Guoxiang Chi

Sources of basinal and Mississippi Valley-type mineralizing brines: mixing of evaporated seawater and halite-dissolution brine

Abstract Origins of basinal brines and Mississippi Valley-type (MVT) mineralizing fluids have been separately attributed to evaporation of seawater or dissolution of halite, although brines originating from the two processes are not mutually exclusive in basins. This study shows that the Na/Br Cl/Br diagram cannot distinguish between evaporated seawater and halite-dissolution fluid. Using the Na deficit Ca excess diagram which was previously proposed to characterize fluid-rock interactions of basinal brines, it is shown that most basinal brines including WT mineralizing fluids of the Viburnum Trend deposits were probably initially a mixture of halite-dissolution fluid and evaporated seawater. Using the same diagram, we suggest that the mineralizing brines of the Gays River MVT deposit were derived from an aquifer of clastics underlying a thick succession of evaporites, where halite-dissolution fluid and evaporated seawater could have mixed.

Hydrothermal dolomitization in the Lower Ordovician Romaine Formation of the Anticosti Basin: significance for hydrocarbon exploration

This study deals with the diagenetic evolution of dolostones of the Romaine Formation, from cores and outcrops on Anticosti and Mingan islands, and reports evidence for hydrocarbon migration in secondary porosity generated by high temperature / hydrothermal alteration. Four types of calcite (grouped into early (C1) and late (C2) assemblages) and six types of dolomite (grouped into 3 different (D1, D2 and D3) assemblages) are distinguished. According to fluid-inclusion and stable C-O isotope data, the early calcite group, which consists of sedimentary micrite, replacement microspar and early pore-filling calcite records near surface diagenetic environments. Early and pervasive replacement dolomites (RD1 and RD2 in the first dolomite (D1) assemblage) were formed during early burial, and are not related to porosity generation. Later replacement dolomites (RD3 and RD4 in the second dolomite (D2) assemblage) and pore-filling dolomite cements (PD1 and PD2 in the third dolomite (D3) assemblage) as well as minor sphalerite were formed from warm, saline fluids, which were likely hydrothermal in origin. Vuggy porosity was produced through brecciation and fracturing, and through some dissolution of the D1 dolomite assemblage by these hydrothermal fluids. Intercrystalline porosity was also created during formation of the D2 dolomite assemblage. The vugs were partly occluded by late-stage pore filling dolomite and calcite cements, pyrite and barite. Bitumen droplets in vugs together with hydrocarbon inclusions recorded in C2 calcite and in contemporaneous barite indicate a hydrocarbon migration event after the porositygenerating processes. The brecciation / leaching of a precursor dolomite host is uncommon in hydrothermal dolomite hydrocarbon fields, but is recognized in the adjacent coeval pool in Newfoundland. This type of alteration in the Ordovician carbonates with the presence of a rich hydrocarbon source rock and favourable maturation are key elements for the on-going exploration efforts in this basin.

Petrogenesis of the Palaeoproterozoic Xishankou pluton, northern Tarim block, northwest China: implications for assembly of the supercontinent Columbia

The Tarim block, one of the largest cratons in China, records an important part of the Proterozoic crustal evolution of the Earth. Many previous studies have focused on the Neoproterozoic magmatism and tectonic evolution of this block in relation to the break-up of Rodinia, although relatively little is known about its earlier tectono-magmatic history. In this article, we present detailed petrographic, geochronologic, whole-rock geochemical, and in situ zircon Hf isotope data for the pre-Neoproterozoic Xishankou granitoid pluton (XBP), one of several blue quartz-bearing granitoid intrusions well exposed in the Quruqtagh area, and discuss these intrusions in terms of their tectonic environment. Zircon LA-ICP-MS dating indicates that gneissic quartz diorite and granodiorite of the XBP crystallized at 1934 ± 13 and 1944 ± 19 Ma, respectively. Both underwent metamorphism essentially coeval with emplacement, a time that is compatible with the globally distributed 2.1–1.8 Ga crustal amalgamation during formation of the supercontinent Columbia. Petrographic and geochemical evidence suggest that the XBP is a continental-arc-type granite and may have been generated by the partial melting of Archaean thickened lower crust; this would suggest that the northern Tarim block was a continental-type arc at ca. 1940 Ma. Our new data, together with previous regional geological studies, indicate that a series of Palaeoproterozoic (ca. 2.0–1.8 Ga) tectono-magmatic events occurred in the northern Tarim attending the assembly of Columbia.

Massive dolomitization of a pinnacle reef in the Lower Devonian West Point Formation (Gaspe Peninsula, Quebec): An extreme case of hydrothermal dolomitization through fault-focused circulation of magmatic fluids

Devonian pinnacle reefs of the West Point Formation in Gaspe Peninsula (eastern Canada) were built on paleotectonic highs in a foreland basin. Of the nine pinnacles known in outcrop, one is dolomitized and occurs at the junction of two Acadian faults. The petrography of the dolomitized facies has revealed the presence of three dolomite phases and one late calcite cement. A first dolomite phase of small crystals is volumetrically minor; the following dolomite phase dominates and consists of centimeter-size replacive saddle dolomite crystals that contain fluid inclusions with homogenization temperatures ranging between 301 and 382C. The third dolomite consists of millimeter- to centimeter-size saddle dolomite crystals that fill late fractures; this phase is characterized by lower temperature fluid inclusions (159–171C). A lower temperature calcite phase (107–123C) fills some voids. Fluid inclusions are saline (8.7 to 13.3 wt.% NaClequiv). The dolomite and calcite phases are characterized by very negative 18OVPDB (Vienna Peedee belemnite) values (between 19 and 14) and negative 13CVPDB values (between 8 and 1). The replacive saddle dolomite phase originated from a fluid with very positive 18OVSMOW (Vienna standard mean ocean water) values (+8), whereas the following dolomite and calcite precipitated from fluids with lower 18OVSMOW values (+3.4 and +4.5). We propose that fault-focused circulation of magmatic fluids is responsible for the very high-temperature massive dolomite replacement of the calcite host, and high-temperature burial fluids later used fractures to circulate in the dolomitized host to precipitate late dolomite and calcite. Regionally, hydrocarbon migration is recognized at the time of late calcite cementation.

Diagenetic history and porosity evolution of Upper Carboniferous sandstones from the Spring Valley #1 well, Maritimes Basin, Canada–implications for reservoir development

Abstract Eighty-two core samples were collected from the Spring Valley #1 well which penetrates the Upper Carboniferous strata in the Late Devonian–Early Permian Maritimes Basin. The strata consist of alternating sandstones and mudstones deposited in a continental environment. The objective of this study is to characterize the relationship of sandstone porosity with depth, and to investigate the diagenetic processes related to the porosity evolution. Porosity values estimated from point counting range from 0% to 27.8%, but are mostly between 5% and 20%. Except samples that are significantly cemented by calcite, porosity values clearly decrease with depth. Two phases of calcite cement were distinguished based on Cathodoluminescence, with the early phase being largely dissolved and preserved as minor relicts in the later phase. Feldspar dissolution was extensive and contributed significantly to the development of secondary porosity. Quartz cementation was widespread and increased with depth. Fluid inclusions recorded in calcite and quartz cements indicate that interstitial fluids in the upper part of the stratigraphic column were dominated by waters with salinity lower than that of seawater, the middle part was first dominated by low-salinity waters, then invaded by brines, and the lower part was dominated by brines. Homogenization temperatures of fluid inclusions generally increase with depth and suggest a paleogeothermal gradient of 25 °C/km, which is broadly consistent with that indicated by vitrinite reflectance data. An erosion of 1.1–2.4 (mean 1.75) km of strata is inferred to have taken place above the stratigraphic column. δ 18 O values of calcite cements (mainly from the late phase) decrease with depth, implying increasing temperatures of formation, as also suggested by fluid-inclusion data. δ 13 C values of calcite cements range from −13.4‰ to −5.7‰, suggesting that organic matter was an important carbon source for calcite cements. A comparison of the porosity data with a theoretical compaction curve indicates that the upper and middle parts of the stratigraphic column show higher-than-normal porosity values, which are related to significant calcite and feldspar dissolution. Meteoric incursion and carboxylic acids generated from organic maturation were probably responsible for the abundant dissolution events.

Hydrocarbon Potential of the Mississippian Carbonate Platform, Bechar Basin, Algerian Sahara

The western Bechar basin contains a 1500-2000-m-thick Mississippian succession of carbonates consisting of superposed shoaling-upward sequences, whose porosity or diagenesis have never been investigated to establish the reservoir potential of these carbonates. In this paper, we present a diagenetic and porosity evolution of the Visean part of the Mississippian carbonate platform that may lead to the development of new exploration fairways in the region. A diagenetic model is proposed in which ooid and crinoid grainstones may have a reservoir potential if hydrocarbon migration occurred before the end of the Serpukhovian, prior to complete occlusion of porosity.

An overpressured fluid system associated with the giant sandstone-hosted Jinding Zn-Pb deposit, western Yunnan, China

The Jinding Zn-Pb deposit is hosted in continental clastic rocks in the Meso-Cenozoic Lanping-Simao Basin in western Yunnan. The deposit has been compared to SEDEX, MVT, and sandstone-hosted types (SST), all assuming intra-basinal origins for the ore-forming fluids. The driving forces of fluid flow and the pressure system have not been systematically investigated. This paper presents field and fluid-inclusion evidence to show that the Jinding mineralizing system was strongly overpressured. The common occurrence of CO2-rich fluid inclusions and the high fluid pressures suggest a mineralizing system markedly different from most other sediment-hosted base metal deposits including SEDEX, MVT and SST. Numerical modeling of basinal fluid flow indicates that sediment compaction alone in the Lanping-Simao Basin cannot produce any significant overpressure. Tectonic thrusting can significantly increase fluid pressure, but is not enough to cause the high fluid overpressure indicated by fluid inclusions. Input of extra-basinal fluids (e.g. deepsourced CO2) into the basin may have contributed to the development of the overpressured mineralizing system.

A New Method for Determining Fluid Compositions in the H2O-NaCl-CaCl2 System with Cryogenic Raman Spectroscopy

Raman peaks of various hydrates in the H2O-NaCl-CaCl2 system have been previously identified, but a quantitative relationship between the Raman peaks and XNaCl (i.e., NaCl/(NaCl+CaCl2)) has not been established, mainly due to the difficulty to freeze the solutions. This problem was solved by adding alumina powder to the solutions to facilitate nucleation of crystals. Cryogenic (-185 degrees C) Raman spectroscopic studies of alumina-spiced solutions indicate that XNaCl is linearly correlated with the total peak area fraction of hydrohalite. Capsules of solutions made from silica capillary were prepared to simulate fluid inclusions. Most of these artificial fluid inclusions could not be totally frozen even at temperatures as low as -185 degrees C, and the total peak area fraction of hydrohalite is not correlated linearly with XNaCl. However, the degree of deviation (XNacl) from the linear correlation established earlier is related to the amount of residual solution, which is reflected by the ratio (r) of the baseline bump area, resulting from the interstitial unfrozen brine near 3435 cm-1, and the total hydrate peak area between 3350 and 3600 cm-1. A linear correlation between XNaCl and r is established to estimate XNaCl from cryogenic Raman spectroscopic analysis for fluid inclusions.

Regional Stratigraphic, Depositional, and Diagenetic Patterns of the Interior of St. Lawrence Platform: The Lower Ordovician Romaine Formation, Western Anticosti Basin, Quebec

Lower Ordovician to lower Middle Ordovician (upper Ibexian to lower Whiterockian; upper Sauk III supersequence) subtidal to peritidal carbonates of the Romaine Formation in the western Anticosti Basin record the evolution during the early Paleozoic of the low-latitude passive margin of eastern North America. A regional paleokarst unconformity, the super-Romaine unconformity corresponding to the North American Sauk-Tippecanoe megasequence boundary developed on top of the Romaine carbonates during the early Middle Ordovician. The regional distribution of the passive-margin carbonates below the unconformity, however, suggests that significant foreland basin tectonic activity influenced the facies patterns in the uppermost Romaine Formation before the final demise of the Lower Ordovician great American carbonate bank, leading to its eventual subaerial exposure and erosion.The Romaine Formation is mostly composed of peritidal and open-shelf carbonate rocks similar to those in age-equivalent El Paso, Ellenburger, Arbuckle, Knox, Beekmantown, and St. George Groups found along the present southern and eastern flanks of the North American craton. Flooding of the Precambrian basement for the first time in the area allowed deposition of a deepening to shallowing carbonate succession in the late Ibexian. A narrow coastal belt of peritidal carbonates onlapped onto the basement with time, but the Romaine platform was mostly covered by open-marine subtidal carbonate deposits. The latter, as sea level receded and offlap began, gave way to peritidal deposition in the latest Ibexian. However, a succession of lower Whiterockian subtidal limestone found locally in the offlapping carbonates indicates that open subtidal conditions resumed briefly before the super-Romaine unconformity formed. This Romaine stratigraphy suggests that two large-scale, third-order, transgressive-regressive sequences are present and can be correlated basinward into the subsurface beneath the northern part of Anticosti Island.Petrographic and geochemical interpretations combined with other geologic and geophysical data provide evidence that the Lower Ordovician carbonates were hydrothermally altered at a regional scale to form porous, structurally controlled dolostone reservoirs. These structurally controlled hydrothermal dolomite reservoirs in the Romaine Formation provide a local but significant trapping mechanism for migrating hydrocarbons along the relatively undeformed, southwesterly dipping homoclinal succession. Their signature has been recognized along several seismic lines and has served as an exploration guide in the recent round of exploration on Anticosti Island.

Synchronous egress and ingress fluid flow related to compressional reactivation of basement faults: the Phoenix and Gryphon uranium deposits, southeastern Athabasca Basin, Saskatchewan, Canada

Previous studies on unconformity-related uranium deposits in the Athabasca Basin (Canada) suggest that egress flow and ingress flow can develop along single fault systems at different stages of compressional deformation. This research aims to examine whether or not both ingress and egress flow can develop at the same time within an area under a common compressional stress field, as suggested by the reverse displacement of the unconformity surface by the basement faults. The study considers the Phoenix and Gryphon uranium deposits in the Wheeler River area in the southeastern part of the Athabasca Basin. Two-dimensional numerical modeling of fluid flow, coupled with compressional deformation and thermal effects, was carried out to examine the fluid flow pattern. The results show that local variations in the basement geology under a common compressional stress field can result in both egress and ingress flow at the same time. The fault zone at Phoenix underwent a relatively low degree of deformation, as reflected by minor reverse displacement of the unconformity, and egress flow developed, whereas the fault zone at Gryphon experienced a relatively high degree of deformation, as demonstrated by significant reverse displacement of the unconformity, and ingress flow was dominant. The correlation between strain development and location of uranium mineralization, as exemplified by Gryphon and Phoenix uranium deposits, suggests that the localization of dilation predicted by numerical modeling may represent favourable sites for uranium mineralization in the Athabasca Basin.