Master of Science (MS)
Number of Pages
The Johnnie district, in the northwestern Spring Mountains, Nye County, Nevada, may have produced a little under 100,000 troy oz of gold, since the discovery of the district in 1890.
An approximately 13,000-ft-thick (4,000 m) section of east-dipping upper Precambrian through Middle Cambrian miogeosynclinal clastic and carbonate rocks is exposed in the district. The strata are, in order of decreasing age, the Johnnie Formation, Stirling Quartzite, Wood Canyon Formation, Zabriskie Quartzite, and Carrara and Bonanza King Formations. These are overlain by Cenozoic units which include and older unit and a younger unit of fanglomerate, the older containing a megabreccia deposit, and Quaternary alluvium.
The rocks were deformed by the Late Cretaceous Sevier orogeny and by subsequent tectonic events, which include Basin-and-Range faulting, of Miocene age. The oldest structures, formed in conjunction with Sevier tectonism, are disharmonic folds in the Johnnie Formation, which folds underwent rotation by later (Sevier orogeny) eastward tilting of the district during larger scale folding. Simultaneously with the later folding, competent rocks were translated across less competent ones along zones of tectonic readjustment; the most notable zone is at the top of the Johnnie Formation in the western part of the district. High-angle fractures -- in extension, conjugate, and pressure-release orientations -- developed at about the same time and were the ancestors for most younger high-angle structures, including quartz veins.
Longitudinal faulting occurred at the end of the Sevier orogeny; and concurrently, the related Congress low-angle normal fault developed. The district was dropped down to the west along the Grapevine fault system during Basin-and-Range normal faulting at the west face of the Spring Mountains. Throughout the structural development of the district, displacement occurred along transverse faults in secondary readjustment to displacement along other features and also low-angle faults transposed younger rocks across older ones.
Some of the tectonism caused local carbonatization of the Bonanza King Formation.
The district was eroded in a series of stages during Basin-and-Range faulting. finally, sometime between latest Miocene to middle Pleistocene times, pediments developed at the edges of the resultant basins, then were buried by bajadas of older fanglomerate. Later, parts of the pediments were exhumed by erosion.
High-angle and concordant quartz-bearing structures were emplaced during hydrothermal activity, probably between the Paleocene and early Miocene epochs. High-angle quartz veins, the average of which strikes ENE and dips north, dominate and are the hosts for most economic, mesothermal mineralization in the district. Three ore mineralogic suites are present: gold-chalcopyrite-pyrite, chalcopyrite-galena, and galena (-calcite). Additionally, chalcopyrite occurs with specularite in stratabound quartz-poor lodes of apparent hydrothermal origin; these oxidize to low-grade malachite deposits. Placer gold deposits formed in the older fanglomerate during post Basin-and-Range faulting erosion of the district. The characteristic wall-rock alteration mineralogic suites in the hypogene deposits are sericite and pyrite in clastic rocks and also sericite, alone, in dolomite; the alteration minerals, chlorite, calcite, and specularite, occur locally. The ore mineralogic suites define a district-wide pattern of hypogene mineralogic zonation about gold centers at the Johnnie and Congress mines, the main producers in the district.
The fundamental control which admitted the hydrothermal fluids into the district is obscure. However, trains of quartz veins are concentrated within 2.5-mi-long (4 km), ENE-trending principal mineralized structures which lie astride an inferred 13-mi-long (21 km) N.-35o-E.-trending major longitudinal structure, which may be a manifestation of the fundamental control. The gold mineralization is localized in the Zabriskie Quartzite and the dolomitic rocks near the top of the Wood Canyon Formation, in part, by the retention of hydrothermal fluids beneath a blanket of shaly rocks at the base of the Carrara Formation. The base of the dolomitic rocks is also the base of the Cambrian section, which is recognized as a favorable stratigraphic site for ore deposition at other places in the southern Great Basin.
Although this report develops a theme in which the hydrothermal fluids were introduced from a remote source below the district, as the broad concepts of ore genesis change with time, it may become accepted that many of the constituents of the hydrothermal fluid were derived from local, syngenetic sources.
Geology; Nevada -- Johnnie District; Nevada -- Nye County; Nevada -- Spring Mountains; Ore deposits
Geology | Stratigraphy | Tectonics and Structure
Ivosevic, Stanley Wayne, "Geology and ore deposits of the Johnnie District, Nye County, Nevada" (1976). UNLV Theses, Dissertations, Professional Papers, and Capstones. 1454.