deposit forms in the root zones of volcanic domes from acid waters that contain residual magmatic volatiles. The adularia-sericite-type deposit forms in a geothermal system where surficial waters mix with deeper, heated saline waters in a lateral flow regime, high above and probably offset from a heat source at depth; neutral to weakly acidic, alkali chloride waters are dominant.

Petrology, geochemistry and age relations of Triassic and Tertiary volcanic rocks from SW Egypt and NW Sudan

Petrological, geochemical and K/Ar radiometrical whole rock age data of volcanic rocks from SW Egypt and N Sudan in the area of Gebel Kamil-Lake Nasser-Laqia Arbain-Nile are presented. These rocks are (i) trachytic-phonolitic rocks and (ii) alkali olivine basalts occurring mostly as volcanic plugs. Their petrographic and their major and trace element patterns are typical for continental within-plate volcanism. Two groups of ages were determined: Triassic (240, 233 Ma) with predominantly silicic, differentiated magmas, and Tertiary (60-20 Ma) with predominantly basaltic magmas. Major element abundances of the basalts show a regional trend from west to east from low to high fractionation irrespective of the age of the rock. Their occurrence can be correlated with two W-E uplift zones which are important for the structural development of the large sedimentary basins of the Libyan desert.

Tectonic implications of paleomagnetic and geochronologic data from the Yukon-Koyukuk province, Alaska

The paleomagnetic and geochronologic record of Alaska is complicated by overprints that make it difficult to relate the terranes of northern and southern Alaska to each other and to North America. To better understand these relationships and the overprinting, paleomagnetic and K-Ar dating samples were analyzed from the three major geologic units of the Yukon-Koyukuk province (YKP) of central Alaska. These units consist of a basal Jurassic-Early Cretaceous volcanic (island-arc) assemblage, Albian-Cenomanian clastic sedimentary rocks, and bimodal Eocene volcanic rocks. K-Ar age determinations of the early Tertiary volcanic rocks form two distinct groups. The older group (65–49 Ma) is mostly felsic and probably represents the maximum inboard penetration of low-angle subduction-related volcanism. These older rocks were deformed during the regional deformation of the YKP. The younger group (44−43 Ma) is post-deformational and composed dominantly of basalt. These results bracket a time (49−44 Ma) when deformation ceased in the province. This time interval is coincident with a major transition from convergence to strike-slip and extensional tectonic style throughout Alaska. The geologic units of the YKP display two characteristic paleomagnetic signatures. Primary directions (based on positive reversal, conglomerate, and/or bedding tilt stability tests) were obtained from the Albian-Cenomanian sedimentary rocks and Eocene volcanic rocks. The mid-Cretaceous sedimentary rocks show about 15° of poleward motion between 90−56 Ma (a time of high rates of convergence between Alaska and the Pacific and Eurasian plates). Age-equivalent rocks from neighboring regions (St. Matthew Island and the Alaskan North Slope) also indicate similar amounts of poleward (northward) motion, which suggests that the whole of western and northern Alaska may have moved northward about 10° relative to North America as a more or less coherent block. Paleomagnetic results from the Eocene volcanic rocks indicate that the YKP was in place by 56 Ma and formed part of the accretionary nucleus that served as a backstop for the accreted terranes of southern Alaska. Secondary (remagnetized) directions are recorded in the basal island-arc assemblage and in Albian-Cenomanian sediments near younger igneous occurrences. The clustering of secondary directions in the geographic reference frame indicates that they were acquired after deformation ceased (⩽49−44 Ma), which is consistent with the coincidence of the mean of these directions and the 54−44 Ma North American reference direction.

Geochemistry and tectonic setting of pre-collision Cretaceous and Paleogene volcanic rocks of Ecuador

Pre-collision Cretaceous–lower Tertiary volcanic rocks of western Ecuador from the coastal area (Pinon and San Lorenzo Formations) and Western Cordillera (Macuchi and Celica Formations) consist of three rock suites. The first suite includes tholeiites of ocean-floor affinities that have distinct light REE-depleted patterns and low La/Nb, La/Hf, and Th/Hf ratios. The island-arc tholeiites of the second suite have lower contents of Ti, Zr, Hf, and Nb. The third suite comprises calc-alkalic rocks, mainly continental-margin andesites and dacites. The continental volcanic-arc rocks crop out in an area east of the tholeiitic rocks. The boundary between the two areas is the major suture separating continental South America from accreted terrenes to the west. A model is suggested for the geodynamic evolution of what is now western Ecuador during the Late Cretaceous. It includes two east-dipping subduction zones of Turonian to Santonian ages and the accretion of the oceanic island arc to the continent.

The crustal heritage of silver and gold ratios in Arizona ores

The ratios of produced grades of silver and gold in ore deposits of Arizona, representing nearly a century of mining, correlate with their geologic setting. The deposits occur south and west of the Colorado Plateaus where two geological domains are recognizable on the basis of type and age of rocks and geological history. The eastern or “Naco” domain has a basement of Proterozoic Pinal Schist, formed from a protolith of mostly clastic sedimentary rocks, and widely scattered younger Proterozoic granites, overlain by Paleozoic platform strata and Mesozoic volcanic and clastic rocks. The “Western Deserts” domain is characterized by an older Proterozoic section of largely submarine volcanic and volcaniclastic strata, together with Proterozoic granites; it is largely devoid of Paleozoic strata, but Mesozoic clastic and volcanic strata are widely exposed. In both domains, middle and late Tertiary volcanic rocks are extensive. Ore deposits of many genetic types, of Proterozoic, Nevadan, Laramide, and younger Tertiary ages, are present; deposits of Laramide and younger Tertiary ages occur in both domains, but deposits of Proterozoic age are confined mostly to the Western Deserts, and those of Nevadan age are restricted to the Naco domain. The ore deposits evolved at different times under different conditions of tectonic stress. The produced grades of silver and gold, and the silver to gold ratios, reveal a bimodal distribution. Most deposits of all epochs and genetic type in the Naco domain show Ag enrichment relative to the ratio of the Ag:Au crustal abundance (clarke ratio) (17.5:1). Except for two Laramide porphyry copper deposits and the Proterozoic massive Cu-Pb-Zn ores, whose ratios are close to the clarke ratio, most other districts of the Western Deserts domain are enriched in gold with respect to the ratio. The regional zoning transgresses ore-deposit type, time, tectonic style, and reported igneous rock types and metal associations. It does not appear to be related to the hypothetical results of subduction processes wherein metals and magmas are directly derived from partial melting of a descending slab of oceanic rocks. Rather, the precious-metal characteristics may represent inheritance from specific kinds of crust.

Evidence from mantle xenoliths for an enriched lithospheric keel under the Outer Hebrides

Several independent lines of evidence suggest the presence of old regions in the Earth's upper mantle, particularly beneath stable cratons, which are enriched in incompatible elements such as barium and the light rare earth elements (LREE). Such 'lithospheric keels' are well documented beneath the Archaean of South Africa1–4 and North America5,6. Recent studies7 have shown the Lewisian of Scotland to be underlain by heterogeneous mantle, fragments of which are entrained by magmas that later intrude the crustal rocks of north-west Scotland. Here we present chemical and isotopic data from megacryst suites in these magmas, which point to niineralbgical complexity in the lower crust and upper mantle. Moreover, variable trace-element ratios in the xenoliths are now recorded as extreme isotopic variations. The Sr and Fb isotopic compositions are similar to those of lamprbites6 and micaceous kimberlites8 respectively. In contrast, the Nd isotopic ratios are highly variable (eNd = −3.4 to −32.6) and overlap with those observed in garnet inclusions9 in diamonds (South Africa) and Iamproitic6–potassic5 volcanic rocks (USA). We suggest that the Hebridean lithospheric mantle has evolved in a manner totally unlike that of the mantle source regions tapped by Tertiary volcanic rocks erupted in the North Atlantic volcanic province.

Indication of migrated hydrocarbons in Tertiary volcanic rocks from western Nûgssuaq, central West Greenland

During the field season of 1985 the author briefly visited Marrait kitdlit at the southern part of the entrance to the Itivdle valley on Nugssuaq (fig. 1). This paper briefly describes the occurrence of possibly migrated organic material in a vein in Tertiary basalt and presents it in a regional context.

Geology and geochemistry of the Deer Trail Pb-Zn-Ag-Au-Cu manto deposits, Marysvale District, west-central Utah

The principle sulfide deposits exposed in the Deer Trail mine, Marysvale district, west-central Utah, form a semicontinuous group of elongated strata-bound Pb-Zn-Ag-Au-Cu bodies (mantos) that replaced as many as 15 carbonate beds in the lower part of the Permian Toroweap Formation. This formation is part of a thick section of Paleozoic and Mesozoic sedimentary rocks that underlies middle Tertiary volcanic rocks of the Marysvale volcanic field. The mantos are stacked along a prominent, steeply dipping axial vein and thin symmetrically away from it. Where unoxidized, individual mantos consist almost entirely of sulfides (pyrite-galenasphalerite-chalcopyrite), have sharp contacts with the country rocks, and range from <1 cm to 3 m in thickness. Hydrothermally altered rocks form indistinct envelopes around the veins and mantos. A K-Ar age of 14 m.y. on sericite from one of these altered envelopes indicates that the manto formed during the final stages of igneous activity in this area.In addition to the major metallic constituents Pb, Zn, Cu, Ag, and Au, the ores contain substantial quantities of Mo, As, Sb, Te, Se, Bi, and Hg. Average delta 34 S values for galena, sphalerite, chalcopyrite, and pyrite are -1.1, 0.6, 2.0, and 3.9 per mil, respectively, and a single galena sample had 206 Pb/ 204 Pb of 18.43, suggesting an igneous source for both the sulfur and the metals. Fluid inclusions in main-stage sphalerite homogenize at 250 degrees to 300 degrees C with salinities ranging from near zero to 17 to 22 equiv wt percent NaCl. The inclusions locally contain liquid CO 2 and show evidence of boiling. Temperatures based on delta 34 S sphaleritegalena values range from 245 degrees to 267 degrees C. The delta D (sub H 2 O) and delta 18 O (sub H 2 O) values of the hydrothermal fluids determined from isotopic analyses of quartz and inclusion fluids in fluorite, quartz, and sphalerite range from -131 to -75 per mil and -20.3 to -2.2 per mil, respectively, and correlate with inclusion salinities, temperatures of deposition, and position of the host mineral in the paragenesis. The deposit formed during the interaction of two types of heated aqueous fluids: a high-salinity, probably magmatic and a dilute meteoric fluid. Magmatic water was significant during main-stage base metal mineralization but was replaced almost entirely by meteoric water in the late stages of the paragenesis.One kilometer west of the mantos, an area of hydrothermally altered rock coincides with a domed area cut by radial fractures, suggesting a hidden igneous intrusion at depth. The Deer Trail deposits are part of a halo of metallic deposits that surrounds this altered and deformed core. Many geologic and geochemical relations at Deer Trail are similar to those in a number of the economically more important manto deposits in the western United States (Tintic, Park City, Leadville, Gilman, Pioche), which, by analogy, may have had a similar origin.

Early Tertiary igneous activity west of the Outer Hebrides, Scotland — Evidence from magnetic anomalies and dredged basaltic rocks

Much of the continental shelf off the Outer Hebrides, Northwest Scotland, is underlain by a basement platform which has not undergone the subsidence typical of many parts of the margin west of Britain. Although seismic data suggest that the platform is largely an extension of the Precambrian Lewisian gneiss of the Outer Hebrides the pattern of magnetic anomalies indicates that other lithologies are present. Dredging operations in an area of basement outcrops between St. Kilda and the Flannan Isles have resulted in the recovery of a collection of basaltic rocks in addition to granitic gneisses. Analysis of the freshest basaltic samples reveals that they are olivine tholeiites, typical of “within-plate” basalts, which are chemically distinct from basic intrusions on St. Kilda and other Tertiary volcanic rocks nearby. Whole-rock K-Ar dating yields a mean age of 43.4 ± 1.0 Ma. The association of the basaltic material with one of several prominent linear belts of magnetic anomalies crossing the shelf points to an extensive invasion of the margin by basic igneous intrusions during the Early Tertiary. The dredged basalts appear to represent a late-stage volcanic episode in the evolution of the Tertiary igneous province of Britain. One block of gneiss in the dredge haul has given ages of 404 ± 9 and 431 ± 7 Ma, perhaps indicating reworking of the Precambrian basement by Caledonian tectonic events on the Lewisian foreland. However, further work is necessary to establish the provenance of this gneissic material.

Paleomagnetism of Middle Tertiary volcanic rocks from the Western Cascade Series, northern California

The Western Cascade Series (WCS) is a 3.5‐km‐thick, crudely homoclinal (east dipping) calcalkaline volcanic sequence of mid‐Oligocene to early Miocene age that crops out near the southern tip of the Cascade Range in northern California. The mean direction of remanent magnetization in the WCS is D, 4.9°; I, 57.6° (N, 53; k, 14.4; α95, 5.3°). When compared to a reference direction for the North American craton, the WCS direction indicates that the southern Cascade Range has rotated 14.0° ± 9.0° since the WCS accumulated. A difference in mean direction between the lower and upper halves of the WCS suggests that much of this rotation occurred during the late Oligocene. Six other paleomagnetic studies of rock units of an age roughly comparable to the WCS also are available from western Oregon, northwestern California, and southwestern Washington. All show statistically significant clockwise rotation (inclinations of all but one are concordant). Comparison of directions indicates that the Pacific Northwest did not rotate as a rigid body, but neither did it behave as a collection of small, independently rotating domains. The amount of rotation found throughout the area increases to the west or northwest, suggesting a driving force for rotation operating at the continental margin.

Stable isotope geochemistry of a porphyry-style hydrothermal system, West Silverton District, San Juan Mountains, Colorado, U.S.A.

A porphyritic monzonite (25.1 Ma) intrudes Tertiary volcanic rocks of pre- and post-San Juan Caldera age along the western margin of the Silverton Caldera and the northern side of the Sultan Mountain monzonite stock (25.9 Ma), within an area of strong sericitic to argillic alteration (25.0 Ma). These alteration facies, which are genetically related to the monzonite, are characterized by quartz-MoS 2 mineralization (Type 1), are surrounded by pyritic and propylitic facies alteration within which quartz/ base-metal sulfide (Type 2), quartz pyrite (Type 3), and barren quartz (Type 4) veining occurs. Whole-rock H- and O-isotope compositions are variable throughout the district (δD= − 148 to −72%; δ 18O= − 3.5 to +9.3%). The propylitized country rocks are strongly depleted in 18O and D but more highly altered samples have “normal” δD and δ 18O values. Coarse-grained sericites that also exhibit a very large range of δD values to −60%) with respect to biotites and clay-sized sericites that also exhibit a very large range of δD values (−137 to −98% and −139 to −84%, respectively). Type 1 veins have a very restricted range of 18O/ 16O ratios (+7.6 to 9.3%), whereas veins of Type 2 and Types 3 and 4 have very large, overlapping, δ 18O ranges of −2.7 to +12.0% and −2.8 to +9.0%, respectively. Type 1 veinlets are characterized by halite-bearing and vapour-rich types, whereas liquid-rich types (excluding halite-bearing types) are dominant in quartz and sphalerite of vein Types 2, 3, and 4. Homogenization and salinity data indicate higher temperatures and much higher salinities of quartz fluids in the central alteration zones (∼350°C, ∼40 ≡wt%NaCl) ompared with those of the peripheral pyritic and propylitic (∼60°C, ≦5 ≡wt%NaCl). The δD values for quartz fluid inclusion waters are quite variable: Type 1 (−125 to −79% 0); Type 2 (−130 to −67% 0); and Types 3 and 4 (−128 to −98% 0). A “magmatic” fluid component is indicated by the calculated composition of sericitic fluids. Of the quartz fluids, those associated with MoS 2-mineralization lie closest to the field of “Primary Magmatic Water”; those associated with the other vein types lie within a large field which ranges from the composition of nearly unexchanged, local meteoric water to compositions reflecting extensive hydrothermal exchange and mixing.

Morphology of Clay Minerals in the Smectite-to-Illite Conversion Series by Scanning Electron Microscopy

AbstractThe morphology of illite/smectite (I/S) from deeply buried bentonites and hydrothermally altered Tertiary volcanic rocks from Japan changes in parallel with the proportion of expandable layers in the I/S. As viewed by scanning electron microscopy, the morphologies range from the typical “cornflake,” “maple leaf,” or “honeycomb” habit of smectite to the typical platy or scalloped (with curled points) habit of illite. Although the changes are more subtle near either end member, at a composition of 60-70% illite layers, the morphology changes from sponge-like or cellular to platy or ribbon-like. The change of morphology at this composition correlates with a change in layer stacking from turbostratic to rotational ordering of the 1Md type. Turbostratic stacking can be thought of as randomly distributed translations of successive layers by any magnitude and in any direction. The rotationally ordered structure, which allows nearly precise juxtaposition of quasihexagonal oxygen surfaces from adjacent layers, probably permits more crystalline regularity in the a-b plane, which promotes a more plate-like or sheet-like habit.

A geological and geophysical appraisal of the Baca geothermal field, Valles Caldera, New Mexico

Abstract The Baca location #1 geothermal field is located in north-central New Mexico within the western half of the Plio-Pleistocene Valles Caldera. Steam and hot water are produced primarily from the northeast-trending Redondo Creek graben, where downhole temperatures exceed 260°C at depths of less than 2 km. Stratigraphically the reservoir region can be described as a five-layer sequence that includes Tertiary and Quaternary volcanic rocks, and Mesozoic and Tertiary sediments overlying Precambrian granitic basement. Production is mainly controlled by fractures and faults that are ultimately related to activity in the Rio Grande Rift system. Geophysically, the caldera is characterized by a gravity minimum and a resistivity low in its western half. A 40-mgal gravity minimum over the caldera is due mostly to the relatively low-density volcanics and sediments that fill the caldera and probably bears no relation to deep-seated magmatic sources. Two-dimensional gravity modeling indicates that the depth to Precambrian basement in Redondo Canyon is probably at least 3 km and may exceed 5 km in eastern parts of the caldera. Telluric and magnetotelluric surveys have shown that the reservoir region is associated with low resistivity and that a deep low-resistivity zone correlates well with the depth of the primary reservoir inferred from well data. Telluric and magnetotelluric data have also identified possible fault zones in the eastern and western sections of the production region that may form boundaries to the Redondo Creek reservoir. These data also suggest that the reservoir region is located at the intersection of lineaments that trend north-south and northeast-southwest. Magnetotelluric results indicate deep low resistivity at the western edge of the caldera which may be associated with deep hot fluids. On the basis of geophysical and well data, we make three estimates of reservoir dimensions. The estimates of the areal extent of the reservoir range from 10 to 30 km 2 , depending on whether the Sulphur Creek or the Valle Seco area is considered to contain deep geothermal resources.

Soil geochemical and biogeochemical studies at the Borealis gold mine, mineral county, Nevada, U.S.A.

The Borealis Mine is a bulk-minable disseminated gold-silver deposit located in Mineral County, Nevada (lat. 118°45′W, lon. 38°23′N). In 1979, Houston International Minerals Corporation discovered this deposit containing 1.4 million metric tons of ore with average grade of 3.4 ppm gold and 16 ppm silver. The main ore hosts are Tertiary volcanic rocks of andesitic and rhyolitic composition. The area receives an anual precipitation of about 45 cm (Houghton et al., 1975), and it is mainly a woodland of pinyon pines ( Pinus monophylla) and Utah junipers ( Juniperus osteosperma). The ground cover is predominantly low sagebrush ( Artemisia arbuscula). The Borealis Mine is located about 2100 m above sea level, and the topography in the vicinity is generally quite gentle. The slopes are commonly less than 15 degrees.