Gardaneh Salavat porphyry copper system:Is it about metal endowment or fertility of the porphyritic intrusion?

Seyyed Reza Mashhadi

Abstract Gardaneh Salavat porphyry copper system is located in the northwest of Iran.It comprises a quartz diorite to diorite porphyritic intrusion that created extensive alteration haloes in the host rocks.Despite all technical considerations,the exploration activities did not yield to ore discovery.As the matter of fact,all of the geological,geophysical,and geochemical characteristics of a porphyry Cu-Au deposit exist but there is no economic mineralization.Detailed examinations suggest that the only reasonable explanation can be related to the infertility or low metal endowment of the porphyritic intrusion.The reasons include(1)anomalous ranges of Cu,Au,and Mo rather than economic ore grades,(2)weakly development of potassic alteration,(3)probably sulfur saturation of magma that occurred before intrusion emplacement in the host rocks,and(4)probably low metal endowment in the regional scale and the lack of known porphyry copper deposits in close neighborhoods.It is tried to test the proposed hypothesis with the introduced fertility indicators based on major and trace element geochemistry.However,current fertility indicators could not prove or reject the proposed hypothesis for Gardaneh Salavat porphyritic intrusion because of the high-K calc-alkaline to shoshonitic nature of the intrusive rocks together with the unavailability of relatively fresh/unaltered samples.This case study is notable to examine because(1)it shows that characterization of fertile from barren intrusions seems to be as important as vectoring towards concealed porphyry copper deposits,especially while dealing with deep exploration targets that are associated with high financial risks,and(2)it highlights the inefficiencies and uncertainties in introduced magma fertility indicators for porphyry copper intrusions.As a result,it is of high importance to conduct an overarching investigation on other possible fertility indicators as a fundamental need for the future of the exploration industry.

Keywords Porphyry·Copper·Fertility·Metal endowment·Barren intrusion

1 Introduction

Porphyry copper deposits are giant low to medium grade orebodies that supply a significant proportion of human needs for multiple elements including Cu,Mo,Au,PGE,Re,Pt,Pd,etc(Tarkian et al.2003;John et al.2010;Crespo et al.2018).Not only the orebody is extensive in dimensions but also it is associated with enormous alteration zones,and extensive dispersion of geochemical haloes(Edwards and Atkinson 1986;Sinclair and Goodfellow 2007;Berger et al.2008).These characteristics make them amenable targets for exploration.On the other hand,these deposits result from complex interactions and feedbacks of many processes(Berger et al.2008).Thus,there might be challenges ahead while searching for undiscovered porphyry copper resources.The mentioned challenges become vital today since giant mineral deposits have already been recognized and we have to focus on small,hidden,deep,and complex mineral resources that have not yet been found.

In any porphyry copper province,there are some barren porphyritic intrusions that may represent similarities with fertile/productive ones (Stringham 1960).Stringham(1960)mentioned nine points regarding the recognition of barren from fertile intrusions.The most important one is that productive intrusions are considerably large and seem to cross-cut host rocks with no or very little disturbance due to intrusion action while barren suites can be variable in size and they have a tendency to be concordant with wall-rock structures or show obvious intrusive structural effects.In recent decades,there have been a number of attempts to assess the fertility of porphyry systems but the problem is inherently so complicated that there is not a certain solution to this problem yet(i.e.Stringham 1960;Edwards and Atkinson 1986;Grancea et al.2001;Richards et al.2012;Williamson et al.2016;Uribe-Mogollon and Maher 2020;Chiaradia 2020;Du and Aude´tat 2020;Pizarro et al.2020).

In this paper,the exploration history of the Gardaneh Salavat porphyry system is briefly examined.It is a porphyritic intrusion representing almost all of the geological,geophysical,and geochemical characteristics of a porphyry Cu-Au deposit while there is no remarkable economic or even sub-economic mineralization.After examining various exploration datasets in detail,it is expressed that the only reasonable explanation is related to the infertility or low metal endowment of the porphyritic intrusion.It has been tried to highlight some pieces of evidence to show that Gardaneh Salavat porphyritic intrusion is barren as it has not contained sufficient metals in the late magmatic fluids or it could not develop potassic alteration efficiently,which in turn,led to anomalous concentrations of elements rather than economic grades of copper and gold.Note that a few porphyry copper fertility indicators are also checked and discussed to challenge the proposed hypothesis,and assess the effectiveness of those indicators in this case study.

2 Theories and methods

2.1 General definition of porphyry copper deposits

Providing a comprehensive definition for porphyry copper systems is elusive because they result from complex interactions;being affected by diverse processes from magma evolution to ore formation and preservation(Sinclair and Goodfellow 2007;Berger et al.2008;Park et al.2021).

According to the descriptive model presented by Berger et al.(2008),porphyry copper deposits have the following characteristics:(1)copper-bearing sulfides occur in a network of fracture-controlled stockwork veinlets as well as disseminated grains in the altered host rock,(2)alteration haloes and ore zones at 1-4 km depth are genetically related to intermediate to acidic magmas emplaced into the shallow crust(6 to more than 8 km)in magmatic arcs above subduction zones,(3)Intrusive rock complexes that are emplaced immediately before porphyry copper deposit formation are mostly in the form of upright-vertical cylindrical stocks and/or complexes of dikes,(4)phyllic,argillic,and marginal propylitic alteration overlap or surround the potassic alteration assemblage,(5)copper mineralization may also be introduced during overprinting phyllic-argillic alteration processes.For more information regarding porphyry copper deposits,refer to various resources available in the literature i.e.Sinclair and Goodfellow(2007),Seedorff et al.(2008),John et al.(2010),and Pohl(2011).

2.2 Methods and available data

In this paper,the Gardaneh Salavat porphyry copper system is studied based on various datasets available from common strategies applied to the exploration of porphyry deposits.The data include geology,lithogeochemistry,geophysics(resistivity,IP,magnetics),different borehole data(assays,petrography samples,fluid inclusion samples,etc.).After an overarching examination of all exploration datasets,the possible geological scenario is mentioned based on the clues anticipated according to the information published in the literature.

3 Study area:Gardaneh Salavat Cu-Au porphyry system

3.1 Location

Gardaneh Salavat porphyry system is located 2 km southwest of Moradlu city,Ardabil Province,Iran.It lies approximately 75 km northwest of Ardebil city(Fig.1).

Fig.1 Location of Gardaneh Salavat Area in northwest of Iran

3.2 An overview of porphyry copper mineralization in the regional scale

Northwest of Iran is known for the occurrence of multiple porphyry copper systems such as Sungun,Haft Cheshmeh,and Masjed Daghi(Maghsoudi et al.2014;Zu¨rcher et al.2015;Parsa et al.2016,2018;Atalou et al.2017;Imamalipour and Mousavi 2018;Kamali et al.2020).In this region,the best regional study for porphyry copper systems exploration is the one performed by USGS(Fig.2).This investigation deals with a regional assessment of porphyry copper systems in the Tethys region;trying to provide a probabilistic assessment of undiscovered resources in western and southern Asia at a scale of 1:1,000,000.Through geoscientific data analysis,they delineated some‘‘permissive tracts’’in the region which is defined as‘‘the surface projection of a volume of rock where the geology permits the existence of a porphyry copper deposit’’(Zu¨rcher et al.2015).

Gardaneh Salavat porphyry system lies outside of the introduced permissive tracts(Fig.2).As stated in Zu¨rcher et al.(2015),the assessment results depend on the used data and limits of the scale.Consequently,a greater areal extent in the northwest of Iran might be considered promising for porphyry copper mineralization because primary field investigations (geological prospecting)indicated the possibility of porphyry copper occurrence at Gardaneh Salavat area.Moreover,according to Jamali et al.(2010),who studied the metallogeny and tectonic evolution of the Cenozoic Ahar-Arasbaran volcanic belt,the Gardaneh Salavat is situated within the‘‘zone A’’of the three introduced metallogenic provinces in northwest of Iran.This metallogenic province(zone A)includes porphyry and skarn-type Cu,Mo,and Au mineralization associated with Oligo-Miocene,shallow-level intrusions.It must be noted that various characteristics of porphyry copper systems are observed in the Gardaneh Salavat area.They are discussed in the next sections of the paper.

Fig.2 Location map of porphyry copper systems in the northwest of Iran.This map is prepared based on available spatial data from USGS scientific report(Jamali et al.2010;Zu¨rcher et al.2015)

3.3 Previous exploration activities in Gardaneh Salavat area

There have been limited geological prospecting and exploration activities in the Gardaneh Salavat area.This area lies in the Lahroud geological map with a scale of 1:100000(Fig.3).According to this map,no mines or mineral indications can be seen.The region is covered by Eocene volcanic and sedimentary sequences that have been intruded by subvolcanic rocks(age:Oligocene)and two series of dikes.Moreover,there are large zones of argillic alteration(alunite and kaolinite minerals).Detailed information on metallogeny,regional tectonic setting,and geology can be found at Jamali et al.(2010)and Ghandchi et al.(1991).

Fig.3 Regional geology around Gardaneh Salavat porphyry system based on Lahroud geological map(Ghandchi et al.1991).The scale of this map is 1:100000 and there are inaccuracies in the map due to great variability of geological units.For more information on regional geology,interested readers are referred to Jamali et al.(2010),and Ghandchi et al.(1991)among others

The latest systematic exploration project in the region is performed in 2015.It included remote sensing of satellite imagery,geological prospecting in preliminary promising zones,geological mapping(scale 1:25000),and stream sediment geochemical sampling survey.Then,more exploration activities are performed to finalize the general exploration stage.All of the investigations focused on the Gardaneh Salavat porphyry system are briefly discussed in the paper.

3.4 Local geology

The geological map of Gardaneh Salavat area is illustrated in Fig.4(scale:1:5000).The oldest rocks in the area are considered to be from the Eocene epoch,the Cenozoic era.These rocks include Eapand Epygeological units comprising volcanic and volcano-sedimentary sequences which cover extensive areal extent of the area.From the viewpoint of lithology,they are mainly andesite and tuff.The Eocene volcano-sedimentary sequence is intruded by several subvolcanic units.The oldest intrusion is related to di unit with microdiorite to gabbro diorite composition.This unit can be seen in various parts of the area,especially the north.The youngest subvolcanic unit is a quartz diorite to diorite that has a limited outcrop in the strongly altered area.

Fig.4 Geological map of Gardaneh Salavat Porphyry Prospect(scale:1:5000),redrawn by permission from Kazemi Mehrnia et al.(2019).Note that geochemical samples,geophysical surveys,and drilled boreholes are also shown

It is anticipated that the east-west trending fault that crosses the strong alteration zone(around BH01 and BH02 boreholes),had a remarkable impact on spreading the alteration haloes.In the center of the strongly altered zone,there are various alterations including clay minerals,carbonate minerals,iron oxides,quartz together with sulfides such as pyrite and chalcopyrite(Fig.5).At this zone,the density of veinlets are high(approximately 20 veinlets in 1 m2).Extensive exploration activities are performed around this alteration zone including lithogeochemical sampling,ground magnetic survey,2D Induced Polarization(IP)and resistivity survey,and finally the drilling of 13 boreholes.

Fig.5 A few photos from geological mapping at Gardaneh Salavat area(A,B:stockwork of veinlets at the main alteration zone;C:hydrothermal breccia)(Kazemi Mehrnia et al.2019)

3.5 Lithogeochemistry

Primary geochemical haloes of porphyry copper deposits are well studied and characterized(Winterburn et al.2020).The geochemical anomalies of these deposit types are zoned with a central Cu±Mo/Au,enclosed by Zn,Pb,and Ag in some cases.It should be noted that Ag and Au anomalies may be above or laterally away from the center of porphyry system(Halley et al.2015).Rather than mapping dispersion haloes of pathfinder elements,it is possible to assess the erosion-level of porphyry system by using zonality coefficient factors(Ziaii et al.2011).Thus,performing a lithogeochemical survey would be beneficial to answer important questions in the exploration procedure.

In Gardaneh Salavat area,a lithogeochemical sampling program was designed.A total number of 255 samples were taken and then analyzed by ICP-OES and Fire Assay methods.The sample spacing was 50 m at the strongly altered zone and 100 m elsewhere.

Geochemical anomalies show a typical pattern consistent with haloes associated with porphyry copper deposits(Fig.6).In the strongly altered zone,there are high anomalies of copper,gold,and molybdenum.Around this center(shown as a pink oval in Fig.6),there are elevated concentrations of zinc and lead.

Fig.6 Results of lithogeochemical sampling at Gardaneh Salavat area.The center of porphyry system is shown by a pink oval for comparison

To estimate erosion-level,the geochemical zonality coefficient proposed by Ziaii et al.(2011)is utilized.In fact,the VZ2zonality index is calculated and used which is formulated as[Pb.Zn]/[Mo.Cu](Fig.6).The concentration of silver in almost all of the samples was lower than the detection limit(1 ppm),thus it was impossible to check VZ2results via VZ1or VZ3zonality factors.According to the values of geochemical zonality coefficient map(VZ2)in Gardaneh Salavat area,the detected anomalies are likely to be related to‘‘upper-ore’’haloes.Actually,there are a few points in the anomalous zone that show values between 0.1 and 5(their effect diminishes during interpolation;therefore it is not visible in the shown scale).These values are representative of upper-ore haloes of porphyry copper deposits(Ziaii et al.2011).

To ensure the obtained results from zonality coefficient method,the range of anomalous elements is also compared to the typical ranges in porphyry copper deposits reported by Halley et al.(2015).Table 1 displays the mean values of pathfinder elements together with the ranges associated with different alteration horizons.This comparison indicates that the recorded anomalies at Gardaneh Salavat area are mainly consistent with deep sericitic to potassic alteration horizon.By evaluating the geochemical zonality coefficient map and the performed comparison,it can be expressed that the expected porphyry orebody is not eroded and there is possibly a hidden ore zone at depth.Additionally,at Gardaneh Salavat area,the ore zone is likely to be found not so far from the surface because a lot of pathfinder elements show the typical ranges related to potassic alteration.

Table 1 Comparing geochemical anomaly values at Gardaneh Salavat area with common ranges in porphyry deposits as reported by Halley et al.(2015)

3.6 Geophysics

There are a variety of geophysical methods that can be used to search for porphyry copper deposits(Thoman et al.1996;Berger et al.2008;Mitchinson et al.2013;Anderson et al.2013;Airo 2015;Guimara~es et al.2019).At Gardaneh Salavat area,magnetic and electrical methods have been utilized that are discussed in the following subsections.

3.6.1 Magnetic surveying

To better understand the geological processes at the subsurface,a systematic grid of magnetic data was recorded(inline spacing:10 m,line spacing:20 m).Figure 7 depicts Total Magnetic field Intensity(TMI),Reduced To magnetic Pole(RTP),Analytic Signal(AS),and Pseudo-gravity(PG)maps.In the TMI grid,irregular distribution of magnetic dipoles with high frequency and medium to high amplitudes are visible,mainly in the eastern part of the map.This kind of magnetic response is related to fresh or relatively unaltered volcanic rocks.Here,it is indicative of volcano-sedimentary rocks with weak propylitic alteration.It seems that AS can better reveal the boundaries of this zone.Note that PG also shows strongly altered zones with relatively high precision,indicating the effectiveness of pseudo-gravity operator for mineral exploration problems(Mashhadi and Safari 2020).In the west of the map,the magnetic field intensities are relatively fixed;showing gradual variations.This response indicates strong propylitic or phyllic alteration haloes.In fact,the replacement of primary magnetite by pyrite in the host rock leads to the decrease of magnetic susceptibility.The observed susceptibility variations have been also reported and discussed by many researchers i.e.Clark(2014),and Honarmand(2016).

Figure 8 illustrates the results of 3Dinverse conventional susceptibility modeling(performed by VOXI earth modeling,Geosoft).The high variability of susceptibility between adjacent cells within unaltered volcanic rocks due to irregular distribution of magnetite in the rock volume is obvious.Regarding the characterization of potassic alteration,there is no high susceptibility anomaly at depth.Here,there is a concern that surficial magnetic anomalies interfered with signals from deeper sources(Dentith and Mudge 2014;Clark 2014);preventing them to be imaged by the magnetic data.However,this is not the case in Gardaneh Salavat area.The borehole data suggest that there is no lithology or alteration at depth that can represent high susceptibility values(it will be discussed later in Sect.4).

3.6.2 Resistivity&induced polarization(IP)tomography

In Gardaneh Salavat area,four profiles were selected to characterize the promising sulfide mineralization zones at depth by 2D Induced Polarization(IP)and resistivity tomography.A combination of dipole-dipole,pole-dipole,and pole-pole was used to achieve a good depth coverage with an acceptable resolution.The unit electrode spacing was 15 m.As it is necessary to remove suspicious noisy IP measurement prior to inversion(Shokri et al.2016;Mashhadi et al.2018;Mostafaei and Ramazi 2019),noisy IP data have been recognized and removed before inverse modeling by using the methodology discussed by Mashhadi and Ramazi(2018).

The result of the profile crossing the center of porphyry system is illustrated in Fig.9.The root mean square(rms)error for resistivity and IP models are 15%and 8.9%,respectively.These values are acceptable while dealing with combined arrays,and high number of datatpoints in the inversion process.Moreover,the characteristics of the utilized arrays make them more susceptible to noise when compared to other configuration types such as gradient or Wenner(Dahlin and Zhou 2004;Loke et al.2013;Mashhadi et al.2018;Loke 2020).

As can be seen from Fig.9,the IP model shows extremely high anomalies.These anomalies are widespread because the alteration haloes are associated with conductive minerals such as pyrite and chalcopyrite.Since IP is dominantly affected by conductive sulfides(Dentith and Mudge 2014),the amount of sulfur in borehole samples can be a great tool for IP model validation(i.e.compare Fig.9 with 13).This fact can be taken for granted in areas such as porphyry prospects where sulfur is almost only contained in conductive sulfide minerals.Clearly,the highest IP anomaly in the area(denoted as‘‘a’’in Fig.9)is consistent with the highest sulfur content in all of the drilled boreholes.

In many geological settings,it is often assumed that resistivity is mainly affected by lithology(Dentith and Mudge 2018).However,it is not always true(Mashhadi 2022),especially while dealing with porphyry mineralization systems.Assessing the variations of resistivity and IP in porphyry systems is inherently elusive but it can be explained by considering two parameters concurrently:(1)alteration,and(2)abundance and style of conductive mineralization.Argillic alteration zones are expected to create low resistivity and/or medium to low IP anomalies because of high content of clay minerals and the absence of metallic sulfides.On the other hand,the effect of alteration on resistivity and IP in potassic,phyllic,and propylitic zones is related to the style and abundance of conductive minerals(metallic sulfides).Consider a single lithotype i.e.an intrusive rock such as granodiorite with three different situations as follows:(1)granite without conductive mineralization,(2)granite containing n%(v/v)conductive mineralization‘‘veinlet’’style,(3)granite containing n%(v/v)conductive mineralization in‘‘dissemination’’style.According to Revil et al.(2015a,b),the following relationship will be expected for resistivity and IP:

Fig.7 Magnetic maps of Gardaneh Salavat area including TMI,RTP,AS,and PG

Regarding Eq.2,it is noteworthy to express that chargeability(IP)only depends on the volumetric content of conductive particles by a linear relationship(for n＜30%(v/v)).For more details about electrical responses in the presence of conductive minerals within the rocks,please refer to Dentith and Mudge(2014),Revil et al.(2015a,b),and Mao et al.(2016).Also,a case study can be found in the literature in which Mashhadi(2022)highlighted the importance of the aforementioned fact for resolving geological structures and the ore itself in a complicated geological setting.

In Fig.9,a dramatic decrease in resistivity in the area denoted as‘‘c’’is visible where a high amount of conductive pyrite dissemination occur.Although the IP parameter in‘‘c’’is high and anomalous,it is lesser than IP values in‘‘b’’which can be discussed by the higher content of sulfides in‘‘b’’(as shown in Fig.9).It should be noted that‘‘b’’and‘‘c’’areas are both affected by phyllic and carbonate alterations.Apart from these variations,there is an electrical signature with high resistivity and very low IP(denoted as‘‘d’’)which seems to be a relatively unaltered lithology i.e.old dioritic intrusion.

Fig.9 Two-dimensional Induced Polarization(IP)and resistivity models over the center of Gardaneh Salavat porphyry system.Note IP consistency with sulfur grades shown at Fig.13

Fig.8 Three-dimensional magnetic susceptibility model of Gardaneh Salavat area(views from different perspectives are shown)

Most of the published research results in the literature have focused on the bulk mineralogy(i.e.whether conductive minerals exist or not)and alteration effects on resistivity and IP(i.e.John et al.2010;Hoschke 2012;Maryono et al.2018).As illustrated here,it seems that the unlike/illusive electrical responses associated with different porphyry deposits can more precisely be interpreted by considering the style of conductive mineralization together with lithology and alteration.In this regard,there are some introduced patterns related to the dominant style of conductive/sulfide mineralization in porphyry copper system but some contradictions could be found in the literature;for example,compare Edwards and Atkinson(1986)with Wies et al.(2020).Consequently,if these patterns are more accurately recognized,electrical resistivity and IP interpretations can also be improved,and proposed drillings may intersect ore zones rather than pyrite enrichments(i.e.pyrite shell).

3.7 Drilling results

After performing geological,geochemical,and geophysical surveys,promising locations for drilling were selected by the exploration team.In the first stage of exploration drillings,five boreholes have been drilled and analyzed.Then,after reviewing exploration data and getting some further clues by analyzing drilled boreholes,seven more locations have been tested to find the expected porphyry orebody.Unfortunately,despite all of the technical considerations by the team,the explorations did not result in ore discovery.

The location of drilled boreholes are shown in Fig.4.Apart from BH05 and BH07 boreholes,other ones had no remarkable result,just intersecting peripheral alteration haloes including phyllic,propylitic,and clay alterations with extremely low grades of copper and gold.It is worth mentioning that clay alteration and carbonate alterations overprinted almost all of the rocks in the area.Figure 10 depicts some pictures from core samples showing different alterations and mineralization detected in boreholes.The best results are related to borehole BH05 and BH07 that are discussed shortly(Fig.11).

Borehole BH05 is drilled up to a depth of 238 m.It includes porphyritic andesite and tuff volcano-sedimentary sequence which is extensively altered by the porphyritic intrusion to clay minerals(mainly kaolinite,illite,and alunite),and phyllic alteration(quartz,sericite,and pyrite).Eventually,this borehole stopped drilling after reaching several meters of the quartz diorite to diorite intrusion that was barren.Among other boreholes drilled after BH05,BH07 had a better result with just a few samples containing economic grades of metals.Borehole BH07 was drilled up to 424 m.In this borehole,the same lithological and alteration haloes were intersected.However,drilling continued to a greater depth in order to evaluate the possible mineralization below the previously recognized enrichment in BH05,and also to investigate the possible deeper unknown and probably younger porphyritic intrusion.However,only the known quartz diorite to diorite porphyritic intrusion which had lots of quartz-sulfide veinlets have been intersected.There were traces of potassic alteration in some of the drilled boreholes especially BH05 and BH07 where they coincide with weak chalcopyrite mineralization.Thus,some petrographic and fluid inclusion samples were taken to check these uncertain macroscopic observations.These samples proved the occurrence of potassic alteration.However,a fundamental question arises here:why there is no indicative copper and gold enrichment in the potassic zone?

Fig.10 A few pictures from core samples of Gardaneh Salavat porphyry system(A chalcopyrite-pyrite veinlet within a porphyritic rock,B chalcopyrite-quartz micro-veinlets,C late stage carbonate alteration,D chlorite-epidote alteration,E disseminated pyrite mineralization,F quartz-sulfide veinlets together with disseminated pyrite)

4 Discussion

4.1 Proposed hypothesis for Gardaneh Salavat porphyry system

As discussed in the previous sections,some of the standard exploration activities,applied to the exploration of porphyry copper deposits,are performed in the Gardaneh Salavat area.One of the reasons for this examination was to ensure that there is no missed target and/or considerable inaccuracy in data interpretation.Consequently,the unsuccessful exploration at the Gardaneh Salavat area can raise lots of questions about this porphyry system.Now,in this section,some detailed notes are presented to unravel the possible geological secret of this porphyry system.

One of the boreholes has been drilled deep into the porphyritic intrusion.Consequently,the potassic alteration must have been recognized if the quartz diorite to diorite intrusion was responsible for alterations and Cu-Au mineralization.Also note that erosion-level estimations based on surface lithogeochemical anomalies revealed that the possible ore is not eroded and the potassic alteration is not so far from the surface.As stated in the previous section,there were traces of magnetite and secondary biotite minerals in some of the boreholes that may show the presence of potassic alteration.These traces can be important to track the history of alterations in a porphyry system since biotite may alter to chlorite after primary formation in the potassic zone(Edwards and Atkinson 1986).In this regard,petrological studies proved the existence of potassic alteration in some samples as demonstrated in Fig.12.Moreover,fluid inclusion samples indicated high salinity and high temperature of the trapped fluids(NaCl=57 wt.%and Th(L-V)＞500°C);indicating the dominance of magmatic fluids as expected in the hypogene zone of porphyry copper systems(Ulrich et al.2002;Zarasvandi et al.2013).On the contrary,the potassic alteration seems to be weakly developed because magnetite is not abundant.This characteristic is not expected in porphyry Cu-Au deposits because they often include abundant magnetite in the potassic zone(Berger et al.2008;Clark 2014).Due to the low magnetite content,the magnetic susceptibility model shows no anomalies at the weakly developed potassic alteration(Fig.13).Correlation of magnetic susceptibility model with drilling results also suggests that the magmatic-hydrothermal alteration decreased the susceptibility of the host rocks but the quartz diorite intrusion itself has even lower magnetic susceptibility.This is possible because the magnetic susceptibility for intrusive rocks can vary significantly as a result of changes in magnetite content i.e.Mattsson et al.(2003).Overall,it can be concluded that the potassic alteration exists but it is not maybe welldeveloped.

Fig.12 A few pictures from thin and polished sections from core samples of Gardaneh Salavat area(a the formation of secondary biotite from the alteration of amphibole in potassic alteration,b,and c formation of pyrite and copper ore minerals including chalcopyrite,tetrahedrite,covellite,and chalcocite)

If the location of inferred potassic alteration zone is examined together with core geochemical data,the increase of copper,molybdenum,and gold assays will be evident(Fig.11).In the best case,a 70 m thick zone with the average values of 0.17%copper,170 ppb gold,and 18 ppm molybdenum can be seen which totally lies within the anticipated potassic alteration zone.It was expected that those boreholes located in the vicinity of BH05 and BH07 would show the same enrichment zones but they are also too low in copper,gold,and other metals.

Fig.11 Geochemical assay results for the best two boreholes at Gardaneh Salavat area

One of the drilled boreholes intersected tens of meters of the barren core of diorite porphyritic intrusion.The‘‘barren core’’which lies at the interior of the inverted-cup or inverted-cone shape of porphyry orebodies and comprises the base of porphyry copper deposits,is a low-grade altered rock(James 1971;Edwards and Atkinson 1986;Berger et al.2008)with high vein-volume(Davies and Williams 2005;Weis et al.2012).Interestingly,the pattern of sulfur in boreholes shows the expected trend in porphyry copper systems as illustrated in Fig.13(Dilles and Einaudi 1992;Berger et al.2008).Generally,the amount of sulfur increases from peripheral alteration haloes to pyrite-shell(strong propylitic to phyllic alteration).Thereafter,the sulfur content decreases towards the ore zone in potassic alteration.Note that in Gardaneh Salavat area,from anomalous copper and gold samples in the potassic zone towards the barren core of porphyritic intrusion,the amount of sulfur experiences an increase again which can be contributed to the higher density of veinlets containing uneconomic sulfides(Weis et al.2012).It may also indicate the high amount of sulfur in the late magmatic fluids originated from the porphyritic intrusion.

Fig.13 A cross section of Gardaneh Salavat porphyry system.Magnetic susceptibility model,borehole data,surface lithogeochemical data,and approximate alteration haloes are shown here

Having all of the information in mind and reviewing all possible scenarios in Gardaneh Salavat area reveal that it is very unlikely to have an undiscovered porphyry orebody.In fact,Gardaneh Salavat is a porphyry system possibly suffered from fertility or metal endowment problem.This is a hypothesis that is hard to prove with certainty.However,pieces of evidence are provided from available data to highlight this hypothesis as the best matching piece for such a complicated exploration puzzle.

All porphyry copper provinces have some barren intrusions.These barren intrusions are similar in age and composition compared to the productive/fertile ones(Edwards and Atkinson 1986).There have been a number of efforts to distinguish fertile from barren magmatic systems.Despite some achievements in this subject,there is almost no straightforward way established to answer this question with high certainty since this is really complex problem(Stringham 1960;Edwards and Atkinson 1986;Grancea et al.2001;Williamson et al.2016;Uribe-Mogollon and Maher 2020;Chiaradia 2020;Du and Aude´tat 2020).

Porphyry copper deposits are intrinsically complicated as they can form in various geodynamic conditions and relating all porphyry copper deposits to subduction seems to be over-simplistic(Sinclair and Goodfellow 2007;Berger et al.2008;John et al.2010;Zu¨rcher et al.2015).The involved ore-related magmas in porphyry copper systems can be generated in various ways ranging from the mantle to melting and/or assimilation(Sinclair and Goodfellow 2007).Moreover,it is not truly known that whether the sources of magma should be enriched in specific metals or the subsequent magmatic process such as fractionation or supplying metals while ascending would be enough for deposit formation (Sinclair and Goodfellow 2007).Chiaradia(2020)showed that metal endowments for porphyry copper deposits grow larger for those cases with longer mineralization process.Grancea et al.(2001)emphasized three factors for a magmatic system to be fertile/productive:(1)Timing of volatile saturation versus crystallization matters.For instance,titanomagnetite crystallization may remove Au from the melt.Magnetite,ilmenite,and biotite do the same to Cu and Au in silicate melts.(2)Silicate melts in mineralized intrusions are enriched in sulfur compared to barren suites.Thus,addition of sulfur or other gases may affect fertility.Here,the role of gases should be considered significant rather than that of fluids.(3)The ratio of(Na+K+2Ca)/Al in the residual melt reflects the effect of chemistry of the residual melts(it can be examined by trapped inclusions,not bulk rock analysis).

Du and Aude´tat(2020)believed that early sulfide saturation is not necessarily detrimental to deposit formation or fertility of porphyritic intrusions.Further proving this fact,in alkaline Cu-Au porphyry deposits like Gardaneh Salavat porphyry system,magma must remain sulfur under-saturated throughout its evolution;preventing chalcophile elements(Cu,Mo,Au,etc.)to be crystallized during magma fractionation because it decreases the metal endowment of the late magmatic fluids of porphyritic intrusion(Clark 2014;Park et al.2019).This is actually the timing of sulfur saturation in the system that is vital for deposit formation(similar to the mentioned criteria by Grancea et al.(2001)).In addition,high fugacity of oxygen is another important factor as it increases sulfur solubility in the magmatic melt.Remember that the solubility of sulfur in the melt also depends on sulfur content(Clark 2014).Another point in this context is that alkaline porphyry system may lack phyllic and propylitic alterations(John et al.2010).As a matter of fact,when the porphyritic intrusion reaches the desired depth of emplacement,the amount of sulfur is not so high(due to the under-saturation of sulfur nature of magma),thus it can only supply sulfur for sulfide ore minerals.

Now,after reviewing some points regarding the fertility of porphyry copper systems,a few reasons are listed to highlight that Gardaneh Salavat porphyry system is a barren intrusion:

· The ranges of copper,molybdenum,and gold enrichment are not noticeable.In fact,the concentrations are only anomalous.

· The weakly developed potassic alteration can be a reason for the low enrichment of metals in the system(Watanabe et al.2018).That is proved both by magnetic susceptibility model,and core logging.Note the existence of potassic alteration is approved by core logging,thin section petrology,and fluid inclusion studies.

· Extensive propylitic and phyllic alteration haloes exist in Gardaneh Salavat area while some alkaline porphyry Cu-Au deposits lack these alterations.Moreover,a considerable amount of pyrite mineralization occur in the widespread peripheral alterations and the barren core itself.Thus,the magmatic-hydrothermal system was highly enriched in sulfur.Accordingly,it is extremely likely that the sulfur saturation might have occurred for the magma before final emplacement and magmatic-hydrothermal activities in the region,leading chalcophile elements to be removed from the magmatic system(i.e.low metal endowment of late magmatic fluids).

· The lack of known porphyry copper deposits in close neighborhoods(i.e.60 km)could be another logical reason for low metal endowment in Gardaneh Salavat porphyritic intrusion.That is because porphyry copper deposits usually have close spatial and temporal relationships(Carranza and Hale 2002;Singer et al.2005;Parsa et al.2018;Safari et al.2018).Besides,there is a general correlation between the relative endowment of the porphyry setting,the level of preservation and exposure,the number of porphyry deposits,and prospects in a promising metallogenic province(Zu¨rcher et al 2015).It is clear that all porphyry copper provinces have some unproductive intrusions but there is no significant porphyry deposit nearby(i.e.60 km)and this fact can be considered as a regional parameter in this context.

4.2 Evaluation of proposed hypothesis based on fertility indicators

To test the proposed hypothesis,some of the main fertility indicators have been examined by means of the available major and trace element analysis of the whole rock.In other words,to assess the geochemical characteristics of primitive source of porphyritic intrusion,twelve samples have been taken from intrusive rocks that were analyzed by both ICP-MS and XRF methods.Note that common geochemical diagrams for rock classification and geotectonic setting evaluation have been plotted by GCD Kit v6.0 program(Janousˇek et al.2006).In addition,some other fertility indicator bi-plots have been drawn in Microsoft Excel.

In Fig.14,there are a few diagrams related to intrusive rock composition,primitive magmatic source characteristics and its fertility.It is important to note that the high Loss On Ignition(LOI)values in XRF analysis due to alteration indicates that some of these diagrams may not be valid/certain for interpretation.However,a brief assessment of these diagrams together with the discussed points can be a basis for proving our hypothesis at Gardaneh Salavat area.Figure 14a and Fig.14b show the nature of intrusion source(i.e.primitive magma)which is categorized as high-K calc-alkaline to shoshonite series.The rock composition is delineated as diorite to gabbro(Fig.14c)that is identical to the results of macroscopic and microscopic studies.Note that weak clustering of samples in Fig.14a-c is the consequence of alteration in analyzed samples.

Loucks(2014)published a paper on distinguishing fertile porphyry copper systems from barren ones by using the available data from 135 deposits and some barren intrusion systems.With his sample selection criteria(i.e.LOI＜3.5%, etc.), he concluded that Sr/Y＞35 and V/Sc＞32.5-0.385*wt.%SiO2(at 58-70 wt.%SiO2)are typical of arc magmas parental to magmatic-hydrothermal copper ore deposits at convergent plate margins(Loucks 2014).Thereafter,Ahmed et al.(2020)reported the effectiveness of Sr/MnO vs.Sr/Y bi-plot for the discriminating productive versus unproductive porphyry copper systems.Wells et al.(2021)suggested another fertility indicator based on immobile elements of Zr and Y.Despite Sr(i.e.the basic component of previously reported fertility indicators),Zr and Y are relatively immobile under common alterations in porphyry copper systems.Thus,having a relatively fresh sample,which is challenging in altered porphyry copper terranes,won’t be necessarily required for fertility assessments,and the fertility assessments will be more reliable(Wells et al.2021).

The above mentioned methods for fertility characterization were performed for Gardaneh Salavat porphyry samples.The Loucks’method seems to be unsuitable for our set of samples since they have high LOI values(LOI＞4.14)while Loucks’samples had LOI＜3.5.However,for comparison,the Sr/Y vs.SiO2plot is illustrated in Fig.14g(Loucks 2014;Corral et al.2021)that shows all Gardaneh Salavat samples in the barren category.The Sr/MnO vs.Sr/Y bi-plot of Ahmed et al.(2020)also highlights Gardaneh Salavat samples within the barren cluster(Fig.14h).Although these two diagrams can prove our hypothesis,the priori assumption is that the alteration in the sample should be minimal while our samples are all affected by alteration fluids,which in turn,leads to uncertain classification in the proposed methodologies.According to Wells et al.(2021)method that utilizes immobile major and trace elements,the intrusion is hydros and can be fertile(Fig.14d).The Zr/Nb vs.Y/Nb trend of Gardaneh Salavat samples don’t follow the two introduced trends for fertile and barren intrusions (Fig.14e).According to Fig.14e,the data of available samples fit into a linear curve(slope or Y/Nb coefficient factor:0.4)that can be considered closer to barren intrusions trend(slope:1.95)with respect to fertile intrusions trend(slope:5.58).In Fig.14f,the Zr vs.Y bi-plot is illustrated with proposed clustering in terms of primitive magma fertility.Accordingly,the samples of Gardaneh Salavat area lie within various clusters,leading to great uncertainty.It is important to note that Wells et al.(2021),after comparing their proposed clustering,mentioned that fertile intermediate intrusive rocks from the Macquarie Arc have 50 ppm＜Zr＜125 ppm and those samples with Zr＜50 ppm were symbolized as unknown.Since Gardaneh Salavat samples within the fertile cluster have less than 50 ppm of Zr,it is questionable if we can consider them truly as fertile.Moreover,the Zr vs.Y fertility indicator is more efficient for low-and medium-K calc-alkaline porphyry systems because:(1)high K2O concentrations delay and diminish the crystallization of hornblende,leading to weaker depletions of Y in the residual melt,and(2)the increase in(K+Na)/Al in silicate melts increases the solubility of Zr in the melt.Overall,Zr and Y depletions in high-K calcalkaline to shoshonite fertile magmas are subdued compared to medium-to low-K calc-alkaline fertile intrusions(Wells et al.2021).

Fig.14 Major and trace element geochemistry diagrams for rock classification and magma fertility characterization(a Th vs.Co diagram(Hastie et al.2007),b K2O vs.SiO2 diagram(Peccerillo and Taylor 1976),c Na2O+K2O vs.SiO2 diagram(Middlemost 1994),d Zr*Y vs.Al2O3/TiO2 diagram(Wells et al.2021),e Zr/Nb vs.Y/Nb diagram(Wells et al.2021),f Zr vs.Y diagram(Wells et al.2021),g Sr/Y vs.SiO2 diagram(Loucks 2014;Corral et al.2021),h Sr/MnO vs.Sr/Y diagram(Ahmed et al.2020))

To conclude,it is worth mentioning that currently developed fertility indicators alone based on major and trace elements geochemistry cannot determine if the primitive magma of Gardaneh Salavat porphyritic intrusion was barren or fertile.That will be a problem for similar porphyry copper prospects(highly altered terranes and/or high-K to shoshonitic intrusions).Overall,despite all of the uncertainties,examination of various data types show that infertility or low metal endowment of Gardaneh Salavat porphyry system is probably the only logical explanation that can be expressed.Maybe the implementation of detailed investigations on porphyry indicator minerals such as zircon(i.e.Lu et al.2017;Shu et al.2019;Cooke et al.2017;Corral et al.2021;Lee et al.2021),which have not yet been performed at Gardaneh Salavat area,can provide further clue to this fertility problem in the region.

5 Conclusions

This investigation presents a brief exploration data analysis at the Gardaneh Salavat porphyry system.Unfortunately,despite all technical considerations during the project,the exploration did not yield to ore discovery.Actually,all of the geological,geophysical,and geochemical characteristics of a porphyry Cu-Au deposit exist but there is no economic mineralization at the subsurface.When examined in detail,it is understood that the only reasonable explanation can be related to the infertility or low metal endowment of the Gardaneh Salavat porphyritic intrusion.The logic behind this hypothesis is discussed in the paper.In brief,the reasons include(1)anomalous ranges of Cu,Au,and Mo rather than economic ore grades,(2)weakly development of potassic alteration,(3)probably sulfur saturation of magma that occurred before intrusion emplacement in the host rocks,and(4)probably low metal endowment in the regional scale due to lack of known porphyry copper deposits in close neighborhoods.To challenge our hypothesis,common fertility indicators of porphyry copper systems are also examined based on major and trace element geochemistry available from the study area.However,the currently developed fertility indicators could not determine if the primitive magma of Gardaneh Salavat porphyritic intrusion was barren or fertile.That’s because the unaltered samples from the intrusion was not available and the geochemical character of the intrusive rocks were high-K calc-alkaline to shoshonitic.Overall,despite all of the uncertainties,infertility or low metal endowment of Gardaneh Salavat porphyry intrusion is the only logical possibility to consider in this study area.

In the mineral exploration industry,we implement different techniques to vector toward orebodies.As illustrated here in this paper,typical geological,geochemical,and geophysical signatures/anomalies would be observed over a barren porphyry system.Consequently,the characterization of fertile from barren intrusions can be as important as vectoring towards concealed porphyry copper deposits in permissive tracts,especially while dealing with deep exploration targets that are inherently associated with high financial risks.Additionally,there are inefficiencies and uncertainties in magma fertility assessments for porphyry copper intrusions.Thus,an overarching investigation on fertility indicators with a broader vision will be necessary for the future.

AcknowledgementsUndoubtedly,writing this article would not be accomplished without the great insights of various geoscientists.Many thanks to IMIDRO and Parsi Kan Kav Campany for providing exploration datasets.I also have to appreciate the members of the above-mentioned groups,especially my colleagues at Parsi Kan Kav,who have had extensive efforts during the Gardaneh Salavat project.

Declarations

Conflict of interestThe author declares no conflicts of interest.