Michel Rocheleau

PALEOGEOGRAPHIC AND PALEOTECTONIC EVOLUTION OF A BASEMENT-CONTROLLED ARCHEAN SUPRACRUSTAL SEQUENCE, CHIBOUGAMAU-CAOPATINA, QUEBEC'

The 7-14 km thick, Archean volcano-sedimentary sequence in the Chibougamau-Caopatina Region (CCR), composed of two mafic to felsic cycles and two thick, essentially epiclastic successions, is divided into a Southern Caopatina Segment (SCS) and Northern Chibougamau Segment (NCS). The sedimentary units which evolved after cycle 1 volcanic rocks in the SCS, and after the combined cycles 1 and 2 in the NCS, document a diachronous sedimentary basin evolution. This criterion has been employed to distinguish an early evolutionary stage in the south and a late evolutionary stage in the north. In each segment the basin episode represents a unique extensional event. Volcanism preceded the major sedimentary-rifting event in both segments. Composition of sediments and sedimentary facies, correlated with the interpreted basin margin faults, support the interpretation of two phases of crustal extension. Volcanic construction and sill intrusion were the prevalent factors governing emergence of volcanic islands in the CCR; emplacement of magma in subvolcanic chambers is considered subordinate. Once volcanism waned, uplift of the consolidated, subvolcanic plutons and rise of the tonalitic gneiss were responsible for the continued unroofing of the volcanic edifices. The abundance of synvolcanic plutons supports this interpretation. Significant plutonic accretion below the volcanic centers transformed the Archean crust in time from a thin transitional crust to a thicker continental-type crust. A late stage K-rich volcanism also suggests the presence of a continental-type crust. Deposition of the Archean supracrustal rocks in the CCR was controlled by tectono-magmatic processes, and they show initial development of a primitive island (arc?) that gradually evolved into a mature island environment. Finally, backarc or marginal basin extension took place, and detritus derived from the volcanic edifices was shed into the adjacent fault-bounded basins. The marginal basins of the western Pacific or Andaman Sea marginal basin may be modern analogues.

Alteration patterns related to gold mineralization and their relation to CO2/H2O ratios

SummaryThe pattern of hydrothermal alteration associated with a gold prospect located within a differentiated iron-rich gabbro sill in the Chibougamau region has been investigated in order to establish the alteration mechanism. The gold mineralization, occurring in east-west trending shear zone, shows a close spatial relationship to zones of intense alteration. Ankerite-sericite-pyrite/ankerite-fuchsite-chlorite/ankerite-sericite-chlorite, and chlorite-calcite-magnetite form four distinct alteration facies which extend outward from the mineralized zones. Detailed mineralogical, petrological and chemical studies show that the iron content of chlorite increases whereas the iron content of the carbonate decreases from the mineralized zone outwards. The hydrothermal alteration pattern is shown to be derived from a single fluid with a high initial CO2/H2O ratio. At first, reducing reactions prevailed and, with progressive fluid-rock interaction, the CO2/H2O ratio was lowered and oxidation reactions dominated.ZusammenfassungHydrothermale Alterationen in Zusammenhang mit einer Goldvererzung eines differenzierten eisenreichen gabbroischen Sills im Chibougamaugebiet wurden untersucht, um den Umwandlungsmechanismus zu ermitteln. Die Goldvererzung in der Ost-West verlaufenden Scherzone ist eng mit Zonen intensiver Alteration verbunden. 4 deutlich unterscheidbare Alterationsfazics, die mit zunehmender Entfernung von der Vererzung auftreten, sind: Ankerit - Serizit - Pyrit/Ankerit - Fuchsit - Chlorit/Ankerit -Serizit - Chlorit und Chlorit - Kalzit - Magnetit. Detailierte mineralogische, petrographische und geochemische Untersuchungen zeigen, daß sich von der Vererzung weg der Eisengehalt im Chlorit, erhöht, gleichzeitig aber in Karbonaten verringert. Die hydrothermale Alteration ist auf eine einzige fluide Phase mit ursprünglich hohem CO2/ H2O-Verhältnis zurückzuführen. Vorherrschend waren zunächst reduzierende Reaktionen. Mit zunehmender Wechselwirkung zwischen Fluid und Gestein erniedrigte sich das CO2/H2O-Verhältnis und oxidierende Reaktionen nahmen zu.