Host Sediments

In addition to Au, a variety of Au-Sb, Au-Ag and Bi-Te alloys accompanied post-D2 arsenopyrite precipitation. Au and Bi-Te alloys are also locally concentrated in the biotite-rich margins of tonalite dykes that were contaminated by the host sediments. These alloys also appear to be concentrated within the HGA and are locally associated with disseminated scheelite and F-apatite. The latter also occur in late-stage veins that cross-cut weakly foliated granodiorite stocks that lie immediately beneath the HGA. A combination of BSE and CL imaging reveals that precipitation of these post-D2 sulfides and alloys occurred within a micro-porosity network that records dissolution-precipitation reactions... 

Logan GA, Hinman MC, Walter MR, Summons RE, Biogeochemistry of the 1640 Ma McArthur River (HYC) lead-zinc ore and host sediments. Northern Territory, Australia, Geochim Cosmochim Acta 65 2317-2336, 2001. 

Spontaneous potential Caliper log Compared to a free gas bearing zone, the spontaneous potential log is less negative, but similar to that of ice An oversized drill hole is indicated by hydrate dissociation. This may also occur with ice in the permafrost. However, as noted by Kleinberg (Personal Communication, July 20, 2006) In some circumstances, hydrates can be a good cement (until it dissociates), and strengthen the unconsolidated host sediment ... 

Hydrate inferred at Site 995 existed as fine-grain, pore-filling accumulations that were widely dispersed in host sediments rather than as concentrated nodules large enough to survive the coring/recovery process. Gas voids and expansions were noted in several core samples, and the amount of methane recovered exceeded that expected from methane saturation of the interstitial waters at in situ pressures. 

Since the Precambrian, the Sr/ Sr of seawater has fluctuated between —0.7070 and —0.7092 as the result of variations in the relative rates of input of Sr-enriched strontium from continental weathering and Sr-depleted strontium from mantle sources. Fluids in sedimentary basins containing Paleozoic strata typically have Sr/ Sr ratios in excess of seawater values that are contemporaneous or coeval with the deposi-tional age of the current host sediment. This is well illustrated by the data of Connolly et al. (1990) for the Alberta Basin, Canada. The enrichment is due to the release of strontium attending the alteration of silicates. Due to the significant increase of Sr/ Sr in seawater since the Jurassic, some formation waters in Cenozoic sedimentary basins actually have Sr/ Sr ratios lower than those of contemporaneous seawater due to the addition of strontium dissolved from older and deeper sedimentary sources... 

Because the SEDEX scheme, like all sequential extraction schemes, is operationally defined, it is important to obtain corroborating evidence for the identity of the separately quantified sedimentary phosphorus reservoirs whenever possible. This can be accomplished by analyzing pore-water chemistry and other solid-phase components of the host sediments, and by linking depth profiles of various solutes and components to SEDEX phosphorus profiles. Some of the most elegant and comprehensive work of this type has been done by... 

Mottled Equivalent of nodular features where the host sediment is carbonate-dominated. It consists of... 

Laminar Consists of sheets of laminated carbonate, in which the laminae are on a millimetre-scale. The term is synonymous with soilstone crust . Some forms are dark in colour. They commonly occur capping hardpan layers but can also occur within chalky layers or in the host sediment or soil. They can be interlayered with pisolitic calcretes on a centimetre-scale. Most are only a few centimetres thick but some forms can reach 2 m and can be... 

Pigments as a Portion of Total Organic Carbon. The value obtained through the division of tetrapyrrole pigment concentration, in pg-sediment dry weight, by the percent organic carbon of the host sediment we have defined as the Pigment Yield Index, or PYI (44). 

Analysis of samples taken from different parts of the carbonate concretions, and from the adjacent host sediment at site 93LD, showed that organic carbon content is higher within the concretions than in the surrounding sediments (Figure 7.17a). Little difference was found between... 

Figure 7.17 Variation in geochemical parameters and indicators of microbial activity within different parts of the concretion and host sediment from site 93LD (a) % organic carbon and viable microbial biomass, (b) different forms of extractable iron, (c) % acid volatile sulphide (AVS) and % total reduced sulphur (TRS).

Figure 7.18 Variation in microbial community structure and indicators of environmental stress within different parts of the concretion and host sediment from profile 93LD (a) community structure -TBS = terminally branched saturates, MBS = mid-branched saturates Poly = polyenoics, Total Mono = total monoenoics, BMono = branched monoenoics. Sedi-far represents the host sediment at some distance from the concretion but at the same stratigraphic level (b) indicators of nutritional stress (c) biomarkers for Desu/fobacfer<10Me16 0) and Desulfovibrio

At temperatures greater than a 100°C, thermal degradation of carboxylic acids produces methane and carbon dioxide (Surdam et ai, 1984). As the carboxylic acid anions are consumed due to increasing temperature, the carbonate system becomes internally buffered, and thus the pH may decrease due to increased in the system, leading to carbonate dissolution and the enhancement of secondary porosity (Surdam et ai, 1984). Factors influencing the thermal destruction rate of organic acids include coupled sulphate reduction and hydrocarbon oxidation, and the mineralogy of host sediments (Bell, 1991) the presence of hematite causes rapid rates of acetic acid decomposition. 

A variety of carbonate cements occurs in the decarboxylation zone depending on the mineralogy of the host sediments and earlier formed carbonates, as well as incursion by deep-seated thermobaric waters. Sediments containing abundant reactive, detrital Fe-minerals result in the formation of... 

Unlike calcite and dolomite, siderite rarely forms as an extensive pore-filling cement, but rather as discrete fine crystals, spherules and nodules scattered in the host sediments. Nevertheless, Baker et al. (1996) found that early diagenetic siderite concretions (0.5-2 mm) form up to 30% of Triassic sandstones and mudstones from eastern Australia. Laterally continuous siderite-cemented offshore shelf sandstone sheets (15 cm thick) occur in Upper Cretaceous sequences from Canada (McKay et ai, 1995). 

Although the presence of cements along fractures is indicative of water flow, precipitation does not necessarily occur by advection, but rather by ionic diffusion from the host sediments. Advective cementation requires the circulation of huge water volumes. For each pore volume of cement, 10 to 10 water volumes are required (Bathurst, 1975 Wood, 1986 Sharp et al., 1988). The distinction between cements formed by diffusive and advective... 

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