Fe-rich Chlorite Research Articles

Heat-pulse calorimetry measurements on natural chlorite-group minerals

Low- and high-temperature heat capacities of five natural chlorite-group samples were measured using the heat-capacity option of the Physical Properties Measurement System (Quantum Design), which is based on the principles of heat-pulse calorimetry, and by differential scanning calorimetry. Comprehensive chemical analyses were performed on these samples by electron microprobe analysis, by inductively coupled plasma mass spectrometry, and by Karl-Fischer titration (for H2O). The natural chlorites span a range in X-Fe from 0.052 to 0.885 with increasing Al-content due to the Tschermak substitution with increasing X-Fe. The measured heat capacities were extrapolated to the end-member compositions of chamosite and clinochlore. Integration of heat-capacity data yields the calorimetric standard entropies of chamosite (Fe5Al)[Si3AlO10](OH)(8) and clinochlore (Mg5Al)[Si3AlO10](OH)(8), with values of 572.0 +/- 0.2 and 425.6 +/- 0.4 J/(mol-K), respectively. The C-p-polynomial for end-member chamosite is C-p = 1151.7 - 8.4564 x 10(3)-T-0.5 - 13.206 x 10(6)-T-2 + 15.233 x 10(8).T-3 [J/(mol-K)], valid in the temperature range of 298.15-900 K, and that for end-member clinochlore is C-p = 1160.5 - 9.9819 x 10(3)-T-0.5 - 5.9534 x 10(6).T-2 + 3.8677 x 10(8)-T-3 [J/(mol-K)], valid in the temperature range of 298.15-1000 K. The Fe-rich chlorites exhibit an asymmetric distribution of the excess heat capacity in a plot of C-p(ex) vs. T, with a maximum at about 52 K. By analogy to annite, we interpret this peak to represent the magnetic ordering temperature. Based on our standard entropy value for chamosite, the enthalpy of formation of berthierine (Fe2.5Al0.5)[Si1.5Al0.5O5Y](OH)(4) was estimated as -3570.30 kJ/mol using a reported onset temperature of 70 degrees C at 16 MPa for the berthierine-chamosite polymorphic transition.

Hydrothermal alteration at the Magdala gold deposit, Stawell, western Victoria*

The Magdala deposit in the Stawell goldfield in western Victoria was formed during the 440 Ma gold event of the Lachlan Orogeny and is hosted by Cambrian quartz-rich turbiditic sedimentary rocks (Albion Formation) that onlap a thick pile of tholeiitic basaltic lavas (Magdala Basalt). Detailed petrographic and geochemical analyses suggest that the host-rock (Stawell Facies) was originally a turbiditic sedimentary rock that was hydrothermally altered in response to seawater interaction with the hot basaltic pile. Subsequent regional greenschist metamorphism and ductile deformation that lasted at least 10 million years culminated in the formation of the Magdala mineralised system and produced a complex pattern of hydrothermal alteration. Evolution of this alteration occurred over six stages: Stage 1, Fe-enrichment of sedimentary rock adjacent to the basalt pre-D1; Stage 2, chlorite (metamorphism), orbicular carbonate and pyrite, syn-D2; Stage 3, muscovite, siderite, ankerite and pyrrhotite, syn-D3 – D4a – b; Stage 4, stilpnomelane, siderite, pyrrhotite, arsenopyrite and pyrite, syn-D4c; Stage 5, silica, minnesotaite and magnetite, post-D4c – pre-D5; and Stage 6, Fe-rich chlorite, muscovite, calcite, arsenopyrite and pyrite, syn-D5. Comparisons with other turbidite-hosted gold deposits in Victoria (e.g. Bendigo and Ballarat) highlight four major differences: (i) presence of a tholeiitic basaltic pile; (ii) ductile deformation (D1 – 4) over at least 60 million years prior to gold mineralisation; (iii) highly evolved hydrothermal alteration; and (iv) source of sulfur. Of these differences the key element is the basaltic pile and its associated heat, which may have promoted the growth of micro-organisms in, and alteration of, the onlapping sedimentary rocks, thereby creating a basis from which an unusual turbidite-hosted orogenic-gold deposit was formed.

The Brunswick No. 6 Massive Sulfide Deposit, Bathurst Mining Camp, Northern New Brunswick, Canada: A Synopsis of the Geology and Hydrothermal Alteration System

The 12.1 Mt Brunswick No. 6 Zn-Pb-Cu-Ag massive sulfide deposit (New Brunswick, Canada) lies between two subaqueous felsic volcanic formations near the base of the Middle Ordovician Tetagouche Group. The footwall comprises rhyodacitic pyroclastic rocks (Nepisiguit Falls Formation), whereas the hanging wall comprises a sequence of rhyolitic flows, breccias, and hyaloclastites (Flat Landing Brook Formation), with an age difference of less than 3 m.y. The Brunswick No. 6 deposit is a proximal autochthonous deposit with a well-zoned massive sulfide body and a basal Cu zone that, at depth, develops into a stockwork stringer sulfide (feeder) system. The massive sulfides are capped sharply by a layered magnetite-chert unit that extends regionally beyond the deposit. The deposit has a keel shape (sheath) formed by the F1F2 interference pattern. The heterogeneous ductile deformation and upper greenschist-grade regional metamorphism has transposed many of the epigenetic stockwork structures and alteration within the host sequence. Hydrothermal alteration is much more extensive, both vertically and laterally, in the footwall (a few 100 m) than in the hanging wall (<100 m). Anomalies farther up in the hanging wall sequence seem to be associated with independent alteration systems related to the rhyolite domes. The least-altered footwall units exhibit keratophyric alteration (albite or adularia) and weak Mg enrichment (chlorite). More intense micaceous (sericite and Mg-rich chlorite) alteration occurs around the footwall sequence. The transposed stockwork stringer sulfides are typically composed of Fe-rich chlorite (± sericite, ± silica) with pyrite, pyrrhotite, chalcopyrite, arsenopyrite, and sphalerite. The (Fe2O3T+MgO)/(Na2O+K2O) and base metal alteration indices are the best practical lithogeochemical vectoring tools at this deposit.

ZONED TOURMALINE ASSOCIATED WITH CASSITERITE: IMPLICATIONS FOR FLUID EVOLUTION AND TIN MINERALIZATION IN THE SAN RAFAEL SN CU DEPOSIT, SOUTHEASTERN PERU

The composition of tourmaline in the San Rafael Sn–Cu lode, in southeastern Peru, provides an important record of the early evolution of the hydrothermal system that produced the world’s richest tin deposit. Many forms, colors and compositions of tourmaline, ranging from dravite to schorl, are present in the deposit, but the late tourmaline that accompanied deposition of early cassiterite has an unusual dark green color, and exhibits a strong trend of enrichment in iron. Appearance of this tourmaline in the paragenesis coincided with a marked change in the vein style, reflecting an opening of the vein system, and a dramatic change in the mineralogy of vein and alteration assemblages, evident from the precipitation of other iron-rich minerals (Fe-rich chlorite and cassiterite). This abrupt change in the plumbing of the hydrothermal system was associated with the introduction of dilute, relatively oxidizing, externally derived waters of meteoric origin that mixed with hot magmatic brines carrying high concentrations of dissolved tin and iron. The resulting sudden cooling, dilution, and oxidation of the ore fluids created the conditions required for massive precipitation of cassiterite and formation of a very large, high-grade ore deposit.

Clay mineralogy of the Permo-Triassic strata of the British Isles: onshore and offshore

AbstractThe regional distribution, mineralogy, petrology and chemistry of the detrital and authigenic clay minerals associated with the Permo-Triassic strata (excluding the Rotliegend: see Ziegler, 2006; this volume), of the onshore and offshore regions of the British Isles are reviewed within their stratigraphical framework. The origin of these clay minerals is discussed in relation to current hypotheses on the developments of the Mg-rich clay mineral assemblages associated with the evaporitic red-bed Germanic facies of Europe and North Africa.Composite clay mineral successions are described for seven regions of the British Isles — the Western Approaches Trough; SW England; South Midlands; Central Midlands; the Cheshire Basin; NE Yorkshire; and the Central North Sea. The detrital clay mineral assemblages of the Early Permian strata are variable, consisting of mica, smectite, smectite-mica, kaolin and chlorite, whereas those of the Late Permian and the Trias are dominated by mica, usually in association with minor Fe-rich chlorite. The detrital mica consists of a mixture of penecontemporaneous ferric mica, probably of pedogenic origin, and recycled Pre-Permian mica. In the youngest Triassic strata (Rhaetian), the detrital clay assemblages may contain appreciable amounts of poorly defined collapsible minerals (irregular mixed-layer smectite-mica-vermiculite) and kaolin, giving them a Jurassic aspect. There are two types of authigenic clay mineral assemblages. Kaolin may occur as a late-stage diagenetic mineral where the original Permo-Triassic porewaters of the sediment have been replaced by meteoritic waters. A suite of early-stage diagenetic clay minerals, many of them Mg-rich, are linked to the evaporitic red-bed facies — these include sepiolite, palygorskite, smectite, irregular mixed- layer smectite-mica and smectite-chlorite, corrensite, chlorite and glauconite (sensu lato). The sandstones and mudstones of the onshore regions of the British Isles display little or no difference in their detrital and authigenic clay mineral assemblages. In contrast, the sandstones of the offshore regions (North Sea) show major differences with the presence of extensive chloritic cements containing Mg-rich and Al-rich chlorite, irregular mixed-layer serpentine-chlorite, and mica.

Jurassic and Cretaceous clays of the northern and central North Sea hydrocarbon reservoirs reviewed

AbstractThe principal clays of the northern and central North Sea are illite (sometimes with interlayered smectite) and kaolin. Chlorite is only locally important. Although it has been proposed that kaolin within North Sea sandstones is detrital in origin, the majority of workers have concluded that it is authigenic, largely the product of feldspar alteration. Kaolin is found within a wide range of sedimentary settings (and within shales) apparently defying the notion that kaolin is an indicator of meteoric water deposition. Within sandstones, the earliest authigenic kaolin has a vermiform morphology, the distribution of which is controlled by the availability of detrital mica to act as a nucleus, and the composition of the post-depositional porewaters. This vermiform kaolin formed in meteoric water, the presence of which is easily accounted for below sub-aerial exposure surfaces in non-marine formations, and below unconformities over marine units. In fully marine sands, and even marine shale units, kaolin still occurs. It has therefore been suggested that even these locations have been flushed with meteoric water.Early vermiform kaolin recrystallizes to a more blocky morphology as burial proceeds, at least in the Brent Group. Blocky kaolin has been reported as growing before, synchronously with, and after the formation of quartz overgrowths, though oxygen isotope studies support low-temperature growth, pre-quartz. Blocky kaolin may form during meteoric flushing associated with lower Cretaceous uplift and erosion, though it is found in fault blocks that are thought to have remained below sea level. Here, the kaolin may form in stagnant meteoric water, relics of the post-depositional porewater. It has also been proposed that the blocky kaolin grew in ascending basinal waters charged with carboxylic acids and CO2, though this hypothesis is not supported by stable oxygen isotope data. Some of the blocky kaolin is dickite, the stable polymorph above ∼100°C.Fibrous illite occurs almost ubiquitously within the clastic sediments of the North Sea. An early pore-lining phase has been interpreted as both infiltrated clastic clay, and as an early diagenetic phase. Early clays may have been quite smectite-rich illites, or even discrete smectites. Later, fibrous illite is undoubtedly neoformed, and can degrade reservoir quality significantly. Both within sandstones and shales, there is an apparent increase in the K content deeper than 4 km of burial, which could be due to dilution of the early smectite-rich phase by new growth illite, or to the progressive illitization of existing I-S. Much of the ‘illite’ that has been dated by the K-Ar method may therefore actually be I-S.The factors that control the formation of fibrous illite are only poorly known, though temperature must play a role. Illite growth has been proposed for almost the entire range of diagenetic temperatures (e.g. 15–20°C, Brent Group; 35–40°C, Oxfordian Sand, Inner Moray Firth; 50–90°C, Brae formation; 100–110°C, Brent Group; 130–140°C, Haltenbanken). It seems unlikely that there is a threshold temperature below which illite growth is impossible (or too slow to be significant), though this is a recurring hypothesis in the literature. Instead, illite growth seems to be an event, commonly triggered by oil emplacement or another change in the physiochemical conditions within the sandstone, such as an episode of overpressure release. Hence fibrous illite can grow at any temperature encountered during diagenesis.Although there is an extensive dataset of K-Ar ages of authigenic illites from the Jurassic of the North Sea, there is no consensus as to whether the data are meaningful, or whether the purified illite samples prepared for analysis are so contaminated with detrital phases as to render the age data meaningless. At present it is unclear about how to resolve this problem, though there is some indication that chemical micro-analysis could help. It is a common belief that illite ages record the timing of oil charge, and so can be used to calibrate basin models.Grain-coating Fe-rich chlorite cements can preserve exceptional porosity during burial. They are found in marginal marine sandstones, formed during diagenesis from precursor Fe-rich clays such as berthierine or verdine.

Infrared spectral reflectance characterization of the hydrothermal alteration at the Tuwu Cu–Au deposit, Xinjiang, China

Short-wave infrared (SWIR) reflectance spectroscopy was used to characterize hydrothermal minerals and map alteration zones in the Tuwu Cu–Au deposit, Xinjiang, China. The Palaeozoic hydrothermal system at Tuwu is structurally controlled, developed in andesitic volcanic rocks and minor porphyries. Hydrothermal alteration is characterized by horizontally zoned development of quartz, sericite, chlorite, epidote, montmorillonite and kaolin about individual porphyry dykes and breccia zones, as is shown by changes outward from a core of quartz veining and silicification, through an inner zone of sericite + chlorite to a marginal zone of chlorite + epidote. The alteration system comprises several such zoning patterns. Silicification and sericitization are spatially associated with Cu–Au mineralization. Zoning is also shown by compositional variations such that Fe-rich chlorite and Al-rich sericite occur preferentially toward the core and the most intensely altered parts, whereas Mg-rich chlorite and relatively Al-poor sericite are present on the margin and the relatively weakly altered parts of the hydrothermal alteration system. The compositions of chlorite and sericite, therefore, can be potentially used as vectors to Cu–Au mineralization. Montmorillonite and kaolinite, of probable weathering origin, are located near the surface, forming an argillic blanket overlying Cu–Au mineralization. Sporadic montmorillonite is also present at depth in the hydrothermal alteration system, formed by descending groundwater. Presence of a well-developed kaolinite-bearing zone on the surface is an indication of possible underlying Cu–Au mineralization in this region. Epidote occurs widely in regional volcanic rocks, as well as in variably altered rocks on the margin of the hydrothermal mineralization system at Tuwu. The widespread occurrence of epidote in volcanic country rocks probably reflects a regional hydrothermal alteration event prior to the localized, porphyry intrusion-related hydrothermal process that led to the Cu–Au mineralization at Tuwu.

Short Wavelength Infrared Spectral Characteristics of the HW Horizon: Implications for Exploration in the Myra Falls Volcanic-Hosted Massive Sulfide Camp, Vancouver Island, British Columbia, Canada

Short wavelength infrared (SWIR) spectrometry has been used to identify previously unmapped hydrothermal alteration zones around volcanic-hosted massive sulfide (VHMS) orebodies at Myra Falls, Vancouver Island, British Columbia. Hydrothermal alteration assemblages are uniformly dominated by fine-grained white mica, with poor development of mineralogical zonation. SWIR spectrometry is an ideal exploration tool for characterizing this fine-grained hydrothermal alteration. At Myra Falls, SWIR spectrometry has identified subtle shifts in the wavelengths of the AlOH absorption feature of white mica, corresponding to compositional changes in altered rhyolite distal and proximal to ore. AlOH absorption occurs at shorter wavelengths (<2,198 nm) and corresponds to lower Fe, Fe + Mg, and Si/Al and higher Na/(Na + K) in strongly altered samples proximal to ore (slightly sodic muscovites). AlOH absorption occurs at longer wavelengths (>2,206 nm) and corresponds to higher Fe, Fe + Mg, and Si/Al and lower Na/(Na + K) in samples distal to ore (nonsodic slightly phengitic muscovites). White mica in siltstone within a meter of VHMS ore has higher Zn, V, Fe, and Mg contents than white mica distal to these altered samples. Chlorite compositions, identified by SWIR, also show systematic changes with intensity of alteration and distance from ore. The average wavelength of the FeOH absorption feature for chlorite in rhyolitic samples proximal to ore is 2,241 nm (intermediate Mg chlorite), whereas wavelengths in background samples average 2,247 nm (intermediate Fe chlorite). Similar changes are observed in footwall and hanging-wall andesites, with samples near the Battle mine containing muscovite to phengitic muscovite (average wavelength of the AlOH absorption feature of 2,200 nm) and Mg-rich chlorite (average wavelength of the FeOH absorption feature of 2,245 nm) to regional andesite samples with phengitic muscovite (average wavelengths of the AlOH absorption feature of 2,209 nm) and Fe-rich chlorite (average wavelength of the FeOH absorption feature of 2,249 nm). In weakly altered rocks white mica compositions also vary with host lithology. The AlOH absorption feature occurs at longer wavelengths in white mica in dacite and andesite compared to adjacent rhyolitic rocks, suggesting that higher Fe and Mg in the host lithology affects the composition of white mica. Two zones of intense hydrothermal alteration above the Battle and HW orebodies have distinctive SWIR spectral characteristics, with the AlOH and FeOH features occurring at shorter wavelengths (<2,197 and <2,240 nm, respectively). Small anomalous zones of alteration were also identified in the Thelwood Valley area, where minor mineralized zones are present. As broad zones of fine-grained white mica (sericite) alteration are ubiquitous throughout the Myra Falls property, alteration proximal to ore cannot be identified simply by visual logging of drill core. Alteration zonation may be determined by subtle shifts in white mica spectral characteristics. This study indicates that SWIR analysis may be an effective field-based exploration tool for quantifying the intensity of alteration associated with VHMS orebodies, and that trends in mineral compositions, even in very fine grained rocks, can be used as mine-scale vectors to ore.

The endogranitic tin zone, Mount Pleasant, New Brunswick, Canada and its metallogenesis

The Mount Pleasant Sn–W–Mo deposits occur within the margin of an eroded caldera underlain by a Devono–Mississippian intrusive-subvolcanic-eruptive complex of granitic, porphyritic and felsic pyroclastic rocks. The separate North and Fire Tower mineralised zones are centred in volcanic necks defined by hydrothermal breccias within this complex. The oldest of three successive granites in both Zones hosts W–Mo mineralisation, whereas the two younger granites in the North Zone contain various Sn deposits. One of these, the endogranitic tin zone, is an irregular but generally tabular body hosted within a distinctive, highly evolved phase of the third, and youngest granite.The endogranitic tin zone was formed by successive stages of sericitisation, chloritisation/topazification-silicification, brecciation, dense chloritisation-topazification and late replacement. It contains quartz, chlorite, topaz, fluorite, cassiterite, arsenopyrite, base metal sulphides, pyrite, stannite, wolframite, native bismuth, molybdenite, kaolinite and carbonates. The ore minerals occur about equally in veinlets and disseminations. Cassiterite was deposited in all stages, but more abundantly during brecciation and particularly during dense chloritisation-topazification where it accompanies a distinctive, avocado-green, Fe-rich chlorite. The host, endogranitic tin zone granite, has undergone significant depletions and enrichments in major and minor elements.Fluid inclusions indicate that the chloritised/topazified-silicified rock formed from early, low salinity dominantly meteoric fluids at ∼200–225°C. Later, saline, magmatic-hydrothermal fluids, probably differentiated in the upper part of the endogranitic tin zone granite, mineralised the brecciated and densely chloritised-topazified rocks at similar temperatures. Latest low salinity meteoric fluids formed the late replacement bodies at 175–185°C. Sn was deposited under acidic conditions at oxygen fugacity of 10−39–10−34 atm from dominant SnCl20 and SnF+ complexes due to fluid oxidation and neutralisation, and cooling.Endogranitic tin zone cassiterite has a trace element signature that is intermediate between that of cassiterite from deep-seated greisens and pegmatites, and shallower cassiterite–sulphide and Bolivian porphyry–tin ores. The late replacement cassiterite, however, is high in Nb, Ta, Ti and Fe, and thus has a pegmatitic signature. This, together with their size and distribution within the endogranitic tin zone and elsewhere in the complex, suggests that the late replacement bodies are aborted pegmatites formed when the magmatic-hydrothermal fluids became largely diluted by the late meteoric fluids. Wood tin and colloform cassiterite contain abundant Fe, resembling wood tin in the high-level topaz rhyolites and Russian subvolcanic tin deposits. Sphalerite contains abundant In, as do other high-level, stanniferous polymetallic ores. The endogranitic tin zone at Mount Pleasant appears to be unique, differing from both greisen– and porphyry–tin deposits and no close analogues are known elsewhere.

A nanopetrographic and textural study of grain-coating chlorites in sandstone reservoirs

AbstractScanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) investigations of chlorite grain coatings from four different sandstone reservoirs indicate a progressive change in both texture and arrangement of chlorite particles from the contact with the detrital substratum to the centre of the pore. Such spatial distribution results from growth by geometrical selection. Geometrical selection of chlorite crystals proceeded during a single event of continuous growth which began before the consolidation of the sandy sediments, lasted through part of the subsequent mechanical compaction and ceased before the occurrence of quartz cement. Nanopetrographic investigations near the detrital quartz-chlorite coating interface demonstrate that inhibition of the quartz cement is due to the limitation of the epitaxial growth of quartz to the interparticular space at the base of the chlorite coating and not an absence of nucleation. It is suggested that these results can be applied to most of the sandstones which contain Fe-rich chlorite grain coatings.

Genesis of the Giant, Bonanza San Rafael Lode Tin Deposit, Peru: Origin and Significance of Pervasive Alteration

The San Rafael Sn-Cu deposit, located at lat 14°13'58 S, long 70°19'18 W, on the upper slopes of the Cordillera de Carabaya, northern Puno Department, southeastern Peru, yielded 38,182 t of fine tin in 2001 and, with reserves of 14.460 Mt at 5.14 percent Sn, the main San Rafael Lode constitutes, both at present and historically, the largest-known bonanza-grade cassiterite repository. Sn and subordinate Cu mineralization were confined to laterally and vertically extensive brittle shear zones generated by regional tectonism that transect a small 24.65 ± 0.20 Ma, epizonal stock of strongly peraluminous, Lachlan S-type cordierite-biotite monzogranite and granodiorite. Early, barren, stage I quartz-tourmaline veins and breccias were emplaced at 24.10 ± 0.15 Ma by high-temperature (<580°C), boiling, saline fluids that plausibly directly exsolved from the granitic melt, whereas both cassiterite (stage II) and, at higher elevations, chalcopyrite (stage III) ores were precipitated from largely nonboiling, cooler (Th = 215°–420°C), and less saline (0–20 wt % NaCl equiv) fluids at 21.9 to 22.7 ± circa 0.5 Ma. Only narrow (<2 m) envelopes of chlorite and weaker sericite and silica are associated with stages II and III veins, but more than 80 percent of the upper 750 m of the San Rafael stock experienced quasipervasive hydrothermal alteration. Early K metasomatism is recorded by widespread replacement of magmatic plagioclase and perthitic alkali feldspar by Na-free orthoclase, while broadly coeval alteration to albite more erratically affected groundmass plagioclase and the rims of alkali feldspar phenocrysts. Despite the intensity of the alkali metasomatism, only minor redistribution of dispersed, magmatic Sn occurred, largely during alteration to orthoclase. Other petrographically similar granitic bodies in the region experienced more intense Na metasomatism, and they host argentian base metal rather than Sn-dominated veins. Superimposed hydrolytic alteration, most intense within 20 to 30 m of the San Rafael Lodes, converted biotite to Fe-rich chlorite and converted alkali feldspar and plagioclase to fine-grained muscovite, but there is no significant dispersed cassiterite or chalcopyrite. Secondary fluid inclusions trapped in quartz phenocrysts of the granitic rocks range in salinity from 0 to 65 wt percent NaCl equiv and in Th from 200° to 530°C; fluid boiling is evident in the higher-temperature populations. First-melting temperatures and decrepitate analyses reveal, in addition to NaCl, widely variable concentrations of KCl, CaCl2 (attaining 40 wt % of the solute) and, apparently most abundant in proximity to the lodes, FeCl2. Entrapment pressures are estimated to have fluctuated in the range 150 to 615 bars for much of the hydrothermal history, but fluid overpressuring to at least 2 kbar occurred, probably cyclically, during the evolution of the hydrothermal system. The salinity vs. homogenization temperature range for the phenocryst-hosted inclusions closely matches that of primary inclusions in quartz and cassiterite in the lodes and, remarkably, small clusters of the former record the entire thermal and compositional trajectory of the mineralizing fluids. Early magmatic brines are inferred to have permeated much of the stock whereas, circa 2 m.y. later, Sn- and Cu-rich, lower-temperature, lower-pH fluids were largely channeled along the evolving shear zones where they mixed with cool, nonsaline, tectonically driven ground water. In conjunction with fluid neutralization through hydrolytic alteration, this channeling resulted in catastrophic precipitation of botryoidal and coarsely crystalline cassiterite and, subsequently, chalcopyrite. The hiatus between initial retrograde boiling and ore deposition, a feature documented in several world-class, high-grade lithophile-element deposits, may record protracted storage of metal-rich brines at depth, perhaps in association with small volumes of highly fractionated melt, such as is represented by small bodies of tourmaline leucogranite exposed on the upper margin of the San Rafael stock. However, the parallel evolution in temperature and salinity exhibited by the early nonmineralizing and later fertile hydrothermal fluids may also indicate that Sn and Cu were largely introduced into the subjacent magma chamber at a late stage.

Equation of state measurements of chlorite, pyrophyllite, and talc

The compressibilities of five sheet silicates-three chlorite compositions, pyrophyllite, and talc-have been measured in situ using energy-dispersive powder diffraction in a multi-anvil apparatus at the synchrotron radiation source, Daresbury Laboratory, U.K. Some high-temperature data were also collected. The three chlorite samples were a natural Mg-rich chlorite [Mg 4 . 4 9 Fe 0 . 2 8 Cr 0 . 1 2 Al 2 . 1 0 Si 2 . 9 5 O 1 0 (OH) 8 ], a natural Fe-rich chlorite [Mg 0 . 2 8 Fe 4 . 1 1 Al 2 . 7 1 Si 2 . 7 7 O 1 0 (OH) 8 ], and a synthetic clinochlore [Mg 5 Al 2 Si 3 O 1 0 (OH) 8 ]. The compressibility data for all three samples, measured up to between 3 and 6 GPa, can be fit to a single equation of state with a bulk modulus of 86.9(16) GPa (using K' = 4). The thermal expansivity measured for the natural Mg-chlorite [α 0 = 2.5(6) x 10 - 5 /K] is similar to previous measurements on various chlorite compositions, suggesting that the thermal expansivity of chlorite also can be described by a single equation, independent of composition. The bulk modulus of the natural pyrophyllite [A[Si 4 O 1 0 (OH) 2 ], measured up to 6.3 GPa, is 37(3) GPa, with K' = 10(1). The few high-temperature points agree with previous measurements. The natural talc [Mg 3 Si 4 O 1 0 (OH) 2 ], measured up to 6.2 GPa, has a bulk modulus of 41(4) GPa, with K' = 6(2). Our bulk modulus of chlorite is much higher than previously determined using X-ray diffraction. This finding explains why calculations of chlorite reactions using the previous data considerably overestimate its stability when compared to experimentally determined reaction positions. The pyrophyllite compressibility data should be useful in calculations of metamorphic reactions involving this mineral. The talc data are essentially identical to previous measurements.

Quantitative X-Ray Diffraction Analysis of Clay-Bearing Rocks From Random Preparations

AbstractAn internal standard X-ray diffraction (XRD) analysis technique permits reproducible and accurate calculation of the mineral contents of rocks, including the major clay mineral families: Fe-rich chlorites + berthierine, Mg-rich chlorites, Fe-rich dioctahedral 2:1 clays and micas, Al-rich dioctahedral 2:1 clays and micas, and kaolinites. A single XRD pattern from an air-dried random specimen is used. Clays are quantified from their 060 reflections which are well resolved and insensitive to structural defects. Zincite is used as the internal standard instead of corundum, because its reflections are more conveniently located and stronger, allowing for a smaller amount of spike (10%). The grinding technique used produces powders free of grains coarser than 20 µm and suitable for obtaining random and rigid specimens.Errors in accuracy are low, &lt;2 wt. % deviation from actual values for individual minerals, as tested on artificial shale mixtures. No normalization is applied and thus, for natural rocks, the analysis is tested by the departure of the sum of the measured components from 100%. Our approach compares favorably with other quantitative analysis techniques, including a Rietveld-based technique.

Alteration Characteristics of the Archean Golden Grove Formation at the Gossan Hill Deposit, Western Australia: Induration as a Focusing Mechanism for Mineralizing Hydrothermal Fluids

The Archean Golden Grove Formation is a 550-m-thick rhyodacitic tuffaceous volcaniclastic succession that hosts the Cu-Zn-rich Gossan Hill volcanic-hosted massive sulfide (VHMS) deposit in the Yilgarn craton, Western Australia. The Golden Grove Formation consists of volcanic quartz and altered pumice and shards, which were deposited during successive episodes of subaqueous mass flow. Coherent volcanics are absent from the Golden Grove Formation at Gossan Hill but form the main rock type in the hanging-wall Scuddles Formation. A massive dacite dome that overlies its volcanic feeder dike is inferred to occupy a synvolcanic structure that focused mineralizing fluids during the formation of the Gossan Hill deposit. The Gossan Hill deposit consists of two stratigraphically separate ore zones interconnected by stockwork. The Cu-rich lower ore zone and the Zn-rich upper ore zone occur in the middle and upper parts of the Golden Grove Formation, respectively. Podiform zones of massive magnetite occur in the Cu-rich ore zone, where the formation of magnetite predates massive sulfide. The asymmetry of massive sulfide, massive magnetite, and alteration zones at the Gossan Hill deposit attest to synvolcanic structural control during mineralization. The principal lithofacies of the Golden Grove Formation are sandstone and pebble breccia, which have a regionally extensive quartz, Fe-rich chlorite, and lesser muscovite alteration. At Gossan Hill, this alteration has resulted in near-complete replacement of tuffaceous components, causing substantial chemical modification of the primary lithologies. Quartz-chlorite (±muscovite) alteration is characterized by severe K2O, Na2O, and CaO depletion, with rocks consisting principally of SiO2, FeO, Al2O3, and MgO, along a quartz-chlorite mixing trend. Widespread preservation of pumice and shard volcanic textures within the Golden Grove Formation indicates that quartz-chlorite (±muscovite) alteration occurred soon after, or possibly during, sedimentation. The absence of diagenetic compaction textures further suggests induration of the succession during this early alteration stage, with the tuffaceous succession largely sealed from the texturally destructive effects of subsequent hydrothermal alteration, except where mineralizing fluids were locally channeled along synvolcanic feeder conduits. Local intense hydrothermal alteration zones surround the Gossan Hill deposit and overprint earlier quartz-chlorite (±muscovite) alteration. These local alteration zones have the same extent as the sulfide vein envelope and represent hydrothermal alteration formed during sulfide-magnetite mineralization. Intense Fe-rich chlorite (ankerite-siderite) alteration occurs as a strata-bound envelope around massive magnetite, Cu-rich veins, and massive sulfide in the lower ore zone. This chlorite-rich alteration has strong FeO and MgO enrichment with minor chloritoid and andalusite that reflect intense acid leaching during hydrothermal alteration. Iron chlorite (ankerite-siderite) alteration grades upward into discordant to strata-bound intense quartz alteration. Intense quartz alteration forms an envelope around Zn-rich veins and massive sulfide in the stockwork and upper ore zone. The trend from Fe chlorite-ankerite-siderite to quartz alteration toward the top of the deposit is consistent with the cooling of hydrothermal mineralizing fluids nearing the sea floor. Rhyodacite and dacite volcanics of the hanging-wall Scuddles Formation have a pervasive muscovite-calcite alteration. Muscovite-calcite alteration led to Na2O depletion and CaO and K2O enrichment associated with burial of the Gossan Hill mineralizing system. We propose that the Gossan Hill sulfide-magnetite VHMS deposit formed during an evolving Archean hydrothermal system that began as part of a regional-scale, low-temperature seawater convection-alteration system. Initially, this system caused extensive replacement of the Golden Grove Formation by quartz and Fe chlorite (±muscovite); a process that sealed and indurated the volcaniclastic rocks by infilling of primary porosity and permeability structures. Due to subsequent impermeability of the host-rock succession, later and hotter mineralizing fluids that generated alteration and massive magnetite and sulfide at the Gossan Hill deposit were constrained to, and focused upward along, a synvolcanic feeder structure.

Transmission Electron Microscopy Study of Conversion of Smectite to Illite in Mudstones of the Nankai Trough: Contrast with Coeval Bentonites

AbstractClay minerals in shales from cores at Site 808, Nankai Trough, have been studied using X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and analytical electron microscopy (AEM) to compare the rates and mechanisms of illitization with those of coeval bentonites, which were described previously. Authigenic K-rich smectite having a high Fe content (∼7 wt. %) was observed to form directly as an alteration product of volcanic glass at a depth of ∼500 meters below seafloor (mbsf) with no intermediate precursor. Smectite is then largely replaced by Reichweite, R, (R = 1) illite-smectite (I-S) and minor illite and chlorite over depths from ∼550 to ∼700 mbsf. No further mineralogical changes occur to the maximum depth cored, ∼1300 m. Most smectite and I-S in shales are derived from alteration of glass, rather than being detrital, as is usually assumed. Discrete layer sequences of smectite, I-S, or illite coexist, indicating discontinuities of the transformation from smectite to (R = 1) I-S to illite. Authigenic Fe-rich chlorite forms concomitantly with I-S and illite, with the source of Fe from reactant smectite.Smectite forms from glass with an intermediate precursor in coeval bentonites at approximately the same depth as in shales, but the smectite remains largely unchanged, with the exception of exchange of interlayer cations (K → Na → Ca) in response to formation of zeolites, to the bottom of the core. Differences in rates of illitization reflect the metastability of the clays. Temperature, structure-state, and composition of reactant smectite are ruled out as determining factors that increase reaction rates here, whereas differences in water/rock ratio (porosity/permeability), Si and K activities, and organic acid content are likely candidates.

Geochemical and isotopic evidence for post-glacial palaeoceanographic changes in Saanich Inlet, British Columbia

Abstract Coring at site ODP 1033B in Saanich Inlet recovered 59.4 m of mainly laminated olive-grey diatom ooze and an underlying 55.15 m of massive grey to olive-grey silty clay. Based on AMS radiocarbon dating, the boundary between the two units is between 11,000 and 13,800 calibrated years BP, and represents the Holocene-Pleistocene boundary. The lower unit represents glaciomarine deposition, whereas deposition of the upper unit began when the modern semi-restricted physiography of the fjord was established following glacial rebound and highly productive marine conditions were established. The glaciomarine clay is almost entirely terrigenous, whereas the diatom ooze contains 2–3 wt.% organic C and 20–40 wt.% biogenous silica; CaCO 3 contributions are minor, but there are several peaks in carbonate abundance in the upper unit. The isotopic composition of organic C and total N suggests that organic matter in the glaciomarine clay is dominantly terrestrial ( δ 13 C organic δ 15 N total =ca. 3‰) and in the diatom oozes it is mainly marine ( δ 13 C organic >−22‰ and δ 15 N total =ca. 10‰). The heavy δ 15 N total values probably record a contribution of isotopically heavy nitrate to the surface waters of the inlet that is transported to British Columbia (BC) coastal waters from the eastern tropical Pacific by the California Undercurrent. Major and minor elemental data suggest that the composition of the terrigenous material and its grain-size has changed over the last 15 kyr, and there are marked enrichments in several redox-sensitive elements in the diatom oozes. Thus, Cu, Mn, Mo, Ni, Pb, V and Zn have higher concentrations in the upper unit; Br and I are also enriched because of their association with organic matter. Mn is enriched in the anoxic diatom oozes due to the presence of manganoan carbonate (Mn peaks generally corresponding with carbonate peaks) formed in the sediment when deep water renewal caused precipitation of Mn oxyhydroxides, which dissolved in the anoxic sediment and was precipitated as a diagenetic phase. The remaining metals are enriched because of their removal to the sediment as sulphides (Cu, Mo, Ni, Pb and Zn) or as particle-reactive reduced species (V). Cr enrichment is obscured by the presence of Fe-rich chlorite. The lag in the enrichment of Mo with respect to organic C in the sediments indicates that anoxia developed some time after marine production increased following the semi-isolation of the fjord.

The Okélobondo natural fission reactor, southeast Gabon: Geology, mineralogy, and retardation of nuclear-reaction products

Research Article| January 01, 2001 The Okélobondo natural fission reactor, southeast Gabon: Geology, mineralogy, and retardation of nuclear-reaction products Keld Alstrup Jensen; Keld Alstrup Jensen 1Department of Earth Sciences, University of Aarhus, DK-8000 Århus C, Denmark Search for other works by this author on: GSW Google Scholar Rodney C. Ewing Rodney C. Ewing 2Department of Nuclear Engineering and Radiological Sciences and Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-2104, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Keld Alstrup Jensen 1Department of Earth Sciences, University of Aarhus, DK-8000 Århus C, Denmark Rodney C. Ewing 2Department of Nuclear Engineering and Radiological Sciences and Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-2104, USA Publisher: Geological Society of America Received: 26 Apr 1999 Revision Received: 29 Nov 1999 Accepted: 22 Feb 2000 First Online: 01 Jun 2017 Online Issn: 1943-2674 Print Issn: 0016-7606 Geological Society of America GSA Bulletin (2001) 113 (1): 32–62. https://doi.org/10.1130/0016-7606(2001)113<0032:TOLNFR>2.0.CO;2 Article history Received: 26 Apr 1999 Revision Received: 29 Nov 1999 Accepted: 22 Feb 2000 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Keld Alstrup Jensen, Rodney C. Ewing; The Okélobondo natural fission reactor, southeast Gabon: Geology, mineralogy, and retardation of nuclear-reaction products. GSA Bulletin 2001;; 113 (1): 32–62. doi: https://doi.org/10.1130/0016-7606(2001)113<0032:TOLNFR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Nuclear fission reactors occur in the Oklo-Okélobondo uranium deposit in southeast Gabon. The Okélobondo reactor zone (RZOKE, ∼310 m depth) was the last reactor zone to be excavated before mining was terminated in December 1997. RZOKE is situated at the base of a 2.5 m deep and 2.7 m wide reactor synform located between the FA sandstone and hydrothermally altered black shales—argile de pile—of the FB Formation. The reactor synform developed by hydrothermal dissolution of the FA sandstone during criticality at ca. 2 Ga. The core of RZOKE is up to ∼55 cm thick and contains ≤90 vol% uraninite (≤91.24 ± 0.91 wt% UO2 and ≤7.22 ± 0.53 wt% PbO) embedded in a matrix of Si-rich illite (6.08 to 7.91 Si per formula unit [p.f.u.]). The argile de pile (typically ≤60 cm) consists mainly of donbassitic chlorite (7.84 to 8.07 octahedrally coordinated Al p.f.u.) cut by fine illite veinlets. Fe-rich chlorite (0.708 to 1.427 Fe p.f.u.) is abundant at the edges of the reactor zone. Organic matter in both the reactor zone and near-field surroundings played an important role in the enhancement of porosity and stabilization of U during formation and operation of RZOKE. Fissiogenic Zr, Ce, Nd, and Th (daughter of 240Pu and 236U generated by neutron-capture reactions) are well retained in uraninite and (U,Zr)-silicate. The (U,Zr)-silicate formed during local migration of Zr, Sr, U, Pu, and lanthanide fission products during reactor criticality. Fissiogenic Ru, including 99Ru—a daughter of 99Tc, was mainly retained as ruthenium arsenide (± Pb, Co, Ni, and S). Although RZOKE generally appears well preserved, partially dissolved galena and uraninite, as well as the presence of accessory anglesite(?) and lead-uranyl sulfate hydroxide hydrate, suggest that recently observed deep, oxidized groundwaters have begun to interact with RZOKE. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Formation and Transformation of Clay Minerals in the Hydrothermal Deposits of Middle Valley, Juan de Fuca Ridge, ODP Leg 169

Mineralogical, chemical, and isotopic characteristics of clays from Ocean Drilling Program (ODP) sites 1035, 856, 1036, and 858, drilled in Middle Valley at the northern end of the Juan de Fuca Ridge during Legs 139 and 169, were studied using X-ray powder diffraction, differential thermal analysis, infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray fluorescence, and inductively coupled plasma-mass spectrometry. Oxygen isotope measurements on authigenic clay minerals provide a record of mineral formation temperatures. Holes 1036A and 858B are located in the present-day hydrothermally active Area of Active Venting and reflect several stages of hydrothermal alteration. Smectite and some mixed-layer chlorite-smectite clays are the dominant phyllosilicates in the upper part of holes 1036A and 858B. Nearly pure chlorite-smectite mixed-layer clays and corrensite mainly occur more than 20 meters below the seafloor. Formation temperatures of these nearly pure chlorite-smectite mixed-layer clays and corrensite were calculated as 250° to 268°C. In the deepest unit, chlorite is the dominant phyllosilicate coexisting with authigenic quartz. The high-temperature alteration stages resulted in gains (50–85%) of Mg and Zn, an almost complete loss of K, Ca, Cu, Rb, Sr, and Ba (> 90%), and depletion (20–40%) in Si, Fe, Na, and Zr. Alteration effects in hemipelagic and turbiditic sediments from hole 856H, drilled near the summit of the Bent Hill massive sulfide deposit, are similiar to those of sediments in hole 1035H, which was drilled 350 m south of hole 856H near the southernmost peak of the Ocean Drilling Program mound. At both holes, the clay fraction of the sedimentary sequence between 140 and 470 m below seafloor is composed of varying amounts of authigenic quartz and chlorite indicating high-temperature alteration. The chlorite formation temperatures calculated from oxygen isotope data lie at 290° to 320°C in hole 1035H and 250° to 264°C in hole 856H. Mineralogical data from hole 1035D, drilled 75 m east of hole 856H, and hole 1035A, drilled 77 m west of hole 856H, indicate lower temperatures of hydrothermal alteration in the upper 50 m of their sedimentary sequences. These sequences are partly altered to chlorite-smectite mixed-layer clays coexisting with several detrital minerals and with a dolomite formation temperature of 74°C. Oxygen isotope evidence indicates authigenic chlorite in the deeper sedimentary section of hole 1035A that formed at relatively low temperatures (180°C), whereas chlorites from hole 1035D formed at higher temperatures (277°–292°C). Chemical changes in the strongly altered clay fractions of hole 856H and 1035D include gains (60–75%) of Fe, Mg, Zn, and Cu, a nearly complete loss (> 95%) of K, Na, Ca, Rb, Sr, and Ba, and a strong depletion (30–70%) in Si, Ti, and Zr. All of the chondrite-normalized rare earth element patterns of chlorite are characterized by negative Eu anomalies and varying levels of light rare earth enrichment. The chloritization observed at the Area of Active Venting is similar to the Mg-rich chlorite and to chloritic minerals observed at sediment-covered spreading centers in Escanaba trough on Gorda Ridge and in Guaymas basin on the East Pacific Rise. This observation suggests intense Mg metasomatism in a mixing zone where hydrothermal fluids interact with seawater and sediment at temperatures above 200°C. The similarity of the Fe-rich chlorites from the Bent Hill massive sulfide deposit to Fe-rich chlorites from several ancient massive sulfide deposits indicates that they probably reflect hydrothermal fluid-rock interaction at high temperatures below the zone of seawater mixing.

Mineralogical and Rb-Sr Isotope Studies of Low-Temperature Diagenesis of Lower Cambrian Clays of the Baltic Paleobasin of north Estonia

AbstractX-ray diffraction (XRD), Rb-Sr isotope analysis, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and Fourier transform infrared (FTIR) methods were used to study diagenetic illite and illite-smectite (I-S) in Lower Cambrian unlithified clays of shallow depth of burial in the northern part of the intercratonic Baltic paleosedimentary basin of the East-European Platform. The studies focused on the &lt;0.06-µm size fraction of the clay. This fraction consists of a highly illitic illite-smectite (I-S) and a poorly crystalline illite (PCI), with some traces of Fe-rich chlorite also present. Rb-Sr isotopic data for the &lt;0.06-µm size fractions suggest that the illitic I-S and PCI have different formation ages. No precise isotopic ages were derived directly owing to the composite illite mineralogy and retention of radiogenic Sr. This retention occurred because of imperfect isotopic homog-enization at low water/rock ratios. The age of burial diagenesis is proposed to coincide with the time of maximum burial depth, which was achieved during the Middle to Late Devonian and continued until Permian-Triassic erosion. Because of the shallow depth of burial (&lt;2 km), diagenesis was probably a low-temperature (&lt;50°C) transformation process. The resident time of 100–150 million years at maximum burial had a major influence in the process.

Selective replacement of mollusk shell by chlorite, Lower Cretaceous Longarm Formation, Queen Charlotte Islands, British Columbia

The shells of molluscan megafauna from Lower Cretaceous coarse-clastic strata of Queen Charlotte Islands, British Columbia, principally ammonites and inoceramid bivalves, commonly show partial replacement alteration to fibrous, Mg- and Fe-rich chlorite. Although Cretaceous volcanic strata are very rare on Queen Charlotte Islands, strata containing the altered shells are generally rich in mafic volcanic detritus, derived from older and widespread Triassic basalts; these basalts are considered to be the source of the Mg and Fe cations present in the chloritic shells. Differences in degree of alteration to chlorite are attributed to local variations in Cretaceous sandstone permeability.