Early Pyrite Research Articles

Boiling and vertical mineralization zoning: a case study from the Apacheta low-sulfidation epithermal gold-silver deposit, southern Peru

The Au-Ag (±Pb-Zn) Apacheta deposit is located in the Shila district, 600 km southeast of Lima in the Cordillera Occidental of Arequipa Province, southern Peru. The vein mineralization is found in Early to Middle Miocene calc-alkaline lava flows and volcanic breccias. Both gangue and sulfide mineralization express a typical low-sulfidation system; assay data show element zoning with base metals enriched at depth and higher concentrations of precious metals in the upper part of the veins. Three main deposition stages are observed: (1) early pyrite and base-metal sulfides with minor electrum 1 and acanthite; (2) brecciation of this mineral assemblage and cross-cutting veinlets with subhedral quartz crystals, Mn-bearing calcite and rhombic adularia crystals; and finally (3) veinlets and geodal filling of an assemblage of tennantite/tetrahedrite + colorless sphalerite 2 + galena + chalcopyrite + electrum 2. Fluid inclusions in the mineralized veins display two distinct types: aqueous-carbonic liquid-rich Lw-c inclusions, and aqueous-carbonic vapor-rich Vw-c inclusions. Microthermometric data indicate that the ore minerals were deposited between 300 and 225 °C from relatively dilute hydrothermal fluids (0.6–3.4 wt% NaCl). The physical and chemical characteristics of the hydrothermal fluids show a vertical evolution, with in particular a drop in temperature and a loss of H2S. The presence of adularia and platy calcite and of co-existing liquid-rich and vapor-rich inclusions in the ore-stage indicates a boiling event. Strong H2S enrichment in the Vw-c inclusions observed at –200 m, the abundance of platy calcite, and the occurrence of hydrothermal breccia at this level may indicate a zone of intense boiling. The vertical element zoning observed in the Apacheta deposit thus seems to be directly related to the vertical evolution of hydrothermal-fluid characteristics. Precious-metal deposition mainly occurred above the 200-m level below the present-day surface, in response to a liquid/vapor phase separation due to an upward boiling front.

Morphology, origin and infrared microthermometry of fluid inclusions in pyrite from the Radka epithermal copper deposit, Srednogorie zone, Bulgaria

Pyrite samples from the Radka epithermal, replacement type, volcanic rock-hosted copper deposit, Bulgaria, have been studied using near-infrared (IR) microscopy. Two generations of pyrite based on their textures, composition and behaviour in IR light can be distinguished. Electron microprobe analyses, X-ray elemental mapping and Fourier transform infrared spectroscopy were used to study the relationship between crystal zoning, trace element contents and IR transmittance of pyrite. The observed crystal zoning is related to variable arsenic contents in massive fine-grained and colloform pyrite from the early pyrite–quartz assemblage, and cobalt contents in pyrite crystals from the late quartz–pyrite vein assemblage. There is a negative correlation between trace element content and IR transmittance of pyrite. The IR transparency of pyrite is thus a sensitive indicator of changes in trace element concentrations. Fluid inclusions have only been found in the second pyrite generation. Scanning electron microscopy observations on open fluid inclusion cavities permitted the crystallographic features of vacuoles to be determined. A characteristic feature of primary fluid inclusions in pyrite is a negative crystal habit, shaped mainly by {100}, {111} and {210}. This complicated polyhedral morphology is the reason for the observed opacity of some isometric primary inclusions. Secondary fluid inclusion morphology depends on the nature of the surface of the healed fracture. Recognition of the primary or secondary origin of fluid inclusions is enhanced by using crystallographically oriented sections. Microthermometric measurements of primary inclusions indicate that the second pyrite generation was deposited at maximum P–T conditions of 400 °C and 430 bar and from a fluid of low bulk salinity (3.5–4.6 wt%), possibly KCl-dominant. There are large ranges for homogenisation temperatures in secondary inclusions because of necking-down processes. Decrepitation features of some of pyrite-hosted inclusions and of all inclusions in associated quartz indicate reheating of the veins to 500–550 °C. The late cobalt-rich quartz–pyrite vein assemblage in the Radka deposit may be the shallow manifestation of deeper and genetically related porphyry copper mineralisation. This is a common observation of many intermediate- to high-sulfidation epithermal replacement-type ore bodies in this ore district and possibly the Cretaceous Banat–Srednogorie metallogenic belt in general.

Diagenesis of the Carrizo Sandstone at Butler Salt Dome, East Texas Basin, U.S.A.: Evidence for Fluid-Sediment Interaction Near Halokinetic Structures

ABSTRACT The Eocene Carrizo Sandstone adjacent to the Butler salt dome in the East Texas basin was cemented locally as the result of processes similar to those that have created salt dome calcite cap rocks throughout the Gulf Coast. The geometry of this anomalous cemented zone was established using Carrizo exposures in a large quarry and cores in the contiguous area. The cemented zone is delimited by a steeply dipping normal fault that is radial to the dome; the cements extend southeast of the fault for about 750 m. The Carrizo is a fine- to medium-grained quartzarenite to sublitharenite with 42 to 50% cement, principally poikilotopic calcite with local pyrite. Pyrite concretions are found on both northwest and southeast sides of the fault, as well as along the fault plane, whereas the calcite cement is present only to the southeast. 13C data for calcite (-19 to -37) suggest that bicarbonate was supplied mainly by bacterial oxidation of methane, whereas 18O data for calcite (-6 to -9) indicate that meteoric water was the principal diagenetic fluid. 87Sr/86Sr values for calcite ( 0.7073) are within the range for Middle Jurassic seawater, suggesting that dissolution of anhydrite from the Louann Salt provided the Ca. Pyrite is more 34S-rich (34S range of 10 to 16) than typical sulfides in Gulf Coast salt dome cap rocks and are interpreted to represent H2S supply from deep sour gas reservoirs. Petrographic and isotope evidence suggests that early H2S supply from deep sour gas reservoirs was channeled upward along the flank of the dome and outward along the radial fault, resulting in early pyrite precipitation along the fault zone and in adjacent sediments. Early pyrite cementation along the fault may have formed a barrier to subsequent fluid circulation, blocking the northwestward movement of both Ca-rich waters from the dome and deep natural gas moving along the fault, thus confining calcite precipitation to the southeast. Bacterial oxidation of this thermogenic gas resulted in calcite precipitation, although mixing with shallow ground water bicarbonate may account for the range in 13C values in the calcite cement. We propose that this type of cementation occurs in similar settings associated with hydrocarbon leakage to shallow levels, including adjacent to many salt domes in the Gulf of Mexico Basin, and may represent a previously unrecognized self-sealing type of hydrocarbon trap.

REMNANT COLLOFORM PYRITE AT THE HAILE GOLD DEPOSIT, SOUTH CAROLINA: A TEXTURAL KEY TO GENESIS

Auriferous iron sulfide-bearing deposits of the Carolina slate belt have distinctive mineralogical and textural features—traits that provide a basis to construct models of ore deposition. Our identification of paragenetically early types of pyrite, especially remnant colloform, crustiform, and layered growth textures of pyrite containing electrum and pyrrhotite, establishes unequivocally that gold mineralization was coeval with deposition of host rocks and not solely related to Paleozoic tectonic events. Ore horizons at the Haile deposit, South Carolina, contain many remnants of early pyrite: (1) fine-grained cubic pyrite disseminated along bedding; (2) fine-grained spongy, rounded masses of pyrite that may envelop or drape over pyrite cubes; (3) fragments of botryoidally and crustiform layered pyrite, and (4) pyritic infilling of vesicles and pumice. Detailed mineral chemistry by petrography, microprobe, SEM, and EDS analysis of replaced pumice and colloform structures containing both arsenic compositional banding and electrum points to coeval deposition of gold and the volcanic host rocks and, thus, confirms a syngenetic origin for the gold deposits. Early pyrite textures are present in other major deposits of the Carolina slate belt, such as Ridgeway and Barite Hill, and these provide strong evidence for models whereby the sulfide ores formed prior to tectonism. The role of Paleozoic metamorphism was to remobilize and concentrate gold and other minerals in structurally prepared sites. Recognizing the significance of paragenetically early pyrite and gold textures can play an important role in distinguishing sulfide ores that form in volcanic and sedimentary environments from those formed solely by metamorphic processes. Exploration strategies applied to the Carolina slate belt and correlative rocks in the eastern United States in the Avalonian basement will benefit from using syngenetic models for gold mineralization.

Preservation of fossils in clay minerals; a unique example from the Upper Ordovician Soom Shale, South Africa

SUMMARY In the Soom Shale, labile soft tissues have been replaced rapidly after death by authigenic clay minerals which now have an illitic composition. There are two possible pathways for this mineralization: (1) initial replacement of soft tissue by kaolinite which was then diagenetically transformed to illite; or (2) direct replacement of soft tissue by illite. Comparison of the compositions of the authigenic illites that replace the soft tissues and of the detrital illites in the host rock show that the former have higher magnesium numbers and enhanced potassium contents. These compositional differences are better explained if illite directly replaced the soft tissues. Kaolinite and/or illite stability are controlled by the pH and the ratio of K+/H+ in the fluid, expressed as: 2(KAl 3 Si 3 O 10 (OH) 2 ) + 3H 2 O + 2H + ↔ 3(Al 2 Si 2 O 5 (OH) 4 ) + 2K + . Although the porewaters of the Soom Shale were anoxic and, at times, euxinic, and so might well have favoured kaolinite authigenesis, the conditions within the local environment of the carcasses could have been very different. Ammonia, produced by protein decay, and potassium, concentrated within the tissues, would have been elevated relative to the surrounding pore waters, so that the K + /H + ratio would have favoured illite authigenesis. Authigenic illites have higher magnesium numbers than detrital sedimentary illites owing to extensive and early pyrite precipitation, indicated by small pyrite framboid diameters (average 4.3 μ), which left the porewaters depleted in iron. The magnesium number in authigenic illites could also have been enhanced by the inclusion of magnesium from seawater.

Origin of iron-rich Mississippi Valley–type deposits

Research Article| January 01, 2001 Origin of iron-rich Mississippi Valley–type deposits James St. Marie; James St. Marie 1Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA Search for other works by this author on: GSW Google Scholar Stephen E. Kesler; Stephen E. Kesler 1Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA Search for other works by this author on: GSW Google Scholar Cameron R. Allen Cameron R. Allen 2Cominco American, Inc., 15918 East Euclid Avenue, Spokane, Washington 99216-1815, USA Search for other works by this author on: GSW Google Scholar Geology (2001) 29 (1): 59–62. https://doi.org/10.1130/0091-7613(2001)029<0059:OOIRMV>2.0.CO;2 Article history received: 22 Feb 2000 rev-recd: 04 Oct 2000 accepted: 04 Oct 2000 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation James St. Marie, Stephen E. Kesler, Cameron R. Allen; Origin of iron-rich Mississippi Valley–type deposits. Geology 2001;; 29 (1): 59–62. doi: https://doi.org/10.1130/0091-7613(2001)029<0059:OOIRMV>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The abundance of iron in Mississippi Valley–type lead-zinc deposits and districts varies greatly; some deposits contain large amounts and others are almost free of iron. Iron in Mississippi Valley–type deposits is largely paragenetically early pyrite or marcasite that was replaced by sphalerite and galena, often in the central part of the deposit or district. Sedimentary exhalative and Irish-type base metal deposits, which also form from basinal brines, have similar variations in iron content. Calculated metal contents of brines in equilibrium with galena, sphalerite, and pyrite show that iron is significantly more abundant than lead and zinc in high-temperature (>200 °C), relatively acid brines with low sulfur contents, whereas zinc dominates under most other conditions, including brines with high temperatures and high sulfur contents. These results suggest that iron-rich Mississippi Valley–type deposits form from brines expelled from the deepest, hottest parts of sedimentary basins. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Re-Os Dating of Low-Level Highly Radiogenic (LLHR) Sulfides:The Harnas Gold Deposit, Southwest Sweden, Records Continental-Scale TectonicEvents

To obtain geologically meaningful and precise results on low-level sulfides that are highly radiogenic, blank corrections that account for the extreme difference in the Os isotope composition of sample and blank are necessary, and the resulting data is best plotted in 187 Re- 187 Os space, much like the approach used for molybdenites. We here introduce the acronym LLHR as a means of characterizing low-level and highly radiogenic sulfides. Use of the traditional 187 Re/ 188 Os vs. 187 Os/ 188 Os plot for LLHR sulfides leads to large analytical uncertainties and highly correlated errors, as 188 Os is poorly determined and can generally be accounted for by the blank. We now recognize that LLHR sulfides, such as those encountered at the Harnas Au deposit, are a common occurrence and characteristic of many crustally affiliated ore systems. The Harnas Au deposit in southwest Sweden is located in the Mjosa-Vanern mineral belt that transects, from northwest to southeast, the prominent geologic fabric of the Sveconorwegian province including the Mylonite Zone. The deposit is hosted in a strongly sheared and locally brecciated sulfide-bearing quartz vein that crosscuts the foliation of orthogneisses belonging to the ~1600 MaA mal granitoid suite. A 187 Re- 187 Os isochron of 973 ± 34 Ma (2 σ ) for samples of pyrite and chalcopyrite from outcrops of the Harnas, Alma, and Silvergruvan veins was obtained. The Harnas and subsidiary Alma veins are Au bearing, whereas the Silvergruvan, ~2.5 km to the southeast, is not of the Au-bearing type. Four replicate analyses of an additional pyrite sample from the Harnas vein yield a model age of 616 ± 40 Ma (95% C.L.). In all samples, Re and 187 Os concentrations are extremely low, 0.35 to 7.1 and 0.005 to 0.074 ppb, respectively, and total common Os (0.002–0.028 ppb) is less than half the calculated radiogenic 187 Os. Two samples of galena from the Harnas vein yielded blank level Re and Os. Neither the ~973 nor ~616 Ma time of mineralization is related to development of the Mylonite Zone at ~920 Ma or intrusion of the anorogenic Bohus-Blomskog granites at ~920 to 900 Ma, previously proposed sources for generating Mjosa-Vanern mineral belt ore fluids. An age of ~973 Ma for Harnas-Alma-Silvergruvan suggests that sulfide deposition was associated with the waning stages (0.97–0.90 Ga) of the Sveconorwegian (Grenvillian) orogeny (1.15–0.90 Ga), postdating metamorphism and deformation that produced a regionally overthickened crust. In the northern part of the Sveconorwegian province, the vein systems for these and numerous other small deposits in the Mjosa-Vanern mineral belt could have resulted from regional exhumation in the Late Sveconorwegian with associated extension and the development of fracture systems at cooler middle to upper crustal levels. In the southern part of the Sveconorwegian province, dominated by granulitic facies rocks, no mineralization is present, perhaps due only to increased exhumation and erosion that has exposed deeper crustal levels. The Late Sveconorwegian age (~973 Ma) records the main period of sulfide deposition for the Harnas-Alma-Silvergruvan vein systems and indicates the robustness of the chronometer within tectonically complex terranes. The Late Neoproterozoic age (~616 Ma) records a later time of pyrite deposition, probably associated with shearing and reopening of the Harnas vein system. Because the Au mineralization at Harnas has been described as associated with shearing and occurring in fracture fillings in early pyrite, we suggest that part or all of the Au mineralization could have been introduced at ~616 Ma during reactivation of the vein system. The 616 Ma age for pyrite suggests a previously unrecognized and much younger mineralizing episode in the Harnas region, most likely reflecting continental-scale extension and the Late Neoproterozoic opening of the Iapetus ocean.

Evidence for an Albian Hudson arm connection between the Cretaceous Western Interior Seaway of North America and the Labrador Sea

Numerous researchers have alluded to the existence of a Cretaceous Hudson Arm connection between the Labrador Sea and the Western Interior Seaway of North America. However, the evidence for this marine connection has been circumstantial. In this paper we present sedimentary geochemical data that indicate a marine influence in the Albian Mattagami Formation of the Moose River basin, James Bay Lowlands, Ontario. The facies associations between dinoflagellate-bearing laminated mudstones, fluvial sandstones, and early pyrite mineralization are interpreted to indicate deposition in the central basin of an estuary. We use the facies association between the estuarine fill and coeval kaolinitic paleosols in the Moose River basin, and in similar deposits in Quebec and Labrador, to reconstruct a southern shoreline of the Albian Hudson Arm to the Cretaceous Western Interior Seaway. We suggest that development of the Hudson Arm connection between the Labrador Sea and the Cretaceous Western Interior Seaway may be related to a regional extensional regime associated with rifting between Labrador and Greenland, and the passage of eastern North America over Cretaceous hotspots.

Mineralogy and fluid inclusion studies of the Au-Cu quartz veins in the Hamash area, South-Eastern Desert, Egypt

Field relations, petrographic observations and fluid inclusion data are used to characterize the mineralizing fluids of gold-copper bearing quartz veins, which are spatially associated with a granite-porphyry, metavolcanics and metagabbro in the Hamash area, South Eastern Desert of Egypt. Four generations of genetically related quartz veins occur in the Hamash mine area. Two types of alteration are developed in vicinity of quartz veins; i.e., sericite-quartz-pyrite and chlorite-epidote-pyrite-sericite alteration. Fe-Cu sulfides in the veins were precipitated in two stages, early pyrite (PI) and chalcopyrite were altered to secondary chalcocite, bornite and digenite and a second generation of pyrite (PII and PIII). PI pyrite and quartz contain inclusions of gold as well as remobilized gold along cracks and microfractures. Two types of fluid inclusions are distinguished: 1) primary H2O-CO2-CH4-NaCl inclusions (type I) and 2) primary and secondary aqueous inclusions (type II). Type II is further subdivided by the inclusions occurrence within different vein types. Type I inclusions entrapped the endmembers of an unmixed fluid which consists of an aqueous phase and a CO2-rich gas phase, respectively. The entrapment conditions of approximately 250°C and 200 bars were estimated by intersecting the isochores of the two coexisting aqueous and CO2-rich fluids and indicate a shallow crustal level. The salinity of type IIa inclusions is generally low (< 9 wt.% NaCleq), they homogenize above 234°C and included heterogeneous and homogeneous fluids over a wide pressure range. Homogenization temperatures of type IIb inclusions vary between 102° and 284 °C, their assumed entrapping temperatures are 200°C and 110°C, respectively. The four generations of quartz veins are related with different inclusion types. The metals including primary gold were probably transported as bisulfide complexes and precipitated due to wallrock sulfidation, fluid mixing and phase separation. The Hamash Au-Cu mineralization shows a combination of porphyry- and epithermal-deposits characteristics.

Origins of pyrites in the ∼2.5 Ga Mt. McRae Shale, the Hamersley District, Western Australia

The Mt. McRae Shale is the footwall rock unit of the Brockman Iron Formation (∼2.5 Ga in age) in the Hamersley Basin, Australia. It is characterized by a high concentration of organic carbon (∼2–8 wt%), an abundance of disseminated pyrite (∼1 to ∼10 wt% S in the bulk rocks), and abundant pyrite nodules (∼1–10 cm radius). We have examined microscale (∼200 μm to 1 cm) variations in sulfur isotopic compositions of pyrite and S and C contents in six rock samples from a ∼27 m drill core section of the base of the Mt. McRae Shale at the Whaleback Mine. The microanalyses of sulfur isotope compositions were performed in situ on single or aggregates of pyrite crystals (eighty-four analyses) using a Nd-YAG laser microprobe. The carbon contents of thirty powdered samples, drilled from different parts of the six rock samples, are similar (∼2 to ∼8 wt% C) with the exception of one sample (0.5 wt%). However, the occurrence, morphology, abundance, and δ 34S value of pyrite reveal distinct differences between the two samples from the upper part and the four samples from the lower part of the drill core section. Pyrite crystals in the upper part occur mostly as disseminated fine grains (∼5 μm). The pyrite S contents are uniform within each sample, but the δ 34S values vary from −6.3 to +7.1‰. These data suggest that pyrite crystals in the upper section were formed by bacterial reduction of seawater sulfate. Pyrite crystals of the lower section occur in the form of veinlets, nodules, or laminae of coarse grains (∼200 μm): the pyrite contents are highly variable within each specimen (0 to >10 wt%), and the δ 34S values vary from +2.2 to +11.8‰. The formation process of pyrites in the lower section appears to have been complicated: pyrite laminae were first formed by bacterial sulfate reduction during early diagenesis, and then some of the early pyrites were dissolved and reprecipitated to form pyrite nodules by later diagenetic or hydrothermal solutions in a closed-system. The sulfur isotope data obtained in this study can be best explained by a model postulating that the seawater about 2.5 Ga ago in the Hamersley Basin had the δ 34S value of +10 to +15‰ and that the kinetic isotope effects accompanying bacterial sulfate reduction were 8–13‰ and 16–21‰, respectively, during the deposition of the lower and upper sections of the McRae Shale. The variable δ 34S values of microscale area and the magnitudes of the kinetic isotope effects suggest that: (1) the sulfate concentration of the 2.5 Ga seawater was already more than one-third of the present seawater value, and (2) the activity of sulfate-reducing bacteria in the 2.5 Ga ocean was generally higher than that in the modern ocean. Suggestion 1 further implies that, by 2.5 Ga, (3) the atmosphere became oxic, and (4) the chemical and isotopic characteristics of sulfur in the earth’s near surface reservoirs (oceans and sediments) were controlled by the Phanerozoic-style biogeochemical cycles, rather than by the mantle-buffer (i.e., magmatic) mechanism. Suggestion 2 may imply that (5) the Archean oceans were generally warmer compared to the modern oceans, and (6) they produced a higher abundance of organic matter that were easily metabolized by sulfate-reducing bacteria.

Fracture Cementation in the North Sea

Fluid migration and distribution are important aspects of the evolution of sedimentary basins. An essential parameter affecting fluid flow is permeability. Even mm thick fractures have permeabilit ies many orders of magnitude above surrounding sediments, and thus have the potential of focusing fluid flow. It is a major task to establish the importance of these fractures as fluid conduits. One aspect is to determine the mechanisms of mass transport and precipitation of the fracture cements. Basically the question is whether the mineral cementing fluids are locally sourced from the wall rock by diffusion or local fluid advection, or have been externally imported and focused into the fractures, or both. Detailed investigation of vein mineral and wall rock may help in establishing the relative importance of the two major cementing processes. Here, we will focus on the use of petrographical observations. In addition, we use microscale stable isotope analysis of carbonate cemented fractures obtained with the laser ablation method (O and C) in order to establish the direction of material transfer. Fracture cements. Cemented fractures occur in various lithologies of Tertiary to Palaeozoic ages within the North Sea, including sandstones, mudrocks, chalk, limestones and volcanics. The fractures are commonly subvertical and 1-2 mm thick, rarely exceeding 1-2 cm, and they are in many cases reactivated. The fractures are cemented by a variety of minerals. However, the about 150 fracture occurrences recorded in our North Sea fracture database are dominantly cemented by carbonates. Other minerals include kaoline, baryte, pyrite and only occasional quartz. This observation contrasts with the common occurrence of quartz veins in metamorphic rocks. Bitumen is also sometimes seen as a fracture filling material. Cemented fractures may be monomineralic, or they are seen to be filled with more than one mineral type. Carbonate cement. Calcite, dolomite/ankerite and siderite are all observed. The grain size of the fracture carbonates is in most cases notably coarser than the grain size in wall rock cements.The wall rock cement is generally also carbonate, but chemically distinct from the vein mineral (e.g. siderite versus dolomite or calcite). Multiple generations of carbonate cementation in fractures occur as well, and also precipitation by carbonate preceding a quite different phase as kaoline is occasionally observed. Carbonate fractures in sandstones which are not cemented by carbonates are recorded in a few cases. Pyrite cement. Pyrite fracture cements are less common, but are observed both within sandstones and altered volcanics. Some fractures have walls lined by early pyrite, and central parts cemented by carbonate, thus documenting the presence of compositionally different fluids at various stages of fracture cementation. Kaoline cement. Fractures cemented by kaoline occur within sandstones and interlayered igneous rocks.The texture is fine grained and appear similar to pore-filling, blocky kaoline cement. Wall rock cements recorded are carbonates or clays. Baryte cement. Baryte cemented fractures are observed within all lithologies. The grains are commonly coarse compared to wall rock, which is cemented by either clays or carbonates. Both equant and elongated grains occur. Dendritic growth is recorded in one case. Quartz cement. Quartz cemented fractures are rarely observed and pure quartz veins are only found within altered igneous rocks. These are probably due to hydrothermal activities. Chert veins are seen within chalk, and carbonate fractures hosted in sandstones occasionally contain a few grains of coarse quartz, which evidence the presence of silicate fluids. Isotopes. The laser ablation method allows for very small-scale recording of variations in isotope composition (C, O) across a carbonate filled fracture. One example of such a carbon isotope profile is given in Fig. 1. The carbon isotopes demonstrate that the carbon source for fracture mineralisation was different from that of the local wall rock.If the fracture fillings has been formed by wall rock dissolution and redistribution, the 613C value of the precipitating mineral would be close to the average value of the

Sulfur isotopic trends and pathways of iron sulfide formation in upper Holocene sediments of the anoxic Black Sea

Down-core trends for extents of iron sulfidation in upper Holocene microlaminated deposits of the Black Sea abyssal plain imply that most of the pyrite in these sediments is formed in the sulfidic water column and/or very close to the sediment-water interface. Sulfur isotopic data for pyrite within microlaminated muds from two localities in the deep basin show striking uniformity between sites and down core, with a mean δ 34S CDT value of −37.2 ± 0.9‰ (1σ, n = 18). These isotopic trends support the hypothesis of early pyrite formation. Isotopic compositions of pyrite sulfur in the microlaminated bottom sediments are similar to those reported for particulate reduced sulfur collected in middepth sediment traps and toδ 34S values for water-column dissolved sulfide within a narrow zone immediately below the O 2/H 2S interface. These three sulfur reservoirs are distinctly different from the isotopic composition of ambient dissolved sulfide of the deep water column (>175 m), with δ 34S values for the deep dissolved sulfide reservoir being consistently lower by ∼2 to 4‰. These isotopic relationships suggest that iron sulfidation is occurring dominantly within a narrow, shallow region at the top of the sulfidic water mass. Isotopic data from a fine-grained turbidite collected in the deep basin and from rapidly accumulating, iron monosulfide-rich muds on the anoxic upper slope are enriched in 34S relative to the microlaminated deposits. The enrichments on the basin margin reflect a large component of iron sulfide formed during rapid burial. A broad suite of geochemical data, including sulfur isotopic results, are consistent with an upper-slope sediment source for the ubiquitous muddy turbidites of the deep basin. The collective effects of reworking, transport, and redeposition result in a complete conversion of AVS to pyrite with little additional iron sulfidation. This conclusion is supported by a detailed isotopic mass balance. The present isotopic study also highlights fundamental differences in the reactivities of the iron reservoirs distributed throughout the anoxic basin. Sediments of the upper slope and the genetically-linked turbidites of the deep basin show similar extents of iron sulfidation that are significantly lower than those of the microlaminated deposits of the deep-basin floor, suggesting dramatically shorter timescales of iron reactivity for the microlaminated sediments of the deep basin. Importantly, isotopic patterns from the modern Black Sea should assist greatly in the recognition and interpretation of ancient anoxia in the stratigraphic record.

A geochemical framework for the application of stable sulphur isotopes to fossil pyritization

Pyrite formation is associated with fossils where the organic material decays by sulphate reduction, or where the carbonate skeleton acts either as a nucleation substrate or induces iron sulphide precipitation by dissolution. These requirements define specific combinations of porewater chemistry and saturation state with respect to carbonates and iron sulphides. A review of modern marine sediments suggests that near-surface porewaters are always over-saturated, or close to saturation, with iron sulphides but are alternately rich in either dissolved iron or dissolved sulphide. However, significant variations in carbonate saturation state may occur, with porewaters being undersaturated in the early stages of sulphate reduction, but subsequently becoming oversaturated until the later stages of methanogenesis. Here undersaturation may return if there is a substantial input of carbon dioxide, and if little alkalinity has arisen from iron reduction. Organic matter pyritization requires that soft-tissue material decays by sulphate reduction to release dissolved sulphide, with dissolved iron supplied by the surrounding porewaters. The modern sediment studies therefore suggest that organic matter pyritization can occur only during the earliest stages of sulphate reduction, where there are iron-rich porewaters which are saturated with iron sulphides. Pyrite replacement of carbonate shell material requires porewater undersaturation with respect to carbonates and analogy with modern sediments suggests that these conditions occur either during the earliest stages of sulphate reduction, or when methanogenesis produces undersaturation. Between these periods porewaters are saturated with respect to carbonates and pyrite of different morphologies can precipitate on carbonate shells. Sulphur isotope data provide support for these associations of porewater chemistry and style of fossil pyritization. The best-described examples are for soft tissue pyritization in Beecher's Trilobite Bed (Ordovician) and the Hünsruck Slate (Devonian), where the isotopic data are consistent with rapid, early pyritization in iron-rich porewaters. More data are needed on shell pyritization with the range of isotopic compositions indicating formation through all stages of diagenesis.

Fluid chemistry and processes at the Porgera gold deposit, Papua New Guinea

The Porgera gold deposit in Papua New Guinea is a world-class example of an alkalic-type epithermal gold system (stage II), which overprints a precursor stage of magmatic-hydrothermal gold mineralization (stage I). Gas and ion chromatographic analyses of fluid inclusions contained in vein minerals from both mineralization stages have been carried out in order to constrain the compositions of the fluids involved in, and the processes attending, ore deposition. These data indicate the presence of three end-member liquids, the most dilute of which was present throughout the mineralization history and is interpreted to represent evolved groundwater of meteoric origin. Its composition is estimated to have been approximately 500 mM Na+, 10 mM K+, 5 mM Li+, 250 mM Cl−, 0.15 mM Br−, and 0.01 mM I−, plus significant concentrations of dissolved gases. More saline liquids were also present during the two main stages of ore formation, and although their compositions differ, both are interpreted to have been derived at least in part from magmatic fluids, and to have been the media by which gold was introduced into the system. Stage I minerals contain fluid inclusions which decrease in salinity towards this dilute end-member composition through the vein paragenesis, reflecting progressive dilution at depth of the magmatic fluid source by groundwaters. Ore deposition is thought to have been caused largely by simple cooling and/or wallrock reactions, although limited in situ fluid mixing may also have occurred. The most saline fluids, present in early quartz and pyrite, contain at least 810 mM Na+, 530 mM Ca2+, 130 mM K+, 12 mM Li+, 87 mM SO4 2−, 960 mM Cl−, 1.1 mM Br−, and 0.05 mM I−, plus significant but variable concentrations of dissolved gases. Fluid inclusions from stage II hydraulic breccia veins reveal the presence of two distinct liquids with contrasting salinities, which were present at different times during vein formation. A higher salinity liquid appears to have predominated during mineralization, whereas lower salinity groundwaters filled the structures during intervening periods. The ore-forming fluid may have been forcibly injected into the veins from depth during fracturing and depressurization events, displacing the resident groundwaters in the process. The original composition of this fluid is estimated to have been at least 1770 mM Na+, 59 mM K+, 180 mM Li+, 210 mM SO4 2−, 680 mM Cl−, 1.4 mM Br−, and 0.09 mM I−, plus 1.5 mol% CO2, 0.19 mol% CH4, and 0.04 mol% N2. Gas chromatographic analyses of fluid inclusions from stage II samples show a decrease in total gas content between early unmineralized veins and post-mineralization vuggy quartz (suitable samples could not be obtained from the ore stage itself). Post-mineralization samples plot along an experimental gas-saturation curve in the CO2-CH4-H2O-NaCl system, obtained at conditions similar to those attending stage II ore deposition at Porgera (200–300 bar, ˜165 °C). These results are interpreted to indicate a period of depressurization-induced phase separation during hydraulic fracturing, which resulted in rich ore deposition. Volatile gases such as CH4 and N2, in addition to CO2 in solution, are shown to have a significant negative effect on total gas solubility. This effect may be of critical importance in lowering the temperature and increasing the depth (pressure) at which phase separation can occur in epithermal systems.

Bismuth-bearing assemblages from the Northern Pennine Orefield

AbstractBismuthinite-bearing quartz veins from the Alston Block of the North Pennine Orefield are all close to, or above, the Rookhope and Tynehead cupolas of the buried Weardale Granite. They are uniform in composition and paragenesis and are earlier than the main fluorite-baryte-galena-sphalerite mineralization of the orefield. Rhythmical crystallization of quartz, chalcopyrite and minor pyrite is followed by fluorite-quartz-chalcopyrite-minor sphalerite-altered pyrrhotite mineralization. Early tin-bearing (up to 0.29 wt.% Sn) chalcopyrite encloses trace amounts of bismuthinite (Bi2S3), synchysite (CaREE(CO3)F2), argentopentlandite (Ag(FeNi)8S8) (close to being stoichiometric), pyrrhotite, cubanite and cosalite (Pb2Bi2S5), while early pyrite carries monoclinic pyrrhotite (close to Fe7S8) and tungsten-bearing cassiterite (up to 1.03 wt.% WO3). Bismuthinite is macroscopically visible and is associated with native bismuth and small, fine-grained, spherical aggregates that qualitative analysis suggests may be cosalite crystals. Synchysite and more rarely monazite, xenotime and adularia are intergrown with bismuthinite. These mineralogical data form part of the basis for an increasing awareness of the contribution of the Weardale Granite to the early phases of mineralization in the Alston Block.

Lithologic and Diagenetic Controls on Intergranular Porosity of the Ivishak Sandstone, Northstar Unit, Beaufort Sea, Alaska: ABSTRACT

The Triassic lvishak Sandstone has a bimodal pore system consisting of (1) intergranular pores and (2) intragranular microporosity within white chert grains. Capillary modeling indicates that throughout the Ivishak at Northstar (offshore Beaufort Sea) microporous chert is water saturated, and only the intergranular pore system is available to hydrocarbons. The volume of intergranular porosity is largely related to depositional sorting. Well-sorted sandstones retain an average of 4% greater intergranular porosity than poorly-sorted sandy conglomerates despite extensive diagenesis. Paragenesis follows the sequence: pyrite > apatite > siderite > siderite dissolution > quartz > kaolinite > barite > calcite. Early diagenetic pyrite is concentrated near the top of the Ivishak, directly below the organic-rich, marine Shublik Shale, and is a result of bactedal sulfate reduction in the presence of marine pore fluids. Siderite cement has depleted [delta][sup 18]O[sub PDB] (-5.9 to -6.6 [per thousand]) and [delta][sup 13]C (-3.4 to -8.3[per thousand]) and highly radiogenic [sup 87]Sr/[sup 86]Sr (0.71064), reflecting the influx of meteoric waters and precipitation at 25 -45[degrees]C. Partial dissolution of siderite cement restores only minor amounts of porosity. Continued invasion by meteoric water led to the formation of abundant quartz cement ([approximately]7.5%) and associated kaolinite ([approximately]2%). Calcite forms a pervasivemore » cement in discrete 4 - 25 foot thick beds and is characterized by moderately depleted [delta][sup 18]O[sub PDB] (-6.3 to -7.1 [per thousand]), positive [delta][sup 13]C (0.9 to 4.3 01.), and highly radiogenic [sup 87]Sr/[sup 86]Sr (0.71088). These isotopic data are consistent with precipitation from continentally derived meteoric waters at moderate burial depths.« less

Experimental study of formation mechanisms of hydrothermal pyrite

Hydrothermal experiments were performed to synthesize pyrite crystals in the system HCl + NaCl + Fe 1− x S + H 2 O ± CaSO 4, in sealed silica capsules at temperatures between 150 and 350°C, for periods of up to eight weeks. The Fe 2+ and H 2S (aq) necessary for the nucleation and growth of the pyrite crystals were generated in situ in the capsules, by the reaction of HCl with pyrrhotite crystals. The solutions were either saturated or undersaturated with respect to pyrrhotite, with anhydrite present or absent from the systems (four experimental series). In the presence of anhydrite in pyrrhotite saturated experiments, considerably larger amounts of pyrite, marcasite, and elemental S formed compared to those without anhydrite present, and has been attributed to substantial sulfate reduction at T > 200° C by H 2(aq) and H 2S (aq) generated in situ in the reaction capsules. Two distinct pathways for pyrite nucleation and growth were recognized in the experiments, and resulted in seven pyrite types differing in crystal habits. The SEM study showed micron-sized pyrite cubes with stepped crystal faces which nucleate oriented on the surfaces of partially dissolved pyrrhotite crystals. Cube-octahedral pyrite crystals overgrow the earlier pyrite cubes, utilizing H 2S (aq) and Fe 2+ from the dissolving pyrrhotite grains. Pyrite crystals also nucleate and grow on the exterior of liquid S droplets, which formed from H 2S (aq) in the experimental capsules. The earliest pyrite crystals recognized in the SEM study on the sulfur surfaces are micron-sized cubes with distinct hopper (i.e., dendritic) morphology. Nucleation appears to be initiated by the reaction between Fe 2+ ions in solution with polysulfides, which occur concentrated on the liquid S droplet surface. Two mechanisms are introduced to account for the polysulfide concentration on sulfur droplet surfaces. The first mechanism suggests that polysulfide liquid, which is solved by liquid sulfur, is expelled to the S droplet surface due to the limited solubility of polysulfides in liquid S. A second mechanism to explain the accumulation of polysulfides in liquid S droplets involves the concept of equilibrium polymerization of S at T > 159° C. This process results in the formation of radical chain S. Polysulfides may form when the sulfur radical reacts with H 2S (aq) from the experimental solution upon which nucleation of pyrite occurs. The early pyrite habits represent transitional habit types which develop into equilibrium pyrite crystals characterized by a pyritohedral habit. This study observed that the type of nucleation surface, either pyrrhotite or S surfaces, controls the habit type of the experimental pyrite crystals. It is suggested that the type of precursor surface, and hence the specific chemical environment, controls the pyrite types that formed in the experiments.

Mineralogy, geochemistry, and paragenesis of the Eastern Metals serpentinite-associated Ni-Cu-Zn deposit, Quebec Appalachians

The Eastern Metals deposit, located in the southern Quebec Appalachians, occurs at the contact of a slice of ultramafic rock enclosed within ophiolitic melange sediments of Cambro-Ordovician age that are slightly metamorphosed to lower greenschist facies. Disseminated to massive sulfides are hosted by intensely carbonatized and silicified rocks at the sheared contacts between serpentinite and graphitic slate. The deposit comprises two mineralized zones, the Ni-Zn North zone and the Cu-Ni-Zn-Co-Au South zone. Textural and structural evidence suggests that the deposit formed in two main phases: early alteration and mineralization in serpentinite; and late main-stage Cu-Ni mineralization.Hydrothermal alteration by CO 2 -, Ca-, S-, and As-rich fluids during a relatively late stage of serpentinization took place as ophiolite slices were obducted during the Taconic orogeny. Serpentinite was initially transformed into talc-carbonate schist, and then, in a progressively more oxidizing and lower temperature environment, into listwaenite (quartz-carbonate rock), and finally birbirite (pyrite-bearing siliceous rock). All hydrothermally altered rocks are rich in Cr, Ni, and Co. The presence of relict chromite and ghosts of typical serpentinite net texture confirms the ultramafic nature of the protolith. The isotopic compositions of carbonate (delta 13 C = -2.4 to -7.0ppm) suggest a juvenile source of carbon, whereas sulfur isotopes from pyrite (delta 34 S = 7 to 19ppm) indicate interaction between seawater or meteoric water from sedimentary country rock.Late main-stage copper and nickel sulfides replace early pyrite. The sulfide assemblage in the North zone, comprising pyrite, violarite-polydymite, and millerite, is compatible with maximum temperatures of formation of 356 degrees and 282 degrees C. Massive sulfides in the South zone are composed principally of pyrite and chalcopyrite. Cubanite exsolutions in chalcopyrite indicate a minimum temperature of formation of approximately 250 degrees C. Gold, present in the South zone only, exhibits good positive correlations with lead, arsenic, antimony, and mercury. Metal zonation in the Eastern Metals deposit developed in response to changes in the redox potential in the host-rock environment.This deposit provides a unique opportunity to study a polymetallic serpentinite-associated deposit which has not been significantly overprinted by supergene alteration, intense metamorphism, or tectonic fabrics that occur in other examples of this deposit class such as Bou Azzer in Morocco or Outokumpu in Finland.

Alteration and fluid inclusion study of the Groundhog vein system, Central mining district, New Mexico

Polymetallic vein and skarn orebodies in the Groundhog mine, Central district, New Mexico, produced more than $590 million in silver, zinc, copper, lead, and gold between 1870 and 1977 (1990 prices). Mineralization occurs along the northeast-trending Groundhog fault zone, and underground workings extend to within 2 km of the Santa Rita porphyry copper deposit. Vein orebodies were mined from the upper 200 m of the deposit whereas extensive skarn and replacement orebodies were mined between 400 and 700 m below the surface.In the upper 200 m of the deposit, early Tertiary granodiorite porphyry dikes, injected along the Groundhog fault zone, crosscut Upper Cretaceous sandstone, shale, and two stages of diorite sills. Propylitic alteration characterized by albite, chlorite, calcite, epidote, and sericite accompanied each igneous intrusion.Hydrothermal fluid circulation followed a stage of normal faulting, resulting in quartz-base metal-silver precipitation in open fissures such as the Crescent ore shoot (330 X 120 X 7 m in size). Next to veins, diorite sills are altered to quartz-sericite-pyrite-clay, with local silicification. Away from veins, sericite decreases and chlorite-pyrite becomes more abundant. Vein-related chlorite is 5 to 16 times more manganese rich (2.8-8 mole % MnO) than chlorite formed in the propylitic alteration stage (0.4-1.4 mole % MnO).Paragenetic vein relationships indicate that early quartz and pyrite were followed by mainstage base metal mineralization, consisting dominantly of sphalerite, galena, chalcopyrite, and pyrite. Sphalerite is more iron and manganese rich at depth in the vein system; FeS increases from 0.8 to 7.3 mole percent and MnS increases from 0.2 to 1.2 mole percent over a 350-m vertical sampling range.Fluid inclusions in vein quartz homogenize between 270 degrees and 405 degrees C, with evidence for local boiling at 390 degrees C. Vapor-rich inclusions were only observed in preore quartz. Primary inclusions in sphalerite homogenize between 270 degrees and 340 degrees C, with a peak at 305 degrees C. The hydrothermal fluid contained an average of 7 equiv wt percent NaCl. Daughter minerals, possibly including dawsonite, anhydrite, and carbonate, are present in some inclusions.Alteration zoning, fluid inclusion data, and element zoning within Groundhog vein orebodies suggest that mineralization occurred during mixing with late-stage meteoric water. Fluid flow in a southwest direction has been documented in the underlying skarn ore zone (Meinert, 1987), whereas fluid flow in the vein ore zone appears to have been primarily vertical, with a smaller component of lateral flow. A pressure-depth estimate determined from fluid inclusion analyses suggests that 2 km of volcanic rocks may have been present above the Groundhog area at the time of mineralization and has since been removed by erosion. Comparison with other base and precious metal vein systems suggests that precious metal mineralization may have been present in the overlying, now-eroded volcanic rocks and may still constitute an unexplored target in the peripheral parts of the district.

Evidence for Cu-(Ag) mineralization by magmatic-meteoric fluid mixing in Keweenawan fissure veins, Mamainse Point, Ontario

Locally rich (up to 40% Cu) Cu-(Fe) sulfide mineralization with minor Ag occurs in fault-related fissure veins cutting, but probably penecontemporaneous with, Keweenawan plateau basalt lava flows at Mamainse Point, Ontario. Paragenetic studies of samples from the Coppercorp mine define four stages of mineralization: (1) early pyrite-(chalcopyrite) with strong silicification and sericitic alteration; (2) a transitional stage of chalcopyrite-hematite and bornite-hematite assemblages, with propylitic alteration; (3) main-stage chalcocite-hematite, often replacing earlier sulfides, with propylitic alteration; and (4) late-stage, low-temperature calcite deposition, with local Cu degrees , Ag degrees , Cu arsenides, and cuprite. This paragenetic sequence suggests an increase in f (sub O 2 ) , perhaps accompanied by an increase in pH and a decrease in total sulfur activity.The mineralized fissures crosscut but are coeval with regional-scale low-grade (zeolitic to local epidotic) alteration in the plateau basalt sequence; their mineralogy, particularly the early sericitic wall-rock alteration, is at variance with the regional alteration mineralogy. This contrast is supported by fluid inclusion and light stable isotope data. Temperatures of the regional alteration reached little higher than 300 degrees C at the base of the lava pile, and fluid salinities were generally below approximately 2 equiv wt percent NaCl, suggesting the dominant presence of meteoric water. However, in the fissure veins, fluids varied from or = 450 degrees C and approximately 22 equiv wt percent NaCl. Fluid inclusions representing the hot saline fluids were only found in the presence of copper mineralization, and it is therefore concluded that these fluids were responsible for metal transport. Analysis of decrepitation residues from fluid inclusions in chalcocite indicate that the fluids were dominantly NaCl-CaCl 2 -H 2 O solutions; suppression of the critical curve by the presence of Ca (super +2) permits the interpretation that these saline, high-temperature fluids were near their critical point at approximately 450 degrees C. Their high temperatures compared to the ambient fluids suggest that they were derived from below the Keweenawan lava pile and that they may have been associated with intrusive magmatic activity. Approximately positively correlated arrays of homogenization temperature vs. salinity data from fluid inclusions in ore samples suggest that mixing between the hot saline fluid and the cooler meteoric ground waters was an important mechanism for deposition of Cu-(Fe) sulfides. The combined effects of rapid cooling, dilution, neutralization, and oxidation resulting from this process would cause a significant drop in copper solubility with consequent deposition of ore; this process would also explain the observed paragenetic sequence of reduced to oxidized assemblages.The delta 13 C values of calcite from the fissure veins average -3.9 + or - 0.9 per mil (n = 12), and fluid delta 18 O ratios calculated from seven samples of quartz and epidote intergrown with sulfides range from 10.7 to 2.5 per mil (+ or -2ppm due to temperature uncertainties). These isotopic ratios overlap the ranges for magmatic carbon and water, respectively. Eight samples of late calcite from mineralized veins indicate fluid delta 18 O values ranging from 4.8 to -3.1 per mil (+ or -0.5-4.5ppm). Calcite from barren veins (n = 3) and from veinlets associated with the regional alteration (n = 4) extend this range down to -6.9 per mil; delta 13 C ratios of between -2.7 and +0.7 per mil were obtained from these last four samples. Negative fluid delta 18 O values and delta 13 C values of carbonates near zero per mil suggest the dominant presence of meteoric waters and atmospheric carbon, respectively, in the regional system and the dominance of meteoric water in barren veins in the fissures; this interpretation is in agreement with the fluid inclusion data. The range of delta 18 O from magmatic to lighter values in the mineralized veins suggests that meteoric water was also present during ore deposition and was mixed in varying proportions with a magmatic fluid. The isotopic data, therefore, support a model of meteoric ground water and magmatic (or highly exchanged) fluid interaction in the fissure veins.Comparisons are made to the nearby Keweenawan Jogran Cu-(Mo) porphyry, and the Tribag Cu-(Mo) breccia pipes. Similarities between these deposits and the Mamainse Point fissure veins in mineralogy, fluid properties, and light stable isotope compositions suggest a genetic link. It is proposed that the Coppercorp, Tribag, and Jogran deposits represent successively deeper levels of exposure in Keweenawan porphyry-related hydrothermal systems. Richest ore deposition occurred in the high-level fissure veins at Coppercorp, due to focused mixing of hot metalliferous fluids with meteoric ground waters.