Partial Melting of Sulfide Ore Deposits During Medium- And High-Grade Metamorphism

Sulfide minerals (Po + Py + Ccp >> Sp + Gn + Apy) are common constituents of the Meguma Supergroup (MSG) country rocks, and are also common in the marginal facies of the South Mountain Batholith (SMB), Nova Scotia, relative to the interior of the batholith. The distribution and relative modal proportions of sulfides in the granites near the contact indicate that they are probably contaminants from the MSG country rocks. We trace the textural and chemical changes that occur in Py, Po, and Ccp from the country rocks through their xenoliths to the granite during their assimilation in the granite. Pyrrhotite in the country rock ranges from large polycrystalline porphyroblasts with many inclusions to small individual grains free of inclusions, all with low concentrations of Co and Ni. Chalcopyrite in the country rocks occurs mainly as anhedral inclusions in pyrrhotite, and as small isolated grains in the silicate matrix. Pyrite in the country rocks occurs as euhedral cubes or as anhedral cores to pyrrhotite grains. Relative to their occurrences in the MSG country rocks, characteristics of sulfide grains in the granitic rocks include generally smaller grain-sizes, Po grains with fewer silicate and oxide inclusions, development of some Po grains with an inclusion-rich core and an inclusion-free rim, Po grains that have a lower Fe:S ratio and contain higher concentrations of Co (up to 0.17 wt%) and Ni (up to 0.41 wt%), local increases in the Ccp:Po ratio, better development of crystal faces on Ccp inclusions in Po, development of micro-dropper or silicate-melt-displacing textures, and a general absence of Py except for its association with Ccp in microgranitoid enclaves. The grain-size reduction is consistent with disintegration of polycrystalline MSG sulfide grains, either in the solid state or in a partially melted state, as well as dissolution in the SMB magma. The higher concentrations of Co and Ni in the SMB pyrrhotite are consistent with cation exchange with the granitic melt. Most of the remaining characteristics are consistent with partial melting of the multiphase MSG sulfide blebs related to their incorporation in the SMB magma. From known phase-relations in the system Cu-Fe-S, we deduce that temperatures of the SMB magma exceeded the temperature of beginning of melting of the composition-dependent multiphase MSG sulfide aggregates, and thus at least some of them underwent partial melting. Other more refractory grains, which have undergone only textural (annealing, ductile deformation) and chemical modification in the solid state, are simply modified xenocrysts.Keywords: sulfide, contamination, assimilation, granite, melting, texture, composition, South Mountain Batholith, Nova Scotia. SOMMaireLes minéraux sulfurés (Po + Py + Ccp >> Sp + Gn + Apy) sont répandus dans les roches encaissantes du Supergroupe de Meguma (MSG), et aussi dans les faciès de bordure du batholite de South Mountain (SMB), en Nouvelle-Écosse, par rapport aux roches typiques de l'intérieur du batholite. La distribution et les propor...

Section: Interpretation Of the Modal And Textural Differences Betweenmentioning

confidence: 57%

Contamination of the South Mountain Batholith by Sulfides From the Country Rocks

Clarke¹,

Erdmann²,

Samson³

et al. 2009

“…Sample 8315-65.9 contains a complex suite of sulfide, sulfosalt, and alloy assemblages that probably represent the last-stage crystallization of a polymetallic melt (Frost et al 2002). We have identified acanthite or argentite (Ag 2 S), dyscrasite (Ag 3 Sb), and a complex domain enriched in Ag, Sb, and S that we were not able to characterize well because of its fine grain-size and poor polish (Fig.…”

Section: Sulfide Sulfosalt and Alloy Assemblagesmentioning

confidence: 98%

“…The pressure effect on the melting temperature of this assemblage is not known. The experimentally determined melting reactions for sulfides have a dT/dP of 17°C/kbar or less (Frost et al 2002). Because melting of native Sb has a negative dT/dP, we consider it reasonable to assume dT/dP slope for the melting reaction was low enough (i.e., less than 27°C/ kbar) that these Ag-Sb-S phases were molten at the time of regional metamorphism.…”

Section: Sulfide Sulfosalt and Alloy Assemblagesmentioning

confidence: 99%

“…For example, Lawrence (1967) postulated that some portions of the orebody were remobilized in the form of "sulfide neomagmas" during metamorphism. This hypothesis has received little attention in the past 30 years, but recent experimental work (Mavrogenes et al 2001) and textural studies (Frost et al 2002) indicate that the Broken Hill orebody was partially molten at peak metamorphism. As part of a study to the determine the role of melts in the evolution of the Broken Hill orebody, we have undertaken a detailed study of the metamorphism in the vicinity of the deposit.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Prolonged Existence of Sulfide Melt in the Broken Hill Orebody, New South Wales, Australia

Frost¹,

Swapp²,

Gregory³

2005

Self Cite

The Broken Hill orebody, in Australia, is hosted in pelitic and psammitic gneisses that underwent metamorphism in the granulite facies. Peak metamorphism in the area occurred around 1600 Ma and exceeded temperatures of 850°C at pressures of 5-7 kilobars. The orebody is located in an area of intense deformation that occurred 5 to 15 million years after the peak metamorphism. Highly foliated pelitic schist directly adjacent to the Broken Hill orebody contains assemblages ranging from quartz -K-feldspar -biotite -garnet -sillimanite to quartz -muscovite -biotite -staurolite -garnet -chlorite. The latter assemblage equilibrated at temperatures as low as 600°C. Silicates in the low-temperature assemblages are intimately associated with galena that, in places contains clusters rich in Ag and Sb alloys, sulfides, and sulfosalts. One-bar experiments in the system Ag-Sb-S indicate that the assemblages seen in these rocks would have been molten at 500°C. These relations suggest that a polymetallic melt persisted at Broken Hill for up to twenty million years after peak metamorphism.Keywords: metamorphism, orebody, massive sulfides, melting, Broken Hill, Australia. SOMMAIRELe gisement de Broken Hill, en Australie, est situé dans un hôte gneissique métapélitique et métapsammitique qui a subi un métamorphisme au faciès granulite. Le paroxysme métamorphique dans la région a eu lieu il y a environ 1600 millions d'années, et a impliqué une température au delà de 850°C à une pression entre 5 et 7 kilobars. Le gisement est situé dans une zone de déformation intense active de 5 à 15 million d'années après la culmination métamorphique. Des schistes pélitiques fortement foliés directement adjacents au gisement de Broken Hill contiennent des assemblages allant de quartz -feldspath potassiquebiotite -grenat -sillimanite à quartz -muscovite -biotite -staurolite -grenat -chlorite. Ce deuxième assemblage s'est équilibré à une température aussi basse que 600°C. Les silicates des assemblages de plus faible température sont intimement associés à la galène qui, par endroits, contient des amas riches en alliages, sulfures et sulfosels d'argent et d'antimoine. Des expériences à un bar dans le système Ag-Sb-S indiquent que les assemblages de ces roches pourraient avoir été à l'état fondu à 500°C. Ces relations font penser qu'un liquide polymétallique a persisté à Broken Hill jusqu'à vingt millions d'années après la culmination métamorphique.(Traduit par la Rédaction)

“…The high Mn content of garnet in metamorphosed pelitic rocks surrounding Cannington has been interpreted to either indicate a reconcentration of Mn from sedimentary detritus during metamorphism (Bodon 1998), an addition of fluid or fluids during postpeak-metamorphism metasomatism (Richmond 1994, Chapman & Williams 1998, or a product of a metamorphosed sulfide orebody (Frost et al 2002). Kilometer-scale continuity of horizons bearing high-Mn garnet and gahnite from the nearby Pegmont Pb-Zn deposit (Vaughan & Stanton 1986;Fig.…”

Section: Introductionmentioning

confidence: 99%

VEIN AND SKARN FORMATION AT THE CANNINGTON Ag Pb Zn DEPOSIT, NORTHEASTERN AUSTRALIA

Roache¹,

Williams²,

Richmond³

et al. 2005

Synchronous deformation and metamorphism at the Cannington Ag-Pb-Zn deposit, in northeastern Australia, involved a phase of Ca and Mn mobilization that accounts for the distribution of pyroxene-and garnet-bearing rocks, which were preferentially overprinted by sulfide mineralization. Garnet composition and its textural association in relation to three generations of planar fabric (S 1 to S 3 ) show that skarn-related grossular-rich garnet formed in the last garnet-stable event, D 3 . The D 3 Cl-rich biotite in the gneiss is intergrown with garnet in veins, and within skarn rocks is continuous along strike with veins of garnet ± pyroxmangite, hedenbergite or amphibole, which indicates a period of anhydrous through to hydrous metasomatism. Compositional maps show that infiltration extended from alteration fronts through fractures, which indicates the high permeability of S 3 ; these maps and X Grt profiles across skarn-gneiss contacts show that there was large-scale transport of mass by infiltration and varying degrees of hydrodynamic dispersion. Precipitation of oscillatory Ca-and Mn-zoned grossular-rich garnet in veins parallel to veins filled with Cl-rich biotite, in conjunction with the distribution of biotite-filled veins across the deposit, suggest that the mass transfer during D 3 may be responsible for deposit-scale Ca-Mn skarn-like zoning.Keywords: garnet, zoning, metasomatism, skarn, shear zones, veins, Cannington, Australia. SOMMAIREUne déformation et une recristallisation métamorphique synchrones dans le gisement à Ag-Pb-Zn de Cannington, dans le nord-est de l'Australie, a impliqué une mobilisation de Ca et de Mn qui rend compte de la distribution de roches à pyroxène et grenat, qui sont devenues par la suite les hôtes ciblés d'une minéralisation en sulfures. La composition et l'association texturale du grenat aux trois générations de foliation (S 1 à S 3 ) montrent que le grenat lié à la formation du skarn, riche en grossulaire, s'est formé lors du dernier épisode de déformation où le grenat était stable, D 3 . La biotite de la génération D 3 dans le gneiss, riche en Cl, montre une intercroissance avec le grenat en veines, et dans les roches du skarn, montre une continuité le long de la gneissosité avec des veines à grenat ± pyroxmangite, hedenbergite ou amphibole, indications d'une période de métasomatose anhydre à aqueuse. Des cartes de répartition d'éléments montrent que l'infiltration s'est propagée à partir de fronts d'altération le long de fissures, indication d'une grande perméabilité le long de S 3 ; ces cartes et les profils de X Grt perpendiculaires aux contacts skarngneiss montrent qu'il y a eu transfert important de masse par infiltration et par dispersion hydrodynamique à divers degrés. D'après la précipitation de cristaux de grenat à zonation oscillatoire en Ca et Mn dans des veines parallèles aux veines remplies de biotite riche en chlore, ainsi que la distribution de telles veines de biotite de part et d'autre du gisement, le tranfert de masse au cours de l'événement D 3 aurait...