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Metallogenic Model of the Ruwai Fe-Zn-Pb-Ag Skarn Deposit, Central Kalimantan:
Understanding the Complexity from Proximal to Distal Base Metal Mineralization
Doly R. Simbolon, Cendi D. P. Dana, Laurie E. Whitehouse
Department of Geology and Survey, PT. Kapuas Prima Coal. Tbk.
Jl. Kapuk Pulo, Ruko Elang Laut, Jakarta Utara
Abstract
The Ruwai Fe-Zn-Pb-Ag Skarn Deposit, located within the
Central Kalimantan metallogenic belt, is one of the best
examples of skarn base metal mineralization in Indonesia. It
has 4 main mining centres, including Gojo-Karim (Fe-
proximal skarn) and Ruwai-Southwest Gossan (Zn-Pb-Ag-
distal skarn). The Gojo block is located in the northeast part
of the Ruwai mining complex. It is characterized by several
intrusions and endoskarn facies. E-W trending magnetite
skarn contains both prograde and retrograde mineral
assemblages. The massive magnetite zone contains the
highest Fe content, typically up to 65% Fe and the highest
Cu grade, up to 0.5% in various intervals from 1-15 m thick.
The Karim block is located to the west of the Gojo block and
has relatively similar characteristics. Exoskarn facies with
massive Zn-Pb mineralization also can be found down dip,
especially in the western part of the block with average
grades of 5m of 3% Pb, 10% Zn and 400g/t Ag. This skarn
zone is overlain by thick pelitic sedimentary facies
dominated by hornfelsed siltstone. The Ruwai block is
located between Karim and SWG blocks. It is characterized
by the presence of mineralized breccia zones. Significant
Zn-Pb-Ag mineralization is found in gouge-clay breccia up
to 5.2% Pb, 4.6% Zn and 1.5% Cu. The Southwest Gossan
block is located in the southwestern part of the Ruwai mining
complex. There are two types of massive sulfide
mineralization styles in this area: Sphalerite-galena
mineralization typically has grades up to 15% Pb, 20% Zn,
and 500g/t Ag, whilst sphalerite-pyrrhotite mineralization
typically has grades up to 0.05% Pb, 5% Zn, and 15g/t Ag.
In general, the mineralization generally occurs within a NE-
SW trending compressive regime. It has been formed by
moderate temperature and low salinity hydrothermal fluids
generated by stock-sized quartz diorite intrusion bodies.
Typical zoning in this area include an endoskarn zone which
can be correlated to the Gojo block, a garnet zone which can
be correlated to the Karim block, a pyroxene and pyroxenoid
zone which can be correlated to the West Karim to Ruwai
area and a gossan zone which can be correlated to sulfide
replacement zones in the Ruwai and Karim blocks.
Introduction
The Central Kalimantan metallogenic belt is an ancient
magmatic arc where many economic mineral deposits have
been discovered, including epithermal low and intermediate
sulphidation gold, volcanogenic massive sulphide and skarn
base metal deposits (Fig.1). The economically viable Ruwai
Fe-Zn-Pb-Ag skarn is one such deposit. It is located in the
Lamandau Regency in Central Kalimantan Province.
Exploration of the deposit was carried out in 1980’s and
1990’s by PT Tebolai Seng Perdana, whilst the exploitation
of iron ore and zinc-lead-silver has been carried out by PT
Kapuas Prima Coal Tbk since 2007 and 2013 respectively.
The Ruwai Fe-Zn-Pb-Ag Skarn Deposit has 4 main mining
centres, including the Gojo and Karim blocks (Fe-proximal
skarn) which were mined for iron ore by open cut mining
methods from 2007 until 2012 and the Ruwai and Southwest
Gossan blocks (Zn-Pb-Ag-distal skarn) which were mined
by open pit mining methods from 2013 to 2016 and by
underground mining methods since 2017.
As one of the best examples for skarn base metal deposit in
Indonesia, it is important to recognize the Ruwai deposit’s
key features as a guide for further exploration in surrounding
or other areas in Indonesia. Numerous research studies have
been conducted on this deposit. However, many questions
still remain unanswered. This study is aimed to understand
and simplify the intricate characteristics of this deposit.
Figure 1: Location of the Ruwai Skarn within the Central
Kalimantan metallogenic belt (Setijadji, et al., 2010).
Geological Framework
The Ruwai Fe-Zn-Pb-Ag Skarn Deposit is located within a
complex geological and structural setting in the eastern part
of the Schwaner Mountain Range in Central Kalimantan.
Various lithological facies can be found, including granitoid
intrusions, metamorphic complexes, and volcanic and
sedimentary formations. Regionally, the oldest stratigraphic
formation in this area is the Late Triassic Kuayan Formation
consisting of hornfelsed volcanic facies, with relict textures
indicating a crystal-lithic tuff origin (Fig.2). These volcanics
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are mostly found in the Southwest Gossan and Karim blocks
and are interfingered with siltstone in upper part. These
siltstones are part of the Late Triassic-Middle Cretaceous
Ketapang Complex which consists of pelitic and calcareous
sedimentary rocks that have been contact metamorphosed.
However, the calcareous and/or carbonate facies within this
complex have not been mentioned in any regional geological
maps as yet. Pelitic sedimentary facies consist of shale,
siltstone and sandstone. They are relatively less altered due
to lack of permeability. Calcite, pyrite and chlorite are the
most common hydrothermal minerals in those facies.
Graphite also can be found within the shale facies in some
locations. Limestone underlies the pelitic facies and has been
recrystallized although petrographic studies indicates the
presence of foraminifera fossil relicts (Fig.7). The most
common features in these facies is carbon bands and stylolite
structures.
The intrusion complex in this studied area is dominated by
I-type calc alkaline granitoid intrusions related to Cretaceous
magmatism activity. There are three main intrusion
complexes in the area studied, including felsic and
intermediate intrusions being part of the Cretaceous
Sukadana Granitoid and mafic intrusions being part of the
Miocene Sintang Intrusion. Two felsic intrusions can be
identified in this area including granodiorite and monzonite
which are mostly found in the Gojo and Southwest Gossan
areas. K-feldspar alteration is common in these intrusions.
Intermediate intrusions consist of diorite and quartz diorite.
They vary both in texture and mineralogical assemblage and
can be either less altered (chl-ep and kspar-bio) or
pervasively silicified. Mafic intrusive bodies in this studied
area comprise of andesite, basalt and diabase and most show
very weak to weak propylitic alteration (chl-ep-mag). Well
preserved basaltic dykes cutting limestone can be found in
the Southwest Gossan open pit.
The youngest stratigraphic formation in the Ruwai area is the
Late Cretaceous Kerabai Volcanics which unconformably
overlay the Ketapang Complex. It consists of various types
of pyroclastics and lavas which are mostly undifferentiated
and hard to distinguish due to intensive alteration and
metamorphism. Relict textures with primary crystals may be
preserved indicating crystal tuff origin. Lithic fragments in
various size ranges can also be found
Based on the aeromagnetic structural interpretation, there are
two sets of major structures in the Ruwai area. They can be
divided into two tectonic phases, including NE-SW thrust
faults cut by later NNW-SSE strike-slip faults. The thrust
faults are possibly the main control to the ore deposit
distribution while the strike-slip faults separate each block.
Minor WSW-ENE lineaments, represented by strike slip
movements, can also be identified in this area, especially in
the Ruwai block.
Proximal Zone
The proximal skarn in the Ruwai mining complex is located
in Gojo and Karim blocks and covers an area of about 2.05
Figure 2: General geology of the Ruwai Skarn Fe-Zn-Pb-Ag deposit and the occurrence of gossan.
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square kilometers. The two blocks are separated by a stock-
sized quartz diorite intrusion. Fe skarn is the predominant
mineralization and mostly occurs as extensive gossan and
argillic alteration on the surface.
1. Gojo Block
The Gojo Block is located in the northeast part of the Ruwai
mining complex. It has been mined for iron ore in two open
pits. Several exploration drilling programs based on ground
magnetic and soil sampling anomalies have demonstrated
the continuity of magnetite skarn mineralization in this area
with depth.
Figure 3: Typical lithologies found in Gojo block including (A)
granodiorite, (B) quartz diorite, (C) magnetite skarn and (D)
endoskarn.
Several lithological facies can be found in Gojo area
including weakly altered intrusive, endoskarn, magnetite
skarn, siltstone and acid volcanics (Fig.3). The Gojo
intrusion complex consists of several granitoid intrusions
including granodiorite, quartz diorite, monzonite and diorite
with various alteration styles and textures. They are mostly
clay altered at surface whilst drill core data shows alteration
styles ranging from chlorite-epidote-actinolite to k-feldspar-
biotite-magnetite (Fig.7). Intensively silicified intrusive also
can be found in some areas. Endoskarn is mostly found
within quartz diorite intrusions adjacent to the magnetite
skarn facies. This also indicates that the quartz diorite is
most probably a syn-mineralization intrusion. Garnet is the
most common prograde mineral phase in this endoskarn
facies together with epidote as the retrograde phase.
Chalcopyrite and bornite also can be found within this
endoskarn in minor amounts (Fig.4).
Acid volcanics are found in the eastern part and are
characterized by the presence of primary bird’s eye quartz
and are mostly argillic altered either into argillic facies or
moderately silicified.
Magnetite skarn can be found together with both prograde
and retrograde mineral assemblages. The massive magnetite
zone contains the highest Fe content, typically up to 65% Fe
with highest Cu grade being up to 0.5% in various intervals
from 1-15 m in thickness. Zn-Pb contents are relatively low
(negligible to several percent). Structural features are mostly
well developed in the eastern part. An E-W structure is
represented by a fault plane (N80°E/80°) with indistinct
movement. It cut by a NW-SE structure represented by an
intensively jointed shear zone. Magnetite skarn is relatively
E-W trending, which possibly indicates that the E-W
structure is syn-mineralization.
2. Karim Block
The Karim Block, located west of Gojo, has been mined for
iron ore (open cut) and is currently being mined for zinc-
lead-silver via two underground tunnels. The open cut area
is characterized by gossan and argillized intrusive, with
diorite and quartz diorite being the most common intrusion
instead of granitoids as at Gojo. Endoskarn is found within
the quartz diorite intrusion and is characterized by the
abundance of both prograde and retrograde calc silicate
minerals and sulphides. Magnetite skarn usually associated
with garnet and pyroxene, although it can also be found
associated with retrograde phase minerals assemblages such
as epidote and chlorite. The magnetite skarn is ENE-WSW
trending and dipping NNE. The highest grades in this zone
are up to 65% Fe with 1.7% Cu in various intervals from 1
m to 10 m in thickness. This magnetite bodies typically has
mushroom-shaped geometry. Several magnetite vein also
can be found in this area.
Exoskarn facies with massive Zn-Pb mineralization also can
be found especially in the western part of the block. Both
prograde and retrograde alteration phases can be found
within the exoskarn. Mineralization can be found within
skarnified limestone with an average width and grade of 5m
of 3% Pb, 10% Zn and 400g/t Ag. Several massive sulfide
bodies can also be found with grades up to 18% Pb and 25%
Zn. However, the average Cu grade at 0.8% within this base
metal mineralization zone is relatively lower than in the
magnetite zone up-dip. This skarn zone is overlain by a thick
pelitic sedimentary facies dominated by hornfelsed siltstone
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(Fig.5). Acid volcanics facies are mostly found in the
southern part of the block characterized by spotted primary
quartz.
Geological structures in this block are mostly NE-SW
trending, with minor NNW-SSE trending offsets. The NE-
SW structure is mostly found in the middle part of the block
and is characterized by intense quartz stockworks which
indicates that this structure is the feeder zone for the
mineralizing hydrothermal fluids.
Fig. 5. (A) Garnet (endo)skarn and (B) epidote (exo)skarn are the
most common skarn facies in Karim block, while wall rock is
dominated by volcano-sedimentary facies including (C) shale, (D)
hornfelsed siltstone and (E) lapilli tuff.
Distal Zone
The distal skarn mineralization is concentrated in the Ruwai
and Southwest Gossan blocks, southwest of Gojo and Karim.
These two blocks cover an area about 2.55 square kilometers
and are structurally controlled. Massive sulfide, dominated
by sphalerite and galena, is the main mineralization style in
these blocks. Geochemically (Pb-Zn in soil) gossanous
zones were the main indicator of the presence of the base
metal deposits in these areas.
1. Ruwai Block
The Ruwai Block is located between the Karim and
Southwest Gossan blocks. It was mined for zinc-lead-silver
mineralisation from two open pits (Ruwai East and Ruwai
West) and one underground tunnel during 2013-2017.
Several drilling exploration and resource infill drilling
programs are currently being carried out to ascertain the
continuity of the ore horizon both in northern and southern
part of the block.
Similar lithologies also can be found within this area
including siltstone, limestone, acid volcanics, intrusive rocks
and skarn facies. Siltstone is mostly silicic and weak-
moderately argillic altered. Limestone shows very weak
propylitic alteration. Acid volcanics facies are mostly found
in the southern part of the two pits, with alteration styles
ranging from propylitic and argillic through to intense
silification. Small bodies of oxidized magnetite skarn also
can be found in the easternmost of the Ruwai East pit.
Diorite and diabase dykes are the most common intrusive
rocks found in this area. They are usually weakly altered
with selective epidote-chlorite replacement.
Figure 4: (A-B) Garnet-pyroxene-magnetite endoskarn from Gojo block containing bornite and chalcopyrite as the main Cu-bearing minerals
along with digenite as the late stage product. (C-D) Intergrowth of sphalerite and galena within epidote-pyroxene exoskarn from SWG block.
(E-F) Pyroxene-magnetite endoskarn from Ruwai block showing the abundance of arsenopyrite.
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Figure 6: (A) Black ore within gouge-clay breccia with siltstone-
limestone fragments. (B) Skarnified limestone dominated by garnet
assemblage. (C) Typical calc silicate skarn with abundance of
fibrous-radial wollastonite. (D) Crystal-lithic tuff facies as the most
common lithology found in Ruwai block.
One of the most interesting features of this block is the
presence of an extensive mineralized breccia zone (Ruwai
East Breccia Zone). Significant Zn-Pb-Ag mineralization is
found in this gouge-clay breccia as indicated by very fine
grained (clay-sized) sulfides (Fig.6). Highest drill hole
grades within this gouge clay is up to 5.2% Pb, 4.6% Zn and
1.5% Cu., although much of which has been called “black
ore” that has been previously mined from the Ruwai East pit
contains grades of up to 44% Zn and 16% Pb. It is
concentrated in the shear (thrust) zone between the limestone
and acid volcanics. Wall rock fragments and copper-oxide
staining (malachite, chrysocolla and chalcanthite) are
commonly found. This gouge clay is NE-SW trending with
a relatively shallow NW dipping direction as controlled by a
major compressional structure.
Mineralization within the Ruwai West pit occurs in different
styles and grades. Typical grades are in the range of 10-20%
of Zn and Pb combined and Ag >100 g/t. Outwards from the
garnet zone, massive sulfides with spectacular grades of up
to 40% Zn, 25% Pb, 3% Cu and 2,000 g/t Ag are found with
varying thicknesses. The thickest zone is about 15 m, located
along the crest of anticline, in which the folded and strongly
fractured limestone is accumulated and it becomes the most
favorable site for mineralization. PT Tebolai Seng Perdana
in its 1997 Feasibility Study quoted a drill indicated resource
for this area of 325,000 tonnes @ 14.33% Zn, 4.81% Pb,
0.55% Cu, 322 g/t Ag. Most of this mineralisation within
the Ruwai West pit has now been mined out by PT Kapuas
Prima Coal from 2013-2016.
There are two major structures in this area. A dextral-reverse
strike slip fault is well represented by the shear zone
(N195°E/80) in the southern wall of Ruwai West pit which
caused the uplift of western pit relative to the eastern pit. A
major compressional NE-SW trending structure (thrust
zone?) with a relatively shallow NW dipping direction is
interpreted as being syn-mineralization as indicated by the
intense mineralization. It triggered the hydrothermal fluid up
flow along with wall rock fragments (up to boulder-sized)
which show alteration from the hot hydrothermal fluids
(Fig.8).
Figure 7: (A) Altered stock-sized quartz diorite intrusion consist several alteration mineralogy including secondary biotite and K-feldspar,
sericite and actinolite indicating inner propylitic to potassic facies. (B) Typical meta limestone showing recrystallization due to contact
metamorphism. (C) Silicified diorite intrusion containing actinolite-epidote-quartz vein with disseminated euhedral pyrite. (D) Hornfelsed
siltstone cut by biotite-quartz-K-feldspar vein. (E) Mymerkitic texture within granodiorite intrusion with calcite-actinolite as the alteration
mineral. (F) Hornfelsed volcanic containing primary quartz and plagioclase with abundance of chlorite concentrated along fractures.
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Figure 8: Large (2m) boulder of wollastonite altered limestone
within the Ruwai East Breccia Zone.
2. Southwest Gossan Block
The Southwest Gossan Block is located in the southwestern
most of Ruwai mining complex. It was mined for zinc-lead-
silver from an open pit in 2016-2017 and is currently being
mined from two underground tunnels at the bottom of the
open pit.
The same lithological facies as found in other blocks can be
found in this area. Undifferentiated volcanic facies are
unconformably overlain by hornfelsed siltstone (Fig.10).
Thin bedded shaly and sandy pelitic sedimentary rocks also
can be found interfingered with the siltstone. Underlying the
siltstone is a limestone facies with manto-type skarn
mineralisation found at the contact between limestone and
siltstone. Both prograde and retrograde skarn mineralisation
can be found. Structurally controlled gouge clay-breccia also
can be found in the eastern-southeastern part of SWG open
pit (Fig.10). Some parts also indicate significant
mineralization which is characterized by dark grey colored
and intense pyritization with copper oxide staining. Diorite
and monzonite dykes are the most common intrusions found
in the Southwest Gossan block together with very late stage
basaltic dykes. An andesite porphyry intrusion also can be
observed in the western wall of the pit. The diorite intrusion
interpreted as the syn-mineralization intrusion in this area.
Base metal mineralization can be found associated with
prograde and retrograde mineral assemblages either in
disseminated form or as massive sulfide bodies (Fig.9).
There are two types of massive sulfide mineralization styles
in this area including sphalerite-galena and sphalerite-
pyrrhotite.
Figure 9: (A) Pyroxene-chlorite skarn with disseminated sphalerite-
galena mineralization. (B) Typical massive sulfide in SWG block.
(C) Hydrothermal breccia composed by silica matrix and siltstone
fragments. (D) Dacitic volcanic facies showing the abundance of
quartz and hornblende as the phenocryst phase. (E) Basaltic dyke as
the most common late intrusion phase.
Sphalerite-galena type mineralization typically has grades
up to 15% Pb, 20% Zn, 500g/t Ag, whilst sphalerite-
pyrrhotite mineralization typically has grades of 0.05% Pb,
5% Zn, 15g/t Ag. Minor magnetite skarn can be found in the
deeper zones (Fig.4).
A NW-SE major anticline structure can be observed very
well in the open pit as the result of compressional stress
associated with a NE-SW reverse fault. A NE-SW sinistral
strike slip fault also can be found in the eastern wall of the
pit. This fault is interpreted to have formed after the reverse
fault and has controlled the formation of late stage basaltic
dykes.
Genetic Model of Deposit
Several aspects need to be considered to understand the
genetic model of Ruwai skarn deposit including geological
control, ore geometry, mineralogical characteristics,
geochemical and hydrothermal fluid signatures. Skarn base
metal mineralization is mostly associated with moderate
dipping (~40) subduction zones where the carbonate
environment is located at the craton’s margins (Meinert,
1992 and Meinert et al 2005 in Pirajno 2009).
The formation of skarn deposits in the Ruwai area is the
result of stock-sized quartz diorite intrusions in the proximal
zone together with small-medium sized dykes in the distal
zone. The skarnization process is formed right after the
contact metamorphism process (isochemical) when silica
rich hydrothermal fluid generated from the intrusion interact
with calcium rich wallrock. The skarn deposit in the
proximal zone is dominated by endoskarn facies hosted by
quartz diorite as mentioned before. The exoskarn is hosted
by limestone as indicated by some limestone relict texture
within the exoskarn bodies. Massive magnetite bodies in the
proximal zone are possibly hosted by dolomitic limestone.
The skarn horizon is mostly controlled by stratigraphy,
where it is mostly found along the contact between siltstone
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and limestone facies. Mineralization generally occurred in
compressive regime where major NE-SW trending thrust
faults become the syn-mineralization structure and control
the localization from proximal to distal skarn (Idrus, 2011).
However, a minor extensional regime can also be identified
and plays an important role in providing hydrothermal fluid
pathways, especially in anticlinal crest zones (Setijadji,
2018). Another essential control is the depth of formation. In
the proximal zone, skarn is mostly formed at shallow depth,
which makes the wallrock tending to be brittle and produce
intense fracturing to provide hydrothermal fluid conduits
(Chang & Meinert, 2008). In the distal zone, two skarn
horizons can be found at shallow and deeper depths
However, brittle deformation also becomes the main
controlling factor in the deeper horizon although of a lesser
intensity than the shallower zone.
A mineralogical study by Idrus et al. (2011) concluded that
the Ruwai skarn is characterized by prograde phase
andradite and wollastonite while the retrograde phase is
comprised of epidote, chlorite, calcite, and sericite. Galena
is typically enriched in silver up to 0.45 wt % and bismuth
of about 1 wt %. Fluid inclusion studies indicates that the
skarn ore body was formed at 250 - 266 °C with a low
salinity of 0.3 - 0.5 wt.% NaCl eq. These characteristics
indicate that Ruwai skarn can be classified as a Calcic Skarn.
A typical alteration zonation sequence of a Zn-Pb skarn
deposit from proximal to distal is altered/endoskarn pluton,
passing through garnet, pyroxene and pyroxenoid into
sulfide/oxide replacement bodies. The endoskarn zone can
be correlated to the Gojo block and the garnet zone can be
correlated to the Karim block. Pyroxene and pyroxenoid can
be correlated to the Western Karim to Ruwai area and the
Southwest Gossan area can be correlated to the sulfide
replacement zone.
Numerous research papers indicate that Zn-Pb skarn can be
considered as a distal zone of porphyry system (e.g. Sillitoe,
2010) where it is formed by small dyke from a larger stock-
sized intrusion. In this studied area, this stock-sized intrusion
can be correlated to the quartz diorite intrusion body located
between Gojo and Karim which is probably the main heat
source of the overall mineralization system at Ruwai.
However, porphyry-style quartz veining is very minor to
absent in this main intrusion body. The skarn mineralization
at Ruwai is possibly more controlled by the chemical
composition of the limestone wall rock. The limestone
provides a large amount of Ca which is much easier to
dissolve and react with siliceous hydrothermal fluid to form
calc silicate minerals than other silicate rocks (i.e. volcanic)
where porphyry-style quartz veins can be well developed
(Chang, et al., 2015).
This study proposes three separate intrusions in time. The
first intrusion, being pre and syn mineralization, is of quartz
diorite and diorite porphyry composition. The second
intrusion is of monzonite-granodiorite composition and is
mostly characterized by a zonation of potassic alteration in
the inner core, grading through propylitic alteration to
endoskarn at contact zones. The last of the intrusions are
basaltic dykes which are post mineralization They are
relatively fresh and the preservation of original rock texture
shows that these dykes hold very little hydrothermal fluid.
Figure 10: A panorama view of SWG open pit with several lithological features including basaltic dykes, anticlinal limestone, gouge clay
breccia and oxidized skarn and siltstone.
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Concluding Remarks
The Ruwai Skarn Fe-Zn-Pb-Ag deposits is one of a good
example to learn the key features of base metal
mineralization as the guide for further exploration. A well-
developed alteration and mineralization zoning in this area
shows a good distribution from proximal to distal part.
Massive magnetite bodies mostly associated with prograde
endoskarn in proximal area while massive sulfide bodies are
associated with retrograde exoskarn phase in distal area.
Compressive structural regime is likely become the main
genetic control of this deposit instead of extensive regime.
Three main intrusion phases from pre/syn- to post-
mineralization can be distinguished by mineralogical
characteristics where the dioritic intrusion as the syn-genetic
intrusion. However, more advanced geochemical and
geochronological studies are still needed to fully understand
the whole genetic model of the deposits.
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Acknowledgements
Authors would like to acknowledge to Dr. Lucas Donny
Setijadji, S.T., M.Sc., Dr.rer.nat. Ir. I Wayan Warmada and
Betseba br. Sibarani, M.Eng who help in preparing the
petrographical and ore microscopy analysis. A valuable
mention is also given to Dr.rer.nat. Ir. Arifudin Idrus, S.T.,
M.T. who gives constructive suggestion to this paper.
