Illite Ages Research Articles

The Role of Sandstone Diagenesis and Aquifer Evolution in the Formation of Uranium and Zinc-Lead Deposits, Southern McArthur Basin, Northern Territory, Australia

The Paleo- to Mesoproterozoic McArthur basin in the Northern Territory, Australia, is filled with mixed carbonate-siliciclastic successions and felsic and mafic volcanic units that contain authigenic minerals that formed during burial diagenesis. Petrographic investigations indicate that some of these lithologic units were cemented early by quartz overgrowths and became diagenetic aquitards, whereas secondary porosity was developed in others that became diagenetic aquifers for basinal brines. The authigenic minerals in both lithology types preserve geochemical information about the types of fluids that occurred in the basin, with those that resided in the diagenetic aquifers having similar geochemical composition to those indicated by alteration minerals in the McArthur River Pb-Zn deposit and the Westmoreland U deposits. Quartz overgrowths formed early during shallow burial in all clastic lithologic units and are particularly common in the well-sorted, marine sediments; these became diagenetic aquitards due to the porosity-occluding cement. In the calcareous units, early dolomite replaced calcite and aragonite at shallow burial depths. Fluid inclusion microthermometry indicates that most quartz overgrowths formed between 80° and 125°C from a fluid with a salinity between 0 and 13.1 wt percent NaCl equiv. The isotopic composition of dolomite indicates δ18Ofluid of −4.5 ± 1.5 per mil and δ13Cfluid of −6.9 ± 1.0 per mil, suggesting formation from a mixed marine and meteoric fluid. Illite cemented by early quartz has a 40Ar/39Ar age of 1738 ± 10 Ma in one diagenetic aquitard, which reflects the age when the host sediment was deposited and confirms that quartz cementation occurred early during burial. With increased burial, the poorly sorted sandstones and conglomerates that were initially poor aquifers at or near the surface experienced framework grain dissolution and the creation of secondary porosity at depth. As a result, the sediments of the Westmoreland Conglomerate, the lower Yiyintyi Sandstone, significant proportions of the Warramana Sandstone, and most of the sandstone interbeds of the Gold Creek Volcanics became diagenetic aquifers and conduits for basinal brines. Illite, dolomite, and chlorite formed late in the paragenesis and filled the secondary porosity. Quartz veins formed last and primarily in faults and fractures. The dolomite and quartz veins contain fluid inclusions that have low eutectic temperatures indicative of Ca2+- and Na+-dominated brines and salinities between 18.5 and 30.9 wt percent NaCl equiv. Illite crystallinity data and chlorite chemical compositions indicate formation between 150° and 250°C. The isotopic compositions of illite and chlorite indicate δ18Ofluid values between 2.9 and 9.1 per mil and δDfluid values between −61 and −25 per mil, which suggest formation from a mixed meteoric and marine fluid. These δ18Ofluid values are indistinguishable from those determined from the isotopic composition of alteration minerals at the McArthur River Pb-Zn deposit (5 ± 5‰) or synore illite at the Westmoreland uranium deposit (4 ± 2‰), suggesting that the diagenetic aquifers and the deposits may be linked. Argon ages of illite from the diagenetic aquifers indicate that fluid migration began as early as 1680 Ma and continued to approximately 1541 Ma, coinciding with and extending past the time when the McArthur River Pb-Zn deposit (1640 Ma) and the Westmoreland U deposits (1655–1606 Ma) formed. The results presented in this study show that clastic sediments dominated by poorly sorted sandstone and conglomerate facies evolved through burial diagenesis to become aquifer lithologic units that hosted basinal brines with chemical characteristics and ages similar to those that are reported from the Pb-Zn and U deposits of the southern McArthur basin.

Comparison of K-Ar Ages of Diagenetic Illite-Smectite to the Age of a Chemical Remanent Magnetization (CRM): An Example from the Isle of Skye, Scotland

AbstractThe clay fractions of Jurassic marls in the Great Estuarine Group in southern Isle of Skye are composed of mixed-layered illite-smectite (I-S) with large percentages (>85%) of illite layers, kaolinite, and generally smaller amounts of chlorite. These marls have not been buried to the depths normally required to convert smectite to illite-rich I-S, so it is possible that the conversion was in response to heat and hydrothermal fluids from nearby early Tertiary igneous activity ∼55 Ma ago. The large percentages of illite layers in I-S, the Środoń intensity ratios, and the Kübler index values appear to be consistent with the formation of diagenetic I-S as a result of relatively brief heating caused by igneous activity. The Jurassic rocks in southern Skye contain a secondary chemical remanent magnetization (CRM) that resides in magnetite and formed at approximately the same time as the Tertiary igneous rocks on Skye. K-Ar age values for I-S based on illite age analysis have been determined to test the hypothesis that the CRM was acquired coincidently with the smectite-to-illite conversion. However, linear extrapolation of K-Ar age vs. percentage of 2M1 polytype (detrital illite) from one marl (EL-6) yields an estimate for the age of diagenetic illite of 106 Ma, which is close to the measured age of the finest subfraction (108 Ma). These estimated and measured age values, however, could be substantially greater than the true age of the diagenetic illite in I-S because of the presence of detrital 1Md illite that was recycled from early Paleozoic shales and whose abundance relative to the diagenetic I-S may have been enhanced because the diagenetic fluid had a low K/Na ratio, limiting the amount of diagenetic illite formed. Nevertheless, most of the illite in the Elgol marls (80% or more in the finest fractions) must be diagenetic and probably formed in response to the early Tertiary magmatism.

On the timing and causes of illite formation and remagnetization in the Cretaceous Marias River Shale, Disturbed Belt, Montana

In this study, we test a burial model for diagenetic illite and whether illitization is related to the acquisition of a chemical remanent magnetization (CRM). A prefolding, reversed polarity, secondary magnetization is found in carbonate concretions in diagenetically altered Upper Cretaceous rocks in the disturbed belt of Montana. Stratigraphically equivalent unaltered rocks to the east do not contain the magnetization. The magnetization could be a CRM or a thermoviscous remagnetization. The pole position is generally consistent with the Early Tertiary K–Ar ages of diagenetic illite–smectite formed under thrust loading, but it is not constrained well enough for a precise age comparison. If this magnetization is a CRM, it could be related to illitization. Alternatively, if the prefolding CRM had been acquired prior to thrust loading then the magnetization is not related to illitization. Geochemical studies are currently underway to test if the magnetization could be related to fluid alteration.

Jurassic and Cretaceous clays of the northern and central North Sea hydrocarbon reservoirs reviewed

AbstractThe principal clays of the northern and central North Sea are illite (sometimes with interlayered smectite) and kaolin. Chlorite is only locally important. Although it has been proposed that kaolin within North Sea sandstones is detrital in origin, the majority of workers have concluded that it is authigenic, largely the product of feldspar alteration. Kaolin is found within a wide range of sedimentary settings (and within shales) apparently defying the notion that kaolin is an indicator of meteoric water deposition. Within sandstones, the earliest authigenic kaolin has a vermiform morphology, the distribution of which is controlled by the availability of detrital mica to act as a nucleus, and the composition of the post-depositional porewaters. This vermiform kaolin formed in meteoric water, the presence of which is easily accounted for below sub-aerial exposure surfaces in non-marine formations, and below unconformities over marine units. In fully marine sands, and even marine shale units, kaolin still occurs. It has therefore been suggested that even these locations have been flushed with meteoric water.Early vermiform kaolin recrystallizes to a more blocky morphology as burial proceeds, at least in the Brent Group. Blocky kaolin has been reported as growing before, synchronously with, and after the formation of quartz overgrowths, though oxygen isotope studies support low-temperature growth, pre-quartz. Blocky kaolin may form during meteoric flushing associated with lower Cretaceous uplift and erosion, though it is found in fault blocks that are thought to have remained below sea level. Here, the kaolin may form in stagnant meteoric water, relics of the post-depositional porewater. It has also been proposed that the blocky kaolin grew in ascending basinal waters charged with carboxylic acids and CO2, though this hypothesis is not supported by stable oxygen isotope data. Some of the blocky kaolin is dickite, the stable polymorph above ∼100°C.Fibrous illite occurs almost ubiquitously within the clastic sediments of the North Sea. An early pore-lining phase has been interpreted as both infiltrated clastic clay, and as an early diagenetic phase. Early clays may have been quite smectite-rich illites, or even discrete smectites. Later, fibrous illite is undoubtedly neoformed, and can degrade reservoir quality significantly. Both within sandstones and shales, there is an apparent increase in the K content deeper than 4 km of burial, which could be due to dilution of the early smectite-rich phase by new growth illite, or to the progressive illitization of existing I-S. Much of the ‘illite’ that has been dated by the K-Ar method may therefore actually be I-S.The factors that control the formation of fibrous illite are only poorly known, though temperature must play a role. Illite growth has been proposed for almost the entire range of diagenetic temperatures (e.g. 15–20°C, Brent Group; 35–40°C, Oxfordian Sand, Inner Moray Firth; 50–90°C, Brae formation; 100–110°C, Brent Group; 130–140°C, Haltenbanken). It seems unlikely that there is a threshold temperature below which illite growth is impossible (or too slow to be significant), though this is a recurring hypothesis in the literature. Instead, illite growth seems to be an event, commonly triggered by oil emplacement or another change in the physiochemical conditions within the sandstone, such as an episode of overpressure release. Hence fibrous illite can grow at any temperature encountered during diagenesis.Although there is an extensive dataset of K-Ar ages of authigenic illites from the Jurassic of the North Sea, there is no consensus as to whether the data are meaningful, or whether the purified illite samples prepared for analysis are so contaminated with detrital phases as to render the age data meaningless. At present it is unclear about how to resolve this problem, though there is some indication that chemical micro-analysis could help. It is a common belief that illite ages record the timing of oil charge, and so can be used to calibrate basin models.Grain-coating Fe-rich chlorite cements can preserve exceptional porosity during burial. They are found in marginal marine sandstones, formed during diagenesis from precursor Fe-rich clays such as berthierine or verdine.

Fault dating in the Canadian Rocky Mountains: Evidence for late Cretaceous and early Eocene orogenic pulses

Fault rocks from the classic Rocky Mountain foreland fold-and-thrust belt in southwestern Canada were dated by Ar analysis of clay grain-size fractions. Using X-ray diffraction quantification of the detrital and authigenic component of each fraction, these determinations give ages for individual faults in the area (illite age analysis). The resulting ages cluster around 72 and 52 Ma (here called the Rundle and McConnell pulses, respectively), challenging the traditional view of gradual forward progression of faulting and thrust-belt history of the area. The recognition of spatially and temporally restricted deformation episodes offers field support for theoretical models of critically stressed wedges, which result in geologically reasonable strain rates for the area. In addition to regional considerations, this study highlights the potential of direct dating of shallow fault rocks for our understanding of upper-crustal kinematics and regional tectonic analysis of ancient orogens.

Kübler Index and K-Ar Ages of Illite in the Yinshan Polymetallic Deposit, Jiangxi Province, South China: Analyses and Implications

Abstract. The Yinshan polymetallic deposit is a hydrothermal vein-type deposit closely related to Late Jurassic felsic-inter-mediate volcanic-subvolcanic activity in Jiangxi Province, South China. Illite is a major alteration mineral observed in the deposit. Our study shows that the Kübler index of the illite has a close relation to ore-forming fluids of different stages of hydrothermal alteration and mineralization. The early Pb-Zn-Ag mineralization dated at 130–136 Ma is characterized by relatively low water/rock ratios and diffusive fluid movement within phyllite, whereas the later Cu-Au-S mineralization at 122 -125 Ma was accompanied by higher water/rock ratios and localized fluid flow through fractures and channels. Illite formed in the early Pb-Zn-Ag mineralization stage contains swelling layers while the illite formed in the later Cu-Au-S mineralization stage has no swelling layers but was associated with intensive chloritization. The last stage of mineralization (at 104 Ma) was minor and did not produce significant amounts of illite.

A Paragenetic and Isotopic Study of the Proterozoic Westmoreland Uranium Deposits, Southern McArthur Basin, Northern Territory, Australia

The Proterozoic sandstone-hosted Redtree and Junnagunna uranium deposits in the Westmoreland uranium field occur in the southern McArthur basin, Australia, at the contact between the Westmoreland Conglomerate and the Seigal Volcanics. Uranium mineralization consists of uraninite with hematite and illite and occurs within a zone of chlorite alteration that formed prior to the uraninite during peak diagenesis. Oxygen and hydrogen isotope ratios on synmineralization illite show that uranium was transported to the site of deposition by a basinal brine with δ 18 O fluid and δD fluid values of 4 ± 3 and −33 ± 10 per mil, respectively. These values are consistent with evolved evaporated seawater but not with hot, oxidized fluids derived from underlying uraniferous granites or volcanic rocks as previously suggested. Illite crystallinity indicates that the uraninite-illite-hematite assemblage formed at 200° ± 50°C. The 40 Ar/ 39 Ar ages of illite and 207 Pb/ 206 Pb ages of uraninite indicate that mineralization occurred between 1655 ± 83 and 1606 ± 80 Ma, coincident with major tectonic events in northern Australia, and was later remobilized between ca. 1150 and 850 Ma. A U-Pb concordia age of 878 Ma is identical to the 870 Ma age recorded by previous workers on Westmoreland uranium deposits. The mineralogy, paragenesis, and geochemistry of the Redtree and Junnagunna uranium deposits are largely indistinguishable from the basement-hosted Nabarlek and Jabiluka uranium deposits in the northern McArthur basin, suggesting that the Westmoreland uranium field might also be prospective for basement-hosted uranium deposits where suitable structural and chemical traps can be found.

A km-scale illite alteration zone in sedimentary wall rocks adjacent to a hydrothermal fluorite vein deposit

AbstractIn a study of wall-rock alteration in the 1.4 km long Würmtal adit at the Käfersteige hydrothermal vein deposit (northern Black Forest, Germany) illite was found to be the only clay mineral within the Bunter sandstone and intercalated claystone. Illite occurs mainly as a detrital mineral in the claystone, whereas it is hydrothermally neoformed in the sandstone, either in the pores or as an alteration product of K-feldspar. The extensive occurrence of authigenic illite along the entire 1.4 km long profile confirms that the fluids migrated far into the sandstone.The authigenic illite formed during a first pulse of high-temperature fluids (Th of ~220°C) with a low salinity (~1 wt.% NaCleq). These fluids also dissolved Sr and Rb from detrital illite of the claystones at the vein. A later hydrothermal pulse with a lower temperature (~70–150°C) and higher salinity (21–28 wt.% NaCleq) silicified the sandstone adjacent to the vein and caused partial substitution of OH- by F- in the structure of the detrital and neoformed illite along the profile.Within analytical error, the K-Ar dates for the neoformed illite of the <2 µm fraction are the same along the profile (~150 Ma). During this hydrothermal process, the age of the detrital illite within the claystone was reset from 310 to 190 Ma. The illite-rich <0.2 µm fractions yield ages of ~142 Ma (sandstone) indicating a Jurassic origin. The uniform age data for illite in the sandstone and in the claystone are probably due to extensive migration of hot fluids through the wall rocks.The hydrothermal fluids are attributed to recycled meteoric water and brines that ascended from the basement into the cover rocks during the opening of the North Atlantic and/or the nearby Tethys area.

40Ar/39Ar and Re-Os Geochronology of Porphyry Copper-Molybdenum Deposits and Related Copper-Silver Veins in the Collahuasi District, Northern Chile

The Rosario and Ujina porphyry Cu-Mo deposits, together with porphyry copper mineralization at the nearby Quebrada Blanca deposit, constitute the third-largest concentration of copper mineralization associated with the Domeyko fault system in northern Chile. At Rosario, fault-hosted Cu-Ag-rich massive sulfide veins are associated with pyrophyllite-alunite-quartz altered rocks. The Rosario massive sulfide veins cut biotitealbite-magnetite, K-feldspar and illite-chlorite altered rocks associated with porphyry-style copper-molybdenum mineralization. Similar massive sulfide veins occur in the La Grande area, 1 to 2 km south of Rosario. Copper ore at Ujina is associated with a K-feldspar-biotite-altered quartz monzonite intrusion. The K-feldspar-biotite assemblage has been overprinted by white mica-chlorite alteration. New geochronologic data presented in this paper constrain the ages of hydrothermal activity in the Rosario and Ujina deposits. The 40Ar/39Ar biotite, illite, and alunite dates (at 2 sigma error), and a Re-Os molybdenite date (at 0.5% error) are reported for porphyritic rocks and hydrothermal alteration from the Rosario and Ujina mineralized centers. A weighted mean 40Ar/39Ar plateau age of 34.4 plus or minus 0.3 Ma is obtained for igneous biotite in a monzonite porphyry that hosts copper mineralization at the Rosario deposit. Illite and hypogene alunite from separate overprinting alteration events yielded 40Ar/39Ar ages of 34.5 plus or minus 0.5 Ma (plateau age) and 32.6 plus or minus 0.3 Ma (plateau age), respectively. An Re-Os age of 33.3 plus or minus 0.2 Ma for molybdenite at Rosario is slightly younger than the 40Ar/39Ar age of illite, but older than the alunite. A weighted mean plateau age of 32.7 plus or minus 1.6 Ma for hypogene alunite from the La Grande Cu-Ag-(Au) vein south of Rosario is indistinguishable from the age of Rosario alunite. At Ujina, the weighted mean 40Ar/39Ar plateau age of igneous biotite for a monzonite intrusion that hosts copper mineralization is 35.2 plus or minus 0.3 Ma. The monzonite is intruded by postmineralization porphyry dikes of similar composition, from which igneous biotite yielded a 40Ar/39Ar age of 34.7 plus or minus 0.3 Ma. Igneous biotite in the Rosario Porphyry cooled through its closure temperature during and after the formation of illite, implying that porphyry-style ore and alteration minerals in the Rosario deposit had formed by 34.3 Ma. The age of alunite at Rosario and La Grande indicate that a second discrete episode of hydrothermal activity was superimposed, 1.8 plus or minus 0.4 m.y. later, onto the earlier-formed porphyry Cu system. Hydrothermal activity at Ujina is constrained by the 40Ar/39Ar ages of igneous biotite in the premineralization and postmineralization intrusions and occurred during a minimum interval of 0.5 plus or minus 0.4 m.y. The biotite granite at La Profunda, 1.5 km east of Ujina, has a igneous biotite age of 81.2 plus or minus 2.9 Ma, indicating that this intrusion is unrelated to the mineralized Eocene-Oligocene porphyry intrusions at Ujina, Rosario, and Quebrada Blanca. The 40Ar/39Ar data for igneous biotite in the Rosario and Ujina porphyries are cooling ages after multiple hydrothermal events. In most cases, these are minimum hydrothermal ages (i.e., ages of the last high-temperature hydrothermal event to have affected the samples), not magmatic ages. However, the biotite age for the Inca Porphyry is possibly only slightly younger than the intrusion age because biotite has not been affected by later hydrothermal alteration.

Reservoir quality and K-Ar age dating of the pre-Khuff section of Kuwait

ABSTRACT Based on studies of petrographic thin sections from core and cutting samples, the pre-Permian siliciclastics in four deep wells in southern Kuwait were found to be tight. Three of these wells are located on the crestal region of the Burgan Arch, and one on the Umm Gudair anticline. These clastics were encountered beneath a thin brick-red shale of unknown thickness, immediately below the pre-Khuff unconformity at the base of the Permian-Triassic Khuff Formation. The pre-Khuff clastics range in thickness from a few tens of feet to more than 4,000 ft, and overlie a Proterozoic argillite (Economic Basement). Based on Illite Age Analysis (IAA) of samples from cores, the depositional K-Ar age of the pre-Khuff clastics is estimated to be younger than 509 Ma (90 percent confidence interval: 544–481 Ma, i.e. Cambrian-Early Ordovician). The argillite was uplifted through the 300°C isotherm at about 611 Ma (90% confidence interval: 635-588 Ma, i.e Late Proterozoic); its deposition and metamorphism preceded this date. During the Paleozoic, the pre-Khuff clastics were buried to depths of 10,000–15,000 ft, but were subsequently uplifted in the Late Paleozoic. IAA diagenetic K-Ar ages of the Economic Basement (421 Ma; 90 percent confidence interval: 442-397 Ma; Late Ordovician-Early Devonian) and pre-Khuff clastics (369 Ma; 90 percent confidence interval: 404–337 Ma, i.e. Devonian-Early Carboniferous) indicate that by these times the pre-Khuff section was already deposited and undergoing burial diagenesis. The interpretation of gravity data indicates that in Paleozoic basinal regions (e.g. between the Burgan Arch and Umm Gudair Anticline), the Paleozoic sedimentary section is likely to be more complete and may exceed 10,000 ft in thickness.

Effects of oil charge on illite dates and stopping quartz cement:calibration of basin models

Oil can fill pores in reservoir sandstones at any burial depth by long or short distance migration. There has been a debate since 1920 concerning the effect of oil charge. We have made detailed local measurements of cements and porosity from individual fields and regional compilations from the North Sea. We consider that oil charge can be dated by K Ar ages of fibrous illite, and oil stops quartz cementation in sands containing less than 20% water. Consequently, abnormally large porosity values can be preserved from 2 km down to more than 4 km of burial. Many oilfields are anomalously porous. K Ar dates within fibrous illite and quartz cement abundance can now be used to calibrate basin models back though geological time.

Illite polytype quantification for accurate K-Ar age determination

PolyQuant, a new method to quantify illite polytypes in physical mixtures, has been coupled with Illite Age Analysis (IAA, Pevear 1992) to determine accurate K-Ar ages of the individual polytypes. K-Ar radioisotope dates from illite in shales and sandstones are typically meaningless because these rocks are mixtures of diagenetic and detrital components in unknown proportions. IAA uses linear extrapolation to unmix measured K-Ar ages and establish the end-member ages. This procedure requires samples to have a diagenetic Illite-Smectite (I-S) component mixed with a detrital discrete illite component because they are separable and quantifiable using X-ray diffraction (XRD) of oriented aggregate samples. We extend IAA for use in mixtures of diagenetic and detrital illite with no I-S component by coupling a genetic algorithm (GA) with the WILDFIRE programs (1993) of Reynolds in a new computer program, PolyQuant, which quantifies illite polytypes in a natural sample. Based on first principles of XRD, WILDFIRE is a forward model that calculates hkl reflections by varying parameters such as rotational disorder, octahedral cation occupancy (i.e., cis- and transvacancies), chemical substitutions (K and Fe), orientation, and crystallite thickness. PolyQuant executes several forward models, using the GA to vary the parameters and weight percents of each polytype to automatically achieve a “best fit” with an experimental XRD pattern. Sensitivity tests reveal that changes in the weight percent of an illite polytype, disorder (P 0 , P cv ), and the orientation function affect the pattern more than other parameters. In addition, PolyQuant successfully quantified illite components in prepared mixtures of 1M illite and 2M 1 muscovite and 1M d illite and 2M 1 muscovite. We show examples to validate IAA from prepared mixtures of end-members with known ages and also to obtain end-member ages of highly illitic clay fault gouge. Using K-Ar ages and PolyQuant to quantify the 1M and 2M 1 components from mixtures of RM-30 (25 ± 1 Ma) and a 2M 1 muscovite from Wards (428 ± 9 Ma), IAA determined end-member ages of 19.9 Ma (0-44 Ma) and 429 Ma (408-459 Ma)-in reasonable agreement with the measured results. In addition, we use clay gouge from the Canadian Rockies to date movement along thrust faults. Regional geologic data indicate movement along these faults occurred between 73 and 48 Ma. IAA determined the ages of faulting to be from 79.2 to 53.5 Ma, in good agreement with the known range of fault dates. Our results indicate that neoformation of illite in the fault zone must have modified the ages to younger dates.

Alkylammonium cations treating and K-Ar dating of illite from mudstones

Two kinds of mudstones from the Tarim Basin and the Tuha Basin were treated with C8 and C18 alkylam-monium cations. Both of them were dated by the K-Ar method. One kind of rock contains mainly diagenetic illite, and the other contains mixed layer illite/smectite (I/S). Sample separation and alkylammonium cations treatment were performed first, and K-Ar dating followed. The result shows that fine-grained minerals have younger K-Ar ages and coarse-grained components have older ages. Plots of K-Ar age versus K2O (%) are linear, the range of diagenetic age can be estimated by extrapolation. The reaction results of the above samples are different after alkylammonium cations treatments. The samples containing mainly illite show a decrease in K2O and an increase in age by 1–25 Ma, suggesting that preferential exchange of young diagenetic clay rather than detrital clay by alkylammonium cations, therefore the age of diagenetic illite can be calculated directly by the % K2O and radiogenic Ar removed. For the I/S dominant samples, detrital illite was opened preferentially by C8 alkylammonium cations treatment. Detrital age can be calculated, but it is hard to obtain the diagentic age directly.

Fibrous illite in oilfield sandstones – a nucleation kinetic theory of growth

Fibrous illite is one of the most important cements that grows within sandstones during burial, and the only one which is commonly dated using the K–Ar age technique. A small quantity of illite can dramatically reduce porefluid flow rates within a sandstone, thus making oil recovery uneconomic. Illite ages potentially correspond to geological events such as hydrocarbon filling of a sandstone reservoir, providing calibration to basin models. Yet the fundamental processes controlling fibrous illite growth are not understood. This contribution presents a new theory for illite growth within sandstones that explains the fibrous morphology of illite, the restricted range of illite K–Ar age dates compared to the age of the host sandstones, and the lack of 0‐Myr‐old illite reported. The growth of fibrous illite as an authigenic cement may be controlled by nucleation kinetics, and not by thermodynamic or growth kinetic considerations as has been previously assumed.

40Ar/39Ar Geochronologic Constraints on the Timing of Massive Sulfide and Vein-Type Pb-Zn Mineralization in the Western Meseta of Morocco

The Meseta domain of Morocco is located on the southern margin of the late Paleozoic Hercynian orogenic zone. 40Ar/39Ar ages of igneous and hydrothermal minerals (biotite and illite) were analyzed from a representative massive sulfide deposit at Hajar (1.5 Mt Zn, 0.45 Mt Pb, 0.11 Mt Cu, and 900 t Ag) and a vein-type deposit at Tighza (1.0 Mt Pb, 0.3 Mt Zn, and 1,000 t Ag) in the western Meseta. A weighted mean age of hydrothermal biotite of the Hajar deposit is 300.9 ± 2.6 Ma (2 σ uncertainty), identical to a hydrothermal illite age of 305.4 ± 4.8 Ma. These two ages are calculated from about 93 percent of the released 39Ar. Another illite age, calculated from 74 percent of the released 39Ar, 313.4 ± 9.1 Ma, overlaps with or is slightly older than these concordant ages. These 40Ar/39Ar results indicate that the biotite and illite alteration of the Hajar deposit formed almost simultaneously at 304 to 300 Ma. This age range is younger than the Visean age previously assumed on the basis of geologic setting. These 40Ar/39Ar ages are interpreted as the age of hydrothermal activity subsequent to massive sulfide mineralization. A magmatic intrusion beneath the deposit may have triggered this hydrothermal activity. Three 40Ar/39Ar weighted mean ages and one plateau age from the Tighza deposit range from 280.6 to 267.7 Ma. Igneous biotite of the Kaolin granite and hydrothermal illite of the Ighrem Aouss and North veins in the deposit indicate ages of 280.6 ± 5.5 Ma, 280.6 ± 3.1 Ma, and 275.5 ± 3.4 Ma, respectively. These three ages are identical within 2 σ uncertainties. The age of the hydrothermal biotite from the Mispickel granite, 267.7 ± 2.3 Ma, is slightly younger. This younger age may have resulted from two periods of magmatic-hydrothermal activity or from resetting of hydrothermal biotite by Ar loss. Regardless, these ages indicate that granite intrusion and hydrothermal alteration associated with galena- and sphalerite-bearing veins occurred mainly at 280 to 275 Ma in the Tighza deposit. The identical ages of the magmatic intrusion and hydrothermal alteration suggest that the vein-type Pb-Zn mineralization, as well as W mineralization, was associated with the magmatic intrusion. The influence of the late deformation and metamorphic overprint may be related to the lowest-temperature steps of the analytical data. The dates obtained from the lowest-temperature steps from both the Tighza and Hajar deposits concentrate between 235 Ma and 220 Ma. This time range is compatible with the geologic constraints on the deformation, and these dates are thus attributed to the post-Visean regional metamorphism.

Origin of illite in the lower Paleozoic of the Illinois basin: Evidence for brine migrations

In the lower Paleozoic of the Illinois Basin, three illite polytypes are found: 2M 1 of detrital origin, and 1M d and 1M of diagenetic origin. Illite polytype quantification of detrital 2M 1 illite and diagenetic 1M d and 1M illite, combined with K-Ar age dating, allows extrapolation to apparent detrital and diagenetic illite ages. Kinetic modeling of smectite illitization, combined with the calculated age of illitization, can evaluate different origins of illite. The diagenetic illite in the lower Paleozoic of the Illinois Basin is interpreted not to have formed solely by burial diagenesis but mainly during multiple brine events. The Upper Ordovician Maquoketa Group contains diagenetic illite (dominantly 1M d with minor 1M ) with an extrapolated age of ∼360 m.y. (356–377 m.y.) and formed from smectite at temperatures of 50–100 °C. This age falls within the span of dates for illite/smectite (I/S) in K-bentonites from the Upper Mississippi Valley and is interpreted to be a combined result of illitization by burial diagenesis and either a hydrothermal brine from the southern and deeper part of the basin or a K-rich brine from the Michigan Basin, Upper Mississippi Valley area, or Forest City Basin. In Ordovician and Cambrian shale partings and sandstone older than the Maquoketa Group, the diagenetic illite ( 1M d in shale and 1M in sandstone) has an age of ∼300 m.y. and formed at temperatures <140 °C. This late Paleozoic age falls within the range of illites from sandstone in the Upper Mississippi Valley and K-bentonites of the Appalachian Basin; it coincides with the Alleghany orogeny and is interpreted as having formed by gravity-driven flow from the uplifted Alleghanian-Ouachita orogenic belt that drove hot (<140 °C) fluids through the Illinois Basin.

Overlapping Cretaceous and Eocene Alteration, Twin Creeks Carlin-Type Deposit, Nevada

We report here new Ar-Ar dates for adularia, illite, and muscovite from the Twin Creeks Carlin-type deposit. All illite samples were obtained from altered intrusive and volcanic rocks that are interlayered with mineralized sedimentary rocks, thus largely avoiding complications caused by detrital illite and muscovite in sedimentary rocks. Interpretation of the illite-age spectra was based, in part, on newly developed models for the behavior of argon in fine-grained illite. The results indicate that all adularia (in the Megapit area) formed at about 42 Ma (Eocene), whereas most illite in the Megapit area and muscovite in alteration zones beneath Chimney Creek formed at about 109 to 103 Ma (Cretaceous). Additional events involving formation of illite at 200 Ma and muscovite at about 310 Ma were also observed but were not widespread enough to be considered significant. Our Eocene date for adularia is in complete agreement with previous measurements on adularia from Twin Creeks, but our Cretaceous dates are considerably older than dates determined in previous studies of the nearby Getchell district, particularly the Osgood stock, which has a date of 98 to 92 Ma. Our results suggest that the Osgood stock is part of a more protracted Cretaceous igneous-hydrothermal event that took place between about 109 and 83 Ma. The relationship of adularia and illite to gold mineralization is not completely clear, making it impossible to use our data to assign a unique age to the Twin Creeks deposit. We favor an Eocene age for some gold mineralization because the adularia that we dated is intimately intergrown with auriferous, arsenian pyrite typical of Carlin-type mineralization. However, adularia has a very restricted distribution in the deposit and temperatures at which it formed were probably not great enough to reset Cretaceous and older ages of illite. Illite is much more widespread and its association with gold is supported by correlation between gold content and K/Al ratios of mineralized material. Thus, Cretaceous illitization was at least an important ground-preparing event at Twin Creeks, and it is not possible to show from our data that gold ore or protore were not deposited during this event.

Further evidence for a Jurassic mineralizing event in central Europe: K-Ar dating of hydrothermal alteration and fluid inclusion systematics in wall rocks of the Käfersteige fluorite vein deposit in the northern Black Forest, Germany

The hydrothermal fluorite vein deposit of `Kafersteige' ranks among the biggest in central Europe. It is located along the suture zone that separates the Moldanubian and Saxothuringian units in the northern Black Forest, and is hosted in Bunter sandstone and underlying granitic basement. K–Ar ages of authigenic illite from the wall rocks give a Jurassic formation age of around 145 Ma for the deposit. Age data scatter down to 80 Ma in illite from the clay gouge and indicate a younger Cretaceous-Tertiary hydrothermal overprint. The pyrophyllite component in authigenic illite from wall rocks and the re-equilibration of illite suggest a formation temperature around 200 °C. Secondary fluid inclusions in quartz from the wall rocks define a syn-mineralization fluid episode involving Ca–Mg–K–Na–Cl-rich brines (about 27 wt% NaCleq) with a T h of about 125 °C, and a post-mineralization Na–Cl-rich fluid overprint with a T h of about 100 °C. Both generations of fluid inclusions relate to the final event of each cycle, while authigenic illite composition and re-equilibration of illite in the clay gouge may document initial temperatures of formation. The Upper Jurassic fluid system can be traced all over western Europe. It is probably an expression of continent-wide rifting and concomitant regional fluid circulation in connection with major tectonic disturbances, magmatism and abnormal heat flow during the opening of the North Atlantic ocean. The younger barren fluid overprint could be related to the onset of the Alpine orogeny.

Clay mineralogy, crystallinity, and K-Ar ages of illites within the Polish Zechstein Basin; implications for the age of Kupferschiefer mineralization

Sir: We are pleased to respond to J. Nawrocki’s comment. It gives us an additional opportunity to articulate our ideas on the significance of the K-Ar ages of illites with respect to Kupferschiefer ore genesis (Bechtel et al., 1999). Nawrocki perhaps has overstated our intent by writing that we used paleomagnetic data to verify our indirect method for measuring the age of the Kupferschiefer mineralization event. The ages in question were estimated ages of diagenetic illite in both the abstract and in the main body of the paper. In the closing sentence of the abstract, we wrote that our results suggest that the formation of diagenetic illite was induced by the mineralizing event. We did not claim verification of the indirect dating method, and in the concluding statement that Nawrocki specifically criticized, we claimed only that the ages we inferred are, within limits of error, in agreement with the ages from paleomagnetic data (Jowett et al., 1987a). We did not (could not) claim agreement with Nawrocki’s then unpublished data that the pole position of …

A multidisciplinary approach to modelling secondary migration: a Central North Sea example

Abstract Numerical simulations assist in understanding secondary migration, and can help estimate which traps in a basin may have received a hydrocarbon charge. This is illustrated by a suite of numerical models used to track the secondary migration of hydrocarbons into an Upper Jurassic Fulmar discovery in the Central North Sea. The models include areal flow calculations using Exxon’s proprietary reservoir simulator, and cross-section calculations using TEMISPACK, a commercial basin modelling package. While the application of conventional reservoir simulation techniques to geologic time-scale problems is itself a novel approach, the most important development has been in the integration of fluid flow modelling with techniques for dating the timing of hydrocarbon charge. These techniques have been used to constrain and validate the model results. In addition, sensitivity studies permit evaluation of the required permeability for effective lateral migration. The areal calculations provide information about migration timing and the fill history of several traps. These fields are charged from mature Kimmeridge source rocks, mapped within the local drainage area. Several model runs were used to evaluate the sensitivity of migration timing to carrier bed permeability and capillary pressure characteristics. Calculations indicated that for reservoir quality carrier beds (permeability ≥ 1 mD) secondary migration can be geologically instantaneous, and that even low permeability silts can serve as effective migration pathways. A cross-sectional model was used to integrate cross-stratal and strata-parallel migration. The model included hydrocarbon expulsion from Kimmeridge source rocks into the Fulmar sandstone and lateral migration up the flank of the structure. Drilling results indicate that leakage into younger Tertiary strata has occurred. The calculation sugests a portion of the top seal breached by 10–20 Ma. Evidence of partial seal loss includes a seismic chimney interpreted above the predicted breached seal, and an appraisal well high on the Jurassic structure which encountered hydrocarbon-stained reservoir rocks with an apparent palaeohydrocarbon contact. Furthermore, hydrocarbons in the overlying Tertiary reservoirs can be geochemically tied to the remaining Jurassic accumulation. Finally, analysis of authigenic cements, particularly the K-Ar ages of fibrous illite, suggests that hydrocarbons occupied the trap until approximately 10 Ma. Analysis of Tertiary hydrocarbon fluid inclusions further indicates that hydrocarbons entered these Tertiary reservoirs approximately 10 Ma. These latter two techniques now enable the modeller to apply temporal constraints to hydrocarbon charge simulation. Such constraints allow ‘history matching’ analogous to field scale reservoir simulations.