Thesis (M.A.)--University of Texas at Austin, 1981. / Vita. Includes bibliographical references (leaves 76-80).

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.

The thermodynamic and the kinetics of the Fe-Mg order-disorder process in orthopyroxene were studied by means of thermal annealing experiments at fix fO₂ condition, and single crystal X-raydiffraction to determine the site occupancies. The behavior of Mn on the equilibrium fractionation of Fe-Mg was studied by a series of annealing experiments using a naturally occurring Mn-rich orthopyroxene (donpeacorite). It was shown that, although Mn and Fe preferentially order to the M2 site relative to Mg, Mn has a significantly stronger M2 site preference than Fe. This result implies that when computing the site fractionation for Fe-Mg in Fe-rich Mn-poor orthopyroxene, the small amount of Mn present in the structure should always be totally ordered in the M2 site. The kinetics of the order-disorder reaction was studied as a function of temperature, composition, and fO₂. The results are compatible with the theoretical predicted variation of the rate constant as (fO₂)¹/⁶. The temperature and compositional dependence of the disordering rate constant, K⁺, can be expressed as: ln K⁺(Ord) = - [(36420)/T(K)] min⁻¹ + 29.03 + 4162(XFe) where XFe is the Fe molar fraction of the sample. The thermodynamic and kinetics data of Fe-Mg order-disorder in orthopyroxene permit retirval of cooling rates natural orthopyroxene crystals from their quenched ordering states around the closure temperature (T(C)) of ordering. Thus, I have applied the data to Central Gneiss Complex, British Columbia, and to a diogenite-meteorite which is believed to have originated on Vesta, to constrain their cooling rates. For the Central Gneiss Complex the cooling rate was found to be ∼10-15°C/My at T(C) ∼290°C. This cooling rate is in excellent agreement with that constrained by geochronological data, and implies an exhumation velocity of ∼0.2mm/y. The thermal history inferred for the diogenite-meteorite GRO95555 suggests that the sample underwent very fast cooling at two different rates: a faster cooling at ∼400°C/year as retrieved from modeling of the compositional zoning the orthopyroxene-spinel couple, followed by a slower cooling at ∼5°C/1000y as obtained from the modeling of the observed quenched state in the orthopyroxene. The implication of these rapid but constraining cooling rates on the excavation and burial of the sample in its parent body has been discussed.

The study of jadeite-hedenbergite binary pyroxenes has profound implications for interpreting the P/T history of eclogites and, in particular, ultra high pressure metamorphic rocks. Because binary pyroxenes are extremely rare in nature, a synthesis method was developed, and various compositions along the jadeite-hedenbergite join were produced. Synthesis products were analyzed using an X-ray diffractometer and microprobe to verify purity and homogeneity. Synthesized binary pyroxene was then mixed with quartz and albite and subjected to eclogitic conditions in order to determine the equilibrium reaction Ab <=> Jd + Qtz as a function of pressure, temperature, and X(jd). Two methods of reaction have been developed; one employs a graphite sample cup, and the other requires the use of welded gold sample cups, which reduce the temperature of reaction and allow for better pyroxene growth. All experiments were performed in a 1/2" or 3/4" piston cylinder apparatus at pressures from 15-39 kbar and temperatures from 650-1300°C.

Nepheline syenite represents the highly differentiated product of fractional crystallization in the Poudrette quarry at Mont Saint-Hilaire, Quebec. It also records a significant subsolidus overprint due to circulation of post-magmatic fluids. Annite, the ferrous-iron-dominant member of the biotite series, represents up to 20% of the typical nepheline syenites. / Hand samples cover the complete range of freshness, from unaltered to hydrothermally modified rocks. Identification of minerals and estimation of their abundances and compositions were obtained with optical microscopy and electron-microprobe analysis. I also used a combined approach of X-ray diffraction and high-resolution transmission electron microscopy. / With respect to annite composition, Fe/(Fe + Mg) varies from 0.74 (porphyntic nepheline syenite) to 0.98 (poikilitic nepheline syenite). Concerning the habit of annite, the melt composition controls the order of crystallization, which then in turn controls whether the annite is bladed and skeletal or oikocrystic. The rock from the Poudrette dyke is a hypersolvus nepheline syenite in which the bladed annite was a liquidus phase; therefore, the order of crystallization was "normal" (mafic mineral first), whereas in poikilitic nepheline syenite, this order was reverse. The proportion of Ti decreases with Fe/(Fe + Mg), whereas that of Mn increases; F and Cl contents are very low in spite of their availability. / The application of n-alkylammonium cation exchange reveals the presence of planar structural defects as a result of dehydrogenation-type reactions. The annite records the results of oxidation during cooling, i.e., open-system behavior in this degassing pluton.

No description available.

The Strange Lake deposit is located on the Quebec-Labrador border. This large deposit of Zr, Y, REE, Nb and Be mineralization is hosted by a Late Proterozoic high-level stock that was intruded into high-grade graphitic gneisses. The intrusive rocks consist of leucocratic and melanocratic peralkaline granites. The occurrence of plastically deformed melanocratic xenoliths in the leucogranite and vice versa indicate that the two granite types were intruded coevally. / The stock is extensively altered, especially in the north. Early alteration is manifested by the replacement of arfvedsonite with aegirine. Later alteration involved an exchange of calcium for sodium. Zr, Ti, Y, REE, Nb, Be are concentrated in Ca-bearing minerals that, together with quartz, commonly pseudomorph Na-bearing minerals. Hematite typically coats the rare-metal enriched minerals. / Pseudomorph outlines are frequently delineated by trains of fluid inclusions. / A model is proposed in which the intrusion of a peralkaline granite to high crustal levels initiated a meteoric water-dominated hydrothermal system in the adjacent gneisses.

The Shasta Ag-Au deposit, located in the Toodoggone River area of north-central British Columbia, has many characteristics of the low sulphidation, adularia-sericite class of epithermal deposit. It consists of quartz-calcite stockworks and breccias, associated with potassic (K-feldspar + sericite) alteration of dacitic tuff host rocks. Ore grade mineralization occurs mainly in Bonanza-style breccias which host acanthite, electrum and native silver, associated with pyrite, sphalerite, chalcopyrite, galena, chlorite and hematite. However, Shasta lacks vein-hosted adularia, and the highest grades of ore are associated with calcite-rich breccias, which are features atypical of the adularia-sericite class. / Hydrothermal breccias, chalcedonic quartz and abundant vapour rich fluid inclusions are strong evidence for boiling of the vein fluid. Fluid inclusion evidence indicates that ore was deposited between 280$ sp circ$ and 225$ sp circ$C at a maximum depth of 750 m, from a relatively dilute hydrothermal fluid. Oxygen and hydrogen isotopic data suggest that the fluid was meteoric in origin. Initial log fO$ sb2$ - pH conditions were estimated to have been between $-$33 to $-$31, and 4.25 to 6, respectively, and to have increased during mineralization. / A model is proposed in which quartz, and subsequently calcite and silver-gold minerals, were deposited due to boiling of the fluid. The consequent decrease in temperature and increase in pH controlled deposition of quartz and calcite, respectively. Precipitation of silver and gold was caused by a combination of increased pH and oxygen fugacity, and a decrease in H$ sb2$S fugacity.

Apatite is an early crystallizing phase in Proterozoic potassic alkaline and shoshonitic magmas, and is used here to model and further the understanding of the source regions, and magmatic and hydrothermal processes of 1.08 Ga potassic plutons in the southwestern Grenville Province, Mont-Laurier area, Quebec. Apatite chemistry and $ sp{87}$Sr/$ sp{86}$Sr ratios were documented in the shoshonitic and potassic alkaline intrusive facies of the Kensington pluton, and in the potassic alkaline series of the Sainte-Veronique pluton; the two plutons are separated by 70 km. Potassic magmas in the belt were derived from separate, although similar mantle regions, while the shoshonitic series may have been derived from a distinct region, or modified by contamination. Assimilation is consistent with zoning patterns in apatite from the Kensington shoshonitic syenite. Inclusion mineralogy at Sainte-Veronique is evidence of saturation of an immiscible sulfide liquid, which may be an intrinsic characteristic of potassic alkaline magmas in the Grenville Province.

Thesis (Ph.D.) -- University of Adelaide, Dept. of Economic Geology, 1978. / 2 microfiche in end pocket.