Calcite veins

A priori considerations. An extensive review of the form and mineralogy of the secondary cave deposits is provided by Hill and Forti (1997), but also see Ford and Williams (1989). By far the most useful speleothems for geological purposes are the most common forms stalactites, stalagmites and flowstones composed of calcite, and occasionally aragonite. Other deposits that have been used to derive useful information include travertines, gypsum crusts, calcite veins and scalenohedral calcite (dog-tooth spar). 

In the Muzo and Chivor mines, in Colombia, calcite veins, in which emerald crystals occur, develop, and they fill the cracks of sedimentary slate rock. Compared with emeralds occurring in basic metamorphic rocks, like those in the Urals, South Africa, or India, Colombian emerald crystals have a higher perfection, fewer inclusions, and attract higher evaluations as gemstones, since crystals grew freely in a hydrothermal solution. 

Ludwig, K.R.. et al. Mass Spectrometric 2J0Th 234U 233U Dating of the Devil s Hole Calcite Vein, Science, 284 (October 9, 1992). 

Ludwig, K.R., Simmons, K.R., Szabo, B.J., Winograd, I.J., Landwehr, J.M., Riggs, A.C. and Hoffman, R.J. (1992) Mass-spectrometric 230Th-234U-238U dating of the Devils Hole calcite vein. Science 258, 284 -287. 

Temiskamite, Ni4As8, is a silver-white mineral, with a tinge of red, which rdpidly tarnishes. Hardness 5-5 density 7-901. It is found in calcite veins in Ontario.2 Possibly this mineral is the same as Maucherite, a nickel arsenide found in Thuringia.3 The diarsenide chloanthite or white... 

Clauer N., Frape S. K., and Fritz B. (1989) Calcite veins of the Stripa granite (Sweden) as records of the origin of the groundwaters and their interactions with the granitic body. Geochim. Cosmochim. Acta S3, 1777—1781. 

Al-Aasm I. S., Muir I., and Morad S. (1993) Diagenetic conditions of fibrous calcite vein formation in black shales petrographic, chemical and isotopic evidence. Bull. Cart Petrol. Geol. 41, 46-57. 

Fig. 3.1.10. Phosphatic concretion from Brazos County, Texas. The large dark object is an appendage of a fossil crab, the tip of which is truncated by a calcite vein. The matrix material is essentially collophane (isotropic francolite), but contains glauconite, quartz, limonite, pyrite, gypsum, etc. in addition to fossil fragments. Magnification 27x. Reproduced with permission (McConnell, 1950).

Data from calcite veins that postdate calcite cement. 

Field studies of calcite veins formed by geothermal water were carried out in the Lower Jurassic Blue-Lias-Formation , a limestone-shale succession cut by many E-W trending normal faults, at the Somerset Coast, Southwest England (cf. Whittaker <6 Green 1983). Most veins trend sub-... 

Figure I. A dense network of calcite veins runs subparallel to a normal fault plane, particularly in the hanging (right) wall, in a lime-stone-shale succession at Kilve, Somerset Coast, 51V England. The veins extend only for a few metres into the damage zone. View west the person provides a scale.

Figure 2. In the fault core of this normal fault at Kilve, 5W England, there is a dense network of white calcite veins. In the damage zone, most of these are restricted to grey limestone layers and end abruptly at the contacts with the darker shale layers. View east the measuring steel tape is 50 cm.

We use the boundary-element program BEASY (1991) to explore why many hydrofractures, such as the observed veins, are stratabound why, for example, the calcite veins in the limestone layers became arrested on meeting with the shale layers. For each model the stress field around the hydrofracture tip is calculated. Tip propagation is simulated in a semi-static way using models that vary slightly in boundary conditions between runs. 

To obtain the fluid overpressure of the hydrofractures that formed the mineral veins in the field area at the Somerset Coast (Figs 1 and 2), we measured the length and aperture (thickness) of 239 calcite veins exposed in subhorizontal limestone layers. We used only veins that have both ends visible in the outcrop, are continuous, and are non-restrictive (that is not dissecting other veins or other discontinuities). All the measured veins are extension fractures that show no evidence of subsequent shear deformation. 

The calcite veins were formed at crustal depths of at least 1 km (Nemcok et al. 1995). Therefore, so they are unlikely to have formed as tension fractures (with <7j negative) but rather as hydrofractures, as is also indicated by their aspect ratios and inferred fluid overpressure. 

Figure 6. Aperture variation of a calcite vein at Kilve, SW England. In the grey limestone layer, the vein is a vertical extension fracture, with ffj as its normal stress. In the shale layer, the vein is an inclined shear fracture, subject to a higher normal stress Oi, and therefore thinner than the vertical part. View east the limestone layer in the upper half of the picture is 25 cm thick.

The fluid overpressure of a hydrofracture can be estimated using analytical models and measurements of its length and thickness. The fluid overpressure of calcite veins at Kilve, SW England was around 20 MPa. From this fluid overpressure the depth to the fluid sources of the hydrofractures is estimated at several hundred metres below the present outcrops. 

Stump (1986) is supported by the occasional presence of apatite, rutile, corundum, kyanite, and tourmaline. Stump (1995) also noted that the La Gorce Formation differs from the Goldie Formation of the central Transantarctic Mountains (Section 5.2) by the absence of carbonate beds except for calcite veins near faults. 

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