Local Hydrothermal Alteration Research Articles

Tectonic and sedimentary evolution of a potential supra-detachment fault basin, Archaean Moodies Group (~3.22 Ga), central Barberton Greenstone Belt

Abstract Greenstone belt dynamics are a key to understanding the formation and evolution of continental crust but the scarcity, commonly poor exposure, strong deformation, and high metamorphic grades of many Archaean supracrustal rocks preserved in greenstone belts often hinder geological insights. Exceptionally, the largely siliciclastic Palaeoarchaean (ca. 3.22 Ga) Moodies Group, uppermost unit of the Barberton Greenstone Belt (BGB) in South Africa and Eswatini, allows a detailed understanding of regional depositional processes and stratigraphic architecture due to its low degree of deformation, considerable thickness, generally good exposure, and diverse sedimentary facies. We here investigate the Powerline Road Syncline (PRS) and the adjacent Maid-of-the-Mists Syncline (MMS), two east-west-trending, tightly to isoclinally folded, locally northwardly overturned synclines in Moodies Group strata in the central BGB. The northern and southeastern margins of the PRS and the southern to southwestern margin of MMS are bounded by the bedding-parallel 24-Hour-Camp Fault. Along this fault, Moodies Group strata and stratigraphically underlying units of the Auber Villiers Formation of the upper Fig Tree Group structurally overlie lower Fig Tree strata above Onverwacht Group strata. Stratigraphic architecture and palaeocurrent analysis of Moodies Group strata in both synclines document a northeastward (depositionally downdip) facies transition from proximal fan delta conglomerates to fluvial- to coastal-plain and estuarine sandstones in which thick foresets likely represent subtidal channel fills. The overall deepening- and fining-upward trend in this sequence, corresponding to an increasing mineralogical maturity of sandstones, is partially obscured by local hydrothermal alteration which was contemporaneous with sedimentation, minor volcanism, and sill intrusion. Moodies Group conglomerates and sandstones were largely shed from the rising Onverwacht Anticline to the southwest. Lithologies and stratigraphy in the PRS-MMS region closely resemble strata of the lower Moodies Group in the Sadddleback Syncline, located nearby to the north but across the Inyoka Fault. If the 24-Hour-Camp Fault was part of a group of extensional faults accompanying the rise of the Onverwacht Anticline and of tonalitic-trondhjemitic-granodioritic plutons around the BGB margins at ca. 3 224 Ma, it would have acted as one of numerous basin-bounding normal faults during early Moodies time. This would place the deposition of Moodies Group strata in the PRS-MMS region in a supra-detachment fault environment which was subsequently tightly folded.

Remote sensing and structural studies of the Central Asir Shear Zone, Western Arabian Shield: Implications for the late Neoproterozoic E-W Gondwana assembly

The Central Asir Shear Zone (CASZ) is a N- to NNE-oriented inhomogeneous strain zone parallel to the Nabitah shear belt in southern Saudi Arabia. It is ~200 km long and 20 km wide, and traverses the Asir terrane. The deformed terrains in the vicinity of the CASZ have experienced a prolonged history involving three successive phases of deformation: arc-collision, dextral transpression and sinistral strike-slip faulting. Remote-sensing data analysis and field investigation provide insights into the geological and structural features along the CASZ. Image transformation and enhancement reveal faults and folds, and discriminate between granitoids, metavolcanics, gabbros and younger (Ablah Group) sediments. The obtained evidence points to polyphase deformation of the area, characterized by narrow zones of high strain, folding and shearing enclosing slices of less deformed rocks. Lineament analysis shows predominantly NNE, NNW, N and NE trends of folds and shear zones, and NW, NE and E fracture trends. The CASZ has a >100 m.y. long deformation history, including a change from dextral to sinistral shearing. Both shearing senses are found on the older N-S and younger NW-SE fault systems. N-S oriented shearing was accompanied by intrusion of granitoids and localized hydrothermal alteration along N to NNE trending shear zones. Strain analysis shows low strain in the Ablah basin sediments, consistent with Ablah deposition after the tectonic climax in the region.

Tectonic evolution of the Grenville Orogen in the central Appalachians

Remnants of the Grenville Orogeny—a globally pervasive tectonothermal event resulting from the amalgamation of the supercontinent Rodinia between ca. 1.3 and 1.0 Ga—are exposed throughout eastern North America for ca. 3000 km from Labrador to Alabama. Although tectonic models for the development of the Grenville have historically been based on observations from the large and well-exposed Canadian Grenville Province, recent investigations suggest that initial continental collision occurred proximal to the Appalachian Blue Ridge. Mesoproterozoic rocks within this portion of the orogen are exposed within a series of allochthonous terranes that were extensively reworked during Paleozoic orogenesis. To better understand the tectonic history of this critical region, we utilized garnet and monazite geochronology to reconstruct the metamorphic history of eleven samples of ortho- and paragneiss from the Appalachian Blue Ridge Province.The resultant dates, together with new results from trace element thermobarometry, define a prolonged record of high temperature metamorphism coincident with the timing of continent–continent collision as recognized in the northern portion of the orogen. Initial garnet and monazite growth commenced between 1100 and 1065 Ma with peak granulite-facies conditions (750–900 °C, 0.55–0.85 GPa) occurring from 1050 to 1010 Ma. Burial and heating of the paragneisses progressed rapidly with peak metamorphism occurring ∼20–30 Myr after deposition of the sedimentary protoliths. Local hydrothermal alteration followed peak metamorphism and occurred at 1000–990 Ma with later cooling through ∼660 °C between 980 and 935 Ma (3 ± 2 °C/Myr). The metamorphic history outlined here, together with insights from previous studies, is consistent with formation of the Blue Ridge rocks within a continental suture zone that had an orogenic architecture comparable to that of the Himalayan-Tibetan orogen.

Age and Origin of Silicocarbonate Pegmatites of the Adirondack Region

Silicocarbonate pegmatites from the southern Grenville Province have provided exceptionally large crystal specimens for more than a century. Their mineral parageneses include euhedral calc–silicate minerals such as amphibole, clinopyroxene, and scapolite within a calcite matrix. Crystals can reach a meter or more in long dimension. Minor and locally abundant phases reflect local bedrock compositions and include albite, apatite, perthitic microcline, phlogopite, zircon, tourmaline, titanite, danburite, uraninite, sulfides, and many other minerals. Across the Adirondack Region, individual exposures are of limited aerial extent (<10,000 m2), crosscut metasedimentary rocks, especially calc–silicate gneisses and marbles, are undeformed and are spatially and temporally associated with granitic pegmatites. Zircon U–Pb results include both Shawinigan (circa 1165 Ma) and Ottawan (circa 1050 Ma) intrusion ages, separated by the Carthage-Colton shear zone. Those of Shawinigan age (Lowlands) correspond with the timing of voluminous A-type granitic magmatism, whereas Ottawan ages (Highlands) are temporally related to orogenic collapse, voluminous leucogranite and granitic pegmatite intrusion, iron and garnet ore development, and pervasive localized hydrothermal alteration. Inherited zircon, where present, reflects the broad range of igneous and detrital ages of surrounding rocks. Carbon and oxygen isotopic ratios from calcite plot within a restricted field away from igneous carbonatite values to those of typical sedimentary carbonates and local marbles. Collectively, these exposures represent a continuum between vein-dyke and skarn occurrences involving the anatexis of metasedimentary country rocks. Those of Ottawan age can be tied to movement and fluid flow along structures accommodating orogenic collapse, particularly the Carthage-Colton shear zone.

Metamorphic and geochronologic constraints on the tectonic evolution of the Central Eastern Desert of Egypt

The Central Eastern Desert (CED) of Egypt which is part of the Arabian–Nubian Shield, is characterized by the unique occurrence of small bodies of banded iron formation (BIF) intercalated with volcanic and volcaniclastic rocks of an “ophiolitic – island arc succession”. All BIFs and their host rocks were regionally metamorphosed under greenschist to amphibolite facies conditions during the Pan-African Orogeny and were intruded by several syn- to post-tectonic magmas. Multiequilibrium calculations, conventional thermobarometry, and petrogenetic grid considerations indicate a northward decrease in metamorphic grade from 520±30°C, 5±2kbar at Um Nar in the south, to 400±50°C, 4±2kbar at Abu Marwat to the north. Areas south of Wadi El Dabbah are characterized by early overturned to recumbent folds verging north and northeast, whereas areas to the north are predominated by southwest-directed folds and thrusts. Cathodoluminescence imaging and LA-ICP-MS analysis lead to the identification of four types of zircon from felsic intrusive and metavolcaniclastic rocks: (i) crystals with oscillatory or sector zoning, and ages of 2.6–1.1Ga; (ii) crystals with oscillatory zoning, and three age populations of 828±5; 772±5; and 728±4Ma; (iii) overgrowths with bright luminescence and ages of 720–650Ma; and (iv) crystals from felsic intrusions with oscillatory zoning, and four age populations in the range of 680–544Ma.These results lead us to suggest that CED represents remnants of an island arc system formed above a southwest-dipping subduction zone. Oceanic crust in the fore- and back-arc basins formed at 850–700Ma, contemporaneously with arc activity. During periods of arc magmatism, these basins filled with volcaniclastic debris, which imposed anoxic conditions leading to increased concentrations of soluble Fe2+ in basin fluids. During periods of arc quiescence, Fe2+ was oxidized and precipitated as BIF precursor minerals in the fore- and back-arc basins. The BIFs and volcaniclastic rocks were affected by localized hydrothermal alteration that began on the ocean floor c. 710Ma. During the Pan-African orogeny, back-arc basin serpentinites, basalts, and overlying volcaniclastic rocks and BIFs were emplaced northward onto the island arc as ophiolitic material. North-directed stacking of wedge shaped thrust sheets resulted in regional metamorphism with highest grade conditions reached in the southern parts of the CED. Transpression along the northeastern margin of the arc during the waning stages of the Pan-African Orogeny (614–550Ma) led to the closure of the fore-arc basin and the emplacement of ophiolites in the northern CED with southwest verging folds. Syntectonic mostly felsic magma intruded the thrust stacks in pulses 680–544Ma. The proposed model for the evolution of the CED provides insights into the origin of small Neoproterozoic BIFs.

국내 백운석 광석의 산상과 광물학적 특성

국내 주요 백운석 광산에서 산출되는 백운석 광석과 수반광물에 대하여 X-선회절분석, 박편관찰, 주사전자현미경 관찰 등을 통하여 광물학적 및 조직 특성을 분석하고, 이들의 산출상태와 성인을 검토하였다. 연구 결과, 백운석 광석은 담회색에서 암회색을 띠며 입자도 세립에서 조립까지 다양하였으나, 주로 거의 순수한 돌로마이트로 이루어졌다. 일부 백운암에서는 소량의 방해석, 석영, 운모가 함유되었다. 세맥, 암맥, 변질부 등에서 국지적으로 방해석, 석영, 일라이트, 장석, 고령토광물, 녹니석 등이 산출되며, 일부 광산 변질부에서 세피올라이트와 규회석이 확인되었다. 그러나 이번 조사에서 온석면과 투각섬석과 같은 석면광물은 확인되지 않았다. 광산내 적갈색 및 황색을 띠는 점토물질들은 주로 일라이트로 구성되며, 카올린광물과 스멕타이트가 수반되기도 한다. 이들은 암맥관입과 관련된 부분적 열수변질과 이후의 풍화작용에 의해 형성된 것으로 보인다. 백운암층의 구성광물, 조직, 산출 상태와 기존의 연구결과로 볼 때, 백운석 광상은 선캄브리아기와 고생대의 얕은 바다에서 퇴적된 석회질 퇴적물이 속성작용에 의해 백운석화 작용을 받아 형성된 것으로 보인다. The occurrence, mineralogical characteristics, and origin of the dolomite ores were investigated from major dolomite mines in South Korea. Mineralogical and textural properties of the ores and associated minerals were analyzed using X-ray diffraction, thin section petrography, and scanning electron microscopy. Dolomite ores were light to dark gray in color and mainly composed of dolomite in varying particle size with minor amounts of calcite, quartz and micas. Calcite, quartz, illite, feldspar, kaolin minerals, and chlorite occurred in local veins, dikes and alteration zones. Sepiolite and wollastonite occurred in the altered part of some mine. Asbestos minerals such as chrysotile and tremolite, however, were not identified in the present study. Reddish brown to yellow clay materials were mainly composed of illite, occasionally associated with kaolin minerals and smectite. These clay minerals might be a product of the local hydrothermal alteration related to the dyke intrusion and subsequent weathering. As well indicated in the previous studies, mineral composition, texture, and occurrence of the dolostone beds suggest their formation through the diagenesis of carbonate sediments deposited in the shallow sea during the Precambrian to Paleozoic period.

Further evidence of widespread Permian remagnetization in the North Patagonian massif, Argentina

A paleomagnetic study on accurately dated (472–476Ma) Early Ordovician undeformed granitoids exposed in the northeastern margin of the North Patagonian massif (41.5°S, 65.0°W) was carried out, with the main goal of comparing the Early Ordovician paleogeographic position of Patagonia relative to Gondwana. About one hundred specimens were processed, from ten sites on the granitoids plus two sites on the quartzites and sandstones of the unconformably overlying Early Devonian Sierra Grande Formation. Seven out of the twelve sites, all in the granitoids, provided consistent directions of a characteristic remanent magnetization. Structural correction of paleomagnetic data from plutons could only be applied at a few sites, from bedding attitudes of overlying Devonian or Tertiary sedimentary rocks. The paleomagnetic study failed to recover the primary remanence. Thermal demagnetization isolated a reversed magnetic component with high unblocking temperatures. Magnetic properties and petrographic analyses suggest that remagnetization is carried by secondary magnetic minerals that grew from circulating high temperature fluids inferred from the pervasive to localized hydrothermal alteration. This led to the formation of magnetite and/or hematite after biotite. A paleomagnetic pole (PP) was computed from the mean site directions. The position of this PP (011°E, 65.0°S; A95=12°, K=24.5) on the apparent polar wander path of South America suggests that this magnetization was probably acquired during the Early Permian. Syntectonic remagnetization during Permian times has been reported for sandstones of the lower member of the Devonian Sierra Grande Formation at nearby outcrops. Our data suggest that the remagnetization was pervasive and affected the Ordovician granitoids as well. Many South American Permian paleomagnetic poles have been derived from syn- to postectonic magnetizations along the southwestern Gondwana margin, from the Argentine Precordillera to the Sierras Australes belt and related to the ca. 290–270Ma San Rafael tectonic phase. Slightly younger remagnetizations have been found affecting sedimentary rocks exposed in the foreland both in Argentina and Uruguay, suggesting a migration of the remagnetization front towards the North-Northeast. The similarity of the position of our new remagnetized pole from the North Patagonian massif to those from units in southern Gondwana confirms that Patagonia was already part of Gondwana by the Early Permian. Whether the remagnetization of the Ordovician granitoids is associated to the widespread and well dated in 290–280Ma magmatism that affected the northern Patagonia and a major part of southwestern Gondwana during the Permian or can be attributed to tectonic processes associated to the collision of Patagonia, or post-collision adjustment of minor plates, is discussed.

Quantifying Metasomatism in Epithermal Au-Ag Deposits: A Case Study from the Waitekauri Area, New Zealand

Major element geochemical exploration for epithermal deposits can extend the range of traditional pathfinder elements to a 1- to 10-km scale, and with knowledge of protolith composition, mass changes associated with hydrothermal alteration can be quantified. In the Hauraki goldfield of New Zealand, altered andesites and dacites host epithermal Au-Ag deposits and prospects. The major element compositions of equivalent unaltered rocks correlate with whole-rock Zr/TiO 2 , an immobile element ratio that is preserved during K metasomatism. We used this feature to estimate the initial composition and calculate a mass balance for veinless altered rocks in the Waitekauri area along a 3-km-wide section that extends from the central Waitekauri fault to the periphery of the alteration zone. The total transferred mass is equal to approximately 11 percent of rock mass in illite-dominated altered rocks, and 24 percent of rock mass in adularia-dominated altered rocks. On average mass losses exceed gains. Potassium was gained in most altered rocks, which contain illite and/or adularia as K-bearing hydrothermal minerals. Silica was gained in adularia-quartz–rich rocks close to the Waitekauri fault. Other major elements are preferentially lost (Ca, Na, Fe, Mg) or effectively immobile (Al, Ti). The greatest K and Si gains occur in adularia-rich rocks that surround Au deposits along the Waitekauri fault, whereas K gains are progressively lower and Si gains are mostly insignificant in deposits and prospects farther east where illite or interstratified illite-smectite is the dominant K-bearing mineral. In contrast, Na and Ca losses do not increase significantly from the periphery to the core of the Waitekauri area, because losses are commonly complete and therefore limited by the initial concentration. However, the K and Si gains correlate with other measures of K metasomatism including K/Sr and Rb/Sr values and molar ( M ) K/(K + Na + 2Ca) values, and together these parameters vector from the barren periphery to the orebody-hosting center of the Waitekauri area. In contrast to major element trends, the pathfinder elements As, Sb, and Hg define more local hydrothermal alteration cells within the larger Waitekauri area, some of which surround Au deposits.

The Cu isotopic signature of granites from the Lachlan Fold Belt, SE Australia

This contribution reports our preliminary work to determine Cu isotope ratios for various granite rocks and examine the Cu isotope systematics within granite suites. A chemical procedure, modified from Maréchal [Maréchal, C.N., Télouk, P. and Albarède, F., 1999. Precise analysis of copper and zinc isotopic compositions by plasma-source mass spectrometry. Chemical Geology, 156(1–4): 251–273.], was used to separate Cu from rock matrix. Quantitative recovery (100.6 ± 1.6%), with a low total procedural blank (2.65 ± 0.66 ng) for Cu, has been achieved, allowing Cu isotopic measurements on samples with as little as 10 ppm Cu. The Cu isotope ratios ( δ 65Cu relative to NIST SRM 976) of 32 rock samples, ranging from mafic to felsic compositions, from 3 batholiths (2 I-type, 1 S-type) from the Lachlan Fold Belt in southeastern Australia, vary from − 0.46‰ to 1.51‰. Most of them cluster around zero, with mean values for the I-type and S-type granites of 0.03 ± 0.15‰ and − 0.03 ± 0.42‰ (2 sigma) respectively. These data, together with Cu isotope ratios of two loess samples, provide preliminary evidence that the baseline Cu isotopic composition of the crystalline part of upper continental crust is close to zero. The tight clustering of Cu isotope ratios of rocks from the I-type suites suggests that high-temperature magmatic processes do not produce significant Cu isotope fractionation. However, two granites with abnormally heavy Cu isotope signatures (up to 1.51‰) appears to be the result of localized hydrothermal alteration. Measurable variation in Cu isotopic composition of the S-type granite may reflect isotopic heterogeneity in the sedimentary source region as a result of redox processes or may be due to hydrothermal overprinting. Thus, Cu isotope geochemistry may be a useful tracer for studying hydrothermal alteration and source heterogeneity of granitic rocks.

Hydrothermal alteration and magnetic properties of rocks in the Carolina de Michilla stratabound copper district, northern Chile

In the Carolina de Michilla district, northern Chile, stratabound copper mineralization is hosted by Jurassic volcanic rocks along the trace of the Atacama fault system. In this study, we present the overall effects of hydrothermal alteration on the magnetic properties of rocks in this district. Two types of metasomatic alteration associations occur, one of regional extent and the other of local hydrothermal alteration associated with copper mineralization (e.g., Lince–Estefania–Susana). Regional alteration is interpreted as a low-grade “propylitic association” characterized by an epidote–chlorite–smectite–titanite–albite–quartz–calcite association. The local hydrothermal alteration is characterized broadly by a quartz–albite–epidote–chlorite–calcite mineral assemblage. The most pervasive alteration mineral is albite, followed by epidote and, locally, actinolite. These minerals contrast sharply against host rock minerals such as chlorite, calcite, zeolite, prehnite, and pumpellyite, but alteration is constrained to mineralized bodies as narrow and low contrast alteration halos that go outwards from actinolite–albite to epidote–albite, to epidote–chlorite, and finally to chlorite. Hydrothermal alteration minerals, compared to regional alteration minerals, show iron-rich epidotes, a lower chlorite content of the chlorite–smectite series, and a nearly total albite replacement of plagioclase in the mineralized zones. Opaque minerals associated with regional alteration are magnetite and maghemite, and those associated to hydrothermal alteration are magnetite, hematite, and copper sulphides. We present paleomagnetic results from nine sites in the Michilla district and from drill cores from two mines. Local effects of hydrothermal alteration on the original magnetic mineralogy indicate similar characteristics and mineralogy, except for an increase of hematite that is spatially associated with the Cu–sulphide breccias with low magnetic susceptibilities. Results indicate that it is impossible to magnetically differentiate mineralized bodies from unmineralized lavas, except for pyrite-rich hydrothermal breccias. In conclusion, for stratabound copper deposits of the Michilla type, the overall effect of hydrothermal alteration on the paleomagnetic properties of rocks is of low contrast, not clearly discernable even at a small scale. From an exploration point of view, magnetic exploration surveys should not discern mineralized bodies of Cu–sulphide breccias except in detailed ground surveys due to the small size of contrasting bodies. Unoriented drill cores with primary ore mineralization record a characteristic remanent magnetization of reverse polarity. Taking into account the azimuth and dip of the drill cores, we were able to compare the magnetization of the mineralized bodies with the characteristic directions from sites drilled in situ from Late Jurassic–Early Cretaceous intrusives mostly. The characteristic direction recorded by the Pluton Viera is similar to the magnetization of the ore bodies of the Estefania mine. If copper mineralization mostly postdates the tilt of the volcanic flows, the low paleomagnetic inclinations suggest an age for the mineralization near 145 Ma, the time of the lowest paleolatitude for the South American plate during the Mesozoic.

Carbonaceous cherts of the Barberton Greenstone Belt, South Africa: Isotopic, chemical and structural characteristics of individual microstructures

Carbonaceous matter occurring in chert deposits of the 3.4–3.2 Ga old Barberton Greenstone Belt (BGB), South Africa, has experienced low grade regional metamorphism and variable degrees of local hydrothermal alteration. Here a detailed study is presented of in situ analysis of carbonaceous particles by LRS (laser Raman spectroscopy) and SIMS (secondary ion mass spectrometry), reporting degree of structural disorder, carbon isotope ratio and nitrogen-to-carbon ratio. This combination of in situ analytical tools is used to interpret the δ 13C values of only the best preserved carbonaceous remains, enabling the rejection of non-indigenous (unmetamorphosed) material as well as the exclusion of strongly hydrothermally altered carbonaceous particles. Raman spectroscopy confirmed that all carbonaceous cherts studied here have experienced a regional sub- to lower-greenschist facies metamorphic event. Although this identifies these organics as indigenous to the cherts, it is inferred from petrographic observations that hydrothermal alteration has caused small scale migration and re-deposition of organics. This suggest that morphological interpretation of these carbonaceous particles, and in general of putative microfossils or microlaminae in hydrothermally altered early Archean cherts, should be made with caution. A chert in the Hooggenoeg Formation, which is older than and has been hydrothermally altered by a volcanic event 3445 Ma ago, contains strongly altered carbonaceous particles with a uniform N/C-ratio of 0.001 and a range of δ 13C that is shifted from its original value. Cherts of the Kromberg Formation post-date this volcanic event, and contain carbonaceous particles with a N/C-ratio between 0.002 and 0.006. Both the Buck Reef Chert and the Footbridge Chert of the Kromberg Formation have retained fairly well-preserved δ 13C values, with ranges from −34‰ to −24‰ and −40‰ to −32 ‰, respectively. Abiologic reactions associated with hydrothermal serpentinization of ultramafic crust (such as Fischer–Tropsch synthesis) were an unlikely source for carbonaceous material in these cherts. The carbonaceous matter in these cherts has all the characteristics of metamorphosed biologic material.

Regional very low-grade metamorphism of basaltic lavas, Disko–Nuussuaq region, West Greenland

The thick Paleocene basaltic lava succession exposed on Disko Island and Nuussuaq Peninsula, central West Greenland, experienced regional metamorphism shortly after volcanism followed by several events of localized hydrothermal alteration. Regional metamorphism resulted in well-developed lower zeolite facies mineral assemblages consisting primarily of mafic phyllosilicates and zeolites. In the upper Paleocene lava formation, the Maligât Formation, alteration is dominated by early mixed dioctahedral–trioctahedral smectite followed by chabazite and thomsonite. This same assemblage persists into the upper portions of the underlying Vaigat Formation. Within the Vaigat Formation, the chabazite–thomsonite assemblage is replaced at depth by an assemblage dominated by mafic phyllosilicates followed by thomsonite + chabazite + analcime + natrolite + gonnardite. Mafic phyllosilicate compositions within the latter zone become systematically more rich in the trioctahedral component with depth; the highest-grade mafic phyllosilicates consist of purely trioctahedral, mixed-layer chlorite–smectite. The mineral assemblage developed in the Vaigat Formation is distinct from similar grade assemblages formed in most large igneous provinces as a consequence of the unusually olivine-rich lava compositions that dominate this formation. The zeolites developed in the Vaigat Formation tend to be more Na-rich and somewhat Si-depleted relative to those that form in feldspar-phyric lavas such as the Maligât Formation. Late hydrothermal alteration led to a wide variety of low-grade metamorphic mineral assemblages largely confined to mineralized veins and their immediate surroundings. The distribution of regional alteration assemblages on Disko and Nuussuaq suggests relatively low geothermal gradients (< 45 °C/km) during metamorphism. The highest regional metamorphic grades occur at the base of the exposed succession on northern Disko, suggesting uplift of this area relative to the rest of the island, consistent with recent seismic data.

Alteration of Volcanic Rocks and Genesis of Kaolin Deposits in the Şile Region, Northern Istanbul, Turkey. I: Clay mineralogy

AbstractThe Şile Region contains discontinuous, cyclic, thin coal beds and industrial clay deposits that were accumulated in lacustrine basins which received extensive volcanoclastic sediments due to transport of highly weathered calc-alkaline volcanic rocks. The Sülüklü area has the largest kaolin deposit in this region. Cyclic kaolinization depended on the degree of leaching of Si and alkalis in cyclic swamp environments and, therefore, kaolinite contents vary in each discontinuous lens-shaped clay bed and underclay within the basin. The kaolins comprise disordered kaolinite, illite, smectite, gibbsite, quartz, pyrite, anatase, K-feldspar and goethite. Depth-related changes in the distribution of clay minerals, associated with coal beds, are indicative of organic acid-mineral reactions. Kaolinite crystallization initiated at the edges of sericitic mica sheets in the form of composite kaolinite stacks. The small size (&lt;1 µm), morphology and poor crystallinity of kaolinite crystals suggest that kaolinization post dated transportation. Primary or secondary origins of particles can be determined from the stacking sequences of kaolinite particles using high-resolution transmission electron microscopy images. Kaolinite stacks always contain a small amount of illite, but smectite is only present in the middle and upper levels. Gibbsite is a main constituent of refractory bauxitic clays locally found as discontinuous lenses and exploited from the lower level of the basin.Genesis of kaolin deposits took place in two stages: first, there was in situ weathering of the oldest andesitic agglomerates, tuffs and ashes at the base of the lacustrine basin coupled with discharge of shallow thermal waters which were initiated by local hydrothermal alteration; second, surface weathering enhanced transportation of altered rocks from the surrounding hills into the lacustrine basin. Kaolinization took place in cyclic swamp environments, as indicated by the presence of cyclic thin- to thick-bedded coals that provided necessary humic and fulvic acids for the post-depositional alteration of altered volcanic rocks to kaolin in dysaerobic, relatively low-pH conditions in saturated groundwater zones.

Géochimie du volcanisme birimien (Protérozoïque inférieur) de l'Unité de Haute-Comoé. N-E de la Côte-d'Ivoire: Premiers résultats

The Lower Proterozoic (Birimian) Haute-Comoé Unit (West African Craton of Côte-d'Ivoire) consists of a metavolcanic and metasedimentary suite. The metavolcanic formation deals with a main T-MORB type tholeiitic serie with intercalations of little-extended calc-alkaline volcanites chiefly composed of andesitic to dacitic rocks. Local hydrothermal alteration is responsible of the genesis of rocks with spilitic affinities. The volcanism setting would be anorogenic, in a tectonic context of frifting in an ensialic environment, trough a strongly slimmed crust, after Kröner's pattern for Precambrian formations.

Alteration and metamorphism of Amitsoq gneisses from the Isukasia area, West Greenland: Recommendations for isotope studies of the early crust

Early Archaean gneisses (3400–3700 Ma) in the Isukasia area of West Greenland have been subjected to a series of local and regional metamorphic processes. Metasomatic alteration accompanied the intrusion of the early Archaean white gneisses (~3600 Ma) into the grey gneisses (~3700 Ma), and resulted in local formation of altered rocks. Pb-Pb isotope results on whole rock and feldspar from the gneisses reveal a major rearrangement of Pb isotopes at about 2600 Ma together with some local third stage Pb changes at a later time. This 2600 Ma event is also shown by Rb-Sr data for phengites in the ~3400 Ma Pegmatitic gneiss sheets and can be correlated with metamorphism accompanying the intrusion of a late Archaean pegmatite (~2600 Ma) swarm. Local hydrothermal alteration by fluids emanating from Proterozoic faults and fractures is petrographically recognised in and near major faults and lithological boundaries. This fault-controlled alteration, though only locally significant, may correlate with low grade thermal metamorphism reflected in a 1600 Ma Rb-Sr mineral isochron on biotites from both altered and unaltered gneisses. Oxygen isotope results show that this alteration probably was associated with influx of a low-δ 18O fluid, perhaps meteoric water. Selection of the least altered Amitsoq gneisses can be made on the basis of petrographic and field criteria, and furnishes the best material to study original geochemical and isotopic systematics in these early Archaean rocks.

Mineral exploration with Skylab photography in central Colorado

Dense linear patterns, intersections of geologic lineaments, hydrothermal and supergene alteration patterns, and vegetation toxicity. Exploration at Weston Pass revealed major faulting, local hydrothermal alteration, and a small zinc-silver-lead deposit. Controls on mineralization are deep-seated faulting, jointing, and permeable and reactive facies in the Mississippian Leadville Formation. Advantages of orbital photography are high resolution and true color pictures, as well as low cost, large area coverage, uninhibited access to remote areas, and confidential reconnaissance.--Modified journal abstract.