LETTERS TO THE EDITOR — New light on Sudbury-type rocks

Sudbury-type (nickel-cobalt) mineralization associated with extensive albitization in Mackelcan Twp., northeast of Sudbury, Ont., was reported by Flag Resources (T.N.M., June 15/92). Noted was the enrichment of these elements in an albitized breccia zone in the Huronian sediments overlying the Wanapitei anomaly. This report sparked a debate in the letters to the editor by Neil Willoughby (T.N.M., June 29/92) and Robin Goad (T.N.M., July 13/92) concerning the significance and timing of albitization with respect to mineralization in the area. Mr. Willoughby suggested the Nipissing intrusions as a possible source for the metals.

Albitization is widespread in the Sudbury-Wanapitei Lake area and is commonly associated with gold mineralization. Recent U-Pb dating of hydrothermal monazite (rare earth element phosphate) in the gold and pyrite-rich albitized rocks from the Sheppard property, Maclennan Twp., Ont., indicates that albitization in the area occurred 1,700 million years ago (Eva Schandl, Donald Davis and Michael Gorton, Ontario Mines and Minerals Symposium, December, 1991) and it postdated the emplacement of the Nipissing diabase by about 500 million years. Although a genetic relationship between albite and gold has not been established to date, the ubiquitous association of gold with albite, sulphides and/or chlorite is common in the region. The extraordinary light rare earth element enrichment in albitized, sulphide-rich, gold-bearing rocks at the Sheppard property and the presence of exotic rare earth element-rich minerals in the albitized and chloritized rocks at the Scadding mine, Scadding Twp., suggest that the sodium-rich fluids probably had a deep-seated origin, and may have been generated by some alkalic or carbonatitic intrusions at depth. If this is true, the relationship between gold and sodium-rich fluids is quite unusual. However, if we consider the proximity of albitized rocks to large fault zones, their common association with in situ brecciation, the abundance of sulphides and extensive chlorite alteration (i.e. Scadding and Norstar mines), we can call on a complex series of events to account for the enrichment of gold or Sudbury-type metals in the sediments. These events probably included protracted tectonism; deep-seated plutonism and faulting. Because albite enrichment occurs only in selected areas (as opposed to other greenschist minerals such as sericite and chlorite) and the rocks are easily identified in the field, albitized rocks are good exploration targets. I suggest that Sudbury-type metal enrichment in the albitized Huronian sediments (as reported by Flag) may well indicate the presence of mineralized source rocks at depth. If the age of these source rocks is comparable to the age of the Sudbury Igneous Complex, the metals could have concentrated in the sediments during later metamorphism/metasomatism.

Eva Schandl

Research Associate

Department of Geology

University of Toronto

Toronto