Early Hydrothermal Stage Research Articles

Breccia- and Carbonate-Hosted Au-Cu-Ag Replacement Mineralization Associated with the Homestake Porphyry Intrusive Complex, New World District, Montana

The Miller Creek, Homestake, and Daisy Pass resources contain 7,406,260 metric tons (t) of mineralized rock averaging 7.44 to 11.97 g/t Au and 0.44 to 0.90 percent Cu. They are spatially associated with the 41 Ma Homestake intrusive complex within the New World district near Cooke City, Park County, in south-central Montana. The Homestake intrusive complex consists of numerous dacitic, porphyry intrusive rocks, and phreatomagmatic breccia bodies that have intruded Cambrian sedimentary rocks and Precambrian crystalline rocks. Diatreme breccia cuts earlier intrusion breccia and contains small zones rich in pulverized and altered rock-flour matrix and volumetrically more abundant clast-rich zones. Clasts within diatreme breccia include Cambrian sedimentary rocks, Precambrian crystalline rocks, and porphyritic intrusive rocks. Although moderate mixing and milling of clasts is present within diatreme breccia, relict Cambrian stratigraphy is commonly preserved. Alteration may be separated into three stages: initial contact metamorphism of sedimentary rocks related to the emplacement of the intrusive rocks and intrusion breccias, an early hydrothermal stage that occurred after emplacement of the intrusive rocks, and a late hydrothermal stage that is postdiatreme. Early hydrothermal alteration of sedimentary rocks and breccia is laterally zoned relative to the Homestake intrusive complex from narrow zones of proximal prograde skarn to more widespread distal calc-silicate hornfels. Calc-silicate hornfels consists of banded epidote, actinolite, chlorite, and orthoclase, lies stratigraphically above and below the mineralized replacement zones, occurs up to 3.3 km away from the intrusive complex, and is barren of Au-Cu mineralization. Intrusive rocks, intrusion breccia, and diatreme breccia are dominated by late, vertically zoned alteration assemblages that destroyed early alteration and consist of deep advanced argillic alteration (including hydrothermal breccia), intermediate-level intermediate argillic alteration, and distal upper-level propylitic alteration. Trace fine-grained epidote associated with early alteration is present locally in the mineralized Homestake replacement zones but is absent at Miller Creek. Replacement zones within the Homestake resource are hosted by diatreme and intrusion breccia containing abundant clasts of Cambrian limestone and are spatially associated with intermediate argillic alteration. Tabular subhorizontal replacement zones at Miller Creek and Daisy Pass are hosted by dolomitic limestone of the Cambrian Meagher Formation at the contact with the intrusive complex and are spatially associated with sills and dikes with argillic and propylitic alteration. Replacement zones associated with late alteration are dominated by magnetite + rutilated (i.e., rutile-bearing) quartz and were partially replaced by later pyrite, clear quartz, carbonate minerals, chlorite, clay, and chalcopyrite. Almost all magnetite in the mineralized replacement zones partially to completely replaced earlier acicular hematite and retains the distinct acicular habit. Native gold and electrum typically occur as micron-sized inclusions and fracture fillings in pyrite and, to a lesser extent, in chalcopyrite. The abundance of chlorite, carbonate, magnetite, and gold increase in the upper part of the hydrothermal system, whereas kaolinite, Cu/(Au × 1,000) and Ag/Au increase in the lower part. Homogenization temperatures of fluid inclusions in clear quartz associated with Au-Cu-Ag mineralization suggest a thermal zoning from the proximal Homestake resource (~369°C) to the peripheral Miller Creek resource (~344°C). A phreatomagmatic event and related structures created permeable zones and conduits for the hydrothermal fluids that selectively replaced nearby limestone-rich protoliths. The alteration mineralogy, geochemistry, fluid inclusion homogenization temperatures, and paragenesis of the replacement zones and breccia-hosted resources are similar, suggesting that they were related to the same hydrothermal system centered on the Homestake intrusive complex.

Hydrothermal alteration in the core of the Yaxcopoil‐1 borehole, Chicxulub impact structure, Mexico

Abstract Petrographic, electron microprobe, and Raman spectrometric analyses of Yaxcopoil‐1 core samples from the Chicxulub crater indicate that the impact generated a hydrothermal system. Relative textural and vein crosscutting relations and systematic distribution of alteration products reveal a progression of the hydrothermal event in space and time and provide constraints on the nature of the fluids. The earliest calcite, halite, and gaylussite suggest that the impactite sequence was initially permeated by a low temperature saline brine. Subsequent development of a higher temperature hydrothermal regime is indicated by thermal metamorphic diopside‐hedenbergite (Aeg3Fs18‐33En32‐11Wo47‐53) after primary augite and widespread Na‐K for Ca metasomatic alkali exchange in plagioclase. Hydrothermal sphene, apatite, magnetite + (bornite), as well as early calcite (combined 3 to 8 vol%) were introduced with metasomatic feldspar. A lower temperature regime characterized by smectite after probable primary glass, secondary chlorite, and other pre‐existing mafic minerals, as well as very abundant calcite veins and open‐space fillings, extensively overprinted the early hydrothermal stage. The composition of early and late hydrothermal minerals show that the solution was chlorine‐rich (Cl/F >10) and that its Fe/Mg ratio and oxidation state increased substantially (4 to 5 logfO2 units) as temperature decreased through time. The most altered zone in the impactite sequence occurs 30 m above the impact melt. The lack of mineralogical zoning about the impact melt and convective modeling constraints suggest that this unit was too thin at Yaxcopoil‐1 to provide the necessary heat to drive fluids and implies that the hydrothermal system resulted from the combined effects of a pre‐existing saline brine and heat that traveled to the Yaxcopoil‐1 site from adjacent areas where the melt sheet was thicker. Limonite after iron oxides is more common toward the top of the sequence and suggests that the impactite section was subjected to weathering before deposition of the Tertiary marine cover. In addition, scarce latest anatase stringers, chalcopyrite, and barite in vugs, francolite after apatite, and recrystallized halite are the likely products of limited post‐hydrothermal ambient‐temperature diagenesis, or ocean and/or meteoric water circulation.

Trace-element analysis of individual synthetic and natural fluid inclusions with synchrotron radiation XRF using Monte Carlo simulations for quantification

The trace element composition of individual fluid inclusions was investigated by using synchrotron radiation X-ray fluorescence for analysis and Monte Carlo simulations for quantification. To validate the input parameters of the Monte Carlo simulation code for the applied spectrometer, area scan measurements on synthetic standard reference glasses (NIST 612 and NIST 610) were performed. Results yielded an accuracy of generally better than 20 % for all elements with Z between 37 and 92 at a standard deviation of less than 7 %. The analytical procedure was then applied to synthetic inclusions in quartz: single point analysis yielded an accuracy between 10 and 30 % with a reproducibility of ± 20 % and a repeatability of ± 7 % for the elements Cu, Rb, Sn and Cs. Ten trace elements (Mn, Fe, Cu, Zn, As, Rb, Nb, Sn, Sb, Cs) with concentrations between 2 and 10500 ppm were determined in fluid inclusions of hydrothermal quartz hosts from the Ehrenfriedersdorf Complex, Germany. Inclusions of three evolutionary stages of the Ehrenfriedersdorf Complex were studied. Vapour-rich inclusions of the pegmatite stage have low concentrations of all elements with Z > 20. Inclusions of the hydrothermal stage record progressive enrichment and differentia- tion in trace elements. Fluids of the early hydrothermal stage are enriched in Sn, Cu and As. Subsequently, they evolve to Fe-, Mn-, Zn- and Cs-rich fluids.

The Origin of Chlorite-Tremolite-Carbonate Rocks Associated with the Thalanga Volcanic-Hosted Massive Sulfide Deposit, North Queensland, Australia

The Thalanga polymetallic massive sulfide deposit is hosted by felsic volcanics of the Cambro-Ordovician Mount Windsor subprovince in North Queensland. The sheetlike sulfide deposit consists of several semiconnected lenses, totaling 6.6 million metric tons (Mt), which lie at a single stratigraphic level between underlying subaqueously emplaced rhyolite and overlying dacitic to andesitic lavas, sills, and volcaniclastic rocks. The ores and host rocks have been deformed and metamorphosed to upper greenschist facies. Associated with the West Thalanga orebody are strata-bound lenses composed of chlorite, tremolite, dolomite, and calcite that previously have been interpreted as exhalites. They extend up to a few hundred meters along the mineralized horizon and locally also exist in the stratigraphic footwall around the fringes of the deposit. Rhyolitic volcanics beneath the deposit are extensively altered. A pervasive sericite ± chlorite ± pyrite zone that extends greater than 200 m into the footwall envelops semistratiform proximal quartz + pyrite ± chlorite stringer zones. Major and trace element geochemical data indicate that Zr, Ti, and Al remained essentially immobile in the footwall rocks despite intense hydrothermal alteration involving significant mass transfers of mobile elements. The chlorite + tremolite + carbonate assemblages associated with ore lenses have immobile element ratios identical to those of the altered rhyolites. Carbonate-rich assemblages retain some relict spheroidal and rhombic textures in dolomite, which suggest formation at an early hydrothermal stage in a matrix of chlorite or in open spaces. The chlorite + tremolite + carbonate rocks probably represent metamorphic equivalents of hydrothermal chlorite + quartz + dolomite + calcite (or Mg smectite + quartz + dolomite + calcite) assemblages. The formation of tremolite by metamorphic reaction of quartz and dolomite is consistent with regional metamorphic conditions. With allowance for metamorphic decarbonation, carbon and oxygen isotope data from dolomite and calcite are consistent with precipitation of primary dolomite at 170° to 250°C from seawater-dominated fluid. The chlorite + tremolite + carbonate rocks at Thalanga are consequently reinterpreted as altered rhyolitic volcaniclastics, containing variable proportions of hydrothermal carbonate. They probably formed in a zone of mixing of hydrothermal fluid and seawater, in a permeable substrate close to the paleosea floor. Siliceous ironstones that are present in distal parts of some Thalanga ore lenses have very low immobile element contents and are probably true exhalites.

Evolution of fluids responsible for iron skarn mineralisation: An example from the Vyhne-Klokoc deposit, Western Carpathians, Slovakia

Vyhne-Klokoc, the largest Fe-Skarnn deposit in the Western Carpathians, is related to the emplacement of a large granodiorite pluton in the central zone of a Neogene stratovolcano. Skarn mineralisation is developed in places where apophyses of the pluton intruded basement carbonates. Granodiorite in the apophyses grades into rocks of granitic composition, involving the replacement of mafic minerals and a concomitant decrease in Fe-content. Ca-magnetite exoskarns (not accompanied by endoskarns) developed in three paragenetic stages. Fluid inclusion (Fl) data for quartz in granodiorite suggest the existence of aqueous fluid immiscibility during the early hydrothermal stages. Three end-members of Fis were recognised, with a continuum between all three types. High salinity, liquid-rich, probably secondary Fls (29-68 wt % NaCl eq., Th 450 to 570'C, composed of NaCl+FeCl2+KCl) coexist with vapour-rich Fls with low but variable salt contents (+CO2). Probably late secondary Fls (1-25 wt % NaCl eq., composed mainly of NaCl+CaCl2, Th 188–283°C) form the other end-member type of Fls trapped in granodiorite quartz. Fls from skarn garnets show a large variation in salinity (4-23 wt % NaCl eq., composed of NaCl±FeCl2+CaCl2+ KCl+MgCl2) and Th (220–370°C), independent of the garnet types, probably reflecting variable amounts of magmatic fluids and low salinity meteoric waters. Fls in retrograde quartz, calcite and sphalerite show progressively more dilute (0-4 wt % NaCl eq, Th 215–380°C), probably dominantly meteoric fluids with evidence for boiling at shallow depth. Chlorite crystallisation temperatures, calculated from the chlorite geothermometer, are in good agreement with the Th data for Fls in associated skarn minerals. Compositional changes in the granodiorite apophyses are the result of subsolidus autometasomatic reactions of accumulated saline magmatic fluid inside the apophyses with pre-existing mafic mineral phases. Reactions add the iron to the fluid -the potential source for magnetite skarn. Later mixing with dilute, cooler probably meteoric waters had the effect of decreasing the salinity and density of the equilibrated magmatic fluid, making it more buoyant and capable of moving out from the apophyses into the country rocks, causing metasomatic reactions and precipitating magnetite. An overlap exists between the FI microthermometry data from primary Fls in garnets and late secondary Fls in the granodiorite quartz indicating the same sources of the hydrothermal fluids - probably mixtures of magmatic and meteoric waters. Based on fluid inclusion, geological, petrological and mineralogical data, an integrated fluid evolution model involving magmatic and meteoric fluids is developed to explain the geological and fluid controls on Fe-skarn mineralization associated with granodiorite intrusions. Die Evolution von Fluiden bei Fe-skarn Mineralisation: Ein Beispiel von der Lagerstatte Vyhne-Klokoc, West-Karpathen, Slowakei.

Mineralogical constraints on magmatic and hydrothermal Sn-W-Ta-Nb mineralization at the Nong Sua aplite-pegmatite, Thailand

The chemistry of silicate minerals provides some constraints on the origins of disseminated cassiterite and Ta-Nb-Ti oxide minerals, and vein-hosted cassiterite-wolframite mineralization at the Nong Sua aplite-pegmatite complex. Zoned tourmaline crystals in aplite and pegmatite have preserved their primary compositions. Similar compositions are observed in hydrothermal tourmaline, suggesting an overlap of P, T, fHF and cation activities between the magmatic and hydrothermal stages. All tourmaline types contain excess (Y-site) aluminum, which is typical of the pegmatite environment. A temperature dependence of Al substitution in tourmaline, proposed for pegmatites elsewhere, is shown to be implausible at Nong Sua, indicating that the alkali:aluminum ratio of the melt and fluid have a greater importance in controlling tourmaline composition than has previously been suggested. Muscovite compositions record a history of post-magmatic re-equilibration down to approximately 400°C. By comparing the fluorine contents of muscovite and tourmaline, which have a contrasting tendency to re-equilibrate, it is apparent that the fHF was relatively high at the magmatic and early hydrothermal stages, and was lower during late hydrothermal activity. Enrichment of Ca and Mg in the rims of magmatic and hydrothermal tourmaline indicates that there was infiltration or diffusion of these elements at the magmatic and hydrothermal stages. Such infiltration-diffusion may have also triggered cassiterite crystallization by increasing fO 2 . Textural and compositional data for garnet suggest that it crystallized from melt. Some garnet cores are defined by primary fluid inclusions. The garnet in these cores contains up to 1500 ppm SnO 2 and provides a link between the late magmatic and the early hydrothermal evolution of the pegmatite system. Columbite, ixiolite and niobian-tantalian-rutile are restricted to being aplite-pegmatite-hosted and commonly occur as inclusions in disseminated cassiterite, or in the cores of Sn-rich garnet. These oxide minerals are interpreted to have crystallized from melt

Chemical differences between minerals from mineralizing and barren intrusions from some North American porphyry copper deposits

Major-element analyses (by electron microprobe) and copper contents (by ion-probe) are reported for primary biotite, amphibole, magnetite, pyroxene, ilmenite, sphene and secondary biotite from intrusive rocks from mineralizing and barren stocks. The districts studied include Christmas, Globe-Miami, Sierrita and Tombstone, in Arizona; Bingham and Alta, Utah; Ely, Nevada; and Brenda, British Columbia. Amphiboles from barren rocks are relatively iron-rich and display only minor compositional variation. In contrast, amphiboles from mineralizing rocks span the range from magnesio-hornblende to actinolite, commonly even within one grain. Barren intrusions (type B) that are temporally distinct from mineralizing intrusions, and barren intrusions outside areas of known mineralization have higher Cu contents in their constituent minerals than do mineralizing intrusions. Barren intrusions (type A) that are deep-level temporal equivalents of Cu-bearing porphyritic rocks are depleted in copper. This suggests that copper is abstracted from not only the apical portions of porphyries but from parts of the deeper parent intrusions. The Cu contents of biotites (av. 23 ppm) and magnetites (97 ppm) from barren type B intrusions contrast with those from mineralizing intrusions, with biotites containing 7 ppm Cu and magnetites 3 ppm Cu. Primary amphiboles from all intrusive rock types have low copper contents, typically 2 to 5 ppm. In the continental North American deposits, the amount of copper available by liberation from or non-incorporation into amphibole, biotite and magnetite during magmatic crystallization or the early hydrothermal stage is low, perhaps too low to be the sole source of copper mineralization, unless copper is abstracted from large volumes (∼ 100 km3) of rock. These results contrast with a study of the island-arc porphyry copper at Koloula, Guadalcanal, where it was argued that sufficient copper for mineralization could have been abstracted from relatively small volumes of host rocks that originally contained as much Cu as the contemporaneous barren rock types.