Vein Dolomite Research Articles

Microbial and hydrothermal dolomite formation in Early Cretaceous lacustrine sediments in Yin'e Basin: Insights from petrology and geochemistry

Dolomite is widely present in geological history, but its origin has always been a prominent problem that troubles sedimentologists. For lacustrine dolomite, current research has not yet provided a reliable explanation for its complex genesis mechanism. The Early Cretaceous lakes in Northwest China host various morphological dolomites, providing valuable materials for exploring the origin of dolomites. According to their petrological and mineralogical characteristics, it can be divided into thick laminated dolomite, thin laminated dolomite, dolomitic mudstone, and vein dolomite. The ratios of trace elements and rare earth elements show that these dolomites precipitated in a brackish–suboxic environment. The high δ13C values (>8 ‰VPDB) of thick laminated dolomite and some thin laminated dolomite suggest the involvement of methane-producing microorganisms in the precipitation of dolomite, and the appearance of microscale/nanoscale spherical dolomite aggregates and the dispersed organic matter around dolomite particles jointly confirm that microbial-mediated biological activity promotes dolomite precipitation. The dolomite stoichiometry (mole % MgCO3) confirms that thick laminated dolomite was deposited in a restricted shallow water environment, while dolomitic mudstone is mainly deposited in relatively open water areas. The thin laminated dolomite in the shale laminae represents short-term or seasonal climatic and environmental fluctuations. In addition, some carbonate minerals of dolomitic mudstone in shallow water environment recrystallized by post-depositional hydrothermal effect, resulting in δ18O value decreased (<−10 ‰VPDB). The vein dolomite is characterized by high rare earth content and low δ13C and δ18O values, and its Sr isotope (0.712894 ± 0.000374) values reflect that the hydrothermal fluid may have been formed by the mixing of infiltrating lake water and crustal magmatic water. According to the characteristics of fluid inclusions, it is inferred that the hydrothermal fluid has the characteristics of low temperature (108.3 °C–159.8 °C), medium salinity (3.5 wt%–14.3 wt% NaCl) and high density (0.95–1.00 g/cm3). The microbial mediation and tectonic hydrothermal fluids play an important role in the formation of the Early Cretaceous lacustrine dolomite.

Origin and evolution of Sr-enriched hydrothermal brines: Insights from vein mineralization of Lower Triassic marine carbonates in the Eastern Sichuan Basin, South China

This paper focuses on celestite-carbonate-cemented veins that developed in the Jialingjiang Formation of the Lower Triassic carbonates from the Libixia anticline in eastern Sichuan Basin, China. Comparative petrography and geochemistry of the host rock and vein minerals provide clues to the nature, origin, and evolution of the vein-cementing fluids. The host rocks are composed mainly of micrite dolomite which yields δ18O, δ13C and 87Sr/86Sr ratios close to the values that characterize contemporaneous seawater. Two generations of vein-mineralization were recognized based on textural relationships, fluid inclusion data and isotope geochemistry. The early generation is dominated by celestite and saddle dolomite, formed from hypersaline, Sr-enriched, hydrothermal fluids. The later generation is represented by calcites, precipitated from less saline fluids at lower temperatures. The 87Sr/86Sr ratios of vein dolomite and celestite vary from 0.7076 to 0.7081, significantly lower than those of host carbonate counterparts (0.7078 to 0.7081; average 0.7080). This isotopic discrepancy suggests that the majority of Sr was sourced from deeper sequences of the lower Triassic carbonates, probably associated with thermochemical sulfate reduction for Sr accumulation. It is argued that the fracture systems associated with the Huayingshan fault provided the heat source and served as conduits for migration of hydrothermal fluids through several hundred meters, and the final mineralization occurred as a result of mixture of Sr-enriched brines with sulfate-bearing solutions. The large celestite ore bodies in the studied area are proposed to form related partially to these hydrothermal fluids.

Dolomitization models in the Sibzar Formation (Middle Devonian), Binalood Mountains (NE Iran): Based on the petrographic and geochemical evidence

Lithostratigraphically, Sibzar Formation (Middle Devonian) in the Kharv section (Binalood Mountains, NE Iran) with a thickness of 334 m consists of three units (in ascending order): dolostone with intercalations sandstone, middle basaltic sill, and upper dolostone. Three types of dolomite have been identified based on crystal size and shape. Fine crystalline dolomite (D1; finer than 30 μm) consist of Nonplanar-a dolomites. Fine to medium crystalline dolomite (D2; 20 to 150 μm) consist of mostly Nonplanar-a dolomites and some dolomites are Planar-s and Planar-e. Vein dolomites (Vd; 40 to 700 μm) have a very diverse crystalline texture and are mostly Planar-s, Planar-e and some Nonplanar-a. Cathodoluminescence technique studies also showed that most of these dolomites have dull red luminescence, in some veins with bright red luminescence and zoning. Based on the X-ray diffraction (XRD) and ICP-OES, all dolomites are nearly stoichiometric dolomite and relatively well ordered. Elemental analysis and stable isotope studies show that the concentration of minor elements such as sodium (D1: 822–1365 ppm; D2: 1016–1241 ppm; Vd: 971–1072 ppm), strontium (D1: 257–963 ppm; D2: 134–1334 ppm; Vd: 144–379 ppm), manganese (D1: 560–3675 ppm; D2: 1039–2168 ppm; Vd: 1818–3454 ppm), and iron (D1: 15997-30080 ppm; D2: 8288–18456 ppm; Vd: 19304-25618 ppm) in these dolomites were high and the values of the stable isotope of oxygen (D1: -9.1 to −5.9‰; D2: -7.6 to −6.1‰; Vd: -11.1 to −7.2‰) and carbon (D1: -3.5 to −2.5‰; D2: -5.2 to −1.3‰; Vd: -2 to −1.6‰) were low. The high values of iron and manganese and depletion of the carbon and oxygen isotopes can be related to the effects of high-temperature diagenetic fluids caused by basaltic sill during the burial. These dolomites formed in tidal environment and burial diagenesis in this Formation. Seawater, connate seawaters, saline basinal fluid, and magnesium-rich hypersaline fluids are the main source of magnesium ion (Mg2+) for dolomites of this Formation.

Diagenetic history of the proterozoic carbonates and its role in the oil field development in the Baikit Anteclise, Southwestern Siberia

The Meso- to Neoproterozoic Kamo Group and Ediacaran carbonate rocks of the Baikit Anteclise, southwestern Siberian platform, Russia, were studied to reconstruct the history of development of the reservoir rocks and oil field. The Kamo Group is composed of 3.5 to 4 km thick dolostone and, rarely, limestone succession that was deformed before the Ediacaran time. Post-depositional processes played the key role in forming the reservoir properties of the Kamo Group. During the stage of deposition and subsidence (1500–720 Ma) silicification, cementation, and recrystallization caused development of a dense and low-permeability matrix. Pronounced recrystallization led to changes in isotopic and geochemical characteristics of the primary sediments, including increase in Mn (up to 0.093%) and Fe (up to 3.11%) contents and 87Sr/86Sr ratios (up to 0.7239), decrease in Sr concentrations (down to 11 ppm), and the development of flat REE-normalized patterns. Folding and uplift in the Baikit basin began about 720 Ma ago and caused fracturing and subsequent healing with vein dolomite and calcite precipitated from hot (70–130 °C) and highly saline (up to 28.7 wt%) fluids.Geochemical (e.g. PAAS-normalized patterns of REE) and isotopic (87Sr/86Sr, δ18O, and δ13C) signatures of host dolostones and crosscutting dolomite veins are very similar suggesting that dolomites within the veins formed through dissolution of the host rocks. Uplift caused subaerial erosion, intense dissolution, and karstification, which penetrated down to 200–300 m below the surface. Folding, uplift, and peneplenization were important for porosity and permeability development in the dolostones of the Kamo Group. The cap rocks were subsequently formed during the Ediacaran and Paleozoic sedimentation cycle.The oil field developed in the Paleozoic. Oil produced from the source rocks of the Madra, Vedreshev, and Iremeken formations of the Kamo Group was dissipated during the uplift and erosion before the Ediacaran sedimentation. After deposition of the Ediacaran and Cambrian strata, source rocks of the Iremeken Formation might have released oil when they passed the oil window, but most of oil in the Kuyumba field was derived from younger Ediacaran source rocks and migrated through fault network from the flanks (the Kureika and Cis-Sayan-Yenisey syneclises) into the reservoir of the Baikit Anteclise that was a topographic high at that time.

Evolution of diagenetic fluid of ultra-deep Cretaceous Bashijiqike Formation in Kuqa depression

Diagenetic fluid types of the Cretaceous Bashijiqike formation are restored based on the analysis of petrographic, electron microprobe composition, inclusions homogenization temperature, salinity and vapor composition and laser carbon and oxygen isotope of diagenetic mineral, and regional geological background. Diagenetic fluid evolution sequence is analyzed on this basis. The crystalline dolomite cement has a low concerntration of Sr, high concerntration of Mn and higher carbon isotope, showing that the crystalline dolomite is affected by meteoric fresh water, associated with the tectonic uplift of late Cretaceous. Similar δ13CPDB, negative transfer of δ18OPDB and the differentiation of the concerntration of Fe and Mn indicate that the diagenetic fluid of the vein dolomite cement is homologous with the diagenetic fluid of the crystalline dolomite cement, temperature and depth are the dominant factors of differential precipitation between these two carbonate cements. Anhydrite cements have high concerntration of Na, extremely low concerntration of Fe and Mn contents. Based on these data, anhydrite cements can be thought to be related to the alkaline fluid overlying gypsum-salt layer produced by dehydration. The barite vein has abnormally high concerntration of Sr, ultra-high homogenization temperature and high-density gas hydrocarbon inclusions, which is speculated to be the forward fluid by intrusion of late natural gas. Coexistence of methane inclusions with CO2 gas proves existence of acid water during the accumulation of natural gas in the late stages. Therefore, the alkaline environment and associated diagenesis between the meteoric fresh water in epidiagentic stage and carbonic acid in the late diagenesis have dominated the process of diagenesis and reservoir, the secondary porosity and fracture zone formed by gas accumulation is a favorable play for the exploration of ultra-deep reservoirs.

Geology and age of the Morrison porphyry Cu–Au–Mo deposit, Babine Lake area, British Columbia

The Morrison porphyry Cu–Au–Mo deposit is genetically and spatially related to Eocene plagioclase–hornblende–biotite porphyry intrusions. One porphyry intrusion yielded a U–Pb age of 52.54 ± 1.05 Ma. Mineralization occurs in three stages: (1) vein-type and disseminated chalcopyrite and minor bornite (associated with potassic alteration and gold mineralization); (2) vein-type molybdenite (associated with weak phyllic alteration); and (3) polymetallic sulfide–carbonate veins (dolomite ± quartz–sphalerite–galena–arsenopyrite–chalcopyrite, associated with weak sericite–carbonate alteration). Re–Os dating of molybdenite yielded ages of 52.54 ± 0.22 and 53.06 ± 0.22 Ma, similar to the age of the host porphyry intrusion. Stage 1 vein fluids were predominantly of magmatic origin: Th = 400–526 °C; salinity = 39.8–47.8 wt.% NaCl equiv.; δ18Ofluid = 3.7‰–6.3‰; disseminated chalcopyrite–pyrite δ34SCDT = 0.2‰ and −0.8‰ (CDT, Canyon Diablo Troilite). Stage 2 fluids were a mixture of magmatic and meteoric water: Th = 320–421 °C; salinity = 37.0–43.1 wt.% NaCl equiv.; δ18Ofluid values range from 0.3‰ to 3.4‰; molybdenite and pyrite δ34SCDT = −2.1‰ and −1.2‰. Stage 3 fluids were predominantly of meteoric water origin: Th = 163–218 °C; salinity = 3.1–3.9 wt.% NaCl equiv.; δ18Ofluid = −2.3‰ to 3.9‰ for early vein quartz, and 1.1‰ to 6.1‰ for late vein dolomite; sphalerite and pyrite δ34SCDT = −7.1‰ to −5.6‰. Morrison is interpreted to be a typical porphyry Cu–Au–Mo deposit related to a calc-alkaline to a high-K calc-alkaline diorite to granodiorite intrusive suite, generated in a continental arc in response to early Eocene subduction of the Kula–Farallon plate beneath North America.

Dashuigou Tellurium Deposit in Sichuan Province, China: S, C, O, and H Isotope Data and Their Implications on Hydrothermal Mineralization

Abstract: Dashuigou, a unique tellurium‐dominated deposit over the world, is located in the western margin of the Yangtze cra‐ton in southwestern China. It is characterized by high‐grade tellurium accompanied by bismuth, gold, silver, and sulfur, and occurs in the area of less than one km2. The mineralization is divided into three stages, i.e. (1) tellurium‐bearing pyrrhotite–pyrite stage, (2) tetradymite stage, and (3) auriferous quartz veins stage. Tellurium mineralization coexisting with bismuth, silver, selenium, and gold predominantly develops in the stage 2, while the stage 1 is enriched only in sulfur and iron, and the stage 3 is very weakly mineralized with gold.The δ34S values of sulfides in the ore of the deposit vary in a narrow range of –3.1 ‐ +2.8 per mil with –3.1 ‐ +2.8 per mil for the stage 1 and –0.5 ‐ +2.1 for the stage 2, showing the isotopic characteristics of mantle derived sulfur. The δ13C values of vein dolomites vary from –5.3 to –7.2 per mil, with –5.3 ‐ –6.6 per mil for the stage 1 and –5.3 ‐ –7.2 per mil for the stage 2, which are significantly different from those of surrounding Triassic marble with δ13C values of –0.3 ‐ +2.8 per mil, and show characteristics of mantle derived carbon. The δ18O values of vein dolomites range from +10.2 to +13.1 per mil, which are higher than those of carbonatite, but lower than those of the marble. Their corresponding δ18Owater values are +0.6 ‐ +3.9 per mil, with +2.7 ‐ +3.8 per mil for the stage 1 and +0.6 ‐ +3.9 per mil for the stage 2. The data implies that these vein carbonates were formed by the mixing fluids of magmatic or mantle source with meteoric or formation water. The δ18O values of ore‐forming fluids responsible for the formation of vein quartz are estimated to be +3.2 to +6.8, the δD values of inclusion fluids of the quartz are measured to be –54 to –82 per mil. All those stable isotopic data suggest the involvement of the fluids from mantle and/or mantle‐derived magmas through fault system in the forming process of the Dashuigou tellurium deposit.

Hydrothermal Dolomitization and Recrystallization of Dolomite Breccias from the Miocene Monterey Formation, Tepusquet Area, California

ABSTRACT Dolomite breccias from the Miocene Monterey Formation, Tepusquet area, California are composed of dolomitic siliceous mudstones that are extensively fractured and filled with white, coarse-grained saddle dolomites. Fracturing and brecciation are much more extensive and intense in the Tepusquet area than in most other outcrops of the Monterey Formation. Despite the intensity of brecciation and its potential importance as an analog to Monterey fractured hydrocarbon reservoirs, no detailed petrographic, crystallographic, or geochemical analyses have been performed on the breccias from the Tepusquet area. In the present study, petrographic, crystallographic, and geochemical analyses show that the vein-filling dolomites were precipitated from hydrothermal fluids that were associated wit hydrocarbon migration, and that the early diagenetic matrix dolomites have been recrystallized, resetting their geochemical and crystallographic properties. Recrystallization of matrix dolomites is indicated by the uniformly negative d18O compositions (x = -7.1 d), low Sr contents (x = 230 ppm), low Na contents (x = 364 ppm), contracted unit cells (x: a = 4.812 A, c = 16.058 A), high degree of cation order, high Mg content (x = 46.1 mol% MgCO3) as compared to most Monterey dolomites, and increasing 87Sr/86Sr ratios with decreasing Sr content. Vein dolomites have large variation in stoichiometry (41.7-49.0 mol% MgCO3), Sr content (124-414 ppm), unit-cell dimensions (a: 4.806-4.829 A, c: 16.013-16.178 A), and cation order (Ca site occupancy: 0.73-0.93, g site: 0.68-0.94). Recrystallization of some vein dolomites is indicated by the covariance of mol% MgCO3 with Sr, d18O, cation order, and unit-cell parameters. The covariance of d18O with mol% MgCO3 is the inverse of the trend expected for the recrystallization of dolomite during burial diagenesis, possibly because of recrystallization during uplift. Vein dolomites have higher 87Sr/86Sr values than matrix dolomites. The least radiogenic matrix dolomites (the least recrystallized as inferred from the d18O composition) have 87Sr/86Sr apparent ages (13.0 and 15.5 Ma ± 0.3) that agree well with the previously mapped age of the brecciated unit. The more recrystallized matrix dolomites have higher 87Sr/86Sr ratios (apparent ages too young for the lower Monterey Formation), thus recording the evolution of Sr isotopic composition of the pore fluid. The close chemical similarity of the vein dolomites and the most recrystallized matrix dolomites, the episodic association between hydrocarbons and the ve n dolomites, and the recrystallization trends in the matrix dolomites, all indicate that evolved formation waters were the source of the hydrothermal fluids that precipitated the vein dolomites.

Stable isotope geochemistry of carbonates from the Apuane Alps mining district, northern Tuscany, Italy

rock, vein filling and ore-gangue carbonates associated with the main stratabound and largely stratiform barite-iron oxides-pyrite deposits of the Apuane Alps were analysed for carbon and oxygen isotpe compositions. The orebodies occur within a metamorphic phyllitic complex with carbonate-bearing intercalations (Scisti di Fornovolasco), and at the contact with the overlying metamorphic dolostones (Grezzoni). Carbonates from the tectonically underlying Nucleo Metamorfico Apuano unit were also analysed. The δ 13 C PDB and δ 18 O SMOW values of dolomite and calcite are in the ranges -0.9 to 2.8‰ and 23.1 to 31.4‰, respectively, for country rocks, -1.9 to 0.0‰ and 22.2 to 26.0‰ for vein carbonates, and -29.3 to -20.3‰ and 16.6 to 19.4‰ for gangue calcites. The main isotopic features are (1) the lowest δ 11 8O values for carbonate rocks are found at the contacts with either schist or orebody, (2) vein dolomite is depleted in both 13 C and 18 O relative to host rock dolomite, and their δ 18 O values very well correlate positively, (3) the δ 13 C of gangue calcites matches with that of carbonates associated to sulphate-reducing bacterial activity. The isotopic data and their spatial distribution can satisfactorily be interpreted either in terms of variable alteration of the protolith isotopic signatures by interaction with metamorphic fluids, or in terms of crystallization at metamorphic temperatures from fluids riven out of the schists. Water to rock mole ratios are tentatively estimated. The present results are compatible with a sedimentary-diagenetic origin for stratiform orebodies, followed by metamorphic recrystallization and mobilization processes

Dolomitisation of the Ardross Limestones (Dinantian), East Fife, Scotland

The Ardross Limestones are two thin marine units within the otherwise dominantly delta-top sediments of the Upper Calciferous Sandstone Measures of East Fife. They are entirely dolomitised bioclastic wackestones which are cut by complex dolomite-ankerite veins. The matrix of these limestones is composed of brown microcrystalline dolomite, together with a significant amount of framboidal pyrite and small amounts of siderite. The matrix dolomite usually has compositions of between 50 and 51 mol% CaCO 3, 8–15 mol% FeCO 3 and up to 1.5 mol% MnCO 3. Matrix δ 18O = −8.31 to +1.58%. The veins comprise Mn-enriched, 18O-depleted inclusion-rich dolomite succeeded by 18O-enriched ankerite. Dolomitisation appears to have been a two-stage process, in which an early fine-grained protodolomite phase recrystallised in hydrothermal fluids associated with Namurian volcanic emplacement. The protodolomite was precipitated during earliest burial from marine-dominated fluids, largely within the zone of sulphate reduction. The later recrystallisation of this material was accompanied by precipitation of the earliest vein dolomites. Later ankerite veins were precipitated from deep diagenetic brines released during some phase of post-Carboniferous regional extension. Geochemical and petrographic differences between dolomite in the Ardross Limestones and that in similar crinoidal wackstones of the overlying Lower Limestone Group in East Fife can be attributed to primary sedimentary differences and contrasting early burial histories.

High-Temperature Hydrothermal Origin for Fractured Carbonate Reservoirs in the Blackburn Oil Field, Nevada: Geologic Note: (1)

Preliminary petrographic and fluid-inclusion studies of drill cuttings and core from the Blackburn oil field reveal that many of the oil-saturated fractures, veinlets, and breccias in the dolomite reservoir rock may be of high-temperature hydrothermal origin. Textures of the earliest formed fracture/veinlet networks and breccias suggest development by explosive hydrothermal fracturing; the veinlets themselves preserve evidence that high-temperature fluids, favorable for such natural hydrothermal rock rupture, once circulated in these rocks. Primary fluid-inclusion homogenization temperatures indicate that early vein dolomite precipitated from hydrothermal brines at temperatures exceeding 350 degrees C (662 degrees F). Younger(?) quartz-sphalerite-galena veinlets developed from dilute aqueous solutions at temperature of more than 225 degrees C (437 degrees F). The high paleotemperatures recorded by early vein dolomite in the now much cooler (about 120 degrees C or 248 degrees F) Blackburn reservoir rock imply an ancient magmatic heat source, in this case almost certainly a small granodiorite stock (Tertiary?) adjoining and possibly underlying the field. Other carbonate-hosted petroleum reservoirs in eastern Nevada are also intimately associated with felsic intrusives. We propose that igneous-related, high-temperature hydrothermal fracturing could have created reservoir porosity and permeability at some or all of these fields, and that future petroleum exploration in the Basin and Range might profitably incorporate the search for concealed plutons.

Dolomite compositions as a guide to epigenetic copper mineralization, Mount Isa Inlier, NW Queensland

Dolomites associated with copper mineralization have higher Fe contents and lower Mn/Fe ratios than their unmineralized equivalents. Over a wide stratigraphic interval and range of original depositional environments, copper-associated dolomite in the Proterozoic Mount Isa Inlier of NW Queensland has Mn/Fe≤0.11. Vein dolomite has a higher Fe content than its corresponding matrix; nevertheless the Mn/Fe ratio of the matrix is a good guide to the maximum Mn/Fe ratio for a dolomitic vein. Thus the simple determination of the Fe and Mn component in the dolomite of a rock gives information about the possible association with copper mineralization even when the rock itself is completely barren.

The silver deposits at Cobalt and Gowganda, Ontario. III: Hydrothermal regimes and source reservoirs–evidence from H, O, C, and Sr isotopes and fluid inclusions

The Ag–, Co–Ni–sulpharsenide deposits of the Cobalt–Gowganda district are characterized by relatively uniform light-stable-isotope systematics, where δ18O in quartz spans 11.1–16.0‰; in K-feldspar, 10.1–12.3‰; in albite, 8.1–14.4‰; in actinolite, 6.0–7.6‰; in chlorite, 3.2–5.6‰; and δD in chlorite = −42 to −35‰. The temperature of hydrothermal silicate deposition was 150–230 °C, as calculated from Δquartz–chlorite, and triple to quadruple isotopic concordancy is locally preserved amongst quartz, chlorite, actinolite, and K-feldspar or albite. Filling temperature modes at 230 and 330 °C exist for primary inclusions in quartz and carbonates. Ore-forming hydrothermal fluids were isotopically characterized by δ18O = −2.5 to + 5‰, δD = −40 to + 5‰, interpreted to reflect isotopically and chemically evolved formation brines from Huronian aquifers, ultimately derived from marine pore fluids, and Proterozoic meteoric water recharge of the sedimentary basin. The restricted range of δ18Oquartz, Δquartz−chlorite, and δDchlorite from a large population of veins implies that the ore-forming fluids were tapped from a large reservoir, or reservoirs, relatively uniform with respect to temperature, δ18O, and δD.Quartzes in silicate selvages, wall rocks, and carbonate-dominated gangue are isotopically comparable, signifying fluid-dominated conditions and the initial precipitation of carbonates from fluids isotopically similar to those involved in the silicate stage and at comparable temperatures. Vein dolomites (δ18O = 21 to 23.1‰) continued to exchange down to temperatures of 110–140 °C in the presence of fluids where δ18O = 3 ± 2‰, during thermal attenuation of the ore-forming reservoir. Vein calcites (δ18O = 1.7 to 15.7‰) record late incursion of meteoric waters where δ18O = −8 to −22‰ at temperatures of ~50 °C. The population of vein carbonates clusters at δ13C = −3.1 to −5.3‰, and this is probably also close to the carbon-isotope signature of the hydrothermal fluid. The source of carbon is uncertain.Actinolites possess age-corrected 87Sr/86Sr = 0.715 to 0.729, for 2200 Ma, close to estimates for the contemporaneous Huronian ratio (0.7053–0.714) but more radiogenic than contemporaneous Archean volcanics (0.7017–0.7021) or the Nipissing diabase (0.7060–0.7061). On this basis, Sr is interpreted to have been derived principally from the Huronian sedimentary reservoir.Fluid inclusions in quartz and calcite of both mineralized and barren veins in the Cobalt and Gowganda mining camps and environs show five different types type I (L), type II (L + halite), type III (L + V), type IV (L + V + H), and type V (V), with types III and IV being most abundant. A histogram of all mine data shows modes around 100, 230, and 330 °C, with a range from &gt; 560 to &lt; 100 °C. No carbon dioxide was observed in the inclusions, although the dominance of calcite and dolomite in the veins attests to its presence during mineralization. Several samples show evidence of aqueous boiling, allowing a direct pressure determination of about 600 bar (60 MPa). The fluids were highly saline NaCl–CaCl2 brines, with up to 54 wt.% NaCl equivalent and highly variable Na/Ca ratios. Fluid inclusions in samples of barren veins from Lundy Township, outside the areas of known mineralization, do not appear to be significantly different from those of the mineralized veins, indicating that the hydrothermal fluids were active throughout a large area of the Huronian basin; this is corroborated by the disturbance of Pb- and Sr-isotope systems in the Nipissing, Huronian, and Archean.The Nipissing diabase likely served as a heat source to mobilize metals and advect formation brines, which may have derived the metals from either or all of the Huronian sediments or the Archean volcanics Nipissing diabase and sedimentary rocks. We suggest a genetic scheme for the veins involving CO2 effervescence and aqueous boiling inducing pH increase and thereby mediating rapid precipitation of ore minerals coeval with and followed by carbonates. This process explains most of the presently known major and minor characteristics of the vein systems and their host rocks, including the chloritic and sodic metasomatism of the Archean and Huronian rocks, abundant calcite, the compositional and mineralogical variability of the ore minerals, the textural variability of both the carbonates and ore minerals, the paragenetic sequence of alteration and mineralization, the distribution of ore minerals with respect to the diabase and other rocks, the relatively narrow vertical extent of mineralization, variations in ore grade and tonnage, and the distribution of economic deposits on the periphery of the Huronian basin.

Stable isotope, petrological, and fluid inclusion studies of minor mineral deposits from the McArthur Basin: Implications for the genesis of some sediment‐hosted base metal mineralization from the Northern Territory

The geology, stable isotopes and fluid inclusions from mineralized and unmineralized Middle Proterozoic sequences of the McArthur Basin, Northern Territory, have been studied at Eastern Creek, Bulman Mines, Beetle Springs, and other localities in the McArthur Basin where disseminated sulphides in unmineralized black shales were available from drill core. At Eastern Creek, galena and minor chalcopyrite (δ34S+3.6 to +11.2%o) occur in an evaporitic sedimentary sequence. Barite (δ34S+18.4 to +24.7%o) also occurs, and saline brines are trapped along healed fractures in the barite. Pressure‐corrected trapping temperatures in the barite (95–138°C), and in vein dolomite (158–168°C) agree with temperature estimates from the degree of maturation of the sedimentary organic matter. The δ18O and δ13CCo2 values of the mineralizing fluid were calculated to be +3.5 to +4.5%o and ‐2.7%o, respectively. Sedimentary dolomite has restricted δ13C and δ18O ranges, within the reported ranges for non‐mineralized Middle Proterozoi...

Dedolomitization in Tectonic Veins and Stylolites: Evidence for Rapid Fluid Migration During Deformation: ABSTRACT

Jurassic through Tertiary thrust-belt deformation of the Mississippian Madison Group has introduced complex fracturing, stylolitization, and carbonate vein mineralization. Host rocks are dominantly dolostone and dolomitic limestone. Tectonic veins are mineralized first by dolomite and then by multiple calcite phases. Dolomite and some generations of calcite which line veins are highly luminescent, while host-rock dolomite is non-luminescent. Both vein-lining dolomite and host-rock dolomite have been corroded and replaced by subsequent generations of calcite mineralization. These textural relationships suggest that fluids associated with thrust-belt deformation were in part extraformational and had not equilibrated with host-rock dolomite. Because thrust-belt deformation moved from west to east with time, the isotopic composition (18O, 13C) of vein and stylolite mineralization can be used to evaluate fluid migration during deformation. In three sections located along an east-west transect in the southern overthrust belt, calcite vein mineralization displays a wide range of isotopic compositions that are distinctly depleted relative to the host-rock composition. These End_Page 458------------------------------ vein-lining calcites exhibit systematic compositional changes with both time of deformation and with geographic position relative to major thrust faults. These isotopic changes in vein mineralization and pressure-solution products, together with the textural evidence for calcitization of host-rock and vein dolomite, suggest that these rocks were open to allochthonous fluid migration during deformation. End_of_Article - Last_Page 459------------

Stable isotope study of base metal deposits from the Eastern High Atlas, Morocco

Tertiary epigenetic lead ± zinc and copper mineralizations occur in the Mesozoic carbonate cover and stibnite, barite and copper mineralization in the Precambrian to Palaezoic basement in the Eastern High Atlas, Morocco. The carbon isotope data from host carbonates range from +3 to -1‰ PDB. The data are typical for marine carbon. There is no difference between surrounding dolomites and younger vein dolomites. The oxygen isotopic composition of the samples is enriched in 16O; the range is from -3 to -11‰ PDB with respect to that of carbonates in equilibrium with marine water. It is postulated that the isotopic composition was changed during dolomitization being caused by transition of connate formation waters diluted to some degree by meteoritic water. The bacteriogenically reduced sulphur from syngenetic sulphides and Mesozoic marine sulphate mobilized during Tertiary orogenesis are thought to be the source of H2S and SO4. Cambro-Ordovicien sulphate is also supposed to be the source of sulphur in the Jebel Zelmou barite deposit in the basement. Isotopic composition of lead from galenas in the Tertiary deposits from the Mesozoic cover are strikingly homogenous. Model ages vary between 230–180 Ma. It is suggested that important mobilization processes are responsible for Tertiary metallogeny in Morocco.