Proterozoic low-sulfidation epithermal Au-Ag mineralization in the Mallery Lake area, Nunavut, Canada

The Mallery Lake area contains pristine examples of ancient precious metal-bearing low-sulfidation epithermal deposits. The deposits are hosted by rhyolitic flows of the Early Proterozoic Pitz Formation, but are themselves apparently of Middle Proterozoic age. Gold mineralization occurs in stockwork quartz veins that cut the rhyolites, and highest gold grades (up to 24 g/t over 30 cm) occur in the Chalcedonic Stockwork Zone. Quartz veining occurs in two main types: barren A veins, characterized by fine- to coarse-grained comb quartz, with fluorite, calcite, and/or adularia; and mineralized B veins, characterized by banded chalcedonic silica and fine-grained quartz, locally intergrown with fine-grained gold or electrum. A third type of quartz vein (C), which crosscuts B veins at one locality, is characterized by microcrystalline quartz intergrown with fine-grained hematite and rare electrum. Fluid inclusions in the veins occur in two distinct assemblages. Assemblage 1 inclusions represent a moderate temperature (Th=150 to 220 °C), low salinity (~1 eq. wt% NaCl, with trace CO₂), locally boiling fluid; this fluid type is found in both A and B veins and is thought to have been responsible for Au-Ag transport and deposition. Assemblage 2 inclusions represent a lower temperature (Th=90 to 150 °C), high salinity calcic brine (23 to 31 wt% CaCl₂-NaCl), which occurs as primary inclusions only in the barren A veins. Assemblage 1 and 2 inclusions occur in alternating quartz growth bands in the A-type veins, where they appear to represent alternating fluxes of dilute fluid and local saline groundwater. No workable primary fluid inclusions were observed in the C veins. The A-vein quartz yields '¹⁸O values from 8.3 to 14.5‰ (average=10.9ǃ.7‰ [1C], n=30), whereas '¹⁸O values for B-vein quartz range from 11.2 to 14.0‰ (average=13.0ǂ.9‰, n=12). Calculated '¹⁸O_H2O values for the dilute mineralizing fluid from B veins range from -2.6 to 0.2‰ (average=-0.8ǂ.9‰, n=12) and are consistent with a dominantly meteoric origin. No values could be calculated for the brine, however, because all A-vein quartz samples contain mixed fluid inclusion populations. However, the fact that A-vein quartz samples extend to lower '¹⁸O values than the B veins suggests that the brine had a lighter isotopic signature relative to the dilute fluid. Hydrogen isotopic ratios of fluid inclusion waters extracted from eleven quartz samples of both vein types range from 'D_FI=-56 to -134‰, but show no particular correlation with vein type. In most respects, the mineralogical and fluid characteristics of the Mallery Lake system are comparable to those of Phanerozoic low-sulfidation deposits, and although the presence of high salinity brines is unusual in such deposits, it is not unknown (e.g., Creede, Colorado). In addition, one of the few other examples of well-preserved, Precambrian, low-sulfidation epithermal deposits, from the Central Pilbara tectonic zone, Australia, contains a similarly bimodal fluid assemblage. The significance of these saline brines is not clear, but from this study we infer that they were not directly involved with Au-Ag transport or deposition.     

Rare earth element diffusion in diopside: influence of temperature, pressure, and ionic radius, and an elastic model for diffusion in silicates

Volume diffusion rates for five rare earth elements (La, Ce, Nd, Dy, and Yb) have been measured in single crystals of natural diopside at pressures of 0.1 MPa to 2.5 GPa and temperatures of 1,050 to1,450 °C. Polished, pre-annealed crystals were coated with a thin film of rare earth element oxides, then held at constant temperature and pressure for times ranging from 20 to 882 h. Diffusion profiles in quenched samples were measured by SIMS (secondary ion mass spectrometry) depth profiling. At 1 atm pressure, with the oxygen fugacity controlled near the quartz-fayalite-magnetite buffer, the following Arrhenius relations were obtained for diffusion normal to (001) (diffusion coefficient D in m²/s): log₁₀D_Yb=(-4.64ǂ.42)-(411ᆠ kJ/mol/2.303RT); log₁₀D_Dy=(-3.31ǃ.44)-(461ᆽ kJ/mol/2.303RT); log₁₀D_Nd=(-2.95DŽ.64)-(496ᇡ kJ/mol/2.303RT); log₁₀D_Ce=(-4.10ǃ.08)-(463ᆳ kJ/mol/2.303RT); log₁₀D_Lu=(-4.22DŽ.66)-(466ᇢ kJ/mol/2.303RT). Diffusion rates decrease significantly with increasing ionic radius, with La a factor of ~35 slower than Yb. The relationship between diffusivity and ionic radius is consistent with a model in which elastic strain plays a critical role in governing the motion of an ion through the crystal lattice. Activation volumes for Yb and Ce diffusion, at constant temperature and oxygen fugacity, are 9.0DŽ.0 cm³/mol and 8.9Dž.2 cm³/mol, respectively, corresponding to an order of magnitude decrease in diffusivity as pressure is increased from 0 to 3 GPa at 1,200 °C. Diffusion of Nd is such that grain-scale isotopic equilibrium in the mantle can be achieved in ~1 My under conditions near the peridotite solidus (~1,450 °C at 2.5 GPa). The equilibration time is much longer under P, T conditions of the lithospheric mantle or at the eclogite solidus (~1 Gy at 1.5 GPa and 1,150 °C). Because of the relatively strong decrease in diffusivity with pressure (two orders of magnitude between 2.5 and 15 GPa along an adiabatic temperature gradient), Nd transport in clinopyroxene will be effectively frozen at pressures approaching the transition zone, on time scales less than 100 My. Rare earth element diffusion rates are slow enough that significant disequilibrium uptake of REE by growing clinopyroxene phenocrysts may be preserved under natural conditions of basalt crystallization. The relative abundances and spatial distributions of REE in such crystals may provide a sensitive record of the cooling and crystallization history of the host lava.     

Palaeotemperatures and Diagenetic Phases of the Upper Triassic Oil-bearing Sandstones in the Eastern Part of the Ordos Basin

The maximum palaeotemperature of oil-bearing sandstones in the UpperTriassic in the eastern Ordos basin has been determined by using many methods including thevitrinite reflectance, fluid inclusion, apatite fission track, illite crystallinity, chlorite polytypeand diagenetic change of authigenic minerals. The thermal gradient in the Late Mesozoic wasabout 2.9-3.0℃/100m. The Upper Triassic was in a mature stage of organic matter andhydrocarbon began to be generated and migrated during this period. The palaeotemperatures ofoil-bearing sandstones were in the range of 88-110℃; those for the generation and migrationof oil ranged from 112 to 122℃. The thickness of the denuded strata overlying the UpperTriassic was 2465-2750m. The present burial depth of oil-bearing sandstones is generally from400 to 1200m. At a depth of ca. 1900m, the temperature may reach 140℃. Below this depth,organic matter was supermature and mainly generated gas.     

The Middle to Upper Jurassic stable isotope record of Madagascar: Linking temperature changes with plate tectonics during the break-up of Gondwana

Stable isotope (δ¹⁸O, δ¹³C) analyses were performed on well preserved belemnites, oysters, and rhynchonellid brachiopods from the Middle to Upper Jurassic of the Morondava Basin in southern Madagascar. Both brachiopods and oysters indicate similar average temperatures of 18.7 to 19.3 °C in the Early Callovian, followed by a temperature decrease towards the Middle Oxfordian (13.9 °C) and a minimum in the Early Kimmeridgian (12.3 °C). In contrast, belemnites from the Oxfordian show lower average temperatures of 10.0 °C, which is likely caused by specific conditions for these organisms (e.g., different fractionation or life habits). Additionally, three oysters from the Upper Oxfordian and Lower Kimmeridgian were used for high-resolution stable isotope analyses. The data show seasonal fluctuations of >6 °C around averages between 14.4 and 14.7 °C. Latitudinal temperature gradients for the Callovian and Kimmeridgian are similar to today at the examined low latitudes of the southern hemisphere. The observed cooling of around 5 °C from the Callovian to the Oxfordian/Kimmeridgian can be attributed to a concurrent southward drift of Madagascar during the break-up of Gondwana. Thus, the study underlines the importance of considering palaeogeography in interpreting stable isotope data as well as the potential of detecting and timing palaeogeographic events by using stable isotope analyses.     

Geochemistry of Mesozoic plutons, southern Death Valley region, California: Insights into the origin of Cordilleran interior magmatism

Mesozoic granitoid plutons in the southern Death Valley region of southeastern California reveal substantial compositional and isotopic diversity for Mesozoic magmatism in the southwestern US Cordillera. Jurassic plutons of the region are mainly calc-alkaline mafic granodiorites with )_Ndi of -5 to -16, ⁸⁷Sr/⁸⁶Sr_i of 0.707-0.726, and ²⁰⁶Pb/²⁰⁴Pb_i of 17.5-20.0. Cretaceous granitoids of the region are mainly monzogranites with )_Ndi of -6 to -19, ⁸⁷Sr/⁸⁶Sr_i of 0.707-0.723, and ²⁰⁶Pb/²⁰⁴Pb_i of 17.4-18.6. The granitoids were generated by mixing of mantle-derived mafic melts and pre-existing crust - some of the Cretaceous plutons represent melting of Paleoproterozoic crust that, in the southern Death Valley region, is exceptionally heterogeneous. A Cretaceous gabbro on the southern flank of the region has an unusually juvenile composition ()_Ndi -3.2, ⁸⁷Sr/⁸⁶Sr_i 0.7060). Geographic position of the Mesozoic plutons and comparison with Cordilleran plutonism in the Mojave Desert show that the Precambrian lithosphere (craton margin) in the eastern Mojave Desert region may consists of two crustal blocks separated by a more juvenile terrane.     

Organic carbon isotope ratios (δ13C) of Arctic Amerasian Continental shelf sediments

Organic matter origins are inferred from carbon isotope ratios ('¹³C) in recent continental shelf sediments and major rivers from 465 locations from the north Bering-Chukchi-East Siberian-Beaufort Sea, Arctic Amerasia. Generally, there is a cross-shelf increase in '¹³C, which is due to progressive increased contribution seaward of marine-derived organic carbon to surface sediments. This conclusion is supported by the correlations between sediment '¹³C, OC/N, and '¹⁵N. The sources of total organic carbon (TOC) to the Amerasian margin sediments are primarily from marine water-column phytoplankton and terrigenous C₃ plants constituted of tundra taiga and angiosperms. In contrast to more temperate regions, the source of TOC from terrigenous C₄ and CAM plants to the study area is probably insignificant because these plants do not exist in the northern high latitudes. The input of carbon to the northern Alaskan shelf sediments from nearshore kelp community (Laminaria solidungula) is generally insignificant as indicated by the absence of high sediment '¹³C values (-16.5 to -13.6‰) which are typical of the macrophytes. Our study suggests that the isotopic composition of sediment TOC has potential application in reconstructing temporal changes in delivery and accumulation of organic matter resulting from glacial-interglacial changes in sea level and environments. Furthermore, recycling and advection of the extensive deposits of terrestrially derived organic matter from land, or the wide Amerasian margin, could be a mechanism for elevating total CO₂ and pCO₂ in the Arctic Basin halocline.     

Petrographic, REE, fluid inclusion and stable isotope study of magnesite from the Upper Triassic Burano Evaporites (Secchia Valley, northern Apennines): contributions from sedimentary, hydrothermal and metasomatic sources

Sparry and microcrystalline magnesite are minor constituents of the Upper Triassic Burano Evaporite Formation of the northern Apennines in Italy. Petrography and geochemistry of magnesite suggest three modes of formation. (1) Evaporitic precipitation of stratified microcrystalline magnesite layers associated with sulfate and carbonate rocks. Most REE are below ICP-MS detection limits. '¹⁸O is +20.2‰ (SMOW) and '¹³C is -2.6‰ (PDB). (2) Hydrothermal infill of Fe-rich (9.78 wt% FeO) lenticular sparry magnesite. This type of magnesite is characterized by very low LREE concentrations, whereas HREEs are relatively high. The fluid inclusion composition is NaCl-MgCl₂-H₂O, salinity is ~30 wt% NaCl equiv., and total homogenization temperatures range from 204-309 °C; '¹⁸O is +17.5‰ and '¹³C is +1‰. (3) The partial or total replacement of dolostones by lenticular sparry magnesite. LREEs are lower in magnesite compared with the partly replaced dolostones. Magnesite yields '¹⁸O and '¹³C compositions of +17.3 to +23.6‰ and +0.5 to +1.4‰, respectively, whereas the partly replaced dolostones yield '¹⁸O and '¹³C values of +25.0 to +26.2 and +1.3 to +1.9, respectively. Complete replacement of dolostones produced massive lenticular sparry magnesite rock containing ooids and axe-head anhydrite relicts; LREEs are depleted compared to unaffected dolostones; '¹⁸O and '¹³C compositions range from +16.4 to +18.4‰ and +0.4 to +0.9‰, respectively. These data and the association between fracture-filling and replacive magnesite suggests a metasomatic system induced by hydrothermal circulation of hot and saline Mg-rich fluids. These processes probably occurred in the Oligocene-Miocene, when the Burano Formation acted as main detachment horizon for the Tuscan Nappe during the greenschist facies metamorphism of the Apuane complex. Thrusting over the Apuane zone produced large scale fluid flow focused at the Tuscan Nappe front. Sources of Mg-rich fluids were metamorphic reactions in the Apuane complex and dissolution of Mg-salts at the thrust front. Considering a maximum tectonic burial depth of 10 km, as inferred from the geometry of the chain, the pressure-corrected temperature of magnesite precipitation (380 to 400 °C) and the calculated fluid composition ('¹⁸O=+13.3ǃ.2‰) are in the range of the published Apuane metamorphic temperatures (300-450 °C) and fluid compositions ('¹⁸O=7-16‰). The results of this study support the hydrothermal-metasomatic model for the formation of sparry magnesite deposits at the expense of dolostone units involved in thrusting and low-grade metamorphism, as proposed for the Northern Graywacke Zone (Alps) and the Eugui deposit (western Pyrenees).     

Oxygen isotope study of the Long Valley magma system, California: isotope thermometry and convection in large silicic magma bodies

Products of voluminous pyroclastic eruptions with eruptive draw-down of several kilometers provide a snap-shot view of batholith-scale magma chambers, and quench pre-eruptive isotopic fractionations (i.e., temperatures) between minerals. We report analyses of oxygen isotope ratio in individual quartz phenocrysts and concentrates of magnetite, pyroxene, and zircon from individual pumice clasts of ignimbrite and fall units of caldera-forming 0.76 Ma Bishop Tuff (BT), pre-caldera Glass Mountain (2.1-0.78 Ma), and post-caldera rhyolites (0.65-0.04 Ma) to characterize the long-lived, batholith-scale magma chamber beneath Long Valley Caldera in California. Values of '¹⁸O show a subtle 1‰ decrease from the oldest Glass Mountain lavas to the youngest post-caldera rhyolites. Older Glass Mountain lavas exhibit larger (~1‰) variability of '¹⁸O(quartz). The youngest domes of Glass Mountain are similar to BT in '¹⁸O(quartz) values and reflect convective homogenization during formation of BT magma chamber surrounded by extremely heterogeneous country rocks (ranging from 2 to +29‰). Oxygen isotope thermometry of BT confirms a temperature gradient between "Late" (815 °C) and "Early" (715 °C) BT. The '¹⁸O(quartz) values of "Early" and "Late" BT are +8.33 and 8.21‰, consistent with a constant '¹⁸O(melt)=7.8ǂ.1‰ and 100 °C temperature difference. Zircon-melt saturation equilibria gives a similar temperature range. Values of '¹⁸O(quartz) for different stratigraphic units of BT, and in pumice clasts ranging in pre-eruptive depths from 6 to 11 km (based on melt inclusions), and document vertical and lateral homogeneity of '¹⁸O(melt). Worldwide, five other large-volume rhyolites, Lava Creek, Lower Bandelier, Fish Canyon, Cerro Galan, and Toba, exhibit equal '¹⁸O(melt) values of earlier and later erupted portions in each of the these climactic caldera-forming eruptions. We interpret the large-scale '¹⁸O homogeneity of BT and other large magma chambers as evidence of their longevity (>10⁵ years) and convection. However, remaining isotopic zoning in some quartz phenocrysts, trace element gradients in feldspars, and quartz and zircon crystal size distributions are more consistent with far shorter timescales (10²-10⁴ years). We propose a sidewall-crystallization model that promotes convective homogenization, roofward accumulation of more evolved and stagnant, volatile-rich liquid, and develops compositional and temperature gradients in pre-climactic magma chamber. Crystal + melt + gas bubbles mush near chamber walls of variable '¹⁸O gets periodically remobilized in response to chamber refill by new hotter magmas. One such episode of chamber refill by high-Ti, Sr, Ba, Zr, and volatile-richer magma happened 10³-10⁴ years prior to the 0.76-Ma caldera collapse that caused magma mixing at the base, mush thawing near the roof and walls, and downward settling of phenocrysts into this hybrid melt.     

Anhydrite-bearing rocks from the Rožná district (Moldanubian zone,Czech Republic): high-grade metamorphosed exhalites?

Several types of anhydrite-bearing rocks have been found in the amphibolite-facies metamorphosed rocks at the north-eastern margin of the Moldanubian Zone. Anhydrite either forms monomineralic bands up to 40 cm thick, or occurs in the form of disseminated grains in surrounding calc-silicate gneiss together with feldspar, scapolite, amphibole, pyroxene, epidote and pyrite. The isotopic composition of sulphur ('³⁴S=30.6 to 32.3‰) and strontium (⁸⁷Sr/⁸⁶Sr=0.70797 to 0.70781) in anhydrite may indicate a marine source of sulphate. The isotopic ratio of strontium is in the same range as that of metamorphosed strata-bound barite-sulphide ores, which have been previously described in the same area. The '³⁴S values of coexisting pyrite range from 21.4 to 22.5‰, the (³⁴S_{anhydrite-pyrite} corresponding to the metamorphic temperature of 600 to 660 °C. In contrast to many submarine-exhalative deposits, the oxygen isotopic compositions of anhydrite ('¹⁸O=9.3 to 10.2‰) are lighter than that of barite ('¹⁸O=10.4 to 13.8‰). This indicates that the both minerals are not in isotopic equilibrium. Therefore, it is probable that anhydrite and barite from the Ro—ná district were deposited from fluids that contained different proportions of seawater and hydrothermal fluids or from hydrothermal fluids that underwent variable extent of oxygen isotope exchange with seafloor rocks. The '¹³C values in calcite ('¹³C=-17.2 to -18.7‰) from anhydrite-bearing rock are lower than those in distant marbles. As graphite is absent in anhydrite- and calcite-bearing rocks, impoverishment in the ¹³C isotope cannot be attributed to the graphite-carbonate isotopic exchange during metamorphism. It is proposed that low '¹³C values in carbonates are caused by pre-metamorphic oxidation of organic matter in course of hydrothermal processes. Anhydrite and anhydrite-bearing calc-silicate gneiss from the north-eastern part of the Moldanubian Zone are interpreted to be the high-grade metamorphosed analogue of anhydrite-rich exhalites commonly found in submarine-exhalative hydrothermal deposits.     

Grain boundary diffusion of Si, Mg, and O in enstatite reaction rims: a SIMS study using isotopically doped reactants

Diffusion-controlled growth rates of polycrystalline enstatite reaction rims between forsterite and quartz were determined at 1,000 °C and 1 GPa in presence of traces of water. Iron-free, pure synthetic forsterite with normal oxygen and silicon isotopic compositions and quartz extremely enriched in ¹⁸O and ²⁹Si were used as reactants. The relative mobility of ¹⁸O and ²⁹Si in reactants and rims were determined by SIMS step scanning. The morphology of the rim shows that enstatite grows by a direct replacement of forsterite. Rim growth is modelled within a mass-conserving reference frame that implies advancement of reaction fronts from the initial forsterite-quartz interface in both directions. The isotopic compositions at the two reaction interfaces are controlled by the partial reactions Mg₂SiO₄=0.5 Mg₂Si₂O₆+MgO at the forsterite-enstatite, and MgO+SiO₂=0.5 Mg₂Si₂O₆ at the enstatite-quartz interface, implying that grain boundary diffusion of MgO is rate-controlling. Isotopic profiles show no silicon exchange across the propagating reaction interfaces. This propagation, controlled by MgO diffusion, is faster than the homogenisation of Si by self-diffusion behind the advancing fronts. From this, and using % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiramaaDa % aaleaacaWGtbGaamyAaiaacYcacaWGfbGaamOBaaqaaiaadAfacaWG % VbGaamiBaaaaaaa!3DD2! D_Si,En^VolD_{Si,En}^{Vol} at dry conditions from the literature, results a % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmirayaafa % Waa0baaSqaaiaadofacaWGPbGaaiilaiaadweacaWGUbaabaaaaOGa % eqiTdqgaaa!3CCD! D￠_Si,En dD'_{Si,En}^{} \delta value of 3᎒^-24 m³ s^-1 at 1,000 °C. The isotopic profiles for oxygen are more complex. They are interpreted as an interplay between the propagation of the interfaces, the homogenisation of the isotope concentrations by grain boundary self-diffusion of O within the rim, and the isotope exchange across the enstatite-quartz interface, which was open to ¹⁸O influx from quartz. Because of overlapping diffusion processes, boundary conditions are unstable and D´_Ox,En' cannot be quantified. Using measured rim growth rates, the grain boundary diffusivity D´_MgO' of MgO in iron-free enstatite is 8᎒^-22 m³ s^-1 at 1,000 °C and 1 GPa. Experiments with San Carlos olivine (fo₉₂) as reactant reveal lower rates by a factor of about 4. Our results show that isotope tracers in rim growth experiments allow identification of the actual interface reactions, recognition of the rate-controlling component and further calculation of D´' values for specific components.     

Isotopic analysis of belemnites and brachiopods from the Cretaceous (Albian) Hunstanton Red Chalk Formation (Hunstanton,Norfolk, UK)

Cretaceous brachiopods (Moutonithyris dutempleana) and belemnites (Neohibolites minimus) from the (Albian) Hunstanton Red Chalk Formation (Hunstanton, UK) were isotopically analysed with the aim of identifying palaeoecological and palaeotemperature trends. Shell preservation was assessed via thin section petrography and geochemical analyses. Oxygen isotopic compositions (and corresponding temperature interpretations) of well-preserved belemnites are similar in comparison to the brachiopod shells. Assuming calcite precipitation in isotopic equilibrium, they are interpreted to have occupied the same or similar warm (15–19 °C) shallow marine environment. Further, these findings indicate that the belemnites mineralised in relatively warm waters and not in deep and cool waters as suggested for some belemnite species. The isotope data are thus inconsistent with the belemnites being associated with a cool water pulse, contrasting with events associated with the Cenomanian chalks. A difference δ¹³C between the belemnites and brachiopods is interpreted to originate from differences in metabolic rates.     

Kankan diamonds (Guinea) III: δ13C and nitrogen characteristics of deep diamonds

Diamonds from the Kankan area in Guinea formed over a large depth profile beginning within the cratonic mantle lithosphere and extending through the asthenosphere and transition zone into the lower mantle. The carbon isotopic composition, the concentration of nitrogen impurities and the nitrogen aggregation level of diamonds representing this entire depth range have been determined. Peridotitic and eclogitic diamonds of lithospheric origin from Kankan have carbon isotopic compositions ('¹³C: peridotitic -5.4 to -2.2‰; eclogitic -19.7 to -0.7‰) and nitrogen characteristics (N: peridotitic 17-648 atomic ppm; eclogitic 0-1,313 atomic ppm; aggregation from IaA to IaB) which are generally typical for diamonds of these two suites worldwide. Geothermobarometry of peridotitic and eclogitic inclusion parageneses (worldwide sources) indicates that both suites formed under very similar conditions within the cratonic lithosphere, which is not consistent with a derivation of diamonds with light carbon isotopic composition from subducted organic matter within subducting oceanic slabs. Diamonds containing majorite garnet inclusions fall to the isotopically heavy side ('¹³C: -3.1‰ to +0.9‰) of the worldwide diamond population. Nitrogen contents are low (0-126 atomic ppm) and one of the two nitrogen-bearing diamonds shows such a low level of nitrogen aggregation (30% B-centre) that it cannot have been exposed to ambient temperatures of the transition zone (̿,400 °C) for more than 0.2 Ma. This suggests rapid upward transport and formation of some Kankan diamonds pene-contemporaneous to Cretaceous kimberlite activity. Similar to these diamonds from the asthenosphere and the transition zone, lower mantle diamonds show a small shift towards isotopic heavy compositions (-6.6 to -0.5‰, mode at -3.5‰). As already observed for other mines, the nitrogen contents of lower mantle diamonds were below detection (using FTIRS). The mutual shift of sublithospheric diamonds towards isotopic heavier compositions suggests a common carbon source, which may have inherited an isotopic heavy composition from a component consisting of subducted carbonates.     

Tin–polymetallic sulfide deposits in the eastern part of the Dachang tin field (South China) and the role of black shales in their origin

The Dafulou and Huile vein and stratabound cassiterite-sulfide deposits and sheeted ore veins at the Kangma cassiterite-sulfide deposit are located in the eastern part of the Dachang tin field. These deposits are hosted in a sedimentary sequence containing significant concentrations of organic matter in the form of Lower Devonian calcareous black shales and hornfels. These rocks together with the younger intrusion of Longxianggai granite (91DŽ Ma) actively participated in the formation of Sn-polymetallic deposits. The following three major stages have been distinguished in stratiform and vein-type orebodies at Dafulou, Huile and Kangma: stage I (cassiterite, pyrrhotite, arsenopyrite, tourmaline, carbonate), stage II - main sulfide stage (quartz, cassiterite, arsenopyrite, pyrrhotite, sphalerite, stannite, pyrite, carbonates) and stage III (native Bi, galena, electrum, sulfosalts). Stage IV (post-ore), recognized at Huile is represented by barren carbonates and zeolites. Whole rock geochemistry has revealed that at Dafulou, Bi and Cu correlate strongly with S, whereas V and Pb correlate well with C_org (organic carbon). The similar distribution patterns of selected elements in average slightly mineralized low-Ca black shales indicate a fluid composition similar for all deposits studied. Studies of graphitization of the organic matter in black shales adjacent to orebodies indicate that d₍₀₀₂₎ and FWHM (full width in half maximum)/peak height values gradually decrease in the following sequence: Dafulou deposit M Kangma deposit M Huile deposit. The pyrolysate of wall rocks at the Dafulou deposit is relatively enriched in asphaltenes and maltenes (55.6-72.0% of the pyrolysate) comparable with pyrolysate obtained from more distal black shales (19.2-28.5%). Typical GC-MS spectra of pyrolysate from distal black shales are dominated by alkanes in the n-C15 to n-C25 range, aromatic molecules being represented mostly by alkyl-naphthalenes. In contrast, only traces of aliphatic hydrocarbons in the n-C14 to n-C18 range and elemental sulfur were identified in pyrolysates from pyrrhotitized wall rocks. The earliest fluid inclusions of the studied system occur in the quartz-tourmaline-cassiterite assemblage of stage I at Dafulou. These inclusions are H₂O-CO₂-CH₄-rich, with 10 to 20 vol% of aqueous phase. P-T conditions of the trapping of inclusions are estimated to be up to 400 °C and 1.3 to 2.0 kbar (between 5.0 and 7.5 km under lithostatic pressure). In contrast, the presence of a low density gaseous CO₂-CH₄ phase indicates relatively low pressures during the formation of the breccia-type quartz-calcite-cassiterite-sulfide mineralization (stage II), when P-T conditions probably reached approx. 380 to 400 °C and 0.6 kbar (up to 6 km under hydrostatic pressure). Fluid inclusion data and oxygen isotope thermometry indicate that cassiterite-sulfide ores of the main sulfide stage (stage II) formed from aqueous-carbonic fluid (CO₂/CH₄ =ᄺ) at temperatures of up to 390 °C at Dafulou and in a temperature range of 250 to 360 °C at Huile and 260 to 370 °C at Kangma. The '³⁴S values of sulfides from Dafulou range mostly between -1 and -6‰, whereas sulfides from the Kangma and Huile deposits are characterized by more negative '³⁴S values (between -8 and -11‰, and between -9 and -12‰, respectively). These data suggest that bacteriogenic sulfides of black shales were a dominant source of reduced sulfur for epigenetic (vein and replacement) mineralization. Oxygen isotopic compositions of five quartz-cassiterite pairs from Dafulou and Huile show a relatively narrow range of calculated oxygen isotope temperatures (250-320 °C, using the equation of Alderton 1989) and high '¹⁸O_fluid values between +8 and +10‰ (SMOW), which are in agreement with fluid derivation from and/or high temperature equilibration with the Longxianggai granite. The carbon and oxygen isotope composition of carbonates reflects variable carbon sources. Stage I calcite is characterized by narrow ranges of '¹³C (-7.0 to -9.5‰ PDB) and '¹⁸O (+15.0 to +17.5‰ SMOW). This calcite shows ubiquitous deformation, evidenced by intense development of twins. Fluid compositions calculated at 330 °C for the Dafulou and Huile deposits and at 270-300 °C for the Kangma deposit ('¹⁸O_fluid between +10.0 and +11.5‰ SMOW, '¹³C_fluid between -5.5 and -7.5‰ PDB), agree with fluid derivation from and/or equilibration with the peraluminous, high-'¹⁸O Longxianggai granite and suggest a significant influence of contact metasedimentary sequences (carbon derived from decomposition and/or alteration of organic matter of calcareous black shales). The '¹³ C values of organic matter from the Lower to Upper Devonian host rocks at the Dafulou deposit (-24.0 and -28.0‰) fit with a marine origin from algae. However, organic matter adjacent to the host rock-ore contact displays a slight enrichment in ¹³C. The organic carbon from the Huile and Kangma deposits is even more ¹³C enriched (-24.6 to -23.5‰). The most heavy '¹³ C values (-16.5‰) were detected in hornfels sampled at the contact of the Upper Devonian sediments with the Longxianggai granite. The '¹³C data broadly correlate with the degree of structural ordering (degree of graphitization) of organic matter, which indicates that both variables are related to thermal overprint.     

Origin of a large breccia-vein system in the Sanerlin uranium deposit, southern China: a reinterpretation

The early Tertiary Sanerlin uranium deposit is located near the southwestern margin of the Chaling-Yongxing pull-apart basin defined by the Chaling-Yongxing and Chenxian-Linwu sinistral strike-slip faults in southern China. The uranium ores are hosted in 15 breccia-vein bodies, which are separately located in the cores of three secondary anticlines of the Upper Permian Dangchong Formation. Individual breccia-vein bodies are composed of fragments of silicified shale and sandstone from the Dangchong Formation, and quartz veinlets as cements. These fragments, together with quartz veins, form a mosaic texture. Hydrothermal pitchblende is the only commercial uranium mineral, mainly occurring as disseminated grains within quartz veins or coating fragments. Other metallic minerals include molybdenite, pyrite, chalcopyrite, galena, sphalerite, and red microcrystalline hematite. Fluid inclusions in quartz veins have homogenization temperatures ranging from 150 to 280 °C, and calculated salinity values between 5.6 and 13.4 wt% NaCl equivalent. Stable isotope analyses show that the mineralizing fluid was characterized by '¹⁸O values of -2.2 to +2.6‰ and 'D_H2O values of -134 to -110‰. These analytical data demonstrate that hydrothermal fluids were mainly derived from formation waters (brines) of the Chaling-Yongxing basin. Fluid overpressuring was caused by an abnormal geothermal gradient and impermeable shales in the deposit area. The geometry, texture, and structure of the breccia-vein system, along with the fluid pressure estimates, suggest that hydraulic fracturing generated the mineralized breccia-vein system. Pitchblende and associated minerals were deposited when gaseous phases were released abruptly from the ore fluids due to the hydraulic fracturing.     

Cretaceous climate oscillations in the southern palaeolatitudes: New stable isotope evidence from India and Madagascar

Palaeotemperatures for the Cretaceous of India and Madagascar have been determined on the basis of oxygen isotopic analysis of well-preserved Albian belemnite rostra and Maastrichtian bivalve shells of from the Trichinopoly district, southern India, and Albian nautiloid and ammonoid cephalopods from the Mahajang Province, Madagascar. The Albian (possibly late Albian) palaeotemperatures for Trichinopoly district are inferred to range from 14.9 °C to 18.5 °C for the epipelagic zone, and from 14.3 °C to 15.9 °C for the mesopelagic zone, based on analyses of 65 samples; isotopic palaeotemperatures interpreted as summer and winter values for near-bottom shelf waters in this area fluctuate from 16.3 to 18.5 °C and from 14.9 to 16.1 °C, respectively. The mentioned palaeotemperatures are very similar to those calculated from isotopic composition of middle Albian belemnites of the middle latitude area of Pas-de-Calais in Northern hemisphere but significantly higher than those calculated from isotopic composition of Albian belemnites from southern Argentina and the Antarctic and middle Albian belemnites of Australia located within the warm-temperate climatic zone. Isotopic analysis of early Albian cephalopods from Madagascar shows somewhat higher palaeotemperatures for summer near-bottom shelf waters in this area (20.2-21.6 °C) in comparison with late Albian palaeotemperatures calculated from southern India fossils, but similar winter values (13.3-16.4 °C); however, the latter values are somewhat higher than those calculated from early Albian ammonoids of the tropical-subtropical climatic zone of the high latitude area of southern Alaska and the Koryak Upland. The new isotopic palaeotemperature data suggest that southern India and Madagascar were located apparently in middle latitudes (within the tropical-subtropical climatic zone) during Albian time. In contrast to the Albian fossils, isotope results of well-preserved early Maastrichtian bivalve shells from the Ariyalur Group, Trichinopoly district, are characterised by lower δ¹⁸O values (up to −5.8‰) but normal δ¹³C values, which might be a result local freshwater input into the marine environment. Our data suggest that the early Maastrichtian palaeotemperature of the southern Indian near-bottom shelf waters was probably about 21.2 °C, and that this middle latitude region continued to be a part of tropical-subtropical climatic zone, but with tendency of increasing of humidity at the end of Cretaceous time.     

Telescoped porphyry Cu-Mo-Au mineralisation, advanced argillic alteration and quartz-sulphide-gold-anhydrite veins in the Thames District, New Zealand

Porphyry Cu-Mo-Au mineralisation with associated potassic and phyllic alteration, an advanced argillic alteration cap and epithermal quartz-sulphide-gold-anhydrite veins, are telescoped within a vertical interval of 400-800 m on the northeastern margin of the Thames district, New Zealand. The geological setting is Jurassic greywacke basement overlain by Late Miocene andesitic-dacitic rocks that are extensively altered to propylitic and argillic assemblages. The porphyry Cu-Mo-Au mineralisation is hosted in a dacite porphyry stock and surrounding intrusion breccia. Relicts of a core zone of potassic K-feldspar-magnetite-biotite alteration are overprinted by phyllic quartz-sericite-pyrite or intermediate argillic chlorite-sericite alteration assemblages. Some copper occurs in quartz-magnetite-chlorite-pyrite-chalcopyrite veinlets in the core zone, but the bulk of the copper and the molybdenum are associated with the phyllic alteration as disseminated chalcopyrite and as molybdenite-sericite-carbonate veinlets. The advanced argillic cap has a quartz-alunite-dickite core, which is enveloped by an extensive pyrophyllite-diaspore-dickite-kaolinite assemblage that overlaps with the upper part of the phyllic alteration zone. Later quartz-sphalerite-galena-pyrite-chalcopyrite-gold-anhydrite-carbonate veins occur within and around the margins of the porphyry intrusion, and are associated with widespread illite-carbonate (argillic) alteration. Multiphase fluid inclusions in quartz stockwork veins associated with the potassic alteration trapped a highly saline (50-84 wt% NaCl equiv.) magmatic fluid at high temperatures (450 to >600 °C). These hypersaline brines were probably trapped at a pressure of about 300 bar, corresponding to a depth of 1.2 km under lithostatic conditions. This shallow depth is consistent with textures of the host dacite porphyry and reconstruction of the volcanic stratigraphy. Liquid-rich fluid inclusions in the quartz stockwork veins and quartz phenocrysts trapped a lower salinity (3-20 wt% NaCl equiv.), moderate temperature (300-400 °C) fluid that may have caused the phyllic alteration. Fluid inclusions in the quartz-sphalerite-galena-pyrite-chalcopyrite-gold-anhydrite-carbonate veins trapped dilute (1-3 wt% NaCl equiv.) fluids at 250 to 320 °C, at a minimum depth of 1.0 km under hydrostatic conditions. Oxygen isotopic compositions of the fluids that deposited the quartz stockwork veins fall within the 6 to 10‰ range of magmatic waters, whereas the quartz-sulphide-gold-anhydrite veins have lower '¹⁸O_water values (-0.6 to 0.5‰), reflecting a local meteoric water (-6‰) influence. A '¹⁸O versus 'D plot shows a trend from magmatic water in the quartz stockwork veins to a near meteoric water composition in kaolinite from the advanced argillic alteration. Data points for pyrophyllite and the quartz-sulphide-gold-anhydrite veins lie about midway between the magmatic and meteoric water end-member compositions. The spatial association between porphyry Cu-Mo-Au mineralisation, advanced argillic alteration and quartz-sulphide-gold-anhydrite veins suggests that they are all genetically part of the same hydrothermal system. This is consistent with K-Ar dates of 11.6-10.7 Ma for the intrusive porphyry, for alunite in the advanced argillic alteration, and for sericite selvages from quartz-gold veins in the Thames district.     

Geology and genesis of Variscan porphyry-style gold mineralization, Petráčkova hora deposit, Bohemian Massif, Czech Republic

A large number of Variscan mesothermal gold deposits are located in the central part of the Bohemian Massif, close to the Central Bohemian Plutonic Complex. The Petrá)kova hora deposit has many features that distinguish it from other deposits in the region and suggest its mineralization is closely related to the late magmatic processes associated with the Petrá)kova hora granodiorite. The gold ores occur as sheeted arrays of quartz veins and veinlets hosted by the small Petrá)kova hora granodiorite stock. Gold is found mainly as free grains of >900 fineness, and is accompanied by abundant pyrrhotite and chalcopyrite, and accessory pyrite, arsenopyrite, loellingite, and molybdenite. Molybdenite from the Petrá)kova hora deposit has been dated by the Re-Os method at 344.4DŽ.8 Ma. Hydrothermal alteration in the Petrá)kova hora deposit exhibits a distinct temporal paragenesis. Selectively pervasive, early K-alteration and silicification are the oldest hydrothermal phases. These were followed by early quartz veins (Q₁ to Q₄) that contain most of the gold mineralization. Late quartz veins (Q₅) and fracture-controlled silicification are gold-poor or barren. Barren calcite veins are the youngest hydrothermal product. Extensive low-temperature, meteoric-water dominated alteration, as is typical of classic porphyry deposits, is absent. However, the lower '¹⁸O whole rock values for Petrá)kova hora granodiorite and aplite (+2.4 to +5.1‰ SMOW) compared to other intrusions in the region reflect either interaction with isotopically light external fluids or magma assimilation of small volumes of hydrothermally altered country rock. The '¹⁸O isotopic compositions for quartz, scheelite and hornblende (7.7 to 13.4‰ SMOW) and the '³⁴S compositions for sulfide minerals (-1 to +3.5‰ CDT) from early, gold-rich quartz veins indicate formation at high temperatures (590 to 400 °C) from fluids with a magmatic isotopic signature ('¹⁸O_FLUID of 5.7 to 7.2‰). Fluids related to late quartz veins (Q₅) suggest the presence of a significant component of non-magmatic water ('¹⁸O_FLUID: +2.5 to +4.0‰). The '³⁴S values of post-Q₅ sulfide minerals (-4.5 to -3.5‰) reflect at least partial derivation of late-stage sulfur from a source external to the intrusions. Aqueous, aqueous-carbonic and nitrogen-bearing fluid inclusions were identified in hydrothermal and igneous quartz, with the aqueous inclusions being the most common. In hydrothermal vein quartz, the salinity of primary aqueous inclusions falls into ranges 6 to 23 and 33 to 41 equiv. wt% NaCl; in igneous quartz, populations in salinity were observed between 5 to 16, 35 to 40 and 62 to 70 equiv. wt% NaCl. The salt component of these fluids is best, and minimally, approximated by the NaCl-KCl-CaCl₂ system. Low- and high-salinity aqueous-carbonic inclusions are accessory in many of the analyzed samples. Three large successive pulses of fluids are recognized. Each pulse begins with a high-salinity (>30 equiv. wt% NaCl) magmatic fluid and evolves toward a lower salinity (~5 equiv. wt% NaCl) fluid. Data suggest that external (meteoric?) water(s) were significant for only the third fluid pulse, which formed the late Q₅ quartz veins and the calcite veins. Polyphase fluid inclusions hosted by igneous quartz of the Petrá)kova hora granodiorite indicate minimum trapping conditions of about 3 kbar and 550 °C. The gold-rich Q₁ to Q₄ veins may have formed along a quasi-isobaric cooling path at 2.5 to 1.5 kbar and 590 to 400 °C. This was followed by uplift, and formation of late Q₅ quartz veins (0.5 to 1.5 kbar; ~300 °C) and post-ore calcite veins (<0.5 kbar; 100 to 140 °C). The characteristics of the Petrá)kova hora deposit suggest that it may represent a position intermediate between intrusion-related gold systems (e.g., Fort Knox deposit, Alaska) and gold-rich, copper-poor porphyry deposits (e.g., Maricunga Belt in Chile). As such, the Petrá)kova hora deposit might be an example of the reduced gold sub-type of porphyry deposit.     

Experimental investigation of zoisite–clinozoisite phase equilibria in the system CaO–Fe2O3–Al2O3–SiO2–H2O

The system Ca₂Al₃Si₃O₁₁(O/OH)-Ca₂Al₂FeSi₃O₁₁(O/OH), with emphasis on the Al-rich portion, was investigated by synthesis experiments at 0.5 and 2.0 GPa, 500-800 °C, using the technique of producing overgrowths on natural seed crystals. Electron microprobe analyses of overgrowths up to >100 µm wide have located the phase transition from clinozoisite to zoisite as a function of P-T-X_ps and a miscibility gap in the clinozoisite solid solution. The experiments confirm a narrow, steep zoisite-clinozoisite two-phase loop in T-X_ps section. Maximum and minimum iron contents in coexisting zoisite and clinozoisite are given by X_ps^zo (max) = 1.9*10 ^{- 4} T+ 3.1*10 ^{- 2} P - 5.36*10 ^{- 2}{\rm X}_{{\rm ps}}^{{\rm zo}} {\rm (max) = 1}{\rm .9*10}^{ - 4} T{\rm + 3}{\rm .1*10}^{ - 2} P - {\rm 5}{\rm .36*10}^{ - 2} and X_ps^czo (min) = (4.6 * 10 ^{- 4} - 4 * 10 ^{- 5} P)T + 3.82 * 10 ^{- 2} P - 8.76 * 10 ^{- 2}{\rm X}_{{\rm ps}}^{{\rm czo}} {\rm (min)} = {\rm (4}{\rm .6} * {\rm 10}^{ - {\rm 4}} - 4 * {\rm 10}^{ - {\rm 5}} P{\rm )}T + {\rm 3}{\rm .82} * {\rm 10}^{ - {\rm 2}} P - {\rm 8}{\rm .76} * {\rm 10}^{ - {\rm 2}} (P in GPa, T in °C). The iron-free end member reaction clinozoisite = zoisite has equilibrium temperatures of 185ᇆ °C at 0.5 GPa and 0ᇆ °C at 2.0 GPa, with (H_r⁰=2.8ǃ.3 kJ/mol and (S_r⁰=4.5ǃ.4 J/mol2K. At 0.5 GPa, two clinozoisite modifications exist, which have compositions of clinozoisite I ~0.15 to 0.25 X_ps and clinozoisite II >0.55 X_ps. The upper thermal stability of clinozoisite I at 0.5 GPa lies slightly above 600 °C, whereas Fe-rich clinozoisite II is stable at 650 °C. The schematic phase relations between epidote minerals, grossular-andradite solid solutions and other phases in the system CaO-Al₂O₃-Fe₂O₃-SiO₂-H₂O are shown.     

Evaluation of bulk carbonate δ13C data from Triassic hemipelagites and the initial composition of carbonate mud

Bulk carbonate samples of hemipelagic limestone–marl alternations from the Middle and Upper Triassic of Italy are analysed for their isotopic compositions. Middle Triassic samples are representative of the Livinallongo Formation of the Dolomites, while Upper Triassic hemipelagites were sampled in the Pignola 2 section, within the Calcari con Selce Formation of the Southern Apennines in Southern Italy. Triassic hemipelagites occur either as nodular limestones with chert nodules or as plane‐bedded limestone–marl alternations which are locally silicified. In the Middle Triassic Livinallongo Formation, diagenetic alteration primarily affected the stable isotopic composition of sediment surrounding carbonate nodules, whereas the latter show almost pristine compositions. Diagenesis lowered the carbon and oxygen isotope values of bulk carbonate and introduced a strong correlation between δ¹³C and δ¹⁸O values. In the Middle Triassic successions of the Dolomites, bulk carbonate of nodular limestone facies is most commonly unaltered, whereas carbonate of the plane‐bedded facies is uniformly affected by diagenetic alteration. In contrast to carbonate nodules, plane‐bedded facies often show compaction features. Although both types of pelagic carbonate rocks show very similar petrographic characteristics, scanning electron microscopy studies reveal that nodular limestone consists of micrite (< 5 μm in diameter), whereas samples of the plane‐bedded facies are composed of calcite crystals ca 10 μm in size showing pitted, polished surfaces. These observations suggest that nodular and plane‐bedded facies underwent different diagenetic pathways determined by the prevailing mineralogy of the precursor sediment, i.e. probably high‐Mg calcite in the nodular facies and aragonite in the case of the plane‐bedded facies. Similar to Middle Triassic nodular facies, Upper Triassic nodular limestones of the Lagonegro Basin are also characterized by uncorrelated δ¹³C and δ¹⁸O values and exhibit small, less than 5 μm size, crystals. The alternation of calcitic and aragonitic precursors in the Middle Triassic of the Dolomites is thought to mirror rapid changes in the type of carbonate production of adjacent platforms. Bioturbation and dissolution of metastable carbonate grains played a key role during early lithification of nodular limestone beds, whereby early stabilization recorded the carbon isotopic composition of sea water. The bulk carbonate δ¹³C values of Middle and Upper Triassic hemipelagites from Italy agree with those of Tethyan low‐Mg calcite shells of articulate brachiopods, confirming that Triassic hemipelagites retained the primary carbon isotopic composition of the bottom sea water. A trend of increasing δ¹³C from the Late Anisian to the Early Carnian, partly seen in the data set presented here, is also recognized in successions from tropical palaeolatitudes elsewhere. The carbon isotopic composition of Middle and Upper Triassic nodular hemipelagic limestones can thus be used for chemostratigraphic correlation and palaeoenvironmental studies.     

Thermal history of the Hodgkinson Province and Laura Basin,Queensland: Multiple cooling episodes identified from apatite fission track analysis and vitrinite reflectance data

Apatite fission track analysis and vitrinite reflectance data from outcrop and well samples in the Hodgkinson Province and Laura Basin reveal regional Cretaceous cooling. Apatite fission track analysis appears to define two discrete cooling episodes, in the mid‐Cretaceous (110–100 Ma) and Late Cretaceous (80–70 Ma), although in most samples data allow only definition of a single episode. Rocks now at outcrop cooled from Cretaceous palaeotemperatures generally between 50 and 130°C in the south of the region, and from >100°C in the north. Some samples from the Hodgkinson Province also show evidence for an Early Jurassic cooling episode, characterised by maximum palaeotemperatures varying from at least 95°C (from apatite fission track analysis) to ～200–220°C (from vitrinite reflectance), with cooling beginning at around 200 Ma. Apatite fission track analysis data do not reveal the earlier event in the Laura Basin, but on the basis of vitrinite reflectance data from Permian? units this event is also inferred to have affected the pre‐Jurassic basin units in this region. The regional extent of the Cretaceous cooling episode in the Hodgkinson Province suggests that the elevated palaeotemperatures in this region were most likely due to greater depth of burial, with subsequent cooling due to kilometre‐scale denudation. For a palaeogeothermal gradient of 30°C/km and a palaeosurface temperature of 25°C the total degree of Cretaceous cooling experienced by the samples corresponds to removal of between ～0.8 and >3.0 km of Triassic and younger section removed by denudation, beginning some time between ca 110 and 80 Ma. Higher palaeogradients would require correspondingly lower amounts of removed section. The geology of the Laura Basin suggests that an explanation of the observed Cretaceous palaeotemperatures in this region in terms of deeper burial may be untenable. Heating due to hot fluid flow may be a more realistic mechanism for producing the observed Cretaceous palaeothermal effects in the Laura Basin.