At Sams Creek a gold-bearing, peralkaline granite
pophyry dyke intrudes Ordovician-Silurian metapelite and quartzite. The granite
porphyry dyke has a thickness of up to 40 m, extends discontinuously for about 8
km, and intrudes thin lamprophyre dykes along its contacts. The granite dyke and
gold-bearing sulphide and quartz-siderite veins within it have been deformed and
recrystallised during several phases of folding, the youngest of which is Early
Cretaceous.

The unaltered granite porphyry is composed of phenocrysts
of perthite, arfvedsonite, aegerine and quartz in a groundmass of the same
minerals, with accessory ilmenite, rutile and fluorite. K-Ar dates of amphibole
from the granite gave ages of 226 and 246 Ma, but these ages may be a minimum
due to resetting during the thermal and deformation events that have affected
the area. Unpublished Ar-Ar data on amphibole from the granite show disturbed
spectra with high temperature steps around 300-315 Ma, which is consistent with
the age of A-type granites in the Buller region. Further age dating of primary
and hydrothermal minerals in the granite is continuing. At least three stages of
hydrothermal alteration have been identified in the granite. Stage I alteration
is characterised by high fO2
magnetite-siderite biotite, which is cut by thin quartz-pyrite
veinlets (Stage II). These are cut by Stage III sulphide, quartz and siderite
veins, and pervasive silicification represented by quartz-perthite-albite-rutile
pyrite-siderite-sericite assemblages overprints the earlier alteration stages.
Lamprophyre is altered to a siderite-chlorite-sericite assemblage. Stage III
sulphide veins are composed of arsenopyrite + pyrite gold galena
sphalerite chalcopyrite pyrrhotite graphite, deposited by low fO2
H2S
fluids. The gold contains 18-30 wt% Ag and up to 1 wt% Bi. Temperature of vein
formation was estimated from vein quartz fluid inclusion homogenisation
temperatures (Th
260-355oC,
average 330oC),
arsenopyrite-sphalerite thermometry (310-365oC)
and quartz-albite stable isotope thermometry (340oC).
The altered granite and the veins have been deformed, and recrystallised and
competent sulphide minerals are fractured and brecciated. Alteration and
mineralisation are confined to the granite porphyry and adjacent
lamprophyre.