Authigenic potassic silicates in the Rathdowney Trend,southwest Ireland: New perspectives for ore genesis from petrography of gangue phases in Irish-type carbonate-hosted Zn-Pb deposits

Petrographic studies of Zn-Pb ore zones hosted by Lower Carboniferous dolomitized Waulsortian reef limestones in the Rathdowney Trend reveal a paragenetic association between sphalerite and potassium silicates: (Ba, K)-feldspar, adularia, and albite as well as rare quartz, illite and phengite. The dominant mineral assemblage is composed of 1) sphalerite ± (K,Ba) feldspar ± pengite/illite near the putative feeder conduit for the Lisheen Main Zone, and 2) a sphalerite ± pengite/illite assemblage distal to the major normal faults (NW of Main Zone, Island Zone and Rapla occurrence). In addition, clay minerals have been identified in fault gouges located at the interface between the Waulsortian reef and argillaceous limestone of the Ballysteen Formation. This mineral assemblage has provided additional constraints on the physico-chemical conditions of ore formation (eH, pH, sulfur and metal species). Another implication of the mineral assemblage, is a revision of fluid rock interaction processes, in which mineral solution-precipitation reactions are contributing to the development of hydrothermal breccias, with dolomitic black matrix breccias interpreted as a by-product of massive sulfide precipitation. Additionally, textural studies of the breccias through fractal dimension analysis of fragment geometry extracted from large scale scanning electron microscope element maps (1 cm² areas or more), indicates the prevalence of chemical brecciation-corrosion processes in the generation of the black matrix breccias. Integration of these new observations allows for comparison of ore forming processes across the Irish Midlands from the perspective of gangue mineralogy and may provide further links with classic Mississippi-type deposits from the mid-continent region of the United States of America.     

陕西双王金矿床角砾岩动力学成因探讨

双王金矿可拼性含金角砾岩是流体致裂角砾岩,而不是构造角砾岩或隐爆角砾岩。该区的深源异常高压流体是角砾岩产生的必要条件。角砾岩的动力学成因主要是区域变质作用、地壳隆升剥蚀作用和地下岩浆活动等形成异常高压流体,在重力、浮力和地热梯度的驱动下,在能干性强的岩石中产生裂隙,并向浅部增殖,被碳酸盐矿物等充填胶结。周而复始的流体压裂—沉淀愈合作用,使金多次富集,形成矿体。     

Réflexions sur la « révolution danienne » dans les Pyrénées

This work disproves the magmatic (ophitic rises) and sedimentological (submarine trans-Pyrenean trough filled with breccias and hemipelagites) arguments presented in favour of a Danian distension step following a major Upper to Late Cretaceous Pyrenean compression phase. In the western Pyrenees (Bearn area) the tholeiitic magmatism is really Triassic or Lowermost Liassic in age. The ophites cross mechanically the Jurassic and Cretaceous enclosing sedimentary beds without any contact metamorphism, which could give proof of a Palaeocene age for the magmatic emplacement. As for the supposed submarine breccias rich in planktonic foraminifera, they really correspond to diapiric Early Cretaceous breccias, to Cretaceous or Tertiary tectono-karstic breccias or to Quaternary colluvial deposits. The Danian/Selandian trough does not exist. The proposed interpretation assigns that the Palaeocene interval must be included within the long compression (transpression) period, which begins in the Upper Cretaceous times and increases during the Early Cenozoic, leading to the main structural step of the Pyrenean cycle, towards the Middle–Upper Eocene. To cite this article: J. Canérot, C. R. Geoscience 338 (2006).     

Origin of the vertically orientated clasts in brecciated shallow‐marine limestones of the Chaomidian Formation (Furongian,Shandong Province,China)

Numerous shallow‐marine limestone layers of the Furongian (Late Cambrian) Chaomidian Formation in the Jiulongshan section (Shandong Province, China) are breccias. Some of these breccias show abundant vertical to sub‐vertical clasts. Typically, these clasts end abruptly at the contact plane with the overlying deposit, either abutting the overlying sedimentary bed or via an erosional plane that truncates the clasts. A few exposures show concentrations of clasts that must have been uplifted to the extent that they transgressed the then sedimentary surface or (possibly) penetrated the overlying sediment which, in this case, consists of muds or marls. The clasts tend to show clusters with respect to the enclosing fabric. All clasts are parallel to each other in a specific cluster, while the various clusters may show different orientations of the clasts. It is deduced that both the exceptional position and the exceptional orientation of the clasts must be ascribed to the upward movement of the clasts under the influence of pore water escaping under high pressure through fluidized sediment.     

A detailed fluid inclusion study in silicified breccias from the Kombolgie sandstones (Northern Territory, Australia): inferences for the genesis of middle-Proterozoic unconformity-type uranium deposits

The relative chronology and detailed chemistry of paleofluids circulating at the base of the Kombolgie Sub-basin were investigated in the East Alligator River district (Northern Territory, Australia), where world-class unconformity-type uranium deposits are located. The chemistry of fluid inclusions was determined using in-situ analysis (Raman microprobe and laser-induced breakdown spectroscopy [LIBS]) and by observing the melting sequences by microthermometry. This study revealed the occurrence of three distinct fluids: (i) a sodium-rich brine that corresponds to a diagenetic fluid percolating at the bottom of the Kombolgie sandstones at a temperature close to 150±15 °C; (ii) a calcium-rich brine, probably corresponding to a residual brine in evaporitic environment that has evolved by fluid–rock interactions with the basement lithologies; and (iii) a low salinity fluid, heated in the basement, injected into the base of the sandstone cover. H₂ and O₂ and/or traces of CH₄ were detected in the vapor phase of some fluid inclusions, especially in the low salinity ones in quartz breccia samples taken above mineralized areas. Hydraulic brecciation of the sandstone was associated with a pressure decrease favoring fluid mixing and the subsequent cementation of breccias. According to the fluid inclusion study and other geologic constrains, the minimum thickness of the Sub-Kombolgie Basin is estimated at 4 km. Drusy quartz breccias with evidence of fluid mixing are quite common at the base of the Kombolgie Basin, but not necessarily linked to U-mineralization. However, it is proposed that the presence of gases such as H₂ and O₂ in fluid inclusions, which results from water radiolysis, constitutes an indicator of gas linked to significant U concentrations deeper in the basement rocks.     

Dolomite formation in breccias at the Musandam Platform border, Northern Oman Mountains, United Arab Emirates

The presence of dolomite breccia patches along Wadi Batha Mahani suggests large-scale fluid flow causing dolomite formation. The controls on dolomitization have been studied, using petrography and geochemistry. Dolomitization was mainly controlled by brecciation and the nearby Hagab thrust. Breccias formed as subaerial scree deposits, with clay infill from dissolved platform limestones, during Early Cretaceous emergence. Cathodoluminescence of the dolostones indicates dolomitization took place in two phases. First, fine-crystalline planar-s dolomite replaced the breccias. Later, these dolomites were recrystallized by larger non-planar dolomites. The stable isotope trend towards depleted values (δ¹⁸O: − 2.7‰ to − 10.2‰ VPDB and δ¹³C: − 0.6‰ to − 8.9‰ VPDB), caused by mixing dolomite types during sampling, indicates type 2 dolomites were formed by hot fluids. Microthermometry of quartz cements and karst veins, post-dating dolomites, also yielded high temperatures. Hot formation waters which ascended along the Hagab thrust are invoked to explain type 2 dolomitization, silicification and hydrothermal karstification.     

Brecciation processes in fault zones: Inferences from earthquake rupturing

Surface-rupture patterns and aftershock distributions accompanying moderate to large shallow earthquakes reveal a residual brittle infrastructure for established crustal fault zones, the complexity of which is likely to be largely scale-invariant. In relation to such an infrastructure, continued displacement along a particular master fault may involve three dominant mechanical processes of rock brecciation: (a)attrition brecciation, from progressive frictional wear along principal slip surfaces during both seismic and aseismic sliding, (b)distributed crush brecciation, involving microfracturing over broad regions when slip on the principal slip surfaces is impeded by antidilational jogs or other obstructions, and (c)implosion brecciation, associated with the sudden creation of void space and fluid-pressure differentials at dilational fault jogs during earthquake rupture propagation. These last, high-dilation breccias are particularly favorable sites for hydrothermal mineral deposition, forming transitory low-pressure channels for the rapid passage of hydrothermal fluids. Long-lived fault zones often contain an intermingling of breccias derived from all three processes.