Fluid inclusion evidence for the formation of main stage polymetallic base-metal veins, Butte, Montana, USA

The Butte porphyry Cu-Mo deposit is cut by the Butte Main Stage, a system of veins that constitute one of the world’s largest Cordilleran-style base-metal lode deposits. The vein system is zoned from a central Cu-rich zone containing covellite, chalcocite, digenite, and enargite to an intermediate zone containing both Cu and Zn sulfides, to a peripheral zone dominated by sphalerite, galena, and rhodochrosite.\ud \ud We examined fluid inclusions in ~50 veins from throughout the lateral and vertical extent of the deposit and conducted microthermometry on 13 of these samples. Fluid inclusions in Main Stage veins are similar in appearance throughout the central, intermediate, and peripheral zones such that only one type of fluid inclusion dominates all samples observed. At room temperature the fluid inclusions are liquid-rich, with 20 volume % bubble (B20 inclusions). Most inclusions analyzed contain between 1 and 4 wt. % NaCl equivalent, and between 0.2 and 1 mol % CO2 . Most inclusions homogenize to liquid between 250°C and 300°C. Even though there is considerable overlap, there is a weak trend from higher to lower homogenization temperatures and salinities from the central zone to the peripheral zone.\ud \ud Vapor-rich inclusions are rare and were identified in only one Main Stage vein, thus, we infer that nearly all inclusions were trapped in the liquid field at pressures\ud above the boiling curve. Maximum estimated depth of formation for Main Stage veins is 6 km. At such pressures, an isochoric temperature adjustment of up to about 50°C is\ud required, indicating that most Main Stage veins formed at temperatures between about 250°C and 350°C. \ud \ud We suggest that Main Stage veins formed where single-phase B60 fluids, which formed pre-Main Stage pyrite-quartz veins with sericitic alteration, decompressed and mixed with meteoric water in a hydrostatic pressure regime. Pre-Main Stage brines were not likely involved in Main Stage vein formation, and the role of pre-Main Stage vapor in the formation of Main Stage veins is not known

Improved electron probe microanalysis of trace elements in quartz

Quartz occurs in a wide range of geologic environments throughout the Earth's crust. The concentration and distribution of trace elements in quartz provide information such as temperature and other physical conditions of fomation. Trace element analySes with modern electron-probe microanalysis (EPMA) instruments can achieve 99% confidence detection of - 100 ppm with fairly minimal effort for many elements in samples of low to moderate aVerage atomic number such as mauy common oxides and silicates. HoweVer, trace element measurements below 100 ppm in many materials are limited, not ouly by the precision of the background measurement, but also by the accuracy with which background levels are determined. A new "blank" correction algorithm has been developed and tested on both Cameca and JEOL instruments, which applies a quantitative correction to the emitted X-ray intensities duriug the iteration of the sample matrix correction based on a zero level (or known trace) abundance calibration standard. This iterated blank correction, when combined with improved background fit models, and an "aggregate'' intensity calculation utilizing multiple spectrometer intensities in software for greater geometric efficiency, yields a detection limit of 2 to 3 ppm for Ti and 6 to 7 ppm for AI in quartz at 99% t-test confidence with similar levels for absolute accuracy

Trace elements in hydrothermal quartz: relationships to cathodoluminescent textures and insights into vein formation

High-resolution electron microprobe maps show the distribution of Ti, Al, Ca, K, and Fe among quartz growth zones revealed by scanning electron microscope-cathodoluminescence (SEM-CL) from 12 hydrothermal ore deposits formed between ~100 and ~750 °C. The maps clearly show the relationships between trace elements and CL intensity in quartz. Among all samples, no single trace element consistently correlates with variations in CL intensity. However in vein quartz from five porphyry-Cu (Mo-Au) deposits, CL intensity always correlates positively with Ti concentrations, suggesting that Ti is a CL activator in quartz formed at >400 °C. Ti concentrations in most rutile-bearing vein quartz from porphyry copper deposits indicate reasonable formation temperatures of <750 °C using the TitaniQ geothermometer. Titanium concentrations of <10 ppm in all veins that formed at temperatures <350 °C suggest a broad correlation between Ti concentrations and temperature of quartz precipitation. In quartz from most deposits formed at 2000 ppm, but in high-temperature quartz, Al concentrations are consistently in the range of several hundred ppm. Aluminum concentrations in quartz reflect the Al solubility in hydrothermal fluids, which is strongly dependent on pH. Aluminum concentrations in quartz therefore reflect fluctuations in pH that may drive metal-sulfide precipitation in hydrothermal systems

Paragenesis of the Paroo Fault, Mount Isa, Queensland, Australia

The sigmoidal 23km long Paroo Fault forms the footwall to the Mount Isa copper orebodies which lie within a silica and dolomite halo. Orebodies are predominantly located directly above the fault in the Proterozoic (c. 1650 Ma) Urquhart Shale which the fault juxtaposed against the\ud metabasalts and psammites of the Haslingden Group (c.1750 Ma), though some are "perched" tens to hundreds of meters above the fault. The Paroo Fault is a probable conduit for ore-forming fluids but the paragenesis of the fault rocks has never been studied. Here we present results of\ud transects across it from the basement to the overlying shales using petrography, microstructural analysis and cathodoluminescence (SEM-CL).\ud The first generation of quartz (Q1) is brightly luminescent and consists of angular to sub-rounded clasts (10-100μm), formed during early faulting. A subsequent deformation caused fracturing and rounding of Q1 clasts. Then a weekly luminescent generation of quartz (Q2) formed angular to sub\ud rounded clasts around Q1. Cross cutting Q2 are fine (10-30μm) veins of moderately luminescent quartz (Q3) containing sulphide. Coarse carbonate (C1) and some sulphide mineralisation formed (in the fault) and were subsequently deformed. Q4 is luminescent quartz which forms large veins (300-1000μm) and is cross cut by the final stage of quartz (Q5) comprising small (1-5μm) brightly\ud luminescent veins. C2 carbonates form along the edge of the large recrystalised quartz (Q1-5) grain boundaries in association with microfracturing and grain size reduction. C2 is associated with chlorite and carbonaceous material and is prevalent at the edges of the Paroo Fault.\ud Reverse movement is recognized on the Paroo fault from macro-scale observations and from fluid inclusion plane (FIP) analysis. Normal movement is recognized from gouge marks on graphitic mylonite and quartz fish, drag structures, and the FIPs. Normal postdates reverse movement as the lineations are best preserved. Although the reverse movement determined from FIPs postdates Q1-3, this movement is consistent with the flat part of the Paroo Fault acting as a dilatant bend. We therefore concluded that the quartz (Q3) and carbonate (C1) veins which are associated with sulphide mineralisation may have been caused by reverse movement creating a dilatational jog during D3

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.\ud \ud 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.\ud \ud 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.\ud \ud 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.\ud \ud 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.\ud \ud 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.\ud \ud 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

District-scale fluid evolution in the Main Stage veins at Butte, Montana

To constrain the fluid evolution in the polymetallic Main Stage (MS) veins at Butte, USA, we present microthermometric measurements of fluid inclusions assemblages (FIAs) in co-genetic ore and gangue minerals through the different mineralization stages. Early quartz-hosted FIAs indicate the highest homogenization temperatures (Th, between 290° and 320 °C) with salinity similar to the intermediate density FI of the pre-Main Stage (4 to 6 wt. % NaCl equiv). During the Cu-rich mineralization stage, a drop in salinity to 2 wt.% NaCl equiv. at temperature ~280°C, precedes locally developed phase separation, witnessed by coexisting vapor-rich and liquid-rich FI with similar Th but with higher salinity (2.5 to 7 wt.% NaCl equiv.). Primary FIAs in sphalerite and rhodochrosite from the late mineralization stage indicate fluid temperature (Th~210°C) and salinity (1.7 to 4 wt.% NaCl equiv.) decrease through time, towards the periphery of the district. This study suggests that mixing of a high-temperature fluid with salinity similar to fluids from the pre-Main Stage quartz-pyrite veins with cooler and lower salinity fluids (potentially of meteoric origin) combined with local boiling processes are responsible of the ore precipitation in the MS veins

Orientations of fluid inclusion planes in the Paroo fault and their relation to macro-scale structures at Mount Isa, northwest Queensland

The Paroo Fault is a prospective conduit for the oreforming fluids that created the Mt Isa copper deposit, but its kinematic history and relationship to copper mineralisation are unclear. Fluid inclusion planes in quartz from the fault zone occur in two prominent sets; one set dips more steeply, and the other set less steeply than the fault zone. They may indicate reverse and normal movements, which can potentially be timed with respect to ore-forming fluids. Different rock types within the fault and the different orientations of fluid inclusion planes are evidence of a complex history on the Paroo Fault, which may be the key to copper mineralisation

In situ quantitative analysis of individual H2O–CO2 fluid inclusions by laser Raman spectroscopy

Raman spectral parameters for the Raman ν1 (1285 cm−1) and 2ν2 (1388 cm−1) bands for CO2 and for the O–H stretching vibration band of H2O (3600 cm−1) were determined in H2O–CO2 fluid inclusions. Synthetic fluid inclusions containing 2.5 to 50 mol% CO2 were analyzed at temperatures equal to or greater than the homogenization temperature. The results were used to develop an empirical relationship between composition and Raman spectral parameters. The linear peak intensity ratio ( IR=ICO2 / ( ICO2+IH2O)) is related to the CO2 concentration in the inclusion according to the relation: Mole% CO2 ¼ e−3:959 IR2 þ8:0734 IR where ICO2 is the intensity of the 1388 cm−1 peak and IH2O is the intensity of the 3600 cm−1 peak. The relationship between linear peak intensity and composition was established at 350 °C for compositions ranging from 2.5 to 50 mol% CO2. The CO2–H2O linear peak intensity ratio ( IR) varies with temperature and the relationship between composition and IR is strictly valid only if the inclusions are analyzed at 350 °C. The peak area ratio is defined as AR=ACO2/(ACO2+AH2O), where ACO2 is the integrated area under the 1388 cm−1 peak and AH2O is the integrated area under the 3600 cm−1 peak. The relationship between peak area ratio (AR) and the CO2 concentration in the inclusions is given as: Mole% CO2 ¼ 312:5 AR The equation relating peak area ratio and composition is valid up to 25 mol% CO2 and from 300 to 450 °C. The relationship between linear peak intensity ratio and composition should be used for inclusions containing ≤50 mol% CO2 and which can be analyzed at 350 °C. The relationship between composition and peak area ratios should be used when analyzing inclusions at temperatures less than or greater than 350 °C (300–450) but can only be used for compositions ≤25 mol% CO2. Note that this latter relationship has a somewhat larger standard deviation compared to the intensity ratio relationship. Calibration relationships employing peak areas for both members of the Fermi diad (ν1 at 1285 cm−1 and 2ν2 at 1388 cm−1) were slightly poorer than those using only the 2ν2 (1388 cm−1) member owing to interference from quartz peak at approximately 1160 cm−1

Orientations of fluid inclusion planes in the Paroo fault\ud and their relation to macro-scale structures at Mount Isa, northwest Queensland

The Paroo Fault is a prospective conduit for the oreforming\ud fluids that created the Mt Isa copper deposit, but its kinematic history and relationship to copper mineralisation are unclear. Fluid inclusion planes in quartz from the fault zone occur in two prominent sets; one set dips more steeply, and the other set less steeply than the fault zone. They may indicate reverse and normal movements, which can potentially be timed with respect to ore-forming fluids. Different rock types within the fault and the different orientations of fluid inclusion planes are evidence of a complex history on the Paroo Fault, which may be the key to copper mineralisation

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\ud (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\ud 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\ud 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\ud 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