Exhalative origins of iron formations

Hafnium isotopes in zircon: A tracer of fluid-rock interaction during magnetite–apatite (“Kiruna-type”) mineralization

We present results for direct dating of Fe oxide-(Cu-Au) (iron oxide, copper, gold—IOCG) mineralization by U/Pb zircon geochronology. Constraining the timing of mineralization has important geodynamic implications for the processes involved in the genesis of these types of deposits and the tectonic evolution of rocks associated with these deposits. Hydrothermal zircon crystals were separated from four IOCG-type ore deposits associated with the Lyon Mountain Granite in the Adirondack Mountains, United States. Zircon grains from these low-Ti magnetite deposits reveal at least two periods of mineralization; one episode ca. 1039 Ma and a second between ca. 1015 and 1000 Ma. Previous age determinations of these deposits were constrained by the age of the altered host granitoids, and the assumption that Fe mineralization was directly related to pluton emplacement. Zircon crystals extracted from the ore yield ages that show that ore mineralization was episodic and younger than pluton emplacement.

http://www.geo.umass.edu/petrology/Valley%20et%20al-Geol-2009-DatingMin.pdf

Direct dating of Fe oxide-(Cu-Au) mineralization by U/Pb zircon geochronology

The integrated approach of field work, microscopy, whole-rock and mineral-scale geochemistry, and in situ U-Th-Pb zircon geochronology has proven to be useful for recognizing the type, timing, and sequence of complex Na and K fluid alteration related to the development of Kiruna-type magnetite-apatite deposits and the tectonic evolution of the granites that host these deposits. The Lyon Mountain Granite in the northeastern Adirondack Mountains of New York State has undergone multiple episodes of hydrothermal fluid alteration and Fe mineralization. Perthite granite containing ubiquitous 1060–1050 Ma zircon grains was overprinted by potassic alteration, which in turn was overprinted by pervasive Na alteration. During the Na alteration, preexisting orebodies, consisting of magnetite, clinopyroxene, and apatite, were overprinted and remobilized to form new deposits that contain magnetite, apatite, quartz, and zircon. The U-Th-Pb zircon geochronology data suggest that the Lyon Mountain Granite intruded the Adirondack Highlands during the Ottawan orogeny between ca. 1060 and 1050 Ma. However, subsequent alteration obscured much of the prehistory of the LMG. Amphibolite layers within the Lyon Mountain Granite and granitic dikes and pegmatites that crosscut the foliation in the Lyon Mountain Granite have been dated between 1045 and 1016 Ma. These ages coincide with previous published zircon age data from second-generation orebodies associated with Na alteration.

New insights on the evolution of the Lyon Mountain Granite and associated Kiruna-type magnetite-apatite deposits, Adirondack Mountains, New York State

A complete set of new optical and x-ray data is given for eleven analyzed alkali amphiboles [Na2(Mg, Fe″)3(Al, Fe‴)2Si8O22(OH)2]. Nine new wet chemical analyses are reported. Using additional selected data from the literature, variation in refractive indices, extinction angles (γ-α), optic angles, density, lattice constants and cell volume are expressed graphically as a function of composition in the glaucophane-riebeckite and magnesiorie-beckite-ferroglaucophane series. Four orientations (G, C, O, and R) of the optical indicatrix within the structure are described and shown to be characteristic of the chemical species glaucophane (G), crossite (C), magnesioriebeckite (O), riebeckite (O), and riebeckite-arfvedsonite (R and O).

/pdf/optical-properties-and-cell-parameters-in-the-glaucophane-22qys22nfe.pdf

Optical properties and cell parameters in the glaucophane-riebeckite series

This investigation is a geological and geochemical study of several magnetite deposits in the Ausable magnetite district of New York. Geologic maps were prepared of part of the district and of these deposits, and laboratory investigations were carried out on the host rock gneiss and its constituent minerals.The Lyon Mountain granite gneiss appears to have been the source rock from which the Palmer Hill and Jackson Hill ore bodies were derived. Modal analyses indicate that accessory magnetite in the gneiss decreases near the ore bodies. Here, also, clinopyroxene has been recrystallized to garnet plus silica and iron. During regional metamorphism, accessory magnetite, and iron from fractured accessory magnetite and from recrystallized clinopyroxene, migrated as a dispersed phase of ions, molecules, or atoms along grain boundaries to shear and crush zones and were concentrated into magnetite bodies. The ore zones were regions of low pressure and high oxygen activity.In skarn areas in the northern part of the map area, small magnetite bodies apparently formed when clinopyroxene and hornblende gneisses rich in iron were recrystallized to yield pyroxene skarn relatively poor in iron plus magnetite.

Magnetite ore formed during regional metamorphism, Ausable magnetite district, New York

The Precambrian iron formation near Atlantic City, Wyoming, lies within a large southwestward-plunging syncline of metasedimentary and metavolcanic rocks. The ore is at least 2,400 million years old, and may have been deposited more than 2,680 million years ago, in which case it is roughly equivalent in age to the older iron formation deposits of the Canadian Shield. The geology of the Atlantic City deposit is similar to that of Precambrian iron formations of the Shield. Magnetite is the chief ore mineral at Atlantic City, and the gangue consists of quartz, actinolite and actinolitic hornblende.Chemical analyses of drill core material average in weight percent: total iron, 33.45; Fe 2 O 3 , 30.75; FeO, 15.49; Al 2 O 3 , 3.85; CaO, 1.31; MgO, 2.80; MnO, 0.09; Na 2 O, 1.22; K 2 O, 0.36; and SiO 2 , 44.13. The values are similar to published averages for Precambrian iron formations from the Canadian Shield.Comparisons of Ca/Mg, Ca/Na, Ca/K, Mg/Na, Mg/K and Na/K ratios from the Atlantic City deposit with various present-day geologic environments and with average crustal percentages has provided chemical information which may help in establishing a mode of origin for the Atlantic City iron formation. Analyses of fumarolic gases, of certain acidic volcanic waters, and of Dead Sea water samples are shown to be statistically similar to analyses of the Atlantic City iron formation. Close similarities in terms of these ratios with average crustal percentages also add significantly to an understanding of the possible genesis of the deposit.It is suggested that the iron formation at Atlantic City, Wyoming, may have been derived through subaerial weathering of moderately low-lying crustal rocks in an atmosphere low in oxygen, and that the leachate was then carried to and concentrated in a closed basin, where it may have received additional contributions from volcanic emanations. The question of whether the basin of deposition was marine or continental remains unanswered.

Arthur F. Hagner

Papers

Magnetite ore formed during regional metamorphism, Ausable magnetite district, New York

Geochemistry and Origin of the Precambrian Iron Formation Near Atlantic City, Fremont County, Wyoming

Optical and chemical variations in minerals from a single rock specimen

Refractive index field diagrams and mineral relationships for coexisting ferromagnesian silicates