No abstract

The amber-bearing coaly shale from the La Cumbre deposit (Cordillera Septentrional, Dominican Republic) contains a large quantity of altered, coalified plant detritus. The coals in these shales are in the transition stage from meta-lignite to subbituminous coals. They are composed mainly of inertinite macerals such as fusinite, semifusinite, macrinite and secretinite. Fossil resin found in the deposit occurs in two forms: detrital grains up to several centimetres in size (type I) and very fine authigenic grains, of a few micrometers in size, inside the humic laminae (type II, resinite). The detrital fossil resins are transparent, with few mineral and organic inclusions. In their composition they contain sulfides, which may come from sulfate reduction, inclusions of plants and/or insects or be caused by volcanic activity developed in surrounding coal series. The resinites are strongly saturated with various inclusions and spatially associated with framboidal pyrite aggregates. Both fossil resin types were probably deposited in a shallow coastal lake environment in the zone bordering the floodplain of the river, with periodic floods. The marine environment conditions, which were progressively changing from oxidizing to reducing, are likely associated with the formation of the fossil resin.

Comparative studies of fossil resins of various ages, botanical sources, geological environments, and provenience were provided via a handheld portable Near-Infrared (NIR)-Raman spectrometer and benchtop instrument both working with laser line 1064 nm. The recorded Raman spectra of individual fossil resins were found to be sufficiently similar irrespective to the device type applied, i.e., handheld or benchtop. Thus, the portable equipment was found to be a sufficient tool for the preliminary identification of resins based on botanical and geographical origin criteria. The observed height ratio of 1640/1440 cm−1 Raman bands did not correlate well with the ages of fossil resins. Hence, it may be assumed that geological conditions such as volcanic activity and/or hydrothermal heating are plausible factors accelerating the maturation of resins and cross-linking processes.

Agate samples collected from the vicinity of Asni and Agouim (Western Atlas, Morocco) were investigated using microscopic observations supported by Raman micro-spectroscopy. The agates are marked by the presence of various microtextures typical of epithermal vein deposits, including jigsaw-puzzle, feathery, and lattice-bladed. The first two indicate that the formation of agates was likely marked by recrystallization of metastable silica phases (i.e., opaline silica or massive chalcedony). The presence of lattice-bladed (after barite and calcite) quartz may be, in turn, ascribed to the boiling-related conditions that could have triggered the formation of abundant copper and iron sulfides found within silica matrix. Additionally, the local occurrence of growth lines (so-called Bambauer quartz) and intergrowth of length-slow and length-fast chalcedony are linked to the variations of physico-chemical conditions during rock formation (alkaline-acidic). According to Raman spectroscopy, silica matrix of the agates is made of α-quartz with a local admixture of moganite (from 0.0 up to 78 wt.%), but also contains numerous solid inclusions of hematite, celadonite, as well as poorly-organized carbonaceous material and rutile. These phases were likely emplaced during low-temperature hydrothermal activity of SiO2-bearing fluids that originated from post-magmatic hydrothermal activity developed within host rocks and/or meteoric waters.

No abstract