Butte Quartz Monzonite Research Articles

Geochemistry and Stable Isotopes of the Flooded Underground Mine Workings of Butte, Montana

Over a century of mining and smelting of the world-class porphyry lode ore deposit at Butte, Montana, has resulted in extensive environmental damage. In addition to its being the location of one of the world’s largest and most acidic mining pit lakes, Butte is host to over 16,000 km of flooded underground mine workings. Of the more than 60 mine shafts that have historically operated in Butte, approximately one dozen are presently accessible for groundwater sampling. The geochemistry of the mine shaft waters is zoned and roughly coincides with a district-wide zonation in hydrothermal alteration and mineralization. Mine waters in the so-called “Central zone” of intense phyllic and advanced argillic alteration have lower pH and very high concentrations of As, Fe, Mn, and Zn, but very low concentrations of dissolved Cu. The scarcity of Cu is attributed to cementation onto scrap iron left in the mines, and/or to replacement of preexisting sulfide minerals below the water table in a manner analogous to supergene enrichment processes. At the other extreme, mine waters in the “Peripheral zone” of weakest alteration have near-neutral pH, low metal concentrations, and contain dissolved sulfide (H2S, HS–). These waters are close to equilibrium with calcite, siderite, crystalline or amorphous MnCO3, and mackinawite (poorly crystalline FeS). A suite of deep groundwater monitoring wells completed in fractured and mineralized Butte Quartz Monzonite, unassociated with the mining complex, shows a similar range in groundwater chemistries to the mine shaft waters, suggesting a fundamental control of bedrock geology on water quality. Based on its isotopic composition, aqueous sulfate in the Butte mine waters was sourced from a combination of pyrite oxidation and leaching of hydrothermal anhydrite associated with early, porphyry-style Cu-Mo mineralization. Aqueous sulfide in the Peripheral zone mine workings is 28 to 50 per mil depleted in 34S relative to coexisting aqueous sulfate, consistent with microbial sulfate reduction. Dissolved inorganic carbon in the majority of the waters sampled appears to have isotopically equilibrated at low temperature with hydrothermal rhodochrosite, an abundant mineral at Butte. Waters with the highest H2S concentrations also have unusually high dissolved inorganic carbon concentrations that are depleted in 13C, consistent with an influx of CO2 from microbial oxidation of organic carbon. The source of organic carbon is not known, but may include timbers used to reinforce the tunnels and stopes. In contrast to the large horizontal gradients in mine water chemistry on a district scale, vertical gradients in chemistry and temperature within each individual shaft at Butte are negligible, possibly due to vertical water circulation. The circulation model is consistent with data on the local geothermal gradient in Butte, and explains why the deepest mine shafts tend to have the warmest water. The Kelley mine has an anomalously warm temperature (~35°C), and some of the excess heat in this mine shaft may have come from pyrite oxidation, a highly exothermic reaction. The flooded underground mine complex of Butte has potential for heat recovery using modern heat pump technology.

Fluid Inclusion Evidence for Magmatic-Hydrothermal Fluid Evolution in the Porphyry Copper-Molybdenum Deposit at Butte, Montana

The porphyry Cu-Mo deposit in Butte, Montana, formed where magmatic hydrothermal fluids, introduced with injections of porphyrytic dikes, fractured and permeated the Butte Quartz Monzonite. These fluids formed a stockwork of quartz and quartz-sulfide veinlets with a variety of styles of potassic and sericitic alteration envelopes. The distribution of vein and alteration types and the distribution of fluid inclusions in these veins record the progressive pressure, temperature, and compositional evolution of the hydrothermal fluids that formed this world-class deposit. Deep drilling and 1,300 m of offset along the Continental fault provide a vertical view of almost 3 km through the Butte deposit. Deep veins within and below the highest Mo grades are quartz dominated with thin K-feldspar or, less commonly, biotitic alteration rims. Fluid inclusions in deep veins trapped a single phase aqueous fluid containing 2 to 5 wt percent NaCl equiv and 2 to 8 mol percent CO2 at temperatures between 575° and 650°C and pressures between 200 and 250 MPa, corresponding to depths between 6 and 9 km. Although Cu grades are low in this region, abundant chalcopyrite daughter minerals in fluid inclusions indicate that the fluids were Cu rich. Fluids that formed these veins transported Cu from the magma below, upward into the region of Cu mineralization with only minor Cu precipitation. Over a kilometer above the bulk of deep quartz and quartz-molybdenite veins, the highest Cu grades are in and around chalcopyrite-bearing quartz-sulfide veins with biotitic alteration (early dark micaceous veins), and their upward, equivalent magnetite-chalcopyrite-pyrite-quartz veins with wide K-feldspar, green sericite, and chlorite alteration (pale-green sericitic veins). These veins contain more evidence for brine-vapor unmixing than any other vein type. The upward progression of early dark micaceous veins to pale-green sericitic veins formed where low salinity, CO2-bearing fluids, similar to those trapped in deep quartz veins, ascended, de-pressurized, sometimes unmixed, and cooled from ~650°C at 90 MPa to ~475°C at ~50 MPa. As low salinity, CO2-bearing, aqueous fluids, similar in composition to fluids trapped in deep quartz veins, cooled at shallow depths, they formed late pyrite-quartz veins with sericitic alteration. These veins formed from fluid cooling at temperatures between 370° and 450°C at transiently hydrostatic pressures between 40 and 70 MPa, corresponding to depths of 4 to 7 km. Most pyrite-quartz veins formed at pressures and temperatures above the H2O-NaCl-CO2 solvus, but evidence for brine-vapor unmixing is also present. Pyrite-quartz veins formed at progressively greater depths as the hydrothermal system cooled, overprinting much previous mineralization. Late Cu-Pb-Zn-Ag-As-rich Main stage veins formed from dilute fluids containing <3 wt percent NaCl equiv and <2 mol percent CO2. These fluids were trapped between 230° and 400°C under hydrostatic pressures between 20 and 60 MPa and depths of 2 to 6 km. No evidence of boiling is observed in Main stage veins. Fluid inclusion phase relationships indicate that the Butte porphyry Cu-Mo deposit formed at 5 to 9 km depth, greater than any other porphyry-type deposit. At Butte, the similarity in bulk composition of fluids trapped in early quartz-rich veins with potassic alteration and late pyrite-quartz veins with sericitic alteration implies that an underlying magma continually provided low salinity, CO2-bearing fluids of relatively constant composition during the entire life of the hydrothermal system. We hypothesize that rather than resulting from changes in fluid chemistry due to magma crystallization, the entire suite of vein and alteration types and the ore metal distribution reflect the path of cooling, depressurization, and wall-rock interaction of a parental magmatic-derived fluid of relatively constant initial composition. Fluid inclusions, vein and alteration relations, and ore metal distribution indicate that Cu and Mo were introduced into the hydrothermal system by the same fluids, but that the mechanisms of precipitation of these metals were decoupled. Early dark micaceous and, to a greater extent, pale-green sericitic, veins have wide alteration envelopes and contain more evidence for fluid unmixing than any other vein type, which suggests that chalcopyrite precipitation was driven by a combination of fluid unmixing, fluid-rock reaction, and fluid cooling between 650° and 475°C. Most molybdenite mineralization, however, is in quartz-dominated veins with little or no alteration that are dominated by low salinity inclusions. These veins formed in response to pressure decrease rather than cooling. After chalcopyrite and molybdenite precipitation, low salinity fluids cooled, usually at temperatures and pressures above the H2O-NaCl-CO2 solvus, to produce significant acid and voluminous sericitic alteration accompanied by pyrite-quartz vein formation that overprints much of the deposit and contains anomalous but noneconomic Cu.

Intensity of quartz cathodoluminescence and trace-element content in quartz from the porphyry copper deposit at Butte, Montana

Textures of hydrothermal quartz revealed by cathodoluminescence using a scanning electron microscope (SEM-CL) refl ect the physical and chemical environment of quartz formation. Variations in intensity of SEM-CL can be used to distinguish among quartz from superimposed mineralization events in a single vein. In this study, we present a technique to quantify the cathodoluminescent intensity of quartz within individual and among multiple samples to relate luminescence intensity to specific mineralizing events. This technique has been applied to plutonic quartz and three generations of hydrothermal veins at the porphyry copper deposit in Butte, Montana. Analyzed veins include early quartz-molybdenite veins with potassic alteration, pyrite-quartz veins with sericitic alteration, and Main Stage veins with intense sericitic alteration. CL intensity of quartz is diagnostic of each mineralizing event and can be used to fingerprint quartz and its fluid inclusions, isotopes, trace elements, etc., from specific mineralizing episodes. Furthermore, CL intensity increases proportional to temperature of quartz formation, such that plutonic quartz from the Butte quartz monzonite (BQM) that crystallized at temperatures near 750 °C luminesces with the highest intensity, whereas quartz that precipitated at ~250 °C in Main Stage veins luminesces with the least intensity Trace-element analyses via electron microprobe and laser ablation-ICP-MS indicate that plutonic quartz and each generation of hydrothermal quartz from Butte is dominated by characteristic trace amounts of Al, P, Ti, and Fe. Thus, in addition to CL intensity, each generation of quartz can be distinguished based on its unique trace-element content. Aluminum is generally the most abundant element in all generations of quartz, typically between 50 and 200 ppm, but low-temperature, Main Stage quartz containing 400 to 3600 ppm Al is enriched by an order of magnitude relative to all other quartz generations. Phosphorous is present in abundances between 25 and 75 ppm, and P concentrations in quartz show little variation among quartz generations. Iron is the least abundant of these elements in most quartz types and is slightly enriched in CL-dark quartz in pyrite-quartz veins with sericitic alteration. Titanium is directly correlated with both temperature of quartz precipitation, and intensity of quartz luminescence, such that BQM quartz contains hundreds of ppm Ti, whereas Main Stage quartz contains less than 10 ppm Ti. Our results suggest that Ti concentration in quartz is controlled by temperature of quartz precipitation and that increased Ti concentrations in quartz may be responsible for increased CL intensities.

Compositions of magmatic hydrothermal fluids determined by LA-ICP-MS of fluid inclusions from the porphyry copper–molybdenum deposit at Butte, MT

We analyzed 85 fluid inclusions from seven samples from the porphyry Cu–Mo deposit in Butte, MT, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The Butte deposit formed at unusually great depth relative to most porphyry deposits, and fluid inclusions in deep veins trapped a low-salinity, CO 2-bearing, magmatically derived, supercritical fluid as a single aqueous phase. This fluid is interpreted to be the parent fluid that cooled, decompressed, unmixed, and reacted with wall rock to form the gigantic porphyry Cu deposit at Butte. Few previous analyses of such fluids exist. Low-salinity, aqueous fluids from the earliest veins at Butte are trapped in deep veins with biotite-rich alteration envelopes (EDM veins). These veins, and the Butte quartz monzonite surrounding them, host much of the Butte porphyry Cu mineralization. Twenty fluid inclusions in one EDM quartz vein are dominated by Na, K, Fe (from ∼0.1 to ∼1 wt.%) and contain up to 1.3 wt.% Cu. These inclusions contain only small amounts (tens of ppm) of Pb, Zn, and Mn, and typically contain Li, B, Ca, As, Mo, Ag, Sn, Sb, Ba, and W in less than detectable quantities. The abundance of Cu in early fluids indicates that a low-salinity, Cu-rich, aqueous ore fluid can be directly produced by aqueous fluid separation from a granitic magma. Similar inclusions (eight) in an early deep quartz–molybdenite vein with a K-feldspar selvage have similar compositions but contain significantly less Cu than most inclusions in the biotite-altered vein. Analyzed inclusions in both veins contain less than detectable concentrations of Mo even though one is molybdenite-bearing. Low-salinity, CO 2-bearing aqueous fluids are also trapped in pyrite–quartz veins with sericitic selvages. These veins cut both of the above vein types and contain inclusions that were trapped at lower pressure and temperature. Thirty-nine inclusions in two such veins have compositions similar to early fluids, but are enriched by up to a factor of 10 in Mn, Pb, and Zn relative to early fluids, and are slightly depleted in Fe. Many of these inclusions contain as much or more Cu than early fluids, although little chalcopyrite is found in or around pyrite–quartz veins. Eighteen halite-bearing inclusions from three veins from both chalcopyrite-bearing and barren veins with both K-silicate and sericitic selvages were analyzed as well. Halite-saturated inclusions are dominated by Na, K, Fe, and in some inclusions Ca. Whereas these inclusions are significantly enriched in Ca, Mn, Fe, Zn, and Pb, fluids in all three veins contain significantly less Cu than early, high temperature, low-salinity inclusions. Analyses of all inclusion types show that whereas bulk-salinity of the hydrothermal fluid must be largely controlled by the magma, fluid–rock interactions have a significant role in controlling fluid compositions and metal ratios. Cu concentrations range over an order of magnitude, more than any other element, in all four samples containing low-salinity inclusions. We infer that variations are the result of fluid trapping after different amounts of fluid–rock reaction and chalcopyrite precipitation. Enrichment, relative to early fluids, of Mn, Pb, and Zn in fluids related to sericitic alteration is also likely the result of fluid–rock reaction, whereby these elements are released from biotite and feldspars as they alter to sericite. In halite-bearing inclusions, concentrations of Sr, Ca, Pb, and Ba are elevated in inclusions from the pyrite–quartz vein with sericitic alteration relative to halite-bearing inclusions from unaltered and potassically altered samples. Such enrichment is likely caused by the breakdown of plagioclase and K-feldspar in the alteration envelope, releasing Sr, Ca, Pb, and Ba.

Geochemistry of the rare-earth elements and uranium in the acidic Berkeley Pit lake, Butte, Montana

Filtered (0.45 μm) and nonfiltered concentrations of rare-earth elements (REE), U, Zr, Th, Ba, Sc, and Y were measured as a function of depth in the Berkeley Pit lake, a large acidic mining lake in Butte, MT. The REE show very little variation with depth, apart from a slight concentration near the surface, presumably due to evaporation. The REE profiles of the pit waters show a depletion in light REE when normalized against NASC and the host Butte Quartz Monzonite (BQM), and a possible enrichment in middle REE vs. heavy REE when normalized against BQM. All of the REE partitioned weakly into secondary ferric precipitates formed by aging of deep Berkeley Pit water. The measured distribution coefficients increased across the lanthanide series from La ( K d=7.7) to Nd ( K d=10.3), and then decreased steadily to Lu ( K d=1.4). Under the conditions of the Berkeley Pit lake, the aqueous speciation of REE and U is dominated by sulfate complexes. Because the stability constants of REE–sulfate complexes show very little variation across the lanthanide series, the observed trends in K d cannot be explained by aqueous complexation. Despite high concentrations of dissolved REE (e.g., 1.1 mg/l Ce) and U (0.85 mg/l), saturation indices for all solid phases were strongly negative, due to the low pH of the Berkeley Pit lake (2.3–2.6). The mobility of REE and U is more likely constrained by adsorption or co-precipitation with strengite, jarosite, schwertmannite, or other secondary minerals forming in the lake.

New insights from reactive transport modelling: the formation of the sericitic vein envelopes during early hydrothermal alteration at Butte, Montana

AbstractA reactive transport computer code has been employed to model hydrothermal alteration of a granitoid rock bordering a discrete vein channel. The model suggests that the grey sericitic and sericitic with remnant biotite alteration envelopes at the porphyry copper deposit at Butte, Montana, can be formed by a reducing, low pH, and low salinity fluid under constant temperature and pressure conditions of approximately 400 °C and less than 100 MPa during a time span of approximately 100 years or less. Hydrothermal alteration has little effect on the porosity of the host rock (Butte Quartz Monzonite), and the diffusivity of the aqueous species also changes little. A sequence of mineral reaction fronts characterizes the alteration envelopes. The biotite dissolution front occurs closest to the vein channel and marks the transition from the grey sericitic to sericitic with remnant biotite envelope. The plagioclase dissolution front occurs farthest into the matrix and marks the edge of relatively fresh Butte Quartz Monzonite. From the properties of the quasi‐stationary state approximation (Lichtner 1988; Lichtner 1991), it follows that once the sequence of reaction fronts is fully established, their relative locations remain constant and the widths of the reaction zones increase with the square root of time.

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.

Paleomagnetism, Rock Magnetism, and Aspects of Structural Deformation of the Butte Mining District, Butte, Montana

Previously, field relations in the Butte District, Montana, have been interpreted to indicate a regional, 40-60° or greater westward tilting of the Late Cretaceous Butte quartz monzonite (BQM), the host rock to mineralization. The tilting was inferred to be Eocene or younger in age, based upon structural features in the overlying Eocene Lowland Creek Volcanics. Extensive paleomagnetic data reveal a more complicated picture. For surface exposure BQM, the in situ mean direction of characteristic magnetization of 13 site means is D = 1.5°, I = 73.3° (K = 27.6, $$\alpha_{95} = 2.6^{\circ}$$). Characteristic directions from BQM affected by early mineralization and high-temperature potassic alteration (antipodes) and two sets of quartz porphyry dikes, which pre-and post-date this mineralization, coincide with results from the surface and Berkeley Pit exposures of BQM. Studies of the magnetic oxides, along with the results of alternating field and thermal demagnetization, indicate a primary thermoremanent origin for the characteristic directions from all units except the high-temperature altered BQM, where a secondary thermoremanent magnetization was imparted. District-wide tilting of the magnitude visualized in the past is precluded by the fact that characteristic magnetization directions of Late Cretaceous units in the Butte district correspond closely to those of time-equivalent units elsewhere in North America. Systematic regional or non-systematic local tilting of as much as 15° could have occurred, but movements of 40-60° or more are ruled out for the BQM units analyzed. The Eocene volcanics overlying the BQM present an interesting tectonic enigma. Immediately northwest of Butte, the units dip steeply to the west or northwest in a graben-like structure; farther west or northwestward, the dips become more shallow. When compared with the predicted Early Tertiary field direction, the data for these units suggest that they have indeed been rotated westward by 30-50° or more about an approximately north-south axis. The immediately underlying BQM is unexposed, but inliers (probably horsts) of BQM surrounded by the volcanics within the graben give characteristic directions identical to those of untilted BQM elsewhere in the district. The discrepancy must relate to movements within the Eocene volcanic units after the magnetic carriers had acquired a stable thermoremanent magnetization, possibly during differential compaction and slumping. The study supports long-standing interpretations that the zonal patterns of mineralization and hydrothermal alteration in BQM have not been extensively reoriented by tectonic tilting.

Preliminary Petrographic, Chemical, and Age Data on Some Intrusive and Associated Contact Metamorphic Rocks, Pioneer Mountains, Southwestern Montana

Petrographic and chemical data on five types of intrusive rocks of the so-called Pioneer batholith in the northeastern Pioneer Mountains of southwest Montana show that the rocks range from quartz diorite to granite. The volumetrically most important body is a coarse-grained biotite-hornblende “granite” that superficially resembles the Butte Quartz Monzonite of the Boulder batholith, some 60 km to the northeast. Their K 2 O content puts the rocks into Tilling9s “sodic series” of the Boulder batholith. K-Ar dating on biotite and hornblende shows that the date of intrusion of all but the quartz diorite is about 70 m.y. The hornblende age of the quartz diorite indicates that it was intruded 76.5 m.y. ago. Its biotite age of 70.5 m.y. agrees with those of the younger intrusions. High-grade contact metamorphic rocks of the Silver Hill Formation (Cambrian) at one locality yielded the same 70-m.y. age on biotite porphyroblasts. These ages compare closely with those of the Boulder and Philipsburg batholiths and show the same tendency for the less mafic rocks to be younger.

Composition and Time Relations of Plutonic and Associated Volcanic Rocks, Boulder Batholith Region, Montana

Research Article| December 01, 1974 Composition and Time Relations of Plutonic and Associated Volcanic Rocks, Boulder Batholith Region, Montana ROBERT I. TILLING ROBERT I. TILLING 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 96718 Search for other works by this author on: GSW Google Scholar Author and Article Information ROBERT I. TILLING 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 96718 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1974) 85 (12): 1925–1930. https://doi.org/10.1130/0016-7606(1974)85<1925:CATROP>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ROBERT I. TILLING; Composition and Time Relations of Plutonic and Associated Volcanic Rocks, Boulder Batholith Region, Montana. GSA Bulletin 1974;; 85 (12): 1925–1930. doi: https://doi.org/10.1130/0016-7606(1974)85<1925:CATROP>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Comparison of areally weighted bulk compositions for the Boulder batholith and prebatholith volcanic rocks (Elkhorn Mountains Volcanics) shows a close match in terms of K2O-Na2O-CaO-SiO2 variations. Detailed examination of available chemical data suggests, however, that the constituent units of the volcanic rocks differ among themselves, as well as from many of the batholith units. Most sampled volcanic rocks are chemically and isotopically similar to the volumetrically minor mafic members of the putonic main series of Tilling (1973). The sodic-series plutonic rocks, which form about 15 percent of the exposed batholith, apparently have no compositionally similar counterparts in the Elkhorn Mountains Volcanics in terms of bulk composition.The Butte Quartz Monzonite, which makes up about three-quarters of the batholith in terms of individual samples and bulk composition, is chemically distinct from most volcanic rocks. This relation suggests that only small amounts of magma of Butte Quartz Monzonite composition erupted onto the surface. The voluminous silicic ash flows of the middle member of the volcanic rocks were probably derived from the same magma that mostly crystallized subsurface to form the Butte Quartz Monzonite. Field, chemical, K-Ar age, and paleomagnetic evidence, however, strongly suggest that a time gap separated the extrusion of the middle-member volcanic rocks from the intrusion of the Butte Quartz Monzonite and related younger silicic variants.Compositional and time relations between the plutonic and genetically associated volcanic rocks are generally compatible with the concept that the volcanic rocks are eruptive equivalents of a shallow evolving batholith but seem incompatible with some specific aspects of the “extrusive complex” or “floored sheet” hypothesis as applied to the Boulder batholith by Hamilton and Myers (1967, 1974). This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

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.

On the Nature of the Boulder Batholith of Montana

In a recent review of the nature of batholiths, Hamilton and Myers (1967) interpreted the Boulder batholith of western Montana to be “in effect a gigantic mantled lava flow .... only a few kilometers thick,” that flowed, under a crust of its own ejecta, across a broad structural basin. Such an interpretation is inconsistent with abundant geologic and geophysical data. The main mass of the batholith, the Butte Quartz Monzonite, does not have the characteristics of a lava flow or a laterally emplaced sheet. Its volcanic cover was not a floating cap but a laterally stable roof that was part of a volcanic plateau which occupied at least twice the area of the batholith. It does not thin toward its edges but is generally steep sided. Its flow structures are predominantly steep rather than near horizontal. It is separated from two smaller flanking plutons by thin vertical septa kilometers long. Its emplacement required more than 4 m.y., a rate orders of magnitude too slow for a single sheet only a few kilometers thick, however extensive. The batholith is more than a few kilometers thick. Recent gravity studies (Burfeind, 1967; Bonini, 1969) suggest a maximum thickness of 9 to 15 km to their authors, but the calculations are based on (1) assumed lateral and vertical homogeneity of the batholith, whereas in reality the Butte Quartz Monzonite core is discontinuously rimmed by more mafic, denser plutons; and (2) inappropriate densities, leading to excessive apparent density contrasts. The gravity data suggest to us that the batholith is more than 15 km thick. Heat flow, cooling rate, and seismic data also are compatible with a thickness of at least 15 km, but are difficult to reconcile with a thickness of only a few kilometers. Convincing examples of extrusive or quasi-extrusive thin batholiths must be sought elsewhere.

Copper content of biotites from the Boulder Batholith, Montana

Copper content (by instrumental neutron activation analysis) of sixty-one biotites from rocks of the Late Cretaceous, composite Boulder batholith ranges from 6 to 4,395 ppm and averages 700 ppm. High copper values for the samples studied can generally be accounted for by the presence of minute inclusions of sulfide minerals in the biotite, rather than to extensive isomorphic substitution of copper in the crystal structure.Although the distribution pattern is variable, the highest copper contents in biotite are observed for samples from the Butte Quartz Monzonite, the largest single pluton of the batholith and the host rock for the rich deposits at Butte and many other major ore deposits of the region. Plutons older or younger than the Butte Quartz Monzonite have less copper (fewer or no sulfide inclusions) in their biotites. Within the Butte unit itself, higher values of copper in biotite tend to be found for rocks from a broadly defined, NNE-SSW-trending belt, which parallels the long dimension of the mass and contains the Butte district in the south and the Wickes and Clancy districts in the north.

Lead and strontium isotope studies of the Boulder Batholith, southwestern Montana

Late Cretaceous batholith, quartz monzonite ore, Precambrian isotope age, partial-melting source material model; Isotope ratios of Pb in feldspar range widely from pluton to pluton: 16.9-18.1 for 206/204, 15.4-15.7 for 207/204, and 37.7-38.5 for 208/204.Whole-rock initial Sr-87/Sr-86 is 0.706-0.709. Variation in Pb isotope ratios within Butte Quartz Monzonite, plus differences in ratios between megacryst and groundmass feldspars in Donald pluton suggest incomplete isotopic mixing; similarity of Butte ore Pb to feldspar Pb of Butte Quartz Monzonite, therefore, has genetic significance. Isotope data suggest that batholith rocks result from partial melting of lower crust or upper mantle; assimilation of upper crustal material is necessary if source material resembled oceanic tholeiites.