The present study investigates the effects of basement variations on the geochemical composition of the Laramide plutonic belt of northwestern Mexico, and implications for the development of the associated porphyry copper mineralization. In the north part, the belt intruded rocks of the North America and Caborca terranes representing cratonic basements juxtaposed by Mid-Jurassic strike-slip faulting. The central part of the belt was emplaced in the Cortes terrane which contains Paleozoic eugeoclinal sequences accreted to North America between Mid-Permian and Late Triassic times. The southern part of the belt intruded island arc-related sequences of the Guerrero terrane, accreted to North America during the Late Cretaceous. A suite of 30 samples of granitoids were studied to characterize the geochemical composition of the belt. The samples range from 56% to 75% SiO₂ and have mid to high-K calc-alkaline and mostly metaluminous compositions. REE results show more evolved chondrite-normalized plots in the north part of the belt, characterized by higher ΣREE and La(N)/Yb(N) ratios, and more pronounced negative Eu anomalies. Southward, the samples show a progressive flattening, eventually with almost no Eu anomalies in the Guerrero terrane. ⁸⁷Sr/⁸⁶Sr initial ratios higher than 0.7062 and initial εNd below -4 characterize the granitoids from the north part of the belt. Slightly less evolved isotope signatures occur in the central part, whereas ⁸⁷Sr/⁸⁶Sr ratios below 0.7063 and more positive εNd values characterize the samples from the south. The data suggest a garnet-bearing source for the granitoids emplaced in the north and central parts of the belt, whereas much less garnet fractionation is needed in the southern part. The effect of different crustal domains on the porphyry copper mineralization is less obvious. Hypogene Cu grades do not vary much along the belt, but the associated metals such as Mo, W, and Au show a certain degree of spatial control. Higher Mo grades are associated with the North America and Caborca terranes. Tungsten deposits are not terrane-constrained, but the more conspicuous mineralization occurs in central Sonora, mainly associated to the Caborca terrane. Higher Au values seem to occur in porphyry copper and associated breccia deposits in the Guerrero terrane. In general, the associations Cu-Mo-WO₃ in terranes of North America affinity, and Cu-Au in the Guerrero terrane suggest basement control.

Lower Paleozoic, passive margin strata host more gold deposits than other Paleozoic strata. Geochemical analysis of unaltered, unmineralized lower Paleozoic sedimentary rocks from Nevada, Arizona, and Australia suggests that the abundance of gold mineralization in lower Paleozoic strata is related to geochemical conditions during sedimentation. Siliciclastic strata show primary gold enrichments of 21 parts per billion (ppb) and 7 ppb (mean values) in Ordovician Valmy and Vinini strata, respectively. Gold contents of lower Paleozoic carbonate platform, shelf, and transitional facies are comparable to the average crustal abundance of ∼2 ppb. Gold concentrations of the siliciclastic rocks correlate stratigraphically with high total organic carbon, total metal contents, vanadium ratio [V/(V+Ni)], and authigenic uranium, and with low δ¹³C(org) and cerium anomaly. These geochemical parameters are indicative of a low-O₂ paleo-environment, interpreted to result from high organic productivity in upwelling zones. The origin of the gold enrichments in lower Paleozoic, siliciclastic strata is related to global conditions in the marine environment during the early Paleozoic. Hydrothermal and detrital inputs of gold to the oceans were higher than at other times. Organic activity in upwelling zones contributed to the accumulation and preservation of gold in marginal marine sediments. Lower Paleozoic strata are favorable hosts for gold mineralization because they are associated with gold-enriched, carbonaceous siliciclastic strata that may act as a source and/or trap for gold-bearing hydrothermal fluids.

Geologic endeavor is a continual search for windows into the past. Our ability to characterize ancient geological processes is only as good as the techniques we have at our disposal; often the desire to uncover new information drives the development of new geologic methods or the modification of old techniques. This dissertation is composed of a series of projects focused on gaining new insights into the history of orogenic systems through the application of existing techniques or the development of new methods. Three primary projects were undertaken, the first a paleoelevation study aimed at determining the elevation of the Colorado Rockies 28 million years ago (Ma). This study was an attempt to both constrain the paleoelevational geometry of North America and test a new paleoaltimetry technique. This "one-isotope" technique relies on the relationship between cosmogenic isotope production rates and elevation; while a paleoelevation for the Colorado Rockies was not derived, the technique should prove useful in other geologic situations. From this initial technique, a two-isotope technique was derived which bypasses some of the difficulties inherent to the one-isotope method. A complete theoretical development of this two-isotope technique is included. Finally, a project documenting the wide-spread remagnetization of a suite of pre-Cenozoic Bolivian red sedimentary rocks reflects the impact orogenesis can have on an entire region. Taken as a whole, these projects focus on both the effects of mountain-building events and the techniques we can use to better understand them.

Supergene enrichment in the Morenci porphyry copper deposit was formed as a result of the coupled processes of erosion and chemical weathering that accompanied five stages of landscape evolution in the Cenozoic Era. During Stage 1 (64 to 53 Ma), low-grade primary chalcopyrite and pyrite mineralization was deposited as a result of Laramide magmatic and hydrothermal processes at about 55 Ma. During Stage 2 (53 to 30 Ma), initial unroofing and erosion removed approximately 1.8 km of rocks overlying the deposit and shed detritus to the north in the Eocene and to the south in the early Oligocene. During Stage 3 (30 to 18 Ma), the deposit was preserved under 640 to 950 meters of volcanic rocks as a result of mid-Tertiary extension and volcanism. During Stage 4 (18 to 2 Ma), most of the supergene copper enrichment at Morenci appears to have been formed as a result of Basin and Range deformation between ∼13 and ∼4 Ma. Sixteen new ⁴⁰Ar/³⁹Ar ages from alunite, jarosite, and potassium-bearing manganese oxides in the district recorded three cycles of enrichment and leaching that peaked at about 7.3 Ma. Microbiological and geological studies revealed that acidophilic iron oxidizing bacteria and dissimilatory sulfate reducing bacteria contributed to leaching and enrichment of copper in the supergene environment, at least since the late Mocene. During Stage 5 (2 Ma to present), destruction of the current enriched blanket accompanied base-level drop and stream incision as a result of progressive drainage integration in southern Arizona in the late Pliocene and Pleistocene.

The variation of equilibrium Fe²⁺-Mg fractionation between orthopyroxene and spinel as a function of temperature was experimentally determined in the range from 1250-850°C and 0.8-1.3 GPa. The effect of chromium on the equilibrium fractionation was constrained by limited experiments at 1000°C and 1.2 GPa. The experimental data can be cast into a thermometric expression as: T(K) = [(1373±155)+121· P(GPa)+(2558±181)·Xˢᵖ(Cr3+)]/[ln K(D) + (0.55± 0.13)] where K(D) = (Fe²⁺/Mg)ˢᵖ/(Fe²⁺/Mg)ᴼᵖˣ. The self-diffusion of Fe²⁺ and Mg in spinel was determined from binary diffusion couple experiments at 1325-950°C and 2.0 GPa. These experiments yield for Fe²⁺: Dₒ = (1.8 ± 2.8) x 10⁻⁵ cm²/sec and Q = (198 ± 19) kJ/mol; and for Mg: Dₒ = (1.9 ± 1.4) x 10-5 cm²/sec and Q = (202 ± 8) kJ/mol. The above data were applied to compositional profiles of coexisting orthopyroxenes and spinels in diogenite samples from the Antarctic meteorite collection to constrain their thermal histories. The modeling suggests that diogenites cooled to a temperature of ≥ 885°C at a rate of ≤ 3800°C/Ma at 1000°C. Modeling of the compositional zoning of spinel indicates that it cooled at ∼(6-20) x 10³ °C/Ma at 700°C corresponding to a burial of ∼65-130 m within a 100-200 m thick regolith blanket. This supports the idea that the samples were ejected and buried within a blanket of regolith material after initial cooling from the magmatic stage. Application of the diffusion data for spinel to the cooling rates of terrestrial ultrabasic rocks suggests that Ozawa (1984) cooling rates derived from the modeling of compositional profiles between olivine and spinel should be revised downwards by approximately two orders of magnitude.

The study of iron-oxide-rich mineralization in the Great Basin demonstrates that these occurrences are widespread and are associated with a wide range of host rocks and associated igneous suites. Two well-exposed areas containing similar types of iron-oxide±(Cu-Co-REE) mineralization and voluminous sodium-rich hydrothermal alteration are hosted by contrasting coeval, mid-Jurassic igneous suites and demonstrate a complex, but similar hydrothermal system evolution. One area is hosted by mafic plutonic and volcanic rocks of the Humboldt mafic complex in west-central Nevada. The second area is hosted by felsic granitoids and volcanic rocks in the Cortez Mountains in north-central Nevada. Alteration and iron-oxide±(Cu-Co-REE) mineralization in both systems is synmagmatic and consists of high-temperature alteration assemblages composed of either scapolite or albite/oligoclase-rich sodic-calcic assemblages. Lower temperature alteration assemblages are composed of sodic and hydrolytic assemblages in addition to potassic alteration in the Cortez Mountains. Mass-balance studies show that all of the components observed within the hydrothermal system are compatible with derivation from an external fluid or from the associated igneous host rocks. Sodium is added in both systems and is approximately 5 times greater than the amount that could be sourced from a plausible magmatic fluid. Components such as iron, phosphorous, copper, cobalt, nickel, barium, and REE are leached from intensely altered portions within each system and are partially trapped up section at the deposit scale. Other elements such as manganese, zinc, and lead that are also mobilized during deep alteration are not trapped within the exposed portions of either hydrothermal system and are presumably lost to the hydrothermal system. Comparisons of mass-transfer in iron-oxide-rich hydrothermal systems with better-known igneous-related hydrothermal. systems demonstrates that mass-transfer systematically among systems and is strongly controlled by the composition of the hydrothermal fluid and the size of the heat source. Thus systems dominated by saline fluids show the largest amounts of mass-transfer. Mass-transfer estimates show that iron-oxide hydrothermal systems are second only to marine hydrothermal systems in terms of total amounts of materials transferred.

This study uses Re-Os isotopes from sulfide minerals (molybdenite, pyrite, chalcopyrite, bornite, sphalerite, arsenopyrite, and galena) and magnetite from high (400-500°C) and low and low ( < 300°C) temperature mineralization assemblages associated with the intrusion of igneous rocks to trace the source of Os and by inference the other metals. The source of Os is used as a proxy for the source of copper and gold in these systems. Concentrations of Re-Os for all sulfides except molybdenite in various deposit types vary from 0.005-2 ppb Os and 0.120-500 ppb Re. Initial ¹⁸⁷Os/¹⁸⁸Os ratios derived from isochron plots ranges from 0.15-50. These data show interesting relationships among deposits of different styles within the district, region, and worldwide scales. On the district-deposit scale, Re-Os isotopes illustrate a complex behavior for the source of Os and by inference other transition and noble metals. The source of Os can be linked to one intrusive event, multiple intrusive events, and the sedimentary rocks into which the magmas intersect. On a regional scale, in the Andean Cordillera, Re-Os isotopes reveal a correspondence between the initial ¹⁸⁷Os/¹⁸⁸Os ratios from sulfides of the ore deposit and amount of copper present in porphyry copper deposits. Eleven deposits sampled from different crustal blocks and different mineralization ages form a trend in which deposits that contain large amounts of copper have less radiogenic ¹⁸⁷Os/¹⁸⁸Os initial ratios, suggesting a greater mantle component. Smaller deposits have more radiogenic Os suggesting greater crustal components. On a global scale, the significant observation for the deposits that form in arc environments is that all of the calculated initial ¹⁸⁷Os/¹⁸⁸Os ratios are greater then the chondritic mantle. This indicates that there must be a crustal source (recycled in a metasomatized mantle, lower/upper continental crust, or subducting slab) that contributes Os and by inference other metals present in porphyry coppers. Another feature of the data set is that gold-bearing systems in arc environments associated with porphyry intrusions have crustal signatures (0.5-2.5) regardless of the style of the related ore body. For instance, the low/high sulfidation epithermal gold systems in Bucaramanga Columbia (¹⁸⁷Os/¹⁸⁸Os initial 1.2), the massive sulfide ores of the Grasberg, Irian Jaya (¹⁸⁷Os/¹⁸⁸Os initial 0.7--1.2), and Cu-Au skarns in the Hedley District, British Columbia (¹⁸⁷Os/¹⁸⁸Os initial 1.7-2.5) all have large crustal components.

The unique mineral and chemical composition of enstatite chondrites has been difficult to explain. Contrary to the conclusions of other workers, there is no evidence for a significant chemical difference between EH and EL chondrites, except in bulk S. Analyzed splits of the enstatite chondrites vary widely in major element composition, largely as a result of analyzing inadequate sample sizes. The phases in enstatite chondrites most likely to reflect conditions in the solar nebula include enstatite, forsterite, diopside, silica polymorph, albite, metal, troilite, oldhamite, niningerite, schreibersite, minerals A and B, caswellsilverite, and djerfisherite. There is evidence that the enstatite chondrites did not form in complete equilibrium. The Na-Cr sulfides known as minerals A and B contain oxygen, but contain little hydrogen, indicating that these sulfides did not necessarily form under hydrous conditions. The mineralogy and modal composition of type 3 enstatite chondrites are best matched by thermodynamic models that involve two chemical fractionations from a gas of solar or cosmic composition. The first fractionation entailed the removal from the enstatite chondrite formation location of solids in equilibrium with a solar composition gas at &sim;1270K. The second fractionation involved the subsequent removal of water vapor from the enstatite chondrite formation location. These fractionations can be understood if the enstatite chondrites formed in a water-depleted gas from which solids at 1270K had been previously removed, sunward of a water condensation front in the solar nebula. Enstatite chondrites largely equilibrated in the nebula at ∼925K, although sulfidation of metal occurred to temperatures as low as ∼675K.

Detailed mapping of the Sierrita-Esperanza orebody has led to a revised interpretation of the Paleocene intrusive units, and a structural model for the emplacement of those units. A left-lateral wrench fault, expressed through Riedel shears, arranged in a flower structure geometry, is proposed to localize the porphyry intrusions, and control mineralization trends. Miocene extensional deformation has not tilted the orebody more than 10°. The San Xavier Thrust is interpreted to be a low angle detachment, upon which the upper plate has moved to the north-northwest approximately 10 km. When the San Xavier "Thrust" ceased movement, a set of low angle normal faults, striking nearly north-south dissected the Sierrita Range. Molybdenite dating, using the Re-Os system, shows that mineralization of the orebody began at 63.5 Ma (±300 ka, 2σ), and ended sometime after 60.0 Ma (±300 ka, 2σ). An isochron of chalcopyrite and pyrite samples with an age of 59.7 Ma (±17.1 Ma, 2σ), and an initial 187Os/188Os of.152 (±0.33, 2σ) suggests that the lower crust beneath the Sierrita Range has a low 187Os/188Os ratio.

Cordierite-orthoamphibole rocks associated with contrasting polymetallic mineralization occur in a wide range of metamorphic conditions in the Atik Lake, Manitoba, and Nemiscau and Montauban, Quebec, metamorphic terranes. / Lithogeochemistry and geothermobarometry investigations using whole rock major, trace and rare earth elements and mineral chemistries have established the chemical and physical changes that occurred during hydrothermal alteration and subsequent regional metamorphism. The Log(Ratio) method, utilizing ratio profiles of the immobile elements $ rm Al sb2O sb3$, TiO$ sb2$, Zr, Nb, Y V and Cr, was designed to identify the protoliths of the metamorphosed altered rocks. / Detailed geothermobarometry and textural evidence indicate that cordierite-orthoamphibole rocks in the three terranes formed at peak P-T conditions of ${ sim}2.5$ kbar and ${ sim}55 sp circ$C at Atik Lake (andalusite zone), ${ sim}4.5$ kbar ${ sim}650 sp circ$C at Nemiscau (sillimanite zone), and ${ sim}6.5$ kbar and ${ sim}620 sp circ$C at Montauban (kyanite-sillimanite zone). Cordierite-orthoamphibole rocks in the terranes exhibit a range of bulk chemical compositions with the most Fe-rich occurring in the Atik Lake belt, and the most Mg-rich in the Montauban mine sequence. Mineral assemblages in these rocks were in equilibrium with contrasting sulfide-oxide $ pm$ arsenide fO$ sb2$-fS$ sb2$ buffers: pyrrhotite-ilmenite $ pm$ arsenopyrite $ pm$ lollingite at Atik Lake, pyrrhotite-pyrite-magnetite at Nemiscau, and pyrrhotite-rutile-sphalerite at Montauban. / The partitioning of Fe-Mg between cordierite and anthophyllite was studied for use as a geobarometer. The partitioning coefficient ($ rm K sbsp{D}{Fe-Mg}$) is slightly temperature dependant and strongly pressure dependent. This new geobarometer was empirically calibrated using mineral chemistry from the three terranes and from the literature. / High $ rm X sb{H sb2O}$ of the metamorphic fluid is required so that the X$ sb{ rm Fe}$ values of the anthophyllite will be in equilibrium under peak P-T conditions with the appropriate sulfide-oxide $ pm$ arsenide assemblage. Metamorphic dehydration of chlorite-rich hydrothermally altered rocks in the three terranes is compatible with the generation of such water-rich fluids during the crystallization of the cordierite-orthoamphibole rocks.