Major Sulfide Minerals Research Articles

Middle Tertiary Replacement Ore Bodies and Associated Veins in the Northwest San Juan Mountains, Colorado

Middle Tertiary ore deposits of significant size and number replace the Eocene Telluride Conglomerate adjacent to northwest-trending base-metal veins on the north-west side of the Silverton caldera in the San Juan Mountains, Colorado.Sphalerite, galena, chalcopyrite, and pyrite are the major sulfide minerals, and the ores contain some cadmium, bismuth, silver, and gold. Major gangue minerals are quartz, green epidote, chlorite, rhodonite, pink manganese-bearing epidote, and carbonates; there are lesser amounts of sericite and clay minerals. Most of the ore is characterized by complete replacement of the original conglomerate, and consists of 70 to 80 percent massive sulfides and about 20 percent fine-grained greenish gangue minerals. A lesser amount of conglomerate-type ore contains 15 to 20 percent coarse sulfides disseminated in the matrix and in pods of sulfides and gangue; the original conglomerate host is still clearly evident in this type of ore. Some siltstone-type ore is characterized by sulfides disseminated in local sandy and silty units; this type of ore lacks any sulfide-gangue pods.Alteration related to the mineralization changed normally reddish units in the conglomerate to gray and green as chlorite and sericite formed in the matrix. Near the ores the grays and greens give way to pinks and pale green where manganese-bearing epidote and carbonates, rhodonite, and green epidote formed. Sulfide minerals occur as small grains disseminated through the matrix of the altered conglomerates and in pods composed of chlorite, rhodonite, and carbonates. Toward the ore zone, the altered pods are more numerous and the disseminated sulfide grains become larger.All the known replacement ore deposits are adjacent to base-metal veins which acted as the conduits for the mineralizing fluids. Where dikes and veins coincide, replacement ore, if present, is located where a vein is alongside rather than within a dike. Permeable, calcareous, conglomeratic rock units appear to have been the most susceptible rocks for replacement. Permeability was enhanced locally by minor fractures and bedding planes. On a regional scale the replacement ores are part of a zonal pattern of altered and mineralized rocks that formed progressively outward from a probable source for the mineralizing and altering fluids in the vicinity of the Red Mountains altered area.Replacement ore is most common in the Telluride Conglomerate, but some occurs in the underlying Cutler and Dolores Formations as well.

Mineralogy of the Marbridge No. 3 & No. 4 nickel-iron sulfide deposits; with some comments on low temperature equilibration in the Fe-Ni-S system

The Marbridge No. 3 and 4 deposits are associated with lens-like bodies of serpentinized ultramafic rock that occur within a sequence of interbanded felsic volcanic rocks and graywackes. Both deposits consist of sulfide disseminated within the host peridotite, together with smaller amounts of stringer ore. Ore at the No. 3 deposit occurs towards the top of its associated serpentinite while that at the No. 4 occurs towards the base.The major sulfide minerals at the No. 4 deposit are pyrite and pentlandite together with minor amounts of millerite. At No. 3, the ore is zoned parallel to the contact with the host volcanics. The major part consists of pyrite, pentlandite and millerite, but close to the contact the assemblage pyrite-pentlandite-pyrrhotite is developed. Small amounts of pentlandite and heazlewoodite occur at the center of the serpentinite lens at the No. 3 deposit. Pentlandite is extensively altered to violarite throughout much of this deposit.Electron microprobe analyses of pentlandite from the Marbridge deposits, together with samples from the Alexo mine, the Texmont mine and the Sudbury ores indicate that the Fe:(Ni + Fe) ratio of pentlandite varies directly with the minerals with which it is associated, ranging from 0.518 for that in association with troilite and intermediate hexagonal pyrrhotite to 0.341 in association with heazlewoodite. This relationship indicates that equilibrium has been established between pentlandite and its associated minerals and is supporting evidence for the existence of a stable tie-line between pentlandite and pyrite at low temperatures in the Fe-Ni-S system.

Lead-zinc ore deposition in the light of fluid inclusion studies, Providence Mine, Zacatecas, Mexico

In the Providencia area elongate pipe-like bodies of Pb-Zn sulfide ore occur in steeply dipping Upper Jurassic and Cretaceous limestones. Most of the ore bodies have a close spatial relationship to the Providencia granodiorite stock which is continuous at depth with the Concepci6n del Oro stock that has been dated at 40 n 2 million years. The mineralogy of the ore bodies is comparatively simple; sphalerite, galena and pyrite are the major sulfide minerals, and calcite and quartz are the gangue minerals. The bulk of the sulfide ore was formed by replacement of carbonate wall rocks, and the remainder by deposition of minerals in vugs. Ore deposition appears to have been structurally controlled by both fractures and bedding. Sulfur isotope studies on minerals from the ore bodies and veinlets within the stock gave 8 S a4 values within the magmatic hydrothermal range. Fluid inclusion studies were made on sphalerite, quartz, calcite and fluorite, the bulk of the data being obtained from sphalerite crystals. The results are consistent and indicate that the massive ore was emplaced at temperatures above 350 o C, whereas zoned sphalerite crystals were emplaced over the temperature interval from 350-200 o C. Calcite and quartz were generally formed at temperatures below 350 o C. 425oC is considered to be a reasonable upper limit for hydrothermal ore deposition at Providencia. The fluid inclusion data suggest that vertical temperature gradients within the conduits during mineralization were less than 50 o C/Km. Considerations of the cooling history of the Providencia stock and the rocks in its vicinity indicate that at least 10 o years must have elapsed after intrusion of the granodiorite prior to the deposition of sphalerite at temperatures as low as 200 o C. Salinity studies on fluid inclusions demonstrate that the salinity of the ore-forming solutions varied between 5 and 40 equiv. wt. % NaC1 and that the pattern of these variations was complex. The possible mechanisms by which such strong and repeated salinity variations could have taken place during ore deposition are discussed. Simple calculations show that the volume of hydrothermal solution needed to deposit the Providencia ore bodies was probably of the order of 10 km a. The source of these solutions is concluded to be a large magmatic body at some depth.