Minoru Utada

FURTHER INVESTIGATIONS OF A CONVERSION SERIES OF DIOCTAHEDRAL MICA/SMECTITES IN THE SHINZAN HYDROTHERMAL ALTERATION AREA, NORTHEAST JAPAN

A complete conversion series for mica/smectites was found in a hydrothermal alteration envelope around Kuroko-type ore deposits at the Shinzan area, Akita Prefecture, Northeast Japan. The minerals are an alteration product of volcanic glass of Miocene age and are commonly associated with zeolites and silica minerals. Degrees of ordering of interstratification of the minerals change discontinuously from Reichweite g = 0 (100–55% expandable layers) to g = 1 (45–20% expandable layers), and from g = 1 to g = 2 (<20% expandable layers). This pattern of conversion differs from the behavior of mica/smectites during burial diagenesis which undergo a continuous change in ordering type, and from the behavior of rectorite which displays a constant expandability and ordering (45–55%) over a wide range of conditions. Differences between these minerals were also found in the relationships between expandability and total layer charge, and between expandability and number of non-exchangeable interlayer cations. In mica/smectites from the Shinzan area, chemical changes in the interlayers and tetrahedral and octahedral sites are consistent with a reaction in which K-enrichment and K-fixation in the interlayers are controlled by an increase in negative layer charge. This conversion occurred in response to a steep geothermal gradient and migrating hydrothermal solutions.РезюмеПолные серии видоизменения для слюд/смектитов были найдены в кармане гидротермического изменения вокруг Куроко-типа залежей руды в Шинзан области, Акита префектура, Северно-восточная Япония. Минералы явдяются продуктом видоизменения вулканического стекла миоценовой эпохи и обычно связываются с цеолитами и кремнеземными минералами. Степени упорядочения переслаивания минералов изменяются прерывисто от числа Рейхвейта g = 0 (100-55% расширяющихся слоев) до g = 1 (45-20% расширяющихся слоев) и от g = 1 до g = 2 (<20% расширяющихся слоев). Этот образец превращения отличается от поведения слюды/смектитов в течение периода диагенеза погребений, которые претерпевают непрерывное изменение типа упорядочения, и от поведения ректорита, который проявлет постоянную способность к расширению и упорядочению (45-55%) в широким диапазоне условий. Различия межлу этими минералами были также найдены и в области соотношений между расширяемостью и полным зарядом слоя, и между расширяемостью и числом необмениваемых межслойных катиогов. Химические изменения в прослойке и в четырехгранных и восьмигранных местах в слюдах/ смектитах из Шинзан области согласуются с реакцией, в которой обогащение прослойки калием и фиксация калия в прослойке контролируется увеличением отрицательного заряда слоя. Это превращение происхоит как реакция на большой геотермальный градиент и миграцию гидротермальных растворов. [Е.С.]ResümeeEine vollständige Umwandlungsserie von Glimmer/Smektite wurde in einer hydrothermalen Umwandlungszone um Erzlagerstätten vom Kuroko-Typ im Shinzangebiet, Akita Prefecture, NO Japan, gefunden. Die Minerale sind Umwandlungsprodukte von vulkanischem Glas aus dem Miozän und treten im allgemeinen zusammen mit Zeolithen und SiO2-Mineralen auf. Der Ordungsgrad der Wechsellagerung in den Mineralen ändert sich diskontinuierlich von der Reichweite g = 0 (100–55% expandierbare Lagen) bis g = 1 (45–20% expandierbare Lagen), und von g = 1 bis g = 2 (<20% expandierbare Lagen). Dieses Umwandlungsschema unterscheidet sich vom Verhalten Glimmer/Smektite, das durch Versenkungsdiagenese hervorgerufen wird, in dem eine kontinuierliche Veränderung im Ordnungszustand stattfindet. Es unterscheidet sich auch vom Verhalten von Rektorit, der über einen großen Bedingungsbereich eine konstante Expandierbarkeit und Ordnung (45–55%) zeigt. Unterschiede zwischen diesen Mineralen wurden auch in der Beziehung zwischen Expandierbarkeit und Gesamtladung der Schichten und zwischen Expandierbarkeit und Anzahl der nichtaustauschbaren Zwischenschichtkationen gefunden. In den Glimmer/Smektit-Wechsellagerungen aus dem Shinzangebiet hängen chemische Veränderungen in den Zwischenschichten sowie auf den tetraedrischen und oktaedrischen Plätzen mit einer Reaktion zusammen, bei der die K-Anreicherung und K-Fixierung in den Zwischenschichten durch eine Zunahme der negativen Schichtladung kontrolliert wird. Diese Umwandlung ist die Folge eines steilen geothermischen Gradienten und wandernder hydrothermaler Lösungen. [U.W.]RésuméUne série de conversion complète pour des mica/smectites a été trouvée dans une enveloppe d’altération hydrothermique autour de dépôts de minéraux du type Kuroko dans la région Shinzan, Préfecture d’Akita, Japon du Nord-Est. Les minéraux sont un produit de l’altération de verre volcanique d’age Miocène, et sont souvent associés avec des zéolites et des minéraux silices. Les degrés d’ordonnement d’interstratification des minéraux changent de manière discontinue de Reichweite g = 0 (100–55% de couches expansibles) à g = 1 (45–20% de couches expansibles) et de g = 1 à g = 2 (<20% de couches expansibles). Ce procédé de conversion diffère du comportement de mica/smectites pendant la diagénèse d’ensevelissement; celles-ci subissent alors un changement de type d’ordonnement continuel; et aussi du comportement de la rectorite qui montre un potentiel d’expansion et un ordonnement (45–55%) constants sur une large gamme de conditions. Des différences entre les relations de potentiel d’expansion et la charge totale de couche, et du potentiel d’expansion et du nombre de cations intercouche non-échangeables de ces minéraux ont aussi été trouvées. Dans les micas/smectites de la région Shinzan, des changements chimiques dans les intercouches et dans les sites tétraèdraux et octaèdraux sont compatibles avec une réaction dans laquelle l’enrichissement de K et la fixation de K dans les intercouches sont contrôlés par une augmentation de la charge négative de couche. Cette conversion s’est produite en réaction à un raide gradient géothermique et à des solutions hydrothermiques émigrantes. [D.J.]

Smectite-to-illite conversion in natural hydrothermal systems

Abstract Smectite-to-illite (S-to-I) conversion in natural hydrothermal systems including active and fossil geothermal fields can be characterized from the viewpoints of mode of conversion, kinetics, and mechanism and this can be related to the durability of buffer materials of high level radioactive waste. From the viewpoint of conversion, in the active geothermal field which can be used as a case study, the S-to-I conversion occurs in different ranges of present day temperature from drill hole to drill hole, which is related to the thermal history of the field up to two or three discrete illite/smectite (I/S) phases having different percents of illite layers (%I) coexist metastably at intermediate stages of the conversion. This is particularly visible in drill holes where the conversion occurs at high temperatures. The S-to-I conversion from a fossil geothermal field suggests that the greater the thermal gradient, the more the smectite illitization is facilitated. By having the average rate of entire S-to-I conversion, which was calculated from the data of K-Ar and % I in I/S, the activation energy required to complete the S-to-I conversion was estimated to be approximately 30 ± 5 kcal/mol in natural hydrothermal systems, where the S-to-I conversion was assumed as a first order reaction and the pre-exponential factor is the same as Eberl and Hower (1976). The estimated value is close to those deduced previously from laboratory experiments. This suggests that the experiments made by using specific geologic materials and conditions may be applicable to evaluate the kinetics of S-to-I conversion in hydrothermal systems from the viewpoint of an empirical equation. Nevertheless, since the S-to-I conversion in natural hydrothermal systems is in fact controlled by mechanisms of smectite dissolution and recrystallization of illite, it will be necessary in the future to construct the real reaction kinetics based on a proven mechanism.

Mineralogical and geochemical characteristics of hydrothermal alteration of basalt in the Kuroko mine area, Japan: implications for the evolution of a Back Arc Basin hydrothermal system

Abstract Basalt in the Furutobe District of the Kuroko mine area in Japan is characterized by abundant chlorite and epidote. Fluid inclusion studies indicate that chlorite is formed at lower temperatures (230–250°C) than epidote (250–280°C). The seawater/basalt mass ratio for the early chlorite-rich alteration was high (max. 40), but that for the later alteration was low (0.1–1.8). The CaO, Na2O and SiO2 of the bulk rock correlate negatively with MgO, while FeO and Σ Fe correlate positively with MgO. These changes in the characteristic features of hydrothermal alteration from early to late are generally similar to those for a mid-ocean ridge geothermal system accompanying basalt alteration. The MgO/FeO ratios of chlorite and actinolite and the Fe2O3 concentration of epidote from the basalt are greater than those of mid-ocean ridge basalt probably owing to the differences in the Fe2O3/FeO and MgO/FeO ratios of the parent rocks. The lower CaO concentration and the higher Na2O concentration of the bulk rock compared with altered mid-ocean ridge basalt can be interpreted in terms of the difference in original bulk rock compositions. The Furutobe basalt, as well as other submarine back arc basalts, contains more vesicles filled with hydrothermal minerals (epidote, calcite, quartz, chlorite, pyrite) than do the mid-ocean ridge basalts. The abundance of vesicles plays an important role in controlling the secondary mineralogy and geochemistry of hydrothermally altered submarine back arc basin basalts.