Çöpler (Erzincan, İç-Doğu Anadolu) Porfiri-Epitermal Altın Yatağında Arjilik Alterasyona İlişkin Mineralojik Kanıtlar

Abstract

Copler porfiri-epitermal altin yatagi, Tetis Alpin-Himalaya orojenik kusaginda acilmali tektonizma sonucu gelisen orta Eosen yasli plutonik sokulumlarla iliskili bir altin yatagidir. Plutonik kayaclar (granodiyorit porfir, diyorit porfir) Gec Paleozoyik-Mesozoyik yasli metapelit ve metakarbonat kayaclar icerisine sokulum yaparak porfiri-epitermal bir Cu-Au yatagi olusturmus ve hidrotermal alterasyon zonlarinin (potasik, fillik, propilitik, arjilik) gelisimine neden olmustur. Bu calismada, onceki arastirmacilarca yerel ve sinirli alanda gelistigi belirtilen superjen alterasyonun aksine, genis yayilim sunan arjilik alterasyon zonlarinin mineralojik-petrografik ozelliklerinin ortaya konulmasi amaclanmistir. Bu kapsamda, optik ve taramali elektron mikroskop (SEM ve SEM-EDS), X-isinlari kirinimi (XRD), O-H izotop jeokimyasi ve K/Ar yas tayini incelemeleri gerceklestirilmistir. Asiri killesmis granodiyorit porfirlerde bile, ilksel porfiritik doku izlenebilmekte, ince-kristalli kil ve kuvarslar homojen ve yer yer mikrolaminasyonlar seklinde gozlenmektedir. Simektit ve karisik-tabakali illit-simektit (I-S) mineralleri yapraksituysu, illit ve kaolinitler levhamsi bicimler sergilemektedir. Kuvars ve jarositler ozsekilli kristaller, kristobalitler kurtcuk gorunumlu, bu calismada ilk kez belirlenen krandallitler ise ince-taneli granuler topluluklar seklinde gozlenmektedir. Arjilik zona ait ornekler fillik zona yakin ic kesimlerde (ileri arjilik zon) kuvars + I-S, kuvars + krandallit + jarosit ve kristobalit, dis kesimlerde (arjilik zon) ise kuvars + simektit + kaolinit birliktelikleri sergilemektedir. Simektitler dioktahedral bilesime sahiptir (d060<1.500 A, oktahedral Al=1.47-1.66). I-S mineralleri yuksek illit bilesenli (I85-S15) ve R3 tipi ara tabakalanmaya sahip olup, oktahedral Al ve yapraklar arasi K icerikleri sirasiyla 1.66-1.71 ve 0.58-0.75 arasinda degismektedir. I-S’lerin O-H izotop bilesimi, dusuk sicaklik kosullarinda ve magmatik suyun baskin oldugu bir olusumu isaret etmektedir. Jarosit iceren ornegin K/Ar yas verisi (43.6 ± 1.0 My), arjilik alterasyonun plutonik sokulum sirasinda veya hemen sonrasinda (<1 My) basladigina isaret etmektedir. Elde edilen bulgular, Copler porfiriepitermal altin yataginda genis yayilim sunan, dusuk sicaklik (<200 oC) ve asidik kosullarda gelismis, aluminyum fosfat ve demir sulfat minerallerinin de gelistigi bir arjilik alterasyonun varligini ortaya koymaktadir. (The Copler porphyry-epithermal gold deposit is associated with middle Eocene intrusive rocks related to an extensional tectonic phase in the Tethyan Alpine-Himalayan orogenic belt. The intrusive rocks (granodiorite porphyry and diorite porphyry) were emplaced into Late Paleozoic–Mesozoic metapelite and metacarbonate rocks creating the porphyry-epithermal Cu-Au deposit and hydrothermal alteration zones (potassic, phyllic, propylitic, argillic). The mineralogic-petrographic and geochemical properties of the extensive argillic alteration zones (not supergene alteration described previously) were determined by optical and scanning electron microscopy (SEM and SEM-EDS), X-ray diffraction (XRD), O-H isotope geochemistry and K-Ar dating. Within the primary porphyritic texture, fine-grained clay and quartz crystals are present as homogeneous and partly micro-laminations in the more intensely argillized granodiorite porphyry. Smectite and mixed-layer illite-smectite (I-S) minerals exhibit flaky/sponge-like and fibrous shapes, respectively, whereas illites and kaolinites show platy/tabular crystal shapes. Quartz and jarosite have euhedral crystals, cristobalites and crandallites (first determined here) are shown as worm-like and fine-grained granular aggregates, respectively, developed within the pores indicating direct precipitation from the hydrothermal fluids. The samples from the argillic zone display quartz + I-S in the inner parts (advanced argillic zone) close to phyllic zone, whereas quartz + smectite + kaolinite associations are towards outer parts (argillic zone). Smectites have a dioctahedral composition (d060<1.500 A, octahedral Al=1.47-1.66 a.p.f.u). I-S minerals have a high illite component (I85-S15) with R3 type ordering of interlayering. The tetrahedral Al and interlayer K contents are 1.66-1.71 and 0.58-0.75, respectively. Oxygen and hydrogen isotope composition of I-S indicate the low temperature conditions from the magmatic water dominant fluids. K/Ar age data from jarosite-bearing sample (43.6±1.0 My) indicates that the argillic alteration started during or shortly after (<1 Ma) the plutonic intrusion. The data demonstrate the argillic alteration was widely distributed and developed under low temperature (<200 oC), acidic conditions in association with the aluminum phosphate and iron phosphate minerals.)