Felsic Schists Research Articles

An Archean "porphyry-type" disseminated copper deposit, Timmins, Ontario

A disseminated copper deposit occurs in a lens of Archean felsic schist of probable volcanic origin in the Timmins area of northeastern Ontario. The Cu-Mo-Au mineralization, alteration, and alteration-mineralization zoning are comparable in some respects to characteristic features of younger porphyry copper deposits. However, structural, petrographic, and geochemical data suggest that the deposit, as it now exists, may not have evolved in a manner similar to most porphyry copper deposits. The host felsic schist was emplaced during early Archean volcanism, suffered two periods of deformation, and was then altered and mineralized probably during the late Archean. The porphyroidal character, present only in certain parts of the schist, was not a primary feature of the host rock but resulted from metasomatism accompanied by mineralization occurring either during or after the final stage of deformation. The timing of the mineralization-alteration event with respect to host rock emplacement differs from that in many porphyry systems. In addition, the host rock is probably volcanic rather than intrusive and was not the source of the metasomatising fluids. No intrusive source can be identified.

Structural interpretation of the Timmins Mining area, Ontario

A reappraisal of Timmins structure indicates two periods of deformation, one involving N–S compression and a second characterized by a SW–NE couple that reoriented first-period folds and formed the Porcupine Syncline. Major faulting also resulted from this couple. Structure sections reveal that ultramafic rocks and felsic schists are stratigraphically distributed, suggesting that they are volcanic rather than intrusive. Problems concerning the origin, timing, and emplacement of gold and copper mineralization are considered in the context of the present structural interpretation.

Petrochemistry and tectonic origin of the Ammonoosuc Volcanics, New Hampshire–Vermont

The Middle Ordovician Ammonoosuc Volcanics of the Bronson Hill anticlinorium in the upper Connecticut River valley, New Hampshire–Vermont, is composed of amphibolite, greenstone, and felsic schists. Major-element analyses of rocks believed to have been basaltic flows, such as pillowed greenstones and dense amphibolites, reveal that they are tholeiitic in composition. The felsic schists do not have igneous compositions, therefore indicating contamination by sedimentary detritus. Regional metamorphism appears to have been isochemical. However, sea-floor alteration prior to regional metamorphism probably depleted the basalts in MgO and slightly enriched them in SiO 2 and P 2 O 5 . On the basis of trace-element discrimination diagrams (Ti-Zr, Ti-Zr-Y, Ti-Zr-Sr), two distinct basaltic populations exist, suggesting an abyssal oceanic affinity for one group and an island-arc affinity for the other. It is proposed that the opening of the proto–Atlantic Ocean (Iapetus) in early Paleozoic time and its subsequent closure during the Taconic orogeny would explain the interfingering of Cambrian to Middle Ordovician abyssal tholeiite, island-arc tholeiite, and eugeosynclincal metasedimentary rocks in the northern Appalachians.