Geological history of the Precambrian rocks in parts of the Porcupine Mining Area, Canada

Abstract

Archaean rocks of the area make up three depositional series which are separated in time one from another by periods of folding. The oldest rocks, exposed in the southern areas, consist of marine-laid basic lavas overlain by felsic volcanics and clastic sediments, all sheared and folded in a northwesterly direction. During the folding, regional metamorphism to the greenschist and amphibolite facies and intrusion of gabbro and ultrabasic rock, took place. The medium-aged rocks were deposited in a broad geosyncline which deepened to the north. In the central parts of the area, shelf facies deposits consist of basic lavas overlain by felsic agglomerate and coarse clastic sediments, but northward these rocks give way to uniformly banded fine clastic sediments deposited in a deeper sea environment. During the later stages of volcanism and sedimentation the rocks were folded synchronously along north-south and approximately east-west lines and metamorphism to greenschist and amphibolite facies took place. Domal structures controlled the locations of volcanic centres, igneous intrusion, and orebodies. The youngest group of rocks, which occurs in the northwestern area, was folded along dominantly east-west lines. Metamorphism of the original sedimentary rocks to gneisses of the high amphibolite facies may have taken place at this time. Faulting and the related intrusion of diabase dykes along north-south, northeasterly and northwesterly lines took place as uplift occurred in the northern areas during Proterozoic time. Widespread, severe retrograde metamorphism accompanied the faulting. Glacial, glacial-lacustrian, and fluvio-glacial deposits of pleistocene age now cover most of the bedrock to depths which range up to 400 feet.