Gahnite Composition: Implications for Base Metal and Rare-element Exploration

Exploration & Mining Geology, Vol. 6, No. 3, 1997




Gahnite, a zinc-bearing spinel, is a resistate mineral that may be used as
an indicator mineral of polymetallic deposits in high-grade metamorphic terrain.
The Ontario Geological Survey recently completed surficial mapping and sampling
programs in the Separation Lake area of northwestern Ontario and Kinniwabi Lake
area of northeastern Ontario in which gahnite was recovered from samples of
surficial sediments. Ternary plots of the composition (ZnO, MgO, FeO[t]) of
gahnite recovered from these surficial studies reveal three distinct groups:
Group 1 is characterized by low (< 1 wt%) MgO values; Group 2 by higher
(>1 wt%) MgO values; and Group 3 by higher FeO(t) values than Groups 1 and 2.
The composition of in situ gahnite recovered from volcanogenic massive sulfide
mineralization, Zn-rich Sedex-type mineralization and peraluminous rare-element
mineralized granite and pegmatite also was determined. The composition of the
gahnite obtained from surficial samples was compared with that of in situ
material. Gahnite from the peraluminous rareelement granite and pegmatite
overlapped very well with surficial gahnite of Group 1. Gahnite from
volcanogenic massive sulfide mineralization and Sedex-type mineralization
overlapped well with surficial gahnite of Group 2. These results suggest that
determination of gahnite composition in indicator mineral surveys may be useful
in exploration programs when assessing the mineral potential of a
region.
Keywords: Gahnite, Resistate mineral, Polymetallic deposits, metamorphic terrain, Mineralization
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