The Jamestown, Gold Hill, and Magnolia mining districts of Boulder County, Colorado, provide one of the few classic examples of telluride mineralization in this country. These districts are parts of a broad, north-trending belt of telluride mineralization about 5 square miles in area located at the northeastern end of the Front Range mineral belt. The predominant country rocks are Precambrian granites, gneisses, and schists which are bounded on the east by Paleozoic and Mesozoic sediments upturned along the front of the range. The telluride veins represent one stage in a complex sequence of Early Tertiary ore types that show varying degrees of correlation with exposed Early Tertiary intrusives. Erosion has removed any volcanics erupted at the time of mineralization, but has exposed genetically related dikes and intrusion breccias of biotite latite in an area coextensive with that of the telluride mineralization.
A series of major northwest-trending faults called “breccia reefs” apparently served as the main channelways for introduction of the telluride ore fluids which rose from depth along the reefs and spread out into northeasttrending vein fissures where ore deposition took place. Most of the telluride production has come from only a few centers that show a pronounced structural control by the reefs. Local structural controls within the vein fissures include vein intersections, intersections of veins with earlier faults or with igneous dikes, irregularities of the veins as related to wall movements, and the physical character of the wall rocks. The telluride veins were mined over a collective vertical range of 3000 feet, and there is no clear-cut evidence of a bottom limit to the ores.
The telluride veins are typically composed of an interlacing network of pyritic or marcasitic horn quartz seams in which the ore minerals are quite sparse and irregularly distributed. Sixty-seven vein minerals have been identified (forty-one by X-ray methods). Individual polished sections commonly contain a dozen or more metallic minerals in fine-grained, intimate intergrowths filling or coating fractures or scattered vuggy openings in the finegrained vein quartz. The chief ore minerals are sylvanite, petzite, hessite, and native gold and in some mines calaverite and krennerite are also important. Ten other tellurides, as well as native tellurium and a variety of sulfides and sulfosalts formed in the telluride stage of mineralization but contributed little or nothing to the values. The principal gangue constituents are quartz and altered wall rock, but roscoelite, ankerite, calcite, fluorite, and barite are locally present.
A section of the report on problems of telluride identification gives data on the polarization figures, rotation properties, reflectivities, and indentation hardnesses of the tellurides.
Hypogene textures and associations of the telluride ores have in many cases been highly modified by cooling, but these effects, as well as the original depositional sequence itself, are clarified by experimental phase relations in the system Au-Ag-Te. In general, the original sequence was one of early sulfides, followed by native tellurium and a series of tellurides of progressively lower tellurium content, and finally by late native gold. A high degree of local equilibrium was maintained during the initial deposition, but as the ores cooled equilibration seems to have varied among assemblages of different bulk compositions. Certain telluride intergrowths formed upon cooling of unstable high temperature phases once present in the ores, and some of these changes took place long after the period of active mineralization as the mineralized terrane gradually cooled.
The individual telluride veins and the telluride belt as a whole are essentially unzoned. However, many of the separate productive centers have a distinctive mineralogy defined by unusual proportions or associations of minerals that are otherwise widespread in occurrence. These relationships are attributed primarily to variations in the bulk compositions of the ore fluids that mineralized the separate structural centers.
The telluride veins have not been deeply weathered and the residual enrichment of gold is correspondingly slight. Partially oxidized ore contains abundant jarosite, limonite, and tellurium oxides and in places some supergene tellurium, mercury, hessite, and the copper tellurides. Fine spongy gold in limonite (“rusty gold”) is common in the outcrops and is in places associated with native silver and the silver halides. The geochemical behavior of the principal metals, gold, silver, tellurium, and iron is discussed in terms of acidities, oxidation potentials, and chloride ion activities in the oxide zone.
Based on physiographic evidence, the known telluride ores are estimated to have formed under a rock cover 2600 to 4600 feet thick and at confining pressures in the range 78 to 360 bars. This estimate is consistent with confining pressures indicated by the arsenopyrite “barometer.” Numerous “thermometers” are applicable to the vein and wall rock assemblages and indicate depositional temperatures locally as high as 350°C, but generally in the range 250 to 100°C. At any point in the veins, depositional temperatures declined through time.
Tellurium is thought to have been transported along with the other cations as soluble chloride complexes in slowly moving ore fluids released from a biotite latite source underlying the telluride belt. These fluids may have acquired some or all of their Si, Fe, Ca, Mg, and possibly V and Ba from the altered wall rocks, but the other vein components including Te, S, and the precious metals were probably hydrothermal differentiates of the biotite latite.
A brief review of major telluride districts shows that there is no obvious scheme of genetic classification that can be based on tellurium mineralogy. Compared to other world districts, the Boulder County belt has produced ores of unusual variability, and the abundance of both free gold and free tellurium in a single major district is truly exceptional. The Boulder County deposits are best placed in the epithermal class of the traditional intensity scale and are an excellent example of complex Tertiary mineralization in Precambrian terrane.