Alteration Geochemistry of the Hackett River Main Zone VMS Deposit

Katarina Bjorkman - April 2011

The Hackett River Main Zone VMS deposit is one of four Archean VMS deposits along a five kilometre strike length in the Hackett River Area of Nunavut with 43.34 Mt combined iIndicated resources and 14.62 Mt inferred resources.  It occurs near the top of a succession of highly metamorphosed felsic volcanic rocks.  It is hosted by rhyolitic volcaniclastic lithofacies, which is underlain by ~200 m of dacitic lapilli tuff, and extensive rhyolitic flows and domes. 

The footwall felsic volcanic succession of the Main Zone has been variably hydrothermally altered.   The Ishikawa alteration index and chlorite-carbonate-pyrite alteration index were used concurrently in the alteration box plot developed by Large et al. (2001) to model the gains and losses of K, Fe-Mg, Ca and Na associated with hydrothermal alteration around the Main Zone deposit.  These were used together with the sodium sulphide index to differentiate between felsic volcanic rock affected by hydrothermal alteration from those affected by regional metamorphism and diagenesis.

There is a strong unconformable halo of Fe-Mg and K enrichment and corresponding Na depletion within footwall felsic volcanic rocks to within ~200 m of the ore horizon with the highest Fe-Mg enrichment closest to the ore.  This corresponds to mineralogical alteration facies characterized by a quartz-sulphide core, an anthophyllite-biotite-(garnet-staurolite)-K-feldspar inner zone, and a porphyroblastic sillimanite-biotite-quartz-muscovite outer zone.  A semiconformable hydrothermal alteration zone extends several kilometres along strike within the footwall felsic volcanic rocks of the mineralized horizon, showing moderate gains in Fe-Mg, carbonate, and K, corresponding to sericite and Fe-Mg carbonate alteration.  There is a distinct hangingwall alteration signature of Fe-Mg, carbonate, and K enrichment within 80 m of the massive sulphides.  Least altered felsic volcanic rocks affected by diagenesis and regional metamorphism are located in the hangingwall and in distal footwall felsic volcanic rocks along strike; these are affected by variable gains in Ca, Na, Mg, Fe, and K.