• An Analysis of Some Regional Gravity Data in Arizona

      Bhuyan, Ganesh Ch.; Sumner, John S.; Lacy, W. C.; Titley, Spencer R.; Bhuyan, Ganesh Ch. (The University of Arizona., 1965)
      The need for accurate reference bases for any gravimetric work can hardly be overemphasized. During the months of March, April, and May, 1964, about 130 gravity observations were made in Arizona, with LaCoste Romberg Gravity Meter DL-1 and Worden Gravity Meter (Educator) No. 461. The purpose of this program was to establish 1) a 1st order control airport gravity base network, 2) a standard calibration range for the State of Arizona, and 3) to gain a structural interpretation of the Tucson Basin. Gravity data were analyzed as to their accuracies and reliabilities, taking into consideration errors involved in tidal corrections, drift corrections, reference datum, and nonlinearity of scale factor of the meters. It is concluded that the reliability of these data is .1 milligal or better. While correcting for the tidal variation of gravity, it was noticed that there was a discrepancy between the theoretical and observed tidal correction values. For any additional precise work, it is desirable to correct for tidal variations from actual records, if available, in conjunction with the theoretical tables. Causes for this discrepancy in tidal gravity variation need further study. Programs were written for a digital computer to calculate 1) the theoretical gravity values from the International Gravity Formula, and 2) Free-Air Anomalies, Bouguer Anomalies and Special Bouguer Anomalies from field data for various stations. Free-Air and Bouguer Anomaly values for different stations were analyzed as to their implications in terms of isostasy, crustal structures and local geological structures. Results from Simple Bouguer Anomaly values indicate a crustal thickness of 49 km to 33 km for Arizona with broad isostatic compensation for regional surface irregularities. An analysis of residual Bouguer gravity anomalies of the Tucson Basin in terms of local geological structures, indicates a basin and range structure for this region. The thickness of sediments ranges from more than 700 feet on the north to more than 8000 feet towards the south of the basin with faults indicated in it. Application of a limiting -depth interpretation method implies that the tops of the disturbing bodies can be no deeper than 2 miles below sea level. A total mass deficiency corresponding to the residual gravity low in this basin comes out to be 1.8 x 10¹⁷ grams according to two – dimensional form of Gauss' Theorem. This corresponds to a 135 cubic mile volume of material with a density .3 grams per cubic centimeter less than the enclosing rocks. From porosity and volume considerations of the sediments in the Tucson Basin, it is estimated that the total water holding capacity may be of the order of 4.6 x 10⁷ acre feet.
    • Precambrian Geology of the Cottonwood Cliffs Area, Mohave County, Arizona

      Davis, George H.; Beard, Linda Sue; Davis, George H.; Coney, Peter J.; Lucchitta, Ivo; Beard, Linda Sue (The University of Arizona., 1985)
      A belt of Early Proterozoic rocks crops out in the Cottonwood Cliffs area, northwest Arizona. The belt contains an eastern and a western assemblage separated by the Slate Mountain fault. The western assemblage consists of mafic to felsic metavolcanic rocks, metapelites, and metaconglomerates. The eastern assemblage consists of phyllites, felsic to intermediate metavolcanic rocks, metagraywackes, and metagabbro bodies. The belt is bounded to the east by foliated granodiorite. The Valentine granite intruded the belt on the west and north. Steeply-plunging lineations and fold axes, and northeast-trending vertical foliation dominate the structural fabric. The regional elongation direction is near-vertical, as indicated by mineral and pebble lineations, and is parallel to fold axes. Although only one deformational event is evident, the intensity of that event may have obliterated evidence of any earlier deformation. Tertiary basalts and the Peach Springs Tuff locally overly the metamorphic rocks. Cenozoic normal faults in the area are mostly of minor displacement.
    • Paleocurrent Analysis of the Upper Miocene Formation, Los Angeles Basin, California

      Bennett, John Newton, 1943-; Wright, Jerome J.; Pye, W. D.; Harshberger, J. W.; Schreiber, Joseph F. Jr.; Bennett, John Newton, Jr. (The University of Arizona., 1967)
      Almost all sandstone beds occurring in the Upper Miocene formations at the Los Angeles basin were deposited by turbidity currents. Primary textures and structures indicative of turbidites occur in fair abundance throughout all three Upper Miocene formations. All accessible outcrops of the Puente, Modelo, and Upper Miocene portion of the Monterey and Capistrano Formations were scrutinized for sandstone beds containing primary sedimentary structures. Through study of these structures, the direction of current movement was determined. The pattern of current movement displayed reveals that sediment was being transported into the Los Angeles basin from all sides. Current directions and mineralogic studies indicate that essentially three source areas were supplying sediment into the basin. These source areas are 1) the San Gabriel Mountains, 2) an area to the east of the Santa Ana Mountains, and 3) a ridge of metamorphic rock paralleling the present coast line. The majority of sediment was derived from an area in the San Gabriel Mountains located northeast or the basin. This is evidenced by the fact that the thickness, grain size, and total sand content of the Upper Miocene units decrease southwestward across the basin.
    • Radius Effect of the Alkaline Earths on the Rate of Inversion of Aragonite to Calcite

      Bennett, Catheryn MacDonald; Schreiber, Joseph F. Jr.; Bennett, Catheryn MacDonald (The University of Arizona., 1972)
      The effect of magnesium, strontium, and other alkaline earths on the formation and persistence of metastable carbonates in the natural environment was investigated to determine the nature of the controlling mechanism. Barium and beryllium were studied to evaluate the effect of ionic radius; magnesium and strontium, in order to determine if the results correlate with the usual order of stability for complexes and adsorbed species. Known weights of aragonite were placed in contact with solutions of beryllium, magnesium, calcium, strontium, and barium. Samples were covered and periodically both pH and percent composition of aragonite determined; supernatant liquids and precipitates were analyzed for cation concentrations by atomic absorption spectroscopy and titrimetric methods. Results indicated that the order of effectiveness of alkaline earth metals in inhibiting recrystallization is : Be > Mg > Sr > Ba. This is the expected order of effectiveness for both surface and solution effects. A solution effect (i.e., sequestration of bicarbonate ions) is strongly suggested by the chemical behavior of each cation.
    • Regional Structure and Stratigraphy of Sierra El Aliso, Central Sonora, Mexico

      Stewart, John; Bartolini, Claudio; Coney, Peter J.; Ruiz, Joaquin; Bartolini, Claudio (The University of Arizona., 1988)
      Assemblages of Paleozoic age and less significant Triassic and possibly Cretaceous-Tertiary volcanic rocks constitute the Sierra El Aliso, 186 km east-southeast of Hermosillo, Sonora. The Paleozoic section consists of approximately 2000 m of allochthonous Ordovician to Permian pelagic and hemipelagic deposits that accumulated in continental slope, continental rise and ocean floor (?) environments. The lower Paleozoic is characterized by graptolitic black shale and radiolarian chert, quartzite, argillite and local limestone. The upper Paleozoic is predominantly turbidite carbonates rich in benthonic foraminifera, and conodont faunas, subordinate bedded chert, siltstone, sandstone and chert-clast conglomerate. After Early Permian time, but prior to the deposition of the Late Triassic Barranca Group the Paleozoic section was imbricated along south-southeast vergent thrust faults. The Triassic rocks unconformably overlie the Paleo-zoic strata and all thrust faults. The Triassic and older rocks are overlain by the Cretaceous-Tertiary volcanics.
    • Geology of the Owl Head Mining District, Pinal County, Arizona

      Barter, Charles F.; Mitcham, Thomas W.; Barter, Charles F. (The University of Arizona., 1962)
      The Owl Head mining District is located in south-central Pinal County, Arizona, within the Basin and Range province. Land forms, particularity pediments, characteristic of this province are abundant in this area. Precambrian rocks of the Owl Head mining district include the Pinal schist; gneiss; intrusions of granite, quartz monzonite and quartz diorite; and small amounts of Dripping Spring quartzite and metamorphosed Mescal limestone. These have been intruded by dikes and plugs of diorite and andesite, and are unconformably overlain by volcanic rocks and continental sedimentary rocks of Tertiary and Quaternary age. No rocks of the Paleozoic and Mesozoic eras have been recognized. The structural trends of the Owl Head mining district probably reflect four major lineament directions. The dominant structural trends found in the area are north and northwest. Subordinate to these directions are northeast and easterly trends. The strike of the northerly trend varies from due north to N30°E and was probably developed during the Mazatzal Revolution. The northwest trend has probably been superposed over the northerly trend at some later date. Copper mineralization is abundant in the area and prospecting by both individuals and mining companies has been extensive. To date no ore body of any magnitude has been found, but evidence suggests that an economic copper deposit may exist within the area. The copper mineralization visible at the surface consists mainly of the secondary copper minerals chrysocolla, malachite, azurite, and chalcocite with chrysocolla being by far the most abundant. Copper minerals are found to occur in all rocks older than middle Tertiary age. Placer magnetite deposits are found in the alluvial material of this area, and one such deposit is now being mined.
    • An Investigation of the Manner and Time of Formation of Malachite

      Titley, Spencer R.; Beane, Richard Edward, 1942-; Titley, Spencer R.; Damon, Paul E.; Guilbert, John M.; Beane, Richard Edward (The University of Arizona., 1968)
      A group of minerals typical of the oxidation zone of copper deposits was studied using chemical thermodynamics, mineral stability relationships, and petrography. It has been concluded that many processes, such as alteration, can be explained using thermodynamics and are compatible with natural relationships. A mineral assemblage consisting of basic carbonates of copper and zinc was investigated with carbon isotopes and mineral stability relationships. The results obtained are consistent with processes resulting from oxidation and leaching of a sulfide deposit followed by redeposition of copper and zinc in the zone of oxidation. Four processes have been suggested by which copper could be introduced into a near-surface environment from depth. Reactions which would occur under these conditions may result in formation of mesogene or hypogene malachite and chrysocolla. A vein deposit containing minerals typical of the zone of oxidation was investigated and it has been concluded that the minerals may have formed from a hydro-thermal solution related to near-surface volcanism.
    • Small Vertebrates of the Bidahochi Formation, White Cone, Northeastern Arizona

      Baskin, Jon Alan, 1947-; Lindsay, Everett H.; Baskin, Jon Alan (The University of Arizona., 1975)
      Two taxa of amphibians, five taxa of reptiles, and eighteen taxa of mammals were collected by screen-washing sediments from the upper Bidahochi Formation at White Cone peak, northeastern Arizona. Five new species of mammals were recovered. They include Perognathoides bidahochiensis (Heteromyidae), Bensonomys yazhi (Cricetidae), Bensonomys bradyi (Cricetidae), Paronychomys alticuspis (Cricetidae), and Martes (Plionictis) repenningi (Mustelidae). Bensonomys yazhi and Bensonomys bradyi are close to the stem of Central and South Americal hesperomyine radiation. The radiation began in the southwest United States and Mexico during the Hemphillian. The middle member of the Bidahochi Formation is dated at 6.7 m. y. by a basalt from Roberts Mesa. The paleomagnetic data and the White Cone local fauna support this middle Hemphillian date.
    • The Tertiary Igneous Terrain in the Vicinity of the King Tonopah Mine, Tonopah, Nevada: An Exploration Case Study

      Barker, Walter Blaine; Eastoe, C. J.; Ruiz, Joaquin; Titley, Spencer R.; Barker, Walter Blaine (The University of Arizona., 1986)
      Uneconomic epithermal precious metal mineralization and associated alteration occur in the Tonopah Property, and are similar in style, although much less intense, to the deposits of the Tonopah camp two miles south. Mineralization is localized within a set of northwest-trending faults within the Tonopah, Mizpah, and King Tonopah Member of the Fraction-Tuff formations, and is associated with widespread propylitic and sparse fracture-localized potassic and argillic alteration. A younger set of Mn-calcite veins, anomalous in manganese, mercury, arsenic, and antimony, occurs in northeast-trending faults cutting older formations as well as the younger Tonopah Summit Member of the Fraction Tuff. This mineralization is possibly associated with silicification, zeolitization, and clay-alteration of the Fraction Tuff. The Tonopah Summit Member of the Fraction Tuff is reinterpreted as younger than the King Tonopah Member. Mega-breccia and basin morphology in the northeast may indicate an eruptive vent in this area.
    • Structural Geologic Controls at the San Luis Mines, Tayoltita, Durango, Mexico

      Ballard, Stanton Neal; Davis, George H.; Coney, Peter J.; Guilbert, John M.; Ballard, Stanton Neal (The University of Arizona., 1980)
      In the San Dimas district, on the western flank of the Sierra Madre Occidental, near the small town of Tayoltita, Durango, gold and silver epithermal ore deposits are mined from the complex Arana fault system. The structural relationships of the Tayoltita system are well-mapped, but their kinematic relationship to ore deposition is unclear. In plan view and in cross-section, the Arana system has a horsetail or wedge-shaped geometry. Subsurface mapping of slickenside striae as movement indicators suggest that the N13°W-striking Arana fault, forming the eastern boundary of the system, is a normal slip fault with at least 250 m of throw. Subsidiary system faults display normal separation with varying degrees of dextral horizontal separation (which is a function of fault orientation). Experimental modeling of the Arana system indicated that the system formed under simple shear as the σ₂ and σ₃ stress axes rotated in a subhorizontal plane about σ₁. Rotational strain caused the developing fault strands to rotate and to be captured by the Arana fault, forming the typical wedge-shaped geometry. Later, a more complex rotation of the three major stress axes enabled hydrothermal fluids to progressively mineralize faults, which had more northerly strikes, by a process similar to progressive strain. This is documented by mineral assemblages that record the instants of fault opening and by the lack of mineralization along the high-angle, northwest- striking faults.
    • The Geology and Geochemistry of Beryllium in Southern Arizona

      Balla, John Coleman, 1936-; Lacy, Willard C.; Erickson, Einar C.; Balla, John Coleman (The University of Arizona., 1962)
      Nine beryllium deposits were studied in order to determine the geological environment of beryllium mineralization in southern Arizona. Beryllium occurs in two pegmatite areas, two contact metamorphic deposits, two quartz-tungsten veins, two quartz-feldspar veins, and in one quartz monzonite stock. It is associated in almost all of these deposits with purple fluorite and tungsten. Beryllium mineralization is associated with granitic and quartz monzonite intrusions of Laramide age, and generally occurs at the intersection of northwest-trending lineaments and the Texas lineament.
    • Stratigraphy and Depositional History of the Pantano Formation (Oligocene-Early Miocene), Pima County, Arizona

      Schreiber, J. F. Jr.; Balcer, Richard Allen; Schreiber, J. F. Jr.; Dickinson, W. R.; Balcer, Richard Allen (The University of Arizona., 1984)
      The Pantano Formation comprises 1,250 m of alluvial, fluvial, lacustrine, and volcanic rocks deposited in a basin formed in response to regional extension during mid- Tertiary time in southeastern Arizona. During deposition, the locations and composition of sediment source areas varied as contemporaneous uplift occurred adjacent to the basin. The lower half of the formation was deposited as alluvial fans that prograded northward, westward, and southward; the upper half was deposited during southwestward retreat of alluvial fan deposition and the onset of lacustrine deposition. An andesite flow separates the two depositional regimes. Radiometric dates of 24.4 ± 2.6 m.y. B.P. for the andesite and 36.7 ± 1.1 m.y. B.P. for a rhyolitic tuff disconformably underlying the formation indicate that deposition occurred during Oligocene to early Miocene time. Proper stratigraphic sequencing and description, paleocurrent analysis, and gravel provenance study aided in understanding the depositional history of the formation.
    • Geology of the Gore Canyon-Kremmling Area, Grand County, Colorado

      Barclay, C. S. Venable; Mayo, Evans B.; Barclay, C. S. Venable (The University of Arizona., 1968)
      The Gore Canyon-Kremmling area is in the southwestern portion of the Kremmling 15-minute quadrangle, Colorado. Precambrian rocks are biotite gneiss, the Boulder Creek Granodiorite, granophyre dikes, and quartz veins. The Boulder Creek Granodiorite intrudes the biotite gneiss, and both of these units are cut by north-northwest-trending, granophyre dikes and quartz veins. Biotite gneiss contains structure elements of a northwest and a northeast fold system. Lineations and foliations in the Boulder Creek Granodiorite are generally concordant to the northeast fold system of the gneiss. Late Paleozoic to Mesozoic and Mesozoic sedimentary formations, in ascending order and with their approximate thicknesses, are the State Bridge Formation, 15 feet; the Chinle and Chugwater Formations undivided, 0-95 feet; the Sundance Formations 0?-100 feet; the Morrison Formation, 250 feet; the Dakota Sandstone, 225 feet; the Benton Shale, 340 feet; the Niobrara Formation, 600 feet; and the Pierre Shale. Quaternary deposits are terrace, landslide, and modern flood-plain deposits. Laramide rock deformation is related to the Park Reuse uplift and includes faulting and, in the sediments, some folding. Some of the faults, including the regional Gore fault, are Precambrian structures reactivated in Laramide time.
    • Structural Investigations of the Italian Trap Allochthon, Redington Pass, Pima County, Arizona

      Benson, Gregory Scott; Davis, George H.; Coney, Peter; Schreiber, Joseph F. Jr.; Benson, Gregory Scott (The University of Arizona., 1981)
      Italian Trap Allochthon is a rare upper-plate exposure of Paleozoic metasedimentary and Precambrian to Tertiary crystalline tectonites in the Santa Catalina-Rincon metamorphic core complex. Elsewhere in the complex, metasedimentary tectonite is usually restricted to an autochthononous position. The internal structures of the allochthon consist of numerous low-angle faults, tear faults, and overturned asymmetric and upright folds. Close association of the low-angle faults and asymmetric folds, and vergence of the folds, indicates that these folds were formed during westward transport along the low-angle faults. The structures of the allochthon are truncated and rotated to the northeast by a listric (?) normal fault. The probable shape of the fault surface, together with the northeastward rotation of the internal structures, suggests translation of the allochthon from the northeast to the southwest. The fact that metasedimentary tectonites are found in upper- plate position indicates that the listric (?) normal faulting post-dates the metamorphism of the Paleozoic and Mesozoic strata. Metamorphism in turn was part of the development of the Santa Catalina-Rincon metamorphic core complex. It is inferred that the Italian Trap Allochthon was emplaced in the final stages of profound regional extension which prevailed during the mid-Tertiary in southern Arizona.
    • Pollen in Fecal Pellets as an Environmental Indicator

      Bartos, Frances Maribel; Martin, Paul S.; Solomon, Allen M.; Kremp, G.O.W.; Bartos, Frances Maribel (The University of Arizona., 1972)
      Identification of pollen in fecal pellets is a potential technique for describing an animals diet and in turn the vegetation of an area. Mule deer and Bighorn Sheep pellets representing both summer and winter browsing and a variety of habitats were examined using relative percentages and the absolute pollen frequencies. In addition, fossil pellets from Stanton's Cave, Grand Canyon, Arizona, were examined and compared with modern pellets. Absolute pollen frequencies of individual pellets showed higher values and greater variation for summer pellets than for winter pellets. Relative pollen percentages for a specific vegetation type showed more variation in fecal pellets than in soil surface samples. Unless specifically being eaten, arboreal pollen types such as Pinus are less abundant in fecal pellets than in soil samples. Unlike soil surface samples, arboreal pollen types in fecal pellets are frequently limited to the immediate source area.
    • Variable Denudation in the Evolution of the Bolivian Andes: Controls and Uplift-Climate-Erosion Feedbacks

      Pelletier, Jon D.; Barnes, Jason B.; Chase, Clement G.; DeCelles, Peter G.; Barnes, Jason B. (The University of Arizona., 2002)
      Controls on denudation in the eastern Bolivian Andes are evaluated by synthesis of new and existing denudation estimates from basin-morphometry, stream - powered fluvial incision, landslide mapping, sediment flux, erosion surfaces, thermochronology, foreland basin sediment volumes, and structural restorations. Centered at 17.5 °S, the northeastern Bolivian Andes exhibit high relief, a wet climate, and a narrow fold- thrust belt. In contrast, the southeastern Bolivian Andes have low relief, a semi-arid climate, and a wide fold-thrust belt. Basin -morphometry indicates a northward increase in relief and relative denudation. Stream-power along river profiles shows greater average incision rates in the north by a factor of 2 to 4. In the south, profile knickpoints with high incision rates are controlled by fold-thrust belt structures such as the surface expressions of basement megathrusts, faults, folds, and lithologic boundaries. Landslide and sediment-flux data are controlled by climate, elevation, basin morphology, and size and show a similar trend; short -term denudation-rate averages are greater in the north (1- 9 mm/yr) than the south (0.3-0.4 mm/yr). Long-term denudation-rate estimates including fission track, basin fill, erosion surfaces, and structural restorations also exhibit greater values in the north (0.2-0.8 mm/yr) compared to the south (0.04-0.3 mm/yr). Controls on long-term denudation rates include relief, orographic and global atmospheric circulation patterns of precipitation, climate change, glaciation, and fold-thrust belt geometry and kinematics. The denudation synthesis supports two conclusions: 1) denudation rates have increased towards the present 2) an along-strike disparity in denudation (greater in the north) has existed since at least the Miocene and has increased towards the present. Denudation rates and controls suggest that Bolivian mountain morphology is controlled by both its orientation at mid-latitude, and the feedbacks between uplift, kinematics, orographic effects on precipitation, glaciation, and the increased erosion that accompanies orogenesis.
    • The Conodont Biostratigraphy of the Black Prince Limestone (Pennsylvanian) of Southeastern Arizona

      Barrie, Kathleen Ann; Schumacher, Dietmar; Bryant, Donald L.; Schreiber, Joseph F. Jr.; Barrie, Kathleen Ann (The University of Arizona., 1975)
      The Black Prince Limestone of southeastern Arizona has been assigned to the Morrowan on the basis of several long-ranging fossils. Since these were not especially diagnostic, the exact time represented by the Black Prince within the Morrowan was uncertain. To date the Black Prince more precisely, six sections were systematically sampled for conodonts. The condonts found, especially Neognathodus bassleri, Rachistognathus muricatus, Idiognathoides convexus, and Spathoqnathodus coloradoensis, indicate a middle Morrowan to early Derryan age for the Black Prince in the study area. Four conodont zones can be recognized: the Neognathodus bassleri Zone, the Idiognathodus sinuosis.- Streptognathodus anteeccentricus Zone , the Idiognathoides convexus Zone, and the Spathognathodus coloradoensis-Neognathodus columbiensis Zone. These zones compare favorably with the zonation previously established in the type Morrowan. This biostratigraphic evidence suggests that the hiatus between the Black Prince and Horquilla Limestones increases in magnitude from southeastern to south-central Arizona. The Black Prince represents a sequence of tidal flat and shallow subtidal carbonate deposits. Mudstones and sparsely fossiliferous wackestones with low fossil diversity and abundance characterize the tidal-flat facies. Grainstones, packstones, and fossiliferous wackestones with high fossil diversity and abundance characterize the shallow subtidal facies.
    • A Paleocene Paleomagnetic Pole from the Gringo Gulch Volcanics, Santa Cruz County, Arizona

      Barnes, Arthur E.; Smiley; Butler, R.E.; Barnes, Arthur E. (The University of Arizona., 1980)
      Paleomagnetic data from 25 sites (5 samples per site) in andesite flows of the Gringo Gulch Volcanics in Santa Cruz County, Arizona, were analyzed to determine a lower Paleocene paleomagnetic pole. Alternating-field demagnetization to 500 oe peak field was sufficient to erase secondary viscous components. The mean direction of magnetization (inclination = -58.8°, declination = 167.5 °) was obtained by averaging the site mean directions of the 25 sites, which are all reversed. The resultant lower Paleocene pole position is at lat. 77.0 °N, 1on. 201.0 °E (dp = 1.2 °, dm = 1.7 °).
    • Saturated Hydrocarbons in Fatty Tissue of Beef Heart

      Nagy, B.; Bandurski, Eric Lord; Bandurski, Eric Lord (The University of Arizona., 1972)
      Saturated hydrocarbons were extracted from fatty tissue of beef heart and identified by combined gas chromatography-mass spectrometry. A complete series of normal alkanes from C₁₄ to C₃₅ was identified together with three isoprenoid hydrocarbons, 2, 6, 10, 14- tetramethylpentadecane (pristane), 2, 6, 10, 14- tetramethylhexadecane (phytane), and 2, 6, 10- trimethyltridecane. In addition, a C₁₇ branched alkane with an isoprenoid-like fragmentation pattern was identified but the spectrum could not be matched with that of a C₁₇ isoprenoid hydro-carbon reported in sediments. The distribution pattern of the ṉ-alkanes is very similar to that reported in pasture plants, indicating that the ṉ-alkanes are derived from the steer's diet. The isoprenoids have not yet been reported in plant tissues, suggesting that they might be produced in the steer from the phytol side chain of chlorophyll a.
    • The San Alberto Lead-Zinc Ore Body at Cerro de Pasco Mine, Cerro de Pasco, Peru

      Ascencios C., Alejandro; Lacy, W. C.; Ascencios C., Alejandro (The University of Arizona., 1966)
      This thesis presents briefly the geology of the Cerro de Pasco district to acquaint the reader with the general geological setting of the district. A study of ore controls for a typical lead-zinc replacement body at the world famous Cerro de Pasco mine in Peru, 102 km northeast of Lima, was undertaken for purposes of better understanding. The particular body selected, the San Alberto Ore Body, occurs as a northeast extension of a main mass of pyrite, known as the "pyrite body", and is enclosed in Triassic- Jurassic limestone. Primary ore controls were determined to be a "Y"-like intersection formed by NS Longitudinal Faulting with a NE striking bedding fault. Resultant brecciation created the necessary permeable conditions whereby ore fluids were channeled away from the pyrite body into the limestone for ore emplacement. Three phases of hydrothermal rock alteration were identified as silicic alteration, chloritic alteration and an outer zone of bleaching and recrystallization. One peculiarity is found in the mineral composition of the silicic alteration, and a hypothesis is presented to explain it. The paragenetic sequence given for hypogene and gangue minerals was determined from the examination of more than 150 thin and polished sections.