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Recent lithogeochemical studies by accurate analytical techniques (e.g., instrumental and radiochemical neutron-activation analyses) have been used to explore the possibility of using gold distribution in the research for new gold deposits; these show that anomalous gold distribution occurs in some parts of the Val-d'Or Formation in the Val-d'Or mining district of Quebec. Gold lithogeochernistry in the Val-d'Or Formation has shown that it is possible to distinguish: (i) background values (1.4-3.5 ppb Au); (ii) zones of primarily anomalous gold values around the Lamaque-Sigma mines (median: 15 ppb Au); (iii) enrichment halos around gold orebodies (median: 70 ppb Au); and (iv) secondary gold enrichment in shear zones. We conclude that the Val-d'Or Formation is auriferous, i.e., anomalously rich in gold at least in some of its parts and contains the Lamaque -Sigma gold mines, representing 68% of the total gold production in the district. The Val-d'Or Formation is part of a central volcanic complex within an island-arc system. The centre of this complex is located in the main Lamaque plug, and this environment may be compared I to high-temperature active geothermal systems that are commonly responsible for the formation of epithermal gold deposits. Gold mineralization at Sigma and Lamaque is considered to be related to a late hydrothermal phase or a retrograde phase of regional metamorphism. For the formation of the gold deposits, two distinct and successive events are postulated: (i) a gold-rich synvolcanic geothermal activity and (ii) a late remobilisation from the host rocks followed by deposition of gold ore within favourable structures. I Des Ctudes 1ithogCochimiques recentes, effectuCes par des techniques analytiques sensibles (p.ex., analyses par activation neutronique instrumentale et radiochimique), dans le but d'explorer la possibilitk d'utiliser la distribution de Au pour la recherche de nouveaux gisements auriferes, ont permis de constater une distribution anomale de Au dans certaines parties de la Formation de Val-d'Or (FVD) dans le camp minier de Val-d'Or. La 1ithogCochimie de l'or dans la FVD a montrC qu'il est possible de distinguer: (i) la teneur moyenne en or (1.4-3.5 ppb Au); (ii) une teneur en Au primaire ClevCe autour des mines Sigma et Lamaque (mCdiane estimCe: 15 ppb); (iii) des halos d'enrichissement autour des gites d'or (mCdiane: 70 ppb Au); et (iv) un enrichissement en Au secondaire dans les zones de cisaillement. Nous concluons que la FVD est aurifkre, i.e., anomalement riche en or, au moins, dans certaines parties de cette formation et elle contient les mines d'or Sigma et Lamaque qui reprksentent 68% de la production totale de ce camp. La FVD fait partie du complexe volcanique de Val-d'Or dont les paramktres chimiques et physiques correspondent a ceux d'un environnement d'arcs insulaires peu CvoluCs. Le centre de ce complexe se situerait dans la cheminCe principale de la mine Lamaque; cet environnement peut &tre comparC aux systkmes gCothermaux actifs L tempCrature ClevCe qui sont gCnCra...

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Sheets of granite are confined to shear zones in the tectonite peridotite unit of the Thetford Mines ophiolite complex. Their orientation is parallel to the thrust plane at the base of the complex. The granite, which is calc-alkaline and rich in potassium, does not belong to the ophiolite sequence. We show that it was tectonically incorporated within the partially serpentinized peridotite during thrusting and emplacement of the complex.During this process, the granite was strongly deformed and recrystallized at low pressure (500 bar (50 MPa)) and low temperature (500 °C or less), which gave rise to the mineral assemblage orthoclase, albite, hydromuscovite, and hydrothermal biotite. This episode was followed by a rodingite alteration, characterized by the formation of grossular, which was associated with the development of economic chrysotile within the enclosing peridotite. The K–Ar ages of biotite, muscovite, and feldspar from the granite cluster around 450 Ma. These cooling ages suggest that the emplacement and alteration of the granite and peridotite had ended by Late Ordovician time.

Rare high-temperature and low-pressure contact-metamorphic assemblages are known to occur in the Güneyce-Ikizdere area, eastern Black Sea, Turkey. The emplacement of plutonic rocks of the calc-alkaline Rize batholith has caused the development of assemblages belonging to the spurrite-merwinite subfacies of the sanidinite facies. We provide compositional data for spurrite, rustumite, hillebrandite, tilleyite, cuspidine, vesuvianite, monticellite and gehlenite, and the secondary calc-silicates foshagite, dellaite, killalaite, bicchulite, hydroxylellestadite and two mineral species first discovered in these skarns, defernite and trabzonite. Our data point to the possible existence of a fluorine-rich variant of rustumite and a hydroxyl-dominant variant of cuspidine. These minerals are associated with familiar skarn minerals. The temperatures achieved are considered to be in the range 800-850°C on the basis of published phase-equilibrium results. The unusually high temperatures of contact metamorphism next to compositionally intermediate magmas may well reflect the presence of an underplating of basic magma in the vicinity and the efficient transfer of heat by the rising fluid phase. The relatively high concentrations of Cl and F encountered in these skarn assemblages are consistent with hydrothermal activity leading to Kuroko-type mineralization in this orogenic setting.

Un nouveau minéral, la defernite, a été mis en évidence dans des skarns produits par une intrusion granitique dans des calcaires d'âge crétacé près de Güneyce-Ikizdere, province de Trabzon, dans les Pontides orientales, en Turquie. Sa composition chimique est Ca₆(CO₃)₂(OH, Cl)₈ • n H₂O. La defernite est orthorhombique avec une structure basée sur le groupe spatial Pna2₁ ou Pnam. Les paramètres de la maille élémentaire sont a = 17,860(5), b = 22,775(6) et c = 3,658(1) Å. Les principales raies de diffraction correspondent à 11,37(100), 8,29(35), 5,68(30), 3,045(40), 2,899(50), 2,418(35), 1,962(35), 1,894(25) et 1,825(40) Å. Le minéral se présente en grains subidiomorphes et en lamelles centroradiées dont les dimensions varient entre 100 et 200 µm. La densité mesurée est 2,5 g/cm³ contre 2,42 g/cm³ pour la densité calculée. Les indices observés sont α = 1,546, β = 1,572 et γ = 1,576. L'orientation optique est a = γ, b = β et c = α. Associés à la defernite on trouve la vésuvianite, l'andradite, le diopside, la calcite, la spurrite, la rustumite et l'hillebrandite. Ce nouveau minéral est considéré comme un produit d'altération formé aux dépens de la spurrite et de la rustumite. La description de ce minéral ainsi que son nom ont été approuvés par la commission des nouveaux minéraux de l'I.M.A. Le matériel original est déposé au Muséum d'histoire naturelle de Genève.

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