ABSTRACT

A Metallogenic Study of the Central Virginian Gold-Pyrite Belt by Norman A. Duke

The pyritic base metal massive sulphide and  lode gold occurrences of the central Virginian Piedmont are hosted in the Chopawamsic metavolcanic (schist) belt.  This terrane is  characterized  by  strongly  bimodal  volcanic  suites interlayered with  considerable  epiclastic  and  chemical  sedimentary rocks, indicating that it originated as a  volcanically active  rift  setting.   The fault-bounded rift deposits  are  flanked  on  the west by the low greenschist fades  metapelitic  schists  of  the  inner  Piedmont and on the east by  the amphibolite fades gneissic lithology of the outer  Piedmont,  which  includes both Grenvillian basement exposures and late Paleozoic granitoid intrusions.
A three-stage evolutionary model is proposed for the setting.   Laterally extensive fissure volcanism during  latest Precambrian through Cambrian times produced a western metavolcanic suite dominated by felsic pyroclastic rocks, a central metavolcanic suite of  interlayered  epiclastic  sediments and bimodal volcanic lithologies, and an eastern araphibolite gneiss fades.  This  initial stage culminated  in a period of high level intrusion accompanied by regional deposition of lean iron formation, local pyroclastic eruptions, and epiclastic sedimentation into voleant-tectonic  sinks.   The area remained a non-volcanic shale successor basin  during  most  of  the  Ordovidan.  After being greatly foreshortened during late Paleozoic metamorphism, it remained stable until the Mesozoic when the  widespread intrusion of diabase dykes and the formation of deep molassic graben accompanied the opening of the Atlantic.
Both low-K mantle-derived and high-K crustal magmas  formed  the  igneous suites.  Where intruded, the low-K partial  melt  fractions  from  the  mantle commonly underwent  gravitational  differentiation  to  ultramafic  cumulates, layered  gabbro,  and  fractionated  tonalite-trondhjemite  compositions.   In contrast, the low-K metavolcanic suites are strongly bimodal;  possibly  these formed through  liquid  immiscibility  and  the  splitting  off  of  conjugate iron-rich basaltic and trondhjemitic liquids.  The high-K lineage accounts for large diorite to granodiorite stocks,  the  western pyroclastic rocks, and the late ash flow tuff deposits.  The uniform magnesian amphibolite horizons which occasionally accompany the latter fades have ultramafic-igneous compositions; these are tentatively identified as metakomatiite flows.
Heat flow within the rift  peaked  in the near-surface environment during the late high  level  intrusive  and  pyroclastic  volcanic  stage.   Enhanced hydrothermal  activity at this time resulted in pervasive alteration  of  the volcanic rocks and derivation of  the  lithologically diverse iron formation -aluminous  quartzite  association  which  caps the metavolcanic  suites.   The pyritic massive sulphide and lode gold mineralization found in the Chopawamsic can also be directly related to syndepositional hydrothermal  processes.   The marked  increase  in the  spessartine  contents  of garnet with proximity  to sulphide mineral concentrations  documents  the existence of primary manganese enrichment  haloes,  a  characteristic  feature  of  many exhalative  sulphide settings.   The   garnet-grunerite   iron   formation  -   cummingtonite  and tschermakitic   hornblende-bearing  ironstone  -  metakomatiite   lithological association which  hosts  the  gold is  common  to  many  gold  districts  in metamorphosed iron formation terranes.  The  metallogeny of  the  Chopawamsic setting reflects variations  in  the syndepositional hydrothermal regime.  The initial high  water/rock  ratios  existing prior to loss of formational brines resulted in the deposition of massive sulphide  near discharge sites while the much lower water/rock ratios existing subsequent to dewatering  enriched  gold within multifades iron formation.