Origine des lentilles riches en sulfures des gisements de palladium du Lac des Iles, Ontario, Canada

Les gisements de Pd du Lac des Iles (Ouest de l’Ontario, Canada) sont connus pour être riches en Pd et pauvres en sulfures. Il y a un débat de longue date concernant l’origine de ces gisements, avec une école de pensée proposant qu’ils sont essentiellement d’origine magmatique, et une autre suggérant qu’ils sont hydrothermaux. En plus de la minéralisation pauvre en sulfures, des lentilles riches en sulfures sont présentes au travers de l’intrusion du Lac des Iles et celles-ci n’ont pas été étudiées précédemment. Le but de cette thèse de doctorat est d’aborder le problème de l’origine de ces lentilles riches en sulfures. Les lentilles riches en sulfures (0,1 à 3 m de long) recoupent la stratigrphie et suivent des zones de cisaillement syn-magmatiques. La minéralogie des sulfures varie d’un assemblage composé de pyrrhotite (Po) pentlandite (Pn) ± pyrite (Py) ± chalcopyrite ± oxydes de Fe-Ti (magnétite and ilménite), à un assemblage dominé par la pyrite (Py). Les ratios Pd/Ir et Pd/Pt roche totale sont élevés et extrêmement variables (respectivement 5 à 258000 et 2 à 226), indépendamment de l’assemblage minéralogique. Les échantillons les plus riches en Py sont enrichis en As et Bi par rapport aux autres échantillons. Les analyses géochimiques roche totale montrent un découplage du Cu à partir du Ni. La plupart des échantillons ont des ratios S/Se d’approximativement 3000 et des valeurs δ34S proches de celle du manteau. Les analyses LA-ICP-MS montrent que les compositions de Po, Pn et d’oxydes de Fe-Ti sont similaires à celles provenant de minéralisations magmatiques dérivées de magmas évolués. Les Py sont riches en EGPI + Rh et montrent une zonation compositionnelle. Leur composition et leur distribution en éléments sont similaires à celles de Py trouvées dans des gisements de sulfures magmatiques. Il est proposé que des liquides sulfurés magmatiques se soient accumulés au travers de l’intrusion en réponse à l’ouverture de zones de dilatation à un stade magmatique. Ces liquides sulfurés ont subit la cristallisation fractionnée et ont formé des cumulats de solution solide monosulfurée (MSS). Les liquides résiduels enrichis en Pd, Pt et Au complémentaires des cumulats de MSS ne sont plus associés avec les lentilles. La variation des ratios d’EGP est interprétée comme étant le résultat de différents degrés de fractionnement de liquides sulfurés évolués. Lors de la cristallisation de la MSS, l’oxigène a diffusé hors des liquides sulfurés et a réagit avec les magmas silicatés pour former les oxydes de Fe-Ti. Au cours du refroidissement, la MSS a exsolvé pour former Po et Pn. Lors de l’altération, le Fe ± Ni ont été redistribué depuis la MSS ou Po ± Pn vers les silicates intersticiels, entraînant le développement de Py. Il est suggéré que lorsque la Py s’est formée, elle a hérité des éléments trace de la MSS ou de la Po, mais en plus les éléments ont diffusés dans la Py à partir des sulfures alentours. Certains éléments mobiles (e.g., As et Bi) ont été introduits dans la Py par des fluides. The Lac des Iles Pd-deposits (Western Ontario, Canada) are known to be Pd-rich and sulfide-poor. There is a long-standing debate as to the origin of these deposits, with one school of thought proposing that they are essentially of magmatic origin and one suggesting that they are hydrothermal. In addition to the sulfide-poor mineralization, sulfide-rich pods are present throughout the Lac des Iles intrusion and these have not previously been investigated. The purpose of this Ph.D. thesis is to address the origin of these sulfide-rich pods. The sulfide-rich pods (0.1 to 3 m long) cross-cut the stratigraphy and follow syn-magmatic shear zones. The sulfide mineralogy varies from an assemblage that consists of pyrrhotite (Po) pentlandite (Pn) ± pyrite (Py) ± chalcopyrite ± Fe-Ti oxides (magnetite and ilmenite) to an assemblage dominated by pyrite. The whole-rock Pd/Ir and Pd/Pt ratios are high and extremely variable (5 to 258000 and 2 to 226, respectively), regardless of the mineralogical assemblages. The most Py-rich samples are enriched in As and Bi relative to the other samples. The whole-rock geochemical analyses show a decoupling of Cu from Ni. Most of the samples have S/Se ratios of approximately 3000 and 34S of near-mantle value. Laser ablation-ICP-MS analyses show that the compositions of Po, Pn and Fe-Ti oxides are similar to that from igneous mineralization derived from evolved magmas. The Py are rich in IPGE + Rh and exhibit compositional zoning. Their composition and element distribution are similar to those of Py found in magmatic sulfide deposits. It is proposed that magmatic sulfide liquids accumulated throughout the intrusion in response to the opening of dilation zones at magmatic stage. These sulfide liquids experienced crystal fractionation and formed cumulus monosulfide solid solution (MSS). The residual liquids enriched in Pd, Pt and Au and complimentary to the cumulus MSS are no longer associated with the pods. The variation in PGE ratios is interpreted as being the result of different degrees of fractionation of evolving sulfide liquids. During MSS crystallization, oxygen diffused out of the sulfide liquids and reacted with silicate magmas to form the Fe-Ti oxides. Upon cooling, MSS exsolved to form Po and Pn. During alteration, Fe ± Ni were redistributed from MSS or Po ± Pn to interstitial silicates, resulting in the development of Py. It is suggested that as the Py formed it inherited the trace elements from the MSS or Po, but in addition elements diffused from the surrounding sulfides into the Py. Some mobile elements (e.g., As and Bi) were introduced into the Py by fluids

Using sulphide indicator mineral chemistry for ore discrimination and targeting in the Churchill Province, northern Quebec, Canada

The Churchill Province in northern Quebec consists of Archean to Proterozoic basement rocks, which have undergone a complex orogenic and metamorphic history. The vast majority of these rocks are covered by Quaternary glacial deposits that display a complex geomorphology reflecting important variations in the glacial dynamics. Occasional mineralized outcrops have been identified south from the Ungava Bay in the Churchill Province during mapping surveys. However, this area has been underexplored owing to the thick sedimentary cover, which limits the effectiveness of conventional exploration methods. Heavy mineral separation from till and esker samples reveals the presence of thousands of sulphide grains, namely pyrite and chalcopyrite, and lesser amounts of sulfarsenides (löllingite and arsenopyrite), which is indicative of the presence of underlying mineralized rocks. As a result, this area is ideal to test the use of sulphide indicator mineral chemistry for mineral assessment and vectoring. In this study, we focus on integrating sulphide indicator mineral chemistry determined by laser ablation inductively coupled mass spectrometry (LA-ICP-MS) with the geology of glacial deposits. Trace-element signatures of pyrite and chalcopyrite grains indicate two distinct compositions for the sulphide minerals present in the glacial deposits and suggest there is strong potential for magmatic and hydrothermal mineralisation. Integrated maps combining sample locations and sulphide grain compositions and populations allow to delineate vectors toward potential economic targets

Sulfide-rich pods from the Lac-des-Îles Pd-ORE deposits, Western Ontario, Canada : Part 1. A genetic model

Massive sulfide pods from the Lac-des-lies Pd-ore deposits (Western Ontario, Canada) show a variation in sulfide mineralogy and texture from essentially magmatic (pyrrhotite+pentlandite±chalcopyrite) to highly altered (pyrite±pentlandite±pyrrhotite±chalcopyrite). We suggest that the magmatic assemblage formed from crystallization of magmatic sulfide liquid. The pyrite (Py)-rich assemblage formed by Fe-loss to the surrounding silicates

An overview of chalcophile element contents of pyrrhotite, pentlandite, chalcopyrite, and pyrite from magmatic Ni-Cu-PGE sulfide deposits

We have compiled the trace element concentrations in pyrrhotite, pentlandite, chalcopyrite, and pyrite from magmatic Ni-Cu-PGE ore deposits with the aim of understanding their petrogenesis and whether these minerals can be used as indicator minerals. Among the samples, there are some of the most studied world-class Ni-Cu- (Aguablanca, Duluth, Jinchuan, Noril’sk-Talnakh-Kharaelakh, Sudbury, Voisey’s Bay, and others) and PGE-dominated (Bushveld, Lac des Iles, Stillwater, Great Dyke, and Penikat) deposits. Crustal assimilation may be constrained using As/Se and Sb/Se ratios in pentlandite. The degree of interaction between the silicate and sulfide liquids (R-factor) can be estimated by the content of highly chalcophile elements (Dsulf liq/sil liq above 1000) in sulfide minerals. The fractional crystallization of the sulfide liquid can be traced using Se/Te ratios of pentlandite. Pyrite formed by exsolution from MSS has higher Rh, Ru, Ir, and Os than co-existing pyrrhotite, whereas pyrite formed by hydrothermal alteration of pyrrhotite inherits the Rh, Ru, Ir, and Os contents of the pyrrhotite it replaced. Sulfide minerals are preserved in transported glacial cover and their trace element chemistry can be used to discriminate their source. Pentlandite from Ni-Cu deposits has much lower Rh and Pd concentrations than those from PGE-dominated deposits, pyrite from magmatic deposits has higher Co/Sb and Se/As ratios relative to pyrite from hydrothermal deposits, and chalcopyrite from magmatic deposits has much higher Ni and lower Cd concentrations than those from hydrothermal deposits

Chalcophile and platinum-group element distribution in pyrites from the sulfide-rich pods of the Lac des Iles Pd deposits, Western Ontario, Canada : implications for post-cumulus re-equilibration of the ore and the use of pyrite compositions in exploration

The Lac des Iles Pd-deposits are atypical within the scope of platinum-group element (PGE) deposits. This is because the deposits do not resemble a classical PGE deposit in a number of ways: the intrusion is small and concentrically zoned; most of the host rocks to the deposits no longer have a primary mineralogy and equilibrated under greenschist conditions; the textures of the rocks from the ore zones are extremely variable; and the ores have very high Pd/Ir and Pd/Pt ratios. In addition to the disseminated sulfides, there are sulfide-rich pods present throughout the stratigraphy. The sulfide mineral textures and proportions within the pods vary from those which are essentially magmatic to those which consist predominantly of pyrite. The pyrite could have been deposited from hydrothermal fluids or it could have formed by alteration of magmatic sulfides. In order to distinguish between these two origins, the PGE and chalcophile element contents of the pyrite were investigated. It was found that the pyrite contains Os, Ir, Ru and Rh. These elements also concentrate in the magmatic sulfides pyrrhotite and pentlandite. Their presence in the pyrite could be explained by redistribution of Fe from pyrrhotite to silicate minerals that are present within and around the sulfide pods, possibly during cooling. Maps of the distribution of the elements show that there is zoning of the elements. The IPGE–Rh are present towards the cores of pyrite along with As whereas Co and Se are present towards the rims. Mobile elements such as Pb, Bi and Ag are present in thin overgrowths at the edges of pyrite and in a few cases, Pt, Te and Sn are also present in the overgrowths. Comparison of the composition and element distribution with pyrites from other igneous settings (Sudbury and Aguablanca) shows similarities, suggesting a common ore-modifying process. In contrast, pyrites from low-temperature hydrothermal deposits have different compositions. A plot of Co/Se vs Sb/As appears to be effective at separating the igneous pyrites from pyrites found in other settings and could possibly be used in exploration

Petrogenesis of massive sulphides from the Lac-des-Iles Palladium ore deposits, Western Ontario, Canada

Previously characterised as a sulphide-poor Pd ore deposit, massive sulphides have recently been discovered in Lac-des-Iles. The massive sulphides occur across the different lithological units and show variable degrees of alteration. The least altered samples comprise a typical magmatic assemblage of pyrrhotite, pentlandite and chalcopyrite. Chalcopyrite-rich samples are found at the edges of the pyrrhotite/pentlandite-rich pods. Base metal and platinum-group element (PGE) compositions indicate that as a whole they represent a frozen sulphide liquid, different from the one that formed the sulphide-poor samples, that was injected along structural features. The altered samples are rich in pyrite and magnetite. Molecular proportions of base metals and S/Se ratios are the same for the altered and unaltered samples indicating that neither S nor Fe has been remobilized from the system. We propose instead that oxidation was responsible for the observed changes in mineralogy. This alteration event appears not to have affected the PGE content

Sulfide-rich pods from the Lac-des-Îles Pd-ORE deposits, Western Ontario, Canada : Part 2. The origin of platinum-group elementsbearing pyrites

Pyrite from Lac-des-lies sulfide-rich pods host substantial amounts of platinum-group elements (PGE). In this contribution we discuss the origin of pyrite and their PGE content

Trace element distribution in primary sulfides and Fe–Ti oxides from the sulfide-rich pods of the Lac des Iles Pd deposits, Western Ontario, Canada : Constraints on processes controlling the composition of the ore and the use of pentlandite compositions in exploration

There is an on-going debate as to whether the Lac des Iles Pd deposits (Ontario, Canada) are of magmatic or hydrothermal origin. An aspect of the deposits that has not yet been documented is the presence of sulfide-rich pods which occur throughout the host intrusion (the Mine Block Intrusion). The ore mineralogy of the sulfide-rich pods consists of pyrrhotite, pentlandite, chalcopyrite, ± pyrite, magnetite and ilmenite. We present the trace element concentrations of pyrrhotite, pentlandite, chalcopyrite, magnetite, and ilmenite from the pods and compare these results with results from other Ni–Cu–platinum-group element (PGE) deposits. The low concentrations of Si and Ca and high concentrations of V, Ni, and Cr in magnetite are consistent with a magmatic origin of the magnetite. Variations in the V and Cr concentrations indicate that magnetite crystallized from a magmatic sulfide liquid during crystal fractionation of the sulfide liquid. The enrichments in Ni, Co, Os, Ir, Ru, and Rh and depletions in Cu, Ag, Cd, and Zn in pentlandite and pyrrhotite relative to chalcopyrite are also consistent with the formation of the pods by crystallization of a magmatic sulfide liquid. Comparison of pyrrhotite and pentlandite compositions from Lac des Iles with those from other Ni–Cu–PGE deposits shows that pyrrhotite and pentlandite derived from evolved magmas have distinct compositions relative to those derived from more primitive magmas. In addition, this comparison shows that pentlandites from PGE-dominated deposits are richer in Pd and Rh than pentlandites from Ni–Cu sulfide deposits. A plot of Pd vs Rh appears to be effective at distinguishing pentlandites of PGE-dominated deposits from those of Ni–Cu sulfide deposits and could possibly be used to adapt exploration strategies

Trace element distribution in primary sulfides and Fe–Ti oxides from the sulfide-rich pods of the Lac des Iles Pd deposits, Western Ontario, Canada : Constraints on processes controlling the composition of the ore and the use of pentlandite compositions in exploration

There is an on-going debate as to whether the Lac des Iles Pd deposits (Ontario, Canada) are of magmatic or hydrothermal origin. An aspect of the deposits that has not yet been documented is the presence of sulfide-rich pods which occur throughout the host intrusion (the Mine Block Intrusion). The ore mineralogy of the sulfide-rich pods consists of pyrrhotite, pentlandite, chalcopyrite, ± pyrite, magnetite and ilmenite. We present the trace element concentrations of pyrrhotite, pentlandite, chalcopyrite, magnetite, and ilmenite from the pods and compare these results with results from other Ni–Cu–platinum-group element (PGE) deposits. The low concentrations of Si and Ca and high concentrations of V, Ni, and Cr in magnetite are consistent with a magmatic origin of the magnetite. Variations in the V and Cr concentrations indicate that magnetite crystallized from a magmatic sulfide liquid during crystal fractionation of the sulfide liquid. The enrichments in Ni, Co, Os, Ir, Ru, and Rh and depletions in Cu, Ag, Cd, and Zn in pentlandite and pyrrhotite relative to chalcopyrite are also consistent with the formation of the pods by crystallization of a magmatic sulfide liquid. Comparison of pyrrhotite and pentlandite compositions from Lac des Iles with those from other Ni–Cu–PGE deposits shows that pyrrhotite and pentlandite derived from evolved magmas have distinct compositions relative to those derived from more primitive magmas. In addition, this comparison shows that pentlandites from PGE-dominated deposits are richer in Pd and Rh than pentlandites from Ni–Cu sulfide deposits. A plot of Pd vs Rh appears to be effective at distinguishing pentlandites of PGE-dominated deposits from those of Ni–Cu sulfide deposits and could possibly be used to adapt exploration strategies

Textural and compositional evidence for the formation of pentlandite via peritectic reaction: Implications for the distribution of highly siderophile elements

The distribution of highly siderophile elements is used in the study of a wide variety of geological topics, from planet formation and evolution to the formation of ore deposits. Under mantle and crustal conditions, these elements behave as highly chalcophile elements, and pentlandite (Pn) is an important host for most of these elements. Therefore, understanding how Pn forms is important to understanding the processes that control these elements. The classic model for the formation of Pn is that below 650 °C, the high-temperature sulfides—monosulfide solid solution (MSS) and intermediate solid solution (ISS)—are no longer stable and exsolve into pyrrhotite (Po), Pn, and chalcopyrite (Ccp). However, Pn has been shown to be the main host of Pd in many ore deposits, and given that Pd is incompatible with both MSS and ISS, this observation is inconsistent with the exsolution model. Furthermore, experimental work has shown that Pn can form by peritectic reaction between MSS and fractionated sulfide liquid. To date, this type of Pn has not been reported in natural samples. In our study of chalcophile-element concentrations in Pn from iconic magmatic Ni–Cu–platinum-group element deposits, we observed three textures of Pn: contact Pn in between Po and Ccp, granular Pn included within Ccp or Po, and flame Pn included within Po. The contact Pn shows zonation in Mo, Rh, Ru, Re, Os, and Ir, with these elements being enriched toward the Po contact and depleted toward the Ccp contact. In some cases, Pd displays a zonation antithetical to that of these elements. In this contribution, we propose that the contact Pn formed via the peritectic reaction described above, and inherited Mo, Ru, Rh, Re, Os, and Ir from the MSS, whereas Pd was contributed from the fractionated sulfide liquid. We expect that this type of Pn should be present wherever MSS and fractionated sulfide liquid remained in contact