Massive sulfide deposits of the Noranda area, Quebec. III. The Ansil Mine.
Barrett, T.J., MacLean, W.H., Cattalani, S., Hoy, L. and Riverin, G., 1991c.
The Ansil massive sulfide deposit occurs at the contact of underlying Northwest Rhyolite and the overlying Rusty Ridge Andesite, in the lower part of the Central Mine sequence of the Blake River Group. The orebody, which is roughly ellipsoidal in outline and up to 200m x 150m across, contains 1.5 Mt of massive sulfide grading 7.2% Cu, 0.8% Zn, 1.7 g/t Au and 25.9 g/t Ag. Least altered host rocks are low-K basaltic andesites and low-K rhyolites. These rocks have Zr/Y ratios of ù5 and LaN/YbN ratios of ù2.3, typical of tholeiitic volcanic rocks, although their major element chemistry is transitional between tholeiitic and calc-alkaline volcanic rocks.
The Ansil deposit, which dips ù40¡ southeast, is a single orebody comprising two main massive sulfide lenses (up to ù35m thick) connected laterally via a thinner blanket of massive sulfides, with thin discontinuous but conformable massive magnetite units at the base and top of the orebody. Sulfide ore consists of massive to banded pyrrhotite-chalcopyrite. In the down-plunge lens, up to 10 m of massive magnetite are capped by up to 10 m of massive sulfide. Finely banded cherty tuff, with sphalerite-pyrite-chalcopyrite, forms a discontinuous fringe to the deposit. The two main lenses of massive sulfide have the highest contents of Cu, Ag and Au, and are thought to have formed in areas of major hydrothermal input. Altered feeder zones contain either chlorite + chalcopyrite + pyrrhotite ± magnetite, or chlorite + magnetite ± sulfides. Footwall mineralization forms semi-conformable zones ù5-10 m thick that directly underlie the orebody, and high-angle pipe-like zones that extend at least 50 m into the footwall. Ti-Zr-Al plots indicate that almost all altered footwall rocks were derived from a homogeneous rhyolite precursor. Hangingwall andesites were also altered. Despite some severe alteration, all initial volcanic rock compositions can be readily identified, and thus mass changes can be calculated. Silica has been both significantly added or removed from the footwall, whereas K has been added except in feeder pipes. Oxygen-isotope compositions up to at least 50m into the hangingwall and footwall are typically depleted in d18O by 2-6Å. These rocks have gained Fe+Mg and lost Si. Altered samples in general range from light REE-depleted to light REE-enriched, although some samples exhibit little REE modification despite strong alkali depletion. Mineralized volcanic rocks immediately below the orebody are enriched in Eu (as are some Cu-rich sulfides in the orebody).
Contact and petrographic relations generally suggest that the main zone of massive magnetite formed by replacement of a cp-po-rich sulfides, although local relations are ambiguous. Magnetite formation may reflect waning hydrothermal activity, during which fluids mixed with seawater and became cooler and more oxidized. Cu-rich feeder pipes that cut magnetite-rich footwall indicate a renewal of Cu-sulfide mineralization after magnetite deposition. Chloritic zones with disseminated sulfides occur up to a few hundred metres above the orebody, attesting to continuing hydrothermal activity.
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