The ophiolitic basic and ultrabasic rocks of the island of Unst, Shetland
comprise a sequence of harzburgites, dunites, clinopyroxene-rich cumulates, and
gabbro, within tectonic blocks that have been thrust over a migmatite complex
during the Laxer Palaeozoic. Concentrations of chromite are found in the
harzburgite and dunite, and to a small extent in the pyroxene cumulate rocks.
They occur as disseminations, sometimes forming millimetre scale layers, and as
more massive schlieren and pods of chromitite. Five alteration or hydrothermal
events have been recognised in the ultrabasic rocks. These comprise early
pervasive serpentinisation, later fracture controlled serpentinisation, veining
and pervasive carbonation, minor late serpentine veining and talc-carbonate
alteration controlled by fault zones.
Exploration for platinum group element (PGE) mineralisation uas carried out
using a combination of drainage, overburden and rock sampling. Analyses of PGE
were obtained by fire assay followed by either neutron activation analysis or
flameless atomic absorption spectrometry, and up to 20 other elements Here
determined by X-ray fluoresence analysis.
Panned concentrate samples were taken from 73 drainage sites distributed
throughout the complex. Ir, the only PGE determined in all samples, showed a
greater concentration in samples derived from the harzburgite unit than those
from other units. Lox amplitude anomalies are present in three discrete areas in
the harzburgite but the maximum level of 210 ppb Ir is associated with a sample
derived from a prominent N-S zone of faulting and hydrothermal activity markedly
discordant to the regional trend of layering in the harzburgite and dunite. This
discordant zone, which extends for at least 7 km, is also marked by samples
containing enrichments in Fe, Co, Ni, Cu and As. The highest Cr levels are
associated with an area in the north of the harzburgite with no previous history
of chromite working but where many locally derived pieces of chromitite float
have been discovered. Relatively high Cr levels are also associated nith the
area of dunite containing the greatest concentration of visible chromite and old
norkings.
A technique of collecting panned heavy mineral concentrates from overburden
samples was adopted as a reconnaissance exploration technique after orientation
sampling in the harzburgite unit at Cliff, an area with high PGE levels in
chromitite and associated dunite. Systematic sampling in the Cliff area outlined
a zone of coincident Pd, Pt and Rh enrichment near to but separate from the
chromite workings knorrn to be enriched in PGE. In contrast the distribution of
Ru was entirely different with scattered lon amplitude anomalous zones and a
maximum anomaly 300m from the chromite-rich zone. Reconnaissance lines were
sampled at other locations within the harzburgite, dunite and cumulate units.
Lore amplitude Pd and Pt anomalies were detected xithin the dunite unit,
especially in 'a traverse across the trace of the prominent N-S fault zone at
Helliers Uater, adjacent to the outcrop of the cumulate unit. In general the
overburden data suggest some association between PGE enrichment and enhanced
levels of Ni relative to typical silicate levels apparent when expressed as the
ratio Ni/MgC.
Rock samples Here collected from all parts of the complex, including most of the
main chromitite workings. Very high levels of all PGE occur in samples of
chromitite, chromite-rich dunite and dunite from the Cliff area, with a strong positive intercorrelation between all PGE. The proportions of the various PGE
are very similar to those present in deposits in major layered basic/ultrabasic
complexes like Bushveld and Stillwater, irith strong relative enrichment in Pd
and Pt. These PGE proportions are completely different from the Ru-Ir-0s
dominant assemblage typical of ophiolitic rocks. Associated with high levels of
PGE are enrichments in Ni, Cu, As, Sb and Te. There is no correlation rrith Cr
and some samples of chromitite from the Cliff area contain only background
levels of PGE.
High to moderate levels of PGE with the same proportions of elements as the
Cliff samples also occur in samples of chromitite and serpentinised dunite from
the dunite unit and in samples of pyroxenite from the cumulate unit, In contrast
PGE-rich samples of chromitite from the harzburgite unit near Harold's Grave
have entirely different proportions of PGE with Ru and Ir in greatest abundance.
This PGE distribution is similar to that in some background samples of
harzburgite and closely resembles the pattern found in typical ophiolites. The
PGE in the Harold's Grave samples do not exhibit the Ni enhancement noted in the
Cliff PGE mineralisation.
In samples from the Cliff area the platinum-group minerals (PGM) sperrylite,
stibiopalladinite, hollingnorthite, laurite and possibly irarsite have been
identified, mostly as grains less than 10 microns in size. In chromite-rich
rocks these minerals occur Rithin chlorite haloes around chromite, in the
blackened altered rims of chromite grains and in interstitial Ni-rich
serpentine/carbonate intergroxths in association with pentlandite, orcellite and
other Ni sulphides and arsenides, sometimes spatially related to
chlorite-carbonate-magnetite veins. They also occur as fine grains Rithin
magnetite rims around chromite and in magnetite or carbonate veins in dunite.
The Ni sulphide/arsenide assemblage associated Rith the PGH is characteristic of
serpentinisation at temperatures less than 500'C, Rell belox the range of
magmatic conditions. A hydrothermal origin for the PGE mineralisation is
proposed, probably related to the second phase of serpentinisation. This
involved the redistribution of Ni accompanied by the introduction of As, Sb and
Te probably with a StrUCtUral Control. Pre-existing concentrations of chromite
may have acted as a precipitation barrier causing rich PGH deposition in the
alteration haloes around chromite grains.
Continuous borehole or trench sections through mineralised zones are required to
assess the economic significance of the PGE mineralisation. Nevertheless the
high levels of PGE attained and the evidence of xidespread occurrence of the
Cliff-type PGE enrichment are favourable indications. The PGE enrichments found
in the cumulate complex are of potential interest as they may originally have
been of magmatic origin. Larger tonnage targets may therefore be present in this
unit compared Rith the likely size of structurally-controlled mineralisation
elswhere in the complex
