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GIS AS MEANS OF ESTIMATING
THE ORE-FORMING POTENTIAL OF INTRUSIVE
FORMATIONS
IN THE VERHKOYANSK FOLD BELT (EAST YAKUTIA)
Kostin A.V.
Diamond
and Precious Metal Geology Institute SB RAS, Yakutsk, Russia,
kostin@diamond.ysn.ru
The use of
GIS technologies in estimating the ore-forming potential of
intrusions is a new step in development of unique distributed model
of areal geological structure in Yakutia. The GIS employment for
estimation of pluton ore-forming potential enabled to combine various
geological data in spatial analysis, prepare and develop basic and
thematic maps, solve the problems related to spatial analysis of
geological information and modeling.
The
Verkhoyansk Fold Belt is extremely rich in mineral resources. About
11000 deposits and ore occurrences are known here. Among them, the
deposits of silver, gold and tin predominate. Some of them are large
and unique, and it makes to consider this area the largest precious
metal province in Russia. Spatial association with intrusive
formations has been established for a number of ore deposits. It
stimulates to estimate their potential ore-bearing. The study carried
out is related to the estimate of intrusion potential for the
copper-porphyry type of mineralization, its genetic relation with
intrusions having been proved. For this, GIS project has been made
based on the licence ArcGis 9.2 that includes:
1. Intrusive
massives (deciphered from geological maps in scale 1:500000 and
1:200000)
2. Deposits
After
deciphering, the band “intrusive massives” was
transformed into geodatabase of Access format that consists of the
table describing the geometry of plutons with calculated areas of day
outcrops, their spatial associations and names. The table with
chemical analyses of intrusive rocks (more than 4000 analyses) is
included to geodatabase. The processing of tables is carried out with
the help of inquiries, one of them grouping the plutons by name and
summing up the areas of their outcrops to make up a new table. The
unique names of massives are kept in its key field that meets “the
first normal form of databases and allows making relations with other
tables of chemical analyses. The character of relations among the
tables is one to many.
Classificator
is used for band “ore deposits”, where visualization of
deposits is simultaneously realized due to the two features, i.e. ore
formation and size of deposit.
When
combining the bands “deposits” and “intrusive
massives”, it was revealed that the more the intrusives were
eroded, the less the deposits were left within their contours and in
the periphery. This is the total model of ore zonation, by which
deposits form over tops of massives and that is supported by numerous
examples of the world class deposits.
In analyzing
the potential of intrusions for the Au-Cu and Cu-Mo mineralizations,
the model of Mongolian copper-porphyry deposits (G. 1985) was
used, with K2O/Na2O
ratio corresponding to Cu-Mo ore-magmatic system in interval from 0.3
to 0.7, and Au-Cu one within 0.7-1.3.
From Access
geodatabase, selection of Au-Cu ore-magmatic systems with high
potential due to given K2O/Na2O
ratio from 0.7 to 1.3, and their selection due to area of day outcrop
have been made with the help of the inquiry. Therefore, classificator
of GIS band was made based on the feature “pluton area”,
and the selected groups characterized extent of their erosional
section.
To reduce the number of objects
that are potentially perspective for the Au-Cu mineralization, the
filter is used that selects the plutons belonging to high potassium
calc-alkaline and shoshonite series.
Algorithm
of pluton filtration is based on formulae for the curves that bound
series of magmatic rocks in diagram SiO2-K2O,
their general appearance being described by equation K2O=k*
SiO2-b,
and inquiries are like:
for
rocks of tholeiite series:
K2O
< (0,033462*[SiO2]-1,5)
for
rocks of calc-alkaline series:
K2O
between (0,033462*[SiO2]-1,5) and (0,066507*[SiO2]-2,5)
for
rocks of high potassium calc-alkaline series:
K2O
between (0,066507*[SiO2]-2,5) and (0,169054*[SiO2]-7,12)
for
rocks of shoshonite series:
K2O
> (0,169054*[SiO2]-7,12)
To the
inquiry on selection of high potassium calc-alkaline series massives,
the condition that limits their area to 5 sq.km and K2O/Na2O
in intervals from 0.3 to 0.7 (for Cu-Mo systems) and from 0.7 to 1.3
(for Au-Cu systems) is added.
The next step of GIS analysis is
to evaluate rows of ore formations that are known within large
ore-magmatic zones. The 5km-buffer zone is constructed by
SpatialAnalyst ArcGis dilation for plutons, and buffer zones with
small to large deposits that belong to Cu-Mo, Au, As and Pb-Zn ore
formations are selected.
As a result of the selection,
the West Verkhoyansk, San-Yuryakhskaya and Derbeke-Nelgesinskaya
ore-magmatic zones, and Abyrabytsky, Talannakhsky and Yakutsk
ore-magmatic knots are considered as being perspective for Cu-Au
mineralization. The West Verkhoyansk ore magmatic zone, where
Nyuektaminsky ore knot is distinguished, is the most perspective
among the enumerated ones on the number of large precious metal
deposits included into the buffer zones of plutons. Within the ore
knot, the copper-molybdenum, gold-copper-porphyry,
gold-(arsenic)-sulfide, gold-sulfide and gold-silver ore formations
were determined (Kostin, Lysenko 2006). Their location is controlled
by the Kygyltassky pluton being not less than 5 km long due to the
size of magnetic anomaly.
The study
carried out showed the possibility of using the GIS analysis to
estimate the potential ore-bearing of plutons. These data can be also
used in GIS models, when taking into consideration that large volumes
of information on isotope datings, geochemistry of rare elements,
composition of fluids, etc. have been accumulated. It will increase
possibilities of studying the factors of precious- and rare metal
mineralization formation in magmatic provinces.
References
G. O. (1995) Mineral resources of
the western part of the Mongol-Okhotsk Fold Belt.
In Ishihara S. & Czamanske G.K. eds. Resource Geology Special
Issue 18, P.151-157.
Kostin A.V., Lysenko M.S. (2006) Ore
knot geological model as basis for development of
prediction-metallogenic GIS (on example of Nyuektaminsky ore knot,
East Yakutia) Regional
Geology, P.29-32.
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