Leached capping of the Dos Pobres orebody

Brant Wilson
Department of Geosciences, University of Arizona
Email: bwilson@geo.arizona.edu

The Dos Pobres orebody, of the Lone Star district of Arizona is characterized in its upper levels and exposures by oxidized iron and copper minerals. These characteristics are a subject of this study with the intent of developing criteria for recognition of Dos Pobres type ores in a region of arid climate. Ore grades (Langton and Williams 1982) are believed to be on the order of 0.7% copper in the hypogene zone and a complex weathering process is believed to have resulted in local copper grades in excess of 0.5%.

The surface of Dos Pobres is typical of that found across this region in copper deposits of comparable styles and compositions. In these deposits, a characteristic profile of weathering/oxidation, leaching, and copper precipitation occurs and results from effects of acid production from pyrite during the weathering of upper parts of the copper-iron sulfide systems. Acid produced from this weathering dissolves and alters minerals across the subjacent profile, transporting copper to deeper levels where it is believed contrasts in reduction/oxidation conditions results in precipitation of enriched copper sulfides, the enriched blankets that have given rise to the high value of these ores.

The supergene mineralogy at Dos Pobres indicates that we are viewing this surface at some level within the zone of leaching. The mineralogy is characterized by hematite, which often forms as a product of chalcocite weathering, is spatially widespread and most commonly found in thick veins and fractures with an envelope of acid alteration. Interpreted as the weathering product of chalcopyrite and pyrite, goethite also is locally abundance in the leached cap at Dos Pobres and can be found in association with or as a mixture with hematite. Neither sulfide minerals nor jarosite, an iron oxide common in low pH conditions associated with the weathering of pyrite, are found in abundance at the surface.

Neotocite and chrysocholla represent the major copper bearing minerals in the leached cap. Neotocite, a complex copper bearing silicate, on the surface is commonly found in areas with hematite abundance and is only slightly less spatially prolific as hematite. Chrysocholla, another copper bearing silicate, is also found at the surface but is limited to a few locations. The subsurface, as seen in locations with relatively deep exposures, may have chrysocholla in higher abundance.

Fractures existing prior to intrusion influenced intrusion geometry and are host to the products of primary sulfide mineral weathering. Oxidation and transport processes are also heavily influenced by fractures formed prior to, during and after mineralizing events. On the surface fracture abundance varies but higher fracture abundance tends to be associated with hematite rich areas.

La Caridad porphyry Cu-Mo deposit: A complete hydrothermal system?

Victor A Valencia1, Joaquin Ruiz1, Lucas Ochoa2 and Enrique Espinoza3
1 Department of Geosciences, University of Arizona
2 University of Sonora
3 Grupo Mexico

Here we present data that suggest that La Caridad, which is a porphyry copper-molybdenum deposit and la Caridad Vieja, which is a high-sulfidation epithermal system are related.
La Caridad porphyry is a world-class copper-molybdenum deposit, and is the most productive Cu deposit in Mexico (~150,000 ton Cu/y). It is located in northeastern Sonora, Mexico, 240 kilometers southeast of Tucson. Reserves at 2001 were ~4 million tons of Cu and 250, 000 tons of Mo.

The deposit occurs within andesites to dacites that are intrude by diorite, granodiorite, quartz monzonite porphyries and pegmatites of Laramide age.

The main stages of alteration and mineralization comprise an early episode that is representated by K silicate veins with orthoclase-quartz-anhydrite-biotite in the intrusives complex and a pervasive biotization of andesites and diorites, with a propilitation around this biotitic zone. This early alteration include weak mineralization of magnetite, chalcopyrite, molybdenite, sphalerite and pyrite. This episode was followed by the emplacement of a hydrothermal breccia at the contact between the andesite-porphyry and porphyry-granodiorite.

A second hydrothermal mineralization event is represented by quartz veins associated with pyrite, sericite and chlorite, and also occurs as pervasive replacements. Tourmaline occurs as acicular radiating crystals intergrown with sericite, pyrite and quartz. This alteration type is associated with the main mineralization event with mineralization of chalcopyrite, pyrite and less molybdenite

A late hydrothermal alteration episode includes open spaces filling, stockworks and cementation of breccia. Lead-zinc-silver mineralization was emplaced as peripheral veins during the final stages. Tourmaline-pyrite-quartz veins and breccias are present in the volcanic rocks.

Pegmatite stocks and veinlets with mineralization of coarse molybdenite and pyrite with minor chalcopyrite and locally tennantite-cpy-py veins cut the hydrothermal alterations zones and superimposed a new alteration.

Mineralization of Mo increase at depth commonly associated with K-alteration.

Supergene mineralization, which has been mined, was present as a blanket of about 2 km in diameter with an average thickness of approximately 50 m and ranging from 10 to 230 m (Seagart et al, 1974).

La Caridad deposit is cut by La Caridad postmineral fault in the eastern part of the deposit, this is a NW-SE striking normal fault with a 45o NE dip.

La Caridad Vieja is located 3 km east of la Caridad ore body. This deposit was mined at the beginning of the past century and is hosted in latites and rhyolites (52.2±1.9 Ma, Livingston, 1973) that are intruded by a small porphyry quartz feldespatic intrusion. This deposit present an acid-sulfate epithermal mineral association, that includes pyrophillite, kaolinite, alunite, quartz and barite as alteration minerals and a variety of sulfides that include, chalcopyrite, pyrite, enargite and tennantite. This event generated massive silicic alteration that extend for more than 100 m laterally away from the deposit.

La Caridad Vieja is believed to represent the high level portion of a porphyry copper system with a gap between both deposit of ~2-3 Km. The intimate spatial association and the identical ages for the host rocks of the porphyry and epithermal mineralization together argue for a genetic relation between the La Caridad and La Caridad Vieja ore bodies.

Given the relatively short period of hydrothermal activity, (53.6 ± 0.3 Ma, Re-Os in molybdenite in K and phyllic alteration, Barra et al. In preparation), it is highly improbable that the two deposits are independent and unrelated. It is more likely that the two deposits were formed from a single magmatic-hydrothermal system that evolved from porphyry Cu-Mo deposit at depth to high sulphidation epithermal at shallow levels.

Acknowledgements: We are most grateful to Grupo Mexico for allowing access to the mine operations and logistics and to the Arizona Geological Society for generous financial support. In particular, we would like to thank to Ing Remigio Martinez, Ing Jose Contla, Fernando Barra, Sergio Castro and Dr Chris Eastoe for their help and for interesting discussions.

Investigation of color and coloring mechanisms in golden barite

John Porter1 and Dana T. Griffen2
1Dept. of Geosciences University of Arizona
2Dept. of Geology, Brigham Young University
Email: jporter@geo.arizona.edu

Barite (BaSO4) is a mineral commonly found in with hydrothermal ore deposits, and other environments. Common barite colors include white, blue, yellow and colorless. The cause of color in yellow barite, called "golden barite", has been determined by electron spin resonance spectroscopy (ESR) to be due to a color center. The color is bleached when heated to 500° C for 20 minutes or more. In some samples the color began to fade after 400° and the samples were all colorless after being heated to 600°. Heated colorless samples were then irradiated using Cu x-rays for three hours, inducing a pale blue color. The induced color fades with exposure to uv light. Efforts to reintroduce the golden color to heat-bleached samples have been unsuccessful. Esr spectra and optical absorption spectra were taken at all three stages, golden (untreated), colorless (heated) and blue (irradiated). The absorption spectra of the samples varied widely with absorption peaks from 380 to 520 nm in the untreated sample and a broad peak around 600 nm in the irradiated sample. The ESR spectra confirmed that the color centers provide the mechanism for coloring in these barites. Although experimental setup was not adequate to identify the geometry of the hole centers, the ESR spectra show some interesting differences. The ESR spectrum of the untreated sample shows a much more complicated signal than that of the irradiated sample. However, the peaks found in the signal of the irradiated sample can be seen in the untreated spectrum, suggesting that the hole center causing the blue color is present in the golden sample, but is masked by the larger signal. Spectra of heated samples show no signal that can be resolved from the background.

The geochemical characteristics and significance of Laramide volcanics, southeastern Arizona and adjacent areas

Elizabeth Wilson and Spence Titley
Department of Geosciences, University of Arizona, Tucson, AZ 85721
Email: ewilson@geo.arizona.edu

Volcanic rocks and related plutons are common in southeastern Arizona and surrounding areas and relate to Laramide and mid-Tertiary regional magmatism. The oldest of these rocks were produced during the Laramide Orogeny, associated with the subduction of the Pacific Farallon plate beneath North America, approximately 80-50Ma. This extensive magmatism was followed by mineralization of plutons, forming porphyry copper deposits. Such deposits in Arizona, New Mexico, and Sonora, Mexico are extremely large, accounting for 10% of world copper production. Laramide plutons in this region were studied extensively over the course of the twentieth century due to their economic value but on an individual deposit or district-scale, and Laramide volcanics, direct precursors to porphyry copper deposits, have never been coherently studied on a regional scale. Stratigraphy, timing, and composition of individual volcanic units in Arizona are well known, however similar volcanics in New Mexico and Sonora, Mexico are not well understood and require extensive study.

This study provides a better understanding of 1) Laramide magmatic processes in the southwest; 2) porphyry copper deposit lifecycles; and 3) possible environmental impacts caused by the exposure of these volcanic rocks. By characterizing the geological and geochemical trends of Laramide volcanics on a regional scale, a survey of major, minor, trace, and rare earth elements coupled with stratigraphy provides a regional view of spatial and chemical trends in Laramide volcanics across the west. Laramide volcanic rocks within the study area are intermediate to silicic in composition, and occur largely as andesite to dacite or rhyolite sequences. Type localities for Laramide volcanics include the Salero Formation of the Santa Rita Mountains and the Silver Bell Formation in the Silver Bell Mountains. Many locations do not exhibit a complete volcanic sequence, thus volcanics across the region can be correlated using these type localities.

All samples plot within the calc-alkaline portion of the AMF diagram and trend towards the alkali end member. Of the alkalis, Na2O remains relatively constant over the range of silica, 48-79%, though K2O increases with silica content. All other major oxides demonstrate decreases as silica increases into rhyolitic compositions. MgO, Fe2O3, Al2O3, and TiO2 all decrease with increases in SiO2 content, with Fe2O3 and MgO having a sub-parallel relationship. Of the trace elements, Th, La, Nb, and Y concentrations are relatively constant for all samples, despite lithology variations. Though Hf concentrations occur within a small range, 0-5ppm, rhyolitic rocks contain greater amounts relative to andesitic-dacitic samples. Rb and Sr vary widely for all samples, however andesitic units have higher concentrations of Sr, up to 1600ppm. Ba is enriched in rhyolites compared to intermediate rocks. Rare earth element trends based on sample to chondrite ratios for Laramide volcanics reveal that magma compositions were enriched in all rare earths and especially LREE. Overall, a slight negative Eu anomaly exists. Additionally, when rare earth element concentrations are examined on a west to east projection, concentrations of LREE appear to fluctuate regularly.

By characterizing Laramide-age volcanics, a regional picture of magmatic chemical behavior emerges. Chemical anomalies will also indicate if weathering of these exposed rocks pose any adverse environmental impacts, such as enriched Pb in soils or groundwater, to the Sonoran region.

The Ertsberg intrusive system: Sleeping giant or restless beast?

Stacie Gibbins1, Spence Titley1 and K. Friehauf2
1University of Arizona, Tucson, AZ 85721
2Kutztown University, Kutztown, PA 19530
Email: sgibbins@geo.arizona.edu

The Ertsberg Mining District is located in the Papuan Mobile Fold and Thrust Belt in the Central Ranges of Irian Jaya, Indonesia. The district is comprised of a number of carbonate- and intrusion-hosted mineralized centers, with the super-giant Grasberg Cu-Au porphyry deposit being the largest (2 Bt of 1.2%Cu and 1.2g/t Au) and (likely) the most well known. Mineralization in the district is spatially and temporally related to Pliocene intrusions, which are the result of continent-island arc collision between the Australian Craton and the Pacific Plate during the Miocene.

The original discovery in the mining district was the Gunung Bijih orebody, a magnesium skarn with 33 Mt of 2.5% Cu and 0.8 g/t Au. The Gunung Bijih orebody has been proposed to be a carbonate roof pendant within the Ertsberg Intrusion. Subsequent exploration on the periphery and within the Ertsberg Intrusion identified the Ertsberg East Skarn System (EESS), a magnesium skarn deposit boasting 250 Mt resource of 1.06 %Cu and 0.81 g/t Au, and Dom, a calcic skarn with 30 Mt of 1.67%Cu and 0.42 g/t Au. Thus while carbonate-hosted mineralization was recognized as being associated with the Ertsberg Intrusion, intrusion-hosted mineralization was not considered until the early to mid-1990's.
Similarly, until the late 1990's, the Ertsberg Intrusion was viewed as homogeneous, with a composition ranging from monzdiorite to quartz monzodiorite. Detailed mapping along a 038( trending cross section of this intrusion (perpendicular to regional structures) has identified multiple igneous phases (and their variants) that range texturally from phaneritic-porphyritic, to porphyritic-aplitic and ultimately aplitic. In K2O + Na2O vs. SiO2 plots the phaneritic-porphyrytic rocks cluster in trachyandesite compositional space, parallel to and just below the alkaline subdivision. Porphyries with aplitic groundmass appear to be the most evolved rocks in the Ertsberg Mining District. They range in composition from trachyandesite to trachyte/dacite, with two of the samples being alkaline.
Hydrothermal alteration is prevalent within the Ertsberg Intrusion, occurring as both (selectively) pervasive and vein controlled. The three pervasive alteration zones recognized are: 1. Actinolite-magnetite-epidote+(sphene?), 2. Biotite-magnetite-anatase(?)+(k-feldspar-epidote), and 3. Texturally destructive garnet-epidote endoskarn.

Cu and Au mineralization is deposited within early biotite-anhydrite-bornite(chalcopyrite veins, and later quartz-bornite-chalcopyrite//green sericite-bornite-chalcopyrite veins. Ore grade is contained almost exclusively within the pervasive biotite-magnetite-anatase(?)+(k-feldspar-epidote) alteration zone. Mapping mineralization/alteration as absent or present within recognized igneous phases provides a time-space picture of the interplay and evolution between magmatic and hydrothermal processes within an intrusive complex.

Inorganic Cu isotope fractionation in natural Cu-(Fe)-S minerals

Steve Young and Joaquin Ruiz
Department of Geosciences, University of Arizona
Email: stevey@geo.arizona.edu

Acid ferric sulfate dissolution experiments on Cu-(Fe)-S minerals and supergene chalcocite, Cu2S, ore reveals appreciable Cu isotope variability can occur as Cu is redistributed during weathering of porphyry copper deposits. Solutions collected from reaction columns constructed to model conditions of Morenci, AZ, copper ore heap leaching exhibit d65Cu variability as much as 3‰ as aqueous Cu2+ is progressively extracted in a two-stage dissolution process. The first stage of copper extraction exhibits near-linear dissolution kinetics as well as a rapid 2-3‰ decrease in d65Cu of derived copper-sulfate solutions. Parabolic reaction kinetics and gradually increasing d65Cu of solutions (approximately 1‰) characterize second-stage copper recovery. Except for increased reaction rates in mesophilic bacteria-mediated experiments, there is no resolvable chemical or isotopic difference between leachates collected from biotic or abiotic experiments.

Molecular models for chalcocite, covellite (CuS) and intermediate non-stoichiometric copper sulfide minerals (1 < Cu:S < 2) show that Cu-S bonds occur in both three- and four-fold coordinations. In addition, Cu-S bond geometry changes as a function of Cu:S ratio. Features of Cu-S coordination number and d65Cu curves, with respect to Cu concentration variation, are nearly coincident, suggesting copper isotopes are fractionated between distinct bonding environments within the crystal structure. The likelihood of copper isotope fractionation between mineral and sulfate solution is greatly reduced through recognition of the invariant d65Cu pattern obtained by leaching chalcopyrite (CuFeS2), a mineral in which Cu is in tetrahedral-coordination with S, exclusively. Dissolution experiments on pure chalcocite and bornite (Cu5FeS4) mineral separates were used to assess relative contributions from equilibrium and kinetic fractionation processes. Naturally-produced variation in Cu isotopes and stable isotopes of other transition metals have been well-documented through prior examination of geologic and extraterrestrial materials. However, this is the first instance in which observations permit construction of a model describing their fractionation at a molecular scale in nature. Understanding mechanisms which fractionate stable transition metal isotopes is ultimately important for describing geochemical processes responsible for distributing chalcophile and siderophile elements during terrestrial evolution.

Age and duration of hydrothermal systems in northern Mexico: constraints from Re-Os systematics

Fernando Barra, Lukas Zurcher and Victor Valencia
Department of Geosciences, University of Arizona
Email: fbarra@geo.arizona.edu

One of the most fundamental questions in metallogenesis is the age of mineralization and duration of hydrothermal systems. This problem has been addressed using different techniques such as K-Ar, U-Pb, Ar-Ar and more recently using Re-Os isotopes. The advantage of Re-Os over the other systems is that with this system we can directly date the sulfides and not the silicate assemblages that probably are unrelated to the mineralization event. Molybdenite has ppm levels of Re and no initial 187Os and hence has been used as a high-precision geochronometer. Although the system is reliable the suitability of the sample must always be assessed before analysis (McCandless et al., 1993). Here I use Re-Os systematics applied to molybdenite from four different deposits located in different tectono-stratigraphic terranes of northern Mexico. The Cuatro Hermanos prospect is located in the Caborca terrane (Campa and Coney, 1983) and has a Re-Os molybdenite age of 55.7 ± 0.3 Ma. The Malpica prospect in the Guerrero terrane shows a Re-Os age of 54.1 ± 0.3 Ma and the La Caridad deposit in the North American terrane has a Re-Os age of 53.8 ± 0.3 Ma. These results support the notion that during the interval between 50 - 60 Ma an important mineralization event occurred in northern Mexico and in southern Arizona as previously stated by McCandless and Ruiz (1993). These data are also in good agreement with previous geochronological information using the K-Ar isotopic system (Damon et al., 1983). The longevity of a porphyry system can also be assessed using this technique, and as shown for the Tameapa deposit (located in the Guerrero terrane) a protracted hydrothermal activity that spans from 57 Ma to 50 Ma is observed. These results add to the growing notion that some porphyry systems are the result of multiple intrusions and mineralization events as has been demonstrated for Bagdad in northern Arizona and El Teniente in Chile.

Campa MF, Coney PJ (1983) Tectono-stratigraphic terranes and mineral resource distributions in Mexico. Canadian Journal of Earth Sciences 20: 1040-1051.
Damon PE, Shafiqullah M, Clark KF (1983) Geochronology of the porphyry copper deposits and related mineralization of Mexico in Dawson KM (ed.) Symposium : Metallogeny and tectonics of the North American Cordillera. Canadian Journal of Earth Sciences 20: 1052-1071.
McCandless TE, Ruiz J, Campbell AR (1993) Rhenium behavior in molybdenite in hypogene and near-surface environments: implications for Re-Os geochronology. Geochim Cosmochim Acta 57: 889-905.
McCandless TE, Ruiz J (1993) Rhenium - Osmium evidence for regional mineralization in Southwestern North America. Science 261: 1282-1286.

Temperature, pressure and oxygen fugacity of crustal xenoliths from Rincon of Arangeo xenoliths, Michoacan-Guanajuato volcanic field, Mexico

Victor A. Valencia 1, John Chesley 1, Kevin Righter 2, Joaquin Ruiz 1 and Luca Ferrari 3
1 Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA
2Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
3Instituto de Geologia, Universidad Autonoma de Mexico, Apdo. Postal 70-296, Ciudad Universitaria, 04510 Mexico. D.F.
Email: victorv@geo.arizona.edu

Lower and middle crustal xenoliths are found in volcanic breccias from Rincon de Parangueo maar, near Valle de Santiago, in the central Trans-Mexican Volcanic Belt (TMVB). The major and trace element geochemistry of the crustal xenoliths are more similar to those observed in arc environments in South America than other xenoliths of rift environments of Mexico, suggesting that these xenoliths are more representative of arc environments of Mexico.

The lower crustal assemblages are predominately mafic granulites and include intermediate granulites, gneisses and cumulates. The xenolith composition changes progressively from mafic at depth to intermediate at shallow levels. Pressure (P) and temperature (T) for the mafic granulites assemblages (pl - cpx - opx ± sp ± Ilm ± ol), are 980-1030 °C and 10- 12 kbar, consistent with a lower crustal origin. Intermediate granulite (pl - cpx - opx ± sp) and cumulate assemblages (pl - cpx - ol - opx ± sp ± ilm ± hnbl ± biot) are consistent with middle crustal P-T conditions of 4-7 kbar and 950-1066 °C. Pressure of gneisses were difficult to constrain due mainly to lack of a reliable geobarometer, but have temperature of 725-760 °C.

The mafic xenoliths have higher fO2 than the other xenoliths within the Rincon de Parangueo complex and than upper mantle xenoliths elsewhere in Mexico, most likely the result of interaction of the lower crust with slab-derived, oxidized fluids or melts.

The fate of magmatic sulfides during intrusion or eruption, Bingham and Tintic districts, Utah

William J. A. Stavast, Jeffrey D. Keith2, Eric H. Christiansen2 and Michael J. Dorais2
Department of Geology, Brigham Young University Provo, Utah 84602
Department of Geosciences, University of Arizona Tucson, Arizona 85721
Email: wstavast@geo.arizona.edu

Magmatic sulfides in 300 samples of volcanic and intrusive rocks from the Tertiary Bingham (Cu-Au-Mo) and Tintic (Ag-Pb-Zn-Cu-Au) districts, Utah were examined to help better understand preservation and removal processes of magmatic sulfides. Our findings show that shallowly emplaced dikes and sills have erratic, but locally high, concentrations of sulfides. Sulfide concentrations appear to vary according to cooling rate and inferred pressure at the time of quenching or crystallization of matrix. Volcanic rocks from these districts typically have at least two orders of magnitude fewer sulfides than very unusual quenched dikes. In addition, sulfide concentrations vary dramatically across these dikes and sills; for example, sulfide abundances in one sill in Castro Gulch, Bingham district, increase from 9 ppm by volume in the center to more than 2000 ppm near the margin. This change in abundance is due to destruction of sulfides in the center of the sill. Chalcophile metals such as copper show corresponding changes in abundance. For example, the whole-rock copper content of the same sill ranges from 23 ppm in the center to 35 ppm along the margins. The concentrations of base metals lost from the center of the sill have ratios that are similar to the ratio of the metals in the entire deposit (production and reserves). The textures of sulfide grains (recrystallization, resorption, and degassing) even in the most sulfide-rich samples commonly are indicative of alteration, suggesting that no sample preserves all of its original magmatic sulfide content. Immiscible liquids of monosulfide solid solution crystallized as pyrrhotite, pyrrhotite and chalcopyrite, or pyrite and chalcopyrite with declining temperature and pressure. These may recrystallize to pyrite and chalcopyrite or to pyrite and an Fe-oxide. The alteration and preservation textures change from sub-spherical sulfide blebs near the margins of dikes and sills, to partially altered sulfides farther in, to no remnant of the altered sulfides in the vast majority of intrusions (except where small sulfides are completely enclosed by phenocrysts). The majority of the sulfides along the quenched margins of these dikes and sills occurs in the matrix. These data suggest that slow cooling coupled with removal of magmatic volatiles, including sulfurous gases (H2S, SO2, etc.), allows the resorption or degassing of magmatic sulfides to occur almost universally during final crystallization of a magma. This removes greater than 90 percent of the original endowment of magmatic sulfides. This probably explains the low magmatic sulfide abundances (1-15 ppm) of slowly cooled, large porphyritic intrusions. Most importantly, this removal process allows metals and sulfur to participate in the formation of porphyry deposits.

GIS and Geosciences: Universal applications to the fields of Hydrogeology and Geomorphology

Adam Bingham
Department of Geosciences, University of Arizona
Email: abingham@geo.arizona.edu

Geographic Information Systems are an extremely powerful analysis tool that is under utilized in the Geosciences but is becoming more prevalent in research today. Due to the varied spatial nature of geologic data, GIS are more and more able to process and analyze data and graphically reproduce it into interactive dynamic maps, which enable the creator to represent multiple data layers of a single system. The power of a GIS comes from its ability to integrate and overly many different data sets all linked by the same spatial coordinates allowing the user to analyze and gather important information about relationships along with many other important functions.

Some of the more universal applications of GIS as applied to Hydrogeologic and Geomorphologic studies include database construction, conceptual modeling and map creation. Examples include groundwater flow, surface water flow, depositional environments and morphology as well as landscape evolution. The use of GIS for conceptual groundwater modeling and hydrogeologic studies will be reviewed in some detail. An Assured Water Supply study for the City of Oro Valley was performed using GIS for model development, database development, map creation, and analysis of spatial data. Examples of multi-layered sets of spatial data will be examined for there impact on the water supply study as well as other related groundwater studies. GIS applications for well impact studies and infiltration studies will also be reviewed. Other possible applications of GIS to different geological fields will be discussed along with the benefits of spatial analysis. The power and versatility of Geographic Information Systems will prove to be an asset to almost any geologic endeavor.

A survey of Martian hillside gullies

Daniel C. Berman
Department of Geosciences, University of Arizona, Tucson, AZ 85721
Email: bermandc@geo.arizona.edu.

Background: The discovery of hillside gullies on Mars, interpreted by Malin and Edgett [1] to be evidence for recent water release, has caused much excitement and debate. At current atmospheric pressure and surface temperatures, liquid water is unstable, and scientists had previously assumed that water was not currently active in geologic processes on Mars. The giant outflow channels observed in early photographs gave rise to the theory that the climate of Mars had undergone some change in the past, and that liquid water had at some point been stable. Before the Mars Global Surveyor (MGS) mission, no evidence for recent water activity had been observed.

Since the beginning of its mission, the MGS Mars Orbiter Camera (MOC) has captured images of hundreds, if not thousands, of examples of apparently young, gully-like features. The gullies are mainly observed on the sloping walls of large craters or channels. These features commonly share three major characteristics: head alcoves, downslope channels, and debris fans. There is also a geographic correlation; most of the channels are poleward of 30 degrees latitude (most in the southern hemisphere) and usually are seen on pole-facing slopes.

As of this date, 112,000 MOC images have been publicly released. I have begun a comprehensive survey of the images to catalogue all that contain gullies. I have so far catalogued over 350. The latitudinal trend in the initial observations has held; the majority of the gullies are found between 30 and 60 degrees, primarily in the southern highlands. Also, most are found on the pole-facing slopes of craters and channels.

Crater counts show these features to be extremely young. Few, if any intact craters are observed on the slopes. Although it is difficult to draw conclusions from crater populations on a strongly sloping surface, the sheer lack of any observable craters, intact or not, is meaningful. Further, the debris fans of many of the gullies are seen to be underlying patches of transient dunes, which furthers the argument that they are very young features.

Once I have examined all MOC images for gullies, I will then make statistical comparisons between their locations and factors such as water depth, altitude, and insolation. These correlations will help us better understand where the water is on Mars, and how and when it is reaching the surface.

References: [1] M. C. Malin, and , K. S. Edgett (2000). Science 288, 2330-2335.

Using GIS technology to assess the McCall Glacier as a benchmark glacier for the Eastern Brooks Range, Alaska

Hector R. Hinojosa1 and William F. Manley2
1 Department of Geosciences, University of Arizona
2Institute of Arctic and Alpine Research (INSTAAR)
Email: pachuco411@hotmail.com

GIS technology is becoming an essential tool in the effort to understand the processes of global change. Glaciers in Alaska have experienced record warming for the past 50 years, and for their topographic and meteorological reasons, they are a good source of climate change indicators in Alaska. Because one benchmark glacier in each glaciated region regulates the physical processes that govern glacier evolution and stream flow in that region (Fountain et al, 1997), every glaciated range in Alaska should contain a benchmark glacier that can help understand adjacent glaciers, or the less intensively studied glaciers, providing information on the range of the glacier variation (Fountain et al, 1997). I explored the assessment of labeling the McCall Glacier, Eastern Brooks Range (EBR), Alaska, as a benchmark glacier based upon glacier area; basin area; perimeter; compactness; slope angle; aspect; median elevation; and hypsometric curves values. I used the Student's t-Test to statistically assess whether such glaciological parameters could make the glacier representative from the EBR. Based upon the Student's t-Test, the McCall Glacier is not a representative one from the EBR. However, the McCall Glacier and the EBR Elevation Line Altitude (ELA) elevation value share almost the same value; this suggests that the McCall's Glacier ELA elevation value might be representative for the EBR; and, in terms of previous and intense mass balance studies, the McCall Glacier is representative for the EBR.

Spatial and temporal complexity of climate-change controlled arid-land stream aggradation: Cuyama Valley, California

Stephen B. DeLong and Jon D. Pelletier
Department of Geosciences, University of Arizona, Tucson, AZ 85721
Email: sdelong@geo.arizona.edu

Stream aggradation due to Quaternary climate-change induced variation in hillslope sediment delivery is widely recognized on a variety of timescales in both tectonically active and inactive terranes. The sedimentary fills that result from this cyclic aggradation tend to be preserved as spatially and temporally complex fan and river terraces within geologic basins. Unraveling what controls the deposition of these terraces necessitates detailed field, chronologic and modeling approaches. Here we present initial field observations from the topographically and tectonically complex Cuyama River basin, which lies at the junction of the Coast Ranges and the Transverse Ranges near the "Big Bend" of the San Andreas Fault Zone in southern California.

Cuyama Valley contains an ephemeral through-going river that in its middle reaches drains two asymmetrical WNW-trending mountain ranges. Extensive steep coarse-grained alluvial fans slope from the 1700-m north-facing Sierra Madre range, filling approximately 80% of the lateral distance between ranges. These fans contain multiple Pleistocene terrace surfaces which are testimony to cyclic aggradation in an overall downcutting fluvial regime. The modern piedmont channels draining this range appear generally underfit and lack extensive Holocene terraces. In contrast, the south-facing 1500-m Caliente Range lacks an extensive suite of Pleistocene fan terraces, but has a more-developed suite of Holocene fan terraces and currently aggrading range-front fans. The Caliente Range appears to have supplied sediment through the late Holocene in quantity sufficient to cause main-stem Cuyama River aggradation.

This out-of-phase geomorphic response is interpreted as evidence for hillslope vegetation density being the primary transmitter of climatic perturbation to channels. Slope aspect and orographic precipitation are the primary controls on modern vegetation density. Although the north-facing Sierra Madre range receives greater precipitation at higher elevations, it appears the lower and drier Caliente range has a higher rate of Holocene sediment supply to the fluvial system due to greater hillslope sensitivity to low-amplitude Holocene climate variation. In contrast, the large-scale valley asymmetry likely results from millennial and longer time-scale sediment delivery being higher from the Sierra Madre range, owing to their higher relief. This higher relief allowed for more dramatic hillslope response to large-amplitude Pleistocene climate variation.

Thrusting within Greater Himalayan rocks in the Annapurna range, Central Nepal Himalaya

Aaron J. Martin , P. Jonathan Patchett, George E. Gehrels, Peter G. DeCelles and Clark Isachsen
Department of Geosciences, University of Arizona, Tucson, AZ 85721
Email: amartin@geo.arizona.edu

Our recent work in the central Nepal Himalaya suggests the need to relocate the boundary between Greater and Lesser Himalayan rocks, the Main Central Thrust. Ever since geologic research in the Himalaya began seventy years ago, researchers have debated the location of the Main Central Thrust (MCT). Confusion about the location stems in part from a compound definition of the MCT. One part of the definition treats the MCT as a lithotectonic boundary between Upper Proterozoic Greater Himalayan (GH) metasediments to the north and Lower Proterozoic Lesser Himalayan (LH) metasediments to the south. Another part of the definition treats the MCT as a structural feature: a ~5 km thick zone of ductile shear that accommodated top-to-the-south displacement of GH rocks onto LH rocks. This compound definition obfuscates the fact that the lithotectonic boundary need not be a thick zone composed of rocks different from GH or LH rocks. Since GH rocks are usually gneisses and LH rocks are usually schists, field workers typically place the MCT at the boundary between gneiss and schist. Our work indicates, however, that the contact between Greater and Lesser Himalayan rocks lies south of the gneiss-schist boundary, within ultra-deformed fine-grained rocks (phyllonites). The lithotectonic boundary is sharp and thin (less than 100 m thick). Thus at least two faults involve GH rocks: one fault between GH and LH rocks, and one fault within GH rocks at the gneiss-schist boundary. This is the first recognition that the gneiss-schist thrust involves Greater Himalayan rocks both in its hangingwall and in its footwall.

In addition to adding to shortening estimates across the Himalaya, the recognition of other faults near the MCT has profound implications that challenge the paradigm of Himalayan inverted metamorphism, at least within LH rocks and the lowermost GH rocks. An inverted metamorphic sequence is defined as an undisrupted package of rocks that exhibits increasing pressure and/or temperature with decreasing structural elevation. The recognition of faults within a package of rocks previously interpreted to exhibit inverted metamorphism obviates the need to call upon any unusual metamorphic or tectonic process to explain geologic observations. Instead, successively structurally higher thrust sheets carry higher-grade rocks, as is the norm in fold-thrust belts around the world.

We use Neodymium isotopes on whole rocks and detrital zircons from quartzites to accurately locate the position of the MCT, defined as a lithotectonic boundary. Because LH and GH rocks are different in age and provenance, they have different geochemical characteristics. Lower LH rocks contain zircons with ages between about 1600 and 2550 Ma, and have very negative eNd(0) values (average -24.5). GH rocks contain zircons as old as 3200 Ma and as young as 650 Ma, and have less negative eNd(0) values (average -15.5). Detailed mapping and tight sampling in five transects allows us to accurately locate the MCT and to map it across 60 km in the Annapurna Range.

Water content of the upper mantle

Andrew C. McCarthy and Mihai N. Ducea
Department of Geosciences, University of Arizona, Tucson, AZ 85721-0077

The upper mantle is one of the driest volumes of the solid earth. The common upper mantle (~ 40-70 km) minerals olivine, orthopyroxene and clinopyroxene are considered "nominally anhydrous"; i.e., H2O or OH are not written as part of their mineral formulae. However, water is present in these "anhydrous" minerals at small but measurable concentrations. In upper mantle peridotite, typical whole-rock water solubilities are ~100 - 200 ppm by weight. Olivine, the dominant upper mantle mineral (~ 60% by volume), can incorporate at least 135 ppm H2O at upper mantle temperatures and pressures of 1100° C and 2.5 GPa. Under the same conditions the solubility of water in orthopyroxene is similar to olivine at 133 ± 12 ppm. Solubility in clinopyroxene is much higher: 1080 ppm H2O has been measured in clinopyroxene from a natural eclogite sample. Water content of minerals in xenoliths sampling the upper mantle varies from the maximum solubility limits down to <5 ppm as measured by infrared spectroscopy. Spatial and temporal variations in upper mantle bulk water content may be due to partial melting (water is highly incompatible in upper mantle minerals), or the flux of fluids (e.g. melts or water-based fluids) through the mantle volume in question. Mineral water content, more than any other single factor, controls the rheology of the upper mantle. The addition of ~100 ppm H/Si to olivine has been observed to enhance dislocation mobility by a factor of ~100, significantly reducing the creep strength of the mineral. The macroscopic result of such a weakening of minerals is to increase the strain rate at much lower stresses and/or temperatures (i.e., lower the viscosity of the rock). Additionally, intragranular water appears to control the development of mantle anisotropy. Here we will present results from xenoliths brought to the surface in two disparate geologic environments: a hotspot situated below oceanic lithosphere (Hawai'i), and a continental area (Mojave).

The Sierra Nevada controversy: Youthful uplift or ancient topography?

Robinson Cecil and Clement G. Chase
Department of Geosciences, University of Arizona
Email: mrc@geo.arizona.edu

Until recently, the majority of the geologic community has been in agreement that the Sierra Nevada mountain range experienced middle Miocene or later uplift. This hypothesis was supported principally by geomorphic evidence, regardless of the fact that magmatic activity had come to an end in the Cretaceous. The concept of late uplift of the Sierra Nevada has come under fire as of late because of the emergence of new geophysical, geochemical and paleoaltimetric data. This data suggests that the Sierra Nevada, despite the association between regions of high topographic relief and rapid rates of erosion, remained at or near its present elevation through the Cenozoic. The use of new paleobotanical studies will make it possible to determine Eocene elevation of the Sierra Nevada and, therefore, to decide between early and late uplift models. Paleobotanically-based paleoaltimetry will allow for the placement of numerical constraints on the elevation of the Sierra Nevada through the Tertiary, thereby providing essential constraints on the geophysical model of the range's evolution. Knowledge of the topographic history of mountains belts is very difficult to ascertain, and the Sierra Nevada are particularly puzzling because they are presently at a high average elevation, yet a significant amount of time (~70 Ma) has passed since their last major formational event. Could the Sierra Nevada have remained high, in spite of high rates of erosion, or were they worn down and later uplifted by enigmatic upper mantle/lower crustal processes? Paleobotanical research is one of the best means of solving such a riddle, as the data it yields is independent of geomorphic evidence that is susceptible to many alternative explanations.

Cenozoic exhumation and the development of the Sierra Madre del Sur, southern Mexico; Evidence from apatite fission track and (U-Th)/He thermochronometry

Sarah Shoemaker1, Mihai Ducea1, J. Garver2, P. Reiners3, M.F. Campa4 and Joaquin Ruiz1
1University of Arizona, Dept of Geosciences, Tucson, AZ 85721
2Geology Department, Union College, Schenectady, NY 12308, USA
3Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USA.
4Escuela Regional de Ciencias de la Tierra, Universidad Autonoma de Guerrero, Apartado Postal 197, Taxco de Alarcon, Guerrero, Mex.
Email: sarahs@geo.arizona.edu

The southern slopes of the Cenozoic Sierra Madre del Sur Mountains, located in southern Mexico, comprise the Xolapa complex; a Mesozoic-Cenozoic magmatic arc reaching 50-100 km inland to the north and stretching 600 km from Zihuatenjo in the west to Salina Cruz in the east. There are no quantitative data to constrain either the timing and amount of Cenozoic uplift or the possible relationships between rock exhumation and plate kinematics in the region. In order to address this problem, 38 samples of undeformed and slightly deformed plutons were collected from six N-S transects along the length of the Xolapa complex (Zihuatenejo, Atoyac, Acapulco, Pinotepa N., P. Escondido, and P. Angel), and analyzed for apatite fission track (AFT) thermochronometry; in addition, 20 (U-Th)/He ages were obtained from the Acapulco and Puerto Angel transects. AFT and (U-Th)/He data do not show consistent age-elevation correlations along any of the sampled transects, but suggest slow average exhumation rates of 0.19 mm/yr since 25 Ma in the west, increasing eastward to 0.34 mm/yr since 12 Ma. These rates suggest that mountain building commenced in the region in the Miocene or, more probably, later. The proposed eastward passage of the Cocos-Caribbean-North American triple junction leading to a plate boundary change from transform (no uplift) to subduction (uplift) in the Eocene would predict an eastward propagating Eocene uplift event, which is not apparent in our data. There is little change in exhumation rates in response to postulated reorganization of the plate boundary west of the Sierra Madre del Sur (currently the Acapulco trench). The complicated age data could be the result of extended duration in the partial annealing (AFT) or retention ((U-Th)/He) zones, or may reflect cooling too shallow to be recorded by known thermochronologic systems. The data could also be influenced by the presence of unrecognized late Cenozoic faults that may exist along the analyzed transects.

Upper crustal deformation in southern Tibet before and during the Indo-Asian collision

Shundong He and Andrew Leier
Department of Geosciences, University of Arizona, Tucson, AZ 85721
Email: donghe@geo.arizona.edu

The high elevation of the Tibetan plateau (> 5km) is largely supported by its thick crust (>65 km). However, the crustal thickening history, and hence the uplift history, of Tibet is poorly known. It has been proposed that the southern Tibetan crust was thickened by (1) homogeneous shortening (~50%) during the Cenozoic Indo-Asian collision (Dewey and Burke, 1973), (2) northward underthrusting of India beneath a rigid Tibet (Argand, 1924; Powell and Conaghan, 1973), or (3) pre-Indo-Asian collision deformation (Murphy et al., 1997). In southern Tibet, there is a widely preserved early Tertiary angular unconformity, which is a perfect marker from which the magnitude and timing of upper crustal strain can be determined. Thrust faults in Linzou basin and at the north side of Nyqintanghla, which have been considered as depositional contact, below the early Tertiary unconformity have been mapped. U-Pb dating the dikes, that cut through late Cretaceous mudstone but not the unconformity, and the early Tertiary volcanic rocks help to constrain the age of the unconformity and the timing of the deformation accurately. The mapping work and a cross-section indicate that there is shortening below the unconformity, hence the eastern southern Tibet has been shortened before early Tertiary. The previous unrecognized shortening before early Tertiary is not result from the Indo-Asian collision, maybe caused from the Lhasa-Qiangtang collision. The mapping and dating results are consistent with what Murphy mapped and dated in western southern Tibet. Although the mapping and dating results are exiting, there are lots of more mapping work need to be done in a wider area before we can conclude what happened in southern Tibet during late Cretaceous and early Tertiary. Mapping the deformation below and above the early Tertiary unconformity, and the deformation of the unconformity itself along a longer section in southern Tibet are critical for further reconstructing tectonic deformation history of the southern Tibet.