Gold Distribution in Porphyry Copper Deposits of Kerman ... · Kerman region, which is known as the most important porphyry Cu-bearing metallogenic province in Iran [14,15]. The variations

Journal of Sciences, Islamic Republic of Iran 19(3): 247-260 (2008) http://jsciences.ut.ac.irUniversity of Tehran, ISSN 1016-1104

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Gold Distribution in Porphyry Copper Deposits ofKerman Region, Southeastern Iran

B. Shafiei1,* and J. Shahabpour2

1Department of Geology, Gorgan University of Agriculture Sciences andNatural Resources, Gorgan, Islamic Republic of Iran

2Department of Geology, Shaheed Bahonar University of Kerman, Kerman, Islamic Republic of Iran

AbstractGold contents of the Kerman porphyry copper deposits are generally low,

ranging from 0.010 to 0.190 g/t. The copper ores exhibit Cu/Au atomic ratiosmuch greater than 40000, and Au/Mo ratios below 30, which signifies them asgold-poor porphyry copper deposits. The gold-poor signature of the Kermandeposits complies with features such as higher contents of chalcopyrite and pyriteover bornite and magnetite in the hypogene ores, widespread phyllic and argillicalterations, weak correlations between Au and Cu, poor correlations between Auand Mo, and their association with adakitic-like intrusions derived from partialmelting of the lower crustal rocks at a thickened continental arc. These featuresreflect rather low temperature (less than 600°C) and low oxygen fugacity of theprimary ore-forming fluids involved in the formation of the Kerman porphyrycopper deposits, indicating the low solubility of gold in low temperature fluids incomparison with higher temperature (~700°C) and more oxidized fluidsresponsible for the formation of gold-rich porphyry copper deposits. Present studyindicates that a strong positive correlation exists between the gold and themagnetite contents of the porphyry copper deposits. These relationships indicatethat deposits with higher gold and magnetite contents (e.g. Sar Kuh, Meiduk andAbdar), were emplaced at higher crustal levels and formed from more oxidizedfluids in comparison to the gold-poor deposits (e.g. Sar Cheshmeh). Thesefeatures can be used as exploration tools for identification of the gold-richporphyry copper deposits in the porphyry copper metallogenic provinces.

Keywords: Gold; Porphyry Cu deposits; Kerman; Iran

* Corresponding author, Tel.: 09133432822, Fax: +98(171)4427040, E-mail: [email protected]

IntroductionMost porphyry copper deposits contain some gold as

by-product [1-6]. The average gold contents of the

porphyry copper deposits is rather low, between 0.012to 0.38 g/t [4,7]; however the huge tonnages ofporphyry deposits provide a sizeable resource of goldwith a significant economic value [5]. Several studies

Vol. 19 No. 3 Summer 2008 Shafiei and Shahabpour J. Sci. I. R. Iran

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have been carried out in the field of gold mineralizationin the porphyry copper deposits worldwide [1-4,6,8,9-12]. Sillitoe [3] suggested that the border line for Aucontent in gold-rich porphyry copper deposits is about0.4 g/t. Based on the Au/Mo ratios, Cox and Singer [4]divided the porphyry copper deposits into three groups:(1) porphyry Cu-Au deposits (Au/Mo>30); (2) porphyryCu-Au-Mo deposits (3<Au/Mo<30); and (3) porphyryCu-Mo deposits (Au/Mo<3). Kirkham and Sinclair [13]defined gold-rich porphyry copper deposits as thosewith Cu(wt.%)/Au(g/t) atomic ratios below 31,000.Based on the available data from many porphyry copperdeposits, Kesler et al. [6] suggested a Cu(wt.%)/Au(g/t)ratio of about 40000 as a boundary for discriminatingbetween gold-rich and gold-poor porphyry copperdeposits. Several studies indicated that the gold contentin porphyry copper deposits is controlled by factorssuch as tectonic setting, depth of emplacement, and theore-forming fluid characteristics [1-6,10-12].Thesefactors along with other characteristics such as the oretonnage, morphology, and the age of the ore depositshave been used to distinguish between different types ofporphyry copper deposits (e.g. porphyry Cu-Mo, andporphyry Cu-Au deposits).

The distribution of gold in the porphyry copperdeposits of Iran, and the factors controlling the goldcontents, are poorly understood. To this aim, weselected eight porphyry copper deposits from theKerman region, which is known as the most importantporphyry Cu-bearing metallogenic province in Iran[14,15]. The variations of Au, Mo and Cu are used toexamine the gold distribution and its metallogenicimportance in these deposits.

The Kerman porphyry copper region with severaldeposits such as Sar Cheshmeh, Meiduk, Darreh Zar,Now Chun, Sar Kuh, Dar Alu as well as many smallerand sub-economic reserves (Table 1) [15-22] have beendeveloped in a volcano-plutonic assemblage, 40-50 kmwide and over 400 km long [15,23-26] in southeasternsegment of the central Iran Cenozoic magmatic arc(Urumieh-Dokhtar belt) (Fig. 1). It is generally acceptedthat this magmatic arc has been generated in response tosuccessive stages of the Tethyan ocean closure duringPaleogene and a continent-continent collision in latePaleogene-Neogene between Arabian plate and CentralIran micro-plate [27-31]. Genetically, porphyry coppermineralization in the Kerman region is associated withMiocene- Pliocene porphyritic stocks [19-21,32,33],known as the Kuh-Panj type granitoids [23]. The ore-hosting intrusions are porphyritic in texture and vary incomposition from diorite to quartz diorite, granodiorite,and locally quartz monzonite [18,20,21,34]. The rocksare per-aluminous (Fig. 2a) and all belong to the I-type

magnetite granitoid series (Fig. 2b) [25,26]. TheKerman ore-hosting porphyries display geochemicalfeatures comparable to those of the adakitic rocks (Fig.2c), such as high Na2O contents (3.5-4.8 wt. %), high Sr(≥400 ppm), low Y (≤18 ppm), high Sr/Y (>40), strongREE fractionation (Yb=0.36-1.5 ppm;La/Yb=19-53),slightly positive Eu anomalies(Eu/Eu*≥1), and relativelynon-radiogenic Sr isotopic signatures (87Sr/86Sr=0.704253-0.704800) [25,26]. The ore-related granitoidsare characterized as orogenic granitoids rather thanfractionated granitoids (Fig. 2d), and mostly belong to apost-collisional environment rather than a pre-platecollision tectonic setting (Fig. 2e) [25,26]. This isconsistent with the lack of a subduction zone during theNeogene time beneath central Iran micro-continent [27-31]. The adakitic-like signatures displayed by the ore-related granitoids are suggestive of the presence of highpressure hydrous magmas derived from partial meltingof a source with residual amphibole+garnet typical ofcompressional tectonic regimes, and continental crust>40km thick, rather than subducting hot slab melting(Fig. 2f) [25,26].

Most of the porphyritic intrusions are associated withwell-developed potassic (K-feldspar and biotite), sodic(albite), sericitic, silicic, propylitic, and locally argillichydrothermal alterations [18,20,21,34]. The mineraliza-tion occurs as quartz stockworks, veins, anddisseminated sulfides in both the host stock and theolder volcanic and pyroclastic rocks [18,20,21,25,26,35]. The common hypogene ore minerals arechalcopyrite, pyrite, and subordinate molybdenite;bornite and magnetite are scarce. A common feature inthe Kerman porphyry copper deposits, is the occurrenceof mineralized and barren dykes [18,20,21] and brecciapipes [36]. The supergene oxidation and secondary Cu-enrichment occurred in all the deposits; but it is onlywell-developed in the giant and large deposits, such asSar Cheshmeh, Meiduk, and Darreh Zar.

Materials and MethodsFor the present study, a database, consisting of Au

(g/t), Mo (%), and Cu (%) assays for 106 hypogene oresfrom drill cores and outcrops of eight porphyry copperdeposits in the Kerman region was established (Table2). Large samples (~3 kg) from ores with potassic andphyllic alteration were prepared by jaw-crushing (≤12.7mm), followed by grinding in a Cr-steel ring mill (≤20µm) in the Sar Cheshmeh Copper Complex.Representative samples were analyzed for Au, Mo, andCu at the Central Laboratory of the Sar CheshmehCopper Complex. Gold was analyzed by GraphiteFurnace Atomic Absorption Spectroscopy technique

Gold Distribution in Porphyry Copper Deposits of Kerman Region, Southeastern Iran

249

Table 1. The summary of location, reserve, grade and size of some porphyry copper deposits in Kerman region

Ore deposit Geographic location Reserves and grades Ore deposit size ReferencesSar Cheshmeh Pariz area

29° 56′ N55° 52′ E

1200Mt,0.7%Cu, 0.03%Mo,

Giant size(~8,400,000t of fine Cu)

[14,17,24,66]

Meiduk(Lachah)

Shahr Babak area30° 20′ N55° 10′ E

150Mt,1.15%Cu

Large size(~1,725,000t of fine Cu)

[17,24,66]

Darreh Zar Pariz area29° 51′ N55° 53′ E

49Mt,0.64%Cu, 0.004%Mo

Medium size(~317,000t of fine Cu)

[17,24]

Now Chun Pariz area29° 55′ N55° 51′ E

80Mt,0.32%Cu


[17,24]

Iju Dehaj area30° 33′ N54° 57′ E

25Mt,0.46%Cu


[17,24]

Dar Alu Sarduieyh area29° 25′ N57° 06′ E

18Mt,0.47%Cu

Small size(~84,600t fine Cu)

[17,24]

Sar Kuh Pariz area29° 55′ N57° 46′ E

16Mt,0.46%Cu

Small size(~64,000t of fine Cu)

[17,24]

Bagh Khoshk Pariz area29° 33′ N55° 58′ E

24Mt,0.27%Cu

Small size(~64,800t containing Cu)

[17,24]

(GFAAS) with a detection limit of 0.01 ppm, after pre-concentration by a Lead Fire Assay technique (LFA)from 25 gr samples. Mo and Cu contents were measuredby Direct Current Plasma technique (DCP) with adetection limit of 5 ppm, and Atomic AbsorptionSpectroscopy technique (AAS) with a detection limit of10 ppm, respectively. Also, forty two samples frompotassic zones were examined for magnetite contents(wt.%), as an index for the oxygen fugacity of theprimary ore-forming fluids, by a magnetic scale(SATMAGAN analyzer, Model 135, Corrigan CanadaLtd.) after induction of electromagnetic field on samplecell at the Central Laboratory of Sar Cheshmeh CopperComplex. saturation magnetization measurements ofsamples were achieved through the amount of handle'smovement by the magnetic force applied on. Theobtained data from these movements were multiplied bythe coefficient obtained from the SATMAGAN analyzercalibration curve which was achieved throughmeasuring standard samples; and therefore themagnetite content of the samples was determined. Themagnetite content (wt.%) of these samples was alsomeasured by quantitative microscopic analysis (particlesize distribution) on powder samples at the Laboratoryof mineralogy in Sar Cheshmeh Copper Complex. Thevalues obtained from this method were close to thoseresulted from the SATMAGAN analyzer. Analyticaldata are presented in Table 2.

ResultsGold content in the hypogene-ore samples is

generally low, ranging from 0.010 to 0.190 g/t. Thehighest average gold content of about 0.09 g/t, wasobtained from Sar Kuh deposit (Table 3). Molybdenumcontents in the samples range between 0.0001% and0.196 % (Table 3). The Sar Cheshmeh samples showthe highest Mo contents. Distribution of the elements inthe variably altered and mineralized samples, indicatethat the highest gold contents occur in the potassic oreswhich are affected by strong phyllic alteration. Incontrast, the highest Mo grades are associated withpotassic alteration. There is a weak correlation betweenAu and Cu (r=0.3) in the ore samples from the potassiczone (Fig. 3); however, the Au contents show nocorrelation with Cu and Mo in the phyllic zone (Figs. 4and 5). The Mo values indicate a strong positivecorrelation with Cu (r=0.7) in the potassic zones, but donot show positive relationship with Cu in the phylliczones (Figs. 3, 4 and 5).

The Kerman porphyry copper deposits exhibit lowAu/Mo ratios (≤9); the highest values are obtained fromthe Sar Kuh deposit (27) (Table 3). The Cu/Au atomicratios are much greater than 40,000 (Table 3).Accordingly, the Kerman deposits are typical of gold-poor porphyry copper deposits. Furthermore, theporphyry copper deposits of the Kerman region mostly


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plot in the Au-poor (porphyry Cu-Mo or PCD-Mo) andthe Au-Mo bearing (porphyry Cu-Au-Mo or PCD-Au-Mo) fields of the Cox and Singer’s diagram (Fig. 6).They plot entirely below the lines of Au-rich porphyrycopper deposits, close to the porphyry copper depositsof the Chilean Andes and the North AmericanCordillera, as defined by Sillitoe [3] and Kirkham andSinclair [13] (Fig. 7). Exceptionally, Sar Kuh plotswithin the field of Au-rich porphyry copper deposits(porphyry Cu-Mo or PCD-Au) as defined by Cox andSinger [4] (Fig. 6). However, the mean gold content ofthe deposits (~0.19 g/t) is lower than the average goldcontent of the gold-rich porphyry deposits (≥0.4 g/t).This is due to the lower Cu and Mo contents compared

Figure 1. (a) Map showing the regional distribution of theUrumieh-Dokhtar magmatic arc and location of the Kermanporphyry copper region in southeast Iran, (b) simplifiedtectonic map of the Kerman porphyry copper region showingthe three main litho-tectonic zones, regional distribution ofgranitoid groups and porphyry copper deposits (modified fromRio Tinto Ltd. [15]; Dimitrijevic [23]).

with other porphyry copper deposits of the Kermanregion.

DiscussionThe reasons for the low Au-content of the Kerman

porphyry copper deposits are discussed below.

Ore-Forming Fluid Characteristics

In most porphyry copper systems, gold is closely

bA

/NK

A/CNK0.5 1 1.5 2

0.4

1

3

2

Peralkaline

Metaluminous PeraluminousGranodiorite

Diorite & Quartzdiorite

a K O (wt.%)

Na

O

0 1 2 3 4 5 6 70

1

2

3

4

5

6

7

I-type

A-type

S-type

Granodiorite


2

2(w

t.%)

c Zr+Nb+Ce+Y

FeO

t/MgO

50 100 10001

10

100

Orogenic Granitoids

Fractionated Granitoids

Granodiorite


500 1000 1500 2000 2500 300000

500

1000

1500

2000

2500

5Ca+

2Mg+

Al

4Si-11(Na+K)-2(Fe+Ti)

1-Mantle Fractionation2-Pre-Plate Collision3-Post-Collision4-Late-Orogenic5-Anorogenic6-Syn-Collision7-Post-Orogenic

e

Typical arc ornormal calc-alkaline rocks

Adakites

Sr/Y

Y (ppm)

0

20

40

60

80

100

120

140

0 10 20 30 40 50

SNP

SP

KDP IJP

ABDP

MPSCP

DAP

SKPDZP

KPP

GKP

RAMP

LZP

NP

d

Subducted oceanic crust-derivedadakites

Thickened mafic lowercrust-derived adakites

Sr/ Sr

eNd(

t)10

5

0

-5

-10

-15

-200.702 0.703 0.704 0.705 0.706 0.707 0.708 0.709 0.710

87 86f

IJP

MPSCP

KPP

Figure 2. Geochemical and tectonic classification of the Cu-hosting porphyries from Kerman region: (a) A/NK vs. A/CNKdiagram [A/NK=molar Al2O3 / (Na2O+K2O); A/CNK= molarAl2O3 / (CaO+Na2O+K2O)] [67], (b) - Na2O vs. K2O diagram[68], (c) Y vs. Sr/Y diagram [69] showing adakitic affinity ofthe Neogene Cu-hosting porphyries from the Kerman region.SCP= Sar Cheshmeh porphyry stock; MP= Meiduk porphyrystock; DZP= Darehzar porphyry stock; SKP= Sar Kuhporphyry stock; SP= Sara porphyry stock; IJP= Iju porphyrystock; SNP= Serenu porphyry stock; GKP= God-e-Kolvaryporphyry stock; KDP= Kader porphyry stock; NP= Now Chunporphyry stock; DAP= Dar Alu porphyry stock; ABDP=Abdarporphyry stock; LZP= Lalleh Zar porphyry stock; RAMP=Razi Abad-Madin porphyry stock; KPP= Kuh Panj porphyrystock, (d) FeOt vs. Zr+Nb+Ce+Y diagram [70], (e) 4Si-11(Na+K) -2(Fe+Ti) versus 5Ca+2Mg+Al [71], (f) Plots ofεNd vs. 87Sr/86Sr for ore-hosting porphyries from the Kermanporphyry copper belt. Fields for adakites is derived fromWang et al. [72].


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Table 2. Au, Mo, Cu, and Magnetite contents of ore samples from the porphyry copper deposits of Kerman region

SampleNo. Deposit Depth of sampling

(Bore hole/outcrop) Ore-type Cu (%) Mo (%) Au (g/t) Fe3O4 (%)

AB-1 Abdar BH-AB3;185-186m Potassic 0.23 0.001 0.040 0.69AB-2 Abdar BH-AB3;268m Potassic 0.20 0.010 0.050 0.81AB-3 Abdar BH-AB3;274m Potassic+Phyllic 0.32 0.040 0.090 0.86AB-4 Abdar BH-AB5;79.5-81.5m Phyllic+Argillic 0.31 0.014 0.080 -AB-5 Abdar BH-AB5;81.5-83m Phyllic+Argillic 0.28 0.015 0.070 -AB-6 Abdar BH-AB3;119-121m Potassic+Phyllic 0.29 0.012 0.070 0.75AB-7 Abdar BH-AB3;130m Potassic+Phyllic 0.31 0.008 0.060 0.79AB-8 Abdar BH-AB3;121-122.5m Potassic+Phyllic 0.36 0.011 0.090 0.86AB-9 Abdar BH-AB3;259-261.5m Potassic+Phyllic 0.35 0.009 0.090 0.94AB-10 Abdar BH-AB5;86-88m Phyllic+Argillic 0.30 0.009 0.080 -DA-1 Dar Aloo BH-D2;50-73m Phyllic+Argillic 0.50 0.010 0.030 -DA-2 Dar Aloo BH-D2;120-125m Phyllic+Argillic 0.70 0.009 0.040 -DA-3 Dar Aloo BH-D2;160-186m Phyllic+Argillic 0.14 0.012 0.010 -DA-4 Dar Aloo BH-D2;192-196m Phyllic+Argillic 0.17 0.001 0.020 -DA-5 Dar Aloo Outcrop Phyllic+Argillic 0.56 0.008 0.040 -DA-5 Dar Aloo Outcrop Phyllic+Argillic 0.49 0.006 0.040 -DA-6 Dar Aloo Outcrop Phyllic+Argillic 0.42 0.005 0.030 -DA-7 Dar Aloo Outcrop Phyllic+Argillic 0.53 0.004 0.040 -SR-1 Sara BH-49;514m Phyllic+Argillic 0.25 0.003 0.030 -SR-2 Sara BH-49;533-536m Phyllic+Argillic 0.39 0.003 0.040 -SR-3 Sara BH-49;553m Phyllic+Argillic 0.28 0.013 0.030 -SR-4 Sara BH-49;558m Phyllic+Argillic 0.40 0.007 0.050 -SR-5 Sara BH-49;562m Phyllic+Argillic 0.25 0.008 0.020 -SR-6 Sara BH-49;568m Phyllic+Argillic 0.59 0.001 0.040 -SR-7 Sara BH-49;575m Phyllic+Argillic 0.51 0.002 0.040 -SR-8 Sara Outcrop Phyllic+Argillic 0.36 0.004 0.030 -SR-9 Sara Outcrop Phyllic+Argillic 0.35 0.003 0.030 -SR-10 Sara Outcrop Phyllic+Argillic 0.34 0.002 0.030 -SK-1 Sar Kuh BH-S11;182m Potassic 0.21 0.004 0.050 0.97SK-2 Sar Kuh BH-S11;174m Potassic 0.21 0.003 0.060 0.98SK-3 Sar Kuh BH-S11;163m Potassic+Phyllic 0.41 0.003 0.190 1.86SK-4 Sar Kuh BH-S11;146m Potassic+Phyllic 0.36 0.002 0.110 1.28SK-5 Sar Kuh BH-S11;120m Potassic+Phyllic 0.36 0.002 0.070 1.42SK-6 Sar Kuh BH-S11;106m Potassic+Phyllic 0.41 0.002 0.060 0.93SK-7 Sar Kuh BH-S11;98m Potassic+Phyllic 0.39 0.002 0.070 1.48SK-8 Sar Kuh BH-S11;72m Potassic+Phyllic 0.33 0.003 0.070 0.99SK-9 Sar Kuh BH-S11;56m Phyllic+Argillic 0.29 0.003 0.060 -SK-10 Sar Kuh Outcrop Phyllic+Argillic 0.35 0.002 0.090 -MP-1 Meiduk BH-44;820-825m Potassic 0.60 0.008 0.060 0.86MP-2 Meiduk BH-44;733-739m Potassic+Phyllic 0.78 0.006 0.070 0.79MP-3 Meiduk BH-55;752-755m Potassic+Phyllic 0.85 0.007 0.050 0.80MP-4 Meiduk BH-55;698-702m Potassic+Phyllic 0.73 0.006 0.060 0.82MP-5 Meiduk BH-56;652-660m Potassic+Phyllic 0.68 0.007 0.050 0.83MP-6 Meiduk BH-56;714-720m Potassic+Phyllic 0.96 0.004 0.070 0.71MP-7 Meiduk BH-53;220-225m Potassic+Phyllic 0.74 0.006 0.050 0.72MP-8 Meiduk Outcrop Phyllic+Argillic 0.49 0.005 0.030 -MP-9 Meiduk Outcrop Phyllic+Argillic 0.41 0.004 0.040 -

MP-10 Meiduk Out crop Phyllic+Argillic 0.43 0.004 0.050 -SDP-1 Seridun BH-S1;220-234m Phyllic 0.25 0.010 0.020 -SDP-2 Seridun BH-S1;193-200m Phyllic 0.26 0.012 0.020 -SDP-3 Seridun BH-S1;205m Phyllic 0.28 0.017 0.030 -SDP-4 Seridun BH-S1;359m Phyllic 0.19 0.009 0.020 -


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Table 2. Continued

SampleNo. Deposit Depth of sampling

(Bore hole/outcrop) Ore-type Cu (%) Mo (%) Au (g/t) Fe3O4 (%)

SDP-5 Seridun BH-S1;178-185m Phyllic 0.31 0.007 0.030 -SDP-6 Seridun BH-S1;234-246m Phyllic 0.24 0.013 0.020 -SDP-7 Seridun BH-S1;200-209m Phyllic 0.26 0.011 0.030 -SDP-8 Seridun BH-S1;185-193m Phyllic 0.31 0.010 0.020 -DZP-1 Darreh Zar BH-DZ51;279-281m Phyllic 0.42 0.002 0.040 -DZP-2 Darreh Zar BH-DZ51;281-288m Potassic+Phyllic 0.46 0.010 0.050 0.34DZP-3 Darreh Zar BH-DZ51;292-294m Potassic+Phyllic 0.43 0.012 0.040 0.38DZP-4 Darreh Zar BH-DZ51;188-191m Phyllic 0.51 0.030 0.020 -DZP-5 Darreh Zar BH-DZ51;152-156m Phyllic 0.48 0.010 0.030 -DZP-6 Darreh Zar Outcrop Phyllic 0.40 0.028 0.050 -DZP-7 Darreh Zar Outcrop Phyllic 0.45 0.021 0.030 -DZP-8 Darreh Zar Outcrop Phyllic 0.46 0.024 0.030 -DZP-9 Darreh Zar Outcrop Phyllic 0.34 0.022 0.040 -DZP-10 Darreh Zar BH-DZ51;252-257m Phyllic 0.43 0.021 0.030 -SCP-1 Sar Cheshmeh BH-979;175-179m Propylitic 0.29 0.001 0.017 -SCP-2 Sar Cheshmeh BH-979;179-184m Propylitic 0.26 0.001 0.016 -SCP-3 Sar Cheshmeh BH-979;185-186m Propylitic 0.22 0.000 0.029 -SCP-4 Sar Cheshmeh BH-986;82.5-90m Weakly Biotitized 0.18 0.003 0.006 0.48SCP-5 Sar Cheshmeh BH-979;90-91.7m Weakly Biotitized 0.22 0.006 0.009 0.38SCP-6 Sar Cheshmeh BH-979;102-105m Weakly Biotitized 0.27 0.011 0.017 0.33SCP-7 Sar Cheshmeh BH-979;108-110m Weakly Biotitized 0.12 0.005 0.010 0.36SCP-8 Sar Cheshmeh BH-1006;195-198m Intensely Biotitized 0.52 0.016 0.036 0.46SCP-9 Sar Cheshmeh BH-1006;200-204m Intensely Biotitized 0.54 0.009 0.035 0.43SCP-10 Sar Cheshmeh BH-991;436-438m Intensely Biotitized 0.52 0.015 0.069 0.55SCP-11 Sar Cheshmeh BH-991;438-439.8 Intensely Biotitized 1.32 0.048 0.074 0.48SCP-12 Sar Cheshmeh BH-991;439.8-440m Intensely Biotitized 1.16 0.038 0.094 0.59SCP-13 Sar Cheshmeh BH-991;440-442m Intensely Biotitized 0.9 0.036 0.022 0.39SCP-14 Sar Cheshmeh Outcrop Intensely Biotitized 1.04 0.043 0.048 0.53SCP-15 Sar Cheshmeh Outcrop Intensely Biotitized 1.07 0.084 0.046 0.49SCP-16 Sar Cheshmeh Outcrop Intensely Biotitized 0.85 0.040 0.047 0.47SCP-17 Sar Cheshmeh BH-991;220-225m Intensely Phyllic±Argillic 1.22 0.040 0.089 -SCP-18 Sar Cheshmeh BH-991;247-248m Intensely Phyllic±Argillic 1.21 0.057 0.120 -SCP-19 Sar Cheshmeh BH-997;260-263.5 Intensely Phyllic±Argillic 0.78 0.088 0.020 -SCP-20 Sar Cheshmeh BH-997;268-271m Intensely Phyllic±Argillic 0.53 0.196 0.049 -SCP-21 Sar Cheshmeh BH-997;271-278m Intensely Phyllic±Argillic 0.39 0.180 0.051 -SCP-22 Sar Cheshmeh BH-997;278-284m Intensely Phyllic±Argillic 0.43 0.500 0.024 -SCP-23 Sar Cheshmeh BH-997;290-292.5 Intensely Phyllic±Argillic 1.09 0.063 0.040 -SCP-24 Sar Cheshmeh BH-1000;391-394m Intensely Phyllic±Argillic 0.36 0.145 0.040 -SCP-25 Sar Cheshmeh BH-1003;554-564m Intensely Phyllic±Argillic 0.29 0.002 0.017 -SCP-26 Sar Cheshmeh BH-1002;606-609m Intensely Phyllic±Argillic 0.7 0.014 0.055 -SCP-27 Sar Cheshmeh BH-1002;615-618m Intensely Phyllic±Argillic 0.54 0.067 0.038 -SCP-28 Sar Cheshmeh BH-1002;638-642m Intensely Phyllic±Argillic 0.57 0.067 0.024 -SCP-29 Sar Cheshmeh BH-1002;643.5-646m Intensely Phyllic±Argillic 1.28 0.007 0.091 -SCP-30 Sar Cheshmeh BH-1002; 646.5-651m Weakly Phyllic±Argillic 0.34 0.057 0.020 -SCP-31 Sar Cheshmeh Outcrop Weakly Phyllic±Argillic 0.39 0.058 0.025 -SCP-32 Sar Cheshmeh Outcrop Weakly Phyllic±Argillic 0.41 0.086 0.024 -SCP-33 Sar Cheshmeh Outcrop Weakly Phyllic±Argillic 0.43 0.074 0.056 -SCP-34 Sar Cheshmeh BH-1003;781-793m Weakly Phyllic±Argillic 0.22 0.003 0.013 -SCP-35 Sar Cheshmeh BH-1003;795-800m Weakly Phyllic±Argillic 0.27 0.009 0.028 -SCP-36 Sar Cheshmeh BH-1003;850-860m Potassic±Phyllic 0.15 0.002 0.0١۵ 0.39SCP-37 Sar Cheshmeh BH-1003;860-870m Potassic±Phyllic 0.26 0.001 0.010 0.43SCP-38 Sar Cheshmeh BH-1003;870-880m Potassic±Phyllic 0.03 0.001 0.0١٠ 0.38SCP-39 Sar Cheshmeh BH-1003;880-890m Potassic±Phyllic 0.25 0.000 0.016 0.39SCP-40 Sar Cheshmeh BH-1003;890-900m Potassic±Phyllic 0.6 0.008 0.030 0.36


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Table 3. The average Au, Mo, Cu and magnetite contents together with Cu/Au (atomic) and Au/Mo ratios in some porphyry copperdeposits from Kerman region compared with different type of porphyry copper deposits [4,6]

Ore deposit Au (g/t) Mo (%) Cu (%) Cu/Au (atomic ratio) Au/Mo Fe3O4 (%)Sar Cheshmeh 0.040 0.050 0.65 504061 0.80 0.40

Meiduk 0.053 0.006 0.67 392145 8.83 0.79Darreh Zar 0.036 0.018 0.44 379135 2.00 –

Sar Kuh 0.083 0.003 0.33 123328 27.00 1.24Dar Alu 0.031 0.007 0.44 440286 4.42 –

Sara 0.034 0.005 0.37 337556 6.80 –Abdar 0.072 0.013 0.30 129247 5.50 0.80

Seridun 0.024 0.011 0.26 336034 2.18 –Porphyry Cu-Mo 0.12 0.016 0.41 >>40000 <3 0.05

Porphyry Cu-Au-Mo 0.15 0.015 0.48 – 3-30 1Porphyry Cu-Au 0.38 0.003 0.55 <40000 >30 2.6

associated with copper. It appears that gold has beenintroduced with copper during the earliest stage ofalteration and mineralization. The highest copper andgold grades commonly coincide in central part of thedeposits (potassic zone), and there is a strong positivecorrelation between Au and Cu. Kesler et al., [6]indicated that gold in porphyry copper depositscommonly is carried by bornite and chalcopyrite wherethey are sufficiently abundant. In deposits that containabundant bornite, it is the preferred host for gold. Wherebornite is rare or absent in the early alteration andmineralization stage, gold is associated withchalcopyrite [38,40-42]. In porphyry copper deposits,gold is found mainly as native grains in the form ofinclusions in Cu-Fe-S sulfides, but more commonly it isfound along their grain margins. The experimental datahave indicated that bornite and chalcopyrite in manyporphyry copper deposits are saturated with goldbetween about 700 and 400°C [6,10]. Since thistemperature range is considerably higher than those atwhich porphyry copper deposits commonly form, manyof the above mentioned researchers believed that goldoriginally have been deposited as solid solution in Cu-Fe-S sulfides when the porphyry copper depositsformed at high-temperature (700-600°C), and thenexsolved to concentrate along sulfide grain boundariesat the temperatures typical of ore deposition (400-250°C). The extent of such solid solution can beevaluated with recent experimental data on thesolubility of gold in high-temperature phases in the Cu-Fe-S systems, especially in bornite rather thanchalcopyrite [10]. Kesler et al., [6] noted that if ore-forming fluids starts deposition at 700°C, the goldsaturated Cu-Fe-S sulfides will contain 800 ppm gold.

This will decrease to 200 ppm at 600°C.They believedthat this decrease in temperature of about 100°C couldrelease almost 75% of the gold in the Cu-Fe-S sulfides.Also, they indicated that if ore deposition startedat 600°C with bornite-chalcopyrite assemblage toform chalcopyrite-bornite-pyrite at lower temperature

Figure 3. Correlation between Au and Cu (a), Au and Mo (b),and Mo and Cu (c) in potassic-ore samples from Kermanporphyry copper deposits.


254

alteration-mineralization stage (400-200°C), this drop intemperature could release only 200 ppm gold from Cu-Fe-S sulfide solid solution. These relations not onlyindicate that the Au-content in high-temperature ore-forming fluids will be more due to higher solubility ofgold but also suggest that Au is exsolved from Cu-Fe-Ssulfides during the cooling of deposits. Fluid inclusionand sulfur isotope studies in Sar Cheshmeh and Meidukdeposits showed maximum ore depositional tempera-tures between 600 and 400°C [21,34]. Therefore, wecan assume that the highest temperature of ore-formingfluids in these ore deposits was less than 600°C.Considering these temperatures and based on theobserved positive correlation between Cu and Au inores samples from the potassic zones (Fig. 3), thepresence of gold in the structure of chalcopyrite andalso as independent phases in Sar Cheshmeh deposit[48], suggest that gold had originally been substitutedfor copper in the relatively high-temperature Cu-Fe-Ssulfides and then exsolved from chalcopyrite due todecrease in temperature from 600°C to 250°C. This is inline with the experimental data of Simon et al., [10] andKesler et al., [6]. Therefore, it seems the low content ofgold in the Kerman porphyry copper deposits can be theresult of the originally Au-poor hydrothermal fluids dueto their low temperature which let little gold to enter thestructure of the early formed Cu-Fe-S sulfides(chalcopyrite). The low temperature of the early ore-forming fluids for the Kerman porphyry copper depositscan be confirmed by the low content of highertemperature Cu-Fe-S sulfides such as bornite.Furthermore, Nedimovic [24] based on paragenesis ofore minerals have classified Kerman porphyry copperdeposits as deposits generated in moderate temperature[24]. Simon et al., [10] noted that primary porphyrycopper deposits that form at low temperatures (belowabout 500°C) will consist dominantly of chalcopyriteand pyrite. These deposits will contain less gold, whichis hosted by chalcopyrite relative to higher temperatureporphyry copper deposits which contain more goldwhich is closely associated with bornite and magnetite.

According to Kesler, et al. [6], one of the mostimportant processes which influence the gold content inthe porphyry copper ore bodies, is the late events ofmineralization which cause the remobilization ofdeposited copper and gold through destruction ofbornite and chalcopyrite in high temperature (potassic)zone. Studies have shown that the late high-temperaturehydrothermal processes which are recognized by thepresence of quartz-sericite assemblage in phyllicallyaltered rocks caused the Cu to remobilize more than Auin comparison with low-temperature hydrothermalprocesses (carbonate-sericite assemblage) [6]. Presence

Figure 4. Correlation Au and Cu (a), Au and Mo (b), and Moand Cu (c) in potassically altered rocks affected by phyllicalteration from Kerman porphyry copper deposits.

Figure 5. Correlation between Au and Cu (a), Au and Mo (b),and Mo and Cu (c) in strongly phyllic-ore samples fromKerman porphyry copper deposits.


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of pervasive and high temperature quartz-sericite-pyritealteration (350°C-300°C; [21,34]) in the Kermanporphyry copper deposits [17-19,21,34,50], the high Cuconcentrations (and Cu/Au ratio) in potassic-phyllicores, along with the poor correlation between Au andCu (Figs. 4 and 5) indicates different behavior of Cuand Au in response to relatively high-temperature ore-forming solutions (350°C-250°C; [21,34]) in the middleand late stages of ore-deposition (phyllic zone). It seemsthat the Cu re-mobilized during the destruction of earlyformed Cu-Fe sulfides by the late stage ore-formingsolutions, was re-deposited along with quartz-sericitemineral assemblage which overprinted potassic zone.This is supported by the similarity of lead isotopecomposition of early (206Pb/204Pb = 18.53-18.59,207Pb/204Pb = 15.59-5.61,208Pb/204Pb = 38.59-38.69) andlate sulfides (206Pb/204Pb=18.57-18.61,207Pb/204Pb =15.60-15.65,208Pb/204Pb = 38.66-38.82) in Kermanregion [25]. This late stage process caused an increasein Cu concentration in the potassically altered rockswhich were overprinted by the phyllic alteration (e.g.,Cu grades up to 1.4% in biotitized rocks affected byphyllic alteration in Sar Cheshmeh deposit) [17,33,50].

Kesler et al., [6] believes that gold might bescavenged from Cu-Fe-S solid solution by later, low-temperature ore-forming solutions. This process wouldlead to the release of free particles of gold and itsdispersion in the whole ore body or concentration ofgold in polymetal and epithermal veins outside the Cuore zone. This process can explain the presence of Au inthe form of free particles in the whole ore body [48] andits presence within the epithermal as well as polymetalveins in country rocks surrounding the porphyry coppersystems in Sar Cheshmeh [50], Abdar [21], and Meidukdeposits [24].

The low grades of Au in the Kerman porphyrycopper deposits can be explained by their derivationfrom originally Au-poor magmatic and hydrothermalfluids. Gold which is not accommodated within thechalcopyrite at low temperature (phyllic zone), willprobably be associated with pyrite as well as in the formof free gold grains and also associated with epithermalveins which are scattered outside the copper ore zone[10,51]. Many Au-poor porphyry copper deposits insouthwest USA, Central Asia, and west of SouthAmerica are associated with widespread phyllicalteration, implying that the low temperaturehydrothermal solutions responsible for the phyllicalteration removed the gold from the central parts of theporphyry systems. The remobilized Au is concentratedas epithermal veins in the country rocks [6,49,52-54].

Magnetite Content

Magnetite is a common mineral of the potassicalteration assemblage in the porphyry copper ores [55].The magnetite content of the potassic zones rangesbetween 0.05 and 10 percent [4]. Sillitoe [3] and Cox

Figure 6. plots of Kerman porphyry copper deposits on theCu-Mo-Au discrimination diagrams of Cox and Singer [4].Data for other deposits derived from Vila and Sillitoe [73].

0 0.5 1.0 1.5 2.00

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Cu (wt%)

Au

(g/t)

FSE

MiamiSanta Rita

El Teniente

Grasberg

Cadia

Ray

WafiAfton

Escondida

Copper Canyon

Ajo

Seridun

Galore Creek

Endeavor Sillitoe (1979)Batu Hijau

SkouriesKingking

Cerro CoronaDizon

AbdarSar Kuh Sar Cheshmeh

Meiduk

Darreh ZarSara Dar Alu

Kirkham &

Sinclair (

1995)

Figure 7. Location of the Kerman porphyry copper depositson Cu vs. Au diagram on which the porphyry Cu-Au depositsare plotted above the diagonal line of Kirkham and Sinclair [6]and the horizontal line of Sillitoe [3].


256

and Singer [4] have proven a strong positive correlationbetween magnetite and gold contents of the porphyrycopper deposits. The high magnetite content (3-10wt.%) indicates high oxidation condition of the ore-forming fluids responsible for gold and coppermineralization in the gold-rich porphyry copper deposits[3,4]. The low gold grades in the Kerman porphyrycopper deposits are consistent with the low magnetitecontent in the potassic zones. The magnetite contents(wt.%) in the potassic ore samples from the Kermandeposits are as follows (Table 3): Sar Kuh ,1.24 wt.%;Abdar ,0.80 wt.%; Meiduk, 0.79 wt.%; and SarCheshmeh ,0.4 wt.%. These values are similar to thoseof the gold-poor porphyry copper deposits (≤1wt.%) [4].Although both Au and magnetite contents are low butinterestingly our data indicate that there is a significantpositive correlation (r=0.93) between average Au gradesand average magnetite content of the potassic ores insome porphyry copper deposits of the Kerman region(Fig. 8). This reflects the higher fO2/fS2 of ore-formingfluids in deposits with higher gold and magnetitecontents (i.e., Sar Kuh, Abdar and Meiduk) comparedwith the gold-poor deposits (i.e., Sar Cheshmeh andDarreh Zar). One possible mechanism for maintaininghigh fO2/fS2 during the early gold-deposition stage (i.e.,potassic alteration), might be the dissociation of H2O toH2 and O2 [4,56]. This process would be favored by thehigh temperature and low pressure that would beassociated with quartz diorite and other intrusionsemplaced at high levels in the crust [4]. Escape of thesmaller hydrogen molecules would result in the highfO2/fS2 required to form magnetite and restrict goldmobilization [4]. The high magnetite content in thecenter of porphyry systems can be significant as anexploration tool for exploring gold-rich porphyry copperdeposits [3,10,11,57].

Emplacement Depth

According to Cox and Singer [4] there is a significantnegative correlation (r=-0.90) between gold content andemplacement depth of porphyry copper deposits. Theyshowed that gold-rich porphyry copper deposits tend tobe emplaced at very shallow levels (0.4-1.5 km) in thecrust relative to the gold-poor porphyry copper deposits(3.6-4.6 km). Etminan [34] and McInnes et al., [33]showed that Sar Cheshmeh deposit was emplaced atdepth between 4 and 5 km. Also, McInnes et al. [33]estimated a depth of 2.8 and 2.5 km for emplacement ofAbdar and Meiduk deposits, respectively. The shallowemplacement depth of the Abdar and Meiduk depositstogether with their higher gold contents compared to SarCheshmeh that is characterized by lower Au content

(average 0.04 g/t) and deeper emplacement, indicates anegative correlation between emplacement depth andthe amount of gold in the Kerman porphyry copperdeposits. The higher level emplacement of gold-richporphyry copper deposits may cause the high oxidationconditions (SO2/H2S>1) of ore-forming fluids [49]. Thiscondition increases Au solubility in hydrothermal fluidswhich finally form the Au-rich porphyry copperdeposits [49]. In the deeply emplaced deposits (3-5kilometers), abundant occurrence of pyrite indicatesreduced conditions in the magmatic-hydrothermalsystem(high fS2/fO2), in this situation, gold will betransported as bi-sulfide complexes in hydrothermalfluids [49], and can migrate to the peripheral zones ofporphyry systems where the gold-rich epithermal veinsare formed.

Tectonic Setting

Although, the tectonic setting appears to be one ofthe most important factor that controls the gold contentof porphyry copper deposits [1,8], but it is not generallyconsidered as a critical factor [2,3,7,9,11]. Island archosted porphyry copper deposits (e.g., Santo Tomas,Dizon, Kingking) have elevated Au content incomparison with the continental margin porphyrycopper deposits (Escondida, Morenci, santa Rita)[1,6].The Kerman porphyry copper region is located on thesoutheastern segment of subduction/collision-relatedUrumieh-Dokhtar magmatic arc constructed on thesoutheastern margin of the central Iranian continental

R2 = 0.9221

0

0.4

0.8

1.2

1.6

0.00 0.02 0.04 0.06 0.08 0.10

Au content (ppm)

Mag

netit

e co

nten

t (w

t.%)

SKP

ABDPMP

DZP

SCP

Figure 8. Plots of the average magnetite contents of thepotassic zone against the average gold grades for the Kermanporphyry copper deposits. The correlation coefficient is 0.98.ABDP=Abdar porphyry copper prospect; DZP= Darreh Zarporphyry copper deposit; MP=Meiduk porphyry copperdeposit; SCP=Sar Cheshmeh porphyry copper deposit;SKP=Sar Kuh porphyry copper prospect.


257

block [21,22,25,26]. Geochemical and isotopicsignature (Sr, Nd and Pb) of Kerman ore-hostingintrusions indicated that productive adakitic-likemagmas and associated metals (Cu) originated from themelting of the lower crustal mafic rocks rather than themelting of the subducting hot slab (Fig. 2f) [25,26].Mungall [58] believes that adakitic magmas which havebeen formed as the direct melting of subducting oceanicslab due to their higher temperature and the remarkablehigher oxygen fugacity, in comparison to those whichhave been originated from melting of thickened lowercrustal rocks, have a considerable potential to generateAu-rich porphyry copper deposits. Due to the lowertemperature and the lower oxygen fugacity of theKerman ore-hosting magmas, the porphyry copperdeposits cannot be enriched in gold [58]. Thischaracteristic also seems to comply with Kessler’sopinion [1] regarding the low Au content of porphyrycopper deposits generated in continental margin arcs.

Other Signatures of the Gold-Poor Porphyry CopperDeposits

One of the most important features which willsupport the gold-poor nature of Kerman porphyrycopper deposits is the presence of a considerableamount of Mo-bearing ores in the copper ore zones [18,34], and the lack of geochemical relationship betweenAu and Mo (Figs. 3 and 4). Sillitoe [3] believes thatgold-rich porphyry copper deposits are Mo-free. Coxand Singer [4] proved a negative correlation betweenAu and Mo in porphyry copper deposits (r= -0.45).These might be attributed to the relatively low oxidationcondition (SO2/H2S<1) of the ore-forming fluidsresponsible for formation of porphyry Cu-Mo depositswith respect to the gold-rich porphyry copper deposits[4,59-62]. Molybdenum is concentrated within bothcopper ore shell and the peripheral pyrite zones in theporphyry Cu-Mo deposits [4]. Gold would migrate tothe surroundings of the system under the samecircumstances and forms limited gold mineralizationassociated with epithermal and poly-metal veins[4].These features are observed in the porphyry copperdeposits of the Kerman region and can indicate thatsuch deposits are of porphyry Cu-Mo type. Recognitionof a poly-metal vein with a high gold grade (0.5 g/t)outside the copper ore zone of Sar Cheshmeh deposit[50,63], as well as the presence of gold-rich poly-metalveins around Meiduk deposit (Chah Mesi deposit)[21,24] and Abdar deposit [21] are consistent with thescarcity of gold in the Kerman porphyry copperdeposits.

Ore deposit morphology has been introduced as one

of the criteria which can distinguish the Au-richporphyry copper deposits from those of Cu-Mo or gold-poor deposits. Sinclair et al., [9] applied the porphyryclassification scheme of Sutherland Brown [64] for agroup of porphyry deposits in the British Columbia andfound out that porphyry copper-gold deposits tend tooccur in two forms, namely classic-type and volcanic-type systems. According to this classification, theclassic-type deposits are centered on small cylindricalplutons. They are commonly associated with brecciapipes and have concentric zones of alteration andmineralization, and coeval volcanic rocks are commonlyabsent. Volcanic-type deposits are related to irregular ordike-like igneous bodies that have intruded a coevalvolcanic pile. In contrast, the plutonic-type porphyrydeposits are characterized by mineralization as integralzones of medium-sized plutons having phaneritictexture. Cox and Singer [4] showed that all gold-richporphyry copper deposits are grouped in the volcanic-type deposits in comparison with Mo-rich depositswhich show classic-type or plutonic-type morphology.Considering the proposed classification and thegeological and radiometric age data [15,17,19,21,25,26,33,35,63,65], the porphyry copper deposits of theKerman region formed in the late stages of arcdevelopment (middle-late Neogene), especially whenvolcanism ceased and a compressive tectonic regimeprevailed. Moreover, and the lack of coeval volcanicrocks with the ore-hosting intrusions [25,26] andabundant occurrences of breccia pipes [36,50,65] at thesite of ore deposition indicate that Kerman porphyrycopper deposits belong to the classic-type of porphyryCu deposits. This is consistent with Au-poor signatureof porphyry copper deposits in the Kerman region.

Some researchers [1,2,9] have mentioned thattonnage of the Au-Cu porphyry deposits are far lessthan Cu-Mo deposits. Cox and Singer [4] confirmed theidea that the increase for ore-tonnage with decreases theamount of gold in porphyry copper deposits. Theyidentified the Au-rich porphyry copper deposits have atonnage of about 100 to 200 million tons or lower andMo-rich porphyry copper deposits have a tonnagearound 500 to 1000 million tons. Therefore, the lowertonnage of Au-rich porphyry copper deposits incomparison to Mo-rich porphyry copper deposits in acomprehensive worldwide scale is accepted by most ofthe researchers. Based on the existing data (Table 1),there is a negative correlation between tonnage and goldcontent of Kerman porphyry copper deposits. Depositswith lower tonnage (Abdar, Sar Kuh, Meiduk) are richerin gold compared with the giant Sar Cheshmeh deposit(Fig. 9). However, the gold grades are still at the levelof gold-poor porphyry copper deposits.


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R 2 = 0.45

0.00

0.04

0.08

0.12

0.0 0.1 1.0 10.0

Cu Tonnage (Mt.)A

u co

nten

t (pp

m)

SCP

MP

SKP

DZP

Figure 9. Inverse correlation between Cu-tonnage andaverage gold contents of Kerman porphyry copper deposits.Cu-tonnages are derived from Table 1. DZP= Darreh Zarporphyry copper deposit; MP=Meiduk porphyry copperdeposit; SCP=Sar Cheshmeh porphyry copper deposit;SKP=Sar Kuh porphyry copper prospect.

AcknowledgmentsWe thank the NICICO for providing access to drill

core samples of the Kerman porphyry copper deposits,as well as Au, Mo, Cu, and magnetite analyses. Theauthors acknowledge constructive reviews from threeanonymous reviewers of the Journal of Sciences,Islamic Republic of Iran that helped to improve themanuscript.

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Gold Distribution in Porphyry Copper Deposits of Kerman ... · Kerman region, which is known as the most important porphyry Cu-bearing metallogenic province in Iran [14,15]. The variations