Lithogeochemistry of Intrusive Rocks in the Halo Porphyry Copper-Molybdenum Prospect, Northeast Cambodia

The Halo copper-molybdenum prospect is a porphyry system in Ratanakiri province, northeastern part of Cambodia. There is only one research was car-ried out on this prospect about geological mapping and short wave infrared (SWIR) spectroscopy on alteration mineral identification. The purpose of this research is to confirm the deposit type from previous Angkor Gold’s report and find the centre of porphyry deposit based on characteristic of intrusive rocks at surface and subsurface, characteristics of the intrusive rocks and alteration lithogeochemistry of intrusive and volcanic rocks by using Pearce Element Ratio (PER) analysis. PER analysis was used to examine the nature and extend of the alteration halos in the porphyry Halo copper-molybdenum prospect. The intrusive rocks and volcanic rocks in Halo, range from diorite to granite (quartz feldspar porphyry) in composition as well as dacite to trachyandesite (andesite porphyry) in composition, respectively. They were formed in a subduction-related tectonic setting, likely a volcanic arc. Trace elements spider diagrams were normalized to primitive mantle display strong enrichment in large-ion lithophile elements such as Rb, Ba and K and depletion alteration zone is vectoring center of the hydrothermal system which may represent the locus of mineralization. Therefore the geochemical signature of potassic alteration within the quartz feldspar porphyry, andesite porphyry and granodiorite porphyry with high grad of copper ranges up to 2670 ppm and molybdenum ranges up to 5297 ppm represents a character for further exploration in the Halo porphyry copper-molybdenum prospect.


Introduction
The Halo porphyry copper-molybdenum prospect is a porphyry system in Ratanakiri province, northeastern part of Cambodia. The province has a potential for porphyry-type deposits such as porphyry copper-molybdenum (Halo prospect, China Wall prospect) and porphyry copper-molybdenum-gold (Okalla prospect) (Figure 1(a); [1]). The Halo porphyry copper-molybdenum prospect lies 2 km southeast of a strike slip fault trending NE-SW, known as the Phum-Syarung-Dok Yong Fault corridor. The Halo prospect is hosted by intermediate to felsic intrusive and volcanic rocks. However, detailed geochemical characteristics of rocks and alteration lithogeochemistry were not studied yet. This is the first study that focuses on the characteristics of the intrusive rocks (major elements, trace elements, and rare earth elements) to constrain petrogenesis and tectonic setting, and lithogeochemistry of intrusive and volcanic rocks to demonstrate elements transportation during hydrothermal alteration at the Halo prospect. The main purpose of this study is to confirm the deposit type based on the datafrom by present study and to find the centre of porphyry deposit in the Halo prospect.

Regional Tectonic Setting and Ore Deposits
Cambodia is located in the southern part of Indochina Terrane, mainland of Southeast Asia. The mainland SE Asia comprises several Gondwana-derived terranes including Indochina, South China, Sibumasu and West Myanmar Terranes, which assembled and amalgamated by subduction-collision processes during the Late Palaeozoic to Mesozoic (Figure 1(a); [2]). The Indochina Terrane is made up of several tectonic units, which host mineralization belts such as Truong Son Fold Belt (TSFB), Loei Fold Belt (LFB), Dalat-Kratie Belt (DKB) [3]. The Halo porphyry copper-molybdenum prospect in northeastern Cambodia lying on the DKB consists of Triassic to Cretaceous sedimentary rocks intruded by Cretaceous (125 -75 Ma) volcano-plutonic rocks, which are overlain by Quaternary intraplate basalts. The DKB extends across the region from Cambodia to southern Vietnam and overlies the southern continuation of the TSFB and LFB ( Figure 1(a); [4]). The Cretaceous belt is chronologically comparable to the plutono-volcanic rocks of the late stage of the Yanshanian Orogeny in SE China (140 -65 Ma) [5]. Thus, this belt could have formed in a similar tectonic setting as a southern continuation of the Yanshanian belt, which was formed by the subduction of the Palaeo-Pacific Plate beneath the Eurasia Plate (including the Indochina Terrane) ( [5] & [6]). The ore deposits in the DKB are intrusion-related gold (Figure 1(a); e.g. Okvau, Snoul in Cambodia [7] and TienThuan in southern Vietnam [8]), porphyry copper-gold, skarn lead-zinc, sediment-hosted gold (e.g. North Kratie in Cambodia; [7] & [9]), and porphyry copper-molybdenum, gold-silver epithermal veins over-printing porphyry copper-molybdenum-(gold) (Canada Wall and OkallaEast in Cambodia; [1]) systems.

The Halo Porphyry Copper-Molybdenum Deposit
The Halo prospect is located on the edge of a monzogranite pluton to the south which is covered with a prominent silica cap outcrops on hilltops in the area.
The host rocks of the Halo prospect are composed of felsic and intermediate intrusive and volcanic rocks. The volcanic rocks were intruded by diorite, granodiorite, and a quartz feldspar porphyry stock, which brought copper and molybdenite mineralization in veins and veinlets in both the intrusive and volcanic units [1]. The quartz feldspar porphyry exposed on the surface is medium to coarse-grained with abundant disseminated sulfides and appears to be the intru-  [10]. Field observations of the core indicate that pyrite, molybdenite and chalcopyrite veinlets and veins exposed on surface exposures continue to the bottom of drill holes HD1 and HD2 ( Figure   1(b)). Drill hole HD1 shows appreciable copper and silver mineralization in the top 99 metres, with anomalous levels of molybdenum corresponding to disseminated sulfides observed in the same interval. Included in the 99m interval is an intercept of 2345 ppm Cu, 1.34 g/t Ag, as well as 261.4 ppm Mo over 88.9 m which includes 7.9 m of 8043 ppm Cu, 2.24 g/t Ag, and 320.7 ppm Mo from 10.1 to 18.0 m [11].
A major magnetic, concentrically ringed "doughnut" anomaly is present in the central part of the Halo porphyry copper-molybdenum prospect [10]. This magnetic anomaly is a typical expression of a porphyry system with a magnetic high in the center surrounded by a zone of lower magnetic response. The high magnetism typically represents an increase of the abundance of magnetite associated with the mineralization in the centre of porphyry type deposit, with surrounding low magnetism which reflects a depletion of the abundance of magnetite in the mineralized host rocks [12]. Termite mound geochemistry revealed anomalies of copper and molybdenum at the central part of Halo (Figure 1 with the high up to 700 ppm Cu while the anomaly is generally >250 ppm Cu,

Analytical Methods
Whole-rock major elements oxides, trace and rare earth elements concentrations of 42 samples including diorite, granodiorite, quartz feldspar porphyry, hornblende diorite collected from surface and four drill holes HD1, HD2, HD3 and HD4 (Figure 2(b)) were analyzed by X-ray fluorescence (XRF) spectroscopy using a RIGAKU RIX-3100 and by inductively coupled mass spectrometry (ICP-MS) Alilent Technologies 7500, respectively, at the Center of Advanced Instrumental Analysis, Kyushu University. Loss on ignition (LOI) was determined by heating the samples at 1000˚C for 2 hours to determine relative weight loss.
Thirty petrographic thin sections were prepared to identify textural characteristics and alteration minerals of rocks and observed using a Nikon Eclipse E600  POL microscope equipped with an AdvanCam-U3II camera. Both bulk and clay fraction X-ray diffraction analysis were conducted to identifyalteration mineral assemblage using a RigakuUltima IV X-ray diffractometer at Department of Earth Resources Engineering, Kyushu University.

Lithology in the Halo Prospect
In order to determine textural and mineral composition of each rock type, petrography was conducted on the relatively fresh samples, least altered samples and also altered samples including outcrop samples and drill core samples.

Outcrop Samples
In the central part of Halo, diorite, granodiorite, andesite and rhyolite were cut by faults trending NE-SW. In the southwestern part of Halo, andesite, rhyolitic tuff, granodiorite and syeno-granite were cut by strike slip fault trending NE-SW, known as PhumSyarung-Dok Yong Fault (Figure 1(b)). Diorite is dark-gray to greenish light gray in color and medium-grained, and consists mainly of plagioclase (up to 1.2 mm across), quartz (less than 1mm across), K-feldspar (up to 0.8 mm long) and biotite (less than 1 mm across). Granodiorite is dark-gray to greenish light gray in color and medium-grained, and consists mainly of anhe-

Drill Core Samples
Drill hole HD1 is composed mainly of dacite and andesite which were cut by numerous pink porphyry dykes (quartz feldspar porphyry), and fine-grained Drill hole HD2 consists of diorite, granodiorite porphyry with patches of phyllic and weak potassic alteration. Mineralization in this drill hole occurs as quartz-molybdenite veins, quartz-pyrite-chalcopyrite veins, magnetite veinlets, quartz-chalcopyrite-pyrite-magnetite vein, pyrite veinlets and dissemination of pyrite and chalcopyrite. The granodiorite porphyry and the diorite were cut by fine mafic dykes, andesitic dykes, aplite dykes, and rhyolite dykes. The granodiorite is a dark-gray, pinkish dark-gray, pinkish light green in color, and medium to coarse-grained and consists of euhedral to subhedralphenocrystic plagioclase (up to 7 mm across), phenocrystic quartz (up to 1.6 mm), K-feldspar (less than 0.3 mm), biotite (less than 1.4 mm across), and hornblende (up to 1.2 mm long). Bitotite was partially altered to chlorite and secondary biotite ( Drill hole HD3 consists of sheared andesite at the top of the hole and transitioned to granodiorite cut by fine-grained mafic dykes. Alteration consists of phyllic and prophylitic alteration. Mineralization occurs as quartz-molybdenite veins, quartz-pyrite-chalcopyrite-molybdenite veins, quartz-magnetite-chalcopyrite-pyrite vein, quartz-pyrite-chalcopyrite-sphalerite-galena veins and disseminated pyrite and chalcopyrite. The granodiorite is light pinkish green in color, and medium-grained, and consists mainly of euhedral to subhedralphenocrystic plagioclase (up to 2.4 mm), phenocrystic quartz (up to 1.4 mm), and groundmass mainly of K-feldspar (less than 0.2 mm) (Figures 3(G)-3(g)).
Drill hole HD4 is dominated by unaltered hornblende granodiorite and hornblende diorite, and transitioned into propylitic and phyllic altered diorite cut by massive polymetallic veins consisting dominantly of pyrite, quartz-cha-Open Journal of Geology lcopyrite-sphalerite-galena veins and quartz-anhydrite-pyrite veins. The hornblende granodiorite and the hornblende diorite were cut by rhyolite dykes, and mafic dykes. The hornblende diorite is dark-gray in color, and medium-grained, and consists mainly of subhedral to euhedral plagioclase (up 2 mm across), euhedral to subhedral hornblende (up to 1.5 mm long), minor quartz (less than 1.2 mm across) that fill interstices between hornblende and plagioclase, and trace amount of biotite flakes (less than 0.8 mm across). The hornblende granodiorite is light pinkish gray in color, and medium-grained, and consists mainly of subhedral to euhedral plagioclase (up to 1 mm across), quartz (less than 1 mm across), K-feldspar (less than 0.6 mm long), euhedral to subhedral hornblende (up to 0.8 mm long). Some plagioclases were altered to epidote

Whole-Rock Major and Trace Elements Geochemistry
Whole-rock compositions of 35 intrusive rocks and 7 volcanic rocks from the Halo prospect (Location of sample in Figure 1 and Figure 2) are determined in Table 1. The effects of hydrothermal alteration on whole-rock geochemical compositions have been assessed using a combination of petrography and bivariate plots (Figure 4 LOI (loss on ignition) values of least altered rocks range from 1.5% to 5.6%, while LOI values of the altered rock range from 1.3% to 4.8%. Thus the LOI values may misclassify between least altered and altered sample in this study. However, the result of least altered rocks classified by box plot [13] is consistent with the petrography. The SiO 2 concentrations of intrusive rocks and volcanic rocks in the Halo prospect range from 57.7 to 75.6 wt.% and 61.7 to 68.2 wt.%, respectively. The (Na 2 O+K 2 O) versus SiO 2 diagram [14] indicates that the intrusive rocks in the Halo are classified as diorites, granodiorites and granites (quartz feldspar porphyry) (Figure (4b)). The high field strength element (HFSE) bivariate plot of Zr/TiO2-Nb/Y [15] is used to distinguish geochemically the various volcanic rocks from the Halo prospect ( Figure 5 [16]. Abbreviation: DC1and ANP1: dacite and andesite porphyry, respectively, in drill hole HD1 and other symbols are the same as in Figure 4.
plots. Therefore, major elements are deemed unreliable to classify the rock types as strongly altered.
On the SiO 2 versus K 2 O discrimination diagram for intrusive and volcanic rocks ( Figure 5(b)), intrusive rocks are medium-K calc-alkaline, high-K calc-alkaline and shoshonitic fields [16]. The samples with high-K values contain hydrother- Open Journal of Geology mal K-feldspar and biotite, implying that K was added to the rocks during hydrothermal alteration (potassic alteration). Thus, it is interpreted that the high-K trend is a result of potassic alteration and does not reflect primary igneous composition (e.g. shoshonite). The least altered granodiorite (GRDP2) plots in high-K calc-alkaline to shoshonite. Therefore, the primary igneous compositions of rock belong to the high-K calc-alkaline series. The least altered diorite and granodiorite exposed on surface plot in medium-K to high-K calc-alkaline field.
Thus the primary composition of diorite and granodiorite belongs to medium-K calc-alkaline series. Diorite and granodiorite (DI3 and GRD3) plot on boundary between medium-K and high-K calc-alkaline field. Least altered hornblende diorite (HDI4) plots in medium-K calc-alkaline field except one sample with strong alteration plot in low-K calc-alkaline filed. The primary igneous compositions of the hornblende diorite belong to the high-K calc-alkaline series. In contrast, low-K composition is also affected by hydrothermal alteration. The granodiorite (HGRD4) plots on boundary between medium-K to high-K calc-alkaline, is close to primary igneous rock compositions. Based on the box plot and SiO 2 versus K 2 O (wt.%) diagram, some samples are strongly affected by K mobility. Therefore, we further use Pearce element ratio (PER) analysis to discuss in next section in order to elucidate about alteration lithogeochemistry [17].

Molar Element Ratios
General element ratios (GERs) and Pearce element ratios (PERs) are both molar element ratios (MERs) used to depict geochemical processes such as alteration ( [17] & [20]). PER analysis uses molar element concentrations ratio to the molar concentration of an element which has remained unchanged during mass transfer processes, i.e. a "conserved" element ( [21] & [22]). Thus in order to quantify the intensity of hydrothermal alteration around major mineralized zones, it is necessary to use a technique that distinguish the metasomatic impact of the mineralizing fluids, a prerequisite for which is pre-existing geochemical heterogeneity in the host rocks. It is also necessary to discriminate between the effects of hydrothermal activity and unrelated events such as weathering or metamorphism. A conserved element must be identified for use as a denominator in the PERs. Conserved elements are also used to test the cogenetic character of the rocks and group them accordingly. A PER diagram of Pr versus Nd shows the same behavior during alteration and a best-fit line with a positive slop that intersects the origin indicating that the rocks are derived from a single precursor (Figure 7(a); [23]). The samples are cogenetic, thus PER can be adapted [22]. A PER plot of (2Ca + Na + K) versus Al/Nd (Figure 7(b)) discriminates between unaltered and hydrothermally altered felsic rocks. The least altered samples plot closer to the feldspar-plagioclase control line with slop of 1 from the origin while the completely sericitized samples plot closer to the line with slop of 1/3 from the origin [24]. The dacite samples plot around and toward the muscovite control line with slop 1/3, suggesting the dacite has been affected by quartz-sericite-pyrite (phyllic) alteration (Figure 7(b)) while the quartz feldspar porphyry, andesite porphyry, diorite and granodiorite samples plot between the plagioclase and the muscovite control lines, suggesting that these rocks have been affected by varying degree of sericite alteration. This is consistent with plagioclase replaced by sericite observed in petrography. Potassic alteration such as secondary biotite or secondary K-feldspar in rocks is not discriminated from unaltered rocks on this diagram. Epidote alteration is represented by the control line with slope 16/3. The hornblende diorite samples plot between the epidote and plagioclase control lines consistent with petrography and XRD analysis.

Genetic Implication
The enrichment of LILEs and depletion of Nb and Ti are characteristic feature of magmas generated in a subduction-related tectonic setting ( [25] & [26]). The low Nb depletion is typical of calc-alkaline magmatic rocks formed in subduction zone environments and may be regarded as an indicator of crustal involvement in magmatic process ( [27] & [28]). Moreover, in the (Y + Nb) versus Rb, and Y versus Nb diagrams (Figure 8; [29]), samples plot within the field of volcanic arc granite. It is consistent with the lithochemistry of porphyry copper-(molybdenum-gold) deposits formed by magmatic-hydrothermal fluids generated from subduction related magmatism.

Tectonic Affinity
The fresh and least-altered diorite and granodiorite were selected to discriminate between typically arc rocks and arc adakite. In the commonly used Sr/Y versus Y diagram, the diorite and granodiorite dominantly plot in typically arc ( Figure  9(a); [30]). This result is consistent with relatively unfractionated HREE patterns of the samples plotted in Figure 6, which implies that partial melting occurred at a depth above the garnet stability field that would be shallower than the subducted slab. Furthermore, based on the (La/Yb)N versus YbN diagram, the diorite and granodiorite plot in the field of post-Archean subduction-related granitoids (Figure 9(b); [31]). This is in the good agreement with the conclusion of [32] who assumed that the Southeast Asian margin was an Andean-type volcanic arc from mid-Jurassic to mid-Cretaceous. Open Journal of Geology  [31]. Symbols are the same as in Figure 4.

Alteration Lithogeochemistry
In the PER diagram (2Ca + Na + K)/Nd versus Ca/Nd (Figure 7 Pearce Element Ratios (PERs) diagrams can discriminate between the effects of hydrothermal activity and unrelated events such as weathering or metamorphism. Moreover, these diagrams also can determine the alteration index (AI) by identifying the slop of the line connecting the sample point and its origin. AI is defined by dividing the ordinate value (y-axis) with absicca value (x-axis) [24].
AI values were rescaled so that a value of 1, representing AI of an unaltered samples as 0%, and AI of totally altered rocks as 100%. AI of the quartz feldspar porphyry and the andesite porphyry associated potassic alteration ranges from 11.8% to 32.3% and that of the dacite associated with phyllic alteration ranges from 43.3% to 66.6% while that of two samples of granodiorite porphyry ranges from 5.1% to 11.9%. The highly altered zones are commonly vectoring towards the center of the hydrothermal system which can represent the locus of mineralization (e.g. [22] & [24]).

Conclusions
1) The Halo porphyry copper molybdenum prospect is hosted in diorite, granodiorite, quartz feldspar porphyry, granodiorite porphyry, andesite porphyry, mafic dykes, aplite dykes, and felsic dykes. Quartz feldspar porphyry and granodiorite porphyry are considered as the main intrusive body and most favorable host copper-molybdenum mineralization. They intersected drill hole HD1 and HD2, which are associated with potassic alteration and dominated by Cu-Fe sulfide mineralization, quartz vein stockworks and magnetite veins.
2) The diorite and granodiorite belong to medium-K calc-alkaline to high-K calc-alkaline series while rock-series of volcanic rocks and quartz feldspar porphyries were not determined due to the mobility of K during alteration.