Electrum dendrites from the Ametistovoe epithermal deposit, Kamchatka, Russia

Electrum dendrites and associated minerals from the Ametistovoe Au-Ag epithermal deposit (Kamchatka) were studied by optical and electron microscopy, X-ray computed tomography, electron microprobe and electron back scattered diffraction techniques.Electrum forms dendrites up to 1 mm in size within sphalerite, or, rarely, within galena and chalcopyrite and random particles within quartz.It has a very homogeneous chemical composition (fineness 603 to 615) which correlates neither with the host mineral nor with the electrum morphology.X-ray computed microtomography showed electrum dendrites are 2-dimensional and comprise acicular "trunk" hosting numerous "branches".3-dimensional and "bush-like" dendrites are rare.Angles between "trunk" and "branches" are usually 90 and, less commonly, 60 suggesting twinning along (100) and (111) planes respectively.Electron back scattered diffraction evidenced the dendrites are single crystals with no defects detected while electrum within quartz has a mosaic texture with low pattern quality.A model proposed for the observed phenomena includes: 1) growth of single crystal electrum dendrites within silica gel; 2) overgrowth of dendrites by sphalerite crystals; 3) recrystallization of the opaline silica to crystalline quartz which resulted in destruction of electrum * * Corresponding author.