|
DEPENDENCE
OF THE GRANITOIDS ORE PRODUCTIVITY ON COMPOSITION AND
DIMENSION
OF INTRUSIVE BODIES, THEIR BELTS AND AREALS
Abramovich G.Ya., Rafienko V.V.
Irkutsk
State University, Irkutsk, Russia, agy@geo.isu.ru
Geological
and geochemical studies not always can unambiguously prove the
consanguinity of ore deposits with concrete intrusive bodies (with
the exception of deposits of proper magmatic and contact-metasomatic
types). The problem became increasingly urgent in connection with
available great quantity of determination of the ore radiological age
that not infrequently essentially differs from the age of intrusions
believed to be “parent” ones.
The authors
have performed a spatial-statistic analysis of the granitoids
intrusions relationship of different petrographic, geochemical
composition and age with the deposits of commercial minerals types.
Territory of mobile belts within the South Eastern Siberia (from the
Yenisei river basin in the west to the Aldan and Argun’ rivers
in the east) served as a testing area. About 800 deposits and 9000
ore manifestations of useful minerals have been studied. Calculation
of «normative mineralization density» for 19 species of
useful minerals has been made using special-purpose petrographic maps
and maps of useful minerals. In this case, “mineralization
density” is the quantity of deposits and ore manifestations of
the definite species of useful minerals falling on the unit of
selected area; “normative mineralization density” within
magmatic areal is the density of the deposits and ore manifestations
of definite species within magmatic areal normalized according to
mineralization density within the limits of discussed mobile belt or
region as a whole. The equation for the calculation of the normalized
mineralization density may be represented as follows: NPPi
= PPi/PRi, where
NPPi
is a normalized density of i
useful
mineral for magmatic areal under consideration, PPi
- mineralization density of i
useful mineral for magmatic areal, PRi
-
mineralization density of i
useful mineral for mobile belt or region. Studies are carried out
using special computer program. On the basis of data analysis it is
suggested that the deposits and ore manifestations reliably related
to parent granitoid intrusions are situated as a band at distance up
to 15 km away from its contact.
The results
of analysis on distinct species of useful minerals are listed in
Table 1. Implemented studies allow us to suggest the following.
Table
1. Normalized
density of mineralization for distinct species of useful minerals within areals of the graniitoids associations of different types of the South Eastern Siberia
-
|
Types of
the granitoids associations
|
Area of areal,
belt
(km2)
|
Species of useful minerals
and their normalized
density of mineralization
|
|
Fe
|
Pb
|
Zn
|
Au
|
W
|
Sn
|
Nb
|
|
Plagiogranitic
|
22275
|
2.7
|
3.5
|
4.6
|
2.5
|
0.8
|
0.3
|
0.1
|
|
Diorite-granodiorite-granitic
|
134000
|
0.9
|
0.7
|
0.8
|
1.8
|
0.9
|
0.7
|
1.7
|
|
Granodiorite-granitic
|
149350
|
0.2
|
1.0
|
0.4
|
1.7
|
3.4
|
6.4
|
1.2
|
|
Subalkali-granodiorite-granitic
|
135380
|
0.6
|
0.8
|
0.8
|
0.3
|
0.9
|
0.1
|
0.4
|
|
Subalkali-granitic
|
66470
|
0.9
|
1.2
|
1.7
|
1.2
|
2.1
|
0.6
|
1.0
|
|
Subalkali-leucogranitic
|
129600
|
1.2
|
2.2
|
2.8
|
1.0
|
2.1
|
0.5
|
2.0
|
|
Subalkali-leucogranitic
with silicialkali metasomatites
|
3620
|
0.2
|
1.0
|
1.0
|
0.3
|
4.0
|
12
|
4.6
|
|
Syenite-subalkali-granitic
|
31340
|
0.6
|
0.5
|
0.4
|
0.2
|
2.2
|
0.4
|
2.6
|
Metallogenic
specialization (actual ore content) of granitoids of different types
and age of large areas may be impartially assessed (on the
quantitative basis) by calculation of “normative
mineralization density” of one or other species of useful
mineral within the boundaries of the magmatic areal area.
Metallogenic
specialization of the South Eastern Siberia granitoids depends on
their petrographic and geochemical composition. For example,
plagiogranitic association is productive for iron, lead, zinc, gold
as well as (according to calculations not presented in the table) for
copper, silver and molybdenum and it is extremely depleted in rare
metals; diorite-granodioritic association is enriched in gold and
rare earths; granodiorite-granitic one (calc-alkali type) – in
tungsten, tin, niobium, as well as lithium and beryllium;
leucogranitic association with silicialkali metasomatites is mostly
enriched in tungsten, tin, niobium and beryllium, tantalum and rare
earths and extremely depleted in elements of iron group and
polymetals.
In agreement
with the areal area it can be concluded that the granitoids
associations being not common and most often represented by
intrusive bodies of small size are more productive than common
associations and represented by large bodies. Here, the depth of
erosional section of magmatic columns is probably of considerable
importance.
It should be
noted that a great sample size provides a sufficient reliability of
obtained statistic calculations for proposed method of the granitoids
mineralization assessment of the definite types associations. In this
connection it is first of all suitable for regional metallogenic
analysis. At the same time, the treatment of data begins with
identification and the mineralization density of local magmatic
areals and belts and whose actual productivity may be assessed after
all complexes of studies.
It is
necessary to take into consideration a partial mutual overlap of the
granitoids areals (belts) of different composition and age. The
resulting ambiguity of determination in the deposit belonging to one
or another magmatic areal (belt, magmatic complex) is eliminated to
certain extent from the basis of a careful analysis of geological,
geochemical or other forms of the mineralization relationship with
one or other magmatic formations when studying of concrete deposits
and ore manifestations.
|