Alkaline Rocks and Carbonatites of the World

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Otto Stock

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Occurrence number: 
031-00-076
Country: 
Canada
Region: 
Ontario
Location: 
Longitude: -80.08, Latitude: 48.05

An almost circular intrusion nearly 10 km in diameter, the Otto Stock cuts metamorphosed Archaean volcanic and sedimentary rocks, and in the south granodiorites of the Round Lake batholith. Numerous inclusions of country rocks are found in the stock, particularly near the margins, and there is a contact metamorphic aureole up to 0.4 km wide on the north side. Most of the stock is composed of granite, quartz syenite and syenite porphyries, with a general decrease in quartz towards the margins, nepheline syenite occurring locally along the northern side. Apart from microperthite and quartz, most of the rocks contain amphibole and up to 14% of aegirine-augite. Foliated nepheline syenite occurs as sills, thin pegmatitic dykes and as gneiss. In some rocks nepheline reaches 50%, the other minerals generally being microcline, often perthitic, aegirine-augite, biotite, scapolite, apatite and sphene.

Economic: 
Cu and Au mineralization occurs in the contact aureole.
Age: 
A Rb-Sr whole rock isochron gave 1730+-50 Ma and hornblende gave 1830+-Ma by K-Ar (Purdy and York, 1968).
References: 
CURRIE, K.L. 1976a. The alkaline rocks of Canada. Bulletin, Geological Survey of Canada, 239: 1-228. LOVELL, H.L. 1972. Geology of the Eby and Otto area District of Timiskaming. Geological Report, Ontario Department of Mines and Northern Affairs, 99: 1-34. PURDY, J.W. and YORK, D. 1968. Rb-Sr whole-rock and K-Ar mineral ages of rocks from the Superior Province near Kirkland Lake, northeastern Ontario, Canada. Canadian Journal of Earth Sciences, 5: 699-705
Fig. 39 Otto Stock (based on Currie, 1976a, Fig. 7 - after Lyall, 1972, Map 2230).
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith