ALBERT

Description: Porphyry Cu (±Mo ±Au) and epithermal Au-Ag deposits are major sources of mined metals and are commonly formed by magmatic-hydrothermal fluids derived from hydrous magmas in Phanerozoic convergent margin settings. The igneous rock assemblages associated with porphyry mineral deposits are common in modern convergent margin settings, but while many have produced acidic magmatic fluids, very few, past or present, have produced sufficient metal, chlorine, and sulfur enrichments necessary to engender an ore deposit. The reasons for this remain uncertain. We report SHRIMP-RG ion microprobe analyses of hafnium, titanium and rare earth element (REE) abundances in zircon, a nearly ubiquitous and robust trace mineral in crustal magmas. Comparison of the compositions of zircons in ore-forming and barren granitic plutons indicate that ore-forming granites crystallized at relatively low temperature and have relatively small negative europium anomalies (mostly Eu N /Eu N * ≥0.4). We interpret these small zircon europium anomalies to indicate oxidizing magmatic conditions and hypothesize that in many cases this reflects oxidation due to SO 2 degassing from magmas with a relatively low Fe/S ratio. Oxidation of europium and iron in the melt is produced by reduction of magmatic sulfate (S 6+ ) to SO 2 (S 4+ ) upon degassing. This interpretation reinforces the important role of oxidized sulfur-rich fluids in porphyry and epithermal mineral deposit formation. Zircon compositions thus may be used to identify ancient magmas that released significant amounts of SO 2 -rich gases, and regional surveys of zircon composition are potentially a valuable tool for mineral exploration.

Description: The Blackbird district, east-central Idaho, contains the largest known Co reserves in the United States. The origin of strata-hosted Co-Cu ± Au mineralization at Blackbird has been a matter of controversy for decades. In order to differentiate among possible genetic models for the deposits, including various combinations of volcanic, sedimentary, magmatic, and metamorphic processes, we used U-Pb geochronology of xenotime, monazite, and zircon to establish time constraints for ore formation. New age data reported here were obtained using sensitive high resolution ion microprobe (SHRIMP) microanalysis of (1) detrital zircons from a sample of Mesoproterozoic siliciclastic metasedimentary country rock in the Blackbird district, (2) igneous zircons from Mesoproterozoic intrusions, and (3) xenotime and monazite from the Merle and Sunshine prospects at Blackbird. Detrital zircon from metasandstone of the biotite phyllite-schist unit has ages mostly in the range of 1900 to 1600 Ma, plus a few Neoarchean and Paleoproterozoic grains. Age data for the six youngest grains form a coherent group at 1409 ± 10 Ma, regarded as the maximum age of deposition of metasedimentary country rocks of the central structural domain. Igneous zircons from nine samples of megacrystic granite, granite augen gneiss, and granodiorite augen gneiss that crop out north and east of the Blackbird district yield ages between 1383 ± 4 and 1359 ± 7 Ma. Emplacement of the Big Deer Creek megacrystic granite (1377 ± 4 Ma), structurally juxtaposed with host rocks in the Late Cretaceous ca. 5 km north of Blackbird, may have been involved in initial deposition of rare earth elements (REE) minerals and, possibly, sulfides. In situ SHRIMP ages of xenotime and monazite in Co-rich samples from the Merle and Sunshine prospects, plus backscattered electron imagery and SHRIMP analyses of trace elements, indicate a complex sequence of Mesoproterozoic and Cretaceous events. On the basis of textural relationships observed in thin section, xeno-time and cobaltite formed during multiple episodes. The oldest age for xenotime (1370 ± 4 Ma), determined on oscillatory-zoned cores, may date the time of initial cobaltite formation, and provides a minimum age for the host metasedimentary rocks. Additional Proterozoic xenotime growth events occurred at 1315 to 1270 Ma and ca. 1050 Ma. Other xenotime grains and rims grew in conjunction with cobaltite during Cretaceous metamorphism. However, ages of these growth episodes cannot be precisely determined due to matrix effects on 206 Pb/ 238 U data for xenotime. Monazite, some of which encloses cobaltite, uniformly has Cretaceous ages that mainly are 110 ± 3 and 92 ± 5 Ma. These data indicate that xenotime, monazite, and cobaltite were extensively mobilized and precipitated during Middle to Late Cretaceous metamorphic events.

Description: The Paleocene to Eocene southern Peru porphyry belt contains three significant porphyry Cu-Mo deposits at Cuajone, Quellaveco, and Toquepala. Ten new zircon U-Pb Sensitive High Resolution Ion Microprobe-Reverse Geometry (SHRIMP-RG) ages for Cuajone and Toquepala, together with published ages for Quellaveco, establish a magmatic history characterized by episodic events. Punctuated magmatism at Cuajone is distributed over approximately 13 m.y., at Toquepala over 8 m.y., and at Quellaveco over 6 m.y. The ages of the porphyry intrusions hosting or associated with the introduction of Cu and Mo at the three deposits show remarkable similarity, with emplacement beginning and ending at approximately 56.5 to 53.0 Ma at Cuajone, 57.0 to 54.0 Ma at Toquepala, and at 58.4 to 54.3 Ma at Quellaveco. Field relations coupled with the U-Pb ages for synmineral intrusions suggest very similar timing of the cupriferous hydrothermal systems, with the youngest pyritiferous and Cu-poor hydrothermal systems being associated with porphyry intrusions as much as 2 m.y. younger than significant Cu introduction. Overall, the porphyry Cu-Mo intrusive complexes represent the youngest magmatic complexes formed during the Late Cretaceous and early Tertiary arc, having formed prior to eastward migration of the magmatic locus.

Description: Uranium-lead ages and trace element compositions of zircon from a series of shallow porphyry intrusions document the temporal, chemical, and thermal magmatic evolution of magmatic-hydrothermal porphyry Cu (Mo-Au) ores in the El Salvador district, Chile. Zircons ( n = 240) from 15 Eocene age diorite, granodiorite, and granite porphyry intrusions were analyzed by SHRIMP-RG ion microprobe. The weighted means of 207 Pb-corrected 206 Pb/ 238 U zircon ages span 3 m.y. from about 44 to 41 Ma, with peak magmatic flux at 44 to 43 Ma. The granodiorite porphyries at the Turquoise Gulch copper deposit record waning stages of magmatism at 42.5 to 42.0 Ma and were followed by postmineral latite dikes at about 41.6 Ma. Porphyry copper ores formed contemporaneously with porphyry intrusion centers that progressed temporally from north to south, from the small deposits at Cerro Pelado (~44.2 Ma), Old Camp (~43.6 Ma), and at M Gulch-Copper Hill (~43.5–43.1 Ma) to the main ore deposit at Turquoise Gulch (~42 Ma). The Eocene porphyry intrusions contain a few Mesozoic ( n = 9) inherited zircons and numerous ( n ≥19) antecrystic zircons about 1 to 2 m.y. older than the host intrusion that provide evidence of extensive Eocene magmatic recycling. The Ti-in-zircon geothermometer provides estimates of 890° to 620°C for zircon crystallization and records both core to rim cooling and locally high-temperature rim overgrowths. Most zircon in ore-related K, L, and R porphyries yields near-solidus temperatures of 750° to 650°C and crystallized from compositionally diverse granodiorite porphyries that are a product of crystal fractionation of hornblende, apatite, and titanite with lesser crustal contamination and mixing with high-temperature deep-sourced mafic magma. During a 3-m.y. period, porphyry intrusions tapped an evolving granodioritic magma chamber that was periodically heated, locally remelted, and mixed with mafic magma during recharge events but cooled between recharge events to evolve ore fluids. Europium anomalies (chondrite-normalized Eu N /Eu N * ) in zircons become more pronounced with increased Hf content and cooling but display two distinct evolutionary paths: Eu N /Eu N * of early quartz porphyry evolves from 0.8 to 0.3, whereas the late synmineralization porphyries evolve from 0.8 to 0.65. The Eu N /Eu N * ratio of zircon reflects the Eu 3 +/Eu 2 + ratio of the melt, and therefore the granodiorite porphyries at Turquoise Gulch were the most strongly oxidized of the El Salvador magmas. The strongly oxidized trend porphyry magmas at Turquoise Gulch are apparently directly linked to magmatic degassing at ~700°C to produce large amounts of ore-forming copper, sulfur, and chlorine-enriched magmatic-hydrothermal aqueous fluids.

Description: New U-Pb zircon SHRIMP geochronology confirms that the Coles Hill uranium deposit in Pittsylvania County, Virginia, is hosted within the Late Ordovician to Silurian Martinsville Intrusive Complex. The meta-igneous host rocks at Coles Hill consist of two units of the Martinsville Intrusive Complex: the felsic Leatherwood Granite and the mafic Rich Acres Formation. Two samples of unmineralized Leatherwood Granite orthogneiss yield 206 Pb/ 238 U ages between 444.5 ± 2.5 and 447.5 ± 1.9 Ma. A third sample of unmineralized Leatherwood Granite orthogneiss shows a wider range in 206 Pb/ 238 U ages, possibly due to Pb loss, and a 206 Pb/ 207 Pb age of 452 ± 18 Ma. Unmineralized Rich Acres Formation amphibolite that cuts the Leatherwood gives a mean 206 Pb/ 238 U age (426.2 ± 7.0 Ma), slightly younger than the Leatherwood age. Samples of mineralized orthogneiss and mineralized amphibolite give similar 206 Pb/ 207 Pb ages of 419 ± 19 and 426 ± 21 Ma, respectively. A biotite gneiss unit that underlies the mineralized zone yields a 206 Pb/ 207 Pb age of 415 ± 21 Ma, indicating that it is part of the Martinsville Intrusive Complex and not a member of the early Cambrian Fork Mountain Schist, as has been previously reported. A genetic model for the Coles Hill uranium deposit has not yet been developed, although age constraints indicate that mineralization is either late or postmagmatic, and this is consistent with the epigenetic, fracture-controlled nature of the mineralization. Results obtained here do not preclude either the igneous host rocks (or similar rocks at depth) or the sedimentary units in the adjacent Triassic basin as possible sources for the uranium.