Abstract: A comprehensive study of the Cambrian–Ordovician Arbuckle Group suggests that multiple fluid migration events have affected reservoir porosity via fractures and preferred stratigraphic horizons. Fluid inclusion homogenization temperatures from late-stage precipitates yield temperatures higher than can be explained by burial conditions or an elevated geothermal gradient. Fluid inclusion melting temperatures yield salinity values that indicate multiple fluids evolving through time. Hydrocarbon fluid inclusions in late-stage baroque dolomite suggest oil migration concurrent with hydrothermal fluid flow. Depleted δ 13 C and δ 18 O values provide evidence for a high-temperature basinal fluid source as well as for the preferential flow of hydrothermal fluids through permeable zones in the Mississippian and Arbuckle Group, where pore systems related to paleokarst are overlain by less permeable units. Radiogenic strontium isotopic data support fluid–rock interaction with siliciclastic material or basement rock at some point during the fluid migration history. Variable 87 Sr/ 86 Sr values suggest multiple sources for the fluids responsible for the cements and a transition from an advective fluid flow system to a vertical fluid flow system. The ancient aquifer system was vertically connected during migration of hydrothermal fluids, and a temperature-controlled vertical density gradient appears to have played an important role in late-stage porosity evolution, focusing the hottest fluids in the upper sections of permeable layers.

Abstract Coupling between the physical processes intrinsic to a hydrothermal system can lead to episodic and chaotic behaviour. Such behaviour includes variations in both space and time of the temperature, fluid pressure and activity of H 2 S, which result in the deposition of alteration mineral assemblages, zoned pyrite and gold; these variations are multifractal. In particular, the coupling of deformation and simultaneous endothermic and exothermic reactions with fluid flow leads to the highly localized deposition of gold. We discuss the physical and chemical mechanisms for such episodic and localization behaviour and explore the non-linear dynamic reasons why such mechanisms are recorded in the multifractal paragenetic sequence and deformation history. The synchronization of intrinsic episodicity as described here and extrinsic forcing induced by episodic seismicity provides another mechanism for enhancing the yield of gold deposition processes and hence the grade of orogenic gold deposits.

Abstract: We present a study of vanadium (V) isotope compositions for 17 crude oils spanning a wide range of concentrations and formation ages. Bulk organic geochemical and biomarker compositions are investigated for 11 co-genetic crude oils from the Barbados oil field. About 2‰ V isotope fractionation was observed and they are primarily correlated with V/Ni ratios and most likely reflect the depositional environment of the petroleum source rocks. Factors such as the lithology of the source rocks, Eh and pH of the depositional environment, and possibly the V isotope composition of seawater could all play a role in the V isotope composition. The Eh and pH conditions determine the speciation and coordination of V ions in fluids, whereas the lithology of the source rock defines the competing phases for available V ions in solution. The V isotopes are significantly modified by maturation and biodegradation. The V isotope fractionation during biodegradation has most probably resulted from the microbial activity-induced changes in the species and coordination geometries of V ions in fluids. The progressive decrease of δ 51 V with the increase of maturation might suggest a preferential loss of 51 V during de-metalation and/or a preferential incorporation of 50 V in the newly formed V-organometallic compounds.

Abstract The neighboring sedimentary rock-hosted Agdarreh (~24.5 tonnes [~0.8 Moz] of Au in ore with an average grade of 3.7 g/t Au) and Zarshouran (~110 tonnes of Au [~3.5 Moz] in ore with an average grade of 4.5 g/t Au) Carlin-style gold deposits in the Takab region of northwestern Iran occur within the active geothermal field of the Miocene Urumieh-Dokhtar magmatic arc, which contains an assortment of intrusion-related deposit types (porphyry Cu, polymetallic, and epithermal Au-Ag). Although Agdarreh is hosted in early Miocene reefal limestones and is strongly oxidized and Zarshouran is hosted in exhumed metamorphosed Neoproterozoic black schist, limestone, and dolomite and is relatively unoxidized, they have many features in common. Phyllic alteration is as young as 14 Ma, and the age of local volcanic rocks is between 16 and 11 Ma. Disseminated gold ore in jasperoid occurs as replacement bodies at intersections of normal faults in decalcified and dolomitized carbonate units as well as in schists at Zarshouran. Where jasperoid is fractured or brecciated, it is cemented by drusy quartz. Precipitation of early pyrite (±pyrrhotite), abundant sphalerite, and minor chalcopyrite and galena was followed by a variety of more abundant complex sulfosalt minerals, arsenian pyrite, stibnite, late orpiment-realgar, and cinnabar, with traces of native bismuth, tellurides, and the newly discovered daliranite (PbHgAs 2 S 6 ). Most of the gold occurs as solid-solution or nanometer gold in arsenian pyrite and sphalerite and as native gold associated with late-stage As sulfides and cinnabar. Agdarreh has an abundance of barite, whereas Zarshouran contains fluorite; both deposits are crosscut by minor late calcite, and each deposit is overprinted by a younger Pleistocene to Holocene geothermal system that produced extensive travertine deposits and oxidized sulfide gold ore at Agdarreh. Whole-rock analyses indicate Al was immobile, Ca was depleted, and significant amounts of Fe, Zn, Pb, As, Sb, Hg, Tl, Ag, Au, Ba, ±F were introduced along with lesser K, W, Ni, Cd, Te, and Se. The paragenetic sequence of the ore minerals reflects a decrease in temperature and an increase in f S 2 . Geochemical data suggest that ore fluids transported base metals, trace elements, and Au together or that a saline, H 2 S-poor fluid containing Fe, Zn, Pb, and Ag mixed with an H 2 S-rich fluid containing Au and trace elements. Primary fluid (LV/LV ±CO 2 ) inclusions in drusy quartz, fluorite, and barite with salinities up to 23 wt % NaCl equiv and eutectic first melting temperatures of –19 to –10°C, are suggestive of ore-stage NaCl-KCl solutions. Secondary inclusions have low salinities (<4 wt % NaCl equiv) and higher first-melting temperatures (–13 to 0°C) suggestive of Na 2 CO 3 -Na 2 SO 4 solutions, and those with salinities close to zero and very low T fm (–56 to –30°C) suggestive of CaCl 2 -MgCl 2 -FeCl 2 solutions represent the latest population of one-phase liquid (L) inclusions. Homogenization temperatures of the primary fluid inclusions (200° ± 20°C) and calculated pressures (~150 bar) are indicative of depths around 1.5 km (assuming hydrostatic conditions). The maximum densities of the CO 2 phase present in aqueous-carbonic inclusions (0.18 g/cm 3 at Agdarreh and 0.29 g/cm 3 at Zarshouran) are indicative of pressures between 100 and 260 bar and depths around 1 km (assuming hydrostatic conditions). These data suggest mixing of immiscible brine and CO 2 -bearing vapor with dilute groundwater. Fluid inclusions in Zarshouran orpiment have δ D H 2O , Na/Cl, and Cl/Br values consistent with magmatic fluids. The δ 18 O of water in equilibrium with jasperoid and drusy quartz is either magmatic, as at Agdarreh, or extends toward meteoric water, as at Zarshouran. The δ 13 C of CO 2 in equilibrium with calcite is characteristic of the host rocks with a minor magmatic component. The δ 34 S of H 2 S in equilibrium with pyrite and sphalerite (1–6‰) is likely magmatic, whereas H 2 S in equilibrium with orpiment, stibnite, cinnabar, and getchellite (8–10‰) was derived in part from sedimentary sources. Late barite has high δ 34 S values typical of marine sulfate. These data indicate that the deposits formed in different parts of a hydrothermal system at the interface between ascending magmatic fluids and local meteoric groundwater in sedimentary or metasedimentary rocks. The characteristics of Agdarreh and Zarshouran suggest that they are shallow manifestations of intrusionrelated hydrothermal systems and, therefore, are best classified as distal disseminated deposits.

Abstract The Allchar Au-As-Sb-Tl deposit is situated in the western part of the Vardar zone, the main suture zone along the contact between the Adriatic and the Eurasian tectonic plates. It is spatially and temporally associated with a Pliocene (~5 Ma) postcollisional high-K calc-alkaline to shoshonitic volcano-plutonic center. The Allchar deposit shares many distinctive features with Carlin-type gold deposits in Nevada, including its location near a terrain-bounding fault in an area of low-magnitude extension and intense magmatism. The mineralization is mostly hosted in calcareous sedimentary rocks at intersections of high-angle faults in permeable stratigraphy. The alteration types (carbonate dissolution, silicification, and argillization), ore mineralogy (auriferous arsenian pyrite and marcasite, stibnite, realgar, orpiment, and lorandite), high Au/Ag ratios, and low base metal contents are also typical of Carlin-type gold deposits in Nevada. However, the Allchar deposit differs from Nevada Carlin-type gold deposits as follows: it is an isolated Au prospect with a close spatial and temporal relationship to a shoshonitic volcano-plutonic center in a mineral belt dominated by intrusion-related Cu-Au porphyry, skarn, and hydrothermal polymetallic deposits. The deposit is clearly zoned (proximal Au-Sb to distal As-Tl), it has a significantly higher Tl content, trace elements in pyrite and marcasite are homogeneously distributed, and synore dolomitization is a widespread alteration type. Gold mineralization is most abundant in the southern part of the deposit. It occurs mostly as invisible Au in disseminated pyrite or marcasite and as rare native Au grains. Gold mineralization is accompanied by intense decarbonatization and silicification. Fluid inclusions and the hydrothermal alteration mineral assemblage indicate that Au was deposited from hot (>200°C), saline (up to ~21 wt % NaCl equiv), moderately acidic (pH <5) fluids that carried traces of magmatic H 2 S and CO 2 . In the calcareous host rocks, mixing of such fluids with cool, dilute, near-neutral groundwater triggered deposition of Au and Fe sulfides. In Tertiary tuff, isocon analysis shows that sulfidation of preexisting Fe minerals was a critical factor for deposition of Au and Fe sulfides. Antimony mineralization prevails in the central part of the deposit, and it is mostly associated with dark-gray to black jasperoid. Stibnite, the most common Sb mineral in the Allchar deposit, occurs as fine-grained disseminations in jasperoid and as fine- to coarsely crystalline masses that fill vugs and fracture zones lined with drusy quartz. Fluid inclusions entrapped by stibnite-bearing jasperoid, quartz, and calcite crystals suggest that stibnite was deposited from more dilute and cooled fluids (aqueous-carbonic fluid inclusions: 6.0–3.5 wt % NaCl equiv, T h = 102°−125°C; aqueous fluid inclusions: 14.5 and 17.1 wt % NaCl equiv, T h = 120°−165°C). In contrast to stibnite, As sulfides (orpiment and realgar) and Tl mineralization are associated with argillic alteration. Fluid inclusions hosted by realgar, orpiment, dolomite, and lorandite record deposition from more dilute (2.6–6.9 wt % NaCl equiv) and relatively cold fluids (T H = 120°−152°C) enriched in K. Isocon diagrams show a tight link between Tl and the low-temperature argillic alteration as well as a significant correlation between Tl and K. The spatial relationship of Tl mineralization with dolomite suggests that Tl deposition was also promoted by neutralization of acidic fluids. The δ D and δ 18 O data obtained from gangue minerals and fluid inclusions indicate that magmatic fluid mixed with exchanged meteoric water at deep levels and with unexchanged meteoric water at shallow levels in the system. The δ 13 C and δ 18 O values of carbonate minerals and extracted fluid inclusions suggest mixing of carbonate rock buffered fluids with magmatic and atmospheric CO 2 . The sulfur isotope values of early disseminated pyrite and marcasite show that H 2 S was initially derived from diagenetic pyrite in sedimentary rocks. In contrast, Sb and As mineralization indicate a strong input of magmatic H 2 S during the main mineralization stage. Late-stage botryoidal pyrite and marcasite are depleted in 34 S, which indicates a diminishing magmatic influence and predominance of sulfur from sedimentary sources during the late-mineralization stage. Fractionation of isotopically light sulfide species from isotopically heavy sulfates due to oxidation under increased oxygen fugacity cannot be excluded.