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Dairi Zn-Pb±Ag Deposit, North Sumatra, Indonesia: Preliminary Study on Host
Rock Petrology and Ore Mineralogy
Tomy Alvin Rivai,1 Kotaro Yonezu,1 Syafrizal,2 and Koichiro Watanabe1
1Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan
Faculty of Mining and Petroleum Engineering, Bandung Institute of Technology, Bandung 40132, Indonesia

2Abstract
Dairi Zn-Pb±Ag deposit is outlined along the eastern limb of the Sopokomil dome. As an initial stepin a comprehensive study of the deposit, characterization of host rocks and orebodies is necessary.
Hand-specimen and petrographic observations reveal that the orebodies are hosted by siltstone-shaleand dolostone sequences termed as Julu and Jehe units, respectively. In Julu unit, the orebodies areconcordant and comprise massive, layering, and breccia ores. Conversely, the orebodies in Jehe unit
are discordant and consist of vein, breccia, and disseminated ores. Within these orebodies, ore minerals comprise sphalerite, galena, chalcopyrite, arsenopyrite, pyrite, pyrrhotite, tetrahedrite, tennantite, freieslebenite, bournonite, unnamed Ag-Sb-S and Pb-Sb-S minerals associated with coarsegrained quartz and dolomite. Textural characteristics suggest that the ore mineral precipitation is
likely to occur in early diagenetic and epigenetic stages with reference to the host rocks in Julu andJehe units, respectively. The orebodies are then deformed along with the host rocks. It is postulated
that the discordant and concordant orebodies have formed from the same mineralizing fluid. Stable
isotope, mineral phase relations, and geochronology are essential to confirm this interpretation.

ward sedimentary deposits during the separation from Australia.[1] The area in which Dairi
Zn-Pb±Ag deposit is located was deformed
to form the Sopokomil dome. From bottom totop, the stratigraphy comprises dolostone(Jehe unit), interbedded siltstones and shales(Julu unit), and interbedded siltstones andsandstones with minor shales (Dagang unit).
The deposit is hosted in the eastern flank of
the dome within Julu unit and Jehe unit anddivided into 4 prospects from SE to NW: Anjing Hitam, Basecamp, Lae Jehe, andBongkaras (Fig. 1).
To the best knowledge of the authors,
this deposit is the only sediment-hosted ZnPb deposit that has been discovered in Sumatra. It formed in Paleozoic age syn-genetically with the host sedimentary formation.[4]Therefore, a thorough research is essential toelucidate the nature of the deposit which canbe further used to guide exploration in the Island. In this paper, we would like to present
our latest petrological and mineralogical results and interpretation on Dairi Zn-Pb±Ag
deposit as the initial stage of this research.

1. Introduction
Dairi Zn-Pb±Ag deposit is located in
Dairi Regency, North Sumatra, Indonesia,
approximately of 290 km to the southwest ofMedan City. This deposit lies on theSibumasu Block portion of Sumatra originating from Australia. The block was separated
from Australia in Permo-Carboniferous andsubsequently drifted toward Cathaysia Blockand collided with East Malaya Block. TheWest Sumatra Block slid along Medial Sumatra Tectonic Zone to be emplaced toward thewest of the Sibumasu Block in Triassic. Closing of Meso-Tethys Ocean led to the collisionof these amalgamated blocks with the Woyla
Arc in Cretaceous.[1] During its tectonic evolution, the Sibumasu Block has undergone
extensional and compressional settings since
its separation from Australia.
The deposit is hosted within a basal formation of Sumatra so-called the Kluet Formation. This formation consists of slates,metawackes, phyllites, metaquartzose arenite,and metalimestones[2],[3] representing sea-61

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hosted in Julu unit are concordant, multilayers, and strata-bound with thickness of up
to 30 m. In contrast, those are hosted in Jeheunit are discordant although they are stratabound and multilayers in deposit-scale withthe maximum thickness of 15 m. In terms ofore types, the Julu-hosted orebodies comprise
pyritic massive sulfide (PyMs), sphaleriterich massive sulfide (SpMs), galena-richbreccia (GnBx), sulfide layer (SL), and vein
(V-Ju; Fig. 2). On the other hand, the Jehehosted orebodies consist of vein (V-Je), breccia (Bx), and disseminated (Ds; Fig. 3).
PyMs is predominated by euhedral pyrite with sphalerite-galena intergrowth filling
its interstices. Fine-grained pyrite was disaggregated from larger pyrite grains in which
densely interconnected quartz veinlets occur.
Toward northwest, quartz-dolomite fillingsin pyrite interstices tend to be more abundant.
Overgrowth texture of two pyrite generations
also exists. Galena and sphalerite replaced
pyrite of the first generation. Additionally,
pyrrhotite is present and forms intergrowthwith sphalerite and galena as well as pyrite.
This intergrowth is ubiquitous in the deep
portion of Lae Jehe.
SpMs is predominated by the sphaleritegalena intergrowth. Pyrite and chalcopyrite
occur in minor amount. Toward northwest,
pyrrhotite is more abundant and intergrownby sphalerite and galena. The intergrowthabundance decreases as the incorporated rockfragments and lenses increase. In some portions, sphalerite aligns parallel to the sedimentary bedding. The alignment was disrupted by small-scale faults filled by quartz
veins containing the sphalerite-galena intergrowth.
GnBx is characterized as an ore which
irregular rock fragments are cemented by galena. Pyrite is euhedral with size is up to 3mm. Galena chadacrysts is abundant within
pyrite. Pyrite was intensely fractured and disaggregated. The fractures were healed bychalcopyrite, bournonite, tetrahedrite, galena,
sphalerite, quartz, and dolomite. Additionally,as fracture healing minerals, chalcopyrite,bournonite, and tetrahedrite appears to exist
in the innermost portion followed by galena,Figure 1. Geologic map and sample distribution in Dairi Zn-Pb±Ag deposit.

2. Materials and Methods
Samples used in this preliminary research were selected from 14 drillholes distributed in Dairi Zn-Pb±Ag deposit (Fig. 1).
More than 130 polished sections and 25 doubly polished thin sections were analyzed under Polarizing Microscope ECLIPSE
E600POL. Of this number, chemical composition of sulfosalts from 13 polished sections
were then determined by SEM-EDS. In addition, XRD analysis was also performed to examine carbonate minerals in 17 samples.
3. Results
3.1. Host Rocks
The sedimentary sequence in the Sopokomil dome downward stratigraphy consists of an interbedded dolomitic siltstones
and sandstones with minor carbonaceous
shales (Dagang unit), an interbedded dolomitic siltstones and carbonaceous shales aswell as pyritic, dolomitic, carbonaceousshales (Julu unit), and arenaceous massive
and breccia dolostones (Jehe unit). This sequence order occurs in all prospects except
Lae Jehe which shows the repetition of Dagang unit below Julu unit.
3.2. Orebodies and Ore types
In Anjing Hitam and Bongkaras, theorebodies are hosted by Julu unit while in Lae
Jehe and Basecamp, the orebodies are present
within Julu and Jehe units. The orebodies

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sphalerite, quartz, and dolomite.
SL is defined as an ore with mm-thicksulfide-rich layers intercalated by shale andsiltstone layers. SL consists of two subtypes:
sphalerite-galena-rich SL and pyrite-rich SL.
In the first subtype, the sulfide-rich layers arepredominated by the sphalerite-galena intergrowth while coarse-grained, euhedral pyrite

is predominant in the second subtype. Toward northwest, pyrite is intergrown by pyrrhotite. Together with other sulfides, this intergrowth was cut by late quartz veinlets. Inaddition, folded and piercing ore layers wereobserved and abundant in Basecamp, LaeJehe, and Bongkaras.

(a)
(b)
(c)
(d)
Figure 2. Representative photomicrographs of (a) pyritic massive, (b) sphalerite-rich massive, (c) galenarich breccia, and (d) sulfide layer ores.

V-Ju cut the host rocks and the orebodies in Julu unit. Galena and sphalerite is predominant. Fine-grained, euhedral pyrite isembedded on sphalerite while coarse-grained,corroded pyrite is present in galena-rich portion vein. Sphalerite hosts chalcopyrite disease. In Lae Jehe, pyrrhotite also exists in thisore type.
A crosscutting relationship between a
10-cm-thick vein and PyMs-SpMs in Julu
unit was observed in Lae Jehe. Toward thevein, ore mineralogy is zoned according tomineral abundance and morphology. In theperiphery, PyMs hosts chalcopyrite-bearing
sphalerite. The amount of pyrite gradually
decreases in the transition to SpMs. In SpMs,
the chalcopyrite disease in sphalerite intensifies along with the initial appearance of pyrrhotite blebs. The pyrrhotite blebs become
more abundant toward the vein. As the blebs
form aggregate, pyrite embedded on sphalerite was replaced by pyrrhotite and startedflowing. Pyrite is intergrown by pyrrhotite
and sphalerite as well as Ag-Pb-Cu sulfosalts.
In the vein edge, galena is predominant while
the center of the vein is barren.
V-Je in Lae Jehe has similar ore mineralogy to V-Ju. In Basecamp, minor to abundant tetrahedrite, tennantite, arsenopyrite,and chalcopyrite coexist with sphalerite, galena, and pyrite. Locally, tennantite and tetrahedrite size can be up to 3 mm. In Bx ore, thesphalerite-galena intergrowth with minor
chalcopyrite and pyrite occurs within quartz
cementing arenaceous dolostones.
Different to other ores in Jehe unit, Ds
in Lae Jehe does not contain galena. Sphalerite, chalcopyrite, and pyrite are present in theore. In contrast, galena is abundant in Ds ofBasecamp. Chalcopyrite, freieslebenite, AgSb-S, and Pb-Sb-S minerals form intergrowthwith galena.
4. Discussion
4.1. Sedimentary Basin
Evolution of Zn-Pb±Ag deposit-hostingsedimentary basins can be divided into riftfill and sag-phase sequences.[5] The rift-fillconsists of coarse-grained continental clastic
sediments (e.g. conglomerates, red beds,sandstones, and turbidites) while the sagphase comprises carbonate and fine-grained
clastic rocks (e.g. shales and siltstones). The
deposits are hosted by the sag-phase sequence whose 3-10% total organic carbon
contents.[6],[7] In Dairi Zn-Pb±Ag deposit, theconcordant ores are hosted by the interbedded dolomitic siltstones and carbonaceous

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shales as well as pyritic, dolomitic, carbonaceous shales of Julu Unit. The host rocks suggest that the deposit formed during the sag-phase of the basin in high organic carbon environment.

(a)
(b)
(c)
Figure 3. Representative photomicrographs of (a) vein, (b) breccia, and (c) disseminated ores.

The presence of the sag-phase sequencealso plays an important role in generating
metalliferous brines in the rift-fill sequence.[6] The trapped formational brines inthe deep aquifer strata are compacted by the
relatively impermeable overlying sag-phasestrata resulting in heated brines[8] which convectively flow within the aquifer.[9],[10] During the free convection flow, metals areleached out and the brines become metal-rich.
An extensional deformation resulting indeep-penetrating, syn-sedimentary faults
provides permeable fluid conduits which enable the metalliferous brines to ascend to thenear seafloor environments.[11] Dagang unit
underlying Julu unit in Lae Jehe and Dang
Takkas unit inferred in the deeper portion of
the basin hosting Dairi Zn-Pb±Ag deposit arepotential aquifer units in which the metalliferous brines could be produced.
The fertility of sedimentary basin thathosts Dairi Zn-Pb±Ag deposit is also supported by regional dolomitization. This feature is recorded as extensive dolostone distribution in Jehe unit and dolomitic composition of sedimentary rocks in Julu unit. The regional dolomitization is a robust event forgenerating a higher transmissivity carbonaterocks (Large et al., 2002). Consequently, theascending metalliferous brines could flowlaterally in limited distance from the fluid
conduits resulting in ore mineral precipitation within Jehe unit. In the overlying Juluunit, the brines flowed vertically through thesyn-sedimantary faults. As the brine encountered unconsolidated sediment in the nearseafloor environment, the brines flowed laterally resulting in the concordant ores.
4.2. Ore Paragenesis
In the concordant ores, the strata-boundmorphology of the orebodies is an indicativeof either hydrothermal fluid exhalation orsub-seafloor precipitation. The multilayer nature of the orebodies suggests multiple pulsesof the metalliferous brines have reached thenear seafloor environment. The sulfide precipitation was initiated by framboids. This
early stage was then followed by the mainstage of subhedral and euhedral pyrite as wellas anhedral sphalerite, galena, chalcopyrite,pyrrhotite, tetrahedrite, and bournonite.
Quartz and dolomite were also precipitatedalong with the main stage sulfides. In thisstage, pyrite was precipitated slightly earlier
than other sulfides and gangue minerals.
Considering that the early stage is minor, textural characteristics suggest that the ore mineral precipitation occurred in sub-seafloor
during early diagenetic stage is more predominant in Dairi Zn-Pb±Ag deposit.
In the discordant ores, the occurrence ofBx and V-Je ores indicate the escape of metalliferous brines and open-space fillings. Ds
ore around veins and breccia occurs in highpermeability portions of dolostones. Sulfide
paragenesis in these ores is similar to that in
the concordant ores with an exception thatthe early stage is absent in the discordant ores.

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Euhedral pyrite was slightly earlier precipitated than galena, sphalerite, chalcopyrite,pyrrhotite, arsenopyrite, tetrahedrite, tennantite, freieslebenite, Ag-Sb-S, and Pb-Sb-S
minerals.
It is postulated that the discordant ores
acted as a feeder zone for the overlying concordant ores. Ag-sulfosalts and Pb-sulfosalts
were mainly precipitated in the feeder zone.
Above the feeder zone, massive ore commonly occurs and evolve to layer ore in distal.[5].[6] Stable isotope study, mineral phaserelations, and geochronology are essential toprove this postulate.
As the Sibumasu block has experienced
multiple compressional events[1], syn-diagenetic Dairi Zn-Pb±Ag deposit has undergone
polyphase deformation and metamorphism.[12] The tilted layers of concordant ores
are considered as a mega-scale indication. In
hand-specimen and microscopic scales, theorebodies also record the deformation.
In hand-specimen scale, the deformation is indicated by the folded ore layers,
their sulfide segregation, and pyrrhotite
piercing veins. Pyrrhotite was concentrated
in fold hinges and piercing veins as well as
sphalerite-galena was concentrated in foldlimbs suggesting that the sulfide remobilization occurred in ductile condition. An indicative of hand-specimen scale deformation andthe sulfide remobilization are also present in
the disrupted mineralized beds and offset
fractures filled by the sphalerite-galena intergrowth.
In metamorphic environments, pyrite
tends to recrystallize to form exceptionallycoarse-grained, euhedral grains.[13] This recrystallization is also supported by the presence of rounded galena matrix inclusions inpyrite. Fractures formed resulting from thebrittle behavior of pyrite. Additionally, pyrite
can be rotated and disaggregated under lowgrade metamorphic environment.[14] Thesepyrite characteristics could be observed in
Dairi Zn-Pb±Ag deposit as an indicative of
deformation documented in microscopicscale.
Another indication is the fracture-fillingminerals of pyrite. Pyrite fractures in Dairi

Zn-Pb±Ag deposit were filled by galena,
sphalerite, chalcopyrite, bournonite, quartz,
and dolomite. These soft sulfides can be remobilized into pyrite fractures. The order offracture filling is in accordance with the readiness of soft sulfides to flow.[15] In addition,
the presence of chalcopyrite as the fracturefilling mineral indicates that the remobilization occurred in fluid-state condition[14], although solid-state remobilization for galena
could not be ruled out in local scale.[16]5. Conclusions
Sedimentary basin hosting Dairi ZnPb±Ag deposit is filled by sag-phase rocks
which are rich in organic carbon and have undergone regional dolomitization. The ore formation is predominated by precipitation during early diagenetic stage in sub-seafloor environment. The discordant ores in Jehe Unit
and the concordant ores in Julu Unit may
have been precipitated from the same mineralizing fluid. In addition, the ores have been
deformed and remobilized in mega-, handspecimen, and microscopic scales.
Acknowledgements
We would like to thank management of
PT Bumi Resources Minerals, Tbk. for allowing us to conduct fieldwork within theDairi concession, to access logging data andsupplementary documents, and to collectsamples from drill cores. We are also gratefulto the company’s geologists and local fieldassistants for their assistance during the fieldwork. We extend our gratitude as well toGreen Asia Program which has contributed toour research significantly.
References[1] A.J. Barber and M.J. Crow, Gondwana Res.
2003, 6(1), 1-28.
[2] N.R. Cameron, J.A. Aspden, D.McC. Bridge, etal., “Geologic Map of the Medan Quadrangle,Sumatra, Scale 1:250,000”, GDRC, 1982.
[3] D.T. Aldiss, R. Whandoyo, A.G. Sjaefudien,
Kusjono, “The Geology of the Sidikalang and(Part of) Sinabung Quadrangles, Sumatra, Scale
1:250,000”, GDRC, 1983.

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P-23[4] M.J. Crow and T.M. van Leeuwen, in: A.J. Barber, M.J. Crow, J.S. Milsom, “Sumatra: Geology, Resources, and Tectonic Evolution”, The
Geological Society, 2005.
[5] P. Emsbo, R.R. Seal, G.N. Breit, S.F. Diehl, A.K.
Shah, “Sedimentary Exhalative (Sedex) ZincLead-Silver Deposit Model”, USGS, 2016.
[6] J.W. Lydon, in: O.R. Eckstrand, W.D. Sinclair,R.J. Thorpe, “Geology of Canadian MineralDeposits”, GSC, 1996.
[7] W.D. Goodfellow and J.W. Lydon, in: W.D.
Goodfellow, “Mineral Deposits of Canada: A
Synthesis of Major Deposit Types, District
Metallogeny, the Evaluation of Geological
Provinces and Exploration Methods”, Geol. Assoc. Canada, 2007.
[8] J.W. Lydon, in: D.F. Sangster, “SedimentHosted Stratiform Lead-Zinc Deposits”, Mineral Assoc of Canada, 1983.
[9] G. Garven, S.W. Bull, R.R. Large, Geofluids

2001, 6, 289-312.
[10] J. Yang, R.R. Large, S.W. Bull, “Chapman Conference”, AGU, 2001.
[11] R.R. Large, S. Bull, D. Selley, et al., in: D.R.
Cooke and J. Pongratz, “Giant Ore Deposits:Characteristics, Genesis, and Exploration”,
CODES, 2002.
[12] N.A. Reynolds and P.M. Geerdts, “StructuralGeology and Resources”, Australian Institute ofGeoscientists, 2012.
[13] J.R. Craig and F.M. Vokes, Mineralogical Mag.
1993, 57, 3-18.
[14] L.B. Gilligan and B. Marshall, Ore Geol. Rev.
1987, 2, 205-229.
[15] F.M. Vokes and J.R. Craig, Mineralogical Mag.
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[16] B.E. Hobbs, Ore Geol. Rev. 1987, 3, 37-45.

Email: tomyalvinrivai@mine.kyushu-u.ac.jp

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