Looks like metasomatism in a sedimentary rock. Isn't this consensus?

Magnesite is principally produced by alteration and metasomatism. It is a common by-product of serpentinization of ultramafic rocks some of which are turned into talc-magesite +/- chlorite assemblages. It has also been reported as a primary mineral in some rare chemical sediments

Wolfgang, yes indeed, so it is. Thank you for this comment.

In a recent paper Dong et al. (2016)  seem to suggest that Mg-rich brines accumulating at the bottom of a lagoon reacted with newly-formed carbonate sediment producing magnesite. They envisage this process  system to take place in one spot (forming a deposit) and more or less synchroneously. In the magnesite community’s terminology this would be “early diagenetic” magnesite metasomatism. In contrast, the features displayed by pinolite magnesite show that the metasomatic crystals originate in fissures of a hard rock (“late diagenetic”). The Mg-rich fluids flowed through a network of fissures and crystals grew from the fissures into the rock. Mg-isotopes would not differentiate between early and late-diagenetic metasomatism, but to me, the pinolite textures  are a decisive argument.

Therefore I am interested to learn, if similar textures have been seen in the Chinese magnesite deposits.

Photograph shows pinolite magnesite from Hohentauern Mine (coin diameter 2.5 cm).

Dong, A., Zhu,  X.-K., Li, Sh.Zh., Kendall, B., Wang, Y., Gao, Zh. (2016)  Genesis of a giant Paleoproterozoic magnesite deposit: Constraints from Mg isotopes. Precambrian Research 281, 673–683.

M. Qasim Jan, thank you for pointing out that part of the magnesite produced for industrial use is derived from ultramafic rocks, commonly formed by reaction with CO2 dissolved in geological fluids (carbonation). Most of the global magnesite supply, however, comes from carbonate-hosted deposits. This discussion concerns the latter; forgive me that I did not mention this in the first post.

The dendritic-like pattern of darker areas in this dolomitic limestone (Tyndall Stone) supposed to show the channels that introduced magnesium-rich solutions into an original limestone. Some fossil lines are still visible.

Thank you, Michael. Dolomitization in progress ... The pattern of these flow channels is strange; do you have an explanation for it?

Walter,

The darker areas which show a dendritic pattern are the channels that brought the Mg-rich fluids into the area. Eventually the whole rock became Mg-rich (dolomite) but the original channels have remained (like river channels flowing into a landscape and saturating the ground everywhere with water).

Thank you, Michael - so the strange channels were  produced by Thalassinoides burrows. I am not a sedimentologist, but I do remember now that as students we saw similar burrows in siltstone of Cretaceous age.

https://en.wikipedia.org/wiki/Tyndall_stone

 Walter,

Your description of pinolites is very interesting having spent ages picking pine seeds every September (in older days) for use in cooking.

Thank you for the term of Thalassinoides (sea-like?) burrows which I never knew. 

We recently encountered this Tyndall rock in drilling at depth near an ultramafic pipe and although it looked similar, on closer examination the dolomite had turned into silica (or olivine, we are waiting for results of XRD) and the fossils into calcite mud. So, the rock was subjected to another alteration.

Your translation of Thalassinoides awoke nice memories - learning ancient Greek over 4 years in school. Chaire!

Walter, Χαιρε!

It kind of looks like recrystallization (late diagenetic). I am curious about the oxygen isotopes. These might tell us something about the formation conditions.  Do you have those and local rainwater isotopes?

Ours (central Serbia) are of hydrothermal origin