Vermiculite mineralization associated with ultrabasic-basic rocks of the Mohammad Abad area, NW Iran

The basic-ultrabasic bodies of Mohammad Abad, with an outcropped area of 28 km², are located 7 km southwest of Horand city, NW of Iran. This area is part of the Central Iranian Domain (Agha Nabati, 2004) and Urumieh-Dokhtar Magmatic Arc (UDMA) of Cenozoic. Rock units in the area include Upper Cretaceous-Paleocene flyschoid sandstone, shale, pelagic carbonate rocks and volcanic-volcanoclastic rocks, including andesitic lavas, dacitic breccias and acidic tuffs. The ultrabasic pyroxenite body is intruded by Oligocene gabbroic body and dikes, in which large pyroxenite xenoliths are present. Moreover, several syenitic and nepheline syenitic dikes cross-cut the ultrabasic-basic complex. Considering their mineralogic similarities with the neighboring well-studied nepheline-syenites of Kaleybar, these dikes can be attributed to Oligocene. The occurrence of gabbro and syenite bodies and dikes at the margins of an ultrabasic complex may refer to an alkaline ring complex formation in the area. According to petrographic studies, ultrabasic rocks are composed of clinopyroxene (diopside), olivine, phlogopite and opaque minerals such as magnetite and pyrite, while gabbros comprise plagioclase, clinopyroxene (diopside), amphibole (pargasite to tschermakite) and lesser phlogopite, biotite and magnetite. Gabbros have shoshonitic affinity, with metaluminous nature and post-collisional volcanic arc setting, produced by low-degree partial melting of an enriched spinel-garnet lherzolite mantle, while garnet remained as residual phase within the source area.

Vermiculite mineralization in the Hashtsar area occurred within the ultrabasic-basic bodies, comparable with Palabora (south Africa) and Libby (Montana, USA) deposits. Hypogene and supergene process were incorporated in its genesis. Hypogene hydrothermal fluids have altered pyroxene and amphibole minerals of the host rocks to biotite, which was later converted to hydrobiotite. Hypogene fluids may have provided by the intrusion of syenitic dikes within pyroxenites and gabbros, as vermiculite formation is more evident at the margins of these dikes. Both hypogene and supergene fluids have caused the conversion of hydrobiotite and phlogopite to vermiculite, among which the role of weathering is more prominent for the study area, considering the major occurrence of vermiculite in shallow depths (~5 m) of the ultrabasic-basic rocks and the decreasing ratio of vermiculite/phlogopite with depth. Conversion of phlogopite and biotite to vermiculite requires the removal of K and Si and addition of Mg, Fe and H₂O. Mg and Fe may have provided by alteration of pyroxene and amphibole minerals from ultrabasic-basic rocks. Geochemical analysis data show that both the ultrabasic-basic bodies, and the vermiculite samples have a strong enrichment of LILE and a negative anomaly of HFSE. Moreover, the ultrabasic-basic rocks of the study area show depletion of Mg. It can be concluded that most of the magnesium from the parental rocks entered the mineral structure of vermiculite as a result of ion exchange with potassium.

Key words: Vermiculite, Pyroxenite, Gabbro, Supergene alteration, Mohammad Abad.