This study was undertaken to determine the source of iron in Comus Formation sedimentary rocks that were sulfidized during deposition of gold in the Megapit area of the Twin Creeks Carlin-type deposit. Sedimentary rocks in and near the Megapit contain ferroan dolomite, largely as overgrowths on iron-poor dolomite. Iron to form these overgrowths appears to have been released from mafic volcanic rocks that are interlayered with the sedimentary rocks. These igneous rocks have undergone two stages of hydrothermal alteration. The first stage involved formation of albite and iron-rich chlorite, possibly caused by interaction with seawater. The second stage involved destruction of the iron-rich chlorite by illite or sericite, which released iron to form ferroan dolomite in the sedimentary rocks. Comparisons show that transfer of iron from the igneous rocks to the sedimentary rocks can account for the present distributions of iron in these rocks. Relative to basalts, Comus Formation igneous rocks are enriched in iron and potassium. These results suggest that ferroan dolomite in sedimentary rocks is not solely a product of diagenetic processes and can form when iron is released from adjacent iron-bearing igneous rocks. Recognition of this additional mechanism for formation of ferroan dolomite expands the range of geologic settings that can be favorable for formation of gold deposits formed by sulfidation.

The Central Asian Orogenic Belt (CAOB) is renowned for massive generation of juvenile crust in the Phanerozoic. Mongolia is the heartland of the CAOB and it has been subject to numerous investigations, particularly in metallogenesis and tectonic evolution. We present new petrographic, geochemical and Sr–Nd isotopic analyses on Phanerozoic granitoids emplaced in west-central Mongolia. The data are used to delineate their source characteristics and to discuss implications for the Phanerozoic crustal growth in Central Asia. Our samples come from a transect from Bayanhongor to Ulaan Baatar, including three tectonic units: the Baydrag cratonic block (late Archean to middle Proterozoic), the Eo-Cambrian Bayanhongor ophiolite complex and the Hangay–Hentey Basin of controversial origin. The intrusive granitoids have ages ranging from ca. 540 to 120 Ma. The majority of the samples are slightly peraluminous and can be classified as granite (s.s.), including monzogranite, syenogranite and alkali feldspar granite. Most of the rocks have initial 87Sr/86Sr ratios between 0.705 and 0.707. Late Paleozoic to Mesozoic granitoids (#250 Ma) are characterized by near-zero Nd(T) values (0 to 22), whereas older granitoids show lower Nd(T) values (21.5 to 27). The data confirm the earlier observation of Kovalenko et al. [Geochemistry International 34 (1996) 628] who showed that granitoids emplaced outside of the Pre-Riphean basement rocks are characterized by juvenile positive Nd(T) values, whereas those within the Pre-Riphean domain and the Baydrag cratonic block, as for the present case, show a significant effect of ‘contamination’ by Precambrian basement rocks. Nevertheless, mass balance calculation suggests that the granitoids were derived from sources composed of at least 80% juvenile mantle-derived component. Despite our small set of new data, the present study reinforces the general scenario of massive juvenile crust production in the CAOB with limited influence of old microcontinents in the genesis of Phanerozoic granitoids.

The Southeast Missouri Iron Metallogenic Province is comprised of eight known major and numerous minor magnetite and hematite deposits. It is hosted by the Middle Proterozoic St. Francois granite-rhyolite terrane. Host rocks are rhyolites, trachytes, and andesites. Ore is associated with, although not necessarily hosted by, magnetite trachytes. Deposits are associated with caldera subsidence structures and, sometimes, trachyte ring intrusions. Deposits are within or near margins of these structures. Areal association of the deposits with a major Proterozoic tectonic zone, possibly a transform fault, suggests additional tectonic/ structural control on ore emplacement. Magnetite and hematite have been produced in the province; currently, only magnetite is produced. Potential exists for production of rare earth elements, copper, and gold.

A characteristic alteration suite is associated with the iron oxide mineralization. The suite includes silicification, potassium metasomatism, and alteration of host rock to actinolite, chlorite, garnet and epidote. While several alteration types are associated with each deposit, every type is not seen at each deposit.

Chemistry suggests that magnetite and hematite deposits in the province have a unique chemical signature when compared to magnetite not directly associated with the major deposits. In addition, hematite that is an oxidation product of magnetite has a different chemical signature than presumed primary hematite.

Пространство, структура и случайность. Вклад в честь Джорджа Матерона в области геостатистики, случайных множеств и математической морфологии

Personal Reminiscences of Georges Matheron Dietrich Stoyan
A few words about Georges Matheron (1930-2000) Jean Serra
Introduction From the editors
Part I Geostatistics