Jeffrey L. Mauk

GENESIS OF DIOCTAHEDRAL PHYLLOSILICATES DURING HYDROTHERMAL ALTERATION OF VOLCANIC ROCKS: I. THE GOLDEN CROSS EPITHERMAL ORE DEPOSIT, NEW ZEALAND

To characterize the evolution of dioctahedral interstratified clay minerals in the Golden Cross epithermal deposit, New Zealand, hydrothermally altered volcanic rocks containing the sequence smectite through illite-smectite (I-S) to muscovite were examined by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission and analytical electron microscopies (TEM/AEM).XRD analyses of 30 oriented clay samples show a broad deposit-wide trend of increasing illite content in I-S with increasing depth and proximity to the central vein system. Six representative samples were selected for SEM/TEM study on the basis of petrographic observations and XRD estimates of I-S interstratification. Ca and Na are the dominant interlayer cations in smectite, but as the proportion of illite layers in I-S increases, so does the K content and (IVAl + VIAl)/Si ratio. Layers and packets tend to flatten and form larger arrays, reducing the amount of pore space. Smectite coexists with (R = 1) I-S, rather than being (R = 0) I-S where R is the Reichweite parameter. The highest alteration rank samples contain discrete packets of mica to ∼300 Å thick, but a limited chemical and structural gap exists between illite, which is intermediate in composition between common illite and muscovite, and illite-rich I-S. Selected-area electron diffraction (SAED) patterns of mica show that the 1M polytype dominates, rather than the common 2M1 polytype.Petrographic, SEM, and TEM data imply that all phyllosilicates formed via neoformation directly from fluids. Relatively mature I-S and micas form simultaneously, without progressing through the series of transformations that are commonly assumed to characterize diagenetic sequences during burial metamorphism in mud-dominated basins. Although the overall distribution of clay minerals is consistent with temperature as a controlling variable, local heterogeneities in the distribution of clay minerals were controlled by water/rock ratio, which varied widely owing to different rock types and fracture control.

Relationship between organic matter and copper mineralization in the Proterozoic Nonesuch Formation, northern Michigan

Abstract Comparison of organic and inorganic geochemical data of stratigraphically equivalent, mineralized and unmineralized samples from strata hosting the White Pine stratiform copper deposit in northern Michigan provides insight into the ore forming process. Whole rock and organic extract analyses were performed on mine and core samples from five gray beds (“stripey” shale, “UZV” siltstone, “thinly” shale, “DGM” siltstone, “domino” shale) of the Proterozoic Nonesuch Formation and from the “lower” sandstone of the underlying Copper Harbor Conglomerate. Bitumen is degraded in both mineralized and unmineralized samples of the “lower” Nonesuch Formation. Presumably, this degradation is not a direct result of the mineralizing process, but represents either regional-scale water-washing or biodegradation. In contrast, the pristane/n-C 17 and phytane/n-C 18 ratios are lower in mineralized samples than in unmineralized samples, suggesting thermal maturation of the “lower” Nonesuch Formation during mineralization. Copper mineralization most likely proceeded via three mechanisms: (1) deposition of native copper; (2) conversion of precursor pyrite to chalcocite; and (3) deposition of chalcocite with copper and sulfur derived entirely from mineralizing fluids. A positive correlation between organic carbon and copper contents in mineralized strata implies that organic matter served as a reducing agent during copper mineralization. However, redox reactions must have conserved total organic carbon since the total organic carbon contents of mineralized and unmineralized samples are similar.

40Ar/39Ar ages of adularia from the golden cross, neavesville, and komata epithermal deposits, hauraki goldfield, New Zealand

Abstract New 40Ar/39Ar ages of adularia from three epithermal vein deposits in the Hauraki Goldfield constrain when these deposits formed. Adularia from veins at Neavesville has 40Ar/39Ar plateau and isochron ages of 6.89 ± 0.02 and 6.86 ± 0.02 Ma, respectively, similar to the 40Ar/39Ar plateau and isochron ages of adularia from veins at Golden Cross of 6.96 ± 0.04 and 6.93 ± 0.03 Ma, respectively. In contrast, Komata mineralisation formed at 5.99 ± 0.02 to 6.07 ± 0.03 Ma, based on the 40Ar/39Ar isochron and plateau ages of the adularia in these veins, similar to the previously reported 6.0 Ma age of mineralisation at Waihi/Favona. These preliminary data suggest that mineralisation in the Hauraki Goldfields occurred episodically.

Preservation of Clay Minerals in the Precambrian (1.1 Ga) Nonesuch Formation in the Vicinity of the White Pine Copper Mine, Michigan1

The Middle Proterozoic (1.1 Ga) Nonesuch Formation, host of the stratiform copper deposit at White Pine, Michigan, consists of 200 m of principally dark grey clastic sediments which contain detritus obtained dominantly from underlying mafic to intermediate volcanic rocks. Clay minerals from samples collected from the mine area and drill holes up to 100 km away have been studied using SEM, EMPA, TEM and AEM. Two morphologies of phyllosilicates, both including white mica and chlorite, occur in the ‘lower’ Nonesuch Formation: (1) detrital-shaped and (2) matrix. Detrital-shaped phyllosilicate grains are up to 450 microns long with long axes parallel to bedding. Matrix phyllosilicates occur as packets typically <200 Å thick and as pore-filling cement.TEM images of detrital-shaped chlorite generally display 14-Å periodicity, although 24-Å corrensite- like units occur locally. Most detrital-shaped chlorite from the mine area samples have a relatively restricted range of Fe/(Mg+Fe) ratios from 0.52 to 0.58, but the Fe/(Fe+Mg) ratios of detrital-shaped chlorite outside the mine area range from 0.27 to 0.64. Ti02 crystals occur within and surrounding the detrital-shaped chlorite. Matrix chlorite has Fe/(Fe+Mg) ratios of 0.47 to 0.63, indicating that it is relatively homogeneous and enriched in Fe compared to detrital-shaped chlorite.Detrital-shaped white mica occurs as a 2Mi polytype and generally has a phengitic composition. Matrix illite-rich I/S occurs as a lMd polytype, is K and A1 deficient relative to end-member muscovite and contains significant Fe and Mg.The data are consistent with homogenization of detrital-shaped chlorite in the White Pine mine area by hydrothermal fluids during copper mineralization. The Ti02 crystals and corrensite-like units in detrital- shaped chlorite imply that it is at least in part derived from alteration of biotite. The presence of immature lMd illite-rich I/S and a one layer chlorite polytype with stacking disorder suggests that the matrix clays are in their original, post-smectite state of formation as consistent with an authigenic origin during early burial diagenesis; i.e., they have not undergone subsequent transformation even though sedimentation and ore deposition occurred prior to 1 Ga.

Geophysical exploration of the Puhipuhi epithermal area, Northland, New Zealand

The Puhipuhi epithermal area, which occurs in a region of graywacke basement partially covered by basalt and lake-bed deposits, is characterized by both large-scale and small-scale geophysical anomalies. Known occurrences of locally intense alteration or silicification are typically associated with strong gravity, resistivity or IP anomalies. Gravity data define a complex negative residual anomaly (up to −50 gu) which has been used to identify and delineate a large area (about 20 km2) of low-density, presumably clay-altered, graywacke basement rocks. This zone, modeled as extending to a few kilometers depth, encompasses, but is more extensive than, the known areas of alteration and has a close spatial association with the basalt cover rocks. Short-wavelength gravity minima and maxima, which indicate that the most intense alteration of the basement rocks occurs below the basalt, correlate, in part, with the inferred location of hydrothermal upflow zones. The control on the location of these zones and their relationship to the location of the basalts is not well known; however, if the basalts acted as a cap rock to the geothermal system, then these areas merit further exploration. High (≥100 ohm-m) and low (≤10 ohm-m) resistivity and high (≥30 mS) IP anomalies occur in association with known silicification, clay alteration and sulfide mineralisation, respectively. In addition, magnetic data help constrain the relative timing of hydrothermal alteration and basaltic volcanism and indicate that mineralisation was broadly synchronous with volcanism.

Wüstite in a hydrothermal silver-lead-zinc vein, Lucky Friday mine, Coeur d’Alene mining district, U.S.A.

Abstract The Coeur d’Alene mining district of northern Idaho is one of the major Ag-Pb-Zn producers in the world; it contains Ag-Pb-Zn replacement veins in clastic burial metamorphic rocks of the Middle Proterozoic Belt Supergroup. Electron microprobe analyses and Raman spectroscopy of samples from the Gold Hunter vein of the Lucky Friday mine revealed the presence of the rare iron oxide wüstite (Fe1-xO) coexisting with stable magnetite. An assemblage of galena, pyrite, and siderite is associated with the two iron oxides although these minerals do not share phase boundaries with wüstite and their coexistence is, therefore, thermodynamically precluded. Wüstite is stable under high-temperature conditions (above 570°C at 1 bar). Increasing pressure can lower this temperature significantly. However, the veins in the Coeur d’Alene district formed well outside the wüstite stability field and represent a low-temperature (250-350 °C), low-pressure (1-3 kbars), and low-fO₂ environment. To form wüstite at temperatures around 250-350°C a pressure increase of at least 70 kbars would be required. This corresponds to burial of around 250 km, which is geologically unrealistic. Similarly, to form wüstite, the temperature must have exceeded 570°C, which exceeds the maximum temperatures of vein formation by at least 200 °C. A temperature increase of that scale is not reflected in the mineral assemblage of the veins or the host rocks, which both contain siderite. We conclude that wüstite formed as a metastable phase at 250-350 °C and pressures below 100 bars because of the low-fO₂ conditions in the Coeur d’Alene district

Variations in sandstone diagenesis with depth, time, and space, onshore Taranaki wells, New Zealand

Abstract Study of diagenesis in cores from Kapuni Field, Waihapa‐2, Kaimiro‐2, and New Plymouth‐2 shows many expected features—quartz and feldspar dissolution and precipitation, clay mineral precipitation, and carbonate dissolution and precipitation. Local variations may overprint broader trends; early quartz overgrowth in parts of the basin is not recognisable in Kapuni wells, and feldspars may be etched differentially. In the Kapuni Group, early siderite precipitated from pore waters similar to those in which the host sediment was deposited, whereas late calcite was probably deposited from fresh water. In the Moki Formation, some calcite may be similarly late, but variations in pH modified deposition. Cathodoluminescence of new growth of calcite and quartz shows considerable variety, and some luminescent and non‐luminescent quartz have grown contemporaneously. Most stable isotope data show regular trends. Petroleum has migrated within the Taranaki Basin at least twice: once during early diagenesis of th...

Telescoped porphyry-style and epithermal veins and alteration at the central Maratoto valley prospect, Hauraki Goldfield, New Zealand

Abstract At the central Maratoto valley prospect, southern Coromandel Peninsula, New Zealand, andesite flows and dacite breccias host rare porphyry‐style quartz veins that are telescoped by widespread epithermal veins and alteration. Early porphyry‐style quartz veins, which lack selvages of porphyry‐style alteration, host hypersaline fluid inclusions that contain several translucent daughter crystals, including halite and sylvite. Overprinting epithermal veins and alteration are divided into two stages. Main‐stage epithermal alteration and veins are characterised by the successive deposition of pyrite, quartz, and ankerite‐dolomite veinlets coupled with intense alteration of the wall rock to quartz, illite, interlayer illite‐smectite (≤10% smectite), chlorite, pyrite, ankerite, and dolomite. Late‐stage epithermal veins and alteration are characterised by the formation of calcite and siderite veinlets, coupled with overprinting of the wall rocks by both these minerals. Multiphase fluid inclusions in a porphyry‐style quartz vein formed at temperatures >400°C and trapped hypersaline magmatic fluid. Lower temperature secondary liquid‐rich inclusions in the porphyry‐style quartz vein homogenise between 283 and 329°C and trapped a dilute fluid with <1.8wt% NaCl equivalent. Inclusions in later epithermal quartz and calcite veins homogenise between 240 and 280°C (av. 260°C) and trapped a dilute fluid with apparent salinities of <2.9 wt% NaCl equivalent. Based on homogenisation and salinity data, secondary inclusions in porphyry‐style quartz veins may have formed 700–950 m deeper than telescoping epithermal veins. Main‐stage epithermal ankerite and dolomite have δ18O(VSMOW) values of 13.5–18.1‰, whereas late‐stage epithermal calcite has δ18O(VSMOW) values of 3.1–5.1‰. Calculated isotopic compositions for the fluid in equilibrium with ankerite‐dolomite and calcite at 260°C, averages 6 and ‐3‰, respectively. The enriched value for main‐stage ankerite‐dolomite suggests formation from waters that underwent significant water‐rock exchange, whereas isotopically lighter water that formed late‐stage calcite underwent little water‐rock interaction. We propose a three‐stage model to explain telescoped veins and alteration styles at the central Maratoto valley prospect area. Porphyry‐style quartz veins were the first to form from hot hypersaline multi‐cation magmatic fluids. These are telescoped by later widespread epithermal veins and alteration following descent of the paleowater table possibly due to rapid erosion or sector collapse of a volcanic edifice. Main‐stage epithermal alteration and deposition produced quartz, chlorite, illite, interlayered illite‐smectite, pyrite, and isotopically heavy ankerite‐dolomite from deeply circulating upwelling alkali chloride waters. Late‐stage collapse of the hydrothermal system resulted in the formation of overprinting calcite and siderite from isotopically lighter descending marginal steam‐heated CO2‐rich waters.