MINERALOGY AND CHLORITE CHEMISTRY
CONSTRAINTS ON THE FORMATION CONDITION OF
PROPYLITIC ALTERATION IN THE TUJUH BUKIT
PORPHYRY DEPOSIT.
EAST JAVA.
INDONESIA
POR Vannak1*.
Arifudin Idrus1.
Nugroho Imam Setiawan1.
Ryohei Takahashi2.
Ran Takeda2.
Miftahul Abrar2 Department of Geological Engineering.
Faculty of Engineering.
Universitas Gadjah Mada.
Indonesia Department of Earth Resource Science.
Faculty of International Resource Sciences.
Akita University.
Japan *Corresponding author: porvannak@mail.
AR TI C LE I N F O
AB STRACT
Article history:
Received 20th June 2025 Received in revised from 6th October 2025 Accepted 12th October 2025 Available online November 2025 Keywords:
Chlorite Geothermometer Oxygen Fugacity Propylitic Alteration Sulfur Fugacity Propylitic alteration develops chlorite, epidote, and calcite assemblages commonly found in porphyry copperAegold deposits.
This study aims to understand the formation temperature, oxygen, and sulfur fugacity of chlorite, a characteristic central element of propylitic-related minerals such as epidote and calcite, using a combination of qualitative and quantitative experimental methods.
The propylitic alteration forming condition was analyzed in detail using a superprobe (JXA-iSP.
and Electron Probe Micro Analysis (EPMA) at a voltage of 15.
0 kV.
The chlorite composition is relatively homogeneous, and displayed as Mg-rich chlorite (Type-I) within AlIVMg-Fe, ripidolite with respect to Si vs Fe/(Fe M.
The chlorite geothermometer indicates that the crystallizing temperature ranges 30 to 332.
82AC.
The oxygen and sulfur fugacity log fO2 and log fS2 of chlorite are low ranges (-70.
4 to -51.
85 and (-32.
55 to .
, respectively.
The analyses identify the mineral as true epidote, with spatial variations indicating Fe occurs as FeAA and Mn as MnAA.
However, the examined proportion of moles of the significant elements (Mg.
Ca.
Fe.
indicates that calcite is present in hydrothermal systems.
How to Cite This Article: POR Vannak.
Mineralogy and Chlorite Chemistry Constraints on the Formation Condition of Propylitic Alteration in The Tujuh Bukit Porphyry Deposit.
East Java.
Indonesia.
INTAN:
Jurnal Penelitian Tambang, 8.
, 52Ae63.
https://doi.
org/10.
56139/intan.
typically formed at T > 2800AC.
The spatial extent of chlorite defines the outer limit of the epidote subzone, but occurrences of chlorite also extend into the actinolite subzone.
It may locally overprint potassic alteration in the core of the deposit during late-stage retrograde alteration .
Commonly, calcite is a gangue mineral of low-sulfidation epithermal deposits, occurring as replacement calcium-bearing minerals, volcanic glass, open-space, and propylitic alteration .
Many researchers have studied chlorite chemistry globally, including the application of chlorite thermometry to estimate formation temperatures, the temperature and oxygen activity conditions of chlorite formation, and low-temperature INTRODUCTION Chlorite is a clay mineral that occurs in various geological settings, including sedimentary rocks, low-grade metamorphic rocks, and altered rocks resulting from hydrothermal processes.
It can form by replacing earlier .
ypically iron-and magnesium-ric.
minerals or by precipitating directly from fluids .
The systemAos chemistry, oxygen fugacity, and temperature and pressure conditions control the chemical composition of chlorite, a phyllosilicate, which remains stable at temperatures ranging from 80AC to over 700AC .
Epidote is a visually distinctive component of propylitic alteration, chlorite geothermometry .
, .
, .
Moreover.
Pacey .
investigated epidote chemistry in the porphyry ore system of Australia and epidote geochemistry in the porphyry Cu-Au deposit of China .
Furthermore, the development of textures and trace element signatures in hydrothermal calcite provides evidence for the ore-forming environment in Finland .
>5 Mt C.
Porphyry Cu-Au deposits of the eastern Sunda arc are spatially associated with small, nested, dioritic to tonalitic intrusive complexes with low-K calc-alkaline to weakly alkaline signatures .
The Tujuh Bukit Project is situated in Sumberagung Village.
Pesanggaran District.
Banyuwangi Regency.
East Java Province.
Indonesia.
It lies approximately 60 km southwest of Banyuwangi, the regional center of the regency, and roughly 205 km southeast of Surabaya, the provincial capital of East Java.
Researchers have interpreted the mineralization in the Tujuh Bukit prospect as a tonalitic porphyry CuAeAuAe Mo system accompanied by a high-sulfidation CuAe AuAeAg sulfide and Au-Ag oxide cap .
The discovery of the world-class Cu-Au deposit at Tujuh Bukit.
East Java .
9 Gt at 0.
45% Cu and 0.
g/t A.
, has reinforced the eastern Sunda arc as a significant metallogenic belt that is highly promising for the discovery of substantial porphyry deposits.
The arc hosts three premier porphyry Cu-Au deposits at Batu Hijau.
Elang, and Tujuh Bukit (>300 t Au and Figure 1.
Location of the study area and sample selection.
All samples are diorites selected from sections A-AAo (U.
C-CAo(U19.
, and F-FAo(U1.
This study aims to understand the characteristics of minerals related to propylitic alteration .
hlorite, epidote, calcit.
, including their classification, crystallization temperature, and oxygen and sulfur fugacity, in the Tujuh Bukit porphyry Cu-Au deposit.
Research and Innovation Center (ERIC).
Faculty of Engineering.
Universitas Gadjah Mada.
Indonesia.
RESULTS AND DISCUSSION
Petrography RESEARCH METHODS Different mineral assemblages produced distinct types of hydrothermal alteration, influenced by variations in temperature, pressure, host rock lithology, and fluid composition.
In diorite samples U19_UHGZ-23-143A.
U58_UHGZ-19-009, and U70_UHGZ-22-087 from the study area, alteration minerals commonly associated with quartz, sericite, and opaque minerals indicate phyllic alteration (Figure 5.
1a, c, .
In contrast, sample U1_UHGZ-23149, an older tonalite, contains abundant calcite associated with epidote, quartz, and opaque minerals, as observed under a microscope.
Electron Probe Microanalysis (EPMA) of the same sample confirms this mineral assemblage.
These observations suggest that the assemblage formed within a propylitic, epidote-rich outer alteration zone (Figure 5.
Sample Selection and Preparation In this case study, we used both qualitative and quantitative experimental methods.
Initially, core logging is essential for recording preliminary data such as lithology, alteration, color, and We selected five diorite samples for petrographic thin-section preparation and electron probe microanalysis (EPMA) to characterize alteration minerals and determine the major-element compositions of specific minerals, including chlorite, epidote, calcite, magnetite, and pyrite.
We prepared four samples for petrographic analysis at the Obsidian Laboratory in Bandung.
Indonesia.
We analyzed the remaining two samples using a super probe (JXAiSP.
operated at 15.
0 kV at the Engineering Figure 2.
Photomicrographs of quartz associated with sericite, and opaque minerals .
, .
, and .
When propylitic alteration-related minerals are dominant, calcite associates with epidote, quartz, and pyrite .
Abbreviation: Qtz=quartz.
Ser=sericite.
Cal=calcite.
Epi=epidote.
Op=opaque.
, porphyry and epithermal deposit.
or retrograde overprinting alteration .
Chlorite Chlorite alteration in outcrop is commonly blue-grey to green with the textural pattern .
ncluding massive, patchy, and brecci.
Chlorite alteration typically exhibits strong schistosity, characterized by distinct Chlorite frequently occurs as a hydrothermal alteration mineral, especially within peripheral zones characterized by propylitic According to the Back Scatter Electron (BSE) image of sample U53_MBH-23-052, the appearance of chlorite is medium-grey with a flaky texture.
The chlorite quartz pyrite, which is likely associated with propylitic alteration .
, .
Figure 3.
Back Scatter Electron (BSE) image of chlorite .
ed dot represents beam point analysis positio.
Abbreviation: Ch=chlorite.
Qutz=quartz, and Py=pyrite.
Using EPMA analysis of chlorite within diorite, the structural formula was determined, assuming 36 oxygen atoms per unit cell.
The data indicate that the chlorite composition is relatively homogeneous.
the Al(IV)-Mg-Fe ternary diagram .
, chlorite is plotted in the Mg-rich .
domain (Figure The Si vs Fe/(Fe M.
classification diagram of .
displayed the symbols of chlorite dramatically falling in and ripidolite and corundophilite (Figure Major Oxide of Chlorite and Its Classification The chlorite composition observed was determined by EPMA using a voltage of 15.
0 kV, showing that Si2O ranges from 30.
33 to 34.
61 wt.
Al2O3 ranges 32 to 15.
77 wt.
MgO .
25 to 18.
%), and FeO is about 21.
22 to 24.
25 wt.
Some oxide elements have a lower content, such as MnO (<2.
49 wt.
%).
CaO (<0.
52 wt.
%).
TiO2 (<0.
03 wt.
%),
K2O (<0.
18 wt.
%), and Na2O (<0.
12 wt.
%).
Alkali content (CaO Na2O K2O) is low, not exceeding 1 % (Error! Reference source not found.
Figure 1.
Classification diagrams of chlorite, showing the position of the examined sample.
Al (IV)-Mg-Fe, .
Si vs Fe/(Fe M.
, .
Epidote is susceptible to replacement by calcite in the presence of CO2-rich fluids .
The Back Scatter Electron (BSE) image shows that epidote appears white to grey (Figure 5.
Epidote forms clusters of subhedral crystals with a casting structure, and occurs with other calcium-bearing minerals, including calcite, quartz, and pyrite (Figure Epidote Epidote is a visually distinctive component of propylitic alteration, typically forming at temperatures greater than 280oC.
Its spatial extent defines the outer limit of the epidote subzone.
At the same time, the epidote also occurs in the actinolite subzone and may locally overprint potassic alteration in the deposit core during late-stage retrograde Figure 2.
Back Scatter Electron (BSE) image of .
ed dots are beam point position, .
Association of Ep Cal Qtz Py.
Abbreviation: Ep=epidote.
Cal=calcite.
Qtz=quartz, and Py=pyrite.
replacement calcium-bearing minerals, volcanic glass, open space, and propylitic alteration .
Calcite is primarily composed of Ca along with other elements, and its composition correlates with that of the surrounding country rocks.
The interaction between hydrothermal fluids and the surrounding diverse host rocks has a strong influence on the traceelement composition of calcite.
However, the chemical zoning observed in large calcite crystals and the presence of distinct grain populations within individual samples cannot be explained solely by host-rock control.
These features instead reflect temporal variations in the composition of the hydrothermal fluids .
A Major Element of Epidote The research team analyzed the major-element composition of epidote at the Engineering Research and Innovation Center (ERIC) of the Faculty of Engineering.
Universitas Gadjah Mada.
Table 2 presents the comprehensive EPMA results for The calculations assume that all iron occurs as FeAA and normalize the data to 12.
5 oxygen atoms.
The result illustrates that the data classified as epidote followed the AI-2-Fe-Mn .
In binary plots of significant elements of epidote spatial variation, the trend association of Fe shows an excellent negative correlation with Al, whereas there is no linear correlation with Ca.
Mn, or Ca Mn (Figure 6b-.
The analytical results indicate that all the iron in the analyzed epidote occurs as FeAA and primarily forms a solid solution with aluminum .
, .
The Mn content does not display a distinct correlation with the absolute concentrations of Fe.
Al, or the combined concentrations of Al and Fe.
however, it shows a negative correlation with Ca (Figures 6f and .
The negative correlation between Mn and Ca strongly supports the occurrence of Mn as MnAA.
Furthermore, the combined cation content of Ca Mn exhibits a broad negative correlation with the sum of Fe Al cations, suggesting an overall structural constraint of 0 apfu (Figure 6.
A Major Element of Calcite The primary element composition of the hydrothermal calcite revealed CaO content ranging 23 to 56.
06 wt.
%, indicating relatively pure These samples generally contained low levels of MgO .
ess than 0.
23 wt.
%) and FeO .
%), while MnO concentrations varied between 13 and 3.
69 wt.
% (Table .
The hydrothermal calcites from the study area are represented on a ternary diagram using the molar ratios of the key elements: Ca.
Mg.
Fe, and Mn.
Generally, because the concentrations of Mg.
Fe, and Mn in the hydrothermal calcites from sample U1_MBH-23-052 are relatively low, the data show only moderate variation.
In the Ca-Mn-(Mg F.
diagram (Figure 7.
, the data are aligned along Calcite Calcite is a common gangue mineral in lowsulfidation epithermal deposits, occurring as several parallel trends, where Ca and (Mg F.
vary at a relatively constant Mn concentration (Figure 7.
In the MnAeMgAeFe diagram, several calcite types align along trends with constant Mn/Fe ratios (Figure 7.
Figure 3.
Diagram .
Epidote classification diagram, .
Major element correlation after .
Figure 4.
Ternary plots illustrate the primary element compositions of hydrothermal calcite: .
CaAeMgAe(Mn F.
system, .
CaAeMgAe(Mn F.
, and .
MnAeMgAeFe .
Table 1.
Chemical composition of chlorite in Tujuh Bukit observed by EPMA
Analysis SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K 2O
P2O5
CaO Na2O K2O
Total
Fe 2
MP4
0,03
MP2_9
MP3_1 MP3_2 MP3_3 MP3_4
0,01
0,03
Number of cations based on 36 oxygens MP3_5 MP3_7 MP3_10 Analysis AIIV
XFe
MP4
MP2_9
MP3_1
MP3_2
MP3_3
MP3_4
MP3_5
MP3_7
MP3_10
Table 2.
EPMA data of epidote U1_Epidote MP1
MP2
MP3
MP4
MP5
MP6
MP7
MP8
MP9
MP10
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
Total Atom per formula unit based on 12,5 oxygen Al-2 Table 3.
Calcite primary element data.
Analysis Code
Weight (%) MgO
MnO
MP3_1
MP3_2
MP3_3
MP3_4
Mol (%) CaO
FeO
Total
MP3_5
MP3_6
MP3_7
MP3_8
MP3_9
Mn Fe together with recalculated and averaged data from the Sutton Sea reported by McDowell and Elders .
Cathelineau .
established the following temperature relationship.
T .
C) and AlIV: T = -61.
98 AlIV, which shows that the temperature ranges from .
3 to 332.
82oC).
ItAos considered only AlIV content, which increases in parallel with the increase in temperature, but with a slight deviation (Figure 8.
Propylitic Alteration Forming Condition Chlorite geothermometer Numerous studies have proposed that the chlorite geothermometer can be estimated using primary element data from EPMA analyses.
Researchers have geothermometers that rely on the ionic composition within the chlorite crystal structure .
, .
, .
Initially, the study indicates that the formation temperature influences the polytype of chlorite, the contents of Fe.
Mg.
AlIV, and octahedral vacancies of chlorite, and subsequently .
, .
, .
proposed an empirical chlorite geothermometer based on the temperature and AlIV content.
Subsequent studies have demonstrated that the chlorite crystallization temperature also depends on the Fe/(Fe M.
ratio, which correlates with the Al<sub>IV</sub> content .
On the other hand, geothermometric estimates may overstate the chlorite crystallization temperature.
address this issue.
Kranidiotis and Maclean .
geothermometers based on AlIV, which also factor in the Fe/(Fe M.
Regarding the histogram illustrated, that of temperature vs Fe/(Fe M.
(Figure 8.
However, in the diagram, temperature vs AlIV, both of the thermometer calculations (Cathelineau and Kranidiots & Maclea.
were affected by Al IV content (Figure 7.
Reference .
confirms that the temperature calculated using the Cathelineau .
crystallization temperature .
3 Ae 332.
82AC).
In this study, we employed two empirical equations to calculate the crystallization temperature, following the methods of Cathelineau .
and Kranidiotis and McLean .
Using updated chlorite analyses and fluid inclusion results from Los Azufres.
Figure 5.
Distribution histogram of .
onsidered Al IV conten.
calculated by (Cathelineau, 1.
vs AlIV, .
onsidered Fe/(Fe M.
by (Kranidiots & Maclean,1.
vs Fe/(Fe M.
, .
both geothermometers considered AlIV content .
, .
H-deficient FeAA-chlorite component .
According to Walshe .
, oxygen's fugacity was calculated in the software Minpet2.
Oxygen Fugacity The calculated oxygen fugacity .
OCC /10AA P.
] of chlorite serves as a reliable indicator of the physicochemical conditions during its formation.
Walshe .
and Reference .
proposed two methods to estimate oxygen fugacity .
OCC) conditions.
Some datasets are valuable because they include FeAA/Fe ratios determined through Myssbauer spectroscopy and provide estimated formation temperatures .
Later research has examined the results in considerable depth .
, .
, .
Walshe .
proposed that oxygen fugacity .
OCC) can be evaluated using the H-deficient FeAA-chlorite endmember, expressed as FeCEAAFealCCSiCEOCACA(OH)CN, within the chlorite solid-solution system.
Nevertheless, traditional electron microprobe methods are not sufficient to accurately identify this Table 4 presents the calculated log fOCC values plotted against the formation temperature under the vaporAe liquid equilibrium pressure of water (P<sub>sat L/V</sub>).
The scatter diagram indicates that chlorite (U53_MBH-23-.
has a high fOCC oxygen fugacity, corresponding to a fall in the magnetite field (Figure .
The pyriteAepyrrhotiteAemagnetite equilibrium lowers the log fOCC relative to the hematiteAemagnetite buffer and seems to regulate the prevailing redox environment in .
Chlorite occurring alongside magnetite in veins contained higher FeAA concentrations compared to chlorite associated with hematite or occurring independently .
Table 4.
Chlorite geothermometer.
Sample:
Rock Type:
Alteration:
N0.
Code
Log fO2
MP2_4
MP2_9
MP3_1
MP3_2
MP3_3
MP3_4
MP3_5
MP3_7
MP3_1
-58,49
-62,41
-58,18
-70,4
-51,85
-69,48
-66,03
-67,66
U53_MBH-23-052
Diorite Propylitic alteration Log fS2 Calculated Temperature Cathelineau.
Kranidiotis and McLean, .
-24,2 306,32 339,89 -26,44 322,22 356,08 -23,98 316,92
350,00
-31,55
343,76
-20,65
350,00
-23,63
332,82
363,64
-31,06
322,07
-27,68
344,14
-29,73
308,44
342,16
Figure 9.
Estimated log fOCC values plotted against formation temperature, calculated using the approach of Walshe .
The dashed and solid lines denote the hematiteAemagnetite and pyriteAepyrrhotiteAe magnetite equilibria, respectively, determined at P = Psat L/V based on the relevant thermodynamic data .
, .
Sulfur Fugacity conditions of low oxygen fugacity (Figure 10.
Although the log .
SCC /10a P.
values of chlorite range from -32.
55 to -20.
65, with an average of All projected data points plot below the buffer line, indicating formation under conditions of low sulfur fugacity (Figure 10.
Regarding the average of oxygen fugacity and sulfur fugacity, the type-I chlorite occurred as hydrothermal reductive related .
Chlorite records oxygen and sulfur fugacity, represented as log .
OCC /10a P.
and log .
SCC /10a P.
, respectively, which can provide robust insights into the physicochemical environment present during its formation.
The derived values (Table .
show that the log .
OCC /10a P.
values for chlorite range between -70.
4 and -51.
85, with an average of -62.
5, indicating formation under Figure 6.
The logarithmic values of oxygen fugacity .
OCC /10a P.
] and sulfur fugacity .
SCC /10a P.
] buffers are plotted against temperature.
A diagram showing the variation of oxygen fugacity buffers with temperature is also presented .
The diagram of oxygen fugacity buffers as functions of .
The diagram of sulfur fugacity buffers as functions of temperature.
Abbreviation:
Hm=hematite, mt=magnetite.
Bn=bornite.
Py=pyrite.
Cp=chalcopyrite.
Po=pyrrhotite.
Ni=nickel.
NiO=nickel oxide.
second authors.
The research teams of UGM (Dr.
Arifudin Idru.
and Akita University (Prof.
Ryohei Takahash.
provided the samples used in this study through a collaborative effort.
The management of the Geoscience Division at PT Bumi Suksesindo provided the core samples essential for this research, which is gratefully acknowledged.
CONCLUSION
In the petrographic thin section, most of the samples associated with quartz sericite pyrite represent phyllic alteration.
However, some samples dominated by chlorite, calcite, and epidote suggest the presence of a propylitic alteration zone.
Regarding chemical composition classification, the study classifies propylitic alteration minerals into I-type chlorite .
, true epidote, and hydrothermal calcite.
Chlorite crystallizes at temperatures ranging moderately from 288.
3 to 332.
82AC, according to the geothermometer by Cathelineau .
, which uses AlIV content.
The chlorite oxygen fugacity versus temperature fall in magnetite under a Py-Po-Mg buffer suggests that it is rich in Fe2 .
Otherwise, the oxygen and sulfur fugacity ranges (-70.
4 to .
and (-32.
55 to -20.
, respectively, indicate hydrothermal reduction.
REFERENCES