Tommy B. Thompson

Petrology and petrogenesis of the Bokan Granite Complex, southeastern Alaska

The Bokan Granite Complex is a peralka-line ring-dike complex emplaced into marine shales, volcanic flows and tuffs, and plutonic rocks. Mineralogically and chemically, the Complex is composed of I-type granites. Aegirine- and riebeckite-bearing granite aplites, porphyries, and pegmatites comprise twelve distinct intrusive episodes. The aegirine-bearing rocks occur in an outer annular zone formed during early crystallization. Subsequent rocks are riebeckite-bearing, due to devolatilization of the magma chamber during a collapse-ring-dike emplacement event. Early crystallization of alkali feldspar occurred during magma ascension from a lower crustal-upper mantle source. At shallow depths, subsolvus crystallization allowed microcline and albite to dominate. Local bodies of aegirine syenite were formed during the early collapse-ring-dike emplacement, in response to the magma devolatilization. The riebeckite granites reflect lower Po2 and possibly declining peralkalinity. The granitic rocks at Bokan all exhibit Na2O contents greater than K2O. Lithophile elements are concentrated in all the rocks, especially in zones where hydrothermal albite and chlorite formed. Rb/Sr ratios increase in progressively younger rocks in the Complex. Agpaitic ratios vary from 0.92 to 2.08 for the granitic rocks.

Sierra Blanca Igneous Complex, New Mexico

The Sierra Blanca igneous complex of south-central New Mexico consists of the Oligocene Sierra Blanca Volcanics which have been intruded by Oligocene monzonite to quartz syenite stocks, sills, and dikes. The Sierra Blanca Volcanics are composed of four formations: Walker Andesite Breccia, Nogal Peak Trachyte, Church Mountain Latite, and Godfrey Hills Trachyte. The Walker Andesite Breccia comprises the major volume of the eruptive rocks. It is characterized by monolithic blocks transported and frozen in a matrix of plagioclase microlites, enstatite, hornblende, and hematite. A few K-Ar dates indicate that volcanism began approximately 35 m.y. ago and continued until about 25 m.y. ago. The Walker Andesite Breccia is succeeded by flows, tuff layers, and welded ash flows of the Nogal Peak Trachyte, Church Mountain Latite, and Godfrey Hills Trachyte, which are preserved as widely separated remnants. The younger volcanic formations give evidence of a more explosive period of volcanism compared with the Walker Andesite Breccia stage. Four hypabyssal stocks intrude the Sierra Blanca Volcanics: Rialto stock, Chaves Mountain stock, Bonito Lake stock, and Three Rivers stock. Three are multiple intrusives. Alkali enrichment is pronounced in one of the stocks. K-Ar dates of the stock range from 34.4 m.y. to 25.8 m.y. The volcanic and intrusive rocks show progressive mineralogical and chemical changes typical of alkali-calcic series. The oldest volcanic rocks contain zoned plagioclase (An 77 to An 47 ) and enstatite, but in successive volcanic rocks plagioclase becomes more sodic (An 32 ) and hornblende becomes the dominant ferromagnesian mineral. FeO(total Fe)/MgO ratios increased from 1.45 to >5 during magma differentiation. Fractional crystallization with removal of calcic plagioclase and enstatite during volcanism appears to have been responsible for differentiation and generation of the late-stage silicic intrusive phases. The large volume of andesitic to trachytic volcanic rocks and intrusive syenitic rocks along with the lack of cogenetic basalt suggest that the parent magma at Sierra Blanca was andesitic. Major structural planes of weakness, which intersect in the Sierra Blanca area, would allow magma generated in the mantle to move into the crust and erupt on the surface. Similar controls appear to have been operational along the Rocky Mountain trend and parallel to the Rio Grande depression. Other andesite breccia sheets present along the trend include the Absaroka volcanic field, Wyoming, the Thirtynine Mile volcanic field, Colorado, and the Red River volcanic-intrusive complex, New Mexico. They are Oligocene in age and have many similar characteristics.

Geology and uranium-thorium mineral deposits of the Bokan Mountain Granite Complex, southeastern Alaska

The Bokan Mountain (Kendrick Bay) uranium-thorium deposits are associated with a Late Jurassic peralkaline granite ring-dike complex. The uranium-thorium deposits form pipe-like bodies along contacts, or occur as pods in en echelon northwest-striking shear zones. The host intrusive is a multiphased body consisting mainly of riebeckite granite porphyry and subordinately of pegmatite-aplite, aegirine granite porphyry, and eight other lesser rock types; the ore is closely associated with an albitized aegirine syenite. The ore consists of thorium and rare earth-rich rock containing uranothorite, uraninite, and generally less than 2% of sulfide species. About 100,000 tons (89,000 t) of ore have been mined at an average grade of about 1% U3O8 and nearly 3% ThO2, mainly from the Ross-Adams pipe. The pipe itself lies along the contact of aegirine syenite and aegirine granite porphyry; it is as much as 24 m across and was mined along strike for over 300 m. The ore in shear zones occurs in lenses as much as 3 m thick and 30 m in strike length. Wallrock alteration within and adjacent to orebodies consists of pervasive hydrothermal albite and lesser amounts of chlorite, fluorite, calcite, quartz, sericite, and tourmaline. Hematite is present in the outer-distal parts of the ore zones. The minerals produced to date occur within the outer annular zone of the Bokan Granite Complex in which aegirine is present as the major alkali ferro-magnesian mineral. Ore emplacement occurred during the first subsidence event in association with devolatilization of the magma chamber. The pod-like ore zones appear to have formed during regional faulting synchronous with magma crystallization and subsequent hydrothermal events. Subsequent magma crystallization yielded riebeckite granites that were depleted in U and Th. Filling temperatures in ore stage fluid inclusions within quartz range from 320 to 331°C with pressure corrections increasing the temperature of formation to 420°C or higher, depending on dissolved gas content. Sulfur isotopic analyses on pyrite, galena, sphalerite and pyrrhotite indicate disequilibrium, but δ34SH2S is estimated at 7.6‰. Carbon and oxygen isotopic analyses on ore stage calcite yield δ18OH2O of 6.8 to 8.1‰ and δ13CΣC of −4.3 to −7.0‰. The oxygen and carbon isotopic data support a magmatic origin for the calcite and associated thorium-uranium deposits.

Orbicular Rocks of the Sandia Mountains, New Mexico

Orbicular rocks that occur within biotite-rich Precambrian granite of the Sandia Mountains in central New Mexico are of three types: (1) multishelled orbicules with alternating biotite- and plagioclase-rich shells, (2) plagioclase orbicules with or without a discontinuous biotite shell near the orbicule margin, and (3) orbicules with plagioclase cores surrounded by thin concentric bands of finely crystalline biotite alternating with plagioclase. Cores of the orbicules consist of fragments of biotite monzonite, plagioclase, or hornfels. Petrographic data on fragment reactions during orbicule formation, an aplite dikelet that cuts the orbicule zone, spacing of orbicule shells, and chemical analyses suggest that these orbicular rocks formed by reactions between xenoliths and magmatic fluids during crystallization of the granite.

Petrology and Mineralization of a Molybdenum-Bearing Alkalic Stock, Sierra Blanca, New Mexico

The Three Rivers stock, a molybdenum-related alkalic hypabyssal complex in south-central New Mexico, consists of three major intrusive phases: (1) an early and passively emplaced thick shell of coarse syenite porphyry, (2) nordmarkite intruded along the northeastern margin of the stock, and (3) late equigranular quartz syenite to alkali granite forcibly injected along a northeasterly trending arch in the syenite porphyry shell. These comagmatic phases developed as the result of extended fractional crystallization of hypersolvus alkalic (high albite) melts, in which disequilibrium precipitation of alkali feldspar played a key role. Rock and mineral textures and compositions vary markedly but systematically within and between phases, and record petrochemical trends culminating with emplacement of the residual alkali granite. Significant molybdenum, with related mineralization and hydrothermal alteration, occurs in a narrow belt along the northeastern margin of the stock. Several lines of field and laboratory evidence indicate that the spatially contiguous metal deposition and hydrothermal activity were generated by and occurred within the framework of Three Rivers magmatic activity and cooling. The hydrothermal mineral assemblage is interpreted as a relatively unfractionated and rapidly quenched derivation of the hydrous phase released by the equigranular quartz syenite, the last major intrusive.

Estimating seepage from a reservoir from change in hydraulic head

Abstract An investigation of seepage below floodwater-retarding structures conducted by the U.S.D.A. Science and Education Administration Federal Research resulted in a method for estimating seepage from a reservoir based upon change in head. A typical structure was selected for the study and physiographic and geologic features of the site were identified. Extensive hydrologic and hydrogeologic data were collected and analyzed for the site. A major inflow event was recorded in the reservoir in September 1965 and data from that event were used to develop an equation for estimating the amount of seepage lost from the structure in relation to the hydraulic head behind the structure. The method presented by the authors provides a physically based method for estimating the impact of a reservoir on the groundwater flow system assuming variable reservoir head conditions.

Geology and Mineralization of the Gilman-Leadville Area, Colorado

The central Colorado mineral belt is endowed with an impressive wealth of mineral deposits, including the world-class deposits at Leadville, Gilman, and Climax, that formed in a variety of geologic environments. The geology of the area spans more than 1.8 Ga, commencing with the Early Proterozoic accretion of volcanic arc and back-arc complexes to the southern margin of the Archean craton. These rocks were complexly deformed and intruded by large Early and Middle Proterozoic batholiths. During Paleozoic and Mesozoic time, the Proterozoic basement complex was buried beneath several kilometers of marine and continental sediments, and it was partially exhumed during Pennsylvanian orogenic uplift. Subduction-related calc-alkalic magmatism and uplift affected the region during the Late Cretaceous-early Tertiary Laramide orogeny. Oligocene and younger extension generated the north-trending Rio Grande rift zone, which was accompanied by bimodal magmatic activity. Most of the mineral deposits in the central Colorado mineral belt are associated with Oligocene calc-alkalic magmatism or to later bimodal activity. Deposits of demonstrably Laramide age are relatively small, and a few small carbonate-hosted deposits may have formed during the Mississippian. The mountains of central Colorado contain some of the largest concentrations of mineral deposits, including those at Climax, Leadville, and fiilman, in the Rocky Mountain region. These ores are part of an elongate zone of hydrothermal deposits, known as the Col or ado mineral belt, that extends northeast from the San Juan Mountains to the Front Range north of Denver (Fig. 1). Although most of the deposits are the products of Cenozoic tectonic and hydrothermal processes, the geology of the central Colorado mineral belt represents more than 1.8 billion years of tectonism, plutonism, and mineralized region, world-class cratonic sedimentation. As with any heavily deposits such as those described in this volume are the culminations of numerous unrelated geologic events that occurred over hundreds of mi11ions of years. The intent of this paper is to briefly summarize the geologic history of central Colorado and its relation to mineralization. In general, the region is underlain by a crystalline Proterozoic basement complex on which several kilometers of Phanerozoic sediments were deposited. Orogenic up 1ift occurred in the late Paleozoic and twice during the Cenozoic, and a major rifting event began in the middle Tertiary. Voluminous plutonic rocks were emplaced during several Late Cretaceous and Cenozoic magmatic events. Recurrent orogenic activity throughout the geologic history generated new structures and reactivited many preexisting faults.

Acceptance of the Society of Economic Geologists Ralph W. Marsden Award for 2005

President Dow, fellow members, and guests: I’m deeply honored to receive the Ralph Marsden award recognizing my contributions to the Society. To be placed in the same company with Ernie Ohle, Paul Sims, Clay Smith, Brian Skinner, and 12 other deserving Society members, including John Thoms, is indeed very special. I, also, had the privilege of meeting Ralph Marsden during my first SEG Council meeting in 1986. It has been customary for the Marsden awardees to review some of the events that led to their Society activities, and I will do so briefly. In the fall of 1985, I received a call from the publications chairperson, John Slack, asking me …