Presence Of Pyrrhotite Research Articles

Magnetic discrimination of pyrrhotite‐ and greigite‐bearing sediment samples

By using bulk samples, rock magnetic measurements were performed to discriminate between pyrrhotite‐ and greigite‐bearing shallow marine sediments that are now uplifted above sea level in southwestern Taiwan. Thermal demagnetization of a composite isothermal remanent magnetization (IRM) was found to be effective in differentiating between the two types of sediments. To check the thermal instability and estimate the true unblocking temperature (TB) spectra of sediments containing these minerals, saturation IRMs (SIRMs) were imparted at each temperature step during demagnetization. While pyrrhotite‐bearing samples showed unambiguous TB temperature spectra, greigite‐bearing samples underwent considerable alteration which is responsible for most of the decrease in magnetization during thermal demagnetization. Such thermal instability of greigite is a practical and important clue for its identification. Zero‐field warming of IRM from 5 to 300 K sensitively indicates the presence of pyrrhotite and trace magnetite in bulk samples without any magnetic separation.

The metamorphism of pyrite and pyritic ores: an overview

AbstractPyrite, the most widespread and abundant of sulphide minerals in the Earth's surficial rocks, commonly constitutes the primary opaque phase in ore deposits. Consequently, an understanding of the behaviour of pyrite and its relationships with coexisting phases during the metamorphism of pyritebearing rocks is vital to the interpretation of their genesis and post-depositional history. Metamorphism is commonly responsible for the obliteration of primary textures but recent studies have shown that the refractory nature of pyrite allows it to preserve some pre-metamorphic textures. Pyrrhotite in pyritic ores has often been attributed to the breakdown of pyrite during metamorphism. It is now clear that pyrrhotite can be primary and that the presence of pyrrhotite with the pyrite provides a buffer that constrains sulphur activity during metamorphism. Pyrite-pyrrhotite ratios change during metamorphism as prograde heating results in sulphur release from pyrite to form pyrrhotite and as retrograde cooling permits re-growth of pyrite as the pyrrhotite releases sulphur. Retrograde growth of pyrite may encapsulate textures developed during earlier stages as well as preserve evidence of retrograde events. Sulphur isotope exchange of pyrite with pyrrhotite tends to homogenise phases during prograde periods but leaves signatures of increasingly heavy sulphur in the pyrite during retrograde periods.

Magnetic properties of natural pyrrhotite Part I: Behaviour of initial susceptibility and saturation-magnetization-related rock-magnetic parameters in a grain-size dependent framework

The grain-size dependence of the initial susceptibility (Xin), saturation magnetization (Js), saturation remanence (Jrs), coercive force (Hc), remanent coercive force (Hcr) and remanent acquisition coercive force (Hcr′), is reported for four natural pyrrhotites in a grain-size range from 250 μm down to < 5 μm. Xin decreases with decreasing grain size (range 7 × 10−5−1 × 10−5 m3 kg−1); Hc, Hcr and Hcr′ all increase with decreasing grain size (ranges respectively 7–80, 8–85 and 14–80 kA m−1). The magnitudes of Js (8.9–16.7 A m2 kg−1) and Jrs (2.0–6.0 A m2 kg−1) correspond with the trend indicated by the microprobe analysis (47.8 down to 46.9 at.% equivalent Fe). Jrs shows a maximum in the pseudo-single domain region (range 2.0–6.0 A m2 kg−1). Js shows a marked decrease for the finest grain-size fractions. Typical for pyrrhotite are a large Hc/Hcr ratio and a Jrs/Xin ratio which is larger than Hcr. When the presence of pyrrhotite has been established in a sample, its grain size is preferable determined by the Hcr trend because pyrrhotite compositional differences hardly show up in this trend.

Massive sulfides in a sedimented rift valley, northern Juan de Fuca Ridge

A number of mounds each several hundred meters across and up to sixty meters high have been observed with SeaMARC II acoustic imagery and Seabeam bathymetry in the sediment-filled axial valley at the northern end of the Juan de Fuca Ridge. The mounds are located a few kilometers west of the eastern valley-bounding normal fault scarp where the local sediment fill is approximately 300 m thick. All of the mounds are believed to be of hydrothermal origin, and one is associated with anomalously high heat flow in excess of 1 W m −2. A piston core collected from that mound comprises coarse clastic sulfide units interbedded with sulfidic muds. Hydrothermal minerals present in the 2.3 m section include pyrrhotite, pyrite, marcasite, sphalerite, chalcopyrite, iss (intermediate solid solution in the Cu Fe Zn S system), chalcopyrrhotite, galena, talc, barite, and amorphous silica. Mineral fabrics of the clasts indicate that the material was precipitated at or near the sea floor by mixing of hot hydrothermal fluids with cold seawater. Low concentrations of Zn, Cu, Cd, and Ag relative to those found in unsedimented ridge hydrothermal deposits, and the presence of pyrrhotite as an early phase mineral indicates that the vent fluids have been modified by reaction with sediments beneath the mound. Rapid sedimentation in a rift valley is clearly conducive to the formation of large hydrothermal mineral deposits like those believed to be present within and beneath these mounds. The relatively impermeable sediment cover insulates the crust, inhibits groundwater recharge, promotes long-lived discharge at a restricted number of sites, provides a substrate for the efficient subsurface precipitation of minerals, and through continued sedimentation, protects surficial deposits from the corrosive effects of seawater. No reliable estimate of the bulk composition of the mounds can be made with existing data, but their size is comparable to major hydrothermal mineral deposits found on land; ancient settings in which many land deposits formed are in many ways similar to the one in which the features described here are currently forming.

ChemInform Abstract: In Situ Study of the Hydrogenation of Diphenyl Ether in the Presence of Pyrrhotite and H2S

ChemInform Abstract: In Situ Study of the Hydrogenation of Diphenyl Ether in the Presence of Pyrrhotite and H₂S

Overview of Gold-Bearing Skarns of Southern Alaska: ABSTRACT

Although skarn deposits occur throughout Alaska, skarns with appreciable gold production/reserves are largely limited to an arcuate belt that stretches through southeastern and southern Alaska to the Alaska Peninsula. Within this belt are nearly 100 known gold-bearing skarn prospects and deposits. They occur in a variety of tectono-stratigraphic terranes, with no consistent age or character of the host carbonate unit. These skarns are, however, characterized by specific geologic associations, including intrusives, alteration, ore and gangue mineralogy, and trace metals. Intrusions associated with gold skarns are typically medium-grained, equigranular to slightly porphyritic members of gabbro-diorite-tonalite or diorite-quartz monzodiorite suites. Intrusive alteration, if present, consists predominantly of secondary albite, actinolite, and epidote, with little secondary quartz and sulfide and very rare secondary K-feldspar and muscovite. Typical identifying mineralogies include an abundance of epidote, chalcopyrite, and chlorite; the presence of pyrrhotite, specular magnetite, idocrase, scapolite, hornblende, and albite; and the general absence of bornite, sphalerite, and galena. Generally, only minor amounts of sulfides and magnetite in gold skarns are present in the skarns per se or the associated plutons; most of the (gold-bearing) sulfide and agnetite is present at the marble front. Trace elements found at anomalous levels in gold skarns include Ni, Co, As, V, Mo, Sb, and Bi. Elements that are inevitably low in gold skarns include Zn, Pb, F, Sn, Be, W, and Ba. These elemental characteristics contrast strongly with those of gold-poor skarns and with metamorphosed volcanogenic massive sulfide deposits. End_Page 672------------------------------ Although gold skarns are generally small deposits (< 1 mmt), their consistent geologic characteristics make them relatively predictable ore-deposit targets in Alaska. End_of_Article - Last_Page 673------------

Effect of pyrite and pyrrhotite on free radical formation in coal

The effects of pyrite (FeS 2) and pyrrhotite (Fe 7S 8) on free radical formation in a coal sample (81% carbon content) have been investigated by electron spin resonance (e.s.r.) spectroscopy. Changes in the e.s.r. parameters (spin concentration g -1, n, linewidth Δ H and g-value) were monitored in samples of coal, coal+8% FeS 2 and coal+8% Fe 7S 8, as these samples were heated in vacuum or in hydrogen from room temperature to 500 °C, in steps of 50 °C for a residence time of 30 min at each temperature. In vacuum heating, changes in n begin to occur at 400 °C, 350 °C and 300 °C respectively for coal, coal+8%Fe 7S 8 and coal+8% FeS 2 samples whereas in H 2, the corresponding temperatures are 250 °C, 200 °C and 150 °C. Changes in Δ H and g were also observed at these temperatures. The maximum increase in n occured for coal+8% FeS 2 samples whereas the minimum increase was observed for the pure coal sample. It is argued that enhancement in n is due to two mechanisms: the pyrite to pyrrhotite conversion and the presence of pyrrhotite itself. The detailed nature of the catalytic activity of pyrrhotite is not known.

The Notch Peak Contact Metamorphic Aureole, Utah: Paleomagnetism of the metasedimentary rocks and the quartz monzonite stock

The Notch Peak pluton, a Late Jurassic granitic stock, intrudes early Paleozoic miogeoclinal strata in the House Range, western Utah. As part of a detailed study to determine textural and chemical changes related to the intrusion, a paleomagnetic reconnaissance has been made of the pluton and its contact aureole. Steep, scattered magnetizations with both polarities present are found in the pluton upon alternating field (af) demagnetization, and they may reflect an original thermoremanence in magnetite that was acquired over at least one polarity reversal. Limestones and argillites in the Big Horse Canyon Member of the Orr Formation (Late Cambrian) were sampled at four stratigraphically controlled sites ranging from unaltered country rock to marble. The site farthest from the pluton is well behaved upon progressive thermal demagnetization and yields a low‐inclination, reversed Paleozoic direction by 400°C. The sites nearer the stock generally have a diffusely defined, two‐polarity, high‐inclination magnetization that reflects remagnetization by the pluton. This magnetization is removed above 300°C, and af demagnetization suggests that it resides in fine‐grained hematite. Of these sites, one appeared nearly unaltered in thin section, whereas the others range from hornfels to skarn. The remagnetization resulted from a late oxidation event related to the intrusion, perhaps from mobilization of groundwater; thus, the country rock has been remagnetized chemically rather than thermally. These results suggest that paleomagnetism may be a useful tool for detecting altered zones around plutons. In addition, no consistent, high‐blocking‐temperature magnetization was found in the aureole rocks; the magnetization of the hornfels was easily measurable at higher steps, but directions changed erratically, and this behavior in part results from the presence of pyrrhotite.

Geological setting and genetic aspects of uranium occurrences in the Kaipokok Bay-Big River area, Labrador

The metasedimentary and metavolcanic rocks of the Aphebian Aillik Group host a number of uranium occurrences which are lenticular concordant zones of finely disseminated pitchblende. The occurrences fall into two groups represented by the Kitts and Michelin deposits which are being considered for development. The Kitts deposit and several other occurrences are in an argillaceous-tuffaceous unit, up to 100 m thick and traceable for 20 km. The Michelin deposit and a number of other occurrences are in rhyolitic volcanic rocks which underlie an area 120 km long and 25 km wide.The uranium mineralization predates at least the final phase of the Hudsonian orogeny that affected the Aillik Group approximately 1,600 m.y. ago and is probably older. This is indicated by the geological settings, structures and textures of the occurrences, and radiometric age determinations. It is postulated that the group represented by the Kitts deposit is syngenetic sedimentary in origin and was formed by precipitaton of uranium in a reducing environment as indicated by the presence of pyrrhotite and graphite in the host rocks. The mineralization in the rhyolitic rocks is characterized by a pronounced enrichment of soda and a depletion of potash in the mineralized rocks in comparison with the unmineralized rocks. It is postulated that the uranium mineralization was contemporaneous with the synvolcanic alkali metasomatism of the host rocks. Uranium was precipitated by a titanium-bearing mineral, probably sphene, and occurs as pitchblende intergrown with sphene. Later Hudsonian deformation, metamorphism, and intrusions induced local redistribution of uranium, seen as pitchblende in veins, fractures, shears, and high-grade aggregates in the occurrences of both the groups. The ultimate source of uranium is considered to be the acidic magma that gave rise to the abundant volcanics of the Aillik Group.