Gold vein mineralization occurs in the metamorphosed and
deformed Dalradian (Neoproterozoic) rocks of the Sperrin Mountains, Northern
Ireland. Two structures exerted a control on the location of the
mineralization; the north-south Omagh lineament and the
west-northwest-east-southeast Curraghinalt lateral ramp in the footwall of
the northeast-southwest Omagh thrust. These are Caledonian structures
resulting from the thrusting of Dalradian rocks over a possibly still active
Ordovician arc. Cathodoluminescence microscopy distinguishes four phases
of vein quartz in the Curraghinalt gold prospect. Fluid inclusion studies
and stable isotope geochemistry have defined the probable fluids responsible
for the precipitation of each quartz phase and associated sulfide and
precious metal mineralization. The initial phase (Q1) appears to have been
associated with the main Caledonian metamorphic event (ca. 470 Ma) and is
nonauriferous. The second phase (Q2) forms an extensive cement to brecciated
early quartz and is believed to have involved a fluid (~15 wt % CO 2 ,
10 wt % NaCl + KCl equiv) with a significant magmatic component of 470 to
400 Ma, which underwent phase separation and dilution with a cooler
formation water. This process resulted in precipitation of the main phase of
gold mineralization characterized by an assemblage of electrum, pyrite,
arsenopyrite, chalcopyrite, tennantite-tetrahedrite, and various tellurides.
Similar fluids are observed on a regional scale, concentrated within the
hanging wall of the Omagh thrust, indicating an extensive fluid-flow event.
The relative abundance of gold at the Curraghinalt and Cavanacaw prospects
is thought to be due to higher fluid fluxes in favorable zones of dilation
and closer proximity to the fluid source. The deposit was subsequently reactivated with the
precipitation of later quartz (Q3-Q4) from a formation water believed to be
resident in the Dalradian metasediments, which mixed with a low-temperature,
high-salinity basinal brine, probably during Carboniferous basin inversion.
Brine flow resulted in the remobilization of earlier electrum, reducing its
fineness, and also introduced base metal sulfides, carbonates, and barite.
Again, brine flow is localized by the Omagh thrust, indicating the
long-lived role of this structure in controlling regional fluid migration.
The highly eroded 23 km diameter Rochechouart impact structure, France, has extensive evidence for post-impact hydrothermal alteration and sulphide mineralisation. The sulphides can be divided into four types on the basis of their mineralogy and host rock. They range from pyrites and chalcopyrite in the underlying coherent crystalline basement to pyrites hosted in the impactites. Sulphur isotopic results show that δ34S values vary over a wide range, from −35.8‰ to +0.4‰. The highest values, δ34S −3.7‰ to +0.4‰, are recorded in the coherent basement, and likely represent a primary terrestrial sulphur reservoir. Sulphides with the lowest values, δ34S −35.8‰ to −5.2‰, are hosted within locally brecciated and displaced parautochthonous and autochthonous impactites. Intermediate δ34S values of −10.7‰ to −1.2‰ are recorded in the semi-continuous monomict lithic breccia unit, differing between carbonate-hosted sulphides and intraclastic and clastic matrix-hosted sulphides. Such variable isotope values are consistent with a biological origin, via bacterial sulphate reduction, for sulphides in the parautochthonous and autochthonous units; these minerals formed in the shallow subsurface and are probably related to the post impact hydrothermal system. The source of the sulphate is likely to have been seawater, penecontemporaneous to the impact, as inferred from the marginal marine paleogeography of the structure. In other eroded impact craters that show evidence for impact-induced hydrothermal circulation, indirect evidence for life may be sought isotopically within late-stage (≤120 °C) secondary sulphides and within the shocked and brecciated basement immediately beneath the transient crater floor.
The Pliocene Silver Creek porphyry Mo deposit at Rico, southwest Colorado, is the youngest known porphyry Mo deposit in the Colorado mineral belt. Associated sulfate replacement deposits, epithermal Pb-Zn-Ag vein and replacement deposits, and Recent hot springs also occur. We investigated the S, C, Sr, and Pb sources at the various levels of this well-preserved hydrothermal system by isotopic studies of the mineralization, and of sedimentary and igneous rocks in the area.Hydrothermal sulfide delta 34 S values are tightly grouped at ca. 0 per mil (x = 0.3 + or - 2.0ppm, 1Sigma ; n = 107) and are substantially lower than those of the hydrothermal sulfates (x = 15.2 + or - 1.6ppm, 1Sigma ; n = 14). Pliocene hydrothermal carbonate delta 13 C values (-7.9 to +1.1; x = -4.9 + or - 2.7ppm, 1Sigma ; n = 16) are similar to those for the Pennsylvanian Hermosa Formation carbonate (delta 13 C = -6.7 to +2.6ppm; x = -3.0 + or - 3.2, 1Sigma ; n = 5), which hosts most of the base and precious metal mineralization, whereas Recent hot spring tufas have somewhat higher values (delta 13 C = 1.8-5.5ppm; x = 3.7 + or - 1.3ppm, 1Sigma ; n = 5). Pliocene hydrothermal carbonates have delta 18 O values between -5.1 and 12.3 per mil (x = 5.0 + or - 6.3ppm, 1Sigma ; n = 16). The tufas have higher delta 18 O values (x = 15.4 + or - 2.6ppm, 1Sigma ; n = 5).Hydrothermal carbonates (including tufa) and sulfate have initial 87 Sr/ 86 Sr (sub (i)) values between 0.70700 and 0.71168. Pennsylvanian sedimentary rocks in the Rico district had 87 Sr/ 86 Sr ratios between 0.70877 and 0.71694 at the time of mineralization. Fresh and altered Pliocene monzonite intrusions at Rico have initial 87 Sr/ 86 Sr (sub (4)) values of 0.70581 and 0.70563, respectively, whereas variably altered rholite dikes have initial 87 Sr/ 86 Sr (sub (4)) values of 0.70987, 0.71574, and 0.71716. The 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 206 Pb/ 204 Pb ratios of molybdenite, pyrite, and galena taken together are 18.27 to 19.65, 15.55 to 15.70, and 37.75 to 38.62. Pliocene lamprophyre and rhyolite dikes at Rico have equivalent Pb isotope ratios of 18.26, 15.48, and 37.81 and 18.79, 15.65, and 38.02, respectively. These Pb isotope data are typical of Tertiary volcanic rocks and associated sulfides in southwest Colorado.Our delta 34 S data indicate a magmatic S source for the porphyry Mo and epithermal base metal sulfide mineralization. Most of the sulfide Pb is also of magmatic origin. In contrast, hydrothermal sulfate replacement deposits peripheral to the porphyry Mo deposit, carbonate gangue from the base metal deposits, and the Recent hot spring tufa derived their respective S, C, and Sr from local sedimentary rocks. Our delta 18 O data indicate that the hydrothermal carbonates were deposited from 18 O-shifted meteoric waters. Our comprehensive isotopic data set presents a similar genetic picture to that of other molybdenite deposits in the mineral belt, in that the porphyry Mo mineralization was dominated by magmatic components, whereas the peripheral base metal mineralization contains both magmatic and upper crustal components. These important genetic similarities are independent of fundamental differences in tectonic setting.
The University of Glasgow has a long tradition of scientific endeavour in the Gregory Rift Valley. This paper details some of the history and inspiration behind current hydrological efforts and details results from a 2016 field excursion to this region. A range of surface and ground waters were sampled and analysed for physical, chemical, and stable isotope composition as scoping investigation into geothermal-related hydrological systems. The results allow us to make some initial observations that will be followed up by additional multi-seasonal data collection. Our initial results show clear chemical and isotopic signals for river, lake, hot spring and Menengai geothermal well waters.
Abstract Metalliferous (Fe–Cu–Pb–Zn) quartz–carbonate–sulphide veins cut greenschist to epidote–amphibolite facies metamorphic rocks of the Dalradian, SW Scottish Highlands, with NE–SW to NW–SE trends, approximately parallel or perpendicular to regional structures. Early quartz was followed by pyrite, chalcopyrite, sphalerite, galena, barite, late dolomite–ankerite and clays. Both quartz–sulphide and carbonate vein mineralisation is associated with brecciation, indicating rapid release of fluid overpressure and hydraulic fracturing. Two distinct mineralising fluids were identified from fluid inclusion and stable isotope studies. High temperature (>350°C) quartz‐precipitating fluids were moderately saline (4.0–12.7 wt.% NaCl equivalent) with low (approximately 0.05). Quartz δ 18 O (+11.7 to +16.5‰) and sulphide δ 34 S (−13.6 to −1.1‰) indicate isotopic equilibrium with host metasediments (rock buffering) and a local metasedimentary source of sulphur. Later, low‐temperature ( T H = 120–200°C) fluids, probably associated with secondary carbonate, barite and clay formation, were also moderately saline (3.8–9.1 wt.% NaCl equivalent), but were strongly enriched in 18 O relative to host Dalradian lithologies, as indicated by secondary dolomite–ankerite (δ 18 O = +17.0 to +29.0‰, δ 13 C = −1.0 to −3.0‰). Compositions of carbonate–forming fluids were externally buffered. The veins record the fluid–rock interaction history of metamorphic host rocks during cooling, uplift and later extension. Early vein quartz precipitated under retrograde greenschist facies conditions from fluids probably derived by syn‐metamorphic dehydration of deeper, higher‐grade rocks during uplift and cooling of the Caledonian metamorphic complex. Veins are similar to those of mesothermal veins in younger Phanerozoic metamorphic belts, but are rare in the Scottish Dalradian. Early quartz veins were reactivated by deep penetration of low‐temperature basin fluids that precipitated carbonate and clays in veins and adjacent Dalradian metasediments throughout the SW Highlands, probably in the Permo‐Carboniferous. This event is consistent with paragenetically ambiguous barite with δ 34 S characteristic of late Palaeozoic basinal brines.
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.
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