Late-stage Quartz Research Articles

Stable isotopic and fluid inclusion indications of large-scale hydrothermal paleoflow, boiling, and fluid mixing in the Keno Hill Ag-Pb-Zn district, Yukon Territory, Canada

The Keno Hill vein system of the central Yukon is restricted predominantly to the highly fractured, graphitic Keno Hill Quartzite unit of Mississippian age. Hydrothermal mineral zoning is related spatially to a Cretaceous granitic pluton which intrudes the quartzite. During mineralization, the quartzite acted as a district-scale aquifer. Subsequent erosion has exposed a 40-km long vein system, from its plutonic roots, outward to polymetallic Ag-Pb-Zn veins, and further to assemblages of epithermal character. The δ 18O quartz values from veins near the pluton increase outwards from +10.6 to +20.1%. as a result of cooling of the hydrothermal fluids and exchange with the quartzite. Contours of isotope values outline broad paths of fluid movement within the quartzite. Proceeding further from the pluton, δ 18O quartz values decrease to +10.5%. at the outer edge of the system. The presence of meteoric water is indicated here, where late stage quartz has δ 18O SMOW values as low as −7.l%. The outward decreasing trend appears to have been established by mixing of isotopically light meteoric water with exchanged fluids that were in isotopic equilibrium with the quartzite. Fluid inclusions from quartz in the orebodies demonstrate an evolving H 2O-CO 2-NaCl-CH 4 system. Loss of CO 2 and CH 4 during water vaporization coincides with increasing salinity and decreasing temperature resulting from high enthalpy steam loss. Depressurization during active faulting is the principal mechanism. Late stage fluids are represented by dilute aqueous inclusions with lower homogenization temperatures. Quartz from silver-rich veins has been shifted to higher δ 18O values, by up to 4%. relative to adjacent silver-poor veins, the result of a minimum 10–25% adiabatic boiling and fractionation dominated by water vaporization and associated cooling. Graphite initially buffered the hydrothermal fluids to a high CO 2 content, with variable CH 4. Involvement of organic carbon from the host rocks is indicated by the negative δ 13 C PDB values for the carbonates, from −4.0 to −12.9%. Variations in the carbon isotopes result from fluctuating CO 2 CH 4 ratios, reflecting the contrasting volatility of the gas pair. Siderite formed as a late-stage product of the boiling event, and its formation coincides with a decreasing 18O trend in the water created by the equilibration of graphite and water in replacing exsolved CO 2. The formation of CH 4 during this stage had a reducing effect on the fluid, resulting in an increase in δ 13C siderite values in association with the decreasing δ 18O siderite values. A closed-system boiling model, together with calculations of water consumption during post-boiling CO 2 and CH 4 formation, indicates that greater than 50% of the original water in the ore fluid was removed. Relatively saline mineralizing fluids resulted.

Textures, deformation mechanisms, and the role of fluids in the cataclastic deformation of granitic rocks

Abstract Optical and scanning electron microscopy were used to examine the textures and infer deformation mechanisms in rocks deformed at upper crustal conditions in the Geesaman oblique-slip fault, southeastern Arizona, and the Washakie thrust fault, northwestern Wyoming. Feldspar in slightly and moderately deformed rocks in both faults deformed by formation of extension and shear fractures along cleavage planes, and quartz grains deformed by brittle fracture. Fracture density is greater in feldspar than in quartz. Intergranular fractures and faults developed by the linking of intragranular fractured feldspars, which in turn resulted in the development of foliated cataclasites in the Washakie fault and phyllonites in the Geesaman fault. Clays and mica developed by the syntectonic alteration of feldspar, and with increasing mica or clay content quartz grains deformed by fracture in a progressively softer matrix. Fracturing and healing of fractures in quartz resulted in irregularly-shaped, elongate pods of quartz in a mica matrix in the Geesaman fault and in a clay and feldspar fragment matrix in the Washakie fault. The syntectonic alteration of feldspars, the presence of iron oxides in the faults, and late-stage quartz veins attest to the flow of water during deformation. The fault zones developed by early fracture in feldspar which gave way to cataclastic flow in feldspars and subsequent development of clay-rich cataclasites at low temperature and of phyllonites at high temperatures. Most of the slip in mature cataclasites was localized by slip and cataclasis of clays. Slip in the phyllonites was localized by plastic deformation of micas. The brittle to ductile transition in the rocks was the result of fracture and subsequent cataclasitic flow in feldspars, followed by deformation of clay or mica, and for the most part excluded the plastic deformation of quartz.

Hydrothermal alteration associated with the Iron Hat iron skarn deposit, eastern Mojave Desert, San Bernardino County, California

The Iron Hat deposit, which occurs in the Marble Mountains, San Bernardino County, California, is a typical example of Mojave Desert iron skarn deposits. Three episodes of skarn formation in calcareous wall rocks adjacent to granitic intrusions have been recognized:1. Early contact skarn formation generated iron-poor garnet, pyroxene, and minor vesuvianite, with retrograde epidote, actinolite, calcite, chlorite, and late-stage vein quartz + or - specular hematite at temperatures < or = 600 degrees C; X (sub CO 2 ) varied between <0.001 and 0.1. Prograde fluids were NaCl-H 2 O-rich, with maximum salinities near 50 equiv wt percent NaCl. Some prograde contact skarn growth was associated with boiling or condensing fluids. Isotopic analyses of vein and residual calcite in contact skarn and adjacent marble can be explained by decarbonation of the marble and interaction with an infiltrating fluid in equilibrium with the pluton during skarn formation and by superimposed low-temperature exchange with evolved meteoric fluids.2. Intermediate-age skarn formed around felsic dikes and produced epidotized felsic dikes with alteration envelopes of iron-rich garnet and pyroxene, minor wollastonite, and retrograde actinolite, chlorite, calcite, quartz, and specular hematite.3. Magnesian-magnetite skarn was the last skarn formed and occurs as fault-controlled clinohumite-magnetite-serpentine bodies at contacts between dolomitic marble and granite. Fluid involved in the precipitation of magnetite skarn was enriched in CO 2 and KCl relative to that which formed contact skarn. Relatively CO 2 -rich conditions (X (sub CO 2 ) approximately 0.1) during this skarn-forming event are consistent with delta 18 O analyses of magnetite and calcite in ore zones, which suggest that these minerals precipitated from 18 O-enriched fluids. This is explicable through mixing of H 2 O-rich fluids in equilibrium with the pluton with CO 2 -rich fluids produced during devolatilization of the adjacent marble. Minimum temperatures of 375 degrees C are suggested by mineral assemblages in ore zones and by isotopic fractionation between coexisting magnetite and calcite.Two distinct alteration events are recorded in the associated pluton:1. Pervasive albitization of granite and felsic dikes occurred penecontemporaneously with contact skarn and dike skarn development but prior to iron ore deposition. Fluid inclusion and isotopic data suggest that the fluid responsible for albitization had a large magmatic component or that it isotopically equilibrated with the pluton at temperatures as high as 600 degrees C.2. Late-stage quartz + or - specular hematite alteration is found in all three types of skarn and in all igneous rocks. Temperatures of 200 degrees to 250 degrees C and NaCl-CaCl 2 -H 2 O + or - KCl fluids with salinities of 0 to 22 equiv wt percent NaCl are indicated by fluid inclusion studies. Drastic whole-rock 18 O depletions associated with this event suggest that the fluid responsible for alteration was largely meteoric in origin.

P-T-X evolution of ore veins associated with palaeogeothermal activity at Aïn Barbar (NE Konstantine, Algeria) : reconstruction from fluid inclusion data

Polymetallic (Fe-Zn-Cu-Pb) ore veins at Aïn Barbar are the result of the activity of a high energy liquid-dominated terrestrial palaeogeothermal field between 16 and 15 Ma ago (Langhian). Fluid inclusion studies yield data on both the conditions of ore deposition and the characteristics of fluid circulation within the field. Pressure was essentially hydrostatic in nature, corresponding to a depth of about one kilometre at the beginning and reduced to no more than 0.5 km towards the end due to erosional unroofing. Chalcopyrite deposition resulted from a break in fluid circulation, allowing the mixing of two kinds of fluid inside the veins ; according to a preferred interpretation, these are : — hot (330°-340 °C), "saline" (≥6 wt. % eq. NaCl) aqueous fluids (A) equilibrated with pyrrhotite, coming from the deeper parts of the aquifer, and drained along the ore veins ; — cooler (300° -320 °C) less saline (~ 1.5 wt. % eq. NaCl) fluids (B') equilibrated with pyrite, coming from the upper and external parts of the aquifer. These diluted B' fluids became progressively more abundant, and cooled down to about 250 °C ; they were responsible for the intense pyritization of earlier pyrrhotite in the veins. Further cooling, associated with a renewed increase of salinity (3-4 wt. % eq. NaCl) is recorded in late-stage quartz. Then, a new cycle of fluid circulation is characterized by : — a transient reheating of the system (in response to a recurrence of magmatic activity) up to 300°-310 °C, the salinity being unchanged ; — a new inflow of diluted fluids (~ 1 wt. % eq. NaCl), associated with a renewal of sulphide deposition. Finally, the system cooled down to 150°-100 °C, and eventually died away.

The Moeraki Boulders--Anatomy of Some Septarian Concretions

ABSTRACT The Moeraki boulders are large (to 2 m) calcite concretions with septarian veins of calcite and rare late-stage quartz and ferrous dolomite. The concretions are enclosed by Paleocene marine mudstones with vitrinite reflectance values of 0.29% R suggesting maximum burial temperatures of 25-30°C. Microprobe analyses of these concretions indicate a Ca enrichment-Mg depletion trend with growth. Calcite in the core contains about 88 mole % Ca and about 93 mole % Ca in the rim. Both rim and cores contain less than 1 mole % Fe and Mn. Septarian veins also show a systematic calcium enrichment with growth of the vein. The initial vein fills are similar in composition to the outer rim of concretionary bodies, suggesting septarian cracks form contemporaneously with attainment of final concretion size. The later-stage calcite vein-fills have less than 1.5 mole % Mg and up to 11 mole % Fe + Mn. These elements systematically decrease both into and outward along the vein, with increasing calcium substitution. The above compositional trends in veins suggest continued crystallization during propagation of the fractures. The carbonate composition trends reflect interaction between the growing concretion and the enclosing mudstone pore-fluid system. The observed Fe, Mn, Mg depletion trends probably reflect depletion of elements released by short-time-scale diagenetic events of finite size. The growth time of the larger concretions is estimated at about 4 million years based on published diffusion growth models. Extrapolation of compositional trends versus growth time from these concretion bodies suggests that septarian veins form on a time scale of several million years.

Textural and stable isotope studies of the Big Mike cupriferous volcanogenic massive sulfide deposit, Pershing County, Nevada

The massive ore contains two major generations of pyrite, a fine and a coarse grained, both of which show a striking variety of textures involving quartz. Framboidal pyrite in the argillite host rock has delta 34 S values of approximately -24 per mil indicating the presence of a euxinic environment. The delta 34 S values of fine-grained pyrite in the massive ore range from degrees 6.4 to + or -2.0 per mil; those of coarse-grained pyrite range from -3.5 to + or -5.5 per mil. A significant portion of the isotopically light sulfur for the early, fine-grained hydrothermal pyrite in the massive lens was probably derived from framboidal biogenic pyrite in interflow sediments of the underlying greenstone pillow lavas. Microcrystalline quartz in massive ore, hanging-wall jasper, footwall hydrothermal chert and coarse quartz from hanging-wall and footwall stringer zones have delta 18 O values between 15.6 and 19.6 per mil; one sample of vein hematite has a value of 4.4 per mil. The combined sulfur and oxygen isotope and textural data indicate that much of the material in the massive lens originally precipitated as fine-grained pyrite or as a precursor iron sulfide along with some silica from a hydrothermal plume similar to those recently observed at the East Pacific Rise spreading center at lat 21 degrees N. The primary material underwent recrystallization, mineralization, and late-stage quartz deposition in the presence of later fluids which had distinctly different sulfur isotope compositions.--Modified journal abstract.

Contrasting evolutions of hydrothermal alteration in quartz monzonite and quartz diorite wall rocks at the Sierrita porphyry copper deposit, Arizona

Two wall-rock types of contrasting chemical composition host the causative Ruby Star quartz monzonite porphyry intrusion and hypogene mineralization at the Sierrita porphyry copper deposit. Vein-related hydrothermal alteration in the Harris Ranch quartz monzonite and biotite quartz diorite wall rocks consists of several mineralogically discrete assemblages. Temporal evolution of different alteration assemblages was established, in part, using petrographic relations within, and crosscutting relations among, individual veins. Temperature and salinity characteristics of hydrothermal fluids responsible for filling of individual veins were determined using primary fluid inclusions in vein-filling quartz. Each generation of primary vein filling introduced characteristic secondary fluid inclusions into earlier developed veins as well. Histograms of homogenization temperatures of primary and secondary fluid inclusions from different veins, and accompanying salinity data, permitted temporal correlations to be drawn between veins which either did not exhibit crosscutting relations in individual samples or which formed in different wall rocks and thus exhibited different alteration mineralogies.Evolution of hydrothermal activity in the area sampled commenced with potassic alteration in both quartz monzonite and quartz diorite wall rocks from 10 to 12 molal (37-41 wt %) NaCl equivalent fluids, inclusions of which homogenize by halite dissolution in the approximate temperature range 300 degrees to 370 degrees C. Salinities of later fluids were in the range 2 to 3 molal (10-15 wt %) NaCl equivalent with only minor salinity variations for the remaining time span monitored by this study. Homogenization temperatures of primary fluid inclusions in veins formed from these lower salinity fluids began near 400 degrees C and increased initially to approximately 430 degrees C where boiling occurred. The pressure defined by boiling of these fluids is about 330 bars. A continuous decrease in fluid inclusion homogenization temperatures followed down to about 300 degrees C, during which time deposition of quartz and K-feldspar with accessory biotite and/or hematite occurred in new and reopened veins in the quartz monzonite wall rock. Simultaneously in the quartz diorite, a sequence of veins and adjacent alteration halos formed, each consisting of an early assemblage of potassic affinity (quartz + biotite + K-feldspar + albite) which evolved to a propylitic assemblage (quartz + epidote + chlorite) as vein filling proceeded. With continued cooling of the solution below about 300 degrees C, muscovite took the place of K-feldspar as the stable potassium-bearing mineral in the quartz monzonite. An analogue to this late-stage quartz + muscovite veining in the quartz monzonite could not be established in the quartz diorite but may consist of zeolite (stilbite) + anhydrite.The bulk of hypogene copper mineralization in both wall-rock types was associated with approximately 2 molal (10 wt %) NaCl equivalent solutions. In the quartz diorite, significant chalcopyrite deposition is associated with fluid inclusions homogenizing from 370 degrees down to about 320 degrees C and always occurs with the later stage propylitic minerals in each vein. In the quartz monzonite wall rock, chalcopyrite was deposited during the transition from potassic and into phyllic alteration. Fluid inclusion homogenization temperatures for this mineralization range from about 330 degrees down to 200 degrees C. No primary chalcopyrite was seen to occur with earlier potassic veining formed from either high- or low-salinity fluids in the quartz monzonite. A very late stage of deposition of chalcopyrite, pyrite, and minor bornite filled the center of late phyllic veins in the quartz monzonite; correlated fluid inclusions have salinities from 1 to 5 molal (5-23 wt %) NaCl equivalent and homogenization temperatures in the range 140 degrees to 160 degrees C. The different alteration and sulfide mineral assemblages interpreted to have formed simultaneously in the two wall rocks of contrasting chemical character can be reasonably assigned to different chemical interactions of each rock type with similar, or equivalent, hydrothermal fluids.

Diagenetic History of Norphlet Formation (Upper Jurassic), Rankin County, Mississippi: ABSTRACT

ABSTRACT Most of the Norphlet sandstone examined in four cores (-15,770 to -22,500 feet) is eolian in origin as shown by thick, high-angle crossbeds, bimodal texture with well-sorted laminae, and relict hematite grain coatings typical of desert sand. Norphlet sandstones average 77 percent quartz, 16 percent feldspar, and 7 percent rock fragments. Partial to total loss of halite cement from cores during coring and slabbing operations hampers the interpretation of diagenetic history. However, the inferred sequence of diagenetic events is: cementation by illite, K-feldspar, quartz, calcite, anhydrite, and halite; development of secondary porosity by dissolution of halite, plagioclase, VRF's, and carbonate cement; hydrocarbon migration and pyrite generation; dolomitization; and cementation by late-stage illite and quartz. Halite and anhydrite were derived from the underlying Louann. Pyrite is present in most samples and formed when sour gas passed through the sands and reduced hematite grain coatings to pyrite. The scarcity of quartz cement is attributed to the lack of shale beneath the Norphlet. Illite cement of two ages is present. Early-formed illite is leaflike flakes that coat quartz grains that inhibited the development of quartz overgrowths. It is not a seriously deleterious mineral. Late illite is thread-like and grows in narrow secondary pores where it seriously reduces permeability. Illite cement and sericitic alteration of plagioclase are more abundant deeper than 19,900 feet. Halite was introduced after moderate burial because prior to its development the sandstones underwent modest compaction as shown by sutured quartz grains and pre-cement porosity values of 22 to 30 percent (indicating compactional losses of 15 to 23 percent porosity before cementation). The original texture of the sands strongly influenced diagenetic events.

Evolution of porphyry copper mineralization in an oceanic island arc; Panama

Petrologic, isotopic, and radiometric studies of intrusive rocks in the Panamanian oceanic island arc indicate that magmatism and associated porphyry copper mineralization were episodic and that they exhibit systematic compositional changes. The earliest stage of igneous activity took place about 60 to 70 m.y. ago and involved emplacement of quartz diorites with tholeiitic chemical characteristics, which did not form porphyry copper mineralization. Later magmatism was calc-alkaline in character and commenced with 50 m.y. quartz diorite and K-poor granodiorite. Later events at 35 m.y. and 5 m.y. emplaced granodioritic intrusions that are more potassic than their local predecessors. Porphyry copper mineralization was most intense and most widespread during the last intrusive event.Comparison of the Panamanian results with information on other island arcs indicates that porphyry copper mineralization is essentially absent from earliest stage, tholeiitic(?) intrusions and, with few but prominent exceptions, becomes more important in successively younger stages of arc evolution. Not all arcs exhibit potassium-enrichment trends like Panama, and late-stage quartz diorite-type porphyry copper deposits can develop. Shoshonitic or alkaline porphyry copper deposits, though absent from the tholeiitic phase of arc evolution, can form at any time during the calc-alkaline stage. Gold values are high in shoshonitic and possibly in quartz diorite-related calc-alkaline porphyry copper deposits, and molybdenum appears to be most abundant in granodiorite-related calc-alkaline deposits.

V.—The Leadhills-Wanlockhead Lead and Zinc Deposits

SynopsisThe Leadhills–Wanlockhead mining field is situated within a synclinorial belt of greywackes, bounded to the north and south by anticlinoria in which lower stratigraphical elements are exposed. The universal strike is N.E.–S.W. The southern margin of the northern anticlinorium is delimited by a strike thrust fault, inclined to the north-west. The shear zone caused by this thrust is considered to be the principal structural feature governing the localization of the ore deposits.The complex N.E.–S.W. folding and faulting, imposed during the Caledonian orogeny, is crossed by a series of intersecting joints, whose average strike is N.N.W. The majority of the mineral veins also trend N.N.W. The formation of these discordant structural features under Caledonian stress is demonstrated by the presence of N.N.W. trending Caledonian dykes.Sinistrai movement along the joint pattern, ascribed to a reorientation of the “Caledonian” stress towards an “Hercynian” direction, resulted in the formation of open spaces on the more north-westerly trending members of the joint system in the greywacke belt. This feature is considered to be the secondary structural control responsible for the localization of the ore deposits.Fifty-seven minerals were identified from the deposits. Fifteen of these minerals had not been previously recorded from the locality, including (1) a new chromian mineral; (2) a new variety, chromian leadhillite; (3) a mineral previously recorded only as an artificial product, lead hydroxyapatite; (4) phœnicochroite, not previously confirmed in the British Isles.Two periods of mineralization were distinguished. The first consisted of quartz veins with which are associated small amounts of gold, pyrite and muscovite, tentatively assigned to the Caledonian orogeny, and the second comprised the lead-zinc mineralization. The paragenetic relationships of the primary minerals of the lead-zinc mineralization indicate two generations of sulphides; the second generation is accounted for by reprecipitation of elements derived from the replacement of the first generation by late stage quartz. A study of the distribution of elements through the paragenesis suggests that some elements were derived from other than a magmatic source, and that contamination has probably played a considerable rôle in the control of the character of the gangue minerals.Evidence that the mineralizing solutions had a deep-seated origin is provided by the mineral zones and the geochemical character of the deposit. Emplacement of the minerals took place at a temperature of the order of 143°−281° C, and a depth of the order of 2000–4000 feet below the surface.The Leadhills–Wanlockhead deposits are related to other lead-zinc deposits in Britain. On the basis of geochemical assemblage and the relation to igneous activity, it is concluded that the deposits were probably derived from the top of the tholeiitic crustal layer and the base of the granitic crustal layer, and were genetically associated with the Hercynian orogeny.