The Jurassic Humboldt igneous complex in west-central Nevada consists of a comagmatic suite of intrusive and extrusive rocks and is tectonically intercalated with Triassic to Lower Jurassic shelf sequence and basinal successions. Its plutonic rocks include, from bottom to top, olivine-gabbro, melatroctolite, hornblende gabbro, microgabbro, and diorite transitional upward into quartz diorite, tonalite-granodiorite, and monzonite. Contacts between these plutonic subunits are commonly gradational, but mutual intrusive relations, characterized by the existence of brecciated and altered zones and xenoliths, are also common. Mafic to felsic plutonic rocks are cut by generally N–S to NW–SE striking dikes that form local dike swarms with one- and two-sided chilled margins. Dikes are composed of fine- to medium-grained rocks ranging in composition from basalts to andesites and feed into and/or are overlain by extrusive rocks consisting of lava flows intercalated with volcanic tuff and breccia. Lava flows at stratigraphically lower levels are more mafic and locally display pillow shapes reminiscent of submarine lava flows, whereas lava flows at higher levels are more felsic and are commonly interleaved with a fine-grained tuffaceous material. Volcanic rocks range in composition from basalts, basaltic andesites, andesites, to latites and dacites and mineralogically and texturally are similar to the dikes. The major element compositions of the analyzed rocks suggest relatively evolved basaltic magmas, whereas strongly incompatible trace element ratios (e.g., Ce/Ta) have high values typical of subduction related magmas. Lavas, dikes, and gabbros commonly display similar rare earth element (REE) patterns, although more felsic rocks are light rare earth (LREE) enriched, suggesting a cogenetic suite of rocks. These REE patterns are characteristic of basaltic andesites from volcanic arcs and suggest, coupled with field relations, that the rocks of the Humboldt complex might have evolved from subduction originated magmas in a volcanoplutonic arc setting. The tectonic nature of the contact between the Humboldt complex and the underlying Triassic-Jurassic sedimentary strata indicates that it was displaced eastward from its original arc environment following its igneous evolution. Both the Humboldt complex and the sedimentary strata are intruded at all structural levels by numerous northeast-striking dikes and dike swarms, which strongly altered and metasomatized their country rocks. These dike rocks are Miocene in age and have geochemical characteristics distinctly different from those of the Humboldt rocks. Based on regional correlations and paleogeographic reconstructions, we interpret the Humboldt igneous complex as the northern continuation of a Jurassic continental margin arc that extended from the Sonora Desert region in the south to northwestern Nevada and northern California in the north. This continental margin arc was a site of regional subsidence and crustal extension that accompanied magmatic activity and penecontemporaneous deposition of both arc and craton-derived detritus in submarine to subaerial conditions during much of the Jurassic. We emphasize that the coeval ensimatic arc terrane west and outboard of this extending continental margin arc might have represented a fringing oceanic realm that was subsequently collapsed into the outer continental margin of North America in Middle to Late Jurassic time.

Abstract The Humboldt mafic complex, west-central Nevada, is a large composite Middle Jurassic basaltic-composition volcano-plutonic center that has exceptionally extensive (>900 km 3 ), intense (nearly complete leaching of many elements) sodium-rich hydrothermal alteration. Mapping of exposures at multiple structural levels allows assessment of the time-space development of hydrothermal alteration and cogenetic magnetite and hematite ± copper sulfide mineralization. Alteration varies from early, deep and proximal marialitic scapolite-hornblende to shallow and distal albite-actinolite-chlorite and chlorite-carbonate assemblages. These associations reflect large compositional changes in host rocks (mass-transfer), whereas distal and deep propylitc assemblages are less intensely modified. Substantial quantities of iron are present in massive, breccia-form, and stratabound magnetite and hematite bodies at intermediate and shallow depths. Lesser amounts of copper, cobalt, and other metals are sporadically enriched at shallow levels. Field, petrological, and geochemical constraints require that the fluids were dominantly or entirely non-magmatic, external brines that circulated in response to the heat and permeability increases associated with repeated basaltic intrusion. The Humboldt system represents a mafic end-member among iron oxide-rich copper-bearing hydrothermal systems ( Barton and Johnson, 1996 ) and, in the larger context, and an end-member in the spectrum of igneous-related hydrothermal systems.

The purpose of this portion of the field trip is to examine the spatial and temporal distribution of sodium-rich hy-drothermal alteration and iron oxide mineralization generated by a saline hydrothermal system driven by the Middle Jurassic Humboldt mafic complex. The Humboldt system is of particular interest because it represents a basaltic mag-matic end member of intrusion-driven hydrothermal systems and, in this case, one where the fluids are largely, perhaps entirely, externally derived brines. Outcrops of Jurassic rocks within the complex record multiple, mutually crosscutting magmatic and hydrothermal events at different structural levels as exposed by mid-Tertiary extension. Mapping of selected areas across this large igneous complex allows definition of the relationships among the magmatic, structural, and hy-drothermal features. In turn, these enable an interpretation of the overall temporal and spatial evolution of a large intrusion-driven hydrothermal system ( Johnson and Barton, 2000 ). From geology and geochemistry, one can estimate that upwards of 15 billion tonnes of iron and 35 million tonnes each of copper and zinc were moved by the hydrothermal system ( Johnson, 2000 ). What happened to these metals and what are the implications for other systems? These are among the issues to consider.

Abstract Intrusion-related hydrothermal systems represent a large variety of geologic environments that in some cases form large metallic mineral deposits. The deposits examined in this trip represent the spectrum from systems dominated by magmatic fluid (Birch Creek, California and Yerington, Nevada) to those systems in which intrusions serve as heat engines to drive convectively circulating brines derived from sedimentary rocks (Hum-boldt, Nevada). In these examples, nonmagmatic fluids are largely excluded from more deeply emplaced intrusions in a compressive environment, and the hydrothermal composition and ores (e.g., granite W-F, Cu porphyry and skarn) are dictated by the composition of the magma and its mechanism of crystallization and aqueous fluid generation. Magmatic fluids are less important in the shallow crustal ore environment, but apparently contribute to acidic alteration zones located vertically above source intrusions. Using Humboldt as an example, we propose that the Fe oxide Cu-Au ores in the shallow environment require an abundant source of sedimentary brines (typical of evaporitic environments), high fracture permeability (promoted by an exten-sional setting) to allow aqueous fluid flow and dike emplacement, and shallowly emplaced intrusions to serve as heat sources.