Marco T. Einaudi

Geological characteristics and tectonic setting of proterozoic iron oxide (CuUAuREE) deposits

Recent work on the Olympic Dam CuUAuAg deposit, South Australia, the Wernecke Mountain breccias, Yukon, the Kiruna iron ore district, Sweden, and the southeast Missouri iron ore district, and a review of literature on other iron-rich mineral deposits in Proterozoic rocks, suggest that these occurrences constitute a distinct class of ore deposits characterized by low-titanium, iron-rich rocks formed in extensional tectonic environments. Other examples of this class may include the mineral deposits of the Great Bear magmatic zone of northwest Canada, the Bayan Obo district of China, and perhaps the Redbank breccia pipes of the Northern Territory, Australia. We designate this class of deposits as Proterozoic iron oxide (CuUAuREE) deposits, and propose that the ore deposits generally referred to as ‘Kiruna-type’ should be considered a subset of this larger class. Salient characteristics of this class of deposits are as follows: 1. (1) Age. The majority of known deposits, particularly the larger examples, are found within Early to mid-Proterozoic host rocks (1.1–1.8 Ga). 2. (2) Tectonic setting. The deposits are located in areas that were cratonic or continental margin environments during the late Lower to Middle Proterozoic, and in many cases there is a definite spatial and temporal association with extensional tectonics. Most of the districts occur along major structural zones, and many of the deposits are elongated parallel to regional or local structural trends. The host rocks may be igneous or sedimentary; many of the deposits occur within silicic to intermediate igneous rocks of anorogenic type. However, mineralization in many deposits is not easily related to igneous activity at the structural level of mineralization. 3. (3) Mineralogy. The ores are generally dominated by iron oxides, either magnetite or hematite. Magnetite is found at deeper levels than hematite. CO3, Ba, P, or F minerals are common and often abundant. The deposits contain anomalous to potentially economic concentrations of REEs, either in apatite, or in distinct REE mineral phases. 4. (4) Alteration. The host rocks are generally intensely altered. The exact alteration mineralogy depends on host lithology and depth of formation, but there is a general trend from sodic alteration at deep levels, to potassic alteration at intermediate to shallow levels, to sericitic alteration and silicification at very shallow levels. In addition, the host rocks are locally intensely Fe-metasomatized. In spite of these similarities, many variations occur between and within individual districts, particularly in deposit morphology. Individual deposits occur as strongly discordant veins and breccias to massive concordant bodies. Both the morphology and the extent of alteration and mineralization appear to be largely controlled by permeability along faults, shear zones and intrusive contacts, or by permeable horizons such as poorly welded tuffs. Thus, the variations of morphology are explicable in terms of local wall-rock and structural controls. Similarly, local variations in mineralogy and geochemistry may be largely attributable to wall-rock composition, and to P, T, and fo2 controls related to depth of formation. We believe that these deposits formed primarily in shallow crustal environments (<4–6 km), and that they are expressions of deeper-seated, volatile-rich igneous-hydrothermal systems, tapped by deep crustal structures. The global occurrence of this type of deposit at approximately 1.8 to 1.4 Ga suggests a relation to a global rifting events effecting continental crust, possibly the break-up of a Proterozoic supercontinent. Secular cooling of the Earth insured that subsequent rifting and mineralizing events might generate deposits similar in kind but smaller in magnitude.

Copper deposition by fluid cooling in intrusion-centered systems: New insights from the Bingham porphyry ore deposit, Utah

Quartz veins in porphyry copper deposits record the physiochemical evolution of fluids in subvolcanic magmatic-hydrothermal systems. We have combined cathodoluminescence (CL) petrography with fluid-inclusion microthermometry to unravel the growth history of individual quartz veins and to link this history to copper ore formation at Bingham, Utah. Early barren quartz veins with K-feldspar + biotite (potassic) alteration selvages occur throughout the 2 km vertical exposure of quartz monzonite porphyry stock. At depths of 500 m to at least 1350 m below the orebody, fluid inclusions in these barren veins trapped a single-phase CO 2 -bearing fluid containing ∼2-12 wt% NaCl e q u i v . Within and to depths of 500 m below the orebody, early quartz veins contain abundant hypersaline liquid (38-50 wt% NaCl e q u i v ) and vapor-rich inclusions trapped together at temperatures of 560-350 °C and pressures of 550-140 bar, consistent with fluctuations between lithostatic and hydrostatic pressure at paleodepths of 1.4 to 2.1 km. CL petrography shows that bornite and chalcopyrite were deposited together with a later generation of quartz and K-feldspar in microscopic fractures and dissolution vugs in early barren quartz veins and wall rock. This late quartz contains hypersaline liquid (36-46 wt% NaCl e q u i v ) and vapor-rich inclusions trapped at 380-330 °C and at 160-120 bar hydrostatic pressure. We conclude that a single-phase magmatic-hydrothermal fluid underwent phase separation to hypersaline liquid (or brine) and vapor ∼500 m below the base of the orebody at a paleodepth of ∼2.5 km. Brine and vapor continued to ascend and formed multiple generations of barren quartz veins with potassic selvages. Thermal decline to temperatures below 400 °C was the main driving force for copper-iron sulfide deposition, given the lack of evidence of mixing of brines with low-salinity waters, the lack of correspondence of the ore zone with the initiation of phase separation, and no change in wall-rock alteration style.

Geochemical and mineralogical controls on trace element release from the Penn Mine base-metal slag dump, California

Base-metal slag deposits at the Penn Mine in Calaveras County, California, are a source of environmental contamination through leaching of potentially toxic elements. Historical Cu smelting at Penn Mine (1865–1919) generated approximately 200,000 m3 of slag. The slag deposits, which are flooded annually by a reservoir used for drinking water and irrigation, also may be in contact with acidic ground waters (pH<4) from the adjacent mine area. Slags vary from grey to black, are glassy to crystalline, and range in size from coarse sand to large (0.6×0.7×1.5 m), tub-shaped casts. Metals are hosted by a variety of minerals and two glass phases. On the basis of mineralogy, slags are characterized by 4 main types: fayalite-rich, glassy, willemite-rich, and sulfide-rich. The ranges in metal and metalloid concentrations of 17 slag samples are: As, 0.0004–0.92; Ba, 0.13–2.9; Cd, 0.0014–1.4; Cu, 0.18–6.4; Pb, 0.02–11; and Zn, 3.2–28 wt.%. Leachates from Toxicity Characteristic Leaching Procedure tests (acetic acid buffered at pH 4.93) on two willemite-rich slags contained Cd and Pb concentrations (up to 2.5 and 30 mg/l, respectively) in excess of US Environmental Protection Agency (USEPA) regulatory limits. Analyses of filtered (0.45 μm) water, collected within the flooded slag dump during reservoir drawdown, reveal concentrations of Cd (1.7 μg/l), Cu (35 μg/l), and Zn (250 μg/l) that exceed USEPA chronic toxicity guidelines for the protection of aquatic life. Data from field and laboratory studies were used to develop geochemical models with the program EQ3/6 that simulate irreversible mass-transfer between slag deposits and reservoir waters. These models include kinetic rate laws for abiotic sulfide oxidation and surface-controlled dissolution of silicates, oxides, and glass. Calculations demonstrate that the main processes controlling dissolved metal concentrations are (1) dissolution of fayalite, willemite, and glass; (2) sulfide oxidation; and (3) secondary phase precipitation. Close agreement between model results and measured concentrations of Al, Ba, Cu, Fe, SiO2, and SO4 in the slag dump pore waters suggests that the dissolved concentrations of these elements are controlled by solubility equilibrium with secondary phases. Differences between predicted and measured Cd and Pb concentrations imply that field weathering rates of glass and sulfides are approximately two orders of magnitude lower than laboratory rates. Overprediction of Pb release may also reflect other attenuation processes in the natural system, such as sorption or coprecipitation.

Extreme 34S depletions in ZnS at the Mike gold deposit, Carlin Trend, Nevada: Evidence for bacteriogenic supergene sphalerite

We identified submicrometer-sized framboidal sphalerite (ZnS) below the base of supergene oxidation in a Carlin-type gold deposit of Eocene age in Nevada, United States, where the framboidal sphalerite forms a blanket-like body containing >400,000 metric tons of zinc. Framboidal sphalerite <0.1 mum in diameter, formed in the early Miocene, ranges from <0.1 to 0.35 mol% FeS; the delta(34)S values range from -25% to w-70%, the lowest values measured in a marine or terrestrial environment. These S isotope data demonstrate the involvement of sulfate-reducing bacteria and provide the first documentation that sphalerite can form significant supergene sulfide-enrichment blankets.

Part I. Contrasting Styles of Intrusion-Associated Hydrothermal Systems

Intrusion-related hydrothermal systems represent a large variety of geologic environments that in some cases form large metallic mineral deposits. The deposits examined in this trip represent the spectrum from systems dominated by magmatic fluid (Birch Creek, California and Yerington, Nevada) to those systems in which intrusions serve as heat engines to drive convectively circulating brines derived from sedimentary rocks (Hum-boldt, Nevada). In these examples, nonmagmatic fluids are largely excluded from more deeply emplaced intrusions in a compressive environment, and the hydrothermal composition and ores (e.g., granite W-F, Cu porphyry and skarn) are dictated by the composition of the magma and its mechanism of crystallization and aqueous fluid generation. Magmatic fluids are less important in the shallow crustal ore environment, but apparently contribute to acidic alteration zones located vertically above source intrusions. Using Humboldt as an example, we propose that the Fe oxide Cu-Au ores in the shallow environment require an abundant source of sedimentary brines (typical of evaporitic environments), high fracture permeability (promoted by an exten-sional setting) to allow aqueous fluid flow and dike emplacement, and shallowly emplaced intrusions to serve as heat sources. IGNEOUS-RELATED hydrothermal systems constitute the most varied type of geologic environment, ranging in tectonic setting from spreading centers to collisional belts, in depth from the surface to the deep crust, and in sources of materials from purely magmatic to largely external. They comprise perhaps the single most important ore-forming environment, yet most igneous systems lack economically significant mineralization. This variety is attributable to igneous factors such as volatile content and its evolution from the intrusion, and to external factors that include depth of emplacement, host rocks, tectonic environment, and structural setting, which control permeability and access of external fluids to the crystallized intrusion and its contact aureole. This field trip examines three large but markedly different intrusion-centered hydrothermal systems in the western Great Basin of California and Nevada (Fig. 1, Table 1). Each example represents a major group of these systems worldwide. The field emphasis will be on examining mass transfer features—such as mechanisms for igneous emplacement, degassing of magmatic-aqueous fluids, and fracturing and ductile deformation—that allow variation from near-lithostatic to hydrostatic conditions, incursion of nonmagmatic fluids into the high-temperature environment, and hydrothermal alteration, vein deposition, and wall-rock replacement via aqueous fluids. The broader questions of metallogenic provinces and processes will be raised as a context for the specific sites examined. The overall emphasis of this trip will be on documenting and understanding the dynamics of igneous-related hydrothermal systems.

Acceptance of the R. A. F. Penrose Gold Medal for 2008

President Thompson, members of the Society, ladies, and gentlemen: Thank you, Eric and John, for your very flattering introduction. The Penrose Medal is a great honor, never expected, but which I accept with gratitude and with a very strong sense of humility—with gratitude to teachers, colleagues, and students who have helped me along the way and to the Society for having selected me; humility as I think of the extraordinary contributions of past Penrose medalists. Following an undergraduate degree at Cornell in 1961, I arrived at Harvard in 1963 and my timing could not have been better. Ulrich Petersen had just arrived, full of energy and ideas from Cerro de Pasco, Bob Garrels hadn’t left yet, and Jim Thompson was in his prime. I remember well my first interview with Jim Thompson, who listened quietly to my request to enroll in his course in phase equilibria, and who then encouraged me to come back and see him the following year after I had completed a list of courses in math and chemistry. I could not have received better advice. Bob Garrels, with his unique, laid-back teaching style and superior squash skills, inspired all of us. Ulrich Petersen guided me through the construction of phase diagrams for …

The SEG Silver Medal for 1999 Citation of Murray W. Hitzman

Mr. President, members of the Society, and guests: It gives me great pleasure to present Murray Hitzman for the 1999 SEG Silver Medal. Murray receives the Silver Medal for his outstanding contributions to economic geology on a very broad front: in ore finding, in mine development, in research, in policy planning, and in teaching. Murray entered the Ph.D. program in economic geology at Stanford in the fall of 1978. He arrived with a double major in Anthropology and Geology from Dartmouth, an M.S. degree in geology from the University of Washington, several years of experience in mineral exploration, and a thesis project all lined up and funded by Bear Creek. The thesis was to be the Ruby Creek deposit, in northern Alaska. Instinctively, Murray developed a regional and stratigraphic approach to understanding Ruby Creek. In a paper published in Economic Geology in 1986, Murray pointed out the similarities between Ruby Creek and the Cu deposits near McArthur River and in silica-dolomite at Mt. Isa, and proceeded to place Ruby Creek in the context of linkages between deposit types at the scale of sedimentary basins. This was a preview of what was to come in Ireland 10 years later. On finishing his Ph.D. at Stanford in 1983, Murray joined Chevron Resources Company and convinced the company to explore in Europe. Murray arrived in …