В сборнике освещены некоторые современные тенденции К - Аг и Rb-Sr геохронологии: исследование геохимических особенностей изотопных систем, изучение миграции изотопов в лабораторных и природных условиях. Особое внимание уделено нейтронно-индукционным методам. Приведены результаты исследования изотопных вариаций, вызванных в природе как термодинамическими и кинетическими факторами (углерод), так и ядерными процессами (уран, ксенон, аргон и другие в пробах из природного ядерного реактора). Ряд статей посвящается методике изотопно-геохронологических исследований. Сборник представит интерес для специалистов геохронологов, геологов, геохимиков, а также студентов старших курсов и аспирантов соответствующих специальностей

Рассматривается поведение изотопных систем К-Аг, U-Pb, Pb-Sr, используемых для определения возраста вулканических и осадочных образований. Ряд статей посвящен конкретным примерам изотопного датирования эпох вулканизма в пределах крупных регионов СССР

Определение возраста геологических объектов - основа любых историко-геологических реконструкций и геологических карт. Исторически сложилось так, что временными аспектами истории земной коры занимается стратиграфия. Объектами стратиграфии являются стратиграфические подразделения (стратоны) разного масштаба, совокупность которых составляет стратиграфические схемы. Вначале стратиграфические схемы строились как схемы-классификации историко-геологических процессов. Их положение в таких стратиграфических схемах определялось относительно друг друга (раньше-позже), но точное время формирования и длительность процесса установить было невозможно.

Кратко даются понятия о минералах группы глауконита, их составе, физических свойствах, условиях образования и подверженности воздействию вторичных процессов. Предлагаются методики распознвания аутигенных неизмененных разновидностей минералов, пригодных для изотопного датирования калий-аргоновым методом.

Для геохронологов, литологов, минералогов, а также геологов общего профиля и геологов-съёмщиков.

Arehart et al. (1993) set out to date hydrothermal Au mineralization at the Post-Betze deposit in an attempt to place the formation of this deposit into a metallogenetic setting. Toward this end they have used many modern dating techniques including 40Ar/39Ar, K/Ar, and fission track geochronology. Based on the 39 measurements presented, they conclude that "based upon what we consider to be the most reliable" data this deposit formed at 117 Ma. This date is said to be consistent with similar ages for other intrusions in northeastern Nevada, and although one is not seen at the deposit, it is implied that a major nearby pluton is responsible for the formation of Post-Betze. By extrapolation, it is then suggested that Carlin-type Au systems are all of similar age, and thus, are the result of compressional tectonics operating prior to development of the current extensional environment. Because of the inescapable importance of the inferences and conclusions generated by the assignment of a 117 Ma age to the formation of the Post-Betze deposit, I would like to discuss the data and conclusions presented by Arehart et al. (1993).

We appreciate the comments on our paper on the Post-Betze sediment-hosted disseminated gold deposit which indicate that clarification is required. Ilchik suggests alternative interpretations of our findings with regard to two main issues: the nature of the sill that we have described as "postore," and the absolute age of Post-Betze mineralization in the context of the overall spread in the dates presented in our paper. We address these issues in turn.

The postore sill, which we also loosely originally termed "dike," is important in that it brackets the timing of gold mineralization. Although Ilchik suggests that this intrusion predates mineralization, the weight of evidence is to the contrary. Whereas the postore sill is composition-ally similar to preore Goldstrike stock-equivalent rocks, it is texturally quite distinct. In fact, it was distinctive enough for us to map it out in the subsurface from drill core, something which we were unable to do for any of the other sills. Comparatively, the rock is little altered and it is the only one in the ore zone that retains primary bio-tite. Assays shown in figure 5 of Arehart et al. (1993) are the original assays done on 5-ft intervals before the core was logged; in some samples mineralized inclusions are present within the sample interval. More detailed analysis of this rock type, when clear of any inclusions of mineralized sedimentary rock, yields very low values for gold. This is true of samples from several core holes. In sharp contrast, the preore dikes and sills are all significantly altered and mineralized (i.e., well above background in areas of sedimentary rock mineralization), although they are generally of lower grade than the surrounding sedimentary rocks. We are well aware of the erratic nature of and lithologic control on gold mineralization in sediment-hosted disseminated gold deposits that are also seen at Post-Betze. Even taking this into consideration, it is clear that there is effectively no gold in the postore sill in comparison to significant gold in preore dikes and sills.

The Post-Betze deposit of Nevada is the largest sediment-hosted disseminated gold deposit presently known, both dimensionally and in terms of contained metal. Ore occurs primarily as submicron-sized gold that is disseminated in altered sedimentary rocks of the Lower Paleozoic Roberts Mountains Formation. However, significant portions of the ore are present in altered monzonite of the Goldstrike stock. Alteration and mineralization were controlled by both structure and stratigraphy. Alteration began with early decarbonatization and was followed by silicification and, finally, argillization. Phyllosilicate mineral zoning grades from proximal kaolinite to kaolinite + sericite to unaltered rock.