MINERALIZATION OF Cu-Au PORPHYRY DEPOSITS IN CIHURIP AND SURROUNDING AREA, GARUT REGENCY,
WEST JAVA, INDONESIA
EMMY Suparka 1), M. AZIZ 2), C.I. ABDULLAH 1), SUPARKA 3)
1) Geology and Palaeontology Group – FIKTM ITB, Bandung, West Java, Indonesia
2) Department of Geology – UNSOED, Purwokerto, Central Java, Indonesia
3) Indonesian Institute of Sciences, Indonesia
E-mail : [email protected]; [email protected]; [email protected]; [email protected]
ABSTRACT
The study area is located in Cihurip and surrounding area, Garut Regency, West Java; Indonesia, it is dominated by andesite lava, volcanic breccia, tuff, pyroclastic breccia, andesite and diorite intrusion; which have undergone mineralization and hydrothermal alteration. Fisiography of the study area is part of Southern Mountain of West Java, and tectonically it belongs to Magmatic Arc of Sunda-Banda.
Based on the field observation of some rock samples which have collected from surface area, the study area generally had undergone alteration and mineralization. Alteration zone in the study area have contains of quartz- biotite-magnetite±actinolite; quartz-chlorite-epidote±actinolite±tremolite; quartz-sericite-clay; quartz-chlorite- calcite; and quartz-clay zone. Mineralization generally have occured within argillic and propyllitic zone, another less within phyllic and porphyry zone. Based on mineral assemblages have occured on fifth zone of hydrothermal alteration in the study area was occured within temperatures about 200oC - 360oC; with pH neutral to basa. So that the study area have undergone alteration and mineralization more than one time with mineralization type porphyry to mesothermal.
Mineralization of porphyry is dominated by quartz, magnetite, pyrite and chalcopyrite veinlets with cross cutting (<1 mm to 3 mm). Besides, the texturally in fill wih opaque mineral veinlets include magnetite, pyrite, chalcopyrite;
veinlet quartz; veinlet tremolite; and have contains of disseminated sulphide.
The assay results average for altered rock are ranging from 0,11 – 0,49ppm Au and 0,01 – 0,10% Cu. With refer to mineralogical assemblages its indicated that zone of upper peripheral of porphyry system, it was also indicated the body of mineralization was uplifted at ± 900 m from the time of formation with the upper part of the system have been eroded.
Keyword : alteration and mineralization, mineral assemblages, porphyry system
INTRODUCTION
The study area is part of Southern Mountain of West Java, Indonesia and tectonically it belongs to Magmatic Arc of Sunda-Banda. The area is located between 810000 mE – 816000 mE and 9176500 mN – 9170500 mN (UTM WGS 84) and belongs to ANTAM Ltd. (Fig. 1). The methods used in this study are field observation, rocks sampling from available outcrops. Some rocks samples were analyzed using petrography, PIMA and AAS methods.
Hydrothermal fluid reaction with wall rocks will cause changing in physical and chemical condition of rocks which is intruded. This changing include colour, textures mineral assemblages and permeability. Whereas the intensities of alteration denpend on their hydrothermal fluid characterize and wall rocks condition. The alteration intensities are also influence within ore deposits accumulation. So that the intensities of alteration can be used to determine the accumulation of ore deposits base on the zonation of hydrothermal alteration.
Therefore the study hydrothermal alteration base on the rocks characteristic in Cihurip, Garut is importance to determine the ore deposits mineralization in porphyry type. This paper is to discuss the mineralogy, alteration and style of mineralization as how the deposit was formed.
GEOLOGICAL SETTING Regional Geology
Physiographycally, study area is located in Southern Mountain Zone of West Java, Indonesia and tectonically it is belong to Sunda-Banda Magmatic Arc (Fig. 2). This arc has been formed since Early Tertier and still active until now. The magmatic arc is composed by calc-alkaline volcanic rocks and volcanoclastic interbeded with sedimentary rocks of Paleogen and Neogen in age and intruded by andesite, dacite and microdiorite.
These rocks lie over Late Cretaceous rocks.
Based on K/Ar dating the Tertiary magmatism which leave related with subduction continental margin can be devided into two periods, that is Late Eocene – Early Miocene and Late Miocene – Pliocene (Soeria-Atmadja et al., 1994).
Stratigraphycally, the regional of study area is composed by Jampang Formation (early Middle Miocene) which consists of lava, andesite breccia and propylitic tuff, being a part of the intruded by quartz diorite (late Middle Miocene). The older rocks are unconformably overlain by conglomerate and tuffaceous sandstone with interbeded claystone of the Bentang Formation of Late Miocene – Early Pliocene age. The volcanic rock unit is dominantly composed of glassy tuff, tuff breccia and andesitic dyke is unconformably overlain by Pliocene. The youngest rocks are Plio- Pleistocene which consist of glassy tuff, scoria- tuffaceous breccia, breccia and andesite lava.
Geology of Cihurip
Cihurip area is occupied by sedimentary, volcanic-pyroclastic and intrusive rocks (Fig. 3) with an elevation range of between 700 and 1,400 meter above sea level. The hills are commonly steep which are formed by resistant rocks of intrusives and silicified sediments. The stream
patterns, which are controlled by faults, joints and intrusives, are sub-dendritic pattern.
Two main lithological units consisting of andesitic volcanic and intrusive rocks covered Cihurip area. The andesitic volcanics unit dominantly comprises andesite lava, volcanic breccia, tuff, pyroclastic breccia, tuffaceous breccia, dacite lava and andesite pyroxene lava.
The andesite lava is mainly exposed at a few places in this area. The tuff, dacite and andesite lava are exposed at the nothern part. The lower part of distribution of the andesitic volcanic is occupied by pyroclastic breccia, which turn gradually to coarse grained and bedding tuff. The fragments vary in size from 2 mm to 4 mm, angular – subrounded, grain supported and cemented by clay associated with chlorite.
Tuffaceous breccia was observed in the southern part of the Cihurip area. The volcanic breccia is distributed at the headwaters of S. Cibadak and in the eastern part of the area.
Diorite and andesite intrude both units. The igneous contact with andesitic pyroxene lava found in several places near the intrusive bodies.
Diorite is the dominant intrusive rocks in Cihurip area in the form of stock and dykes which characterized by light to dark grey, fine to medium-grained, in-eguigranular, porphyrytic and composed of interlocking plagioclase, hornblende, pyroxene and biotite. The diorite contains moderate to strong magnetite and is frequently associated with pyrite and veinlets cross cut magnetite as dissemination and fractures filling.
At some places the diorite is weakly to moderately altered to epidote-chlorite-actinolite-pyrite.
Whereas, at the window of S. Ciparay, the diorite is associated with low to moderate density of quartz veinlets stockwork and cross cut magnetite- actinolite.
The dominant structural tends are NNW – SSE, which represented by joint and faults, and reactivation structures. Northwest southeast trending dextral strike slip faults are dominant structures identified in Cihurip area. Interpretation of LANDSAT image, suggesting there are indication of these faults shown by lineaments of creeks and topography striking N 3150 E (Fig. 4).
The exposures of the fault zones are identified at several places at S. Cisanggiri and its tributaries
and in the tributaries of S. Cibadak. At some places the fault zones are filled by diorite dyke.
ALTERATION
Most of the rocks in the area have been altered to some degrees. In the field, observed hydrothermal alteration is characterized by presence of predominantly quartz, clay and chlorite. However, under microscopic observation, several minerals such sericite, epidote, carbonate and kaoline are identified while using PIMA detection, illite and kaolinite (dominant), halloysite, montmorillonite, pyrophyllite, dickite, paragonite and nontronite.
Five main stages of alteration characterize mineral assemblages in Cihurip area: (1) Potassic, (2) Propyllitic, (3) Phyllic, (4) Sub-Propyllitic, and (5) Argillic Alteration. (Fig 5). The boundaries between these different alteration types are quite irregular and difficult to map in detail.
Potassic Alteration
In the Cihurip area, the potassic alteration event only affected the volcanic rock type and probably predates the emplacement of the porphyry dykes.
There are three main aspects to the potasssic alteration of this rock: (1) alteration of mafic minerals to biotite-actinolite, (2) an irregular stockwork of hairline black biotite±magnetite veinlets, and (3) quartz plus magnetite veinlets with no anhydrite. Potassium feldspar alteration is not significant aspect of the potassic alteration event at the Cihurip. Moderate potassic alteration is cropped out in S. Ciparay. There is a roughly cylindrical distribution of potassic alteration, but its shape in level plan in the range of 850 – 900 meters is slightly ellipsoidal with a long axis of at least 400 meters (unconstrained) and a short axis of 250 meters.
Propyllitic Alteration
Propyllitic alteration in the Cihurip consists of quartz, chlorite, epidote, actinolite and tremolite.
There are two main spatial occurences of the propyllitic alteration: (1) within the volcanic rock type at the periphery of the Cihurip system outside the outher edges of the potassic alteration zone, and (2) in the center of the Cihurip system at the
outher edges of the porphyry dykes. These two occurences probably formed at different times: the propyllitic alteration event (i.e., part of the prograde alteration) and the propyllitic alteration of the dykes resulting from the retrograde cooling as the Cihurip hydrothermal system was dying away.
This zone of propyllitic alteration forms an irregular ring around the periphery of the Cihurip hydrothermal system. Moderate propyllitic alteration is cropped out in several places at S.
Ciparay, S. Cihijau, S. Cihideung, S. Cibeureum and S. Ciawi. Inward migration of thermal- chemical boundaries as the prograde hydrothermal alteration event was contracting would explain this relationship of propyllitic alteration at the periphery of the system. In the porphyry dykes of the propyllitic alteration is texturally destructive and has resulted in the conversion of the mafic phenocrysts mineral to chlorite and the groundmass to chlorite + epidote ± actinolite ± tremolite. The final stage of retrograde fluid flow up the contacts between the porphyry dyke and the volcanic rocks would explain the pattern of this propyllitic alteration being confined to the center of the Cihurip hydrothermal system along these contacts.
Phyllic Alteration
Phyllic zone is widespread in this system which is usually confined to closely associated with propyllitic and sub-propyllitic zones. Weak to strong phyllic alteration is cropped out in several places at S. Ciparay, S. Cihijau, S. Cihideung and S. Ciawi. In generally, phyllic alteration which are characterized by the presence of sericite, is commonly observed within altered volcanic rocks.
The altered rocks are white yellowish to light grey associated with low-density quartz veinlets.
However, at one location on the southern side of the area, there is a ellipsoidal shape wide occurrence of this phyllic alteration type, which extends laterally close to the central porphyries.
Sub-Propyllitic Alteration
Sub-propyllitic alteration is widespread in several places in the Cihurip area consists of quartz, chlorite and calcite, which varies from weak to moderate condition. This alteration is developed
within the andesitic volcanics unit around the diorite contact and along the fault zones, which extends laterally outward from the central dykes.
This zone of sub propyllitic alteration is the dominant alteration type throughout the main diorite intrusion. Alteration along intrusion contacts often comprises an early phase of quartz- biotite-magnetite±actinolite, which forms an irregular ring around the periphery of the Cihurip hydrothermal system. Weak to moderate sub propyllitic alteration is cropped out in several places at S. Ciparay, S. Cihijau, S. Cihideung, S.
Cibadak, and S. Cisanggiri.
Argillic Alteration
The argillic zones are widely distributed on the southern - southwestern slope of a ridge between S. Cijambu and S. Cisanggiri, characterized by varying colour from yellowish to brownish white, and commonly observed within andesite. In general distribution of this alteration is developed within silisification. Therefore when the rocks affected by this alteration, it becomes harder and compact, and frequently associated with minor to abundant clay.
The argillic alteration zone extends up to several hundred meters from the diorite and andesite contacts. The most intensive quartz-clay alteration appears along the Northwest trending strike slip fault, which extends from the left tributary of S.
Cisanggiri to the Southeast through the right tributary of S. Ciawi.
MINERALIZATION
Mineralization, which occurs in the area is associated with an early phase of porphyry style quartz-biotite-magnetite±actinolite alteration. The central mineralized zone is also closely associated with a porphyry dyke that is an offshoot of the main diorite intrusion. The emplacement of the diorite dyke in the area was probably influenced by the fracture zone and reactivation of these faults controlled the emplacement of the intrusions and focused related hydrothermal fluids.
Mechanism of mineralizations
When magma rise to the shallow level, freezing
& early crystallization occurred from diorite intrusion followed by heat transfer on wall rock and reaction with residual magma solution as a neutral-alkali pH hydrothermal solution which play a role on a form of quartz-biotite-magnetite- actinolite zone (Showed by no. 1), with temperature of hydrothermal solution about 3600C (Fig. 6). Along with temperature decreasing, quartz-chlorite-epidote±actinolite±tremolite zone was beginning to formed (Showed by no. 2), and hydrothermal solution is still in neutral-alkali pH.
Freezing and crystallization from diorite intrusion resulting fractures mainly on intrusion body and followed by separation of magma volatile. In addition, the increasing of gas pressure while freezing of magma, also can be initiate the form of fracture on diorite porphyry intrusion. These fractures, then be filled by magma residue solution which dominated by silica and metal compound, with the result that form quartz fine vein and magnetite with stock-work structures.
In the last phase of freezing of diorite intrusion, hydrothermal solution which rises through extention zone –caused by fault – start to reach the surface then be reacted with wall rock and mix with meteorite water resulting neutral pH solution (pH 5-6) relatively. At that time, temperature of hydrothermal solution approximately around 2700-3200C. The reaction of hydrothermal solution with rocks, has changed plagioclas, K- feldspar and chlorite into sericite, and then quartz- sericite-clay zone was formed (Showed by no. 3).
Furthermore, when temperature decreased, the reaction of solution with meteorite water re- occurred and O2, CO2, H2 added to the solution, resulting the rise of pH solution from the previous zone, and at that time quartz-chlorite-calcite zone was formed (Showed by no. 4). Along with temperature re-decreasing, H+ gas volatiles were released and the influence of meteorite water become more dominant (H2O), Hydrolysis reaction which change sericite, chlorite, glass dan plagioclas inti clay minerals, at that time quartz- clay zone was formed.(Showed by no. 5)
DISCUSSION
The Cihurip system does not fit the conventional Cu-Au porphyry deposit model, but it contains elements of deposit type. The Cihurip mineralization deposit is a discrete Cu-Au bearing hydrothermal system centered about diorite- porphyry dyke. The distribution of the porphyry dykes along with shearing observed at their edges suggests that the dykes filled a fault or zone of weakness. The fault and the contacts along the porphyry dykes that later filled the fault acted as conduits for the hydrothermal system of the Cihurip. The potassic alteration and its associated peripheral propyllitic halo predated the intrusion of the porphyry dykes. During the final stages of cooling of the Cihurip hydrothermal system, fluids were focused along the contacts of the porphyry dykes causing propyllitic and phyllic alteration and porphyry dyke rock types only within several meters of the contacts.
The assay results of the rock samples indicated values of Au 0.10 – 0.49ppm, and the elevated Cu values maximum 620ppm and 910ppm respectively, appears in some samples. The Cu-Au mineralization, which is postdating the diorite dykes is interpreted to be related to the late stage of magmatic differentiation.
CONCLUSIONS
The knowledge of gold-copper mineralization (van Leeuwen et.al., 1987, 1994; Carlile and Mitchell, 1994) along most parts of the western Sunda-Banda arc confirms the occurrence of major low sulphidation epithermal mineralization.
They are related to crustal extension following aerly andesitic magmatism in areas where intrusive bodies are rarely exposed and porphyry mineralization is absent. A careful study on the young Neogene stratigraphy indicates a remarkable increase in andesitic volcanics within the Pliocene sediments. The relatively wide distribution of andesitic volcanics in this area, which is possibly related to either caldera collapse or graben subsidence.
Mineralization and associated hydrothermal alteration in the Cihurip is hosted and enclosed by a porphyry dyke, which occurs in two stages: the
first stage is associated with the potassic alteration zone which probably predates the diorite porphyry, and the later mineralization stage is a part quartz-pyrite-tremolite veining event with chlorite, sericite-clay which postdates the emplacement of the porphyry dykes. This mineralogy, texture and forms fail to indicate a porphyry type as suggested previously.
ACKNOWLEDGEMENT
The authors would like to acknowledge the support and backing of the management of ANTAM Ltd. Unit GEOMIN who permitted this paper to be published. We are also grateful to those who participated in the field collecting the data, in particular Afdi Rahmandita, Hikmat Nadzaruddin and M. Ubaidillah Baheshti. We wish to record our appreciation to Project RUK BPPT for the financial assistance.
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FIGURE 1: Location of the study area
FIGURE 2: Physiography Map of West Java (van Bemmelen, 1949; vide Martodjojo, 1984)
FIGURE 3: Geology Map of Cihurip Area
FIGURE 4: Interpretation of lineament from LANDSAT image band 4,5,7 using ERmapper v3.5; and roset diagrame from Rockware programme
GEOLOGY MAP OF CIHURIP AREA
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LANDSAT IMAGE OF CIHURIP AREA
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FIGURE 5: Alteration map of Cihurip area
FIGURE 6: Simplified mechanism of mineralizations in Cihurip area
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LEGEND:
ALTERATION MAP OF CIHURIP AREA
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