Breccia Fillings Research Articles

Implications of multiple fluids in the deposition of Pb-Zn-Ba deposits in the Alvand Mountain, Golpayegan, Iran: Evidence from fluid inclusions and O, C, S isotopes

Middle Cretaceous sedimentary carbonates of Alvand Mountain host Pb-Zn-Ba veins. This mineralization occurs mainly as veins, breccia fillings, and to a lesser extent disseminated and host rock replacements. Ore mineralogy is simple and consists of sphalerite, galena, and barite, with minor pyrite, rare chalcopyrite, and tetrahedrite. Host rock dolomitization and hydrothermal silicification are typically associated with ore. Two types of primary liquid–vapor fluid inclusions were distinguished in sphalerite, quartz, and barite. Type I (salinity, 17–23 wt% NaCl; Th, 130 ± 30 °C) is dominant in sphalerite, whereas type II (salinity, 4.5–10 wt% NaCl, Th, 190 ± 40 °C) is dominant in quartz and barite; these fluids have characteristics of basinal brines and show a negative mixing trend in an evolving process of sulfide and gangue saturation. The δ18O and δ13C values for host rock and altered minerals range from 22.0 to 12.9 ‰ and 3.8 to −3.2 ‰, respectively, which suggest the influence of increased temperature fluids, presence of organic carbon, and fluid mixing. The δ34S values of sulfides and barite varies from 2 to 17 ‰ and 23 to 24 ‰, respectively, suggesting that reduced sulfur could be derived by thermochemical reduction of Paleogene seawater sulfate in the presence of organic matter; however, the wide ranges of δ34S sulfide values (15 ‰) exhibits that reduction of sulfur probably originated by different sulfur reduction processes. The data present in this study suggest that during increased tectonic activity in the late Cretaceous-Paleogene, mixing of basinal brines and water–rock interaction resulted in Pb-Zn-Ba deposition in carbonate host rocks. Therefore, it may represent low-temperature mineralization, possibly analogous to MVT depositional systems.

Geology, fluid inclusion, and stable isotope (O, H, S, C) characteristics of the Hazinemağara (Gümüşhane) lead-zinc deposit, NE Turkey

The Hazinemağara (Gümüşhane) Pb-Zn deposit is located in the southern part of the Eastern Pontides Orogenic Belt in northeastern Turkey. It occurs as irregularly shaped ore bodies with breccia fill structure within the intraformational breccia or brecciated horizons of limestone in the Upper Jurassic-Lower Cretaceous Berdiga Formation. The dominant ore minerals are pyrite, sphalerite, galena, and tetrahedrite, and the major gangue minerals are calcite, quartz, and barite. Microthermometric investigations of the fluid inclusions show that the ore-forming fluids contain CaCl2 and MgCl2 in addition to NaCl. The salinity of the fluid ranges from 1.7% to 9.2% with an average of 5.7% (wt.% NaCl equivalent) (n = 93). The homogenization temperature of the fluid ranges from 160.0 to 386.0 °C with an average of 263.5 °C (n = 98). The plots of the δD values of water trapped in fluid inclusions hosted by barite and quartz (in the range of 92‰ to 81‰) and the δ18O values calculated for water in equilibrium with these minerals (4.1‰ to 18.5‰) are closer to the magmatic range than any other water sources and suggest the presence of magmatic water in hydrothermal fluid with some modifications by interaction with fluid from surrounding sedimentary rocks. The sulfur isotope composition of sulfide minerals (in the range of -7.8‰VCDT to +5.4‰VCDT, avg.: -2.0‰; n = 13) indicates a magmatic source for sulfur. It is concluded that the magmatic water derived from deep-seated Eocene plutonic rocks (Kaçkar Granitoids-I and -II) possibly carried metals and the sulfur derived from the source magma or leached from magmatic rocks on the pathway along the fault zones and precipitated mainly within voids between limestone fragments in intraformational breccia horizons of the Berdiga Formation. The Berdiga Formation is observed in large areas within the Eastern Pontide region and the intraformational breccia horizons could provide large stratabound ore potential.

Microbial-dominated carbonate platforms during the Ladinian rifting: sequence stratigraphy and evolution of accommodation in a fault-controlled setting (Catalan Coastal Ranges, NE Spain)

The Upper Muschelkalk sedimentary record constitutes a major transgressive pulse of north-eastern Iberia during the Ladinian. This record is arranged in two transgressive–regressive (T–R) sequences formed by two stepped microbial-dominated carbonate ramp systems where accommodation was mainly controlled by extensional faults. This study seeks to gain new insights into how the evolution of syn-rift subsidence controls the creation of accommodation space, the depositional styles and, especially, the palaeogeographical domains where specific microbialites developed (thrombolites and stromatolites). Thrombolite bodies (at least 40 m thick) display two types of architecture, biostromal and mud-mounded and stromatolite bodies (at least 7 m thick) consist of tabular and domed, head-shaped morphologies. Domed and mounded forms are usually developed during stages of increasing accommodation rates, low-to flat-nelief forms tend to grow in association with periods of low accommodation rates. A sea-level fall of at least 50 m occurred at the end of the Early Ladinian leaving the platform subaerially exposed. As a result, a prominent karst with significant erosional incisions and profuse collapse breccia fillings was formed in the inner and middle ramp settings. The resultant subaerial unconformity bounds T–R sequences 1 and 2. Subsidence curves display two stages of rapid/decelerated total subsidence, constituting two discrete rift/post-rift pulses in the large Triassic rifting period: (i) Buntsandstein – Middle Muschelkalk, and (ii) Late Muschelkalk – Imon Formation (Rhaetian). The second pulse is characterized by a rapid syn-rift subsidence during the Late Muschelkalk, and a decelerated post-rift subsidence throughout the deposition of Keuper facies and Imon Formation. The Late Muschelkalk rapid syn-rift pulse of total subsidence produces gains in accommodation, which controls the development of the stromatolites and thrombolites (biostromes and mud-mounds).

The pilot knob magnetite deposit in the Proterozoic St. Francois Mountains Terrane, southeast Missouri, USA: A magmatic and hydrothermal replacement iron deposit

The Pilot Knob magnetite deposit is located in southeast Missouri within the 1380–1480MaSt. Francois Mountains terrane rhyolitic/trachytic volcanic rocks. The deposit is tabular, dips about 45°, and is sill-like in nature, being approximately parallel to the bedding in the host tuffs. The deposit was uncovered by erosion and exposed to weathering in the late Proterozoic, and is overlain in angular unconformity by the Cambrian Lamotte Sandstone. This Proterozoic weathering cycle apparently had little effect on the deposit with only the updip edge being converted to hematite. Textural and mineralogical features of the deposit suggest a combined magmatic and hydrothermal replacement origin. The magnetite-rich ores that make up the bulk of the deposit are interpreted as having crystallized from an iron-rich magma, and a surrounding envelope of lower- to moderate-grade ores where magnetite has clearly replaced the tuffaceous host rocks are interpreted as hydrothermal in origin. After the development of the higher-grade magnetite ores and the envelope of lower- to moderate-grade ores, late hydrothermal minerals were deposited as cross-cutting veins and breccia fill. The two most abundant minerals in the higher-grade portions of the deposit are magnetite and albitic plagioclase, and petrologically the higher-grade ores could be described as a magnetite sodic syenite. The most abundant gangue mineral within the lower-grade impregnated envelope of ores around the higher-grade ores is K-feldspar, apparently relict from the rhyolites/trachytes. Thin lenses within the higher-grade ores contain calcite as a matrix mineral to the magnetite and are considered to indicate carbonatitic affinity. Rare earth elements are elevated in one of five whole rock chemical analyses of the ores and the REE-bearing mineral ferriallanite has been identified. Minor portions of the deposit, below the Proterozoic weathering cap, consist of high-grade hematite ores having equilibrium textures. At depth, the deposit is intruded by the Shepherd Mountain gabbro, a 120m thick, near-horizontal dike, which resulted in minor contact metamorphism of the ore.

Mineral inclusions in placer zircon from the Ohře (Eger) Graben: new data on “strontiopyrochlore”

After qualitative examination of the inclusion inventory of 80 zircon megacrysts from alluvial placers of the Cenozoic alkali basalt area of northern Bohemia and south-eastern Saxony, three representative megacrysts from the Podsedice deposit were selected for an in-depth study of inclusion minerals. Primary and secondary inclusions were distinguished and used as monitor for zircon-related syn- and epigenetic processes. The trace-element characteristics of the zircon hosts imply an alkali silicatic parental rock, probably nepheline syenites or fenites, which occur as enclaves in placer-near breccia fillings. The genetically most interesting inclusion species is “strontiopyrochlore”, for which its presence within areas of oscillatory zonation suggests a magmatic origin. This mode of origin contrasts to previous observations indicating formation of “strontiopyrochlore” as secondary mineral in altered carbonatite. The Sr concentration of “strontiopyrochlore” from Podsedice varies between 7.4 and 12 wt.% SrO, corresponding to a proportion of Sr of 29–48 at.% of the total A-site cations. The infiltration of late- or post-magmatic Fe-rich hydrothermal fluids along fractures crosscutting the zircon host caused a partial substitution of Nb5+ by Fe3+ in the B-site of the pyrochlore structure and the breakdown of primary fergusonite-like inclusions into undefined Nb–Y–REE–Fe oxide phases usually associated with goethite-like minerals. Thorite and related Y–REE–Th silicate phases are the most frequent secondary inclusions originated by dissolution−re-precipitation. Baddeleyite probably formed in response to desilicification that the zircon host has experienced during the contact with the SiO2-undersaturated alkali basalt melt.

Petrographic Studies of Rocks from The Chesapeake Bay Impact Structure (USA): Implication for Moderate Shock Pressures in Sedimentary Breccias

Shock petrographic investigations were carried out on samples collected from drill cores from the Chesapeake Bay impact structure (USA). The late Eocene Chesapeake impact structure is, at 85 km diameter, currently the largest impact structure known in the United States, buried at shallow to moderate depths beneath continental margin sediments underneath southeastern Virginia. To better define the variety of the samples collected from the shallow drill cores and the shock degrees experienced by the target rocks and breccias in the Chesapeake impact crater, thin section analyses were conducted on more than 50 samples from the various zones of the impact structure. The study involves measurements of the orientations of planar deformation features (PDFs) using a universal stage attached to a petrographic microscope. The aim of this study is to determine the shock pressures of various clasts in the shallow breccia fill of the crater. As a result, we note that the overwhelming numbers of shocked grains, which are now present in the sedimentary breccia, are derived from the basement granitoids. Our studies involved samples from four shallow drill cores (Exmore, Windmill Point, Kiptopeke, and Newsport News).The breccia fill is termed the Exmore breccia, which is dominated by particulates of silt, shocked and unshocked granitic fragments, shale, clay, and free shocked quartz grains. The Kiptopke and Windmill Point samples contained rare fragments showing a variety of different shock effects, whereas the Newporte News samples, show several fragments and impact melt with the evidence shock metamorphism was noted. The most abundantly observed shock indicators are shock fracturing, indicative of shock pressures of less than about 10 GPa, as well as 1-2 sets of PDFs in quartz grains, which is indicative of moderate shock pressures of up to about 20 or 25 GPa. Key words: Chesapeake Bay Crater, PDFs, Shock pressure, Universal stage, Impact structure.

Sedimentology, diagenesis, and stratigraphic occurrence of giant ooids in the Ediacaran Rainstorm Member, Johnnie Formation, Death Valley region, California

The Rainstorm Member of the late Neoproterozoic Johnnie Formation contains several distinctive features of possibly regional to global significance—the Johnnie Oolite, an incised valley with a carbonate breccia fill, and an enigmatic bed of giant ooids (the Rainstorm giant ooids). Although the Johnnie Oolite and the incised valley occur in most Johnnie Formation outcrops scattered throughout the Death Valley region, the giant ooids have only been observed in one area, restricted to the southern Nopah Range. If these giant ooids were deposited as part of the Rainstorm Member, they would represent the youngest known occurrence of Neoproterozoic giant ooids and the only such unit deposited after the last Precambrian glaciation (the Gaskiers). However, since the giant ooid bed is associated with the incised valley fill, it could simply be a large block of oolite derived from the stratigraphically lower Beck Spring Dolomite, known to contain giant ooids. This study uses mapping, petrography, and isotope geochemistry to compare giant ooids from the Rainstorm Member and the Beck Spring Dolomite, with the goal of determining the provenance of the Rainstorm giant ooids. All lines of evidence suggest that the Rainstorm giant ooids form a unit distinctly different from those of the Beck Spring Dolomite. Carbon isotope values from the ooid bed indicate that it was probably deposited after the Shuram δ 13C excursion, making this the youngest known Precambrian giant ooid unit. The most plausible explanation for this bed is that it is a raft derived from a so far unobserved pre-incision Rainstorm Member giant ooid bed, which would also have been the source of many of the giant ooid clasts in the incision breccia.

Johnnie

The Rainstorm Member of the late Neoproterozoic Johnnie Formation contains several distinctive features of possibly regional to global significance—the Johnnie Oolite, an incised valley with a carbonate breccia fill, and an enigmatic bed of giant ooids (the Rainstorm giant ooids). Although the Johnnie Oolite and the incised valley occur in most Johnnie Formation outcrops scattered throughout the Death Valley region, the giant ooids have only been observed in one area, restricted to the southern Nopah Range. If these giant ooids were deposited as part of the Rainstorm Member, they would represent the youngest known occurrence of Neoproterozoic giant ooids and the only such unit deposited after the last Precambrian glaciation (the Gaskiers). However, since the giant ooid bed is associated with the incised valley fill, it could simply be a large block of oolite derived from the stratigraphically lower Beck Spring Dolomite, known to contain giant ooids. This study uses mapping, petrography, and isotope geochemistry to compare giant ooids from the Rainstorm Member and the Beck Spring Dolomite, with the goal of determining the provenance of the Rainstorm giant ooids. All lines of evidence suggest that the Rainstorm giant ooids form a unit distinctly different from those of the Beck Spring Dolomite. Carbon isotope values from the ooid bed indicate that it was probably deposited after the Shuram δ13C excursion, making this the youngest known Precambrian giant ooid unit. The most plausible explanation for this bed is that it is a raft derived from a so far unobserved pre-incision Rainstorm Member giant ooid bed, which would also have been the source of many of the giant ooid clasts in the incision breccia.

Chaotic breccia along the Dent Fault, NW England: implosion or collapse of a fault void?

A body of chaotic breccia along the reverse-oblique Dent Fault zone is ascribed to hanging-wall collapse into persistent voids created by geometric mismatch of fault walls, although some implosion into transient voids is a possibility. The breccia comprises a 20 m wide body of hanging-wall lithologies, with a chaotic clast-supported fabric that contrasts with the fitted-fabric breccias typical of the Dent Fault damage zone. The breccia body has crude bedding defined by clast shape and size contrasts. The void fill is cut by Variscan fault strands, which, together with its ferroan calcite and barite cement, prove its late Carboniferous rather than recent age. It is shown that any fault void, transient or persistent, had a smaller aperture than the final width of the breccia body, and no more than 5 m; a span that can be supported to depths of 2 or 3 km. However, cement zonation in the breccia fill suggests that the void opened in multiple increments, each of an aperture compatible with the maximum displacement in any one event along the Dent Fault. The Dent Fault example highlights the possible general importance of fault-void collapse but also the problems in distinguishing it from implosion processes.

A hydrothermal system associated with the Siljan impact structure, Sweden--implications for the search for fossil life on Mars.

The Siljan ring structure (368 +/- 1.1 Ma) is the largest known impact structure in Europe. It isa 65-km-wide, eroded, complex impact structure, displaying several structural units, including a central uplifted region surrounded by a ring-shaped depression. Associated with the impact crater are traces of a post-impact hydrothermal system indicated by precipitated and altered hydrothermal mineral assemblages. Precipitated hydrothermal minerals include quartz veins and breccia fillings associated with granitic rocks at the outer margin of the central uplift, and calcite, fluorite, galena, and sphalerite veins associated with Paleozoic carbonate rocks located outside the central uplift. Two-phase water/gas and oil/gas inclusions in calcite and fluorite display homogenization temperatures between 75 degrees C and 137 degrees C. With an estimated erosional unloading of approximately 1 km, the formation temperatures were probably not more than 10-15 degrees C higher. Fluid inclusion ice-melting temperatures indicate a very low salt content, reducing the probability that the mineralization was precipitated during the Caledonian Orogeny. Our findings suggest that large impacts induce low-temperature hydrothermal systems that may be habitats for thermophilic organisms. Large impact structures on Mars may therefore be suitable targets in the search for fossil thermophilic organisms.

Geology, mineralogy and fluid inclusion data of the Kizilcaören fluorite–barite–REE deposit, Eskisehir, Turkey

The Kizilcaören fluorite–barite–Rare Earth Element (REE) deposit occurs as epithermal veins and breccia fillings in altered Triassic metasandstones and Oligocene–Miocene pyroclastics adjacent to alkaline porphyritic trachyte and phonolite. This deposit is the only commercial source of REE and thorium in Turkey. Most of the fluorite–barite–REE mineralisation at Kizilcaören has been formed by hydrothermal solutions, which are thought to be genetically associated with alkaline volcanism. The occurrence of the ore minerals in vuggy cavities and veins of massive and vuggy silica indicate that the ore stage postdates hydrothermal alteration. The deposit contains evidence of at least three periods of hypogene mineralisation separated by two periods of faulting. The mineral assemblage includes fluorite, barite, quartz, calcite, bastnäsite, phlogopite, pyrolusite and hematite as well as minor amounts of plagioclase feldspar, pyrite, psilomelane, braunite, monazite, fluocerite, brockite, goethite, and rutile. Fluid inclusion microthermometry indicates that the barite formed from low salinity (0.4–9.2 equiv. wt% NaCl) fluids at low temperatures, between 105 and 230 °C, but fluorite formed from slightly higher salinity (<12.4 equiv. wt% NaCl) fluids at low and moderate temperatures, between 135–354 °C. The depositional temperature of bastnäsite is between 143–286 °C. The local coexistence of liquid- and vapour-rich inclusions suggests boiling conditions. Many relatively low-salinity (<10.0 equiv. wt% NaCl), low and moderate temperature (200–300 °C) inclusions might be the result of episodic mixing of deep-saline brines with low-salinity meteoric fluids. The narrow range of δ 34S (pyrite and barite) values (2.89–6.92‰ CDT)suggests that the sulphur source of the hydrothermal fluids are the same and compatible with a volcanogenic sulphate field derived from a magmatic sulphur source.

The nature of a deformation zone and fault rock related to a recent rockburst at Western Deep Levels Gold Mine, Witwatersrand Basin, South Africa

In August 1998, a major deformation zone was exposed over several metres during mining operations on 87 Level (2463 m below surface) at Western Deep Levels Gold Mine, southwest of Johannesburg, providing a unique opportunity to study the products of a recent rockburst. This zone consists of three shear zones, with dip-slip displacements of up to 15 cm, that are oriented near-parallel to the advancing stope face. Jogs and a highly pulverised, cataclastic ‘rock-flour’ are developed on the displacement surfaces, and several sets of secondary extensional fractures occur on either side of the shear zones. A set of pinnate (feather) joints intersects the fault surfaces perpendicular to the slip vector. Microscopically, the shear zones consist of two pinnate joint sets that exhibit cataclastic joint fillings; quartz grains display intense intragranular fracturing. Secondary, intergranular extension fractures are associated with the pinnate joints. Extensional deformation is also the cause of the breccia fill of the pinnate joints. The initial deformation experienced by this zone is brittle and tensile, and is related to stresses induced by mining. This deformation has been masked by later changes in the stress field, which resulted in shearing. This deformation zone does not appear to be controlled by pre-existing geological features and, thus, represents a ‘burst fracture’, which is believed to be related to a seismic event of magnitude M L=2.1 recorded in July 1998, the epicentre of which was located to within 50 m of the study locality.

Thermal and chemical evolution of fluids during fluorite deposition in the Zaghouan province, north-eastern Tunisia

Several F, Pb, Zn and Ba deposits are located in the province of Zaghouan in north-eastern Tunisia. They are hosted in Lower Liassic or Upper Jurassic reef limestones, and the overlying condensed Carixian phosphatic limestones and Campanian marls, respectively. The mineralization occurs in three types of orebodies: stratiform replacement heaps and lenses (Jebel Stah and Hammam Zriba), breccia fillings and dissolution void fillings (Sidi Taya) and lodes (Jebel Oust). More than one generation of fluorite is observed in the stratiform deposits. Microthermometric analyses of the inclusion fluids observed in fluorite and quartz show that the economic concentrations of fluorite have deposited from moderate to highly saline (12–22.5 wt% NaCl equivalents) hydrothermal (110–160 °C) mineralizing fluids at the center (Jebel Stah, Sidi Taya) and to the east of the province (Hammam Zriba). Late remobilizations, observed in the stratiform deposits, are related to the circulation of a warmer (up to 185 °C) but less saline (10 wt% NaCl equivalents) fluid (Jebel Stah) and more saline (12–22 wt% NaCl equivalents) fluid (Hammam Zriba). The highest temperature (up to 250 °C) and salinity (32–34 wt% NaCl equivalents) are observed to the west of the province of Zaghouan (Jebel Oust). Less saline (3–6 wt% NaCl equivalents) and moderately hot to hot fluids (up to 220 ± 20 °C) and rich in gaseous CO2 invade most of the ore deposits in later stages and give rise to the massive quartz within fractures at Jebel Stah. Chemical analyses of the fluids extracted from the inclusions occuring in fluorite show compositions dominated by the presence of Na+, Ca2+ and Cl− ionic species and allow the mean temperature of the fluids in the source reservoir to be estimated as 275 ± 25 °C. The circulation of the ore-forming fluids is triggered by a regional tectonic extensional phase which occurs within the post-Jurassic to ante-Miocene time interval. The deposition of the economic concentrations of fluorite resulted from the decrease in pressure and temperature of the hydrothermal brines (Jebel Oust), along with the increase in the dissolved calcium activity (Jebel Stah and Sidi Taya), or a decrease in salinity due to the mixing with a hot, less saline and Na-poor, Ca-rich fluid (Hammam Zriba). The mineralogical associations (CaF2, PbS, ZnS, BaSO4) hosted within carbonate rocks, the temperatures and the salinities of the fluids that gave rise to the more important ore deposits (110–160 °C and 12–22.5 wt% NaCl equivalents), their composition (Na, Ca, Cl) and the molar ratios between the major ionic species, as well as the presence of liquid hydrocarbons in the mineralizing fluids, show that the ore deposits of the province of Zaghouan belong to the carbonate-hosted F, Pb, Zn, Ba Mississippi Valley-type deposits.

Evaporites; in situ sulfur source for rhythmically banded ore in the Cadjebut mississippi valley-type Zn-Pb deposit, Western Australia

The Cadjebut orebody, northern Western Australia, is a Mississippi Valley-type deposit with reserves of 3.5 million metric tons of 17 percent combined Zn + Pb. Mineralization is hosted by carbonate-evaporite units in the Givetian lower dolomite sequence of the Pillara Limestone platform and occurs as two types: (1) an early Zn-rich, stratiform, rhythmically banded ore, and (2) a later crosscutting, breccia fill, Pb-rich ore.The rhythmically banded ore comprises 0.1- to 1.0-m-thick layers of repetitive 5- to 15-mm-thick bands of sphalerite, galena, Fe sulfide, and calcite. Three generations of sphalerite are developed symmetrically about a dark brown, hydrocarbon-bearing lamination of sucrose dolomite, detrital quartz, and feldspar. Galena is associated with the second-generation colloform sphalerite, and marcasite occurs as a post-main ore mineral phase. Late-stage blocky calcite fills open cavities.Evidence for precursor evaporitic laminites as the host rock to the rhythmically banded ore includes: (1) preservation of rhythmically banded primary evaporites in stratigraphically equivalent units to the lower dolomite near Cadjebut; (2) the ubiquitous occurrence of ore calcite pseudomorphous after primary lenticular laths, rosettes, and mosaic mesh textures in ore-equivalent horizons, both inside and outside the orebody; (3) antiformal tepee structures within the banded ore; (4) enterolithic structures in unmineralized and mineralized ore-equivalent horizons; (5) solution breccias in the rhythmically banded ore horizons; and (6) homogeneous, heavy sulfur isotope signatures of the banded ore sulfides, with values just below those of lower dolomite evaporites and mid-Devonian seawater.Mineralization at Cadjebut occurred during burial of the reefal platform in the mid-late Carboniferous. During platform burial, the conversion of gypsum to anhydrite is interpreted to have resulted in volume loss and creation of an anastomosing network of microgeode-like cavities along stylolitized cyanobacteria mat laminations. These open cavities provided the passageway for the introduction of early Zn-saturated, hydrocarbon-bearing solutions, focused along regional and local faults, to the evaporites. The hydrocarbons introduced with the ore fluids acted as a catalyst for sulfate reduction of the host evaporites, resulting in the precipitation of base metal sulfides and concomitant enlargement of the geode cavities. The formation of the banded ore is fabric-mimicking, such that primary sedimentary structures remained intact during the process, despite the fact that it did not involve replacement sensu stricto.The presence of evaporites constrains the chemical model for Cadjebut mineralization, in that only a single fluid mechanism is required for base metal transport and precipitation. In situ reduction of oxidized sulfur species acquired at the site of ore deposition is implied. Sulfur isotope values yield disequilibrium, yet rather homogeneous, values that range from 4.5 to 0.5 per mil less than the interpreted source lower dolomite evaporites. The data are consistent with reduction of the host sulfates by thermochemical sulfate reduction. The direct correlation between evaporite host rocks and base metal mineralization at Cadjebut is considered an important relationship, not only for genetic models for this style of mineralization in northern Australia and elsewhere but also for Mississippi Valley-type exploration models in general.

Lead-isotope compositions of galena in carbonate-hosted deposits of western Newfoundland: evidence for diverse lead sources

Carbonate-hosted zinc and lead deposits occur in two settings in the Humber Terrane of western Newfoundland: (i) associated with dolomitized units and breccias in Cambrian and Ordovician shelf-facies carbonate rocks; and (ii) as open-space, vein, and breccia fillings associated with calcite within or near the margins of Carboniferous basins. We report 11 new lead-isotope analyses from deposits of the former type on the Great Northern Peninsula (the GNP suite) and 8 new analyses from deposits of the latter type on the Port au Port Peninsula and the northern part of the Bay St. George subbasin (BSG suite).The isotopic composition of lead in galena from the GNP suite defines linear trends in which the lead-isotope ratios increase with stratigraphic height in the Cambro-Ordovician sequence. These trends are interpreted as mixing lines; samples at the non-radiogenic end have very low 207Pb/204Pb but high 208Pb/204Pb ratios relative to 206Pb/204Pb and are interpreted as representing substantial evolution in high-grade metamorphic rocks of the Grenville Structural Province that underlie the sedimentary succession. The more radiogenic source(s?) is probably located within the sedimentary pile and contributed metals to the fluids during their migration to the deposition sites.Lead isotopes in deposits in the Bay St. George area cluster near average crustal model growth curves. Metal sources for these deposits, considered by previous workers as diagenetic in origin, are interpreted as being clastic sediments within the sedimentary basin.

Geochemical studies on the Boda Pb-Zn deposit in the Siljan astrobleme, central Sweden

Abstract The Boda Pb-Zn deposit is located in the eastern part of the Siljan impact structure in central Sweden. It consists of calcite, baritc, sphalerite, galena, pyrite and marcasite, mainly as fracture and breccia fillings along a thrust zone in Ordovician limestones. Boda, and the smaller but similar Solleron deposit, probably formed in close connection with the tectonic disturbance following the Devonian meteorite impact considered to have created the Siljan Ring structure. A geochemical profile through the ore-hosting limestone at Boda shows that wall rock alteration is not very marked. Lead isotope data point to the Proterozoic basement as the direct or indirect source of the ore lead, and to a large contribution of redeposited Proterozoic lead in the Ordovician limestone, presumably contained in the clastic silicate fraction. The extreme radiogenicity of the Boda ore lead may be explained by selective leaching of the more radiogenic lead fraction from the source rocks, rather than leaching of ave...

Lake Lappajarvi, Central Finland: a Possible Meteorite Impact Structure

SVENSSON1 has drawn attention to a most interesting cryptic structure—one more in the growing list of such cryptoexplosion structures. I would, however, question how much weight should be attached to the recognition of shock lamellae in quartz grains as a criterion of meteorite impact involvement. I have previously endorsed Bucher's scepticism regarding numerous so-called astro-blemes2, and since that publication Snyder and Gerdemann3 have put forward what seems to me to be an overwhelming case for the endogenous origin of the numerous cryptoexplosion structures east of the Appalachians. Currie and Shafiqullah4 have recovered potassic trachyte and alnöite from the Brent Crater, Ontario. Both these accounts confirm my conclusions, and the Brent discovery supports the idea that some sort of highly explosive alkalic vulcanism produced these structures. It is apparent that shatter cones and coesite, both found in association with cryptic structures of this group, are not restricted to meteoritic cryptoexplosion structures. One may well question whether shock lamellation in quartz has any more validity: surely it has been reported from the Brent and Holleford craters, which are clearly of endogenous origin ? (A fortuitous association of alnöite with an astro-bleme is extremely unlikely, for alnöites are very rare igneous rocks associated, in most cases, with localized, highly explosive diatreme activity, involving carbonatites, and such endogenous eruptivity produced the Pretoria Salt Pan which appears just like many of the cryptoexplosion craters attributed to meteorite impact explosion.) Geologists who, like myself, have worked extensively on carbonatites5 must realize that the supposed restriction on the energy involvement involved in volcanic explosions (accepted without question by many authorities because it is repeated so often in the literature) is without foundation. The sort of phase change reaction that must be involved in the production of the immense carbonatite explosion breccia fillings of some carbonatite vents, and of the associated cone sheets—a reaction producing a colossal explosion at a point focus at depth—does not seem to be limited in any way by the strength of the overlying thickness of crustal rocks. Such an explosion, involving adiabatic expansion, could, physicists assure me, involve a virtually unlimited amount of energy, and so we are left with the possibility of two mechanisms, one internal and one of extraterrestrial origin, capable of producing these cryptoexplosion structures. No one would argue that there is much conflicting evidence: and this conflict applies to the Rieskessel, New Quebec Crater, Manacouagan Lake, Clearwater Lakes, Bosumtwi Crater, and even to Wolf Creek Crater. One thing, however, is clear from the literature: there are at present no valid criteria that afford strong support for either theory, except the actual association of meteoritic material; and even where that is present, as at Wolf Creek6,7, one can reasonably harbour just an iota of doubt7.

Fluorite-bastnaesite deposits of the Gallinas Mountains, New Mexico, and bastnaesite paragenesis

Many early reports on the occurrence of bastnaesite indicate that the mineral crystallized under a high-temperature environment. The Gallinas Mountain (New Mexico) bastnaesite deposits, however, occur as epithermal veins and breccia fillings in unaltered Permian sandstone in proximity to early Tertiary alkalic hypabyssal intrusives. The low temperature of formation is indicated by the predominance of open-space fillings, little replacement, extensive crustification, and lack of wall rock alteration. In addition, a geothermometric fluid inclusion study indicated a temperature of formation of 175-185 degrees C.This re-examination of a previously-known deposit and a study of other bastnaesite occurrences shows that the mineral is generally indicative of mineralization of alkalic derivation. Bastnaesite, however, is not a critical mineral diagnostic of any particular geological environment or temperature of formation. Bastnaesite occurs in magmatic, hydrothermal, contact metasomatic, and carbonatite deposits. It is both a primary and secondary mineral.