Page 71 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

70 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
turing is most intense the rhyolite has been kaolinized to a soft white rock. Immediately adjoining the fractures in some places, there are narrow zones in which the white rock has been colored purplish red by iron. Along some of these fractures a mixture of cassiterite and hematite has been found. In numerous spe- cimens gathered by the writer the order of mineralization seems to have been, first, hematite in brilliant black plates and in mas- sive form, followed by a mixture of cassiterite and hematite, and finally scattered plates of hematite over the surface of the earlier minerals. In some places chalcedony has followed the last stage of hematite and forms a thin bluish-white film over the surface of the earlier minerals. The cassiterite is mostly in honey-colored, dark brown or black dense botryoidal masses with colloidal characteristics, but in many places brilliant red crystals of the tin oxide occur in the veins. The hematite is chiefly bril- liant black plates which frequently occur in clusters on the vein wall, but in part it is botryoidal. The mineralized fractures are not as a rule completely filled, the coatings on the walls ranging from a sixteenth to an eighth of an inch in thickness. Rarely a fracture as much as an inch in width is almost entirely filled with the mixture of cassiterite and hematite, but such bunches are always small and do not extend far in any direction. Some nuggets of stream tin would indicate that there are a few frac- tures up to 6 inches in width, as nodular masses of cassiterite evidently built out from one wall to a thickness of 3 inches, have been seen.
Alteration of the rhyolite rarely extends for more than a foot on either side of an individual fracture. Where many closely spaced fractures exist, there may be a greater amount of alteration and mineralization. The altered rhyolite consists essentially of kaolin, which has formed largely from the ground- mass of the original rock, and residual quartz. The feldspar phenocrysts in some places have been entirely altered, but at most places the alteration has not been completed and their out- lines have been preserved. Next to the fractures the altered rhyolite has become porous for as much as an inch, and is stained a purplish-red color. According to Hill, 70 the material of this character seen under the microscope consists of grains of residual quartz, remnants of feldspar, iron-stained kaolin, small flakes of hematite and cassiterite, and a yellowish-green chlorite. Silicification is absent from practically all of the rocks in the vicinity of the tin-bearing fissures. No sulfide minerals and no topaz, tourmaline, or other fluorine- or boron-bearing min- erals were noted in the Taylor Creek deposits.
It is believed by Hill that the fracturing of the rhyolite was caused in part by strains set up in cooling and in part by move- ments subsequent to the extrusion. The development work shows that fracturing and alteration extend to a depth of 300
70 Hill, J. M., op. cit. (U. S. G. S. Bull. 725), p. 355.