The crystal structure of a birefringent garnet (similar to Adr(53)Grs(47)) that occurs as a late-stage rim on andradite from Stanley Butte, Graham County, Arizona is analyzed and refined using single-crystal XRD. The structure has an orthorhombic I 2/a 1 2/d (unconventional setting for Fddd) space group symmetry, with unit-cell parameters of a = b = 11.966(3) angstrom, c = 11.964(3) angstrom, alpha = beta = 90?, gamma = 90.29(2), V = 1713.0(7) angstrom(3), Z = 8. The orthorhombic garnet displays very high birefringence (delta similar to 0.021) produced by the strong Fe-Al ordering in the octahedral sites, with Fe occupancies of 0.804 and 0.221 in Y-1 and Y-2 sites, respectively. Difraction peaks (such as 101 and 103) violating the Ia (3) over bard symmetry of cubic garnet are obvious even in powder XRD pattern. The homogenization temperatures of the fluid inclusions suggest that the low-crystallization temperature is responsible for the ordered orthorhombic structure. The strong ordering state of the structure and the sharp boundaries in the chemical zoning in the crystal (between similar to Adr(53)Grs(47) and similar to Adr(100)) indicate the orthorhombic intermediate grandite garnet is a thermodynamically stable phase at low temperature, separated by wide miscibility gaps from the pure end-members (grossular and andradite) with cubic structures. Most of the previously reported triclinic garnet structures are likely artifacts produced by pseudo-merohedral twinning of less-ordered orthorhombic structure, as indicated by the characteristic pairing pattern of different Y-sites with the same occupancies.

SignificanceThere is a common consensus that lode gold deposits mostly precipitated from metamorphic fluids via fluid boiling and/or fluid-rock interaction, but whether magmatic hydrothermal fluids and the mixing of such fluids with an external component have played a vital role in the formation of lode gold deposits remains elusive. We use garnet secondary ion mass spectrometry oxygen isotope analysis to demonstrate that the world-class Dongping lode gold deposit has been formed by multiple pulses of magmatic hydrothermal fluids and their mixing with large volumes of meteoric water. This study opens an opportunity to tightly constrain the origin of lode gold deposits worldwide and other hydrothermal systems that may have generated giant ore deposits in the Earth's crust.

The Reef Deposit is an anomalous Au-Cu occurrence in the Paleoproterozoic terranes of northern Wisconsin, better known as host to significant Cu-Zn volcanogenic massive sulfide (VMS) deposits. Previous work using lead isotopes and fluid inclusions has identified a protracted development of the mineralization from initial formation as the root zone veins of a VMS deposit during the Penokean orogeny (-1.9-1.8 Ga), with the most recent mineralization/remobilization activity associated with late Paleozoic Mississippi Valley-type (MVT) fluid flow (Haroldson et al., 2018a, 2018b). Here we use the oxygen isotope history of the Reef Deposit to verify and further examine the deposits protracted development. Laser fluorination oxygen isotope measurements of primary mineralized quartz veins range in delta O-18 from 6.8 to 10.0 parts per thousand (VSMOW), and a trend is observed of increasing delta O-18 values of parallel vein zones along a 400-m traverse from northwest to southeast, likely from a temperature gradient during initial formation. Temperature estimates for a VMS deposit setting (230 to 400 degrees C) are consistent with a hydrothermally shifted formation fluid, using seawater delta O-18 estimates during the Penokean Orogeny and evolving to higher delta O-18 by incorporation of magmatic fluids or interaction with local crust. In situ oxygen isotope measurement by Secondary Ion Mass Spectrometer (SIMS) of cross-cutting quartz and carbonate range in delta O-18 from 19.4 to 28.4 parts per thousand for quartz, 25.3 to 28.1 parts per thousand for dolomite, and 9.6 to 29.3 parts per thousand (VSMOW) for calcite. High 8180 values ( > 19 parts per thousand) are measured in a crustiform-textured quartz stockwork microveinlet, in dolomite observed in late carbonate microveinlets, and calcite in settings associated with the late quartz and dolomite and a separate calcite-only setting directly linked with gold mineralization/remobilization.

Low temperature H 2 O-NaCl inclusions associated with late MVT overprint Methane bearing high temperature inclusions associated with ~1470 Ma magmatism Amphibolite grade regional deformation identified by neonate inclusionsFigure 1.Simplified geologic map (modified from Cannon et al., 1997) of Pembine-Wausau subterrane and surrounding area.Triangle -the Reef Deposit, squares -volcanogenic massive sulfide deposits.Solid lines represent faults, squiggly lines represent shear zones.Black lithology areas represent Proterozoic quartzite, the dark grey Marshfield subterrane is Archean in age.

Au-Cu mineralized quartz veins of the Reef Deposit, Wisconsin, USA, originally formed prior to or early in the Paleoproterozoic Penokean orogeny in central North America as either the root zone of a gold-rich VMS deposit, or in an orogenic gold setting. Nearly 400 m.y. later, magmatic hydrothermal fluids associated with a continental scale anorogenic magmatic event remobilized mineralization within the veins. And still later, during the Paleozoic, remobilization and perhaps upgrading of Au and other metals occurred in response to circulation of fluids associated with Mississippi Valley-type deposits found in overlying supracrustal rocks. Fluid constraints on the formation of the Reef Deposit and later overprinting over a 1.5 b.y. timeframe are examined using the fluid inclusion assemblage (FIA) approach. Distinctive assemblages are examined on individual merit, each offering individual stories to aid in our understanding of the protracted development of the Reef Deposit. Presumably primary fluid inclusions in Au- and Cu-bearing quartz-sulfide veins that formed during the Paleoproterozoic event show H2O-CO2-CH4 +/- NaCl compositions based on microthermometry and laser Raman spectroscopy. Possibly primary H-2-NaCl and secondary H2O-CO2-CH4 -NaCl FIA formed prior to regional deformation and are recognized as discrete relict and neonate clusters. The intersection of isochores from the highest density neonate inclusions of either fluid composition indicates re-equilibration at lower amphibolite conditions during regional deformation. Quartz hosted secondary fluid inclusion assemblages characterized by H2O-CH4 and CO2-CH4 compositions formed at high temperatures and have not been re-equilibrated and are interpreted to post-date regional metamorphism. These FIA occur along healed fractures with chalcopyrite mineralization. These inclusions are likely associated with fluid circulation driven by emplacement of the adjacent Wolf River Batholith (ca. 1.45 Ga), that altered pyrite to pyrrhotite and remobilized or emplaced chalcopyrite at temperatures as high as 700 degrees C. This reduced fluid was in equilibrium with locally observed graphitic sediments. Secondary lower temperature (< 50-210 degrees C) calcite and quartz hosted H2O-NaCl inclusions show variable homogenization temperatures and salinities. H2O-NaCl inclusions in quartz homogenize at 78-210 degrees C, with lower salinities of 3.0-13.9 wt% NaCl equivalent, and are observed to crosscut CH4 bearing FIA. H2O-NaCl inclusion assemblages in calcite have maximum homogenization temperatures of 103 degrees C and salinities of similar to 24 wt % NaCl equiv. Lower temperature H2O-NaCl inclusions are most likely related to the Paleozoic-aged MVT fluid overprint that remobilized and possibly emplaced gold mineralization.

Sedimentological observations from the Paleoproterozoic Huronian Supergroup are suggested to mark the rise in atmospheric oxygen at that time, which is commonly known as the Great Oxidation Event (GOE) and typically coupled with a transition from mass-independent fractionation (MIF) to mass-dependent fractionation (MDF) of sulfur isotopes. An early in situ study of S three-isotopes across the Huronian Supergroup by Papineau et al. ( 2007 ) identified a weak MIF-MDF transition. However, the interpretation and stratigraphic placement of this transition is ambiguous. In this study, all four S isotopes were analyzed for the first time in two Huronian drill cores by secondary ion mass spectrometer (SIMS), and both Δ33S and Δ36S were calculated. Based on improved precision and detailed petrography, we reinterpret the dominance of pyrrhotite in the studied sections, which was previously proposed as "early authigenic" in origin, as resulting from regional metamorphism. Small but analytically resolvable nonzero values of Δ33S (from -0.07‰ to +0.38‰) and Δ36S (from -4.1‰ to +1.0‰) persist throughout the lower Huronian Supergroup. Neither pronounced MIF-S signals nor a MIF-MDF transition are seen in this study. Four scenarios are proposed for the genesis of small nonzero Δ33S and Δ36S values in the Huronian: homogenization by regional metamorphism, recycling from older pyrite, dilution by magmatic fluids, and the occurrence of MDF. We argue that the precise location of the MIF-MDF transition in the Huronian remains unsolved. This putative transition may have been erased by postdepositional processes in the lower Huronian Supergroup, or may be located in the upper Huronian Supergroup. Our study highlights the importance of integrated scanning electron microscopy and secondary ion mass spectrometry techniques in deep-time studies and suggests that different analytical methods (bulk vs. SIMS) and diagenetic history (primary vs. metamorphic) among different basins may have caused inconsistent interpretations of S isotope profiles of the GOE successions at a global scale. Key Words: Great Oxidation Event (GOE)-Secondary ion mass spectrometer (SIMS)-Paleoproterozoic-Sulfur isotopes-Mass independent fractionation (MIF). Astrobiology 18, 519-538.

Gold deposits are often observed to have complex histories with multiple mineralization and remobilization events. Lead (Pb) isotopes have been useful in identifying the source and timing of metals during the initial or "primary" mineralization in these deposits; however, the source and timing of metals associated with Pb that has more radiogenic isotope ratios as compared to the primary Pb are less definitive. Here, we used Pb isotope analyses, obtained by whole-rock, microdrilled, and in situ sampling techniques, coupled with detailed petrography to identify distinct fluid histories associated with gold mineralization in the Reef Deposit, which occurs in Penokean-age (1.8 Ga) rocks in north-central Wisconsin. Early Pb-rich minerals, encased in pyrite (as is some gold), have nonradiogenic Pb isotope ratios that match the Pb isotope composition of galena from volcanogenic massive sulfide deposits that occur locally north of the Reef Deposit. In contrast, late Pb-rich minerals, intergrown with gold, have highly radiogenic Pb isotope ratios that closely match those of galena from Paleozoic carbonate rocks in the region. A low-temperature brine fluid was found in fluid inclusion assemblages in late crosscutting carbonate veinlets. Based on these data, we infer that initial gold mineralization in the Reef Deposit was produced during the Penokean orogeny (ca. 1.8 Ga) and was subsequently overprinted by fluids similar to Mississippi Valley-type mineralization during the Permian (ca. 270 Ma). This temporal disparity between primary mineralization and remobilization, as identified by Pb isotopes, has important implications for our understanding of gold and other ore deposits. This study should prove useful to guide future Pb isotope investigations in various disciplines.

Dehydration of water from surface Mg2+ is most likely the rate-limiting step in the dolomite growth at low temperature. Here, we investigate the role of polysaccharide in this step using classical molecular dynamics (MD) calculations. Free energy (potential of mean force, PMF) calculations have been performed for water molecules leaving the first two hydration layers above the dolomite (104) surface under the following three conditions: without catalyst, with monosaccharide (mannose) and with oligosaccharide (three units of mannose). MD simulations reveal that there is no obvious effect of monosaccharide in lowering the dehydration barrier for surface Mg2+. However, we found that there are metastable configurations of oligosaccharide, which can decrease the dehydration barrier of surface Mg2+ by about 0.7-1.1 kcal/mol. In these configurations, the molecule lies relatively flat on the surface and forms a bridge shape. The hydrophobic space near the surface created by the non-polar -CH groups of the oligosaccharide in the bridge conformation is the reason for the observed reduction of dehydration barrier.

23 Nano-precipitates with tripled periodicity along the c-axis are observed in a Ca-rich dolomite 24 sample from Proterozoic carbonate rocks with “molar tooth” structure. This observation is 25 consistent with previous description of d reflections. High-angle annular dark-field STEM 26 imaging (or Z-contrast imaging) that avoids dynamic diffraction as seen in electron diffraction 27 and high-resolution TEM imaging modes, confirms that d reflections correspond to nanoscale 28 precipitates aligned parallel to (001) of the host dolomite. The lamellae precipitates have a cation 29 ordering sequence of Ca-Ca-Mg-Ca-Ca-Mg along the c direction resulting in a chemical 30 composition of Ca0.67Mg0.33CO3. This superstructure is attributed to the extra or d reflections, 31 thus is referred to as the d superstructure in this study. The structure can be simply described as 32 interstratified calcite/dolomite. The crystal structure of the d superstructure calculated from 33 density functional theory (DFT) has a space group of P31c and has a and c unit cell parameters 34 of 4.879 Å and 16.260 Å, respectively, values between those of dolomite and calcite. The 35 detailed structural characteristics and parameters obtained from ab initio calculations are also 36 reported in this paper. The method of combining Z-contrast imaging and ab initio calculation can 37 be used for solving structures of other nano-precipitates and nano-phases. 38

Nano-precipitates with tripled periodicity along the c-axis are observed in a Ca-rich dolomite sample from Proterozoic carbonate rocks with "molar tooth" structure. This observation is consistent with previous description of d reflections. High-angle annular dark-field STEM imaging (or Z-contrast imaging) that avoids dynamic diffraction as seen in electron diffraction and high-resolution TEM imaging modes, confirms that d reflections correspond to nanoscale precipitates aligned parallel to (001) of the host dolomite. The lamellae precipitates have a cation ordering sequence of Ca-Ca-Mg-Ca-Ca- Mg along the c direction resulting in a chemical composition of Ca0.67Mg0.33CO3. This superstructure is attributed to the extra or d reflections, thus is referred to as the d superstructure in this study. The structure can be simply described as interstratified calcite/dolomite. The crystal structure of the d superstructure calculated from density functional theory (DFT) has a space group of P31c and has a and c unit-cell parameters of 4.879 and 16.260 Å, respectively, values between those of dolomite and calcite. The detailed structural characteristics and parameters obtained from ab initio calculations are also reported in this paper. The method of combining Z-contrast imaging and ab initio calculations can be used for solving structures of other nano-precipitates and nano-phases.

The Quadrilátero Ferrífero, Brazil, is presently the largest accumulation of single itabirite-hosted iron ore bodies worldwide. Detailed petrography of selected hypogene high-grade iron ore bodies at, e.g. the Águas Claras, Conceição, Pau Branco and Pico deposits revealed different iron oxide generations, from oldest to youngest: magnetite → martite (hematite pseudomorph after magnetite) → granoblastic (recrystallised) → microplaty (fine-grained, <100 μm) → specular (coarse-grained, >100 μm) hematite. Laser-fluorination oxygen isotope analyses of selected iron ore species showed that the δ18O composition of ore-hosted martite ranges between −4.4 and 0.9 ‰ and is up to 11 ‰ depleted in 18O relative to hematite of the host itabirite. During the modification of iron ore and the formation of new iron oxide generations (e.g. microplaty and specular hematite), an increase of up to 8 ‰ in δ18O values is recorded. Calculated δ18O values of hydrothermal fluids in equilibrium with the iron oxide species indicate: (1) the involvement of isotopically light fluids (e.g. meteoric water or brines) during the upgrade from itabirite-hosted hematite to high-grade iron ore-hosted martite and (2) a minor positive shift in δ18Ofluid values from martite to specular hematite as result of modified meteoric water or brines with slightly elevated δ18O values and/or the infiltration of small volumes of isotopically heavy (metamorphic and/or magmatic) fluids into the iron ore system. The circulation of large fluid volumes that cause the systematic decrease of 18O/16O ratios from itabirite to high-grade iron ore requires the presence of, e.g. extensive faults and/or large-scale folds.

Dolomite crystals in partially dolomitized limestone from the Platteville Formation are both compositionally and microstructurally heterogeneous. A single dolomite crystal usually contains three phases: the host Ca-rich dolomite [Ca1.14Mg0.86(CO3)2], an Fe-bearing dolomite [Ca1.06Mg0.80Fe0.14(CO3)2], and calcite inclusions. These three phases show similar orientations. The Ca-rich dolomite exhibits modulated microstructures with wavelength ranging from 7 to 30 nm. The modulated microstructures are not evident in Fe-bearing dolomite. Modulations in the Ca-rich dolomite have three predominant orientation ranges in the studied sample: from (205) to (104), from (001) to (-101), and ~(110), which are consistent with previous studies. Bright-field (BF) and high-angle annular dark-field (HAADF) images confirm that these modulations are due to chemical variation rather than strain or diffraction contrast. The Ca-rich lamellae are Mg-rich calcite with compositions ranging from Ca0.85Mg0.15CO3 to Ca0.70Mg0.30CO3. The observed results indicate that these Ca-rich exsolution lamellae formed during diagenesis. In this study, three kinds of "c"-reflections, which are weak spots in the halfway position between the principal reflections along the (104)*, (112)*, and (110)* directions, have been found in the diffraction patterns of some Ca-rich dolomite. Mg-Ca ordering in x-y planes was not observed directly in Z-contrast images. FFT patterns from the Z-contrast images do not show "c"-reflections. STEM images confirm that the "c"-reflections could result from multiple diffraction between the host dolomite and twinned Mg-calcite nano-lamellae under TEM imaging and diffraction modes.

The Pojeonri Cu quartz veins occur in the north-western portion of the Hwanggangri Metallogenic Province and consist of two parallel massive quartz veins that fill fractures oriented NW and NE along fault zones in Paleozoic metasedimentary and sedimentary rocks of the Ogcheon and Taebaeg belts. Based on the mineralogy and paragenesis of the veins, only one mineralization episode has been recognized. The ore minerals are mainly chalcopyrite, pyrrhotite, and pyrite with minor arsenopyrite, sphalerite, galena and oxides of those base metal minerals.Systematic studies of fluid inclusions in quartz veins found three types: CO2-rich, H2O-CO2, and aqueous inclusions. Hydrothermal fluids related to the mineralization are composed of H2O-CO2 +/- CH4-NaCl fluids (400-2700 bar, average 800 +/- 550 bar) with Thtotal values of 263 degrees to 443 degrees C and salinities less than about 18 wt.% NaCl equiv. Ore minerals from the quartz veins were mainly deposited as a result of a decrease in sulfur fugacity caused by the separation of carbonic vapor from the parent H2O-CO2 +/- CH4-NaCl fluids. The calculated and measured oxygen and hydrogen isotope compositions (delta O-18(H2O), delta D) of the ore-forming fluids (9.5 to 12.6%. SMOW, -87 to -80%. SMOW, respectively) indicate that the hydrothermal fluids of the Pojeonri quartz veins were probably derived from mostly magmatic water or water that originated as a result of high temperature exchange between the igneous plutons and adjacent high-delta O-18 country rocks. Isotopic and fluid chemistry indicates that the Pojeonri veins and other polymetallic veins in this metallogenic district are genetically related to intrusions emplaced during the Cretaceous orogeny. Most features of the Pojeonri Cu quartz veins and the many polymetallic deposits in the Hwanggangri metallogenic district resemble features of districts hosting porphyry W-Mo deposits Of Climax-type. (C) 2011 Elsevier B.V. All rights reserved.

Approximately 3.4 billion tons (Gt) of iron ores containing > 50 percent Fe were: produced from U.S. mines in the Lake Superior region front 1848 until they were exhausted 20 to 30 years ago. The Vermilion Range in Minnesota produced nearly 100 million tons (Mt) of this ore from Archean greenstone belt-hosted iron formation. The remaining production has come from Proterozoic strata including 2.3 Gt from tire Mesabi and 100 Mt from the Cuyuna Ranges in Minnesota while Michigan and Wisconsin contributed 230 Mt from the Marc quette Range, 290 Mt from the Menominee Range, and 325 Mt from the Gogebic Range. The protore of these direct-shipping ores are carbonate- or oxide-facies banded iron formations that contained 25 to 35 percent Fe prior to undergoing leaching (desilicification), oxidation, and volume loss. The conventional model ascribing these changes to supergene processes has recently been challenged by research showing that hypogene fluids, channeled by faults into structurally favorable horizons and settings, have played a dominant role in producing some of the high-grade (> 60% Fe) ores that are presently providing much of the world's iron ore. Descriptions of the North American iron ores, generally starting with the U.S. Geological Survey monographs published at the beginning of the 2011, century provide many tantalizing clues, suggesting that hypogene fluids have indeed played all important role in the evolution of some of these districts. Application of modern geophysical techniques and structural and geochemical analyses stay well guide the discovery of new high-grade ores either below or adjacent to the historic mining area. The time seems to be ripe for exploration to return to the area that can claim to have begun geologists' understanding of this most important ore deposit type.

The Insizwa sill is an similar to 1 km-thick subhorizontal layered mafic intrusion and part of the Karoo Large Igneous Province in South Africa. This well-exposed intrusion consists of several superimposed petrologically and geochemically distinct units. Magnetic methods were used to study the intrusion in order to constrain the physical processes active in these types of bodies during crystallization. Rock magnetism studies indicate that within different petrologic units bulk susceptibility is controlled by primary magnetite (with minor pyrrhotite) and/or paramagnetic minerals (olivine, pyroxene). New magnetic data based on 659 specimens obtained from 3 vertical borehole cores, each spaced 5 km apart, confirm the prominent vertical zonation in low field magnetic susceptibility (K-1f), degree of anisotropy (P-j) and orientation of the anisotropy of magnetic susceptibility (AMS) axes. The magnetic susceptibility correlates very well with petrographic units and the lateral continuity of magnetic units between boreholes is very consistent. Petrologic units with high, but variable, K-1f, also show moderate anisotropy and dominantly vertical foliations. We interpret these patterns to result from inverse fabrics from single domain magnetite. The degree of anisotropy is low in petrologic units with low K-1f, which also show shallowly dipping magnetic foliations. We interpret that the magnetic properties of these units are dominated by the paramagnetic minerals. These low K-1f petrologic units also show no systematic increase in K-1f, suggesting that only minor differentiation is occurring in these units. The dataset derived from 2 surface sampling traverses are consistent with borehole core AMS data, showing a pattern of dominantly steep magnetic foliation and variably plunging magnetic lineation with a NW-SE trend. (c) 2007 Elsevier B.V. All rights reserved.

The Sonju Lake intrusion (SLI), in northeastern Minnesota, is a layered mafic complex of Keweenawan age (1096.1 +/- 0.8 Ma) related to the Midcontinent rift. The cumulate paragenesis of the intrusion is recognized as broadly similar to the Skaergaard intrusion, a classic example of closed-system differentiation of a tholeiitic mafic magma. The SLI represents nearly closed-system differentiation through bottom-up fractional crystallization. Geochemical studies have identified the presence of a stratabound, 50-100 m thick zone anomalously enriched in An + PGE. Similar to the PGE reefs of the Skaergaard intrusion, this PGE-enriched zone is hosted within oxide gabbro cumulates, about two-third of the way up from the base of the intrusion. We present a petrofabric study using the anisotropy of magnetic susceptibility (AMS) to investigate the emplacement and flow patterns within the Sonju Lake intrusion. Petrographic and electron microprobe studies, combined with AMS and hysteresis measurements indicate the primary source of the magnetic signal is pseudo-single dornain (PSD) magnetite or titanomagnetite. Low field AMS was measured at 32 sites within the Sonju Lake intrusion, which provided information about primary igneous fabrics. The magnetic fabrics in the layered series of the Sonju Lake intrusion are consistent with sub-horizontal to inclined emplacement of the intrusion and show evidence that the cumulate layers were deposited in a dynamic environment. Well-aligned magnetic lineations, consistently plunging shallowly toward the southwest, indicate the source of the magma is a vertical sill-like feeder, presumably located beneath the Finland granite. The Finland granite acted as a density trap for the Sonju Lake magmas, forcing lateral flow of magma to the northeast. The strongly oblate magnetic shape fabrics indicate the shallowly dipping planar fabrics were enhanced by compaction of the crystal mush. (c) 2007 Elsevier B.V. All rights reserved.