Jeffrey N. Rubin

The mobility of zirconium and other immobile' elements during hydrothermal alteration

Abstract Development of zircon and other Zr phases in hydrothermal deposits indicates that Zr can be highly mobile in these systems. Mobility is most common in, but not restricted to, F-rich hydrothermal systems related to alkalic, F-rich igneous suites; these suites can range from peralkaline through metaluminous to peraluminous. A few examples are neither alkalic nor F rich. Three locations in the Trans-Pecos Magmatic Province, Texas, U.S.A., demonstrate this hydrothermal Zr mobility. All three igneous systems are alkalic and F rich but vary in alkali/Al ratios. Peralkaline rhyolites and trachytes in the Christmas Mountains contain as much as 2100 ppm Zr, mostly in aegirine or arfvedsonite; zircon is rare or absent. Fluorspar replacement deposits in limestone at contacts with the rhyolites contain as much as 38,000 ppm Zr, occurring as small, disseminated zircons. The deposits also are enriched in a variety of incompatible elements, including Be, rare-earth elements (REE), Y, Nb, Mo, Hf, Pb, Th and U. The Sierra Blanca intrusions, a series of mildly peraluminous, F-rich rhyolite laccoliths, contain as much as 1000 ppm Zr, mostly as zircon. Hydrothermal zircon occurs as overgrowths on magmatic grains, as veinlets connected to overgrowths, and in fluorspar replacement bodies in adjacent limestone. The highest Zr concentrations in fluorspar are ∼200 ppm. Metaluminous quartz monzonite from the Infiernito caldera contains 400–600 ppm Zr, mostly as zircon. Euhedral zircon in quartz-fluorite veins in the quartz monzonite indicates mobility of Zr. Zirconium concentrations in the veins are unknown, but the paucity of zircon suggests little Zr enrichment relative to the host. Zircon and, more rarely, zirconolite, occur in skarn in the Ertsberg District of Irian Jaya, Indonesia. Unlike in Texas, related igneous rocks are metaluminous, and the hydrothermal system was F poor. Worldwide, hydrothermal zircon, other Zr phases, and Ti- and Al-bearing phases occur in skarn, epithermal precious metal veins, volcanogenic massive-sulfide deposits and mylonites. We propose that differences in Zr mineralogy of igneous source rocks is an important factor in determining the availability of Zr to hydrothermal fluids. Although Zr concentrations in the Sierra Blanca and Christmas Mountains rhyolites are similar, Zr enrichment in fluorspar was much greater in the Christmas Mountains. We suggest that hydrothermal solutions could easily break down aegirine and arfvedsonite to release Zr, but that zircon was only moderately attacked. Therefore, far more Zr was available for transport and subsequent deposition in the Christmas Mountains than at Sierra Blanca. Availability of other trace elements probably is also governed by their mineral host. Although Zr mobility is most common in F-rich hydrothermal systems related to alkalic and F-rich igneous systems, mobility at Ertsberg may have been promoted by sulfate complexing.

Widespread, lavalike silicic volcanic rocks of Trans-Pecos Texas

Several silicic units of the Trans-Pecos volcanic field have outcrop and thin-section scale features of lava flows but areal extents and aspect ratios of ignimbrites. These voluminous rocks (up to hundreds of cubic kilometres per unit) are quartz trachytes to low-silica rhyolites (68% to 72%SiO 2 ). Lava flow features include flow banding and folding, elongated vesicles, and autobreccias and vitrophyres at the base and top of units. Pyroclastic flow features include sheetlike geometry, lateral extents up to 70 km, aspect ratios as low as 1:700, and areal extents up to 3000 km 2 . A few of these units are clearly rheomorphic ignimbrites, but others show no unambiguous evidence of a primary pyroclastic origin. Although no adequate explanation currently exists for the origin of the latter, we evaluate two end-member hypotheses: (1) they are ignimbrites in which extreme rheomorphism has obliterated primary internal features, and (2) they are highly viscous lavas with unusually high heat retention or effusion rates that allowed them to spread over great areas. Either origin requires a rock type and eruptive mechanism not commonly recognized.

Case study of an extensive silicic lava: the Bracks Rhyolite, Trans-Pecos Texas

Abstract Field, petrographic, and chemical data indicate that the Bracks Rhyolite of western Trans-Pecos Texas is a single lava flow that traveled as much as 35 km from a source in the north-central part of its outcrop. With a minimum original extent of 1000 km2 and volume of 75 km3, the Bracks is far more extensive and voluminous than typical silicic lavas. The Bracks crops out in a 55 × 16 km north-trending belt. It thins radially from a maximum thickness of about 120 m. However, except at flow margins where it thins abruptly, it is everywhere at least 25 m thick. Clusters of vitrophyric domes that intrude and are otherwise identical to the dominantly crystalline lava may represent diapiric rise of hotter, less dense, lower parts of the flow. Some domes may overlie fissure vents that fed the flow. The source area is in the north-central part of the flow, on the basis of thickness and flow patterns and distribution of vitrophyric domes. The Bracks is a slightly peralkaline low-silica rhyolite or trachydacite (68–69% SiO2). Whole-rock analyses for major oxides and rare earth elements and microprobe analyses of alkali feldspar, Fe-rich clinopyroxene, fayalite, and magnetite phenocrysts show that the Bracks, including vitrophyric domes, is chemically and mineralogically homogeneous, both laterally and vertically. Evidence that the Bracks was emplaced as a lava flow includes autobreccia at the base and top, steep flow fronts, abundant flow bands and folds, elongate vesicles, trachytic texture, and groundmass textures that indicate crystallization from a liquid. Basal breccia that occurs throughout the extent of the flow, is uniformly coarse, and varies in thickness laterally can only form from a lava that flowed over its entire extent. Evidence of a pyroclastic origin, such as shards, pumice, lithics, or welding zonation, is absent. The low aspect ratio of the Bracks (approximately 1 : 500), although in a range typical of many ash-flow tuffs, is considerably higher than that of unequivocal tuffs in Texas, which have comparable outcrop areas but are much thinner. The great extent and sheet-like geometry of the Bracks probably reflects high eruption temperature (≥ 900°C), low volatile content, moderately low viscosity, rapid eruption, and slow cooling. Unusually low viscosity, on the order of basaltic lavas, was not a factor because, despite peralkalinity and high temperature, the Bracks probably had a low volatile content.

Skarn CuAu orebodies of the Gunung Bijih (Ertsberg) district, Irian Jaya, Indonesia

Abstract The major CuAu skarn deposits of the Gunung Bijih (Ertsberg) district in central Irian Jaya are products of hydrothermal systems that developed in association with Pliocene magma emplacement in an active continental margin. The CuAu skarn orebodies occur within a Cretaceous to Tertiary sedimentary sequence that was deformed as the northern Australian continental margin entered a north-dipping subduction zone at ∼ 12 Ma. The intermediate-composition intrusions consist of fine-grained porphyritic stocks, dikes, and sills that have K-Ar ages ranging from 2.7 to 4.4 Ma. Most intrusions are slightly potassic, but these data could be affected by alteration. The skarn orebodies in the Ertsberg district are hosted in deformed lower Tertiary New Guinea Group carbonate strata along the periphery of the Pliocene Ertsberg intrusion. Major skarn orebodies include the Ertsberg (GB), the Ertsberg East (GBT) complex, including the GBT, the Intermediate Ore Zone (IOZ) and the Deep Ore Zone (DOZ), and the Dom. Chalcopyrite is the dominant ore mineral in the GB and Dom orebodies, whereas bornite dominates in the GBT complex. Native Au occurs within bornite and chalcopyrite in GB and GBT ores. The district calc-silicate alteration assemblages are characterized by high-temperature skarn minerals, including forsterite, monticellite, and minor melilite. Diopsidic clinopyroxene is common, particularly in GBT. Anhydrite and phlogopite are abundant in the GBT complex, and the anhydrite:calcite ratio increases with depth from GBT to DOZ where anhydrite is ubiquitous and calcite rare. At least three types of garnets have been identified at the Dom and show a progressive increase in ferric iron content. Garnet decreases with depth in the GBT complex. Talc, serpentine, tremolite-actinolite, and chlorite are common retrograde minerals. Copper sulfide mineralization is texturally associated with early retrograde alteration. Differences among the skarn orebodies are related in part to variable protolith composition that affected skarn development within different stratigraphic positions. Distinctive fossil replacement textures preserved within skarn indicate that the Oligocene-Miocene Ainod Formation is the most likely protolith for the GB and Dom orebodies. The GBT and upper IOZ orebodies probably are hosted by the Eocene Faumai Formation. The DOZ and lower IOZ orebodies, dominated by magnesian skarn alteration, appear to be developed in a dolomitic unit within the lower New Guinea Limestone Group, which probably is equivalent to the Paleocene Waripi Formation.

Stratigraphic Inheritance Controls of Skarn-hosted Metal Concentrations: Ore controls for Ertsberg-Grasberg District Cu-Au skarns, Papua, Indonesia

orebodies occur as stratabound concentrations in specific stratigraphic units within thick stratigraphic successions. Mineralization systems may range from sedimentary basinal brines to magmatic hydrothermal carbonate replacement and skarn systems. These restricted ore-hosting intervals typically are related to the susceptibility of certain units to postdepositional alteration and porosity enhancement by fluid systems ranging from near-surface to burial diagenesis that commonly are inherited from their depositional framework. Common sedimentary basin examples include stratabound breccia or other porosity zones resulting from differential removal of soluble components that provided permeable zones for subsequent fluid migration, including MVT-mineralizing sedimentary brines as in the Tennessee and Pine Point districts (e.g., Kyle, 1983). In more dynamic tectonic settings, reactive sedimentary successions adjacent to plutons commonly are host to skarn-type metal deposits, but seldom are these relationships pursued from the context of detailed analysis of unaltered strata that can be correlated with the altered and mineralized intervals. The Ertsberg-Grasberg district in Papua, Indonesia, hosts two giant porphyry and skarnhosted Cu-Au systems that formed between 3.3 and 2.5 Ma in the Central Range that forms the Highlands of western New Guinea. These Cu-Au systems are associated with two dioritic intrusive centers, the Grasberg Igneous Complex and the Ertsberg Intrusive Complex, that were emplaced into a deformed sedimentary sequence of Cenozoic carbonate and late Mesozoic siliciclastic strata. The lithologic variations allow the assessment of the development of alteration assemblages for specific rock types and their relationship to economic mineral concentration (Fig. 1). The Ertsberg-related system contains 3.6 Gt grading 0.60% Cu and 0.44 ppm Au (Leys et al., 2012) in four skarn deposits, the Ertsberg, the Ertsberg East Skarn System, the Dom, and the Big Gossan (Mertig et al., 1994). These deposits represent hypogene copper sulfide concentrations with high complementary gold values. The Ertsberg East Skarn System (EESS) is one of the world’s largest skarn-hosted Cu-Au orebodies; economic mineralization is vertically continuous for more than 1,500 m in steeply dipping strata along the flank of the Ertsberg Intrusive Complex. EESS ores are hosted by mixed assemblages of lower Paleogene siliciclastic and dolomitic carbonate strata that have been altered to Mg-rich skarn assemblages. Prograde skarn assemblages in the dolomitic lower Waripi formation and the limestone member of the Ekmai formation are dominated by forsterite and diopside (Rubin & Kyle, 1998). The alteration assemblage of calcareous strata in the younger Faumai and Kais formations is dominated by monticellite and diopside. Unaltered strata from a stratigraphic interval equivalent to the EESS skarn-hosted ores have been characterized with regard to their petrographic features, major element compositions, and petrophysical properties (Gandler, 2006; Gandler & Kyle, 2008). These stratigraphic units are interpreted to be responsible for the varied skarn lithologies within the EESS. The dominant prograde skarn assemblages are controlled by protolith composition, notably the relative abundance of quartz, dolomite, and calcite. Models based on isochemical metamorphism of mixed assemblages of quartz and dolomite suggest that the formation of forsterite-diopside-dominant skarn assemblages resulted in the greatest amount of porosity increase, which served to host Cu-Au concentrations. (Gandler, 2006; Gandler & Kyle, 2008). Additional variables include pre-alteration porosity and permeability, but those are generally low. Magnetite is a component of prograde alteration, preferentially replacing dolomitic strata and commonly hosting high grade Cu-Au ore. Thus, EESS Cu-Au concentrations were locally controlled by J. Richard Kyle, Laurel Gandler, Heidi Mertig, Jeffrey Rubin, and Matthew Ledvina, 2014. Stratigraphic Inheritance Controls of Skarnhosted Metal Concentrations:Ore controls for Ertsberg-Grasberg District Cu-Au skarns, Papua, Indonesia. Acta Geologica Sinica (English Edition), 88(supp. 2): 529-531.