Автор(ы):Буслов М.М., Жимулев Ф.И., Травин А.В.
Издание:Доклады Академии наук, 2010 г., 5 стр.
Язык(и)Английский
New Data on the Structural Setting and 40Ar/39Ar Age of the MP–LP Metamorphism of the Daulet Formation, Kokchetav Metamorphic Belt, Northern Kazakhstan, and Their Tectonic Interpretation

Ultra-high-pressure (UHP) and high-pressure (HP) metamorphic complexes in continental subduc-tion zones are spatially associated with younger medium-pressure (MP) and low pressure (LP) meta-morphic rocks [1]. It is a controversial question whether these MP–LP rocks are formed by the contact effect of exhumed hot nappe stack of UHP–HP rocks on the underlying rocks (by analogy with meta-morphic base of obducted ophiolite allochthon) or result from later syncollisional metamorphism related to crustal thickening and granite magmatism. MP and LP metamorphic complexes with inverted metamor-phic zoning are known in the collisional Himalaya nappe-thrust system [2, 3]. Structurally, they are confined to the overthrusting of the Himalayas onto the Indian continent along the Main Central Thrust. The MP (garnet–kyanite–staurolite) and LP (sillimanite– biotite and biotite) rocks were formed at the initial stage of thrusting (25–15 Ma) and exhumed to the surface by later thrusts and denudation [3].

ТематикаРегиональная геология
Выпуск 92
Автор(ы):Christian J.Grainger, David I.Norman, Matthew T.Heizler, William C.McIntosh
Издание:Economic geology, 1997 г., 22 стр.
Язык(и)Английский
40Ar/39Ar Dating and Mineral Paragenesis for Carlin-Type Gold Deposits along the Getchell Trend, Nevada: Evidence for Cretaceous and Tertiary Gold Mineralization

The orebodies at the Getchell and Twin Creeks mines were studied through mineral paragenesis, geologic relationships, and 40Ar/39Ar dating. Mineral paragenetic relationships are based on observations made during the logging of 18,000 m of drill cuttings and core and crosscutting relationships recognized in the field and Main pit at the Getchell mine. Ages for igneous and mineralizing events were determined through 40Ar/39Ar incremental heating analyses of 15 samples of biotite, K feldspar, sericite, and vein adularia. A thermal history for the area was also developed using K feldspar multiple diffusion domain results and age determinations on cogenetic minerals, which have different argon closure temperatures.

ТематикаПолезные ископаемые
МеткиAr-Ar метод, Carlin type gold, Getchell Trend, Gold, Золото
Выпуск 90
Автор(ы):Robert P.Ilchik
Издание:Economic geology, 1995 г., 3 стр.
Язык(и)Английский
40Ar/39Ar, K/Ar, and fission track geochronology of sediment-hosted dessiminated gold deposits at Post-Betza, Carlin Trend, Northeastern Nevada - a discussion

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).

ТематикаПолезные ископаемые
Выпуск 90
Автор(ы):Arehart G.B., Foland K.A., Kesler S.E., Naeser C.W.
Издание:Economic geology, 1995 г., 3 стр.
Язык(и)Английский
40Ar/39Ar, K/Ar, and fission track geochronology of sediment-hosted disseminated gold depositsat at Post-Betze, Carlin Trend, Northeastern Nevada - a reply

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.

ТематикаПолезные ископаемые
Выпуск 88
Автор(ы):Arehart G.B., Foland K.A., Kesler S.E., Naeser C.W.
Издание:Economic geology, 1993 г., 25 стр.
Язык(и)Английский
40Ar/39Ar, K/Ar, and Fission Track Geochronology of Sediment-Hosted Disseminated Gold Deposits at Post-Betze, Carlin Trend, Northeastern Nevada

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.

ТематикаПолезные ископаемые
Ленты новостей
1836.6