Abstract

In Destor and Duparquet townships, Precambrian quartz-feldspar porphyry intrusives have been injected into Keewatin-type lavas and pyroclastics, Timiskaming-type conglomerate and arkose and post-Keewatin type, pre-Timiskaming type basic intrusives.Surrounding the quartz-feldspar porphyry intrusives are halos containing numerous well developed quartz crystals. These are present in all types of rock with which the quartz-feldspar porphyry is in contact.The presence of quartz crystals in conglomerate boulders adjacent to the porphyry and the presence of quartz-feldspar porphyry fragments and conglomerate boulders along brecciated contacts between conglomerate and quartz-feldspar porphyry had led previous workers to the erroneous conclusion that the Timiskaming-type sediments are younger than the quartz-feldspar porphyry.A study of the above features as well as the halos of quartz crystals developed in other rock types adjacent to the quartz-feldspar porphyry indicates that these phenomena are due to porphyritization.Porphyritization is the development of a porphyritic and porphyro-blastic texture in a previously formed rock by partial magmatic replacement and by metastomatism.The process accompanying porphyritization has been divided into five stages:(1) The injection of quartz-feldspar porphyry.(2) Permeation of the more porous rock types at the contact by a tenuous phase of the quartz-feldspar porphyry liquid fraction.(3) Brecciation and fracturing.(4) Injection of a highly siliceous fraction of quartz-feldspar porphyry into zones of fracturing and brecciation.(5) Injection of the final liquid residue of the cooled quartz-feldspar porphyry into zones of fracturing and brecciation.Gold deposition took place in these zones, at this stage, and is genetically related to the quartz-feldspar porphyry.

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