The chapter addresses the mechanism of growth of druses of faden quartz in a tectonically deformed sandstone. A peculiar feature of this type of quartz, the so-called “white tape” represented by fine subparallel cracks with fluid inclusions, appears highly informative about the genesis of the mineral and tectonic regime of its growth. Two stages of formation of druses of faden quartz are recognized. The suggested mechanism is checked for contradictions by means of the event bush method. The proposed event bush model appears to describe a wider range of quartz formation environments and therefore may serve as a conceptual framework for various models of quartz growth in sedimentary rocks.
TopProposed Model
A few years ago Krisak and Kupenko (2014) found single crystals of faden quartz in druses in a low-amplitude fault in the Donetsk Basin, Lugansk People's Republic (LPR, formerly part of Ukraine). No earlier evidence, published or unpublished, of occurrence of faden quartz in this basin is known to us. Krisak and Kupenko suggested that the formation of crystals was related to plastic and brittle deformations in the rock.
In 2015 we encountered druses orthogonal to the sedimentary layers, which were completely filled with faden quartz, in the south-western suburb of Zorinsk (Perevalsky District, LPR).
The druses occurred in a mid-Carboniferous sandstone and structurally were confined by the crest of an asymmetric, 3000 m long, 1800 m wide and 300 m high anticline fold with a flat-lying northern limb (10° – 15°) and steep southern limb (50° – 70°). The southern limb was complicated by an upthrust. Fold axis dips 15° westward.
Sandstone containing the faden quartz is medium-grained, with its own quartz grains and aggregates ranging from 0.4 to 1 mm across. All grains in the sandstone are dominantly round- or oval-shaped. Veins that host the druses have thickness from 5 to 30 mm with little variation along strike, the contacts are sharp and clear. Sometimes veins are coated in hematite. Faden quartz in the druses is represented by clusters of crystals orthogonal to the vein contact (1). Crystals are 4 to 15 mm, rarely up to 30 mm long, have flattened or polygonal form, with a predominant development of the two prism faces {1010} and weak development of rhombohedron faces {1011}.
Figure 1. Faden quartz in druses: (a) photograph; (b) graphic sketch with faden shown in gray.
Under the microscope, in transmitted light the white tape appears to consist of thin parallel alternating cracks, which can be traced by chains of single-phase or rarer two-phase fluid inclusions (2).
Figure 2. The structure of “white tape” in faden quartz under a microscope