Chert, Quartz (Var: Chalcedony) : SiO2, Parapierrotite : TlSb5S8

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Copyright © Norman King
minID: EQP-9R3

Chert, Quartz (Var: Chalcedony) : SiO2, Parapierrotite : TlSb5S8

Field of View: 7.2 mm

Detail of vuggy chert viewed in thin-section (using cross-polarized light) from Ajax Dolomite at Lookout Pass. This kind of chert apparently results from metasomatic silicification of both carbonate and fine-grained siliciclastic sedimentary rocks, and has been referred to as “jasperoid.” Jasperoids are well-known for hosting gold deposits in the Great Basin of the United States, including those in the Carlin trend and other gold trends in northeastern Nevada. The bright white to orange and pink grains scattered through this matrix are silt-sized quartz grains in what was probably a calcareous shale or shaly micritic (= very fine-grained) limestone. Darker masses are richest in ultra-fine-grained silicate minerals and organic matter. Complex grain-size and shading variations that created the patchy matrix show the rock was brecciated early in its history, well before additional brecciation during the Laramide and subsequent orogenies (produced Rocky Mountains and Great Basin structuring) formed the quartz-filled vugs that are conspicuous here, as well as zones crossed by multitudinous fine fractures. Very fine-grained microquartz (white and grainy) was deposited first in the prominent vug near the center of the photo, followed by coarser microquartz (includes bright white to yellow, orange, and maroon). Then came bluish-gray fibrous radiating chalcedony. Finally, another round of the coarser microquartz was deposited in the center. Thus, silica deposition occurred during successive phases, with conditions reversing and repeating themselves at times. The opaque (black) grains are crystals of parapierrotite, which I have observed in the jasperoid matrix but not in quartz-filled vugs. Collected, photographed, and interpreted by Norman King.

Collected: 2011

This Photo was Mindat.org Photo of the Day - 16th Apr 2018

This photo has been shown 769 times
Photo added:4th Sep 2015
Dimensions:2140x1756px (3.76 megapixels)
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