ABSTRACT New K-feldspar 40 Ar/ 39 Ar and apatite fission-track thermochronological data from the crystalline basement of the western Gulf of Mexico (basement core samples from Tamaulipas Arch, Tuxpan, and Jalapa–Santa Ana highs) and K-feldspar 40 Ar/ 39 Ar from field samples of the Chiapas Massif in southern Mexico provide valuable information on the tectonic history of the region, namely, the rifting and postrifting stages of evolution in the Gulf of Mexico. The onset of rifting was probably as early as ca. 216 Ma and was characterized by extensional faulting that led to cooling of the basement footwall blocks by tectonic unroofing. The Tamaulipas Arch and the Jalapa–Santa Ana High were unroofed and cooled until ca. 160 Ma, whereas rocks from the Chiapas Massif were probably affected only until ca. 180 Ma. The thermochronological data suggest that the Tamaulipas Arch and the Chiapas Massif may have been footwalls to low-angle detachments prior to ca. 180 Ma. By ca. 180 Ma, the Chiapas Massif was arguably attached to Yucatán. Rotation of the Yucatán block (and Chiapas Massif) probably started at ca. 167 Ma and unroofed (exhumed) the Tamaulipas Arch very quickly until 155 Ma, when it was unconformably covered by Kimmeridgian sediments along its flanks. The Tamaulipas Arch was progressively buried until the Eocene (ca. 40 Ma), when it was uplifted, and a portion of its sedimentary cover was eroded. A second pulse of uplift occurred in the late Miocene. Our thermochronological data also show that there are along-strike variations in the vertical movements experienced by the Tamaulipas Arch since the Jurassic. This can have important implications for oil maturation of the source rocks in the region, as there might be zones that remained within the oil window for significant amounts of time.

Abstract Precious metal epithermal, sedimentary-rock-hosted prospects constitute a new class of ore deposits recently described in the Tertiary Eastern Rhodopes of southeastern Bulgaria. The Stremtsi prospect investigated in this contribution is located in a distal location with respect to the main cluster of sedimentary-rock-hosted Ada Tepe and Rosino gold prospects of the Eastern Rhodopes. The Stremtsi prospect is hosted by a Priabonian clastic sedimentary rock sequence, overlying metamorphic rocks of the Central Rhodopean dome. The eastern part of the Stremtsi prospect contains high gold grades, and is characterized by a strongly silicified zone, including adularia and silicified dolomite blades, diagnostic for boiling conditions during ore formation in such low-sulphidation epithermal systems. The western part of the Stremtsi prospect consists of a barite, sphalerite and galena mineralization, associated with silicification, and illite and carbonate alteration. Both parts are underlain by subvertical quartz-carbonate-pyrite veins. Primary and secondary fluid inclusions, respectively, in dolomite and barite yield homogenization temperatures ranging between 90 and 247 °C. The salinity of primary inclusions in dolomite falls between 1.9 and 5.1 wt% NaCl equivalent, whereas the one of secondary fluid inclusions in barite ranges between 0.0 and 3.1 wt% NaCl equivalent. The variable homogenization temperatures reflect post-entrapment re-equilibration of the fluid inclusions, whereas the salinities were preserved and the inclusions in dolomite are interpreted in terms of dilution of a saline fluid in the western part of the Stremtsi prospect. The sulphur isotope compositions of sulphides from Stremtsi range mainly between −4 and +4‰. They are not diagnostic and can be attributed to magmatic, metamorphic, and sedimentary sources. They overlap with the main compositional range of sulphides from other sedimentary-rock-hosted epithermal systems and reveal the existence of hydrothermal fluids with common characteristics during ore formation throughout the Eastern Rhodopes. In addition, at Stremtsi, negative δ 34 S values between −42.6‰ and −8.8‰ combined with framboidal pyrite and elevated δ 34 S values of +7.0‰ to +19.5‰ support locally derived sulphur generated, respectively, by bacterial and thermochemical sulphate reduction. Modelling of O, C, and Sr isotope data of dolomite support the above described ore-forming processes. A positive correlation between δ 18 O (+12.7‰ to +19.7‰ V-SMOW) and δ 13 C (−2.8‰ to +1.5‰ V-PDB) values for dolomite from the eastern, silicified and gold-enriched zone of the Stremtsi prospect is satisfactorily modelled by boiling between 140 and 180 °C of a deeply circulating fluid characterized by δ 18 O and δ 13 C values of +5.5‰ V-SMOW and −1.5‰ V-PDB, respectively, and radiogenic strontium leached from the metamorphic basement rocks or its clastic counterparts in the Priabonian host rocks. By contrast, negative correlations of δ 18 O values (+13.4‰ to +23.3‰ V-SMOW) with δ 13 C values (−0.6‰ to −3.9‰ V-PDB) and 87 Sr/ 86 Sr ratios of dolomite from the western, barite and base metal-rich zone are adequately modelled by a shallow, low temperature (70 °C), intra-formational fluid recharged by meteoric water, which interacted with organic matter, that is, coal layers, and carbonate rocks from the Priabonian host sequence, mixing with a deep, moderate temperature (190 °C), 87 Sr-enriched fluid characterized by δ 18 O and δ 13 C values of +5.5‰ V-SMOW and −1.5‰ V-PDB, respectively. Disequilibrium conditions revealed by sulphur isotope thermometry of two galena-barite pairs yielding discrepant temperatures of 190 and 306 °C are consistent with fluid mixing. A plateau age of 37.57±0.31 Ma obtained by 40 Ar/ 39 Ar dating of adularia from Stremtsi is interpreted as a maximum age because of the saddle-shaped age spectrum. Combined with 40 Ar/ 39 Ar age data from previous studies, it reveals that the sedimentary-rock-hosted epithermal prospects constitute an independent, regional and older ore-forming hydrothermal system, distinct from the younger volcanic-rock-hosted epithermal deposits of the Bulgarian and Greek Eastern Rhodopes.

Abstract The establishment of accurate time scales of mineral systems is essential to construct reliable genetic models about their formation. Time scales of fossil mineral systems are directly determined through radiometric dating of different stages of development of the mineral system. In theory, porphyry systems are, among mineral systems, those whose duration can be bracketed with most accuracy and precision, because of the universal occurrence of ore and gangue minerals that can be dated with the high precision U-Pb zircon, Re-Os molybdenite, and 40 Ar/ 39 Ar dating techniques. Time scales of fossil porphyry systems reported in the literature range between <0.1 to >4 Ma. The long durations (>1 Ma) of magmatic-hydrothermal activity measured in several porphyry systems are likely the result of multiple magmatic pulses in agreement with field observations indicating that porphyry systems are associated with several intrusive events. Nonetheless, estimated long durations could also be affected by methodological problems. One methodological problem is the accuracy of the intercalibration among the three different methods. It has become evident during the last 15 years that 40 Ar/ 39 Ar dates are systematically younger compared to U-Pb dates. This has been attributed to incorrect values of the secondary standard (Fish Canyon Tuff sanidine), most commonly used to calculate 40 Ar/ 39 Ar ages, and/or of the 40 K decay constant. Systematic cross calibrations to check the consistency between Re-Os and U-Pb dates are lacking and should also be carried out. Another possible cause of erroneous long durations of porphyry systems concerns the way to determine the emplacement age of the causative intrusion. The current high precision (≤0.1%) of single zircon U-Pb dating by isotope dilution-thermal ionization mass spectrometry (ID-TIMS) shows that zircon grains extracted from a single sample of intermediate/felsic magmatic rocks do not overlap in age. This is so because zircon grains record a protracted evolution of magmas within the crust lasting several hundreds of thousands of years. Under these conditions, the emplacement age of a magmatic intrusion is best approximated by the youngest ID-TIMS age measured from a population of zircon grains. In contrast, spot ages measured with in situ techniques, due to their lower precisions (1-3%), are not able to discriminate such protracted magmatic evolution recorded by different zircon grains. This allows pooling together spot ages of different zircons, resulting in a statistically significant mean age with a low uncertainty. In reality this is a mixed age that is characteristically older (by up to a few hundreds of thousands of years) than the age of the youngest single zircon grain measured by ID-TIMS. A further problem in estimating the duration of magmatic-hydrothermal activity in porphyry systems derives from the widespread use of 40 Ar/ 39 Ar dating. Because this method does not date the crystallization of a mineral but rather its cooling below its closure temperature, 40 Ar/ 39 Ar dates may be affected by (hydro-)thermal activity that postdates the mineralization.