Pyrite: physical and chemical textures

Pyrite may crystallize initially in forms as diverse as framboids or cubes depending upon the temperatures and pressures. Fluid-rich diagenesis or low-grade metamorphism clearly results in thorough recrystallization and the common formation of cubes. Once these have formed, the pyrite becomes much more refractory and retains many characteristics even in deposits which have undergone penetrative deformation. This is in strong contrast to the behavior of most of the accompanying sulfides, which often undergo ductile deformation, solid state or chemical remobilization, and annealing. Pyrite deforms sparingly until there is brittle failure; however, there may be significant pyrite corrosion and regrowth during metamorphism as the result of sulfur exchange with other minerals, especially pyrrhotite. Pyrite fabrics may also be significantly modified by pressure solution or Coble creep. Optical microscopic examination and electron microprobe chemical mapping of pyrites from a variety of mineral deposits, including several high-grade metamorphic ones, reveals that the pyrites frequently contain both physical and chemical textures that may be interpreted in terms of the depositional and the post-depositional history of the deposits. Inclusions of sulfides or other minerals reveal information on the timing of the crystallization or recrystallization of the pyrite; chemical mapping of elements such as Ni, Co, and As reveals information on the relative time of transport of these elements in the ore fluids. Received: 19 March 1997 / Accepted: 14 May 1998     

Thermobarometry of the Bleikvassli Zn-Pb-(Cu) deposit, Nordland, Norway

The Bleikvassli massive sulfide ore deposit is hosted by Proterozoic pelitic, quartzofeldspathic, and amphibolitic rocks of the Uppermost Allochthon of the Scandinavian Caledonides. Staurolite-garnet-biotite and kyanite-staurolite-biotite assemblages indicate that metamorphism reached the kyanite zone of the amphibolite facies. Geothermobarometry was conducted on rocks in and around the deposit using a variety of silicate and sulfide calibrations. Temperature determinations are most reliant on the garnet-biotite exchange reaction, with analyses obtained from 259 garnet rims and adjacent biotite. Results from nine calibrations of the garnet-biotite geothermometer are considered, but compositional limitations of many calibrations involving high Ca and Mn contents in garnet and Al^VI and Ti in biotite make many of the coexisting mineral pairs unsuitable. Average temperatures calculated from the two calibrations that most closely address the garnet-biotite compositions observed at Bleikvassli are 584 °C ± 49 °C and 570 °C ± 40 °C. The application of two calibrations of the garnet-staurolite geothermometer on a limited number of samples yields 581 °C ± 2 °C and 589 °C ± 12 °C, assuming a _H2O=0.84, based upon calculations of the modal proportions of gaseous species. Pressure determinations are less constrained. Phengite and plagioclase-biotite-garnet-muscovite geobarometers give average pressures of approximately 5.0 kbar and 8.5 ± 1.2 kbar, respectively. Pressures obtained from the sphalerite-hexagonal pyrrhotite-pyrite barometer average 7.7 ± 1.0 kbar. In consideration of these results, the peak metamorphic conditions at the Bleikvassli deposit are estimated to be 580 °C and 8 kbar. Received: 18 June 1997 / Accepted: 14 May 1998     

Linnaeite from the Precambrian Raipas group of Finnmark,Norway

The previously described minerals of the linnaeite series occuring in the Borras and Raipas deposits near Alta, Finnmark, have been re-examined, mainly with the help of the electron microprobe. — As an introduction, a survey is made of the chemistry of the linnaeite series from published data and a new diagram is presented to show the relative variations of the four possible metallic elements. — The analyses confirm the respective chemical natures of the linnaeites from the two deposits and show a rather even chemistry within each deposit. Individual grains show inhomogeneities in element contents from outsides to centres and the forms of the inhomogeneities appear to vary depending on the relative abundance of the element concerned. The most abundant element in each case is relatively depleted in a very narrow outer zone or rim, whereas the minor elements show a more gradually diminishing (often irregular) content from grain boundary to centre. The intermediate elements show varying forms of distribution. The rim zones in the case of the major elements may be the result of diffusion out into the surrounding minerals, whereas the minor element distribution is possibly a primary one related to the growth of the linnaeite grains. — Results from one large, fractured linnaeite grain in the Raipas ore indicate considerable replacement by bornite along the internal fractures, though not at the original grain boundaries.

---

Zusammenfassung Die früher beschriebenen Mineralien der Linneit-Serie, die in den Lagerstätten Borras und Raipas in der Nähe von Alta, Finnmark, vorkommen, wurden erneut untersucht, besonders mit der Elektronen-Mikrosonde. — Zunächst wird eine Übersicht der Daten über die Chemie der Linneit-Serie gegeben und ein neues Diagramm konstruiert, um die Variationsbreiten der vier metallischen Elemente von Linneit zu zeigen. — Die Analysen bestätigen die chemische Zusammensetzung der Linneite von den zwei Vorkommen, und zeigen eine weitgehende Übereinstimmung innerhalb jedes Vorkommens. Die Einzelkörner weisen Inhomogenitäten in der Elementverteilung vom Rand zur Mitte auf und ihre Form scheint mit der relativen Häufigkeit der betreffenden Elemente zu variieren. Das häufigste Element ist in jedem Fall in einer schmalen äußeren Zone relativ verarmt, während die untergeordneten Elemente eine mehr stufenweise Abnahme von Korngrenze bis Zentrum zeigen. Die intermediären Elemente zeigen wechselnde Verteilungsformen. Im Fall der Hauptelemente mag die verarmte äußere Zone ein Ergebnis von Diffusion in die umgebenden Mineralien sein, wogegen die Verteilung der untergeordneten Elemente möglicherweise primär, also an das Wachstum der Linneitkörner geknüpft ist. — Untersuchungen an einem großen aufgespaltenen Linneit-Korn aus dem Raipas-Erz deuten auf beträchtliche Verdrängungen durch Bornit entlang den inneren Spaltrissen, nicht aber entlang den ursprünglichen Korngrenzen.

     

Scandinavian Caledonide Metallogeny in a plate tectonic perspective

Caledonian metallogeny has endowed Scandinavia with abundant metalliferous mineral resources of several genetical and compositional types, many of which have been exploited at various scales from the seventeenth century onwards. Because of this long history of exploration/exploitation, coupled with excellent exposures, the Caledonian orogen in Scandinavia can be taken as a model to illustrate the relationship between metallogenic evolution and plate tectonics. Its orogenic and metallogenic development can be related to a period of plate movements which started about 700–600?Ma ago with rifting and break-up of a Proterozoic megacontinent (Rodinia) followed by the opening-up of a wide ocean (Iapetus), and ending with collision between the Laurentian and Baltic continents (the climactic Scandian phase of orogeny) in Silurian times. Fragments of this sedimentary, magmatic and tectonic history are recorded in various autochthonous, and allochthonous units which were ultimately thrusted eastwards above the Baltic plate margin. The present consensus is that sequences deposited on the margin of the Laurentian plate are represented in the Uppermost Allochthon (UmA) mostly in northern Norway. The Neoproterozoic and Cambrian history of this margin started with the rifting of Rodinia, followed by the development of an Atlantic-type margin which was characterised by an easterly thickening, continental, shelf-wedge of basal quartz sandstones and an overlying blanket of thick carbonate banks reflecting the drift of Laurentia towards equatorial paleo-latitudes. This setting is considered to have been host to two characteristic types of stratabound-stratiform ores: massive to disseminated Zn-Pb-Cu and Cu-Zn sulphides in sedimentary and mixed sedimentary-volcanic lithologies with original ore reserves up to 5–6?Mt (Bleikvassli, Mofjellet); and numerous, laterally extensive, magnetite-hematite deposits in marble-metapelite lithologies. The economically most important of the latter group are the deposits of the Dunderlandsdal area where resources of around 500?Mt were present. Early rift basins preserved on the Baltic side of the rifted megacontinent, were filled with coarse clastites prior to establishment of a passive plate margin in Cambrian times. The passive margin was characterised by shallow marine sandstones and later bituminous Alum Shales deposited in a stable, epicontinental sea. Platform sedimentation was accompanied by local magmatic activity of rift-type tholeiitic through to carbonatitic compositions. This plate margin is presently represented in autochthonous and allochthonous sequences in the lower parts of the Caledonian nappe pile. Metalliferous mineral formation related to this time and setting was of limited importance and comprised previously worked Nb, P and Fe ores in carbonatites of the Fen Complex, vast but uneconomic resources of U, Mo, Ni and V in the Alum Shales, and minor stratabound base metal sulphides and orthomagmatic Cr and Ni-Cu-(PGE) occurrences. Continental margin and oceanic successions, probably developed along the edge of a microcontinent within the Iapetus ocean, are represented in the Gula, Støren and equivalent, sequences in the Upper Allochthon (UA), tectonostratigraphically above and west of the Baltic platform lithologies. These host many Cu-Zn sulphide ores intimately related to tholeiitic basalt units in sedimentary sequences dominated by pelitic and psammitic material, as well as bituminous shales and quartzites of possible ribbon-chert origin. Original ore reserves around 20?Mt were known at the now abandoned Tverrfjell mine and the recently closed Joma deposit. Minor Cu-Ni ores occur in subvolcanic, mafic-ultramafic intrusions. Major plate convergence is first recorded in the Middle to Late Cambrian (about 505?Ma), when subduction along the outer margin of Baltica affected rocks which are now found in or below the basal parts of the UA. The metallogenic significance of this subduction is uncertain; related magmatic and ore-forming processes have yet to be documented. The earliest subduction-related sequences of major ore-forming importance, however, are slightly younger (Early Ordovician) and are presently found in the UA and partly in the UmA. One of these is the Stekenjokk-Fundsjø arc sequence (about 490?Ma) which forms a central unit in the mountain chain, comprising bimodal, immature arc-type volcanites and high-level felsic intrusions. The sequence formed in a primary oceanic setting outboard of the Baltica plate while this lay in an approximately east-west position opposite to, and at some distance from, the Siberian plate, and was amalgamated with the Gula Complex in Ordovician times. Abundant VMS deposits are associated with thick, often graphitic, tuffites and are of the Zn-Cu type with generally high Zn/Cu ratios. The biggest known deposit (Stekenjokk-Levi) contained 26?Mt of ore, four others had individual tonnages of about 3?Mt and have been important base metal producers. Another series of arc sequences are found at higher levels in the nappe pile, in southwestern Norway, the western Trondheim Region, the western Grong District, Leka and Lyngen. Mafic or bimodal volcanic and plutonic lithologies comprise many types characteristic of immature arcs and often include ophiolitic successions. They all formed in oceanic arc-marginal basin systems on the Laurentian side of the Iapetus ocean between 500 and 480?Ma. VMS deposits of the Cu-Zn type are abundant and several of these were important base metal producers in the past. The biggest deposit is Løkken which contained 30?Mt of ore; six fall in the 1–10?Mt class and include the Skorovas and other deposits in the western Grong District and in southwest Norway. Orthomagmatic ores occur in the form of PGE mineralisation in ultramafic cumulates in the basal parts of ophiolite successions and Cu-Ni-PGE mineralisation related to high-level intrusions of boninitic affinity; none of these have hitherto been of economic significance. The immature arc-marginal basin systems near Laurentia continued to evolve and in Early to Middle Ordovician times magmatism gradually changed to predominantly calc-alkaline igneous activity characteristic of an active continental margin. Major VMS ores seem to be scarce in these sequences which are most extensively developed in central and southwest Norway. Known metalliferous deposits are predominantly iron-formations ranging from magnetite- to pyrite-dominated types with varying base-metal contents; Zn-Pb-Cu mineralisation is subordinate. Major granodioritic plutons which intruded the mature-arc sequences at an advanced stage of development (c.460?Ma) occasionally contain vein- and stockwork-type Cu-Mo mineralisation, so far with no economic significance. By the end of the Ordovician, convergence of the Laurentian and Baltic continents had resulted in considerable narrowing of the Iapetus ocean and obduction of the arc/arc-basin systems on to the edge of the Laurentian plate. Material derived from the uplifted sequences supplied detritus to the remaining, narrow, basin. At about the Ordovician/Silurian boundary these thick clastic, often calcareous, sequences and their crustal substrata were intruded by rift-type, mainly tholeiitic to alkaline magmas and minor subduction-related melts; local volcanism also accompanied this activity. This important magmatic event is interpreted to reflect a paleotectonic setting characterised by transcurrent movements and development of local transtensional regimes and fault-controlled sedimentary basins during the initial, oblique, interference of the Baltic and Laurentian plate margins. Its metallogenic manifestation is seen partly in occasional Ni-Cu deposits in mafic intrusions, including the Bruvann, Råna, deposit presently being exploited. More abundant, however, are stratabound sulphides which are associated with the volcanic rocks, or hosted in the sedimentary successions often closely related to mafic intrusions. The most important past-producers were found in the Røros and Sulitjelma districts, and included several individual deposits ranging from 1 up to nearly 9?Mt in size and dominated by Cu and Zn in highly variable proportions and grades. Partly overlapping in time with the rift-related magmatism, an array of large, mafic to granitic batholiths was emplaced in rock sequences which are presently found in the Uppermost and Upper Allochthons. This activity continued through the Late Ordovician and Early Silurian, presumably along the Laurentian plate margin in response to westward subduction of the edge of Baltica. Its metallogenic significance is limited to small and uneconomic skarn-, pegmatite- and quartz-vein mineralisation of Zn-Pb-Cu, Mo, W and Be. The ultimate collision between Laurentia and Baltica occurred during the Silurian, when the accretionary prism with granitic batholiths was obducted onto the Baltic margin. The obduction and subsequent uplift of the subducted edge of Baltica led to the formation of intramontane ORS basins in the Early and Middle Devonian. The Caledonian orogeny terminated with the deformation of the ORS basins in the Late Devonian. The collisional stage is metallogenically represented by three types of deposits, i.e. lead sandstone deposits (Pb, Zn, Ba, F and Cu), carbonate-hosted base-metal deposits, and vein deposits with variable proportions of Au, Ag, Pb, Cu, Zn, Fe, As, Sb, W, Mo and/or U. The first type, which comprises deposits with up to 5.0?Mt lead metal (Laisvall) is by far the most important economically. The vein deposits have in the past been the site for small-scale mining of precious metals and molybdenite. Although the exact timing of the different ore-forming events is poorly constrained, all types seem related to a continuous process of tectonically induced fluid flow during episodes of crustal shortening and uplift. Brief comparisons with other parts of the now-rifted Caledonian-Appalachian orogen reveal both similarities and differences in the relative importance of the ore types generated during the time span covered in the present account.     

Mineralogy and textural relationships among sulphosalts and related minerals in the Bleikvassli Zn-Pb-(Cu) deposit, Nordland, Norway

Several distinct assemblages of Pb-Sb, Pb-As, Cu-Pb-Sb and Cu-Fe-Zn-Sn sulphosalts are identified in sulphide samples from Bleikvassli mine, Norway. Detailed optical microscopy and electron probe microanalysis have permitted investigation of textural relationships between minerals and compositional variations between different ore types. Tetrahedrite, typically containing 10–16?wt.% Ag (rare freibergite containing 25–30?wt.% Ag has also been identified in two samples), stannite (Cu₂(Fe>Zn)SnS₄), and meneghinite, CuPb₁₃Sb₇S₂₄, are widely distributed as trace constituents throughout massive pyritic and galena-rich ores. Native antimony and pyrargyrite occur in trace amounts in all ore types, as the breakdown products of earlier sulphosalts. Several distinct types of wall-rock mineralisation are present at Bleikvassli. Of considerable mineralogical interest are the coarse-grained sulphide mobilisates within the wall rock which contain a distinct?and characteristic suite of Pb-As sulphosalts:?tennantite?+?jordanite (Pb₁₄As₆S₂₃)?+?seligmannite (CuPbAsS₃) ± dufrenoysite (Pb₂As₂S₅). Bournonite (CuPbSbS₃) is the only Sb-bearing sulphosalt recognised in significant amounts within the mobilisates, meneghinite and tetrahedrite being conspicuously absent. These mobilisates display considerable Au enrichment; electrum can be confirmed, intimately associated with jordanite and tennantite. Appreciable Sb (up to 3?wt.%) is contained within galena in the mobilisates, in contrast to galena from massive ores which contains only negligible Sb. Contents of Ag and Bi in galena vary considerably in all ore types, but confirm earlier suggestions that galena is a major Ag-carrier at Bleikvassli. Boulangerite (Pb₅Sb₄S₁₁), jamesonite (FePb₄Sb₆S₁₄) and gudmundite (FeSbS) occur in trace amounts. Sn-sulphosalts are represented by kësterite, (Cu₂(Zn> Fe)SnS₄), but commonly zoned with respect to Zn/Fe ratio, in the mobilisates, rather than by stannite. A rare type of mobilisate, also in the wall rock, in which chalcocite and bornite are the main minerals, contains native Ag, stromeyerite (AgCuS), mckinstryite ((Ag,Cu)₂?S), Ag-free tetrahedrite, an unnamed Cu-Ag-Fe sulphide (Cu₃Ag₂FeS₄) and native Bi, myrmekitically intergrown with chalcocite. Although a comprehensive genetic model for the wall-rock mineralisation at Bleikvassli is largely impossible given the limitations in the present state of knowledge regarding mechanisms involved in remobilisation processes, a multi-stage model of remobilisation during regional metamorphism is considered to best explain the observations. An interplay of different solid- and liquid-state remobilisation mechanisms, in various combinations, is required to account for the macro- and microscopic observations. Remobilisation probably began during the earlier stages of metamorphism, with crystallisation and further remobilisation taking place during the entire metamorphic cycle, giving rise to the extensive chemical and mineralogical diversity observed today. Preserved mineral assemblages and their textural relationships reflect a complex sequence of replacement and decomposition reactions taking place during the latest phase of late-metamorphic crystallisation and subsequent cooling.     

Stromeyerite and mckinstryite from the Godejord polymetallic sulphide deposit,central Norweigan Caledonides

The Cu-Ag-S minerals, stromeyerite and mckinstryite, have been found for the first time in a stratabound polymetallic pyritic deposit in the Caledonides of central Norway. The surface specimens examined contained approximately 0.5% Ag, 1.8% Cu, 15.0% Zn and over 10 g/t Au and showed the mineral association pyrite, sphalerite, chalcopyrite, galena, tennantite, bornite, Cu-Ag sulphides, covelline, native Au, a Cu-Sn sulphide, and a new mineral of composition Ag₅CuTeS₂. The Cu-Ag sulphides appear to be replacing preexisting sulphides, with the exception of pyrite and sphalerite. The nature of this replacement is discussed. Analyses, by microprobe, of the Cu-Ag-S phases are reported and compared with published data. The stromeyerite shows an average composition Cu_1.01Ag S, the mckinstryite Cu_0.77Ag_1.19S. Values are reported of the reflectance at 542 nm for both minerals. The data indicate that stromeyerite is optically positive with Rg: 30.7%, Rm: 27.3%, Rp: 25.8% while mckinstryite is negative with Rg: 32.5%, Rm: 31.9%, Rp: 27.6%.     

The effects of sulfidation and oxidation during metamorphism on compositionally varied rocks adjacent to the Bleikvassli Zn–Pb–(Cu) deposit, Nordland, Norway

Petrographic, electron microprobe, and bulk-rock geochemical analyses indicate that the distribution and composition of ferromagnesian silicates (biotite, garnet, and staurolite) in and adjacent to the metamorphosed Bleikvassli Zn–Pb–(Cu) volcanogenic massive sulfide deposit, Norway, are dependent upon the competing effects of f O₂–f S₂ and host-rock composition. The enrichment in magnesium content of these silicates within the orebody and at distances of as much as 5–10 m away is due to the increased f O₂ and f S₂ conditions imposed on the silicates in zones subject to minor hydrothermal alteration during regional metamorphism. Alternatively, within pelitic country rocks at distances >5–10 m from ore, the host-rock chemistry controls the composition of metamorphic silicate minerals. Also, country rocks within a few meters of ore are distinguished by the common presence of zinc-bearing staurolite (up to 9 wt% ZnO) coexisting with biotite ± garnet. Rocks in the Bleikvassli deposit were hydrothermally enriched in zinc and fluorine prior to metamorphism. The fluorine resides mainly in biotite, which is an additional contributing factor to the magnesium enrichment of that mineral due to Fe²⁺–F avoidance. Our inference that the sulfidation–oxidation halo around the Bleikvassli ore deposit is only meters in width contrasts with the view of Maiga (1983), who proposed the effects of sulfidation could be identified at distances >159 m from ore. It is evident that the delineation of a sulfidation–oxidation halo bordering a metamorphosed massive sulfide deposit must be done carefully in order to discriminate between the effects due to variations in primary rock composition versus those resulting from a sulfur and oxygen fugacity gradient between the massive sulfides and the sulfur-poor country rocks. Received: 1 March 1998 / Accepted: 3 May 2000     

Some aspects of the regional metamorphic mobilization of preexisting sulphide deposits

The paper first reviews some of the more recent literature dealing with the mobilization of preexisting sulphide ores during regional metamoprhism. There is a fairly general consensus that mobilization of ore and gangue minerals is widespread under regional metamorphic conditions and that distances of the order of millimeters to meters are involved, though there are advocates of long-distance mobilization (rejuvenation) involving many kilometers. Most writers, too, speak of an order of relative mineralogical differentiation, though this view is not without its opponents either. While creep or plastic flow of the softer sulphide minerals would appear to account for some forms of stress-induced mobilization, most authors consider that the mobilized components have moved in the form of a solution, though little serious thought has been given to the nature of the fluids involved. The writer, following certain other workers, proposes that the melting of systems, of the type met with in many metamorphosed sulphide ores under conditions of the epidote-amphibolite facies and above, must also be seriously considered as a probable mechanism leading to the formation of the mobilisates.

---

Zusammenfassung Zuerst wird neuere Literatur über Remobilisationen in präexistierenden Sulfidlagerstätten während der Regionalmetamorphose zusammengefaßt. Im allgemeinen stimmen die Autoren überein, daß während dieser Art von Metamorphose eine Mobilisation im mm- bis m-Bereich verbreitet ist, wobei es auch Vertreter gibt, die von Wanderungen über viele km sprechen (Rejuvenation). Die meisten Autoren sprechen auch von einer differentiellen Mobilisation (Abfolgen), wobei auch diese Ansicht ihre Gegner gefunden hat. Während ein gewisses Fließen (creep or plastic flow) der weicheren Mineralien für eine Art von Mobilisation verantwortlich gemacht wird, sind sich die meisten Autoren einig, daß der Transport über eine flüssige Phase stattfand, wobei wenige Versuche vorliegen, die Natur dieser Lösungen zu untersuchen. Es wird dann im Anschluß an Arbeiten anderer Autoren vorgeschlagen, daß die Schmelzphasensysteme, wie sie z. B. in vielen metamorphen Sulphidlagerstätten der Epidot-Amphibolit-Fazies und bei höherer Metamorphose auftreten, auch in Betracht gezogen werden müssen, als ein wahrscheinlicher Mechanismus bei der Bildung von Mobilisaten.