Filamentary native gold from the Elatsite porphyry Cu-Au deposit ...

Filamentary native gold fromthe Elatsite porphyry Cu-Audeposit, Srednogorie zone,Bulgaria

Au-Ag-Te-Se depositsIGCP Project 486, 2005 Field Workshop, Kiten, Bulgaria, 14-19 SeptemberIvan K. Bonev, Rumen Petrunov, Radostina AtanassovaGeological InstituteBulgarian Academy of Sciences1113 Sofia, Bulgaria;E-mail: bonev@geology.bas.bg

Native gold is known in highly variable morphology- As octahedral, cubic, dodecahedral and more complex crystals- In skeletal and dendritic forms- As irregular grains- As prismatic elongated or platy grains- Rarely, as hair-like and fine fibrous formationsPetrovskaya (1973) assigned them to a morphogenetic type of notwellbounded and rounded crystals

Bundles of goldfilaments in a smallcavity of magnetitebornite-chalcopyriteore

A group of filamentsAn enlargeddetail

10 µmThe twin-peakedtermination of one of thefilaments:cubic-octahedral habit

Single crystal filament with changeable growth direction andstepped lateral faces

10 µmA bundle of fibers in twinned positions, bounded by cubic faces;1 and 2: twinned along (111) crystals in two orientations

A fibre with longitudinal (111) twinplane

10 µmSurface morphology :complex vicinals on o(111)10 µm

cDifferent tips of goldfilaments: a) moreindividualised distortedcrystal termination; b) and c)splitted terminations withcomplex shapes (triple in b,and double in c).Crystal forms: d{110}, o{111}, a{100}, m{311}, f{310}. Scale bars - 10 µm

Gold filaments withchangeablethickness

Complex steppedsurfaces of somefilaments10 µm

A bundle of mechanically distorted subparallel filaments(A detail on right: translation gliding)

The ElatsitedepositChemical composition of filamentary gold and compositional fields ofgold from the main ore assemblages: mt-bn-cp, q-cp-py + q-py and q-ga-sph (after data of Kehayov and Bogdanov, 2005, and others). Inparticular, mt-bn-cp-II concerns to visible gold cutting early bornite

System Au-Cu-Fe-SAu in Cu sulphidesAu transport:Simon, Kesler, Essene, 2000 Gammon & Williams-Jones, 1997Stefansson & Seward, 2003, 2004bornitechalcopyrite600 o C 4,875 ppm 104-125 ppm AuCl 2-400 o C 13-80 ppm 2-4 ppm sulphide complexes

10 µmFilamentary crystals ofdifferent thickness.The Chelopech deposit

Galena filaments andstraight whiskersMadan ore district

Conclusions- The rare in nature filamentary gold in Elatsite occurs in small cavities of theearly magnetite-bornite-chalcopyrite-The filamentary native gold in Elatsite forms not straight crystals boundedby small alternating faces of the main crystal forms: a, o, d, f, m- The chemical composition of the filamentary gold is comparable with this ofthe main chalcopyrite-pyrite stage- Most probably, the high-temperature (600-530 o C) Au-rich bornite ss is earlycollector for gold, by cooling of which (~400 o C) gold is separated and redepositedin the close vicinity- The extremely non-equilibrium filamentary crystals of gold were depositedfrom highly supersaturated heterogeneous boiling solutions of high salinity