52 GEOSCIENCES 2007 Statistical analysis of the distribution of platinum-group minerals in the ophiolite chromitites from the Dobromirtsi Ultramafic Massif (SE Bulgaria): implications for their origin and postmagmatic evolution Ñòàòèñòè÷åñêè àíàëèç íà ðàçïðåäåëåíèåòî íà ìèíåðàëè îò ãðóïàòà íà ïëàòèíàòà â îôèîëèòîâèòå õðîìèòèòè íà óëòðàìàôè÷íèÿ ìàñèâ Äîáðîìèðöè (ÞÈ Áúëãàðèÿ): èçâîäè çà òåõíèÿ ïðîèçõîä è ïîñòìàãìàòè÷íà åâîëþöèÿ Jose-Maria Gonzalez-Jimenez 1 , Thomas Kerestedjian 2 , Fernando Gervilla 1 , Joaquin Proenza 3 Õîñå-Ìàðèÿ Ãîíçàëåñ-Õèìåíåñ 1 , Òîìàñ Êåðåñòåäæèÿí 2 , Ôåðíàíäî Õåðâèÿ 1 , Õîàêèí Ïðîåíöà 3 1 University of Granada-CSIC, Dept. of Mineralogy and Petrology and Instituto Andaluz de Ciencias de la Tierra, Avd. Fuentenueva, s/n 18002 Granada, Spain; E-mail: gervilla@ugr.es, jmgonzj@ugr.es 2 Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia; E-mail: thomas@geology.bas.bg 3 University of Barcelona, Dept. of Cristalography, Mineralogy and Mineral Deposits, Mati i Franques, s/n 08023 Barcelona, Spain Key words: platinum group minerals (PGM), cromitites, ophiolites, Dobromirtsi Introduction The statistical analysis of the distribution of plati- num-group minerals (PGM) has been carried out on a population of 175 mineral grains identified in 61 polished sections. These polished sections are rep- resentative of 10 chromitite showings of variable size, distributed across the ultramafic massif of Dobro- mirtsi. PGM were identified by means of ore micros- copy; photographed, measured and qualitatively analyzed by environmental scanning electron micro- scope; quantitatively analyzed by electron micro- probe. The statistical analysis focuses on the distri- bution of grains, nature of the mineral assemblage, grain size (integral surface) and nature of the dif- ferent PGM. This analysis was made considering the textural location of the different PGM grains: i.e. included in unaltered chromite, included in altered chromite or located in the altered silicate matrix of the chromite. Results The distribution of PGM in the studied chromitite samples is very irregular since they are present in only 35 of the 61 polished sections studied. This ir- regular distribution is also observed when the dif- ferent chromitite bodies are compared: while only 20 PGM grains were identified in the 12 studied sam- ples of the D1 chromitite, the 2 samples investigated from D9 chromitite contain 37 PGM grains. PGM occur as single inclusions (87% of the iden- tified grains) or forming part of diphase (25%) or polyphase (8%) inclusions in chromite or in the sil- icate matrix. In unaltered chromite, PGM preferen- tially occur as single grains (74%), and minor as diphase (20%) or polyphase (6%) inclusions. These proportions change in altered chromite and espe- cially in the silicate matrix where the abundance of single PGM grains decrease (down to 41%) and diphase grains increase (up to 47%). Grain size of PGM grains varies from 1 µm 2 to 300 µm 2 . However, grain size distribution is again irregular since 85% of the grains are smaller than 25 µm 2 . Studying in detail the population of grains with grain size smaller than 25 µm 2 , we observed that the smaller the grain the higher its abundance. This distribution defines a polynomial curve which, once extrapolated to low grain size regions, predicts the presence of abundant small (<1 µm 2 ) PGM grains. These grains could not be identified by the optical or electronic instruments used.