Orebody
modelling and resources estimation of the hydrothermal gold-silver
mineralization
at
Cikidang, West Java, Indonesia
Abstract
Cikidang
is one of a number of recently discovered Tertiary volcanic-hosted gold deposits
which occur within a one kilometer-long, northeast-trending belt in the central
part of Lebak district, West Java, Indonesia. It consists of several steeply
dipping quartz veins, namely Cikidang, Barat, Tengah and Timur veins that formed
within the ephitermal environment. In terms of morphology and geometry the veins
occurs as single, thin and tabular shapes.
The
geology of the deposit consists of andesite intrusion, tuff and breccia/lapilli
tuff which comprise the dominant host rock for vein structures and underlie most
of the Cikidang area. The vein textures (banded colloform, comb structure and
brecciated) and mineralogy (quartz, calcite, adularia) are typical of those
encountered in low-sulphidation vein deposits.
The
Cikidang vein was selected for modeling the geology and the reserve by using
Microlynx which is a 2D/3D GIS modeling package. Three different data sets were
available, drill holes, channel and tunnel data, but only the drill holes data
set was used to model the geology and the resource of the Cikidang vein. Four
ore zones, namely zone A,B,C and D were outlined using drill holes data. Zone B
was then selected for modeling the geometry of the vein. The perimeter
geological modeling method which is provided within the Microlynx package was
selected to model the geometry of the vein.
Univariate
statistical analysis of the Cikidang data set showed the data values for Au and
Ag to be lognormally distributed. The lognormal data values were then used for
computing a 3D semivariogram in Microlynx to extract the parameter which are
used for value estimation using IPD. To analyze the relation between gold and
silver, bivariate statistical analysis was used. A positive correlation was
observed between the two metals.
The
grid model was selected for the resource estimation of the Cikidang vein as it
is appropriate for tabular ore bodies. The grid cell dimensions based on the
sample density were 25m * 20m. The inverse power of distance weighting method (IPD)
was then applied to interpolate the grade values of gold and silver as
potentially economic metals.
Resources
were estimated in two categories based on two different search radii. Radius of
75 m is categorized as category 1 and radius of 150 m as category 2. The total
tonnage computed from the resource modeling was 2.6 Mt at average grades of 4.6
g/t Au and 27.7 g/t Ag by using estimation category 2, and 2.1 Mt at average
grades of 2.9 g/t Au and 18 g/t Ag by using estimation category 1. Applying a
cut-off grade of 4 g/t Au gives a total of 0.4 Mt ore at average grade of 10.2
g/t Au and 43 g/t Ag from resource category 1, and a total of 0.6 Mt ore at
average grade of 9.6 g/t Au and 42.8 g/t Ag from resource category 2.
The
results of this study demostrate the usefulness of the 2D and 3D modeling and
geostatistical approach for the evaluation of resources of the Cikidang vein.
These methods can be applied to the other prospective veins within the Cikidang
area and others similar region.