Intrusive differences spelled out

There seems to be some confusion in The Northern Miner and within the investment community at large with respect to the term “layered intrusions.”

In most cases, I see authors and investment commentators describing zoned intrusions as being “layered intrusions,” and there is a distinct difference between the two. Specifically, there are major differences in the size, grade and types of mineral deposits associated with these distinct intrusion types.

A layered intrusion is also known as a “stratiform intrusion.” It is an intrusive body in which there are layers of varying mineralogical composition. These intrusions are further defined as having distinct layers of igneous rock, which show banding that simulates the stratification of a sedimentary sequence.

Examples of these intrusions would be Bushveld in South Africa, Stillwater in Montana, Skaergaard in Greenland and Big Trout Lake here in Ontario.

A zoned intrusion is an intrusive body in which there are areas of latitudinal character, which, by some special or distinctive characteristic, such as chill margin, contact breccia, crystal size or mineralogy, distinguish it from the surrounding area.

I wish to stress that there are no distinctive layers that simulate the stratification of a sedimentary sequence. Examples of these intrusions would be Lac des les, Ajax, and, from what I have read and heard at meetings, River Valley, all of which are in Ontario.

Lorne Burden, P.Geo.

Peterborough, Ont.