Mineralogical, petrological and geochemical characteristics of bauxites from 'Minjera' deposit near Buzet

Ovim radom istraživane su mineraloške, petrološke i geokemijske značajke boksita iz ležišta „Minjera“ kod Buzeta u Istri. Semikvantitativni mineralni sastav 8 uzoraka sivih boksita, uzorkovanih duž profila ležišta, određen je metodom rendgenske difrakcije na prahu (XRD). Petrološke karakteristike 11 uzoraka vapnenaca iz podine, krovine te crnog boksita u krovini ležišta određene su metodom polarizacijske mikroskopije. Spektrometrijskim metodama (ICP-AES, ICP-MS, XRF) određen je kemijski sastav analiziranih uzoraka. Podinu boksita čine naslage cenomanskih rudistnih vapnenaca, a u krovini im se nalaze paleocenski slatkovodno-brakični vapnenci s gastropodima i ostacima bilja. Crni boksit iz vršnog dijela boksita, sadrži boksitične piritizirane ooide. U uzorcima sivih boksita iz profila određeni su sljedeći minerali: dijaspor, böhmit, kaolinit, pirit, rutil, anatas, gips i goethit. Pojedini uzorci sadrže mješanoslojne/nepravilno interstratificirane minerale (moguće illit-smektit) i 14Å minerale (vermikulit i/ili smektit). Kemijskom analizom određeno je da SO3 i Fe2O3, nemaju jednoliku raspodjelu duž profila, što ukazuje na fluktuaciju vodnog lica tijekom procesa nastajanja boksita. Obogaćenje REE i povećanje udjela Al2O3 prema podini ležišta ukazuju na „in-situ“ boksitizaciju.
Mineralogical, petrological and geochemical characteristics of bauxites from deposit „Minjera“, located near Buzet in Istria, were investigated. Semiquantitative mineral composition from 8 samples of grey bauxites, which are sampled in the profile within the deposit, is determined by XRD. Petrological characteristics of limestones in the footwall and hangingwall of bauxite deposit were determined from 11 samples by polarising microscopy. Microscopical investigations also included one sample of black bauxites from the top of the sampled profile, near the hangingwall limestones. Chemical composition of bauxites is determined by various spectrometric methods (ICP-AES, ICP-MS, XRF). Footwall of bauxites are rudist limestones of Cenomanian age, and in the hangingwall of bauxite deposit Paleocene freshwater to brackish limestones with gastropods and plant remains appear. Black bauxite from the top of the profile contains bauxitic pyritized ooids. In the samples of grey bauxites following minerals are determined: diaspore, böhmit, kaolinite, pyrite, rutile, anatase, gypsum and goethit. Some samples contain mixed-layer/irregularly interstratified minerals (possibly illite-smectite) and 14Å minerals (vermiculite and/or smectite). The trends of distribution SO3 and Fe2O3 along the profile can’t be clearly distinguished from the results of the chemical analyses, which implies the fluctuation of water levels during the process of bauxitization. Enrichment in REE and increased Al2O3 toward the footwall of the deposit implies an “in-situ” bauxitization.