Orientador: Norberto Morales / Banca: Antonio Misson Godoy / Banca: Hans Dirk Ebert / Banca: Ticiano José Saraiva dos Santos / Banca: José de Araújo Nogueira Neto / Resumo: O Ceará Central caracteriza-se pela ocorrência de uma rede de zonas de cisalhamento preferencialmente orientada na direção NE-SW e/ou NNE-SSW. Associados espacialmente a estas zonas de cisalhamento ocorrem rochas intrusivas de dimensões variadas. Entre estas ocorrem os Batólitos de Quixeramobim, Quixadá e Senador Pompeu. Estes batólitos apresentam-se com forma alongada na direção NE-SW, subparalela às zonas de cisalharnento principais (Zonas de Cisalhamento de Senador Pompeu e Quixeramobim, por exemplo). O Batólito de Quixeramobim apresenta rochas agrupadas em seis diferentes fácies. As rochas desde batálito constituem uma série cálcio-alcatina granodiorítica, com dioritos predominantes e granitos 3A e quartzo-dioritos subordinados, ocupando a quase totalidade da área de ocorrência. O Batólito de Quixadá é composto por monzonitos e qz-monzonitos e o Batálito de Senador Pompeu por granodioritos e monzogranitos As características deformacionais, levantadas através de estudos das feições planares e lineares impressas nestas rochas, são sugestivas de um desenvolvimento progressivo em condições de estado magmático passando ao estado sálido, o que caracteriza as zonas de cisahamento principais como mobilizadoras de magmas e responsáveis pelo alojamento final das rochas. Datações radiométricas U-Pb e Pb-Pb forneceram idades de cristalização (alojamento) de 585 l 4,7 Ma (Quixadá), 587 l 14 Ma (Quixeramobim) e 561 l 15 Ma (Senador Pompeu). O mecanismo de alojamento sugerido para o Batólito de Quixadá compreende expulsão de magma de porções mais profundas para o local de alojamento final em regime transpressivo. Para os Batólitos de Senador Pompeu e Quixeramobim, sugere-se geração de espaço por nucleação de fraturas em extensão associada a transcorrência. Este último registrando variação de regime transtrativo para transpressivo. / Abstract: The Central Ceará (NE Brazil) has, as typical characteristic, a shear zone system network with directions in NE-SW and for NNE-SSW. In association with these shear zones occur intrusive rocks like Quixeramobim, Quixadá and Senador Pompeu Batholiths. They have elongated shape in NE-SW direction, broadly parallel to major dextral ductile shear zones. The Quixeramobim Batholith has rocks grouped in six different facies, characterizing a calc-alkaline series with diorites as major rock types. Subordinated granite and quartzdiorites also occur. The Quixadá Batholith is formed by monzonite e and quartzmonzonite. The Senador Pompeu Batholith includes granodiorites and monzogranites. These rocks have foliaton and lineation organization pattern that are suggestive of progressive fabric development in magmatic and solid state. The orientation of these features is broadly parallel to the shear zones that promote emplacement of lhe plutons. U-Pb and Pb-Pb minimum emplacement ages are 585 l 4.7 Ma (Quixadá), 587 l 14 Ma (Quixeramobim) e 561 l 15 Ma (Senador Pompeu). The emplacement of the Quixadá Batholith mechanism suggested is the squeeze of magma from deeper places into lhe crust to the final emplacement site in trasnpression. For the Quixeramobim and Senador Pompeu Batholiths, we suggest an emplacement mechanism with space generation by fracture nucleation in extension, into the transcurente framework. / Doutor

D. Phil. / A Mesoproterozoic basement and a Cambrian-Ordovician carbonate platform characterize the Precordillera terrane. These characteristics and its distinct geologic history mark a difference between this suspected exotic-to-Gondwana terrane and the Gondwanan autochthonous, leading to speculation that the Precordillera was derived from Laurentia. The surprising similarities of the carbonate sequences between the Precordillera and certain parts of southeast Laurentia suggest a common geological history. However, other models interpret the origin of the Precordillera terrane as being para-autochthonous with respect to Gondwana. All these models are still controversial. A combination of several methodologies including petrography and heavy minerals analysis, geochemistry, Sm-Nd and Pb-Pb isotopes and zircon dating were applied to several Ordovician and Ordovician to Silurian units of the Precordillera terrane. Geochemistry and petrography indicates that all the Formations studied have similar characteristics, with at least two sources providing detritus to the basin. The dominant source has an unrecycled upper continental crust composition whereas the other component is more depleted. The study of detrital chromian spinels suggests that mid-ocean ridge basalts, continental intraplate flood basalts and ocean island basaltrelated rocks were among the sources for the detrital record of the Precordillera terrane. Nevertheless, the mafic sources and their ages remain unknown. Nd isotopes account for negative εNd values and TDM ages in a range of variation found elsewhere within Gondwana and basement rocks of the Precordillera. The Sm/Nd ratios of certain samples indicate fractionation of LREE. Pb isotopes indicate that a source with high 207Pb/204Pb was important, and point to Gondwanan sources. Detrital zircon dating constrain the sources as being dominantly of Mesoproterozoic age (but with a main peak in the range 1.0 to 1.3 Ga), with less abundant populations of Neoproterozoic (with a main peak in the range 0.9 to 1.0 Ga), Palaeoproterozoic, Cambrian and Ordovician ages in order of abundance. i The uniformity shown by the provenance proxies indicate that there were no important changes in the provenance from the Lower Ordovician until the Early Silurian. Several areas are evaluated as sources for the Precordillera terrane. The rocks that fit best all the provenance constraints are found within the basement of the Precordillera terrane and the Western Pampeanas Ranges. Basement rocks from the Arequipa-Antofalla area (Central Andes) also match the isotopic characteristics, but a northern source is less probable, except for the Western tectofacies. On the other hand, areas such as Antarctica, Falklands/Malvinas Microplate, the Natal-Namaqua Metamorphic belt and the Grenville Province of Laurentia can be neglected as sources. The proposal of these areas as sources is in agreement with palaeocurrents and facies analyses and suggests proximity between them and the Precordillera since at least the Late Arenig to Early Llanvirn. This has important implications for the proposed models regarding the geotectonic evolution of the Precordillera terrane. The models would need to be adjusted to the here proposed youngest timing of collision.

The Nsuta manganese deposit is located in the Western Region of Ghana, approximately five kilometers south of Tarkwa Goldfields. The deposit has been an important source of manganese ore since mining began in 1916. The purpose of this project was to produce a concise model of the stratigraphy, sedimentology and structural evolution of the deposit in support of future exploration projects. The manganese ores occur as an up to 45m thick carbonate bed in a thick turbidite-greenstone succession that is part of the ~2.2 Ga Birimian Supergroup. Calc-alkaline volcanics, volcaniclastics, turbidites, argillites and phyllites are thought to have been deposited in a backarc basin environment. The entire sedimentary succession, including the manganese orebody, is a thick turbidite package hosted between an upper and lower greenstone unit consisting predominantly of volcaniclastic material. The entire lithological succession at Nsuta is interpreted to have been deposited within the middle to lower reaches of a submarine fan environment. Field evidence suggests a simple stratigraphy, commencing with a lower greenstone unit composed largely of volcaniclastic material. This is followed by an upward-fining lower turbidite unit deposited in response to a marked transgression and sea level rise. Maximum rate of sea level rise provided ideal conditions for manganese precipitation and concentration, as detrital influx ceased. The central portion of the carbonate orebody that formed hosts the manganese orebody. An upward-coarsening turbidite unit follows above the carbonate unit. This upward-coarsening succession reflects a regression and a highstand systems tract in terms of sequence stratigraphic principles. It is capped by an unconformity that formed during a period of rapid relative sea level fall. It is overlain by a second upward-fining turbidite succession. This succession is not fully preserved as there is a sheared contact between it and the overlying upper greenstone unit. Post-depositional deformation and metamorphic alteration are largely attributed to the Paleoproterozoic Eburnean Orogeny. A first phase of compression was directed along a NW-SE axis and produced a series of isoclinal anticlines and synclines (F1) with NE-SW striking axial planes. This was followed by thrusting between the anticlines and synclines. The age of this deformation and closely associated greenschist metamorphism can be accurately constrained between 2.09 Ga and 2.07 Ga. E-W oriented oblique listric faulting has a prominent effect on the appearance of the Nsuta manganese deposit, as it produced a series of imbricate fault blocks dipping to the north. Associated with this period of deformation is small-scale cross folding with axes plunging to the east (F2). The faults post-date the Eburnean Orogeny and must be associated with a second major tectonic event. Finally, a NNE-SSW striking normal fault, locally known as the German Line, caused further block rotation, notably in the northern parts of the mining concession. Late Mesozoic deep lateritic weathering and incision of the lateritic peneplane by modern rivers have resulted in the complex dissected appearance of the Nsuta orebody. However, based on the detailed structural analysis provided in this study, a feasible target for future exploration of manganese ore buried beneath Late Mesozoic and Cenozoic sediments and soils, has been identified. This target is located to the west of Hills A and B. / Dr. J.M. Huizenga Prof. Nic Beukes Prof. J. Gutzmer

>Magister Scientiae - MSc / The study area Koekoeb B is a farm that falls under the Kakamas Terrane which in turn falls under the Gordonia Subprovince in the Namaqua-Natal Metamorphic Province, South Africa. This area was chosen due to lack in literature about its lithology. Koekoeb B is comprised of metasedimentary rocks of the Biesje Poort Subgroup and granitoids of the Keimoes Suite. The Kakamas Terrane was deposited in an intracratonic basin between the Kaapvaal Craton and the Namaqua continental mass. The sediments were buried with time and experienced metamorphism due to burial pressures and temperatures. The area experienced folding as a result of the collision of the newly formed Kakamas Terrane and the Bushmanland segment with the Kaapvaal Craton during a Wilson Cycle. During subduction and collision the metasedimentary rocks were intruded by what is known as the Keimoes Suite. The most abundant intrusive rock in Koekoeb B is the Friersdale Charnockite. It is considered the youngest with Rb-Sr ages around 1080-1090 Ma. The Gordonia Subprovince experienced such intense deformation that continuous folds formed and there is even evidence of parasitic folds. Northwest striking shear zones developed as a result of the continued compression of the Namaqua mass with the adjacent north easterly Kaapvaal Craton. The folds and shear zones formed under four major deformational events Two months were spent acquiring orientation data (direction of dip and dip) in the field. A Clar compass was used to measure the dip direction and dip readings of bedding, cleavage, joints and lineations. The orientation data was imported into Move® software to create a geological map. Samples collected from the field were used to produce thin sections for petrography studies using the petrographic microscope. Conclusions were drawn from the analysis of the data. Koekoeb B experienced regional metamorphism and folding when the Kakamas Terrane collided with the Kaapvaal Craton. The area was subdivided into four subareas based on the strike and dip data generated on the geological map. The synoptic β-axis diagram determined that the subareas are of the same generation but the fold axes orientations vary slightly. Because the study area did not include the shear zones no conclusive reason can be given but it can be assumed that the variation is due to movement along the shear zone or as a result of the intrusion of the Keimoes Suite. The area later experienced brittle deformation which is evident from the large number of joints found; the joints cut across the folds and show a different stress regime from the folds. Conjugate joints were observed on the field and plotted on stereonets. The results showed a vertical sigma two which confirmed that Koekoeb had been affected not only by compression tectonics but by the strike-slip movement on the shear zone.

The development of an open pit mine at the Endeavour 42 (E42) epithermal gold deposit, situated in the Junee-Narromine Volcanic Belt of the Ordovician Macquarie Arc, central New South Wales, has provided a 3D view of the structurally controlled deposit which was hitherto not available due to the paucity of outcrop in the region. Outcropping geological relationships present a complicated history of overprinting structural deformation and vein events, including the spatial characterization of the gold-mineralizing system. Host rocks consisting of interbedded sedimentary and resedimented volcaniclastic facies, trachyandesite and porphyritic andesite lavas and intrusions (coherent and autoclastic facies), intruded by a large diorite sill, were initially tilted and faulted, followed by the emplacement of multiple dyke phases along faults. Economic gold mineralization at E42 is restricted to faults, fault-hosted breccias, and veins, and was deposited over a period spanning two distinct structural regimes. Early gold-bearing veins are steeply dipping and interpreted as forming coevally along two sets of faults and dykes within a tensional stress regime. High grade fault-hosted, hydrothermally cemented breccia intervals are included temporally with early gold-bearing veins based on comparable mineralogy and steep, fault parallel orientations. Crosscutting the early steep gold-bearing vein sets are two populations of coeval inclined gold-bearing veins, dipping moderately to the southwest and northwest, respectively, which formed in a compressional stress regime with tension directed subvertically. The E42 epithermal deposit likely developed in the period of overall crustal extension, ca. 443-433 Ma, following Phase 1 of the Late Ordovician – Early Silurian Benambran Orogeny. The generation of permeability, styles of fracture propagation, and the reactivation of pre-existing planes of weakness in the rock package are key factors in the development and current geometry of the E42 gold deposit. High grade veins and faults are commonly flanked by sericite-quartz ± carbonate alteration haloes, which exhibit consistent geochemical patterns for metals and pathfinder elements, both laterally away from structures, and vertically within the deposit. Au, Ag, As, Hg, Sb, Tl, Cu, Pb, and Zn, all display increasing concentrations towards high-grade structures, as well as higher up in the epithermal system, with varying dispersion haloes. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Master of Science / Department of Geology / Brice LaCroix / The Los Burros mining district (LBMD) is located within the Lucia subterrane (Underwood et al., 1995) along the Sur- San Gregorio-San Simeon-Hosgri fault system of California’s Franciscan complex. The LBMD presented an attractive gold prospecting and mining area during the late 19th and early 20th centuries. Recent interest in the area has focused on fault-offset modelling through thermochronology (Underwood et al., 1995, Chapman et al., 2016). However, the mechanism, conditions, and timing of ore formation in the area are poorly constrained due to a lack of academic interest and documentation. This research seeks to arrive at a better understanding of gold emplacement mechanisms in the LBMD through an examination of lithologic and structural controls coupled with source fluid composition and peak P/T constraints. Sampling and mapping of lithologies and structures within the vicinity of the LBMD were conducted during the summer of 2016. Samples were collected for clay-fraction and bulk-rock mineralogy via XRD analyses, petrographic inspection and interpretation, including fluid inclusion microscopy/microthermometry as well as Raman spectroscopy. These analyses were conducted in an endeavor to constrain and explain a previously identified regional thermal anomaly within the vicinity of the LBMD with the intention of characterizing and modelling the impacts of structural controls on gold deposition. The results of this study suggest gold deposition from a gas-poor, metamorphic source-fluid with approximately 300 ppm CH₄, CO₂ density of 0.15 g/cm³, and an average salinity of 1.7 wt % NaCl equivalent. These data, along with peak P/T conditions of ~285°C and 680 bars were inferred using input from illite crystallinity data, fluid inclusion microthermometry, and the application of the Raman spectroscopy of carbonaceous material (RSCM) geothermometer used by Lahfid et al. Gold mobilization from sulfide and carbonaceous-rich sediments through polyphase deformation events led to emplacement along structural and lithologic contacts likely as a syn-orogenic event. Gold emplacement occurred during the reorientation of the regional structure by transpression.

Stone stripes are linear accumulations of rock debris separated by finer material and oriented down the steepest available slope. In north central Oregon, the stone stripes are composed of basalt fragments and occur where the soil mantle averages less than 1 m in depth. They are best developed in areas of higher elevation, usually from about 900 to 1100 m, and are most prominent on convex to straight slopes of 15 to 30 degrees. The stripes vary from a few meters to over 150 in length, and their widths range from 0.3 to over 3 m. The depth of the stone stripes ranges from 20 to over 65 cm. Morphological investigations of stone stripes in north central Oregon reveal that: (1) surface rock orientation is not very strongly related to slope direction or gradient; (2) stripe depth does not seem to be connected to width of the stripe nor to position on the slope; (3) vertical sorting occurs in a rock size ratio of about 5:3:2 from surface, to middle, to base; (4) the introduction of cattle has resulted in considerable stripe modification; and (5) vegetation encroachment and infilling with soil are stabilizing the stripes from the edges inward. The stone stripes found throughout north central Oregon are of such a size and magnitude that they cannot be adequately explained by geomorphic processes currently operating in the region. Although these processes are sufficient to cause some stripe movement and modification, the initial striped patterns were probably developed under periglacial conditions during the cooler, moister phases of the Quaternary. Evidence which lends credence to a cold climate origin for these stone stripes includes: (1) other forms of patterned ground, i.e." sorted circles, nets, and polygons; (2) soil mounds; (3) talus slopes; (4) terracettes; (5) convex slopes; and (6) stripe stabilization through vegetation encroachment, soil infilling, and heavy lichen growth.

Geothermal energy potential depends on locating highly porous and permeable zones that support fluid flow to extract heat. Hot springs in the playa of Surprise Valley, CA are distributed along gaps and bends in magnetic anomalies interpreted as sub-cropping mafic dikes (Glen et al., 2013). In addition to these dikes in the Valley Playa, dikes outcrop in the Hays Canyon Range (HCR) that defines the eastern margin of the valley. Dikes in the HCR have two distinct attitudes (1) N-S striking dikes (~180) that dip 60oW, and (2) NNW-SSE striking dikes (~330) that dip 85oE. Both attitudes are spatially associated with locally high fracture density and minor hydrothermal alteration that may have formed from dike emplacement. This study tests whether the distribution of hot springs can be explained by elastic distortions around an array of opening dikes that promotes localized dilation to support a network of open secondary structures focusing fluid flow to supply the hot springs. This is done through two mechanical model experiments which use boundary elements in an elastic half-space. The first model is a sensitivity study investigating the role of dike dimensions and position in the density stratigraphy on their opening. Field analysis constrains the strike-length, thickness, and the upper tip position of dikes, although height is unknown. The model reproduced the mapped dike-length (4000 m) and thickness (2.0 m) with a dike-height of 60000m and a magma density of 2500 kg/m3 which is consistent with mafic dikes. The second model applies the dike dimensions and calculates the resulting stress state and fracture potential around an array of dikes conforming to both attitudes of the dike array mapped magnetic anomalies and informed by from field results of dike orientation in the Hays Canyon Range. Simulations of the N-S trend predict regions of enhanced Coulomb stress and tension that promote fracture formation and opening near dike tips where segments are isolated and where two closely spaced dike segments underlap. Conversely, compression is enhanced along the dike walls and where the segments closely overlap. The NNW-SSE trending array of dikes predict increased Coulomb stress and tension at similar locations in the array, but with more extreme values. Thus, the NNW-SSE dike array geometry better matches areas of enhanced fracturing with locations with active hot springs (as well as regions of enhanced compression with their absence) than the geometry of the N-S dike array. / Geology / Accompanied by 3 *.M files: 1)Sawyer_temple_0225M_171/WORKFLOW_p3dResults_Sensitivity_positionbc_interrogation_3d_NCD.m 2)Sawyer_temple_0225M_171/WORKFLOW_p3dResults_Sensitivity_positionbc_figures.m 3)Sawyer_temple_0225M_171/WORKFLOW_p3d_SV_toy_dikes_Sensitivity_positionbc.m

Examination of deformation temperature, flow vorticity, and strain symmetry in the Loch Eriboll, Loch More, and Assynt regions of the Moine thrust zone (MTZ) in northwest Scotland allows quantitative kinematic and thermal characterization of a crustal-scale shear zone at the base of the Scandian (435-425 Ma) orogenic wedge. Quartz crystal fabrics, kinematic vorticity (Wm), and strain estimates from the ductile thrust sheets in this region are used to determine how pure and simple shear components of deformation are partitioned, and indicate that these processes may be thermally, structurally, and lithologically dependent. Vorticity analysis of samples collected along strike in the MTZ and overlying Moine nappe indicate that Scandian thrusting and deformation involved a considerable pure shear component. Integrated strain and vorticity estimates indicate that significant sub-vertical foliation normal shortening has occurred as nappe stacking progressed. Along strike Wm variation could not be directly correlated with changes in footwall structural architecture, lithology, deformation temperatures, or structural depth and are thus interpreted to reflect local variability driven by a complex interplay of these and possibly other factors. Quartz c- and a-axis fabrics indicate that deformation in the footwall of the Moine thrust involved plane strain to general flattening strain with only a minor rotational (non-coaxial) component. In contrast, deformation in the Moine nappe was strongly non-coaxial, as indicated by asymmetric single girdle c-axis fabrics. Quartz c-axis opening angles and microstructures suggest that deformation temperatures increase from north to south and from structurally lower to structurally higher levels in the footwall to the Moine thrust. Vertical ductile thinning must be accommodated by either volume loss or extrusion of material towards the synorogenic topographic surface. Extrusion towards the synorogenic topographic surface implies a causal link between upper and lower crustal processes, with significant implications for the kinematic, geometric, and kinetic (deformation rates) evolution of the Scandian orogenic wedge. New thermobarometric and deformation temperature estimates are combined with structural and kinematic investigations to characterize the thermal structure of the Moine, Ben Hope, and related nappes. At the leading edge of the Moine thrust, subhedral garnets with prograde compositional growth zoning yield peak temperatures (grt-bio) of 440-492 °C at 4.5-6.0 kbars from Creagan Meall Horn to northern Assynt. Three samples collected at similar structural positions along the leading edge of the Moine nappe yield deformation temperatures of 420-460 °C, as determined from quartz c-axis opening angles. At the structurally highest position in the Moine nappe, garnet prograde compositional zoning profiles are preserved and samples yields P-T estimates of 565-571 °C (grt-bio) and 4.5-5.1 kbar (GRAIL barometer, minimum pressure due to absence of Al2SiO5 phase). Quartz c-axis fabrics of samples collected at similar structural positions yield deformation temperature estimates of 490-565 °C. In the structurally higher Ben Hope nappe, two phases of prograde compositional growth zoning are preserved in garnet bearing amphibolite near Portvasgo. The later outer garnet rim records P-T conditions of 655-672 °C at 3.9-5.1 kbars. The inverted metamorphic sequence from the Moine to the Naver thrust is mostly intact and is interpreted to be Scandian (435-420 Ma) in age. It is likely that the formation of this inverted sequence is due to a combination of progressive accretion of successively lower grade thrust sheets onto the base of the Scandian wedge and heating of the Moine and Ben Hope nappes from above by the relatively higher temperature migmatites of the Naver thrust. Vertical ductile thinning, in conjunction with erosion and normal faulting, likely led to rapid exhumation of the Scandian nappe pile and in turn preserved the inverted metamorphic sequence. / Ph. D.

Epithermal vein and carbonate replacement deposits in Cunningham Gulch are located within the western San Juan Tertiary volcanic field in southwestern Colorado. The Pride of the West epithermal vein system is hosted within the intracaldera facies of the Sapinero Mesa Tuff, a voluminous ash-flow tuff that erupted from and resulted in the formation of the San Juan Caldera at 28 mybp. The Pride of the West vein system is developed along a radial fracture formed during resurgence of the San Juan Caldera prior to eruption of the Crystal Lake Tuff (27.5 mybp). This eruption led to the concomitant collapse of the Silverton Caldera, nested within the larger San Juan Caldera. The Pride of the West, Osceola, and Little Fanny mines are positioned near the intersection of the Pride radial fracture system and the buried structural margin of the San Juan Caldera, suggesting that ore concentration was controlled by this structural setting. Large limestone blocks of the Mississippian Leadville Formation are incorporated into the intracaldera fill volcanics in the mine area. These blocks appear to have been engulfed within mudflow breccias of the Tertiary San Juan Formation (32.1 mybp). They were then emplaced in their present structural position within a caldera-collapse breccia which caved from the oversteepened margin of the San Juan Caldera. Regional propylitic alteration of the hosting volcanics to a chlorite-calcite-pyrite assemblage preceded vein-associated alteration and mineralization. The veins are enveloped by a narrow phyllic alteration assemblage of quartz, sericite, illite, kaolinite, and pyrite. The veins are comprised of sphalerite, galena, chalcopyrite, pyrite, hematite, magnetite, quartz, pyroxmangite, calcite, and minor barite. Substantial bodies of replacement ore are present where the vein structures intersect the limestone blocks; the mineral assemblages of the replacement deposits are identical to those of the feeding vein structures. Commonly, replacement textures are spectacular concentrations, especially the "zebra ore" which primarily consists of regularly spaced, alternating bands of sulfides and quartz. These "zebra" laminations are stratigraphically controlled and appear to represent replacement of a depositional or diagenetic fabric. Main ore-stage mineralization began with widespread deposition of quartz with or without pyrite, followed by sphalerite, chalcopyrite, and galena. Post ore-stage brecciation and silicification events are evident and were followed by deposition of calcite and minor barite during the waning stages of the hydrothermal system. The distributions of Fe, Mn, Pb, and Ca suggest a lateral component of fluid flow from northwest the southeast, away from the structural margin of the Silverton Caldera. Fluid inclusion data from both vein and replacement-type sphalerite and quartz indicate that mineral deposition occurred over a range of 200 to 312°C (mean 243°C) from solutions containing 1 to 5% total salts. The high base metal to precious metal content of the ore, the phyllic alteration assemblage, and the temperature and composition of the ore-forming fluid indicate that the mine workings are within the lower portion of a fossil geothermal system. / text