Igneous Quartz Research Articles

Oxygen and hydrogen isotope studies of a Precambrian granite-rhyolite terrane, St. Francois Mountains, southeastern Missouri

Isotopic analyses were made on whole-rock samples and minerals from 25 granites, 21 rhyolites, and 10 basaltic dikes and sills from the l,500-m.y.-old St. Francois Mountains terrane. The δD of chlorite is relatively uniform at −44 to −65, but the whole-rock and feldspar δ18O values systematically increase from +7 in the northeast to +14 in the southwest, correlating with an increasing intensity of “brick-red” alteration of the K-feldspars. Coexisting quartz and feldspar are typically not in isotopic equilibrium, except in the northeast. The coarser grained (>1 mm) quartz (δ18O = 8.8 to 10.6) is isotopically similar to “normal” igneous quartz, but the δ18O of the finer grained quartz correlates with the whole-rock δ18O pattern. Although the St. Francois terrane is geologically similar to many low-18O Tertiary volcanic-plutonic complexes that have interacted with meteoric-hydrothermal fluids at high temperatures, only a single locality (Stono Mountain) contains low-18O quartz and feldspar (δ18O quartz = 4.6 to 5.4). Even at this locality, the feldspars were later enriched in 18O by the same alteration event that affected the rest of the region. This second stage of hydrothermal activity apparently involved aqueous fluids having δ18O ≈ 0 to −6 and δD = 0 to −25. The temperature of alteration may have been as low as 50° to 100°C in the southwest (upper part of the volcanic section). This event affected some of the basaltic dikes and sills, but others were not altered and must have been intruded afterward. Rb-Sr and K-Ar age data suggest that the regional alteration occurred as late as 1,100 to 1,200 m.y. ago, long after the age of primary igneous activity. The hydrothermal fluids apparently originated as meteoric surface waters, indicating that during late Precambrian time such waters were isotopically similar to present-day meteoric waters in warm climates; this interpretation implies that the late Precambrian oceans must have been isotopically similar to present-day ocean.

Hydrogen and oxygen isotope ratios in minerals from porphyry copper deposits

The O 18 /O 16 , D/H and C 13 /C 12 values, where applicable, were measured on quartz (40 samples), K-feldspar (12), biotite (16), sericite (23), and calcite (7) from potassic, sericitic and argillic alteration assemblages and from fresh igneous rocks and veins in 9 North American porphyry copper and molybdenum deposits, and in 4 other hydrothermal mineral deposits. The porphyry copper deposit at Santa Rita, New Mexico was sampled in detail.Both sericites and biotites appear to preserve their O 18 /O 16 and D/H values, as these isotope ratios are not affected by later low-temperature exchange with local ground waters. A systematic correlation is, however, observed between the D/H ratios of sericites (and clays) from most Tertiary porphyry deposits and those of meteoric ground waters; this requires the presence of a significant meteoric water component in the hydrothermal fluids involved in sericitization (and argillization). In contrast to the sericites, the very restricted range of D/H ratios of biotites from Bingham, Ely and Santa Rita (--74 + or - 10ppm) indicates the probable dominance of magmatic waters in the fluids associated with biotite alteration. The oxygen isotope data for the quartz-K-feldspar-biotite-chalcopyrite assemblages also support a high temperature of formation in the presence of waters with delta O 18 -values in the magmatic water range. However, at the Butte deposit meteoric waters were apparently associated with the early biotitization.Many problems are associated with the application of isotopic geothermometers to the alteration assemblages because of the general presence of: (1) retrograde exchange, especially of the K-feldspars, (2) isotopically heterogeneous minerals, such as quartz of different origins, that are intimately intergrown and thus difficult to separate physically, and (3) the common lack of independent criteria for recognizing isotopic equilibrium. In the quartz-sericite assemblage the delta O 18 of the igneous quartz is preserved and is different from that of quartz precipitated during the sericitization processes.The Santa Rita isotopic data are discussed in terms of two simplified end-member models to evaluate the importance of temperature variations relative to variations in the O 18 /O 16 ratio of the hydrothermal fluids; both types of variations are probable. Calculated temperature ranges for the combined models are 580 degrees to 390 degrees C for potassic alteration (note that these orthoclases are very K-rich, Or (sub 90-96) ), and 390 degrees to 285 degrees C for sericite alteration, with delta O 18 -water ranging from 7.3 to 3.9 per mil. Similar temperatures for sericite formation were obtained independently by direct isotopic analysis of some quartz-sericite assemblages, utilizing the quartz-muscovite O 18 /O 16 geothermometer. C 13 /C 12 values of hydrothermal vein carbonates in the porphyry copper deposits (delta = -- 2.6 to --5.9) tend to be a little heavier than primary igneous calcites from carbonatites (which typically have delta = -- 5 to --8ppm). The similarities between the isotopic variations in the calculated hydrothermal waters associated with sericitization and argillization and those of saline formation waters of North America suggest that heated Na-Ca-Cl brines originally present in the neighboring sedimentary and volcanic sections may be an important component of the hydrothermal fluids. The quantities of meteoric ground water that have interacted with the hot igneous stocks of the porphyry ore deposits, though very significant, are much smaller than the amounts involved in the vein systems at Butte and in many epizonal Tertiary igneous intrusions emplaced into permeable volcanic country rocks.

Oxygen isotope studies on the origin of tektites

O^(18)/O^(16) ratios of thirteen tektites from Czechoslovakia, Libya, Texas, Indochina, Philippine Islands, Australia, Java, and Peru have been determined by the fluorine-extraction technique. All but one of these lie in relatively restricted range of O^(18)/O^(16) ratio, with δ = 9.6 to 10.4 per mil relative to the ocean-water standard. Tektites are therefore 0.5 to 1.5 per mil heavier than average granitic igneous rocks or about the same as average igneous quartz. The sample from Macusan, Peru (δ = 12.0), is anomalous and is probably not a true tektite. Six analyzed shales are isotopically heavier than tektites and show a much wider range in values (δ = 14.2, 14.8, 15.3, 15.5, 17.6, 18.2). Several detrital sedimentary rocks previously analyzed by Silverman and the present authors have δ values of 10.2 to 15.5; analyzed metasedimentary rocks have δ values of 12.1 to 15.7 per mil. Therefore, tektites are not sedimentary or metasedimentary material fused by lightning or by impact of an astronomical body with the earth. Chemically, tektites are unlike terrestrial igneous rocks, and their oxygen isotopic composition is unlike that of sedimentary or metasedimentary rocks. They therefore may be extraterrestrial objects.

Inclusions in the Quartz of Igneous and Metamorphic Rocks

ABSTRACT The in the quartz from 35 igneous rocks and 16 metamorphic rocks were studied in detail in thin sections. They were grouped into regular, acicular, globular, and irregular inclusions following a modified scheme proposed by Mackie in 1896. In this study, regular characterize the quartz of schists, irregular predominate in igneous quartz, and globular and acicular are most abundant in igneous quartz although they also occur in metamorphic quartz. Spectrographic analyses were rim on 35 samples of petrogenic quartz. Even clear colorless quartz is rich in elements other than silicon and oxygen. Probably ordinary rock-forming minerals in sub-microsopically fine particles are abundantly included in most petrogenic quartz.