The mechanism of heat extraction from the lower oceanic crust near the ridge axis is poorly constrained despite its importance for understanding both the process of accretion of the plutonic complex and the mass fluxes associated with ridge hydrothermal systems. We have investigated the role of zones of focussed fluid flow in the plutonic complex of the Oman ophiolite in the near-axis cooling of the oceanic crust. Lineaments identified on aerial photographs, that occur at ∼1 km spacing, show evidence for extensive hydrothermal fluid flow through regions ∼10 to 50 m wide. Fluid flow is initiated in these regions at ∼800°C and continues at least into the lower greenschist facies. Strontium-isotope analyses indicate that the fluid flux through these zones is sufficient to transport a metasomatic front from the base of the sheeted dike complex to close to the Moho. Computed *minimum* fluid fluxes to transport a metasomatic front through the focussed fluid flow zones are ∼1x10^8^ kgm^−2^. Modeling of diffusive exchange of calcium from olivine to clinopyroxene indicates enhanced cooling rates adjacent to the focussed fluid flow zones. Heat fluxes estimated from the enhanced cooling rates are broadly consistent with the fluid fluxes determined from modeling the Sr-isotopic composition of samples from the focussed fluid flow zones. The combination of independent estimates of the fluid and heat fluxes, such as these, can provide more rigorous constraints on the thermal history than either approach used in isolation. Our results show that focussed fluid flow could play a major role in the cooling in the lower oceanic crust. Significant focussed fluid flow in the lower oceanic crust has important implications for predicting the total mass flux associated with hydrothermal circulation at mid-ocean ridges. This is because fluids flowing through channels become chemically rock-buffered at smaller fluid fluxes than those flowing pervasively through a rock mass. Thus, if focussed fluid flow is an important mechanism of heat loss from the lower oceanic crust the chemical fluxes from ridge hydrothermal systems into the oceans may be smaller than currently thought.
Significant tellurium enrichment occurs in many orogenic gold deposits but the factors causing this are little understood; some authors suggest this demands a magmatic input whereas others suggest it need not. Fractionation of Te from Se and S could offer insight into source/pathway processes of auriferous fluids. The metasedimentary-hosted Cononish vein gold deposit, Scotland, is unusually Te-rich compared to many orogenic gold deposits with Te/Au ≈ 2.4 whereas most orogenic deposits have Te/Au < 1. Here, Ag in Au-Ag alloy increases from ∼10 to 90 wt% through the paragenesis, correlating with decreasing hessite (Ag2Te) abundance. This suggests the Au-Ag alloy composition was controlled by the fluid Te activity, and that this decreased through time. This is coupled to an increase in pyrite δ34S from −2.0‰ to +11.4‰ through the paragenesis. Thus, the deposit formed from a primary fluid with a low-δ34S and high Te + Au + Ag that evolved to a high δ34S-low Te, Pb + Cu bearing fluid. The high δ34S of the later fluid suggests it can only be sourced from specific nearby metamorphosed SEDEX horizons. The early fluid that deposited most of the gold could be sourced from other metasedimentary units in the stratigraphy or be magmatic in origin. We argue that two observations taken together suggest it is most likely that this fluid was magmatic; the age of the mineralisation is identical to the last stage of crystallization of nearby granite batholiths, and the fluid has a S-isotope signature consistent with a magmatic source. Gold deposits in orogenic belts are almost certainly polygenetic and this study demonstrates evidence for Te-rich "orogenic" deposits having a significant magmatic component.
Pyrite is one of the most common minerals in many precious and base metal hydrothermal ore deposits and is an important host to a range of trace elements including Au and Co and the semi-metals As, Se, Sb, Te and Bi. As such, in many hydrothermal ore deposits, where pyrite is the dominant sulphide phase, it can represent a major repository for these elements. Furthermore, the concentrations and ratios of Au, As and Co in pyrite have been used to infer key ore-forming processes. However, the mechanisms controlling the distribution of Te and Se in pyrite are less well understood. Here we compare the Te and Se contents of pyrite from a global dataset of Carlin-type, orogenic Au, and porphyry-epithermal deposits to investigate: (1) the potential of pyrite to be a major repository for these elements; and (2) whether Te and Se provide insights into key ore-forming processes. Pyrite from Carlin-type, low-sulphidation and alkaline igneous rock-hosted epithermal systems is enriched in Te (and Se) compared to pyrite from high-sulphidation epithermal and porphyry Cu deposits. Orogenic Au pyrite is characterised by intermediate Te and Se contents. There is an upper solubility limit for Te as a function of As in pyrite, similar to that established for Au by Reich et al. (2005); and this can be used to identify Te present as telluride inclusions, which are common in some epithermal-porphyry and orogenic Au deposits. Physicochemical fluid parameters, such as pH, redox and temperature, as well as crystal-chemistry control the incorporation and concentration of Se and Te in pyrite. Neutral to alkaline fluids have the ability to effectively mobilise and transport Te. Fluid boiling in porphyry-epithermal systems, as well as wall rock sulphidation and oxidation in Carlin-type (and orogenic Au) deposits can effectively precipitate Te in association with pyrite and Au. In contrast, Se concentrations in pyrite apparently vary systematically in response to changes in fluid temperature, irrespective of pH and fO2. Hence, we propose that the Se contents of pyrite may be used as a new geo-thermometer for hydrothermal ore deposits. Furthermore, the comparison of bulk ore and pyrite chemistry indicates that pyrite represents the major host for Te and Se in Carlin-type and some epithermal systems, and thus pyrite can be considered to be of economic interest as a potential source for these elements.
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