Abstract

A small pyrrhotite-pentlandite-chalcopyrite deposit between Lawrence and Lowell, Massachusetts, occurs in a Paleozoic noritic stock in Cambrian quartzite. The norite varies somewhat in composition. The deposit has been worked at various times for gold, silver, iron, copper, and nickel. Spectrographic analysis of the pyrrotite shows a small content of nickel and cobalt. The ore minerals, formed subsequent to the crystallization of the silicates, are those characteristic of this type of deposit, but show several interesting mineragraphic features. Pyrrhotite consistently has oriented exsolution blades. Pentlandite has been incompletely replaced by violarite, which is seen as a gridwork within the pentlandite crystals. Both magnetite and ilmenite are present, the latter chiefly in the country rock. Magnetite shows several different habits and associations, which indicate that several periods and modes of formation were operative. Marcasite replacement of pyrrhotite was a supergene process. Mineralogic and mineragraphic data coupled with field evidence indicate that the ore was deposited in the period when magmatic action was nearly finished but strong hydrothermal action had not yet begun---forming the type of deposit termed by Bateman (1942) as late-magmatic residual liquid segregation and injection.

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