Excess Alumina of Plagioclase Related to Copper-Gold Enrichment: Study Case at the Humpa Leu East (HLE) Porphyry CU-AU Prospect in HU’U District

Abstract


Introduction
The cooling and crystallization of hydrous and oxidized magmas results in the formation of porphyry Cu deposits, which may also contain Mo and Au.These magmas are typically found in arc or collisional environments and are distinguished by their high-water content and oxidizing conditions (Cooke, 2005;Sun et al., 2013;Hou et al., 2015a;Hou et al., 2015b;Sillitoe, 2010;Sun et al., 2015).Porphyry Cu deposits are often massive and low-grade, supplying a significant amount of copper to the mining industry (Sillitoe, 2010).The lack of fertile magma creation reduces the possibility of mineral development and leads to barren systems (Richards, 2003;Richards, 2015).Assessing magma fertility using mineral assemblages, whole-rock geochemistry, and mineral compositions are essential for assessing potential mineralization in magmatic belts (Loucks, 2014;Richards, 2015).
The high-water content in magma (H2O) is a vital factor in the formation of porphyry Cu-Au deposits, as indicated by various studies including Chiaradia & Caricchi (2017), Nevolko et al. (2021), and Richards (2011Richards ( , 2015) ) and; Richards et al. (2012).Water do not only act as a fundamental component in these deposits but also facilitates the interaction between copper complex ligands (such as Cl-and HS-) and copper, leading to the formation of ore minerals.This interaction is detailed in studies of Richards (2011), Seward et al. (2014), and Chiaradia & Caricchi (2017).The solubility of Cu ligands and sulfur in water is a limiting factor; thus, the available water quantity is a controlling element in the development of porphyry copper deposits.The abundance of water in magma leads to earlier saturation, as explained by Richards et al. (2012), and enhances the extraction of gold from the melt into volatile aqueous phases, as shown in the research by Li & Audetat (2013).Consequently, a high magma water content is instrumental for the migration and concentration of metal elements, playing a significant role in mineralization processes.
Plagioclase is the most prevalent felsic mineral component in volcanic rocks, occurring as prominent phenocrysts with numerous groundmass microlites (Twana et al., 2023).According to Muhammad et al. (2023), plagioclase is one of the silicate minerals that is an early-forming mineral that is essential in volcanic rocks.In other words, plagioclase can represent the history or evolution of the rock itself.Plagioclase geochemistry can be a tool to determine a fertile system related to porphyry mineralization enrichment, which reveals the water content of the magma (Williamson et al., 2016).To understand the evolution of magma and the formation of magmatic-hydrothermal ore deposits, it is crucial to identify the water content in magma, as highlighted (Fiedrich et al., 2018).The concentration and enrichment of water and other volatile substances in the melt play a pivotal role in generating a free volatile phase.This phase is instrumental in the creation of magmatic-hydrothermal ore deposits, including types like porphyry mineralization, as explained by Heinrich and Candela (2014).In their research, Williamson et al. (2016) found a link between high alumina levels and porphyry mineralization.They suggested that plagioclase in fertile regions is distinctive due to its 'excess' alumina content.This excess alumina is associated with increased water content in the melt, as further established by Williamson et al. (2016).The study is focused on the HLE porphyry Cu-Au prospect situated in the Hu'u district of Sumbawa Island, Indonesia.This particular area of study is part of the concession held owned by PT.Sumbawa Timur Mining (PT.STM) company in Sumbawa Island, located in the Dompu region within the province of West Nusa Tenggara, Indonesia.The research site is positioned approximately 30 kilometers south of the Dompu regency and about 75 kilometers southwest of Bima city (Fig. 1).The Hu'u potential is located approximately 440 kilometers east of West Nusa Tenggara's provincial seat.Flights from Lombok to Bima take around 45 minutes, followed by a 2-hour drive to the Hu'u.The HLE project is one of the porphyry Cu-Au prospect in PT.STM's, the concession area that is still being explored.The ONTO prospect was discovered earlier and is connected with porphyry deposits overprinted by high sulfide epithermal deposits associated with covellite-pyrite mineralization with barren thickconsiderable lithocap thickness (Burrows et al., 2020).The HLE potential prospect is a traditional porphyry deposit with pencil-type intrusion pattern that lacks overprints and disruptions from other deposits (Sillitoe, 2012).We have conducted mineral assemblage observations of the causative intrusions and mineral chemistry analysis of plagioclase geochemistry data analysis related to early, intermediate, and late causative intrusions.This study is aimed to examine trace element compositions of plagioclase from those causative intrusions to assess magma fertility for copper and gold mineralization in term of magma water content.

Regional Geology and Mineralization
The Sunda arcs, particularly in Java-Nusa Tenggara, exhibit significant metal mineralization linked to volcanic centers that originated from Oligocene-Miocene subduction events.Notable examples include Selogiri, Trenggalek, and Salakan.Additionally, there are associations with Pliocene-Pleistocene intrusions at sites like Tumpangpitu, Batu Hijau, Elang, and Hu'u (Fig. 2).These mineral deposits are diverse, including porphyry copper-gold, high-low sulfidation epithermal, gold-silverbarite-base metal, skarn, and sediment-hosted types, as Carlile and Mitchell (1994) describe.The Sunda magmatic arc is known for porphyry deposits that are often connected with lithocap, epithermal, and skarn deposits, as discussed by Hammarstrom et al. (2010).These porphyry deposits, associated with porphyry intrusions including andesite, dacite, and tonalite, developed from the Eocene to the Pliocene era (Hammarstrom et al., 2010;Maryono et al., 2018).
The early Miocene period saw a reduction in calc-alkaline magmatism, followed by considerable erosion and sediment accumulation.Mineralization in the Sunda arc is typically linked to magmatism from tectonic island arc volcanoes (Carlile and Mitchell, 1994;Setijadji et al., 2006;Setijadji and Maryono, 2012;Maryono et al., 2018).It's also noted that porphyry types in the Sunda magmatic arc mainly relate to lithocap, epithermal types, and some skarn deposits (Hammarstrom et al., 2010).However, the relationship between the age of volcanism or magmatism and the type of mineralization in Indonesia is complex, as epithermal deposits are not consistently younger than porphyry deposits and are not always located in volcanic centers (Setijadji and Maryono, 2012).Mineralization in the magmatic arc is extensive and occurs through multiple phases of volcanic system evolution.In Java, gold-copper mineralization dates range from the Miocene to the Pliocene-Pleistocene, with initial intrusion mineralization occurring about 5-10 million years ago, and significant amounts of gold (300 tons) and copper (over 5 million tons) forming less than 5 million years ago.The geological nature of the Sumbawa islands, whether an island arc or a continental arc, remains a topic of debate.Hamilton (1979) and Katili (1975) have suggested a continental arc relation, a view supported by several studies, including those by Elburg et al. (2004), Fadlin et al. (2018Fadlin et al. ( , 2021Fadlin et al. ( , 2023)), Gardner et al. (2013), Gertisser & Keller (2003), and Reubi et al. (2002).Sumbawa Island, covering an area of 15,448 square kilometers, stretches approximately 270 kilometers from east to west.The Indonesian Geological Survey has released a simplified geological map and stratigraphic column for Sumbawa Island, as documented by Sudradjat et al. (1998) and Suratno (1994), illustrated in Figure 3.This island is characterized by its rugged terrain, dominated by volcanic mountains, including the towering Mt.Tambora, which reaches an altitude of 2,851 meters.The island's geological and morphological features are segmented into three distinct crustal blocks: the western, central, and eastern segments.These divisions are primarily determined by the Trans-Sumbawa Fault system, along with northwest-oriented arc-transverse discontinuities.The geological foundation of Sumbawa Island is formed by an Early to Middle Miocene volcanic arc, along with associated volcaniclastic deposits, which are laid over the oceanic crust near the Sunda Continental Shelf (Barberi et al., 1987;Garwin, 2000).Garwin, 2000;Suratno, 1994;Sudradjat, 1998).
In the vicinity of Batu Hijau, located in the western block, the thickness of the volcanic crust varies between 14 and 16 kilometers, whereas in the Humpa Leu East area (Hu'u district and its surroundings) in the central block, it extends to a depth of 20 to 23 kilometers.Meanwhile, the eastern block exhibits an intermediate crustal thickness compared to the other two blocks, as detailed by Barberi et al. (1987).The central block's greater thickness is evident in its depth compared to the western and eastern blocks.In the western block, the crust thickens northwards, while in the eastern block, it becomes denser towards the south, as described by Barberi et al. (1987).The southern part of the western block is elevated and slopes northward, just as the northern part of the eastern block is raised and tilts southward.
In contrast, the central and eastern blocks, which slope southwards, display more extensive crustal exposure due to their down-dropped positioning.The geological evidence found on Sumbawa Island, including the types of rocks and hydrothermal alteration patterns, supports this concept as indicated by Garwin (2000).Sumbawa Island is characterized by a variety of intrusive rocks, showcasing a spectrum of compositions from mafic to felsic.In the geological landscape of Sumbawa, the western block and the western part of the central block are distinguished by a more frequent occurrence of mapped intrusions compared to their eastern counterparts and the eastern half of the central block, especially in the area south of Saleh Bay.These intrusions primarily exhibit a calc-alkaline nature, with a compositional range that includes diorite or basaltic andesite through to granodiorite, as reported by Garwin (2000Garwin ( , 2002)).The eastern Sumbawa region, characterized by andesitic and dacitic volcanic layers, has been penetrated by hypabyssal dacite intrusions.Radiometric dating and field correlations indicating cross-cutting relationships suggest that these intrusions date back to a period extending from the Middle Miocene to the Pliocene, as detailed by Garwin (2000).

Geology of HLE Prospect
Based on the findings of PT.STM (2018) and Fadlin et al. (2023) the Humpa Leu East (HLE) porphyry Cu-Au prospect consists of various volcanic rocks, including andesitic tuff, crystalline tuff, volcanic breccias, subvolcanic intrusions, and diorite porphyry (Fig. 4).The occurrence of spotted diorite porphyry within this area is thought to be influenced by reverse faults (PT.STM, 2018).In the vicinity of the HLE porphyry Cu-Au prospect, five distinct types of hydrothermal alterations are observed on the surface: potassic, propylitic, advanced argillic, intermediate argillic, and argillic alterations (PT.STM, 2018).
The potassic alteration found in the HLE prospect has unusual characteristics when compared to other porphyry deposits in the eastern Sunda arc.Notable features encompass abundant M and A veins, secondary K-feldspar, and a substantial presence of carbonate, accompanied by anhydrite and chlorite (Fadlin et al., 2023).The absence of secondary biotite in the study area, potentially transformed into chlorite, further enhances the uniqueness of this alteration.Significant carbonate-rich alteration in HLE is uncommon in porphyry systems, especially in the eastern Sunda arc.The presence of carbonate-rich alteration in the chlorite-sericite and sericite zones adds complexity to the system (Fadlin et al., 2023).Carbonate precipitation in the porphyry system is intricately tied to introducing CO2 from hydrothermal fluids and the availability of sufficient Ca derived from Ca-bearing minerals like plagioclase and mafic minerals.Accordingly, the carbonate-rich alteration in the potassic zone is likely the result of Ca-rich plagioclase reacting with CO2-rich hydrothermal fluids (Fadlin et al., 2022).The HLE porphyry Cu-Au prospect is one of the classical porphyry Cu-Au in the world, characterized by multiple pencil-type intrusions, including early, intermediate, and late intrusive rock without any overprinting of the late epithermal deposits (PT.STM, 2018;Sillitoe, 2012).Fadlin et al. (2023) have reported that the causative intrusions in the HLE prospect can be divided into two types of causative intrusion rocks: quartz diorite and diorite porphyry, developed on an active continental margin with calc-alkaline magma preference.
The HLE porphyry Cu-Au prospect in the Hu'u district primarily exhibits ore mineralization characterized by the presence of sulfide minerals, including bornite, chalcocite, covellite, chalcopyrite, pyrite, along with less common sphalerite and galena, as documented by (PT.STM, 2018;Sillitoe, 2012;Fadlin et al., 2023).This mineralization occurs in two distinct forms: quartz-sulfide veins and disseminated sulfides.Among these, chalcopyrite is the most prevalent copper-bearing sulfide, closely linked with the main stage of mineralization.This association is particularly evident in the intermediate phase or during chlorite-sericite and sericite alteration processes (Sillitoe, 2012).Furthermore, Fadlin et al. (2023) also have reported that the presence of gold (Au) is primarily found in chalcopyrite during the intermediate stage.Quartz-sulfide veins are the most typical mineralization style that contributes to metal precipitation in this study area.The early mineralization stage, characterized by A, M, and AB veins, is often poorly mineralized with low sulfide concentration.It is possible that the veins developed at an excessively high temperature, which is unsuitable for sulfide mineral precipitation.This condition is also observed in central java at the Selogiri porphyry prospect (Sutarto et al., 2015).Sparse bornitechalcopyrite coexisting with magnetite in the M and A veins is evidence that Cu precipitation has begun in the early stage of mineralization.The intermediate stage is the primary mineralization event, distinguished by numerous sulfide-bearing B and C veins.Pyritic D veins are a typical late stage of the quartz-sulfide vein, with lower sulfide abundance.Disseminated sulfide mineralization contributes to the prospect's metal budget.bornite, chalcocite, covellite, chalcopyrite, and pyrite are the most common sulfide elements in this mineralization style (Fadlin et al., 2023).

Materials and Methods
This study uses petrographic and quantitative chemical analysis of the plagioclase by EPMA (Electron Probe Micro-Analyzer).A total of 6 selected polished thin sections samples represented the causative intrusions were selected to be analyzed.Polished thin slices of rock samples were cut and ground to a thickness of 0.03 mm.Subsequently, the thin polished section samples underwent polishing with 3 μm and 1 μm diamond polishes.The petrographic analysis of the polished thin sections aims to determine the mineral assemblage of the causative intrusive rock samples using a Nikon ECLIPSE LV100N POL.This method also checked the texture and other impurities of the plagioclase minerals to minimize contamination at EPMA analysis.Plagioclase was quantitatively examined analysed by utilizing the JEOL-JXA 8230 Electron Probe Micro analyzer machine at Akita University's Department of Resources Science in Japan.The primary elements of plagioclase, Si, Al, Na, and Ca were calibrated using natural albite standards.Plagioclase minerals from the causative intrusions samples were studied using 15 kV acceleration voltage, 20 nA beam current and a 5 μm beam diameter.An analysis of the mineral chemical composition of plagioclase was performed by the author during his PhD program at Akita University, Japan.The mineral chemistry data were incorporated into the PhD thesis, but the discussion of magma fertility according to plagioclase chemistry was not included.

3.1.Mineralogical Characteristics
Mineral analysis was conducted on samples of intrusive rock obtained from various drilling sites.Petrographic studies indicate the presence of a multiphase intrusive event, characterized by two distinct types of intrusive rocks: quartz-diorite and diorite porphyry.The quartz diorite porphyry, primarily associated with the early to intermediate intrusion phases and significant mineralization events, displays a grey hue and a porphyritic texture.Its composition includes plagioclase and minor quartz, along with secondary minerals such as carbonate, chlorite, anhydrite, albite, secondary quartz, and K-feldspar.These intrusions, containing fewer than 80% phenocrysts, are predominantly characterized by subhedral plagioclase (labradorite-andesine), with plagioclase/mafic minerals replaced by secondary quartz and chlorite.The groundmass of quartz diorite porphyry intrusions is microcrystalline and mainly comprises plagioclase with lesser primary quartz.In contrast, samples from intrusions associated with the late phase exhibit a dark grey-grey color with a porphyritic texture.They consist of dominant plagioclase minerals and alteration minerals such as calcite, secondary quartz, chlorite, anhydrite, albite, and Kfeldspar.
In porphyry-related intrusive complexes, magmatic plagioclase occurs abundantly in phenocrysts and groundmasses (Seedorff et al., 2005).Its conditions were also observed in the Humpa Leu East porphyry Cu-Au prospect, where the plagioclase mineral is a dominant phenocryst comprising quartzdiorite and diorite porphyry (~80 % of all phenocrysts, estimated visually in thin sections) in every intrusion phase (Fig. 5).Hydrothermal processes primarily influence the existence of magmatic plagioclase in porphyry Cu deposit systems.It can be replaced with epidote, calcite, muscovite/illite, dickite, kaolinite, alunite, pyrophyllite, orthoclase, or albite (Seedorff et al., 2005;Sillitoe, 2010).A similar condition was confirmed to the HLE porphyry Cu-Au prospect, where the plagioclase phenocrysts in the early phase of intrusion are mostly associated with potassic alteration (K-feldspar, calcite, and albite), some phenocrysts of plagioclase in this phase show overprinted by chlorite, sericite, and anhydrite.Furthermore, plagioclase phenocrysts in the intermediate and late phases mostly show an overprinted by the late alteration processes, such as chlorite, sericite, calcite, and anhydrite (Fig. 5 c1).Furthermore, according to petrographic results (Fig. 5c2), numerous samples of plagioclase phenocryst exhibit evidence of substantial 'excess' alumina, expressed by concentric zones, strongly implying development by magmatic processes.Concentric zones in plagioclase phenocryst may represent fluid injections into the sub-porphyry magma chamber (Fiedrich et al., 2018;Williamson et al., 2016).However, it is difficult to find pure and clean plagioclase without being contaminated by secondary processes such as hydrothermal alteration under petrographic observation.It is unusual to find a clean texture of plagioclase well preserved in porphyry copper environments.Therefore, in this publication, we try to use a combination of observations based on a Back Scatter Electron (BSE) image to confirm and avoid the inclusions or impurities (Fig. 6).Regarding the observations under the Back Scatter Electron (BSE) image showing that the plagioclase mostly shows an intensive fracture, some filled by altered minerals, probably quartz and calcite minerals, and also other impurities such as sulfides minerals.Furthermore, a few plagioclase phenocrysts show a concentric zoning.In addition, the BSE image analysis also shows the mineral sulfides are most abundant within an intermediate phase of intrusion compared to the early or late phase of intrusion (Fig. 6).

Plagioclase Chemistry
Samples EPMA examination of plagioclase phenocrysts from numerous samples, representing the three types of intrusion stages, revealed chemical similarities (Table 1).The SiO2 concentration of plagioclase phenocrysts in every stage of intrusion ranges from 50.8 to 56.4 wt.%, the Al2O3 content ranges from 25.5 to 29.4 wt.%, and the FeO level is less than 0.23 wt.%.Plagioclase phenocrysts in intrusion samples often have a small amount of TiO2 (<0.01 wt.%) and K2O (<0.74 wt.%).CaO concentrations range from 8.56 to 13.2 wt.%, while Na2O ranges from 3.72 to 6.25 wt.% (Table 1; Appendix 1).The plagioclase phenocryst classification (after Deer et al., 2013) shows that the plagioclase related to the early phase of intrusion is mostly labradorite, while plagioclase phenocryst associated with intermediate and late phases of intrusion has a wider compositional range from labradorite to andesine (Fig. 7).

Magma Fertility Assessment
In this chapter, we attempt to compare major element data of plagioclase by electron probe microanalysis (EPMA) from viable samples (HLE prospect) and infertile samples of Kelud, Mt in the Sunda arc (plagioclase chemistry data obtained from Jeffery et al. (2013).The goal is to determine melt water content using extra alumina in plagioclase.According to Williamson et al. (2016), excess alumina in plagioclase can be defined as Al* = ((Al/(Ca+Na+K)-1)/XAn) > 1.The graph of An.% vs. Al/Ca+Na+K (Fig. 8) was obtained using atoms per formula unit (apfu.),demonstrating that plagioclase from fertile and infertile systems can be effectively discriminated, with the blue line linking albite (NaAlSi3O8) and anorthite (CaAl2Si2O8) endmembers.Notably, the blue line, showing Al*=((Al/(Ca+Na+K)-1)/0.01An) = 1, differs systems with 'excess' Al (Al*>1), from infertile ones (Al*<1).The results of the calculations of excess alumina of plagioclase from the HLE porphyry Cu-Au prospect indicated that the igneous plagioclase exhibits more significant Al* or excessive Al concentration when compared to Kelud, Mt. samples that are unrelated to porphyry copper deposits (Fig. 8).The excess Al could be related to the injection of fluid-rich melts into the magma chamber, resulting in high magmatic water content (Williamson et al., 2016).Similarly, Kyono and Kimata (2001) suggested that the excess Alumina in plagioclase can be explained by water replacing vacancy sites Si4O8 due to high water melt.Furthermore, the intermediate phase intrusion of the HLE prospect shows richer excess Al than the early and late phase intrusion.In porphyry ore-forming magmas, high water content is necessary for porphyry mineralization (Loucks, 2014), which is also relevant to the grade of Cu-Au mineralization in this prospect, where the main mineralization is mostly related to the intermediate stage (Fadlin et al., 2023;Sillitoe, 2012) which also confirmed by the BSE image analysis in this research (Fig. 6) Regarding the binary diagram of SiO2 vs. CaO+Na2O+K2O wt.% (Williamson et al., 2016) (Fig. 9), showing the trend is in line with the anorthite line, which can be indicated that most of the data, 'excess' Al of the plagioclase, may not result from alteration.At the same time, the data indicates that the plagioclase is related to magmatism processes, except for a few points in two data groups from the early and intermediate mineralization porphyries, which show partial to K-feldspar and muscovite alteration.In the HLE prospect, 'excess' alumina can be linked to high melt water contents (Fiedrich et al., 2018;Williamson et al., 2016).We propose that the magmatic plagioclase can serve as a guide to the ore in the porphyry Cu-Au system.By using fertility indicators from our results to plagioclase minerals, we can improve reconnaissance-stage exploration for porphyry Cu-Au prospects in greenfield or covered terrains along the Sunda arc, particularly in the eastern Sunda arc.Furthermore, plagioclase minerals analyzed following their compositions can be used to discriminate whether an area lacking geological information has un-prospective intrusions or prospective intrusions, thus indicating potential exploration targets.Furthermore, observing plagioclase 'excess' Al in surface or subsurface (drilled) calc-alkaline rocks is an affordable exploration method.

Conclusions
The excess of alumina (Al*) in mineralized/fertile samples of the HLE porphyry Cu-Au prospect shows exhibits more pronounced Al* than the infertile samples of Kelud, Mt.As 'excess' alumina of plagioclase can be linked to high melt water contents in the HLE prospect.The main mineralization zone, related to the intermediate intrusive, shows higher 'excess' alumina than early and late causative intrusive rocks.The excess alumina (Al*) applies to the magma fertility assessment of Cu mineralization in the Sunda arc.