Con straints on ore for ma tion con di tions at the Mazra’eh Shadi epi ther mal de posit, NE Tabriz, Iran: ev i dences from geo chem is try, sul phur iso tope, quartz tex tures
and fluid in clu sion stud ies
Kaikhosrov RADMARD1, *, Hassan ZAMANIAN1, Mohammad Reza HOSSEINZADEH2, Ahmad Ahmadi KHALAJI1
1 Lorestan Uni ver sity, Fac ulty of Nat u ral Sci ences, De part ment of Ge ol ogy, Khoram Abad, Iran
2 Uni ver sity of Tabriz, Fac ulty of Nat u ral Sci ences, De part ment of Earth Sci ences, Tabriz, Iran
Radmard, K., Zamanian, H., Hosseinzadeh, M.R., Khalaji, A.A., 2019. Con straints on ore for ma tion con di tions at the Mazra’eh Shadi epi ther mal de posit, NE Tabriz, Iran: ev i dences from geo chem is try, sul phur iso tope, quartz tex tures and fluid in clu sion stud ies. Geo log i cal Quar terly, 63 (2): 230–247, doi: 10.7306/gq.1465
As so ci ate Ed i tor – Justyna Ciesielczuk
The Mazra’eh Shadi de posit is one of the most rep re sen ta tive gold de pos its in the Ahar-Arasbaran Belt. The main min er als are ga lena, sphalerite, py rite and chal co py rite. Con cen tra tions of Au-Ag oc cur mainly within quartz veins. Five tex tures (crustiform, comb, microcrystalline, cock ade, and mo saic) are dis tin guished by field re con nais sance and hand spec i men ob - ser va tions. The d34S val ues sug gest an in creas ing role of me te oric wa ter from the deep est lev els to the shal low level and sur - face. Fluid in clu sion data show that the min er ali sa tion at the Mazra’eh Shadi de posit can be clas si fied as a vol ca nic-rock-hosted in ter me di ate-sulphidation epi ther mal de posit. Fluid in clu sions in vein quartz can be dis tinctly di vided into three types ac cord ing to in ter pre ta tion of petrographic fea tures: in tense boil ing, gen tle boil ing and non-boil ing con di - tions. The pres ence of in tense and gen tle boil ing among dif fer ent substages at the same level in the Mazra’eh Shadi de posit in di cates that the base of the boil ing zone likely shifted up ward and down ward dur ing vein for ma tion. The con cen tra tions of Au-Ag oc cur mainly within quartz veins in the shal low level with gen tle boil ing (max. 813 ppb Au) and with in tense boil ing (max. 2420 ppb Au), whereas lower Au-Ag con cen tra tions are as so ci ated with base metal-rich (Pb-Zn) in the deep est lev els with non-boil ing flu ids (max. 52 ppb Au).
Key words: Mazra’eh Shadi, al ter ation, in tense boil ing, supersaturation.
INTRODUCTION
Epi ther mal gold, base metal and por phyry de pos its in Iran are mainly pres ent in two mag matic belts re lated to the geodynamic evo lu tion of the Tethyan realm be tween the Ara - bian and Eur asian plates dur ing the Early Me so zoic–Late Ce - no zoic: NW-trending Urumieh-Dokhtar mag matic belt (UDMB), and the E–W-trending Alborz Belt (Yang et al., 2009). The Ce - no zoic Ahar-Arasbaran Belt (AAB) of north west ern Iran is part of the Alborz Mag matic Belt. The Mazra’eh Shadi de posit is lo - cated in AAB, north ern Iran, which is lo cated ~130 km north-east of Tabriz in the Alborz Mag matic Belt geostructural zone (Fig. 1). Large Cu-Mo por phyry de pos its, Cu-skarn oc cur - rences, and Cu-Mo-Au epi ther mal-vein de pos its in this area in - di cate eco nomic value and po ten tial of min er ali sa tion in this
mag matic belt and hence re quire ment of more sys tem atic stud - ies of metallogenesis and earth re source ex plo ra tion. Quartz gen er ally is a dom i nant gangue min eral as so ci ated with epi ther - mal vein de pos its and re flects dif fer ent hy dro ther mal fluid con - di tions. There fore, quartz tex tures have been con sid ered by many re search ers to cor re spond to the evo lu tion of hy dro ther - mal sys tems (Saunders, 1994; Bobis et al., 1995; Dong and Zhou, 1996; Shimizu et al., 1998; Moncada et al., 2012). A com - mon trend de tected in hand spec i mens of quartz is that its tex - ture var ies from typ i cally coarsely crys tal line at depth to fine - -grained and microcrystalline at shal low lev els of epi ther mal vein sys tems (Dowling and Mor ri son, 1989; Hedenquist et al., 2000; Simmons et al., 2005; Chris tie et al., 2007). Quartz tex - tures in hand-scale sizes of ore sam ples were se lected to show the tex tural char ac ter is tics. The stud ies on the Mazra’eh Shadi min er ali sa tion have been lim ited pro vid ing lit tle in sight into the or i gin of the de posit. In this pa per, we de scribe the geo log i cal set ting and geo chem i cal char ac ter is tics of the Mazra’eh Shadi de posit, in clud ing fluid in clu sions, quartz tex tures and sul phur iso to pic char ac ter is tics, and pro pose a ge netic model for the de posit and guide lines for fu ture ex plo ra tion in the AAB.
* Corresponding author, e-mail: k_radmard@yahoo.com Received: February 22, 2018; accepted: February 9, 2019; first published online: May 17, 2019
GEOLOGY
The Alborz Mag matic Belt in north ern Iran is di vided into the west ern and east ern parts (Azizi and Jahangiri, 2008; Azizi and Moinevaziri, 2009). The east ern zone con sists of ba sic and fel - sic tuffs and lavas with al ka line to shoshonitic af fin i ties (Blourian, 1994), whereas the west ern part con sists of andesitic to dacitic lavas and many calc-al ka line to shoshonitic granitoid bod ies. The west ern part is sub di vided into two min er al ized prov inces, the AAB in the north and the Tarom-Hashtjin met al - lo gen ic prov ince in the south (Fig. 1).
The Ce no zoic AAB hosts pre cious and base metal min er ali - sa tion (Daliran et al., 2007), in clud ing por phyry, skarn, and epi - ther mal cop per, mo lyb de num, and gold de pos its. Ac cord ing to the dis tri bu tion of the ore de posit types, three dif fer ent met al lo - gen ic zones are dis tin guished within the AAB (Fig. 2). The three zones are (Jamalia et al., 2010):
–Zone A con tains Cu ± Mo ± Au por phyry and skarn and stockwork Cu-Mo-Au min er ali sa tion types (Daliran et al., 2007). The Sungun, Anjerd and Mazra’eh mines from west to east are lo cated in this zone.
–Sev eral epi ther mal gold oc cur rences (Mazra’eh Shadi, Safikhanlu, Sarilar and Zaglic mines) are lo cated in Zone B.
Prom i nent ex am ples of this type of min er ali sa tion are hosted by the Eocene vol ca nic rocks (Daliran et al., 2007).
–Zone C de pos its oc cur in the moun tain range from the Sabalan vol cano in the south-east to the Jolfa re gion in the north-west. The main ex am ples in clude the Masjed Daghi and Miveh Rud (Daliran et al., 2007).
Wide spread, WNW-trending al ter ation zones that are par al - lel to the re gional faults oc cur mainly within the Eocene vol ca nic rocks in Mazra’eh Shadi in Zone B. Con sid er ing the al ter ation as sem blages, vein min er als, and fluid in clu sion data, Ebrahimi et al. (2009) rec og nized dif fer ent types of epi ther mal sys tems in the AAB, as fol lows (Fig. 2):
–low-sulphidation (Safikhanlu and Zaglic);
–in ter me di ate (e.g., Mazra’eh Shadi);
–high-sulphidation (e.g., Masjed Daghi).
Geo log i cally, the study area con sists of three rock suites (Fig. 3; Radmard et al., 2017):
–eocene pyroclastic rocks, es pe cially tuffs and andesitic lavas;
–oligocene-Plio cene pyroclastic rocks, acidic domes, and dykes of andesitic and dacitic com po si tions;
–qua ter nary volcanogenic con glom er ate (ag glom er ate) and al lu vial units.
The old est rocks in this area are Eocene pyroclastics, which are cov ered by Oligocene and Plio cene vol ca nic rocks. They com prise an de site and trachyandesite. Nu mer ous dacite to andesitic dykes and stocks in truded into the Up per Eocene vol - Fig. 1. A sim pli fied re gional geotectonic map of Iran (Nabavi, 1976)
Lo ca tion of stud ied area is marked by black rect an gle. Lo ca tion of the Ahar-Arasbaran Belt in the Alborz Mag matic Belt (NW Iran) that is sub di vided into the Ahar-Arasbaran Belt in the north and Tarom-Hashtjin met al lo gen ic prov ince (THMP) in the south (Azizi and Jahangiri, 2008;
Azizi and Moinevaziri, 2009)
ca nic com plex. The youn gest for ma tion in the area is Qua ter - nary al lu vial sed i ments. The Qua ter nary volcanogenic ag glom - er ate (lahar) widely crops out in the vi cin ity of Mazra’eh Shadi in the south east ern part of the study area (Fig. 3A). Subvolcanic rocks, such as quartz monzodiorite – diorite por phyry (mdp), in - truded into Eocene vol ca nic and vol cano-sed i men tary units and caused min er ali sa tion and al ter ation of these units.
Gold min er ali sa tion in the Mazra’eh Shadi re gion is re lated to Eocene vol ca nic and in tru sive rocks. Out crops of the sub - volcanic rocks ex tend over about 1650 m and show a width of about 300–450 m west of Hizehjan (Radmard et al., 2017). The lo ca tions of sam ples are shown in Fig ure 3B.
The main struc ture in the Ahar-Arasbaran metallogeny re - gion formed as re sult of the Al pine Orog eny, es pe cially dur ing the Cre ta ceous Laramid phase (Radmard et al., 2017). The evo lu tion of this re gion has been a con tro ver sial is sue. While some au thors con sider this re gion as an in te gral part of the UDMB, based on sim i lar i ties in the geo chem is try of the vol ca - nic-plutonic rocks (Nogol-Sadat, 1993), oth ers ar gue for a rift set ting (Riou, 1979) or a rift and col li sion-re lated set ting (e.g., Moayyed et al., 2008). The UDMB is dom i nated by calc-al ka line vol ca nic and plutonic rocks and is con sid ered an An dean-type mag matic belt gen er ated by NW-dip ping subduction of Neo - -Tethyan oce anic crust be neath the Cen tral Ira nian mi cro-con ti - nent, and the col li sion of the Af ri can and Eur asian plates dur ing the Al pine Orog eny in the Paleogene (Berberian and King, 1982). The de pos its, which are formed from the sur face to ap - prox i mately 400 m be low the sur face, are typ i fied by veins, stockworks and dis sem i na tions.
The hy dro ther mal ac tiv ity and the pres ence of the faults and highly frac tured brec cia zone caused a va ri ety of al ter ation in the Mazra’eh Shadi de posit. Field map ping, mi cro scopic study and XRD anal y sis re sulted in rec og ni tion of five, zon ally dis trib - uted hy dro ther mal al ter ation as sem blages that con sist of propylitization, phyllic, argillic and ad vanced argillic al ter ations as so ci ated with sili ci fi ca tion and ore de po si tion. Gen er ally, the out crops of the sili ci fi ca tion can be di vided into three main forms; as stockwork in the north east ern part of the area; as dis - tinct sil ica caps in out crops of the cen tral and south west ern part;
as sil ica veins with a 20°N–80°E trend in the south west ern part (Radmard et al., 2017).
MATERIAL AND METHODS
More than 100 sam ples have been col lected from sur face and drill core sam ples in the Mazra’eh Shadi re gion. Lab o ra tory works be gan with prep a ra tion of thin and pol ished sec tions from 95 sam ples which were stud ied by op ti cal mi cros copy at Tabriz Uni ver sity. For de ter mi na tion of the ma jor rock-form ing min er als in the ores, 63 sam ples were se lected for X-ray dif frac tion (XRD) and ana lysed by a SIEMENS model D-5000 diffracto meter with CuKa ra di a tion, volt age 40 kV, beam cur rent 80 mA, con tin u ous scan ning, scan ning speed 8°/min, scan range 2–70°, slit DS – SS – 1°, am bi ent tem per a ture 18°C, and hu mid ity 30% in the Binalod lab o ra tory (Iran). Rock sam ples were crushed to 200-mesh size par ti cles in an ag ate mill. Un al tered ig ne ous rocks (16 sam ples, Ap pen dix 1*) were se lected for XRD from host Fig. 2. Dis tri bu tion of dif fer ent met al lo gen ic zones and min er ali sa tion in the Ce no zoic Ahar–Arasbaran Belt
ex posed in north west ern Iran (Jamalia et al., 2010)
* Supplementary data associated with this article can be found, in the online version, at doi: 10.7306/gq.1465
( aera yduts eht fo pam la c igo loeg – A3 .giFdramdaR7102 ,.la te sel pmas fo noi ta col – B ;) rof el pmas – ISM PCI ,si s ylana DSM rof el pmas – DRX,snoi tces dehs ilop dna niht rof el pmas – TSM ,si s ylana snoi sul cni diulf rof el pmas – FSM
rocks. The chem i cal anal y ses (in duc tively cou pled plasma mass spec trom e try – ICP-MS) were per formed in the Amdel lab o ra tory (Aus tra lia) and the ALS Chemex Can ada for 51 sam ples. Gold con tent was de ter mined by in duc tively cou pled plasma op ti cal emis sion spec trom e try (ICP-OES). De tec tion lim its for the el e - ments are: Au – 1 ppb, Ag – 0.2 ppm, Pb, Zn and Cu – 0.5 ppm.
Over 500 Scan ning Elec tron Mi cros copy with En ergy Dispersive Spec tros copy (SEM-EDS) anal y ses on a Field Emis sion mi cro - scope were used in this study. The SEM-EDS anal y ses were per formed at the De part ment of Met al lur gi cal and Ma te ri als En - gi neer ing, Lorestan Uni ver sity (Lore stan/Iran). Elec tron micro - probe anal y ses (EPMA) of the pol ished and thin-pol ished sec - tions were per formed in the Imidro lab o ra tory (Iran) by a Cameca SX100 at IMPRC. Op er at ing con di tions were: 20 kV and 20 nA, with a beam di am e ter 1–5 mm.
Ten sam ples con tain ing quartz veinlets from drill holes (Hen-07 and Hen-08 bore holes) and sur face sam ples were se - lected for ther mo met ric anal y ses. Microthermometric stud ies were car ried out at the Lorestan Uni ver sity Geo log i cal De part - ment–Fluid In clu sion Lab o ra tory by a Linkam THMSG600 freez ing-heat ing stage mounted on an Olym pus mi cro scope.
They were con ducted on 100–150 mm thick pol ished slabs pre - pared from 10 quartz sam ples from the si lici fied and min er al - ized zones. Most sam ples were from gold-rich and quartz-sul - phide brec cia veins. The pre ci sion of tem per a ture mea sure - ments is ~±0.1°C on cool ing runs and ±2°C on heat ing runs and the tem per a ture range var ies from –196 to 600°C. Sa lini ties of liq uid-rich fluid in clu sions were cal cu lated from the mea sured ice-melt ing tem per a tures (Bodnar, 1993). In ter po la tion of data was done by im ple ment ing the Kriging in ter po la tion func tion built in the Surfer soft ware, ver sion 9, Ex cel, com puter aided de sign (Auto CAD), Flu ids (Bakker, 1999), Clathrates (Bakker, 1997), and Flincore (Brown, 1989). The ba sic sta tis tics, min i - mum, max i mum, mean and stan dard de vi a tion were cal cu lated by SPSS soft ware, ver sion 16.
The sul phur iso tope com po si tions were ex am ined in 11 py - rite sam ples (from the Hen-07 bore hole). Rep re sen ta tive py - rite-rich sam ples from the ore stage vein ma te ri als were ana - lysed for sul phur iso tope ra tios at the G. G. Hatch Sta ble Iso - tope Lab o ra to ries, Uni ver sity of Ot tawa. The an a lyt i cal pre ci - sion is ±0.2‰.
RESULTS
Ig ne ous rocks in the Mazra’eh Shadi de posit in clude an de - site, trachyandesite to andesitic bas alts. These rocks con tain
~21 to 30% pheno crysts of plagioclase (oligoclase-an de sine) and hornblende (Fig. 4).
The sul phur iso to pic val ues of py rite are in a range from –0.2 to 0.7‰, but they are con cen trated in the range of 0.2 to 0.7‰. The d34S value of the fluid was cal cu lated from d34S value of py rite, and the min eral d34S ver sus H2S frac tion ation fac tor (Ohmoto and Rye, 1979) was es ti mated as sum ing H2S as the main sul phur spe cies in the fluid (Ta ble 1).
Microthermometric stud ies were per formed on pri mary in clu - sions in quartz sam ples. The fluid in clu sions are round and are mostly liq uid-rich two-phase ones. The in clu sions have a wide vari a tion in size from 4 to 93 mm with an av er age of 14 mm. The larg est fluid in clu sion of 93 mm was ob served in the out crop sam - ple (MSF-17). Ho mog e ni za tion tem per a tures of the in clu sions in quartz, which were com monly ob served ei ther as in clu sions along growth zones or iso lated sin gle in clu sions in crys tals, vary be tween 160 and 324 in drill core sam ples and be tween 176 and 302°C in sur face out crops, with an av er age of 228°C for sam ples from both drill core and sur face out crops (Radmard et al., 2017).
The ice melt ing tem per a tures vary from –0.1 to –3.2°C, which cor re spond to a sa lin ity range from 0.17 to 5.17 wt.% NaCI equiv - a lent with an av er age of 1.9 wt.%. The microthermometric data (ho mog e ni za tion tem per a tures, ice melt ing tem per a tures and size for fluid in clu sions) and cal cu lated pa ram e ters, such as sa - lini ties and den sity, are sum ma rized in Ap pen dix 2.
Three main types of fluid in clu sions were iden ti fied at room tem per a ture (25°C) in the sam ples from si lici fied and min er al - ized zones. In or der of abun dance, they are as fol lows:
Type 1 – liq uid-rich (LV). In clu sions of this type are filled by liq uid + vapour with the liq uid phase vol u met ri cally dom i nant and lack of daugh ter crys tals. These fluid in clu sions are com mon in all min er al ized quartz veins. The di am e ters of these fluid in clu - sions range from 4 to 93 mm. Liq uid-rich in clu sions (75–80% fill - ing de gree) oc cur in al most all sam ples and make the larg est num bers of the stud ied in clu sions. The last ice-melt ing tem per a - ture (Tm, ice) of these in clu sions is be tween –0.1 and –3.2°C.
These in clu sions yield ho mog e ni za tion tem per a tures from 160 to
Fig. 4. Pho to mi cro graphs of host-rock an de site (XPL)
A – plagioclase and hornblende are ma jor con stit u ents in porphyritic an de site;
B – por phy ritic an de site with fresh plagioclase pheno crysts;
XPL – cross-po larized light, Pl – plagioclase, Hbl – hornblende
308°C and the sa lini ties are 0.17–5.2 wt.% NaCl equiv a lent. This type oc curs widely in quartz with zonal tex ture.
Type 2 – two-phase vapour-rich (VL). In clu sions of this type are com mon in the out crop quartz vein sam ples. The di - am e ters of these fluid in clu sions range from 6 to 31 mm. Their last ice-melt ing tem per a ture (Tm, ice) is be tween –0.1 and –1.1°C. These in clu sions ho mog e nized into the gas phase. The ho mog e ni za tion tem per a tures range from 173 to 324°C, which is rel a tively the high est ho mog e ni za tion tem per a ture range ob - tained in the study area, and the re lated sa lini ties are 0.16–1.8 wt.% NaCl equiv a lent. The gas bub ble in (VL) in clu - sions oc ca sion ally oc cu pies >80% of the in clu sion vol ume. This type is com mon in quartz with comb tex ture and mo saic tex ture.
Type 3 – vapour monophase (V). In clu sions of this type have di am e ters rang ing from 10 to 30 mm.
DISCUSSON
PETROGRAPHIC AND GEOCHEMICAL CHARACTERISTICS OF HOST ROCKS
Un al tered ig ne ous rocks as so ci ated with the Mazra’eh Shadi de posit (16 out of 51 sam ples; Ap pen dix 1) are dis trib - uted in the subalkaline field on the to tal al ka lis ver sus sil ica di a - gram (Le Maitre, 2002), span ning from dacite, an de site to ba - saltic an de site (Fig. 5A). Ig ne ous rocks typ i cally as so ci ated with epi ther mal de pos its world wide in clude andesitic com po nents (Arribas, 1995; Simmons et al., 2005) and show subalkaline af - fin ity (Du Bray, 2014) com pris ing the ba salt-rhy o lite span (Fig.
5A). Ig ne ous rocks as so ci ated with quartz-adu laria epi ther mal de pos its have SiO2 con tents that range con tin u ously to as low as ~48 wt.% SiO2, and the SiO2 con tents of the prin ci pal ig ne - ous rock pop u la tion as so ci ated with quartz-alu nite epi ther mal de pos its only in crease to ~54 wt.%. Sil ica con tents of ig ne ous rocks as so ci ated with quartz-adu laria de pos its,which range to val ues about ~6% less than those of ig ne ous rocks as so ci ated
with quartz alu nite de pos its, are dis tinct and con sti tute the prin - ci pal geo chem i cal dif fer ence be tween ig ne ous rocks as so ci - ated with the two epi ther mal de posit types.
Many of the ig ne ous rocks as so ci ated with epi ther mal de - pos its have Sr/Y greater than ³20 (Fig. 5B) which is char ac ter is - tic of mag mas with adakitic com po si tions; these com po si tions are also con sis tent with greater mag matic wa ter con tents. In the Sr/Y ver sus Y (ppm) di a gram (Defant and Drummond, 1993), more than half of the Mazra’eh Shadi sam ples as so ci ated with epi ther mal de pos its have adakitic com po si tions with Sr/Y >20 (Fig. 5B).
Hornblende pref er en tially in cor po rates Y and there fore horn - blende crystallisation and frac tion ation re sult in pro gres sively de - pleted Y con tents (Sisson, 1994). Greater mag matic Sr con tents re sult from sup pressed crystallisation of plagioclase, which pref - er en tially in cor po rates Sr and, con se quently, the greater Sr/Y in - di cates crystallisation of hornblende and sup pres sion of plagio - clase crystallisation, both con sis tent with greater mag matic wa ter con tents that are ul ti mately con du cive to the mag matic vol a tile exsolution re quired to gen er ate the mag matic hy dro ther mal flu - ids re spon si ble in epi ther mal sys tems. In the Rb ver sus (Y + Nb) tectono-mag matic dis crim i na tion di a grams (Pearce et al., 1984), most of the ig ne ous rocks as so ci ated with epi ther mal de pos its fall in the vol ca nic arc gran ites field. How ever, some of the ig ne - ous rocks as so ci ated with the Mi das quartz-adu laria gold-sil ver de posit (Ne vada) sug gest a within-plate af fin ity (John, 2001). In this di a gram, all Mazra’eh Shadi sam ples fall in the vol ca nic arc gran ites field (VAG; Fig. 5C).
In the Na2O + K2O-CaO ver sus SiO2 vari a tion di a gram (quartz adu laria or quartz alu nite de pos its) the Mazra’eh Shadi sam ples are dis trib uted (Fig. 6) in the calcic field (62.5%) to calc-alkalic field (31.5%). Ig ne ous rocks typ i cally as so ci ated with epi ther mal de pos its con tain ing 50 to ~65 wt.% SiO2 are calcic to calc-alkalic, whereas sam ples with >65 wt.% SiO2 are gen er ally calc-alkalic to al kali-calcic (Frost et al., 2001). How ever, most of the ig ne ous rocks as so ci ated with epi ther mal de pos its world wide are calc-al ka line in terms of IUGS clas si fi ca tion.
T a b l e 1 Sul phur iso tope data of sulphides (py rite in the main stage min er ali sa tion)
from the Mazra’eh Shadi de posit
Sam ple
No. Vein min er al ogy T
[°C] d34S V-CDT [‰]
d34S H2S [‰]
Depth [m]
1 MS Qtz-Mo-Ccp-Py-Sp-Gn 243.7 0.6 –0.9 155
2 MS Qtz-Py 220.4 0.3 –1.3 55
3 MS Qtz-Mo-Ccp-Py-Sp-Gn 243.7 0.7 –0.8 155
4 MS Qtz-Py 219.0 –0.2 –1.9 sur face
5 MS Qtz-Py 219.0 0.1 –1.6 sur face
6 MS Qtz-Py 219.0 0.1 –1.6 sur face
7 MS Qtz-Mo-Ccp-Py-Sp-Gn 243.7 0.7 –0.8 155
8 MS Qtz-Py 220.4 0.4 –1.2 55
9 MS Qtz-Mo-Ccp-Py-Sp-Gn 243.7 0.7 –0.9 155
10 MS Qtz-Py 220.4 0.2 –1.4 55
11 MS Qtz-Py 219.0 0.3 –1.4 sur face
Ccp – chal co py rite, Gn – ga lena, Mo – mo lyb de nite, Py – py rite, Qtz – quartz, Sp – sphalerite; T is based on microthermometric mea sure ments of fluid in clu sions in quartz; d34S H2S val ues are cal cu lated based on H2S-sul phide sul phur iso tope frac tion ation equa tions of Ohmoto and Rye (1979)
Com par i son of the Mazra’eh Shadi Na2O + K2O-CaO ver - sus SiO2 and ma jor ox ides (TiO2/SiO2, Al2O3/SiO2) val ues with pub lished data from some epi ther mal de pos its (21 quartz adu - laria de pos its and 15 quartz alu nite de pos its; Du Bray, 2017) shows sim i lar ity to the quartz-adu laria epi ther mal min eral de - pos its (Figs. 6 and 7).
In low-SiO2 ig ne ous rocks as so ci ated with epi ther mal de - pos its, con cen tra tions of TiO2 and Al2O3 vary con sid er ably, but they de crease in a more si li ceous va ri ety (Fig. 7). The re la tion - ship be tween TiO2 and SiO2 is in ter preted as a func tion of crystallisation and frac tion ation of clinopyroxene, hornblende, bi o tite, and Fe-Ti ox ide (Du Bray, 2017). Plots of TiO2 or Al2O3
ver sus SiO2 for Mazra’eh Shadi sam ples are con sis tent with the quartz-adu laria epi ther mal type world wide (Du Bray, 2014). The re sults of elec tron probe microanalyses of quartz are sum ma - rized in Ta ble 2.
MINERALISATION
The min er ali sa tion is re stricted to sil ica veins and lo cally to the stockwork (Fig. 8A, B). The ore min er als oc cur as dis sem i - na tions, mas sive ac cu mu la tions, and the ma trix of hy dro ther - mally brecciated (Fig. 8C) and si lici fied vol ca nic rocks.
Min er ali sa tion oc curs ex clu sively along Hizehjan vil lage in the east ern part of the area. The main min er als are ga lena, sphalerite, py rite and chal co py rite (Fig. 8D, E). Ore and gangue min er als as sem blages iden ti fied in the Mazra’eh Shadi de pos - its are typ i cal of min er ali sa tion of epi ther mal or i gin (Radmard et al., 2017).
At the Mazra’eh Shadi de posit, Au con cen tra tions oc cur mainly within quartz veins (Fig. 8F) in a shal low level (65 m b.s.l.), whereas lower Au con tents are as so ci ated with base metal-rich deeper lev els (280–155 m). Py rite is the most abun - dant sul phide min eral (Fig. 8G– I).
Ac cord ing to the petrographic fea tures and cross-cut ting re - la tion ships, it is pos si ble to dis tin guish three main stages of quartz min er ali sa tion (Fig. 9). The in ter me di ate stage ap pears to be the most pro duc tive in no ble met als. It is dis trib uted in three spa tial do mains dif fer ing with mean gold con tents and de - duced boil ing con di tions:
a – deep est lev els (155 m with 52 ppb Au and no ev i - dence of boil ing),
b – shal low lev els (65 m with 813 ppb Au and gen tle boil - ing),
c – sur face (with 2420 ppb Au and in tense boil ing).
ORE TEXTURES
Ore tex tures are highly vari able. Ore min er als tex tures are mas sive, dis sem i nated, re place ment, banded (open-space fill - ing), and stockwork. The min er ali sa tion in cludes mainly Au-Ag - -bear ing quartz veins within an de site and trachyandesite, which is con trolled by fault dis tri bu tion (Radmard et al., 2017). The quartz veins con sist mainly of fine- to coarse-grained quartz or quartz and base metal sulphides with mi nor chal ce dony. Geo - chem i cal anal y ses of al tered an de site and trachyandesite in the north ern Mazra’eh Shadi show dis tinct anom a lies of Au, Pb, Ag, Cu and Zn (Radmard et al., 2017). Sul phide min er als con sist mainly of py rite as so ci ated with gold, whereas free gold grains oc cur rarely in sil ica veinlets. Gold at Mazra’eh Shadi is mostly in vis i ble and could be mainly traced by chem i cal anal y sis.
QUARTZ TEXTURE
In this study, the term “tex ture” is used for fea tures ob served in hand spec i mens and in mi cro scopic scale to doc u ment mor - pho log i cal vari a tions of vein quartz at the Mazra’eh Shadi de - posit (e.g., Dowling and Mor ri son, 1989; Barton, 1991; Bobis, Fig. 5. Com par i son of pet ro log i cal di a grams for Mazra’eh Shadi
de posit with epi ther mal de pos its (quartz-adu laria – red sym bols and quartz-alu nite – blue sym bols) epi ther mal min eral de pos its (Du Bray, 2017)
A – to tal al kali-sil ica di a gram of ig ne ous rocks as so ci ated with epi ther - mal de pos its; field bound aries from Le Maitre (2002); B – Sr/Y ver sus Y (ppm) abun dances in ig ne ous rocks (Defant and Drummond, 1993); C – trace-el e ment, tec tonic–set ting–dis crim i na tion vari a tion di a gram show ing the com po si tion of ig ne ous rocks; tec tonic set ting-com po si - tion bound a ries from Pearce et al. (1984); VAG – vol ca nic arc gran ites, WPG – within plate gran ites, ORG – ocean ridge gran ites, Syn-COLG – syn-col li sion gran ites
Fig. 6. Com par i son of Na2O + K2O-CaO ver sus SiO2 di a gram of ig ne ous rocks as so ci ated in Mazra’eh Shadi de posit with
epi ther mal min eral de pos its
A – as so ci ated with quartz-adu laria de pos its; B – as so ci ated with quartz-alu nite de pos its (Du Bray, 2017); bound aries be tween
var i ous rock se ries from Frost et al. (2001)
Fig. 7. Vari a tion di a grams show ing abun dances of ma jor ox ides (TiO2/SiO2, Al2O3/SiO2) in ig ne ous rocks as so ci ated
with quartz-adu laria epi ther mal min eral de pos its Sym bols are the same as those in Fig ure 6A (Du Bray, 2017)
T a b l e 2 Re sults of elec tron microprobe anal y ses of quartz [wt.%]
Point.
No. Lo ca tion Na2O K2O MgO CaO MnO FeO Al2O3 SiO2 TiO2 P2O5 To tal
1 MSF19 0.08 0.09 0.04 0.01 0 0.03 0.57 98.99 0 0 99.83
3 MSF19 0 0 0.01 0 0.01 0.03 0.4 99.39 0.02 0 99.86
4 MSF19 0.04 0.03 0.03 0.01 0 0 0.14 99.12 0 0.03 99.4
6 Hen-08-D143 0 0 0.05 0.01 0.02 0.02 0.12 99.85 0 0 100.1
9 Hen-08-D143 0 0.02 0 0 0.02 0 0.09 98.94 0 0.03 99.1
15 Hen-08-D143 0 0 0 0.03 0 0.04 0.08 99.37 0 0.04 99.56
26 Hen-07-D176 0.02 0 0 0 0 0.06 0.18 99.54 0.09 0.01 99.9
Fig. 8. Field pho to graphs (A and B), hand spec i men (C) and mi cro pho to graphs of ore (D–I)
A – out crop of a vein com posed mostly of quartz and Fe-ox ides–hy drox ides in si lici fied wall rocks in Mazra’eh Shadi de posit; B – out crop of stockwork of cop per-bear ing min er ali sa tion in quartz monzodiorite por phyry at Mazra’eh Shadi; C – brec cia tex ture in drill core sec tion; D – paragenesis of sphalerite and chal co py rite (re flected light) in main min er ali sa tion stage; E – paragenesis of ga lena and sphalerite (re flected light) in main min er ali sa tion stage; F – na tive gold in quartz vein (microcrystalline); G – py rite and Fe-ox ides as frac ture-fill ings in quartz; H – ag gre gate of spher i cal py rite en closed in quartz (pre-main min er ali sa tion stage); I – paragenesis of py rite, chal co py rite and sphalerite in the main min er ali sa tion stage; F–I – (back scat ter elec tron im age); Au – gold, Py – py rite, Ccp – chal co py rite, Sp – sphalerite, Gn – ga lena, Qtz – quartz
Fig. 9. Three main gen er a tions of quartz veins: Qtz1 – early (pre-main min er al iza tion, low in Au), Qtz2 – in ter me di ate (main ore as sem blage) re sult ing from gen tle boil ing, with high Au, Qtz3 – a late stage (post-main min er al iza tion, Au poor), Qtz – quartz, scale in centimetres
1994; Dong et al., 1995; Craig, 2001; Chauvet et al., 2006;
Chris tie et al., 2007).
Min eral tex tures that can in di cate a se quence of pre cip i ta - tion in clude pseudo morphs, inter growths, in clu sions of dif fer ent or i gin (exsolution, relicts) and growth zonation, among oth ers.
The de gree of over step ping of equi lib rium con di tions is re - corded by the dif fer ent form of a min eral, as has been shown ex per i men tally (Okamoto et al., 2010). Dif fer ent rates of min eral growth and the vari able bal ance be tween the rates of the mul ti - ple steps in the re ac tion path ways on the atomic scale of grain nu cle ation and growth is re flected by chang ing forms. Sil ica min er als, in par tic u lar quartz, are char ac ter ized by spe cific prop er ties (e.g., crys tal shape, col our, trace el e ment and iso to - pic com po si tion) which are re lated to the geo log i cal his tory and spe cific con di tions ex ist ing at the time of for ma tion of this hy dro - ther mal ore de posit.
Many chem i cal and phys i cal prop er ties of quartz and other sil ica poly morphs, such as trace el e ment con tent, iso to pic com - po si tion, etc., are de ter mined by their struc ture (Götze and Zimmerle, 2000; Götze, 2009). These dif fer ent prop er ties of quartz and other sil ica min er als re sult in the ex is tence of nu - mer ous va ri et ies, i.e. poly morphs. Nu mer ous stud ies have shown that quartz of dif fer ent or i gin can be dis tin guished by dis - tri bu tion and shape of in clu sions, tem per a ture of ho mog e ni za - tion, and chem i cal com po si tion of fluid in clu sions.
Vein tex tures in the Mazra’eh Shadi de posit were ex am ined at var i ous scales, from mi cro scopic to mesoscopic (hand spec i - mens). Anal y sis over a broad size range is nec es sary to un der - stand thor oughly the in for ma tion con tained in the rock (Craig, 2001). Such stud ies pro vide an in di ca tion of tex tural changes in a sin gle sam ple, as well as tex tural dif fer ences be tween sam - ples in dif fer ent lo ca tions, on the scale of centi metres to hun - dreds of metres. Such char ac ter is tics show the spa tial di ver sity of quartz tex tures, in clud ing those fa vour ing gold min er ali sa - tion, at depths up to shal low lev els or sur face. Five tex tures (crustiform, comb, microcrystalline, cock ade, and mo saic) are dis tin guished from field re con nais sance and hand spec i men ob ser va tion at the Mazra’eh Shadi de posit. Sam ples have been col lected from sur face and drill core sam ples (Hen-07 bore hole in an Au-Ag-bear ing quartz vein). Ex am ples of quartz tex tures ob served in the Mazra’eh Shadi de posit and their po si tion in the clas si fi ca tion of Dong et al. (1995) are shown in Fig ure 10.
Pri mary tex tures ob served in the stud ied quartz veins are os cil la tory zoned euhedral crys tals, chalcedonic and sphe roi dal tex tures, and mo saic quartz. The most com mon tex ture in the hy dro ther mal quartz veins in cludes os cil la tory euhedral growth zones that vary in thick ness and shape (Götze et al., 1999;
Lehmann et al., 2009). Mo sa ics of fine-grained in ter lock ing crys tals of quartz re sult from rapid quartz pre cip i ta tion upon pres sure drop in hy dro ther mal sys tems (Rusk and Reed, 2002). This tex ture may be de rived from recrystallisation or min eral re place ment.
FLUID INCLUSIONS
Ho mog e ni za tion tem per a tures (Th) of fluid in clu sions in epi - ther mal de pos its range from 100 to 350°C, with 90% of the data be tween ~120 and 310°C (Bodnar et al., 2014). The micro - thermometric data and cal cu lated pa ram e ters, such as sa lini - ties, ho mog e ni za tion tem per a tures, and ice-melt ing tem per a - tures, den sity and size for 106 liq uid-rich and vapour-rich fluid in clu sions, are shown in Fig ure 11.
Both base and pre cious metal de pos its show a mode in Th at the ~240–250°C span (Bodnar et al., 2014). Sa lini ties range from 0.1 to <40 wt.% equiv a lent NaCl, clus ter ing of the data be -
tween 0.1 and 17 wt.% NaCl equiv a lent, and plac ing ma jor ity of the sa lini ties <10 wt.% (Fig. 12). Both base and pre cious metal de pos its are dom i nated by flu ids with sa lin ity val ues less than a few weight per cent, al though base metal-rich de pos its (Fig. 12) tend to have a higher pro por tion of fluid in clu sions with sa lini ties
>3–4 wt.%, if com pared to the pre cious metal de pos its (Fig. 12).
Pro jec tion of the fluid in clu sion microthermometric data of Mazra’eh Shadi on the Th ver sus sa lin ity di a gram (Fig. 12A, B) in di cates their char ac ter is tics typ i cal of both base and pre cious metal de pos its, but still dom i nated by pre cious metal de pos its.
Rec og ni tion of fluid in clu sions trapped un der con di tions of boil - ing or immiscibility is valu able for P-T es ti ma tions be cause the ho mog e ni za tion tem per a tures equal the trap ping tem per a tures (Roedder, 1984).
Epi ther mal de pos its are clas si fied into high-sulphidation, in - ter me di ate-sulphidation, and low-sulphidation types ac cord ing to ore, min eral as sem blages, and chem i cal com po si tion of the par ent flu ids (Sillitoe et al., 2003).
Ac cord ing to Bodnar et al. (2014), ho mog e ni za tion tem per - a tures and sa lini ties of fluid in clu sions in high-sulphidation base and pre cious metal de pos its show a range from 100 to 345
±15°C and 0 to 23 wt.% NaCl equiv a lent, re spec tively. In in ter - me di ate-sulphidation, the base and pre cious metal de pos its show the tem per a ture range of 140–310°C and sa lini ties of 0–12 wt.% NaCl, re spec tively. Low-sulphidation de pos its are al - most ex clu sively pre cious metal de pos its and show a Th range from 120 to 320°C and sa lin ity from 0 to 14 wt.% NaCl, but still most data dom i nated within the 200–300°C and <5 wt.% sa lin ity range. Plots of sa lin ity ver sus ho mog e ni za tion tem per a tures for Mazra’eh Shadi fluid in clu sions in Fig ure 13 (for each sulphi - dation state: high, in ter me di ate, and low), in di cate that the de - posit is in ter me di ate with re spect to sulphidation.
The base met als to gold ra tio in creases at depth. Sul phide min er als de pos ited from pri mary ore-form ing flu ids (with the av - er age sa lin ity of 1.34 wt.% NaCl equiv a lent and tem per a ture of 219–281°C). The base-metal sul phide min er als: ga lena, sphalerite and chal co py rite, are com mon. Hy drau lic frac tur ing and a con com i tant rapid pres sure drop caused boil ing that led to out flow of early flu ids to a shal lower level and evo lu tion to - ward higher sa lin ity (av er age 2.49 wt.% NaCl equiv a lent) and lower tem per a ture (Figs. 14 and 15). As this fluid boils and SO2
and CO2 are par ti tioned into the vapour phase, the re main ing liq uid car ries a sur plus of H+, which makes it very acidic (pH = 1;
Hedenquist et al., 2000). Boil ing of an ore fluid in this case will re sult in loss of H2S to the vapour phase, which causes destabilization of the Au(HS)-2 com plex and pre cip i ta tion of Au.
This trend re flects the evo lu tion of flu ids and the de crease in tem per a ture with time.The pres ence of microcrystalline quartz at the Mazra’eh Shadi de posit is con sis tent with the sil ica supersaturation.
If Th data in fluid in clu sion as sem blages are con sis tent (i.e.
90% of the Th data con cen trate within a 10–15°C in ter val) among in clu sions of var i ous sizes and shapes, then the ho mog - e ni za tion tem per a tures of the fluid in clu sions re cord orig i nal con di tions of en trap ment (Goldstein and Reynolds, 1994).
Sum mary of microthermometric data and cal cu lated pa ram e - ters for pri mary fluid in clu sions (Ap pen dix 2, Fig. 16A) in di cate that grad ual evo lu tion of hy dro ther mal flu ids took part dur ing for ma tion of the Mazra’eh Shadi de posit.
The ef fect of con cen tra tion of dis solved el e ments due to los ing of volatiles dur ing fluid boil ing could re sult in in creased sa lini ties (Henley and McNabb, 1978; Wilkinson, 2001; Meinert et al., 2003; Liao et al., 2014). The evolved fluid had higher sa - lin ity (av er age 2.49 wt.% NaCl equiv a lent) and lower tem per a - ture (av er age 227°C), from which gold pre cip i tated (Fig. 16).
Two pop u la tions: high-tem per a ture low-sa lin ity, and low-tem -
Kaikhosrov Radmard, Hassan Zamanian, Mohammad Reza Hosseinzadeh, Ahmad Ahmadi Khalaji
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F Mazra’ehShadide posi t(left )withclas si fi ca tiono fDonge tal .(1995)(right )
Fig. 11. His to grams of the den sity (A), ho mog e ni za tion tem per a ture (B), sa lin ity (C), ice melt ing tem per a - tures (D) and size (E) for 106 stud ied liq uid-rich and vapour-rich fluid in clu sions in Mazre’eh Shadi de posit; Mean – arith me tic mean, Std. Dev – stan dard de vi a tion, Tm ice – ice melt ing tem per a tures
per a ture high-sa lin ity in clu sions mark out. This tem per a ture do - main cor re sponds to a depth of 200 to 380 m be low the wa ter ta ble (palaeodepth in Fig. 15). There fore, the boil ing should be con sid ered as the main mech a nism re spon si ble for ore de po si - tion at the Mazra’eh Shadi de posit (Fig. 16B).This find ing is con sis tent with co ex is tence of vapour-rich and liq uid-rich in clu - sions (Fig. 17).
STABLE ISOTOPES
Sul phur iso to pic com po si tions have been widely used to in - ves ti gate sul phide and sul phate de po si tion pro cesses, and as trac ers of sul phur sources (e.g., vol ca nic rocks, mag matic volatiles and sea wa ter). Sul phur iso to pic com po si tions of sulphides of epi ther mal de pos its gen er ally yield d34S val ues rang ing from –15 to 5‰ de pend ing on their geo log i cal set tings:
(a) –6 and + 5‰ for low-sulphidation with most val ues close to 0‰ (Ohmoto and Rye, 1979; Field and Fifarek, 1985);
(b) –15 to +8‰ for al ka line-hosted epi ther mal de pos its (Rich ards, 1995);
(c) –10 to +8‰ for high-sulphidation de pos its (Arribas, 1995).
The sul phur iso to pic val ues of sul phide min er als (–0.2 to 0.7‰, Fig. 18A) im ply that sul phur in the hy dro ther mal flu ids was de rived from mag matic volatiles (Rollinson, 1993). Sul phur iso tope val ues for sulphides dis play a shift in d34S from around 0.7‰ in deep est lev els (155 m) to –0.2‰ sur face sam ples (Ta - ble 1 and Fig. 18B). This sug gests an in creas ing role for me te - oric wa ter from the deep est lev els to shal low level and sur face.
Fig. 12. Sum mary of pub lished ho mog e ni za tion tem per a ture-sa - lin ity pairs from epi ther mal ore de pos its (Bodnar et al., 2014) with microthermometry data of Mazra’eh Shadi fluid in clu sions
A – fluid in clu sion data from epi ther mal base metal de pos its;
B – fluid in clu sion data from epi ther mal pre cious metal de pos its;
n – num ber of an a lysed in clu sion
Fig. 13. Com par i son of fluid in clu sion data for epi ther mal ore de pos its of high (A), in ter me di ate (B), and low (C) sulphidation (Bodnar et al., 2014) with microthermometry data on Mazra’eh Shadi fluid in clu sions
For each sulphidation state (high, in ter me di ate, and low), the data are also sep a rated with sym bol ac cord ing to the metal bud get in the de posit (base metal-rich ver sus pre cious metal-rich); n – num ber of ana lysed in clu sions
Sev eral epi ther mal gold sys tems are lo cated in AAB, such as those of high sulphidation (Masjed Daghi), in ter me di ate sulphidation (Mazra’eh Shadi) and low sulphidation (Zaglic and Safikhanloo). Sul phur iso tope val ues for sulphides show a shift from ~0‰ (1.2 to –1.1‰) in Masjed Daghi to in creas ingly neg a - tive in Zaglic and Safikhanloo. The d34S val ues sug gest a de - creas ing role for mag matic flu ids from the Masjed Daghi to Mazra’eh Shadi and Zaglic and Safikhanloo epi ther mal sys - tems, i.e. in the NWW–SEE di rec tion (Fig. 18C).
RELATIONSHIP BETWEEN QUARTZ AND ORE TEXTURES, BOILING, AND METAL DEPOSITION
Boil ing of flu ids oc curred re peat edly, lead ing to the so lu tion supersaturation by sil ica, which re sulted in the for ma tion of banded veins with vari able quartz tex tures. In ter mit tent boil ing in epi ther mal de pos its caused re peated se quen tial pre cip i ta tion of pre cious metal min er als with base metal sulphides that are as so ci ated with colloform and microcrystalline quartz (Shimizu, 2014). The pre vi ous stud ies at the Mazra’eh Shadi de posit (Ebrahimi et al., 2009, 2011) did not dis cuss the vari a tion in boil ing con di tions re vealed by the fluid in clu sion study.
At Mazra’eh Shadi, quartz veins con tain ing co ex ist ing liq - uid-rich and vapour-rich in clu sions, as a strong ev i dence of boil - ing dur ing hy dro ther mal evo lu tion, have rel a tively high Au grades (up to 813 ppb). This type is com mon in quartz with crustiform tex ture at shal low lev els and sur face.
Iden ti fi ca tion of de grees of boil ing, as ev i denced by the fluid in clu sion as sem blages formed at dif fer ent stages of the vein quartz pre cip i ta tion, is par tic u larly im por tant to dis tin guish be - tween the nonboiling through gen tle boil ing to in tense boil ing at the same sam ple lo ca tion.
In ef fect of the de tailed mi cro scopic study of fluid in clu sions in vein quartz at the Mazra’eh Shadi epi ther mal de posit three types of fluid in clu sion are rec og nized: (1) type A fluid in clu sion as sem blage along growth zones in quartz that are in dic a tive of nonboilling con di tions; these fluid in clu sions are oval; (2) type B fluid in clu sion as sem blage con sist ing of co ex ist ing liq uid- and vapour-rich in clu sions that are in dic a tive of gen tle boil ing; these in clu sions ex hibit an oval to ir reg u lar shape; (3) type C fluid in - clu sion as sem blage con sist ing en tirely of vapour-rich in clu -
sions that are in dic a tive of in tense boil ing, which ex hibit an oval to subrectangular plan shape. Fluid in clu sion as sem blages in type C at the Mazra’eh Shadi de posit (Fig. 17C) are the re sult of in tense boil ing.
The pres ence of both types of fluid in clu sions (B and C) in the Hen-07-D66 bore hole at the 66 m mine level in di cates that the in ten sity of boil ing fluc tu ated. The ho mog e ni za tion tem per a - tures of the in clu sions re corded orig i nal con di tions of en trap - ment, if a fluid in clu sion anal y sis yielded con sis tent Th data (i.e.
90% of the Th data within a 10–15°C in ter val) among in clu sions of var i ous sizes and shapes (Goldstein and Reynolds, 1994).
Ev i dence for gen tle boil ing or nonboiling con di tions is pro vided by the pri mary fluid in clu sions in zonal and comb quartz. The pres ence of adu laria, blade-shape cal cite, and quartz pseudo - morphs af ter this cal cite are ev i dence of lo cal boil ing (Browne, 1978; Hedenquist et al., 2000; Simmons and Browne, 2000).
The close as so ci a tion of pre cious metal min er als and microcrystalline quartz at the Mazra’eh Shadi de posit was likely the re sult of con com i tant pre cip i ta tion of pre cious met als and amor phous sil ica due to in tense boil ing. This the ory is sup - ported by the fact that large amounts of pre cious met als pre cip i - tate with amor phous sil ica as a con se quence of H2S loss due to in tense boil ing in sur face pipes where there are sharp pres sure de creases ob served in some New Zea land geo ther mal wells (Brown, 1986).
How ever, in tense boil ing is not the only cause for pre cious metal min er ali sa tion. Gen tle boil ing may be an in di ca tion of lower-grade pre cious metal pre cip i ta tion spread over a larger ver ti cal range above the base of the boil ing zone, and the high - est ore grades may oc cur at some dis tance above the base of the boil ing zone (Simmons and Browne, 2000; Moncada et al., 2012). At the Mazra’eh Shadi de posit, con cen tra tions of Au-Ag oc cur mainly within quartz veins at a shal low level in the Hen-07-D66 bore hole with gen tle boil ing (max. 813 ppb Au) and
Fig. 15. Plot of Ag, Au, sa lin ity, d34S and ho mog e ni za tion tem per a tures against depth in the bore hole Hen-07
in Au bear ing quartz veins Lo ca tion of quartz in Fig ure 10 Fig. 14. Plots of ho mog e ni za tion tem per a tures ver sus depth
for fluid in clu sions in bore hole Hen-07
Gen eral trend from base metal sul phide-dom i nated (lead and zinc) with high tem per a ture fluid to gold – dom i nated with
low tem per a ture
in the MSF17 with in tense boil ing (max. 2420 ppb Au), whereas lower Au-Ag con tents are as so ci ated with the base metal-rich deep est lev els in the Hen-07-D155 bore hole with a nonboiling pre cip i ta tion en vi ron ment (max. 52 ppb Au).
The ev i dence of in tense and gen tle boil ing in dif fer ent stages/substages, as well as the pres ence of comb and mo saic tex tures at the same depth in the Mazra’eh Shadi de posit, in di - cates that the base of the boil ing zone likely shifted up ward and down ward dur ing vein for ma tion. Chris tie et al. (2007) sug gested a re la tion ship be tween palaeodepths and quartz tex tures of ma -
jor epi ther mal de pos its in New Zea land. Crustiform tex ture is com mon at shal low to deep lev els and comb tex ture is char ac ter - is tic of deep lev els, whereas microcrystalline tex tures are typ i cal at in ter me di ate to deep lev els of these epi ther mal sys tems. In the Mazra’eh Shadi de posit, crustiform tex ture is com mon at shal low lev els (in dic a tive of in tense boil ing), whereas comb tex tures are typ i cal at shal low and sur face lev els (in dic a tive of gen tle boil ing).
The pres ence of mo saic quartz at the Mazra’eh Shadi de posit is con sis tent with the sil ica supersaturation and clas si fi ca tion of tex - tural fea tures by Dong et al. (1995) (Fig. 10).
Fig. 16. Plots of ho mog e ni za tion tem per a tures ver sus sa lin ity (wt.% NaCl equiv a lent) for fluid in clu sions in bore hole Hen-07
A – each box shows the me dian, quartiles, and ex treme val ues within a cat e gory; two pop u la tion, a – high tem per a ture-low sa lin ity and b – low tem per a ture-high sa lin ity; B – ho mog e ni za tion tem per a ture ver sus sa lin ity (wt.% NaCI equiv a lent) in fluid in clu sion in bore hole Hen-07 (Radmard et al., 2017); ar row rep re sent ap prox i mately boil ing trend ac - cord ing to model of Wilkinson (Wilkinson, 2001)
Fig. 18. Dis tri bu tion of d34S iso tope
A – his to gram of sul phur iso tope abun dance in the Mazra’eh Shadi de posit; B – plots of d34S val ues from Mazra’eh Shadi de posit ver sus depth; C – com par i son of d34S val ues from Mazra’eh Shadi de posit with Zaglic and Safikhanloo (Ebrahimi et al., 2011), Masjed Daghi (Ebrahimi et al., 2009), low-sulphidation epi ther mal de pos its (Field and Fifarek, 1985), pre cious metal epi ther mal de pos its (Cas tor et al., 2003) and Sungun por phyry (Calagari, 2003); NWW–SEE – di rec tion of map in Fig ure 2
Fig. 17. Fluid in clu sion types in quartz from Mazra’eh Shadi de posit
A – fluid in clu sion as sem blage in quartz, con tain ing liq uid-rich in clu sions with con sis tent liq uid to vapour ra tios that are in dic a tive of nonboiling con di tions (bore hole Hen-07-D155); B – a fluid in - clu sion as sem blage of co ex ist ing liq uid- and vapour-rich in clu sions that are in dic a tive of gen tle boil ing (bore hole Hen-07-D66); C – fluid in clu sion as sem blage of vapour-rich in clu sions, which are in dic a tive of in tense boil ing or gen er ally gas min eral-form ing fluid (MSF17); L – liq uid, V – vapour
In the Mazra’eh Shadi de posit the de gree of sil ica super - saturation in creases from deep to shal low lev els. Dis tinct are recrystallisation tex tures that de velop in ver sion of one or more metastable forms of sil ica to fairly sta ble quartz and hence re - cords ini tial pre cip i ta tion of sil ica as a metastable form (Herrington and Wilkinson, 1993). Vugs and euhedral growth into space are in dic a tive of open “fis sure” space in the veins or may in di cate recrystallisation at the pe riod of min eral growth.
Pseudomorphic re place ments of platy cal cite by quartz in di cate a change of pH or other phys i cal and chem i cal con di tions in the vein over the time of min eral pre cip i ta tion, such that, in the first step, cal cite rap idly grew from so lu tion as platy grains so the so - lu tion be came undersaturated with re spect to cal cite, and the car bon ate was re placed by quartz in the later step. Brec cias and veined tex tures ev i dence ac tive fault ing.
CONCLUSIONS
1. The Mazra’eh Shadi rock sam ples are dis trib uted in the subalkaline field of the TAS di a gram from dacite to ba saltic an - de site and show vol ca nic arc sig na tures.
2. Mi cro scop i cally ob serv able fea tures of fluid in clu sions in vein quartz re veal three types of par ent so lu tions: in tense boil - ing (com mon in quartz with crustiform tex ture), gen tle boil ing
(com mon in quartz of comb and mo saic tex tures), and non - boiling within the de posit area.
3. Sul phur iso tope val ues for sulphides dis play a shift in d34S from ~0.7‰ in the deep est lev els (155 m) to –0.2‰ in sur face sam ples, which sug gests an in creas ing role of me te oric wa ter to wards the sur face.
4. Pro jec tion of sa lin ity ver sus ho mog e ni za tion tem per a tures for Mazra’eh Shadi fluid in clu sions (for each sulphidation state:
high, in ter me di ate, and low) shows that the de posit can be clas si - fied as an in ter me di ate epi ther mal de posit of pre cious met als.
5. The close as so ci a tion of pre cious metal car ri ers and microcrystalline quartz (for mer amor phous sil ica) at the Mazra’eh Shadi de posit was likely the re sult of their con com i - tant pre cip i ta tion due to in tense boil ing.
6. Con cen tra tions of Au-Ag oc cur mainly within quartz veins in a shal low level with gen tle and in tense boil ing, whereas lower con tents of Au-Ag are as so ci ated with base met als in deep lev - els where nonboiling con di tions oc curred.
7. The de gree of sil ica supersaturation in creases from deep lev els to shal low lev els.
Ac knowl edge ments. The au thors are grate ful to the re - search com mit tee of Lorestan Uni ver sity and the Uni ver sity of Tabriz for sup port ing this pro ject. Au thors would like to thank the IMIDRO and the Cop per Com pany for tech ni cal sup port.
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