IGCP Board; 10th session; Report of the ... - unesdoc - Unesco

The problem of the metallogeny and exogenic
processes of the Proterozoic was proposed by
the Finnish scientists for discussion at the next
session to be held in Finland. This will enable
the participants of Project 160 to gain a more
profound knowledge of the geological localities
in a typical Precambrian region of the Baltic
Shield.
Activities planned. A meeting on development
and interactions of Precambrian lithosphère,
biosphere, and atmosphere is to be held jointly
with Project 157 in Mexico City in 1982. H.
Clemmey has proposed another meeting on sedimentary
and diagenetic processes in Precambrian
metallogenesis which is to be sponsored
jointly with Project 91 in London in May 1982.
No. 161 SULPHIDE DEPOSITS IN MAFIC AND
ULTRAMAFIC ROCKS
A.J. Naldrett, Department of Geology, University
of Toronto, Toronto, Canada M5S 1A1.
Description. The aims of the Project include:
(1) gathering and compiling systematic data sets
on magmatic sulphide deposits associated with
a wide variety of tectonic environments and host
rock types with a view to recognizing and understanding
the best environments for ore formation.
Unmineralized mafic and ultramafic rocks
will, of course, be included in the compilation;
(2) fostering field conferences and symposia to
stimulate interest in magmatic deposits, to help
educate geologists from countries lacking such
deposits, and to assist in the compilation as outlined.
Summary of activities. The Project Workshop
"The Geology and Exploration for Ni and Pt-
Group Element Deposits" was held at the University
of Zimbabwe, Salisbury, Zimbabwe.
The lecture and laboratory sessions lasted four
days from 20-23 July 1981, and included descriptions
of Zimbabwe deposits, discussion of the
use of predictive genetic models in planning exploration
programmes and an outline of exploration
methods currently in use in Zimbabwe.
This formal instruction was followed by six days
of field trips, during which the participants visited
all of the major Ni deposits in Zimbabwe,
including the classic Belingwe greenstone belt
and the Great Dyke. About sixty geologists participated
in the workshop. In addition, the Geological
Liaison Office at the Commonwealth
Science Secretariat convened a meeting of the
heads of the geological surveys of the African
Commonwealth countries, to coincide with the
workshop so that these people were also able to
participate in all facets of the workshop.
Data compilation is progressing, with the first
data sets already in the hands of the US Geological
Survey for incorporation into their CRIB
storage system. Data compilation of Canadian
nickel sulphide deposits is in hand and a map
52
showing Canadian nickel sulphide deposits, their
size and types is being prepared. The complete
compilation of data on Finnish nickel occurrences
and the codified data from approximately half of
the Norwegian nickel deposits have been sent to
the US Working Group for computer entry.
A special Issue of Economic Geology (No. 6,
1981) on the geology of Western Australian nickel
sulphide deposits was published as a result of the
joint collaboration of IGCP Projects 161 and 91.
The Project's scientific progress can be summarized
as follows:
In the field of nickel sulphide deposits: (1) Environment
of ore deposition. Important progress
in the understanding of Komatiite-related nickel
sulphide deposits at Kambalda in Western Australia,
is reported in the recently published Nisulphide
volume (Loftus-Hills et al., 1981). It
has been known that these same ore deposits occur
at the base of a sequence of ultramafic lava
flows, mostly in topographic irregularities at the
basal contact of the ultramafic succession and, to
a lesser extent, above this contact at the base of
the next one to three overlying flows. The latest
work has shown that, within one hundred metres
of the ore deposits, the normal, regular succession
of flows and interrelated cherty sediments
is less regular. Sediments are either absent or
discontinuous, flows have a greater ratio of vertical
to horizontal dimension, and MgO -rich units
occur throughout the succession, rather than predominantly
at the base, as is the situation away
from ore. Although no vertical feeder conduits
have been distinguished at Kambalda, the ore
deposits are thought to lie close to the feeders.
Ore lying within and on the flanks of a feeder has
been identified at the Langmuir komatiite deposit
(Green and Naldrett, 1981).
(2) Sulphide-Silicate interaction as a guide to ore.
Some years ago it was proposed that if a magma
was saturated in sulphide, the concentration of
chalcophile elements should differ during fractional
crystallization than if it was not sulphide saturated
(Duke and Naldrett, 1978); Leisher et al.
(1981) have shown that Ni decreases more rapidly
with decreasing MgO in flows that are part of
the Kambalda Ni camp than in the Belingwe belt
(Zimbabwe) within which Ni sulphide deposits
have not yet been identified. It also has been
shown that mineralized intrusions in northern
Maine (USA) are characterized by olivine that is
less Ni-rich than olivine characteristic of unmineralized
intrusions (Thompson and Naldrett, 1981).
These observations have considerable implications
with respect to prospection and are being followed
up.
As regards Pt deposits in the 1980 Project report
it was stressed that "the compositions of certain
(Pt-rich) ores still defy quantitative modelling".
Two important advances have been made in this
respect: