The Hemlo gold deposit,Ontario: A geochemical and isotopic study

The Hemlo deposit, near Marathon, Ontario, is one of the largest gold deposits in North America. It is stratiform within Archean metamorphosed volcano-sedimentary rocks. The main ore zone is composed of pyritic, sericitic schist, and massive barite. This is the first report of stratiform barite in the Archean of North America, but other occurrences have since been found west of Hemlo. The mineralization is substantially enriched in Au, Mo, Sb, Hg, Tl and V and lacks carbonate. Because of metamorphism and deformation of the body its genesis is uncertain.${}^{87}\text{Sr}{}^{86}\text{Sr}$ of .7017 for barite from the deposit is similar to that of the sedimentary barite west of Hemlo and to initial ratios of contemporaneous volcanic rocks. At the base of the main ore zone, barite with δ^34S of +8 to +12%. was deposited with ~0%. pyrite. Upward, both barite and pyrite get isotopically lighter, with minimum values for pyrite, to ?17.5%, in non-baritic schist forming the upper part of the ore zone. In drill section, Au grades correlate with the isotopic composition of pyrite. This, and the association of fractionated sulphide with sulphate, suggests that Au, pyrite and barite were deposited contemporaneously. The linked, asymmetric distributions of S minerals and isotopic distributions, which are continuous from section to section, and the isotopic similarity of the Hemlo and western barites are consistent with a syngenetic depositional model.Two sources for the S minerals are considered. In the first, exogenous sulphate from a restricted basin were partially reduced in a geothermal system to form ³⁴S-depleted sulphide. In the second, the sulphate and sulphide are of magmatic-hydrothermal origin. Sulphate and fractionated sulphide are uncommon in Archean rocks, but one or both occur with unusual frequency in major Archean gold deposits. Hydrothermal fluids of moderately high , containing sulphate and permitting isotopic fractionation between oxidized and reduced S species, may have favoured the dissolution, transport and precipitation of Au.     

Dislocation microstructures in naturally deformed silicate garnets

Dislocation microstructures of naturally deformed silicate garnets and olivines in garnet-peridotites and silicate garnets in eclogites from four localities have been observed with a transmission electron microscope (TEM) to clarify the dislocation characteristics of silicate garnets. We have obtained the following results: (1) dislocation densities of garnets in all the garnet-peridotites (ρ = 10⁵−10⁷ cm⁻²) are always nearly an order of magnitude lower than those of co-existing olivines; (2) dislocation densities of garnets in eclogites (ρ = 10⁵−10⁸ cm⁻²) which are embedded in garnet-peridotites are almost an order of magnitude higher than those of garnets in the surrounding garnet-peridotites; (3) the dominant Burgers vector, b, of mainly edge dislocations in garnet is 100 for specimens with dislocation density ρ = 10⁵−10⁶ cm⁻², while b=1/2111 for specimens with ρ = 10⁷−10⁸ cm⁻². Result (1) indicates that the observed dislocations in garnets were formed by plastic deformation under the same stresses as for co-existing olivines, and that there is a similar relationship between applied stress and dislocation density for garnets as for olivines. Result (2) suggests that the stress concentration occurred around eclogites embedded in garnet-peridotites, and the resulting differential stress in garnets in eclogites was further elevated by the interlocking of neighboring hard garnet grains. Finally, result (3) indicates that the dominant Burgers vector of mainly edge dislocations in garnet changes from 100 to 1/2111 with increasing applied differential stress.     

An experimental study on active tendon control of cable-stayed bridges

Active tendon control of cable-stayed bridges subject to a vertical sinusoidal force is experimentally and analytically studied. Emphasis is placed on the effects of linear and non-linear internal resonances on the control (due to the presence of the cable vibration). A simple cable-supported cantilever beam is used as a model. It is found that active tendon control is very effective in vertical girder motion with small cable vibration (girder dominated motion), whereas it is not effective in vertical girder motion with large cable vibration (cable dominated motion). Analytical prediction is very satisfactory except for the latter case.     

Instability due to time delay and its compensation in active control of structures

Time delay problem and its compensation in active control of civil engineering structures were studied. It has been shown by stability analysis of a SDOF system with time delayed feedback that the maximum allowable time delay depends not only on the natural period of the structure but also on feedback gains. We have demonstrated by numerical simulation that the performance of the control system degrades significantly when the time delay is close to this value and it even becomes unstable when time delay is greater than or equal to this value. The maximum allowable time delay decreases with decrease in natural period of the structure as well as with increase in active damping. The paper presents a technique for compensation by modelling time delay as transportation lag. This method ensures the stability of the controlled system as well as the desired response reduction.     

Synchronization of human walking observed during lateral vibration of a congested pedestrian bridge

Observation of human-induced large-amplitude lateral vibration of an actual pedestrian bridge in an extremely congested condition is reported. Walking motions of pedestrians recorded by a video camera are analysed. It is found that walking among 20 per cent or more of the pedestrians on the bridge was synchronized to the girder lateral vibration. With this synchronization, the total lateral force from the pedestrians to the girder is evidently increased and it acts as a resonant force on the girder lateral vibration.     

On a time-symmetric Hermite integrator for planetary N-body simulation

We describe a P(EC) ⁿ Hermite scheme for planetary N -body simulation. The fourth-order implicit Hermite scheme is a time-symmetric integrator that has no secular energy error for the integration of periodic orbits with time-symmetric time-steps. In general N -body problems, however, this advantage is of little practical significance, since it is difficult to achieve time-symmetry with individual variable time-steps. However, we can easily enjoy the benefit of the time-symmetric Hermite integrator in planetary N -body systems, where all bodies spend most of the time on nearly circular orbits. These orbits are integrated with almost constant time-steps even if we adopt the individual time-step scheme. The P(EC) ⁿ Hermite scheme and almost constant time-steps reduce the integration error greatly. For example, the energy error of the P(EC)² Hermite scheme is two orders of magnitude smaller than that of the standard PEC Hermite scheme in the case of an N  = 100,  m  = 10²⁵ g planetesimal system with the rms eccentricity 〈 e ²〉^1/2 ≲0.03.     

Relative durability of chert and rhyolite detritus in conglomerates and sandstones: with reference to the study of provenance geology

Modal changes in both gravel and sands occur because of unequal durability of some clastic materials to abrasion. Selective abrasion causes an elimination of undurable materials and a complimentary enrichment of competent components. The fluviatile Oligocene Noda Group, northeast Japan, consists of two formations that include both the competent and undurable clastics. Modal analyses of rhyolite‐ or chert‐rich sediments in both formations show that the amount of pebble‐ and granule‐size chert detritus is nearly constant, but the sand‐size chert fragments remarkably decrease with the decreasing grain size. The amount of rhyolite pebbles and the sands, however, is nearly unchangeable despite the variable clast size. Results of our study reveal that we need careful thoughts for a restricted size of sediments and the mode of fine‐grained detritus that include undurable chert sands tends to significantly underestimate the outcrop area of chert bedrocks in the provenance.     

Nanometer‐scale anatomy of entire Stardust tracks

Abstract– We have developed new sample preparation and analytical techniques tailored for entire aerogel tracks of Wild 2 sample analyses both on “carrot” and “bulbous” tracks. We have successfully ultramicrotomed an entire track along its axis while preserving its original shape. This innovation allowed us to examine the distribution of fragments along the entire track from the entrance hole all the way to the terminal particle. The crystalline silicates we measured have Mg‐rich compositions and O isotopic compositions in the range of meteoritic materials, implying that they originated in the inner solar system. The terminal particle of the carrot track is a ¹⁶O‐rich forsteritic grain that may have formed in a similar environment as Ca‐, Al‐rich inclusions and amoeboid olivine aggregates in primitive carbonaceous chondrites. The track also contains submicron‐sized diamond grains likely formed in the solar system. Complex aromatic hydrocarbons distributed along aerogel tracks and in terminal particles. These organics are likely cometary but affected by shock heating.     

软流圈熔体地幔交代作用过程中的铂族元素行为

南美洲南部的Pali Aike火山岩区第四纪碱性玄武岩中普遍发育含石榴石的斜方辉石岩包体。这种斜方辉石岩既作为独立的捕掳体存在又以细脉的形式穿插于橄榄岩捕掳体中。斜方辉石岩普遍含富Ti矿物,并且次生斜方辉石含橄榄石和单斜辉石残晶。与含石榴石橄榄岩中的斜方辉石相比,这种次生的斜方辉石以高TiO2、中等含量的Al2O3以及低Mg#为特征,表明它是在一种高度分异演化的富Ti熔体交代作用下通过消耗橄榄石和单斜辉石方式形成的。斜方辉石岩全岩的Co、Ni略低,Cr和铂族元素(PGE)含量与地幔橄榄岩相当,表明这些元素在交代作用过程中相对稳定,而交代介质带入的组分以碱质(K2O+Na2O)、Ti、Si、Al和S为主。交代的斜方辉石在现代活动岛弧和古克拉通的地幔橄榄岩捕掳体中多有报道。与这些环境中地幔样品的斜方辉石相比,PaliAike地区的次生斜方辉石含有相对高的Ti和Al,以及相对低的Mg。高Ti低Mg属性反映了交代介质可能来源于下伏的软流圈地幔并且经历了高度的分异和演化过程。Pali Aike地区所见到的这种交代斜方辉石和斜方辉石岩在其他被上涌软流圈影响的陆下岩石圈地幔中可能普遍存在。这些研究对了解中国华北-东北中生代以来的岩石圈地幔减薄机制有着重要的借鉴意义。