Pre-main Stage Research Articles

Two-event lode-ore deposition at Butte, USA: 40Ar/39Ar and U-Pb documentation of Ag-Au-polymetallic lodes overprinted by younger stockwork Cu-Mo ores and penecontemporaneous Cu lodes

The ore-genesis model for world-class deposits of the Butte mining district, Montana, USA, is deep pre-Main Stage porphyry Cu-Mo and overlying Main Stage Ag-Zn-Cu zoned-lode deposits, both of which formed from hydrothermal fluids driven by minor volumes of rhyolitic magma. The lode-specific model is that hydrothermal processes diminished in intensity outward from district center along lode veins, synchronously forming metal zones. The accepted models are controverted by new geologic and multi-method geochronologic studies.The new data reveal the following sequence of events: (1) Thermal study of country rock indicates that the 76.9-Ma Butte Granite cooled to 350–400 °C by 4 m.y. after emplacement. (2) Five quartz porphyry rhyolite dikes were emplaced at 67–65 Ma and another at 60 Ma (SHRIMP U-Pb) into the cooled Butte Granite without resetting 40Ar/39Ar ages in country rock. (3) Fifty-eight white mica and K-feldspar samples from alteration envelopes adjacent to Ag-Au-polymetallic lodes in outer parts of the district, Zn-rich lodes in intermediate parts, and Cu-rich lodes in the district center yield 40Ar/39Ar ages of 73–70 Ma for Ag-rich lodes, 65–64 Ma for Cu-rich lodes, and complex age spectra of 69–65 Ma for Zn-rich lodes.The data show that Ag-Au-polymetallic lodes occupied cross-district fractures by about 73 Ma, forming the greater Butte mining district. At 67–65 Ma, minor quartz porphyry dikes were emplaced into central and eastern parts of the rejuvenated fracture system but without evidence of related cupola or volcanic rocks or of thermal disturbance in the country rock. At 64.5 Ma, overlapping hydrothermal cells formed two stockwork Cu-Mo domes in deep parts of the fracture system. At 65–64 Ma and closely related to late-stage stockwork Cu-Mo activity, a penecontemporaneous hydrothermal pulse formed a high-sulfidation hydrothermal plume that (1) utilized the large re-opened fractures to cannibalize and remobilize Cu from autologous, stockwork, and older Ag-Au-polymetallic lodes, (2) deposited the rich, high-sulfidation Cu lodes, and (3) mobilized metals from early Ag-Au-polymetallic veins in middle parts of the district, transported the metals outward and redeposited them, enriching early veins, especially in the intermediate Zn plus Cu areas.Metals zones in lodes of the Butte district are the result of an intensely focused, Cu-rich hydrothermal plume that variably reworked the center of significantly larger, 10 m.y. older, Ag-Au-polymetallic lodes.

Cl/Br ratios and stable chlorine isotope analysis of magmatic–hydrothermal fluid inclusions from Butte, Montana and Bingham Canyon, Utah

A bulk geochemical study has been carried out on fluid inclusion leachates extracted from quartz veins from porphyry Cu deposits in Butte, Montana, USA and Bingham Canyon, Utah, USA. The leachates mostly represent low-salinity magmatic–hydrothermal fluid inclusions. Their halogen ratios (Br/Cl) of fluid inclusion leachates were determined by ion chromatography, and δ37Cl values of the leachates were measured by continuous-flow isotope ratio mass spectrometry. Br/Cl ratios from early pre-Main stage and later Main stage veins at Butte range from 0.60 to 1.88 × 10−3 M. Ratios are similar in pre-Main stage veins with sericite bearing selvages and Main stage samples ranging from 0.81 to 1.08 × 10−3 and from 0.92 to 1.88 × 10−3 M, respectively, clustering below seawater (1.54 × 10−3 M) and overlapping mantle values (~1–2 × 10−3 M). Two samples associated with early pre-Main stage potassic alteration yield distinctly lower Br/Cl ratios of 0.60 and 0.64 × 10−3 M. Butte δ37Cl values range from −0.8‰ to −2.3‰ with no significant difference between pre-Main stage and Main stage samples. Br/Cl ratios for quartz veins from Bingham Canyon range from 0.18 to 3.68 × 10−3 M. Br/Cl ratios from Bingham range above and below previously reported for porphyry copper deposits. In contrast to Butte, δ37Cl values for Bingham are lower, ranging from −0.9‰ to −4.1‰. In the absence of any processes which can significantly fractionate chlorine isotopes at high temperatures, we suggest that the porphyry system at Bingham, and to a lesser extent at Butte, have inherited negative chlorine isotopic signatures from the subducting slab generated at low temperatures.

How potassium silicate alteration suggests the formation of porphyry ore deposits begins with the nearly explosive but barren expulsion of large volumes of magmatic water

The rate, duration, and volume of reactive aqueous fluids injected into fractured rock are recorded in the pattern of rock alteration. The volume of altered rock records the total mass of fluid expelled. The width of the transition to fresh rock indicates the rate of expulsion, and division yields its duration. We derive mathematical expressions that capture these relationships and are easy to apply, verify them against finite difference simulations, and use them to determine the time required to produce the early and extensive potassium silicate alteration observed in porphyry deposits at Butte, Montana, and Bajo de la Alumbrera, Argentina. The analysis suggests that potassium silicate alteration was produced, and a larger volume of rock heated to near magmatic temperatures, when magma bodies ∼ 1 km (Alumbrera) to ∼ 8 km (pre-main stage Butte) in diameter lost their volatiles in ∼ 20 and ∼ 900 yr, respectively. Metals were deposited as the venting shut down and the potasically-altered system cooled over a much longer period of time. The voluminous, early, and rapid release of large volumes of magmatic volatiles has broad implications for deposit formation, exploration, and the dynamics of magma devolatization. The analysis illustrates the conceptual and scientific utility of simple mathematical expressions that capture both global and local mass balance relationships, and suggests where better parameter definition and additional kinds of modeling could further constrain estimates of the duration of venting.

Sulfur and oxygen isotopic record in sulfate and sulfide minerals of early, deep, pre-Main Stage porphyry Cu–Mo and late Main Stage base-metal mineral deposits, Butte district, Montana

Typical porphyry-type Cu–Mo mineralization occupies two connected domal centers, the eastern Pittsmont and western Anaconda domes, that predate and largely underlie the well-known, throughgoing, Main Stage polymetallic veins of Butte. Among the sulfur-bearing minerals recovered from deep drill core of this early pre-Main Stage hydrothermal assemblage are anhydrite, chalcopyrite, pyrite, and molybdenite in veinlets bordered by K-silicate alteration, and pyrite from slightly younger quartz–pyrite veinlets with ‘gray-sericitic’ alteration selvages. The ranges of δ 34S values for minerals of the K-silicate assemblage are 9.8–18.2‰ for anhydrite ( n=23 samples), 3.0‰ to 4.7‰ for molybdenite ( n=6), 0.4‰ to 3.4‰ for pyrite ( n=19), and −0.1‰ to 3.0‰ for chalcopyrite ( n=13). Sulfate–sulfide mineral fractionation is consistent with an approach to isotopic equilibrium, and calculated temperatures for mostly coexisting anhydrite–sulfide pairs (anhydrite–molybdenite, n=6, 545 to 630 °C; anhydrite–pyrite, n=13, 360 to 640 °C; and anhydrite–chalcopyrite, n=8, 480 to 575 °C) are broadly consistent with petrological, alteration, and fluid-inclusion temperature estimates. The δ 34S values for pyrite ( n=25) in veinlets of the ‘gray-sericitic’ assemblage range from 1.7‰ to 4.3‰. The δ 34S values for sulfides of the pre-Main Stage K-silicate and ‘gray-sericitic’ assemblages are similar to those of most Main Stage sulfides, for which 281 analyses by other investigators range from −3.7‰ to 4.8‰. Sulfide–sulfide mineral pairs provide variable (−175 to 950 °C) and less reliable temperature estimates that hint of isotopic disequilibria. The sulfide data, alone, suggest a conventionally “magmatic” value of about 1‰ or 2‰ for Butte sulfur. However, the high modal mineral ratios of sulfate/sulfide, and the isotopic systematics of the early K-silicate assemblage, suggest that pre-Main Stage fluids may have been sulfate-rich ( X SO 4 2− ≈0.75) and that total sulfur was isotopically heavy ( δ 34S ΣS≈10‰), which would have required an evaporitic crustal component to the relatively oxidized granitic parental magma that was the source of the hydrothermal fluids and sulfur. Modeling of brine–vapor unmixing of a 10‰ fluid, reduction of sulfate, and vapor loss suggest that these processes may have formed the isotopically heavier (14‰ to 18‰) anhydrite of the western and shallower Anaconda Dome, contrasting with the lighter and more numerous values (9.8‰ to 12.9‰) for anhydrite of the eastern and deeper Pittsmont Dome. Such a process might also have been able to produce the sulfide isotopic compositions of the younger ‘gray-sericitic’ and Main Stage zones, but the limited data for sulfates permit δ 34S ΣS compositions of either 2‰ or 10‰ for these later fluids. Oxygen isotopic data for late Main Stage barite (−0.3‰ to 12.4‰, n=4 samples) confirm variable meteoric water contributions to these fluids, and the data support either the absence of, or limited, sulfate–sulfide isotopic equilibrium in these samples. The δ 34S values for sulfate–sulfur of barite are markedly variable (4.4‰ to 27.3‰), and the unusual 34S depletion indicates sulfur formed by oxidation of H 2S.

SHRIMP U-Pb and 40Ar/39Ar Age Constraints for Relating Plutonism and Mineralization in the Boulder Batholith Region, Montana

The composite Boulder batholith, Montana, hosts a variety of mineral deposit types, including important silver-rich polymetallic quartz vein districts in the northern part of the batholith and the giant Butte porphyry copper-molybdenum pre-Main Stage system and crosscutting copper-rich Main Stage vein system in the southern part of the batholith. Previous dating studies have identified ambiguous relationships among igneous and mineralizing events. Mineralizing hydrothermal fluids for these types of deposits and magma for quartz porphyry dikes at Butte have all been considered to be late-stage differentiates of the Boulder batholith. However, previous dating studies indicated that the Boulder batholith plutons cooled from about 78 to 72 Ma, whereas copper-rich Main Stage veins at Butte were dated at about 61 Ma. Recent efforts to date the porphyry copper-molybdenum pre-Main Stage deposits at Butte resulted in conflicting estimates of both 64 and 76 Ma for the mineralizing events. Silver-rich polymetallic quartz vein deposits elsewhere in the batholith have not been dated previously. To resolve this controversy, we used the U.S. Geological Survey, Stanford, SHRIMP RG ion microprobe to date single-age domains within zircons from plutonic rock samples and 40 Ar/ 39 Ar geochronology to date white mica, biotite, and K-feldspar from mineral deposits. U-Pb zircon ages are Rader Creek Granodiorite, 80.4 ± 1.2 Ma; Unionville Granodiorite, 78.2 ± 0.8 Ma; Pulpit Rock granite, 76.5 ± 0.8 Ma; Butte Granite, 74.5 ± 0.9 Ma; altered Steward-type quartz porphyry dike (I-15 roadcut), 66.5 ± 1.0 Ma; altered Steward-type quartz porphyry dike (Continental pit), 65.7 ± 0.9 Ma; and quartz monzodiorite of Boulder Baldy (Big Belt Mountains), 66.2 ± 0.9 Ma. Zircons from Rader Creek Granodiorite and quartz porphyry dike samples contain Archean inheritance. The 40 Ar/ 39 Ar ages are muscovite, silver-rich polymetallic quartz vein (Basin district), 74.4 ± 0.3 Ma; muscovite, silver-rich polymetallic quartz vein (Boulder district), 74.4 ± 1.2 Ma; muscovite, early dark micaceous vein (Continental pit), 63.6 ± 0.2 Ma; biotite, early dark micaceous vein (Continental pit), 63.6 ± 0.2 Ma; potassium feldspar, early dark micaceous vein (Continental pit), 63 to 59 Ma; and biotite, biotite breccia dike (Continental pit), 63.6 ± 0.2 Ma. Outlying silver-rich polymetallic quartz veins of the Basin and Boulder mining districts probably are directly related to the 74.5 Ma Butte Granite, whereas Steward-type east-west quartz porphyry dikes and Butte pre-Main Stage deposits are parts of a 66 to 64 Ma magmatic-mineralization system unrelated to emplacement of the Boulder batholith. The age of the crosscutting Main Stage veins may be about 61 Ma as originally reported but can only be bracketed as younger than the 64 Ma pre-Main Stage mineralization and older than the about 50 Ma Eocene Lowland Creek intrusions. The 66 Ma age for the quartz monzodiorite of Boulder Baldy and consideration of previous dating studies in the region indicate that small ca. 66 Ma plutonic systems may be common in the Boulder batholith region and especially to the east. The approximately 64 Ma porphyry copper systems at Butte and gold mineralization at Miller Mountain are indicative of regionally important mineralizing systems of this age in the Boulder batholith region. Resolution of the age and probable magmatic source of the Butte pre-Main Stage porphyry copper-molybdenum system and of the silver-rich polymetallic quartz vein systems in the northern part of the Boulder batholith documents that these deposits formed from two discrete periods of hydrothermal mineralization related to two discrete magmatic events.

Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada; a 6-km vertical reconstruction

Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada; a 6-km vertical reconstruction

Sulfur isotopic variations in the A2844 vein and wall rock, Butte, Montana.

Sulfur isotopic analyses on vein pyrite and associated sulfides in a short section of a vein in the Intermediate Zone, Butte, Montana, show remarkable isotopic uniformity. The standard deviation (0.5 ‰) and range of pyrite values (1.4 to 3.9 ‰) are attributed to ore formation in a fO2-pH regime where the isotopic composition was insensitive to large changes in the physico-chemical conditions. The δ34S of the hydrothermal solution responsible for pyrite deposition is estimated to be +3.1 ‰. Large per mil differences and textural relationships between pyrites and associated copper and zinc sulfides show that the different sulfides were not deposited simultaneously and/or in isotopic equilibrium. Disseminated wall rock pyrites are depleted in 34S by as much as 2.4 ‰ relative to the adjacent vein and veinlets. The isotopic relationship between vein and disseminated pyrites may be due to a number of phenomena including diffusional fractionation, the effects of wall rock on the physico-chemical properties of the solution but are probably largely due to the interaction of pre-Main Stage and Main Stage hydrothermal processes. Delineation of these mechanisms is confounded by the presence of many veinlets in the wall rock.

Paleomagnetic documentation of the early, high-temperature zone of mineralization at Butte, Montana

Butte quartz monzonite, host rock to mineralization in the Butte district, Montana, was pervasively altered to a high-temperature K-silicate alteration assemblage during pre-Main Stage, porphyry-type mineralization within the deep levels of the district. Magnetization directions of steep, negative inclination (reverse polarity) are characteristic of Butte quartz monzonite within the zone of pervasive pre-Main Stage alteration and mineralization. Butte quartz monzonite in surrounding, less altered parts of the district and farther from Butte contains only steep, positive inclination characteristic directions (normal polarity). Magnetic, petrographic, and radiometric evidence together with the exact correlation of reversed polarity directions with high-temperature pre-Main Stage alteration assemblages suggest that, in the deep, central portion of the Butte district, the original normal polarity thermoremanent magnetization was completely unblocked by heating to temperatures in excess of 578 degrees C. A secondary thermoremanent magnetization was acquired in subsequent cooling in a field of reversed polarity prior to Main Stage, fracture-controlled mineralization. The detection of reversed polarity directions in a portion of the Butte quartz monzonite pluton serves as a method for delineating the extent of a high-temperature (greater than 578 degrees C) thermal perturbation during mineralization at Butte.

Lithologic determination of mass transfer mechanisms of multiple-stage porphyry copper mineralization at Butte, Montana; vein formation by hypogene leaching and enrichment of potassium-silicate protore

Lithologic determination of mass transfer mechanisms of multiple-stage porphyry copper mineralization at Butte, Montana; vein formation by hypogene leaching and enrichment of potassium-silicate protore

Early fracture-controlled disseminated mineralization at Butte, Montana

Solution transport through intensely fractured quartz monzonite wall rock resulted in the formation of a large-tonnage, low-grade mineralized zone in the Butte district before the formation of the much larger and more continuous high-grade vein structures that have been mined for almost a century. The early, pre-Main Stage, copper mineralization is directly related to the presence of small, alkali feldspar-stable, discontinuous veinlets and alteration halos which carry important amounts of disseminated chalcopyrite. Molybdenite occurs mainly in quartz or quartz and alkali feldspar veins. A general sequence of alteration assemblages, based on cross-cutting relationships, shows a systematic trend of decreasing mineralogic complexity toward the younger pre-Main Stage assemblages. Microscopic fabric studies show that alteration minerals around early veinlets are arranged in zones parallel to veinlet walls. The alteration mineralogy in the bands is very simple near veinlets and increases in mineralogic complexity away from the veins. Partial replacement textures in the alteration zones show that at least some of the discontinuities in mineral assemblages represent reaction fronts. Compositions of hydrothermal alkali feldspars and adjacent muscovites, determined with the election microprobe, are generally constant within individual specimens, but vary considerably from sample to sample. A model is developed in this study of the partitioning of alkalies between muscovite and alkali feldspar at quartz saturation which implies temperatures in the range of 600 degrees to 700 degrees C for early alteration and mineralization. Phase equilibria applied to the early mineral assemblages in Butte are consistent with this temperature range and imply conditions near the triple point andalusite-alkali feldspar-muscovite in the presence of quartz. A genetic relationship between quartz porphyry intrusives and the earliest high temperature hydrothermal features seems reasonable, as these intrusives provide a possible solution source and fracture mechanism for mass transport of the important base metals into the densely fractured equigranular quartz monzonite country rock.

Hydrogen and Oxygen Isotope Evidence for the Origins of Water in the Boulder Batholith and the Butte Ore Deposits, Montana

Biotite, hornblende, and whole-rock delta D values for 23 plutonic rocks from the composite Boulder batholith (age 78 to 68 m.y.) range from --63 to --155 per mil (SMOW). Biotites and hornblende with delta D = --63 to --90 (9 samples) are from rocks which have normal igneous whole-rock delta 18 O values (7.0 to 8.3) and quartz-K-feldspar fractionations. D/H ratios of these unaltered rocks probably approximate primary magmatic values. Samples with delta D = --130 to --155 and/or low whole-rock delta 18 O values, (i.e. --4 18 O values; these rocks probably interacted with very small quantities of heated meteoric waters. Meteoric water-rock interaction was a wide-spread phenomenon during cooling and late-stage crystallization of the Boulder batholith but only locally occurred on a large scale. Correlation between the 18 O/ 16 O and 206 Pb/ 204 Pb ratios indicate that some of the minor 18 O/ 16 O and D/H variations are probably primary features of the magmas.At Butte, the isotopic data for hydrous alteration minerals (biotite, sericite, pyrophyllite, clays) and the calculated delta 18 O water values at the selected temperatures are:Stage delta D Mineral delta 18 O Mineral delta 18 O Water T degrees CPre-Main Stage -160 to -170 +3 to +5 +6 to +9 600Main Stage. Central -115 to --180 --9 to +12 -12 to +8 300Intermediate --130 to -155 -4 to +7 -7 to +4 300Peripheral -130 to -145 -2 to +4 - 5 to +1 300Vein quartz also spans much of this 20 per mil delta 18 O range, which is the largest ever reported from a single ore deposit. Excluding two sericites, the hydrogen isotope composition of Main Stage alteration minerals at Butte is very low and extremely uniform (delta D = -140 + or - 15). Calculated D/H ratios of the hydrothermal fluids at approximately 300 degrees C are --110 + or - 15. The low D/H ratios of pre-Main Stage (EDM) biotites indicate equilibration with similar deuterium-depleted solutions of meteoric hydrothermal origin, in contrast with the isotopic evidence for a magmatic hydrothermal origin for alteration biotite in porphyry copper deposits. It is therefore possible that the hydroxyls of the very fine grained EDM biotites were exchanged during the Main Stage hydrothermal activity. The most 18 O-rich Main Stage fluids are associated with the pervasive sericitization in the Berkeley pit area, and the most 18 O-depleted solutions formed the advanced argillic assemblages and some vein minerals in major through-going fractures. Meteoric waters were dominant in all of these Main Stage hydrothermal fluids throughout the Central, Intermediate, and Peripheral Zones. The large variations in delta 18 O water for the Main Stage mineralization indicate that fluids of radically different isotopic composition were present at different times and/or places during the evolution of the alteration assemblages; this is probably a result of variable degrees of interaction between the wall rocks and the hydrothermal fluids or to mixing of meteoric waters with different histories.

Sulfur isotopic reconnaissance of Butte, Montana

The immense amount of geologic information available for interpreting geochemical data makes Butte, Montana, ideal for a sulfur isotopic study.The mean per mil values for the different Main Stage minerals show the following order of S 34 enrichment.Peripheral Zone: pyrite (3.6) > sphalerite (0.5)Intermediate Zone: pyrite (1.3) > chalcopyrite (--0.1) = enargite (--0.1) >bornite (--0.3) > chalcocite (--1.3)Central Zone: pyrite (0.7) > chalcocite (--1.7) > enargite (--2.0)Deep Level Zone: pyrite (1.0) > chalcopyrite (--0.9) >chalcocite (-1.0) > bornite (--1.2)The order of S 34 enrichment among the minerals is proportional to their bonding strength and similar to the equilibrium results between coprecipitated sulfides of other districts reported by other authors. However, the copper sulfides are somewhat later than pyrite and sphalerite. The above isotopic dissimilarities indicate that the copper sulfides probably do not represent replacement or arrested copper metasomatism of earlier pyrite. The approach toward sulfur isotopic equilibrium between the non-arsenic-bearing copper sulfides possibly may have been achieved during exsolution from a single, high-temperature parental sulfide in the bornite solid solution field. Depositional isotopic equilibrium between sulfides may have been preserved in the Peripheral Zone of the Main Stage mineralization and in the molybdenite-bearing pre-Main Stage mineralizations, where molybdenite is more enriched in S 34 than pyrite.The outward increase in per mil values is best illustrated by pyrite and is the reverse of other published examples of sulfur isotopic zoning. The geological evidence and Sakai9s (1968) calculated isotopic effects suggest that the isotopic trend is primarily due to an increase in the pH of the hydrothermal solutions as they moved upward and outward from the Central Zone. Thus the vein mineralogy at Butte may not be so much due to a decrease in temperature outward as to the chemical evolution of the hydrothermal solutions.The apparent isotopic similarity of the minerals in the E-W and NW vein systems within the same zone is compatible with the geologic evidence that the vein systems were mineralized nearly simultaneously. Preliminary data suggest that the pre-Main Stage mineralizations have a different sulfur isotopic composition than the Main Stage.The mean (0.4ppm) and standard deviation (1.6ppm) of all the samples are similar to other magmatic hydrothermal Cordilleran ore deposits. Our sulfur isotopic data and published lead isotopic data indicate that the ore deposits are genetically related to the Butte quartz monzonite. Published lead, strontium, oxygen, and hydrogen isotopic data permit the speculation that at Butte the sulfur was of crustal rather than subcrustal origin.