The Misema and New Senator calderas: Volcanology, volcano-tectonic structures
and targeting VMS-deposits, Blake River Group
Projet DIVEX-B SC33
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General geology of the Abitibi greenstone belt with the location of numerous calderas including the Misema-New Senator-Noranda complex, and the Hunter Mine, the Normetal, Gemini, and Selbaie calderas. The pie diagrams show the distribution of lithological units (modified from Daigneault et al., 2002, 2004). |
Introduction
Our multi-disciplinary Divex study focuses on the renewed economic potential and interest of the Noranda mining camp in the Abitibi greenstone belt by assessing the various caldera-forming stages, and their link to volcanogenic massive sulfide (VMS) deposits. Apart from a detailed geochemical programme and systematic sampling for U-Pb age determinations, special emphasis is placed on the interpreted summit calderas of the Misema caldera, especially the Montsabrais volcanic complex, because of the VMS potential. The ring dyke complexes, the magmatic roots of the summit calderas are new exploration loci, as mafic summit calderas favour extensive hydrothermal venting. The best documented example is Axial Seamount along the Juan de Fuca Ridge with the Ashes hydrothermal field. The subaqueous summit calderas are ponded magmas characterized by channelled, sheet, pillowed, and pahoehoe flows as well as related inflation structures (Applegate and Embley, 1992). Some of the ponded magmas in ancient sequences may have erroneously been interpreted as thick sills (Moore and Mueller, 2008). The economic potential of these magma ponds should not be dismissed, because modern seafloor analogies are favourable sites (Schmincke, 2004). Associated with the volcanic flow facies of the Montsabrais volcanic complex are 20-30 m-thick volcaniclastic deposits that show an explosive origin and subsequent shallow-water reworking.
Objectives and Methodology
The prime objective is to understand the two calderas so that new VMS targets can be pinpointed. Numerous field and laboratory techniques were employed. Volcanic facies mapping and gridding was conducted at the outcrop scale. A N-S- and E-W-orthogonal baseline with a 5 m spacing were placed over the entire Glenwood rhyolite in the Cap d’Ours section. All other parallel grids were conducted at 20 m intervals. Contouring of outcrops was conducted at a scale of 1:200 and completed in the SW segment with a SX Blue II Bluetooth GPS system. Volcanic facies mapping was done at a 1:400 scale and in some areas at a greater detail (1:200 to 1:100). Representative samples were collected for petrographic, geochronological and geochemical analyses in accordance with field observations. U-Pb geochronological analyses were performed by Dr. R.Friedman at the Pacific Centre for Isotope and Geochemical Research at the University of British Columbia using thermal ion mass spectrometry (TIMS). Major and trace element geochemical analyses were completed at the GEOLABS facility of the Ontario Geological Survey in Sudbury using X-ray fluorescence (XRF) for major elements and induced coupled plasma mass spectrometry (ICP-MS) for trace and rare Earth elements.
Results of 2008
Age determinations
U-Pb age determinations of zircons in mafic and felsic volcanic rocks provide a rigorous manner in which to test the evolution of the Blake River megacaldera complex. The primary focus is to discern the evolution of the Blake River megacaldera complex from early volcanic shield volcano-building to the evolved felsic dominated caldera stage. The presented volcanic ages give the maximum spread of Blake River evolution.
A) Concordia age determination of 2 chemically abraded zircons from a gabbro in the ring dyke complex of the Montsabrais volcanic centre. B) Medium-grained gabbro with small pockets of quartz-amphibole, commonly enriched in zircons. The pockets represent the last magmatic (exsolution-explusion) events in the development of dyke-sills. |
A) Concordia diagramme of Clericy gabbro with the most evolved quartz-rich gabbro phase. The regression line has an upper intercept of 2704.3 ± 2.0, which is considered the age of crystallization. B) Medium-grained quartz gabbro with cm-to mm-scale quartz rich quartz patches. |
The U-Pb age determinations show that the Blake River megacaldera complex evolved over roughly 10-12m.y., from 2706- 2696-94 Ma (if errors are included). The evolution of a single caldera is rarely more than 1-2 m.y., but if one considers caldera multiple events and includes the basal shield volcano phase then time scales of 6-13 m.y. for a complete evolutionary development are common, shown for the continental Valles caldera, and especially for the oceanic island, Tenerife with the Las Cañadas caldera. The study shows that there are numerous ages for the mafic volcanic rocks with the oldest documented event at ca. 2704-2706 Ma. The development of a mafic basalt plain and numerous overlapping shield volcanoes is therefore real and represents the base for subsequent complex caldera evolution. The ring dykes may be somewhat younger at 2701-2702 Ma, but are certainly related to the mafic constructional phase, possibly occurring at the same time as the New Senator caldera between 2703-2700 Ma. A possible scenario is that while a central felsic volcano and caldera system developed in the central Blake River, the mafic dominated rim represented by the Misema caldera, showed the formation of numerous summit calderas along major rift zones. This would be analogous to the modern and active rift zone on Hawaii (Mueller et al., 2009). Felsic volcanism of 2703 Ma, represented by the Glenwood rhyolite, re-enforces the interpretation by Pearson and Daigneault (2009) of the older New Senator caldera.
Calderas occur in clusters and are overlapping events (Mueller et al., 2008). The Blake River megacaldera complex is no exception, and shows a complex history that can only be resolved with detailed facies mapping and geochemistry in conjunction with U-Pb age determinations. As stratigraphic concepts are of little use, volcanic facies must identify intrusive and extrusive volcanic phases, and only then do the ages make any sense. Our major contribution is understanding and documenting the evolution of the mafic rocks in the scheme of caldera formation. The ages of extrusive and intrusive felsic rocks constrain local felsic edifice construction (e.g. Glenwood rhyolite).
Glenwood Rhyolite
Two aspects were considered in 2008: (1) the physical volcanology and geochemistry and (2) the alteration, mineralization and deformation of the Glenwood rhyolite. The former is briefly presented in this report. The Glenwood rhyolite is a 800-1000 m-thick felsic succession located in the town of Rouyn-Noranda proper. The rhyolite is intruded by numerous dyke generations and conformably overlain by pillowed and brecciated mafic volcanic rocks. The felsic and mafic volcanic facies are indicative of a subaqueous setting. The felsic Glenwood rocks at Cap d’Ours are divided into aphyric (aphanitic) and quartz-feldspar-phyric rhyolites. The 100-400m-thick aphyric rhyolites are lava flows construct a subaqueous dome, and contain several volcanic flow facies, which include: (1) massive lobate facies, (2) in-situ brecciated lobate facies, and (3) flow breccia facies. Sericitization, chloritization, and silicification indicate the hydrothermal alteration processes, and this is readily identified in the volcanic matrix. The 5-50m-thick, quartz-feldspar-phyric flow forms are divided into a (2) massive to dyke facies and (2) an endogenic lobate facies. Alteration assemblages are pronounced in the lobate facie for these. This volcanic facies is responsible for the inflation of the dome-flow edifice. The mafic dykes post-date the felsic event and feed the overlying mafic pillowed flows that cap the Glenwood rhyolite.
Volcanic facies of the Glenwood rhyolite (Cap d’Ours). Note the aphyric volcanic construction phase and the quartz-feldspar-phyric inflation stage. U-Pb ages shows that the Glenwood rhyolite is part of the New Senator caldera event (Mapped by Moore, Genna, Mueller, 2008).
Montsabrais volcaniclastic sequence
The volcanic-volcaniclastic deposits of the Montsabrais volcanic complex are the first evidence of a shallow-water depositional setting associated within central ring dykes of the Misema caldera. Dimroth et al. (1985) interpreted the massive and stratified lapilli tuff breccias in the Montsabrais region as pyroclastic pillow breccias, but the here described cross-bedded units had not yet been identified. In the innovative paleographic model of the Blake River Group, Dimroth et al. (1982) advocated a shoaling volcanic centre at Montsabrais that has abundant explosive volcanism. It is possible and logical that the volcaniclastic sequence at Montsabrais is time-correlative with the Dalembert tuff (Tasse et al., 1978), the Jevis and Kino pyroclastic deposits of the Clericy-Bouchard–Hebert area (Pilote et al., 2007, 2008) and the Stadacona breccia of Noranda (Mueller et al., 2009), as these units are compositionally similar (e.g. basaltic to basaltic andesite) and are contain massive to stratified beds with abundant pumice, euhedral and broken crystal and lithic volcanic fragments. The abundant mafic explosive volcanism is constrained between the inner and outer ring faults of the Misema caldera, and this is also where most of mafic summit calderas are located.
Altered m-scale mafic pillow flow units conformably overlain by cross-bedded and planar stratified tuff and lapilli tuffs. Note thin-tuff beds indicating suspension deposits. Scale blue pen: 14 cm.
References
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