Geological Prospection of Placer Chromium Deposits in the Waropen Regency—Indonesia (New Guinea) Using the Method of Indicator Minerals

Indicator minerals (IM) have been used in the research of gemstone and base metals for over 100 years. IMs are a main source of information about the occurrence of deposits, hydrothermal and weathering changes, as well as bedrock source and type. Since 2013, base metal mineral exploration has taken place on New Guinea Island (the Indonesian part). The analysis of chromian spinels as IM in beach and river sediments led to the discovery of rich chromian-bearing deposits in the Botawa River sediments. The dominant detrital minerals include chromian spinels, olivine, pyroxenes and serpentine. The source of chromian spinels, olivine and pyroxenes are most likely peridotites from the Earth’s mantle zone of the ophiolite series, generated under supra-subduction conditions. The terrigenous deposits contain metamorphic minerals, such as staurolites, andalusites, epidotes, amphiboles and fragments of micas, quartz and chlorite schists. Using ICP-MS analysis of the beach sands, the concentration of Cr2O3 was determined to be at the level of 1.17%, while in the heavy minerals concentrate from the Botawa River sediment, the content of Cr2O3 amounts to 24.83%. The authors conclude that the west parts of the Van Rees Mountains are the probable parent rocks for chromium-bearing sediments.


Introduction
Indicator minerals (IM) are minerals that provide important information about potential occurrences of deposits [1], weathering and hydrothermal changes, as well as the source and type of parent rock [2,3]. The IM are macroscopically recognizable and characterized by high density and physical resistance, silt and sand size and are transported over great distances [2]. Geologists have been using IM in the mineral exploration of copper ores [4,5], chromium ores [6], as well as platinum [7,8], gold [9] and gemstones [10,11] for a long time. The presence of IM in the alluvial and sea deposits allows recognizing the type of mineral deposits on a regional scale [12].
The gradual depletion of mineral resources entails conducting extensive geological prospection using both the greenfield and brownfield exploration. One of the most promising world regions with the high geopotential is the area of the Malay Archipelago (Indonesia) situated in the south-east edge of the Eurasian Plate. High seismicity (caused by subduction processes) and volcanism are present in this region [13]. The basement of the west part of the Malay Archipelago is represented by the continental crust, while the east part is dominated by the island arc, ophiolite sequences and younger ocean basins [13]. The deposit occurrences in Indonesia are strongly connected to subduction phenomena, acidic and middle volcanism in the island arches as well as with ophiolite complexes [14]. On Indonesian islands, chromites occur in two types ore deposits [15]-primary deposits of podiform in ophiolite series (South Kalimantan Province-Tanah Ambungan; South Sulawesi Province-Baru; Papua-Jayapura) and placer deposits (North Maluku-Poso; Central Sulawesi-Bungku).
The region with a high mineral potential is the Waropen Regency located in the northern part of New Guinea Island and the adjacent Cendrawasih Bay (Figure 1). The complicated tectonic history of this region (still insufficiently researched) and the confirmed presence of many mineral deposits motivates mining companies to further onshore and offshore exploration on New Guinea Island. Since 2013, a team of Polish geologists [32] has conducted geological prospection in Indonesia and on New Guinea Island. During the geological reconnaissance in the Waropen Regency (Figure 2), samples of beach sediments and alluvial deposits from the Botawa River were collected. Their mineral and chemical composition was determined. Heavy minerals fraction obtained from collected samples consists of characteristic minerals for podiform-type chromium deposits. The rich concentration of chromium spinels indicates the close location of parent rocks. The detailed results of geological prospection in the Waropen Regency are published here for the first time. The main purpose of this research is to identify the rock source and genesis (types of magmas from which this source was formed), as well as the search for chromian spinels placer ore deposits.

Materials and Methods
The geological prospection was carried out along an approximately 8 km long stretch of the Waropen Regency coastline, between the 136°25′27 "E meridian and 136°21′56 "E meridian. The research was conducted in two regions-North and South. Eleven samples ( Figure 2 and Figure 3) of contemporary beach sediments were collected from a depth of 10 to 15 cm b.g.l. (below ground level). Additionally, 5 pits (red stars, Figure 2) in the alluvial deposit of the Botawa River to a depth of 90 cm were excavated and 5 samples of heavy minerals concentrate (obtain by panning) were collected from there. The weight of extracted samples ranged from 50 to 100 g. Using acetic acid 10%, biogenic elements (shell fragments, foraminifera and snails) were removed. Sieve analysis of the grain material was carried out with sieves with a mesh diameter ranging from 2 mm to 0.063 mm. Open source GRADISTAT v.5.11 program was used to perform statistical calculations [36]. Statistical parameters of samples were calculated using the Folk&Ward method [37] (ϕ method). Heavy fraction (from 0.5 to 0.063 mm) has been separated from sediments, using sodium polytungstate of 2.89 g/cm 3 density. The heavy minerals' thin sections were embedded in epoxy resin. ECLIPSE E600 POL polarizing microscope and a scanning electron microscope FE-SIGMA VP (Carl Zeiss Microscopy Ltd., Cambridge, UK) with two detectors EDS (SDD XFlash | 10) (Bruker, Germany) were used to observe mineral concentrates. The research was done with the acceleration voltage of 25 kV, in a high vacuum. An electron microprobe CAMECA SX-100 (Cameca, Cedex, France), equipped with wave-length dispersive spectrometers (WDS), with the acceleration voltage of 15 kV and current beam of 20 nA, was used for point analysis of minerals' chemical composition. Samples for SEM-BSE (Scanning Electron Microscopy-Backscattered Electron Mode) and EPMA (Electron probe microanalysis) analyses were coated with carbon. The EPMA analysis was performed on crystals TAP (thallium acid phthalate), LIF (lithium fluoride), PET (pentaerythritol) and LPET (large pentaerythritol). Reference material, detection limits and standard deviation are presented in Table 1. The research took place in the laboratories of the Department of Geology at the University of Warsaw.
The chemical analysis of bulk samples was performed in a certified laboratory Bureau Veritas (Canada). Two analytical programs were used-LF202, where samples were melted with Na2B4O7/Li2B4O and then dissolved with a mixture of aggressive acids and FA330 program for noble metals analysis with Fire Assay methodology (www.acmelab.com). The samples were tested using ICP-MS (Inductively coupled plasma mass spectrometry) and ICP-ES (Inductively Coupled Plasma Emission Spectroscopy) analyzers.
The geological structure of the Waropen Regency and Cendrawasih Bay is still poorly known (Figures 1,2). Contemporary geotectonic interpretation of the region indicates that this area is placed in the eastern part of the Melanesian collision arc [46] that was formed above the subduction zone [42], [47]. Structurally, the Waropen region (Waropen Basin) and the adjacent Cendrawasih Bay are parts of the large North Basin (Irian). The history of the basement of the terranes still remains unsolved. Analysis of gravimetric anomalies indicates that the basement of sedimentary basin is represented by ocean crust rocks (ophiolite, diorite intrusions) and is covered with a sediment sequence about 6.5 km thick ( Figure  4) [33][34][35]. The oldest analyzed sediments in this sedimentary basin are Paleocene and early-Oligocene rocks from the Awuewa formation. This sequence is discordantly covered by Miocene-Pliocene limestone of the Darante formation [34]. The consequence of the change in the depositional conditions was the development sedimentation of the Makatas formation. The rapid basin subsidence led to a discordant sedimentation, which occurred in almost the whole North Basin. The sediments of the Mamberamo formation are a result of erosion. It is composed of Pliocene-Pleistocene regressivetransgressive sediments that are regionally differentiated (four parts of the Mamberamo formation). The uppermost series of cluster sediments are Neogene deposits assotiated with a system of inflow cones and deltas [34]. Dickinson [48] suggests that the formation of the North Basin and the deposition of cluster deposits is a result of rapid subsidence of the structure (fore-arc basin) due to compression forces caused by the collision of the Indo-Australian and Pacific plates.

Mineralogy and Geochemistry of the Beach Sediments
The macroscopic observations of samples allowed evaluating and classifying the deposit by the color, grain size and the organic components' occurrence. The sieve analysis ( Table 2) has shown that the beach sediments are sands (10 samples) and muddy sand (sample NMI 0115), grey and blackishgrey in color and moderately to well sorted. Skewness for the analyzed samples indicates a negative skew distribution and leptokurtic measure of flatness. Sand from sample NMI 0119 is the only poorly graded one. The heavy minerals concentrate samples have not been included in the sieve analysis. In the beach sediments, biogenic fragments of marine organisms were found (from 2.5 wt% to 23 wt%)mainly shells, corals, foraminifera and snails. All samples have a polymineral composition. On the Southern Region a beach sediment ( Figure 3A,B) contain quartz, potassium feldspar, plagioclase, pyroxenes, amphiboles, epidotes, garnets, andalusite and illite, muscovite, biotite, serpentine, lyssarite (serpentine subgroup), zircon, monazite, xenotime, chromium spinel, magnetite, rutile and ilmenite. In the samples from the Northern Region are dominate mainly ( Figure 3C,D): diopside, hornblende, riebeckite, magnetite, chromian spinel, olivine, serpentine subgroup, albite, sanidine, biotite and quartz. Samples from alluvial sediments of the Botawa River have a similar mineral composition. Based on microscopic analyses, two generations of grains with different levels of roundness have been separated: 1) moderate and well-rounded, discoidal and spherical grains ( Figure 3) poorly rounded ( Figure 5A), automorphic and elongated grains ( Figure 5A,B). The samples contain numerous fragments of metamorphic rocks: chlorite, quartz and mica schists. The content of heavy minerals in the samples (0.05-0.063 mm fraction size) varies from 2.4 wt% to 29 wt%. The content of heavy minerals in 0.25 mm fraction in Northern Region is 34 wt% on average, while in the Southern Region equals 14 wt%.  The chemical composition of beach sediments (Table 3) is relatively homogenous. The contents of Cr2O3 are high, ranging from 0.01 wt% to 1.17 wt%. In the samples from the Northern Region, only higher contents of CaO, from 2.74 wt% to 8.63 wt% are noted, while in the Southern Region, it is at the level of 1.29 wt% to 5.42 wt%. The loss on ignition (LOI) in the samples is very high, measuring from 6.5% to 12.8%. The highest concentration of trace elements in the samples is found for Ni, from 292 to 556 ppm, as well as for V 72-254 ppm. Lower contents were found for Zn 73-181 ppm, Co 24.1-55 ppm, Pb 5.7-21 ppm and Cu 5-12.6 ppm. The Fire Assay Analysis (FA330-Fire-Assay Analysis) results show very low contents of noble metals in the beach sediments. Usually, they are below the detection limit of the method used (ICP-MS). The maximum contents for Pt are up to 6 ppb, Pd up to 2 ppb and Au up to 4 ppb. The higher concentration of Ag up to 6.1 ppm is noted in the samples.

Mineralogy and Geochemistry of Alluvial Sediments from Botawa River
Mineral analysis of the heavy minerals concentrate from the Botawa River sediments shows the linear growth of metals contents in terrigenous sediments, from the coast to the mountain source area (Table 4). There are abnormally high contents of Cr2O3 up to 24.83% ( Figure 6), TiO2 up to 3.93%, Ni up to 492.6 ppm and V up to 2026 ppm. Heavy minerals concentrate is composed of chromite, magnetite, ilmenite, pyroxenes, amphiboles, olivine and secondly quartz, plagioclases and chlorite. In sample WAR 31, a very high content of noble metals is revealed: Au up to 3.05 ppm, Pd up to 11 ppb and Pt up to 8 ppb.

Chromian Spinel
Chromian spinels occur in all grain fractions of the beach sediments. The dominant grain class is the 0.5-0.063 mm fraction. The minerals rarely form isometric crystals. Normally, they are allotriomorphic. Chromian spinels occur in paragenesis with magnetite and serpentinized olivine. Some of the grains were transformed during the post-magmatic processes in low-grade metamorphic processes related to serpentinization ( Figure 7A). The micro-area analysis (EPMA) of minerals shows significant differences in the chemical composition (Table 5). Chromian spinels form a continuous spinel-magnesiochromite-chromite series (Figure 8). Magnesiochromite is the dominant group and the differentiation of Cr, Al and Fe contents is visible in it. Two groups of chromian spinels have been separated based on changes in the Cr# index Cr# = 100 × Cr/(Cr + Al) [49]: high-Al spinel (Cr# 20-60) and high-Cr spinel (Cr# = 60-80).  The Cr2O3 content in the high-Cr spinels varies from 46.03 wt% to 61.79 wt%, with high Cr# 61-82, contrary to high-Al spinels where Cr2O3 is from 29.34 wt% to 46.69 wt% and Cr# 33-59. All spinel grains contain a low amount of TiO2 < 0.6 wt%. High-Al chromian spinels show a wide range of chemical composition-MgO (8.75 wt%-17.46 wt%), FeO (13.31 wt%-29.54 wt%), MnO (0.11 wt%-0.42 wt%). Almost similar contents are present in the high-Cr spinels: MgO (6.37 wt%-14.10 wt%), FeO (13.70 wt%-27.69 wt%), MnO (0.19 wt%-0.47 wt%). In Cr-and Al-spinels, there is enrichment in transition metals: Co, Ni, Zn and Cu (Table 5). Microscopic observations (SEM-BSE) have revealed the presence of olivine and chromite ( Figure 7B) intergrowths but it has not revealed the alloys of noble metals.

Olivine
The microscopic observations of beach sediments have allowed separating two groups of olivine: 1) yellow olivine grains, under the process or already after the process of serpentinization; and 2) green olivine grains without any visible changes. Those minerals dominate in the 0.5-0.063 mm grain class. The fraction >0.5 mm contains no olivine. The olivine serpentinization was accompanied by the development of a crack system, transformation of grains and a growth of new phases-magnetite. The micro-area analysis (

Ortho-and Clinopyroxenes
SEM microscopic observations indicate the presence of numerous pyroxenes in the beach sediments. Analysis of the chemical composition of minerals (Table 7) allowed separating augite, pigeonite, diopside and enstatite. An overwhelming majority of pyroxenes (about 85%) is composed of enstatite and about 12%-15% of diopside. Augite and pigeonite are quite rare (1%-2%). Pyroxenes occur in the 0.1-0.5 mm grain class. Usually, they form subhedral and anhedral crystals, without visible cleavage. They coexist numerously as a paragenesis with amphibole and ilmenite (Figure 9). A characteristic feature of these minerals is the jagged end of the crystal. Pyroxenes undergo slow serpentinization processes that result in the formation of serpentine minerals with a fibrous structure. The chemical composition of enstatite (En86.5-90.4 Wo0.1-4.50) is typical for minerals from this group, which occur in ultramafic rocks and contain SiO2 (54.22 wt%-57.29 wt%) and MgO (31.51 wt%-34.50 wt%). The content of Al2O3 is low (0.89 wt%-4.23 wt%). Additionally, NiO (0.01 wt%-0.15 wt%) and Cr2O3 (0.38 wt%-0.92 wt%) are present in small amounts. These metals commonly occur in magnesiumrich orthopyroxenes from ultramafic rocks [51]. The magnesium index of enstatite is Mg# 0.90-0.92.

Provenance Area
The collision of the Indo-Australian and Euro-Asian plates, formation of numerous subduction zones and related extension processes led to a series of changes on the regional level in SE Asia, including New Guinea Island [38,39,52]. Cenozoic tectonic collisions with island-arc terranes (Tosem, Amanab, Weyland, Gauttier, Cyclops) and closing of marginal basins led to the formation of numerous ophiolites in a supra-subduction environment [46,47,53]. The creation of the New Guinea ophiolites was associated with the formation of two unevenly-aged marginal basins in the zone surrounding the Indo-Australian Plate [52,53]. The development of the basins was determined by convergence processes between the Indo-Australian Plate (which was at first moving to the northeast and then to the north) and smaller oceanic plates: the Philippine Sea Plate and the Caroline Plate [39].
Ophiolite may represent a wide variety of lithological construction and geochemical development. Chromian spinels may be formed in depleted mantle section of ophiolites formed in supra-subduction zone (SSZ) environments due to partial melting and melt-rock reaction. Mantle sequences of many ophiolites contain podiform chromitites composed of magnesiochromite ± olivine ± other silicate phases [46,54,55].
In the Indonesian part of New Guinea Island, five zones with podiform-type ophiolite sequences were localized (Figure 1). Potentially, they could be a source of chromium and cobalt [15]. In the Waropen Regency, no similar deposit structures have been found until now. The Regency area is one of the least geologically known areas of New Guinea. It is due to the morphology of the terrain (strongly hilly area), lack of road infrastructure and rich and dense vegetation which makes it difficult to access particular locations.
The composition of contemporary terrigenous sediment is a result of mutual interactions between the parent rocks and factors acting during the sedimentary cycle [56][57][58]. The beach sediments from the Waropen Regency have polymineralic composition, which indicates a relatively mixed source area. The contents of heavy minerals in the samples reach 20%, showing high concentration of Cr2O3 up to 1.17 wt% and Ni up to 556 ppm. The morphology analysis of heavy mineral grains indicates two generations of different roundness: 1) moderate and well rounded, discoidal and spherical grains-amphiboles, pyroxenes, olivine, andalusite; and 2) poorly rounded, automorphic and elongated grains-chromite, magnetite, rutile and monazite. The spatial distribution and associated grain size and shape depends on the transport, distance, deposition conditions and the physical and chemical properties of minerals. As it is pointed out by Reference [56], olivine, pyroxenes and amphiboles are the least stable minerals, while ilmenite, rutile and chromite -potential indicator minerals -are more physically and chemically stable. The resistant nature of these minerals explains their accumulation in the coastal environment following long-term erosion of the parent rock [59].
Field and satellite analysis of the land surface structure (Figure 1), course and watercourses layout and water sections, indicate that rocks building the western part of the Van-Rees Mountains are the potential source of heavy minerals. The slope gradient direction in these mountains is NNW, similar to the direction towards which permanent watercourses are draining. The New Guinea rivers transport 1.7 × 10 9 t −1 of sediments into the World Ocean yearly [60]. The amount of incoming mass is spatially differentiated throughout the island. The rivers draining the northern and north-western part of the island are the source of sediments containing quartz, clay minerals and heavy minerals [60].
Fragments of metamorphic rocks are present in the beach sand sediments, mainly chlorite-, quartz-and mica-shales. Additionally, in the Botawa River, alluvial fragments of ultramafic rocks are present. The mineral composition of beach sediments from the Waropen Regency is analogous to the clastic material from the Jayapura region, New Guinea [61]. The original source of contemporary terrigenous sediments in the Jayapura Regency are rocks form the Cyclops Mountains terrane-a core composed of calcium-alkaline metamorphic rocks and an ophiolite sequence which was obducing on top. It is possible that the geologically poorly explored Van-Rees Mountains are part of a terrane or an accretionary wedge, unknown until now. According to Reference [34], the Van-Rees Mountains can form an element of a paleo-subduction complex (accretionary prism), separating the North Basin (into the fore-arc basin and back-arc basin) where chromite was generated under suprasubduction conditions.

Source of Indicator Minerals
Based on chromite chemical composition, a range of correlation was determined, which may help in establishing the character of parent rocks and the geotectonic environment. The content of TiO2 is a key indicator for the understanding of the geotectonic environment [62]. In spinels originated from dunites, TiO2 contents are very low and amount to an average of 0.11 wt%, for harzburgites-0.05 wt% and lherzholites-0.06 wt%. The source for chromite is most likely peridotites (Figures 10 and 11A,B) from the Earth's mantle zone, an ophiolite sequence formed under supra-subduction conditions ( Figure 11B). Chromian spinels that formed in the initial phase the forearc basin creation have geochemical indicators of boninites, while chromites that formed in the backarc basin show a geochemical composition similar to MORB [63]. The Al2O3 contents in spinels and pyroxenes are sensitive to the degree of melting rocks from the Earth's mantle. Systematic Al2O3 decrease in these minerals indicates an increase in peridotite impoverishment [64]. A dependence diagram between TiO2 wt% and Al2O3 wt% [62] reveals similarity/affinity of high-Cr spinels generated from the Ti-and Al-depleted magma in the fore-arc basin. Spinels enriched in aluminum, which are genetically related to the back-arc basin, show a geochemically different character. The Cr# versus TiO2 wt% dependence diagram ( Figure 12) indicates that chromite originates from impoverishment peridotites with a slight deviation towards MORB for Al chromite. The Cr/Fe ratio is in the range of 3.40-6.11, on average 4.45. The Cr/Fe ratio is less than 2.5 in most stratiform chromite ores but is high (2.5-4.5) in ophiolite chromite [16].   The EPMA studies of olivine have shown that it is represented by magnesium varieties with a high magnesium index (Mg# 90-92). High Mg# and very low NiO contents (0.29 wt%-0.45 wt%) show its geochemical similarity to olivine from dunites and harzburgites of the Cyclops Mountains [45], [61]. A similar relationship can be observed in pyroxenes, where the chemical composition is similar to that of pyroxenes from ultramafic rocks. The enstatite chemical composition is typical for orthopyroxene from ultramafic rocks. Enstatite show a low content of Al2O3 0.89%-3.03% and have admixtures of nickel (NiO 0.01 wt%-0.15 wt%) and chromium (Cr2O3 0.38 wt%-0.92 wt%). The magnesium index (Mg#) positively correlates with the increasing impoverishment of peridotites. The chemical analysis of the samples indicates that orthopyroxenes are associated with abyssal peridotites (ABP) and the supra-subduction zone in fore-arc basin (diagram according to [67], Figure  13).
High-Cr spinels from the Waropen Regency were most probably formed in the initial stage of the subduction process from chromium-oversaturated boninite magma, gradually changing their geochemical character towards MORB. The modification of chemical composition of Cr# and Mg# spinels may reflect a partial melting of original/primary peridotites, magma generation conditions in rock/melt reaction and the metasomatic process impact occurring in the mantle wedge [78,79].

Further Exploration and Research Destination
The geological prospection enables to identify the most promising area for mineral exploration. The examinations provided evidence for abnormally high concentrations of useful metals in the river samples-Cr2O3 up to 24.83 wt% and Au 3.05 ppm. With current knowledge, it is impossible to adopt reliable parameters of minimum average and marginal contents of useful components in the deposit (cut off). Based on the obtained results, it can be assumed that the parent rocks of metals are the rocks from the western part of the Van Rees Mountains complex. It was determined that the beach sediments are not promising as mineral deposits and further work should be focused on analysis of fluvial sediments. Future research should concentrate on identification of useful minerals and their technological properties, as well as on abundance of mineral resources. The Botawa River sediments show a high concentration of Cr2O3. Therefore, it is likely that further exploration areas located in the upstream part of the Botawa River will contain even higher amounts of chromian spinels. It may lead to the discovery of rich chromium-bearing placer deposits.
The chemical composition analysis of chromian spinels (Table 5) has allowed establishing the preliminary character and source of chromium-bearing rocks that were generated from initial rock melt from magma composed of boninite and MORB. According to contemporary geodynamic models of New Guinea [45,47,83], the northern surrounding zone of the Indo-Australian Plate was a part of the active edge, along with the subduction zone. As a result of structural changes, terranes were accreted and the ophiolites generation took place in the SSZ environment [45,53].

Conclusions
Geological prospection of the Botawa River sediments and beach sediments has demonstrated the presence of indicator minerals, such as chromite, magnesium olivine and pyroxenes, characteristic for chromium deposits of podiform type. The content of Cr2O3 increases linearly towards the potential parent rock source-the Van Rees Mountains. The concentration of Cr2O3 in beach sediments is up to 1.17 wt%, while in heavy minerals concentrate from the Botawa River it increases to 24.83 wt%. The Van Rees Massif may form a previously unknown terrane, which is linked structurally and geochemically to the Cyclops Mountains complex in the Jayapura region (New Guinea). It is certified by chemical analyses of chromium spinels, pyroxenes, olivines and the presence of fragments of metamorphic rocks-mica and chlorites shales. Detailed geochemical research of the complex will allow finding its more specific geotectonic position.
The chemical analysis of chromian spinels indicates their origin from depleted peridotites, formed in back-arc and fore-arc basins under supra-subduction conditions. Chromian spinels form a continuous spinel-magnesiochromite-chromite series. The analysis of pyroxenes indicates that their grains originate from ultramafic rocks. Chromium spinels need further detailed analysis to establish the conditions of melt generation, its changeability and the possibility of potential concentration of metals from the Pt group.
High concentration of Cr2O3 in the alluvial sediments may be a clue to finding a new potential source of chromite in New Guinea Island. To confirm this statement, it is necessary to conduct further detailed studies in the upstream Botawa River valley.
Funding: This research was supported by statutory funds from Polish Geological Institute-National Research Institute and Warsaw University.