1887

oa South African Journal of Geology - Stratabound base metals and gold in iron-rich rocks of a late Proterozoic-early Palaeozoic rift setting, central Appalachia, USA

Volume 89 Number 2
  • ISSN : 1012-0750
  • E-ISSN: 1996-8590

 

Abstract

Pyritic base metal sulphide and lode gold occurrences within the Virginia Piedmont of the Appalachian orogen are hosted by the Chopawamsic Formation. This late Proterozoic to early Palaeozoic succession of thin-banded mafic and felsic metavolcanic rocks, interlayered with epiclastic plus chemical metasedimentary rocks, formed in a volcanically active rift setting. An.evolutionary model is proposed in which laterally extensive fissure volcanism during latest Proterozoic through Cambrian produced a western volcanic suite dominated by felsic pyroclastic rocks, a central suite of interlayered epiclastic sedimentary and bimodal volcanic rocks, and an eastern mafic volcanic suite. This initial stage culminated in high level intrusions, felsic pyroclastic volcanism, and regional deposition of ironstone. The rift was a non-volcanic shale basin during most of the Ordovician. After fore-shortening during late Palaeozoic metamorphism, it remained stable until the Mesozoic when widespread intrusion of diabase dykes and the formation of deep molassic graben accompanied the opening of the Atlantic.

The base metal occurrences are spatially associated with ironstones rich in alumino-silicate porphyroblasts interlayered with epiclastic metasediment and bimodal metavolcanics of the central suite. The lode gold occurrences are spatially associated with a lithologically diverse garnet-grunerite iron-formation, cummingtonite-tschermakitic hornblende ironstone, aluminous quartzite, and ultramafic metavolcanic assemblages occurring along the base of the Ordovician metashale. Heat flow within the rift was greatest during the period of high level intrusion and pyroclastic volcanism. Hydrothermal activity at this time was responsible for alteration of the volcanic rocks, deposition of the diverse ironstone-aluminous quartzite association, and concentration of base metals and gold. The metallogeny of the Chopawamsic belt suggests an initial high water/rock system in which massive base metal sulphides were deposited near discharge sites. Subsequent to compaction and dewatering a low water/rock system concentrated gold in mineralogically complex ferro-aluminous beds.

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1986-05-01
2020-09-29

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