CHROMIAN MUSCOVITE, UVAROVITE, AND ZINCIAN CHROMITE: PRODUCTS OF REGIONAL
METASOMATISM IN NORTHWEST NELSON, NEW ZEALAND
ALVA CHALLIS
Institute of Geological and Nuclear Sciences Ltd., P.O. Box 30 368, Lower Hutt, New Zealand
RODNEY GRAPES
Research School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand
KEN PALMER
Analytical Facility, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand
Abstract
Chromian muscovite ("fuchsite") with up to 8.5% Cr2O3, uvarovite garnet (up to 83% Uv), zincian
chromite (up to 13.7% ZnO), and chromian rutile (up to 3.3% Cr2O3), are variously associated with
chlorite, biotite, epidote, kyanite, and tourmaline, all Cr-bearing, in Lower Paleozoic quartzite, biotite schist,
marble, and carbonated ultramafic rocks in Northwest Nelson, New Zealand. Sulfides and carbonates associated
with the Cr-silicates and oxides include pyrite, galena, gersdorffite, pentlandite, and calcite, siderite, cerussite,
magnesite, dolomite, and ankerite. Chromium mineralization of the host rocks takes the form of
thin seams, and broad zones tens of meters wide that parallel host-rock schistosity and exhibit varying
intensities of green coloration. All the Cr-rich localities occur within 3-4 km of Early Cretaceous granitic
plutons and are close to, or lie within, major fault zones. Field relationships, mineral textures, rock associations,
and compositions indicate that the mineral paragenesis is metasomatic in origin, and the result of
hydrothermal fluids from the granitic plutons interacting with ultramafic rocks or detrital chromite derived
from them.
Keywords: chromian muscovite, uvarovite, zincian chromite, chromian rutile, Cr-silicates, analyses, metasomatism, Nelson, New Zealand.