exercise 1: envi spectral libraries - ige.unicamp.br exercises_unicamp... · zona de alteração...
TRANSCRIPT
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Carlos Roberto De Souza Filho1
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University of Campinas Institute of Geosciences
Department of Geology and Natural Resources
PO Box 6152 13083-970
Ph: 55-19-3788-4535
Email: [email protected]
Exercise 1: ENVI Spectral Libraries There are a variety of software packages for processing field/laboratory spectral data, including:
TSG (the Spectral Geologist) though CSIRO and AUSspec,
PIMAVIEW though Integrated Spectronics Pty Ltd; and
ENVI (Environment for Visualising Images) through KODAK RSI (Research Systems
Inc).
There are also a variety of accessible spectral libraries that can be used as a reference to help
interpret unknown sample spectra, including:
USGS;
NASA JPL;
The Johns Hopkins University;
University of Arizona; and
Specmin through Spectral International.
The first three of these libraries are available in the ENVI software package, which is what we
will be using to help interpret spectra of natural geological samples (Exercise 2).
TASK:
Open up ENVI to reveal the pull down menu.
>Spectral>Spectral Libraries>Spectral Library Viewer to show the:
Spectral Library Input File window
>Open Spec Lib>usgs_min>usgs_min.sli
Press OK to reveal a list of minerals. Left mouse click on any mineral name to open up a
spectrum (0.45 to 2.5 microns or 450 to 2500 nm). Left mouse click inside the spectral plot to
show a vertical line with channel number, wavelength position and reflectance level given at the
bottom left. Multiple minerals can be presented. Become familiar with both narrow and broad
absorption bands for a selection if important alteration minerals discussed in the associated PPT
presentation. Also explore other ENVI functionality using the pull down menus shown at the
top of the Spectral Library plot window including,
Wavelength zoom >Edit>Plot Parameters or control + left mouse click point drag
and drop
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Continuum removed spectra
Plot key (right mouse click in window) drag and drop to a new window
Using the USGS library spectra and the Figures 1 and 2, what are the colours and wavelength
peaks of:
COLOUR WAVELENGTH
Azurite……………….…………………………..………………
Hematite……………………….……………….………………..
Goethite……………………………….………..………………..
Magnetite………………………….…………..…………………
Malachite…………………………….…………………..………
Quartz ………………………………………………………..
Figure 1. Colour and wavelength Figure 2. Additive colour wheel
Exercise 2 – Spectral mineralogy and Alteration Zonation
The spectra provided below are from the hand samples on display. Interpret the mineralogy of
these samples using the USGS library:
BO_11B………………………………………………………….
T-7………………………………………………………….
PYROP………………………………………………………….
SER………………………………………………………….
BO_15…………………………………………………………..
BO 17A………………………………………………………….
BO 18A………………………………………………………….
CALC………………………………………………………….
red
blue
green
magenta
yellow
white
cyan
black
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The figure below stacks five of these samples in order of their location along a transect. What
can be said about the alteration facies shown by these minerals, including alteration zonation
(annotate of the figure) and likely style of mineralisation (see Appendix 1)?
………………………………………………………………………………………………………
………………………………………………………………………………………………………
………………………………………………………………………………………………………
………………………………………………………………………………………………………
………………………………………………………………………………………………………
………………………………………………………………………………………………………
Where would you locate the samples P, C and BO-11b along this transect: Show on above
figure.
………………………………………………………………………………………………………
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………………………………………………………………………………………………………
………………………………………………………………………………………………………
Which direction towards potential mineralisation (up or down)?
………………………………………………………………………………………………………
Exercise 3 : ENVI Mineral Alteration Facies
(I) montagem de bibliotecas espectrais para uso no estudo de:
mineralizações de ouro em greenstone belts Arqueanos
1. composição de rochas não alteradas hidrotermalmente: metamorfismo regional
- TALCO, SERPENTINA: rochas ultramáficas
- ACTINOLITA ou hornblenda clorita: rochas máficas
- sericita (paragonitica): rochas félsicas e alguns sedimentos
- kaolinita, montmorilonita, nontronita (maf – Fe-smectita), saponita (ultram – Mg-smectita):
produtos do intemperismo
2. Zona de alteração hidrotermal (mais externa) – ZONA DA CLORITA
- talco: rochas ultramáficas
- CLORITA, carbonato (calcita ankerita): rochas máficas e alguns sedimentos
- sericita: rochas félsicas e alguns sedimentos
3. Zona de alteração hidrotermal – ZONA DO CARBONATO
- FE-CARBONATO (siderita e ankerita)
- chlorita ()
- sericita ()
4. Zona de alteração hidrotermal (mais interna) – ZONA DA MUSCOVITA-PIRITA
- SERICITA (fengita)/MUSCOVITA
- biotita ()
- ankerita/siderita ()
- pirita
(II) montagem de bibliotecas espectrais para uso no estudo de:
mineralizações de cobre-porfiro em ambientes de cordilheira
- zonas de alteração clássicas:
(1) propilitica (mais externa)
- EPIDOTO, CLORITA, carbonato, actinolita – IMPORTANTES
- quartzo, zeolita, feldspato – não apresentam feições diagnósticas
- magnetita e sulfetos –mascaram feições de absorção no espectro.
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(2) argílica
- ILITA-ESMECTITA, ESMECTITA, , ILITA, kaolinita, clorita, carbonato -
IMPORTANTES
- quartzo– não apresenta feições diagnósticas
(3) argílica avançada
- ALUNITA, kaolinita, dickita, diásporo, pirofilita, mica – IMPORTANTES
- quartzo
(4) fílica (phyllic)
- ILITA 2M (muito cristalina; ‘quase’ sericita)
- sericita/mica; carbonato; clorita,
- quartz, feldspato
- sulfetos
(5) potássica
- BIOTITA
- K-feldspato
- magnetita
(Outros)
- GYPSO
- barita
- anidrita
‘TASKS’
(A) Construa uma biblioteca espectral, com base naquela do USGS, para cada associação,
salvando-as no disco rígido, num diretório criado previamente. Analise os espectros de
reflectância entre os minerais da associação e discuta qual a possibilidade de separação entre os
mesmos. Tente, finalmente, avaliar a separabilidade espectral entre as várias zonas de alteração
hidrotermal.
(A) TESTE DE MISTURAS ESPECTRAIS
- Faça uma análise, utilizando misturas representativas para o modelo de deposito,
sobre o aumento e/ou diminuição de contraste de algumas feições típicas de minerais
puros.
- Spectral => Spectral Math => enter an expression => s1+s2+s3+ ... Sn
Obs: Utilize recursos do ENVI para fazer essa análise, como por exemplo, a remoção do
contínuo, estaqueamento entre os espectros e etc.
(B) Re-amostre as bibliotecas que você constituiu para as resoluções do ASTER.
Para faze-lo:
1. Spectral => Spectral Libraries => Spectral Library Viewer
- Spectral library input file (carregue as bibliotecas que
voce montou anteriormente – uma de cada vez)
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2. Spectral => Spectral Libraries => Spectral Library Resampling
- Input file (use a biblioteca carregada)
- Spectral Resampling parameters
o user definied filter function
o open spectral library
o ASTER …. FILTROS
3. Spectral => Spectral Libraries => Spectral Library Viewer
- Spectral library input file (carregue o arquivo da
biblioteca reamostrada que voce acabou de criar)
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Exercise 4 – Mesothermal Alteration Mineralogy
The aim of this exercise is to spectrally map alteration mineralogy and zonation in a fresh rock
drill core taken from a mesothermal gold deposit in greenstone rocks.
Figure 6 presents a stacked profile of PIMA spectra taken from a diamond drill core (Figures 7
provides a selected wavelength expansions). Note that this drill core intersected only one rock
type that was variably altered during hydrothermal alteration
Using your Spectral Libraries, answer the following questions.
(a) What minerals or mineral groups are evident? (Note that there may exist mixtures of two
or minerals). Annotate your interpretation on one of the figures.
.....................................................……......................................................................................
...................................................……........................................................................................
(b) What evidence (if any) is there for the oxidation state of the iron and the type of water?
..............................................……............................................................................................
...........................................……...............................................................................................
...........................................……...............................................................................................
(c) Is there evidence for the mineral cation composition? Explain.
.................................................……........................................................................................
......................................................……....................................................................................
............................................................…….............................................................................
(d) What was the likely composition of the host rock (give reasons)?
..................................................................…….......................................................................
........................................................................…….................................................................
.............................................................................…….............................................................
(e) Account for the alteration mineralogy and identify the most “prospective” samples.
...............................................................……..........................................................................
.....................................................................……....................................................................
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Figure 6 : Stacked PIMA reflectance spectra from a diamond drill core from the Eastern
Goldfields.
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Figure 7: The same spectra as Figure 6 though displayed from 2150 to 2500 nm.
Exercise 5 - Regolith Mapping
This exercise examines VNIR and SWIR spectral variations through a lateritic profile. A
schematic lateritic cross-section (Figure 9) shows the types of physicochemical changes that may
be expected to be evident in the spectral data.
A stack plot of IRIS VNIR reflectance spectra (Figure 10) spans samples from the top of the
lateritic duricrust down to the mottled zone. Broad "electronic" absorptions in the VNIR spectra
show variations in iron-related mineralogy.
(a) What is the dominant iron oxide mineralogy in the lateritic duricrust and mottled zone.
Apart from the feature at approximately 900 nm (0.9 µm), what other features appear to change
with the iron oxide composition (excluding the features at 1400 and 1900 nm).
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.........................................................................................................……..................................
...................................................................................................……........................................
.............................................................................................……..............................................
.......................................................................................……....................................................
Figures 11 to 14 present various zoom sections of stacked PIMA spectra measured from a
complete laterite profile similar to that shown in Figure 9.
(b) What minerals are evident in the PIMA spectra and how did you identify these?
...............................................................................................................……...........................
.....................................................................................................................…….....................
...........................................................................................................................……...............
.................................................................................................................................…….........
.......................................................................................................................................……...
..........................................................................................................................................……
..............................................................................................……............................................
(c) Identify the primary and weathering minerals?
PRIMARY:...................................................................................................……......................
WEATHERING:.................................................................................................……...............
...................................................................................................................................……........
(d) What is the nature of the parent rock?
............................................................................................……...............................................
(e) What information is there about water, iron oxides, cation substitution and clay
crystallinity?
..................................................................................................…….........................................
........................................................................................................……...................................
..............................................................................................................…….............................
....................................................................................................................…….......................
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..........................................................................................................................…….................
................................................................................................................................……...........
......................................................................................................................................…….....
(e) Annotate the various lateritic units (lateritic duricrust, mottled zone, saprolite and fresh
rock) on Figure 11. What primary mineral coexists with the weathered minerals and
explain the significance?
..............................................................................................……............................................
....................................................................................................……......................................
..........................................................................................................…….................................
................................................................................................................……...........................
Figure 9: Schematic lateritic profile. The degree of iron oxide development is dependent on the
parent rock composition with granites generally producing less iron oxide than mafic and
ultramafic rocks.
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Figure 10: Stacked IRIS reflectance spectra taken from samples collected down a lateritic
profile.
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Exercise 6 : ENVI ASTER Processing - Ratios
Coverage:
Image display and linking
Z-profiling
Contrast stretching
2-D scattergrams
Log residuals
Band maths and ratios
Masking
Open up the Cuprite ASTER reflectance image:
>File>Open Image File>*****
From the available bands list select RGB and three bands of your choosing. Three image
windows appear, including a Zoom, Scroll and (full resolution) Image.
Open a Z-spectral (spectral bands) profile for the Image pull down menu:
>tools>profiles>z-profile
Move the curser position around the image to check for changes in spectral shape.
Contrast enhance the image from the Image Pull down menu:
>enhance>[image] Linear 2%
Open a second image window (New Display) in “Gray Scale” from the available bands selecting
any single band.
Apply a colour density slice to the gray scale image using the Image Window pull down menu:
>Tools>Colour Mapping>ENVI Colour Tables. Select Rainbow.
Link the two image windows using the any one of the Image Windows pull down menu:
>tools>link>link display
Right mouse click inside the image window to reveal the second image.
Contrast stretch the single band image using:
>enhance>interactive stretching.
Experiment with changing the threshold values.
Generate band math or ratio ASTER products for the following :
3/2 : green vegetation
4/3 : iron oxide abundance
5/4 : ferric/ferrous iron (in silicate/carbonate) ratio
(6+8)/7 for chlorite, epidote
(5+7)/6 : Al-OH abundance
7/5 with mask of (5+7)/6 : Al-OH type (Group 1: alunite, pyrophyllite, kaolinite,
dickite); Group 2: muscovite; Group 3: phengite)
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(6+9)/(7+8) : Mg-OH + carbonate abundance
6/5 or 7/5 : pyrophyllite, alunite, kaolinite
11/(10+12), 11/10, 13/12 and 13/10 : SiO2 abundance
13/14 : carbonate abundance
7/8 mask with 13/14 for calcite vs dolomite
12/13 : “basic” minerals (garnet, CPX, epidote, chlorite)
To generate simple 2 band ratios:
>transform>band ratios
To generate multi-band bands ratios or “continuum” band ratios:
>basic tools>band math>enter and expression
To preserve sufficient dynamic range write:
Float(b1+b2)/float(b3)
Insert selected bands (recommend ASTER Bands V1: 5 V2: 7 V3: 6 for ALOH depth
calculation) into each available variable for a new image window.
Link image windows using ether image pull down menus:
>tools>link>link display
Apply density colour slice to the ratio products using image pull down menu:
>tools>colour mapping>ENVI colour tables>rainbow
To contrast stretch this product using the image window pull down menus:
>enhance>interactive stretching
Establish interactively suitable minimum and maximum threshold values. From Interactive
Stretch Window:
>Options>histogram parameters insert optimum min and max>apply
To generate a masked product which can remove those pixels that complicate the information
from a desired end product (e.g. green vegetation from AlOH depth), main ENVI menu:
>basic tools>mask>mask build>display (the image to build the mask)
From the Mask Definition window:
>Options>Import data range>Band Min Value (XXX determined from Interactive
histogram threshold assessment). Save file to disk (label with *_mask)
From the main ENVI window:
>basic tools>mask>mask apply Insert both image file and mask file accordingly and save
file (labelled accordingly).
Can coherent and interpretable alteration zonation be recognised from these images. These will
be compared with the Hourglass products generated later on so keep a good file naming
convention for these images.
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Appendix 1: Alteration Mineralogy. Summary of assemblages of alteration minerals, commonly used terminology, and the environment of formation.
Most of the key minerals in this table are dealt with individually in the Atlas; some minerals have multiple entries
because their characteristics change in different environments (from Atlas of Alteration, Editors AJB Thompson and
JFH Thompson, Special Publication, Geological Association of Canada, 119 pages, 1996).
Mineral Assemblage (Key minerals are in bold)
Standard
Terminology
Environment of Formation
Intrusion-related
biotite (phlogopite), K-feldspar
(orthoclase), magnetite, quartz,
anhydrite, albite-sodic
plagioclase, actinolite, rutile,
apatite, sericite, chlorite, epidote
potassic (biotite-
rich), K-silicate,
biotitic
Generally found in the core of porphyry deposits,
particularly those hosted by more mafic intrusions
(diorite, monzonite, granodiorite), or mafic to
intermediate volcanic/volcaniclastic wallrocks. May
form a large peripheral alteration zone in wallrocks
(without K-feldspar) that zones out to propylitic
alteration.
K-feldspar (orthoclase or
microcline), quartz, albite,
muscovite, anhydrite, epidote
potassic, K-silicate Found in the core of porphyry systems, particularly
hosted by felsic intrusions (granodiorite – quartz
monzonite, granite, syenite).
albite (sodic plagioclase),
actinolite, clinopyroxene
(diopside), quartz, magnetite,
titanite, chlorite, epidote,
scapolite
sodic, sodic-calcic Occurs with minor mineralisation in the deeper
(peripheral in some cases) parts of some porphyry
systems and is a host to mineralization in porphyry
deposits associated with alkaline intrusions.
sericite (muscovite-illite),
quartz, pyrite, chlorite, hematite,
anhydrite
phyllic, sericitic Commonly forms a peripheral halo around the core
of porphyry deposits; it may overprint earlier
potassic alteration and may host substantial
mineralization.
sericite (illite-smectite),
chlorite, kaolinite (dickite),
montmorillonite, calcite,
epidote, pyrite
intermediate
argillic, sericite-
chlorite-clay
(SCC), argillic
Generally forms a structurally controlled to
widespread overprint on other types of alteration
(potassic) in many porphyry systems; precursor
textures are usually preserved. Argillic is often used
for texturally destructive alteration that has a similar
clay-rich mineralogy, and which occurs in and
around structures in the upper parts of porphyry
systems.
pyrophyllite, quartz, sericite,
andalusite, diaspore, corundum,
alunite, topaz, tourmaline,
dumortierite, pyrite, hematite
advanced argillic Intense alteration, often in the upper part of porphyry
systems, but also form envelopes around pyrite-rich
veins that cross-cut other alteration types.
topaz, muscovite, quartz,
tourmaline
greisen Localized high-temperature alteration associated
with peraluminous granites and related
mineralization.
garnet, clinopyroxene,
wollastonite, actinolite-
tremolite, vesuvianite, epidote
calcic skarn Generally forms replacement zones in wallrocks
(exoskarn – typically in limestone or occasionally
mafic to intermediate volcanic rocks), or within
intrusions (endoskarn). Andradite and diopside occur
in oxidized assemblages related to porphyry Cu
systems; grossular and hedenbergite are more
common in reduced skarns (Au, W, and Sn).
forsterite-diopside or
serpentinite-talc, calcite,
magnetite, tremolite
magnesium skarn Magnesium skarns are developed as metasomatic
replacements of dolomitic limestone. High-
temperature magnesium skarns are characterized by
forsterite and diopside and low-temperature
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magnesium skarns contain serpentinite and talc, both
of which occur as retrograde minerals after forsterite
and clinopyroxene.
calcite, chlorite, hematite, illite-
smectite, montmorillonite-
nontronite, pyrite
retrograde skarn Commonly replaces earlier skarn alteration but may
also affect adjacent wallrock – limestone.
chlorite, epidote, albite,
calcite, actinolite, sericite, clay,
pyrite
propylitic Commonly forms the outermost alteration zone at
intermediate to deep levels in porphyry systems. In
some systems, propylitic alteration is mineralogically
zoned from inner actinolite-rich to outer epidote-rich
alteration.
Intrusion-related – High-sulphidation Epithermal
quartz, rutile, alunite, native
sulphur, barite, hematite, pyrite,
jarosite
vuggy silica,
vuggy quartz
Typically occurs in structural zones or as
replacement bodies in permeable lithologies, usually
in the core of zones of advanced argillic alteration.
This extreme form of leaching can occur in the upper
parts of porphyry systems (telescoped) but is more
common at higher (epithermal) levels.
quartz, chalcedony, alunite,
barite, pyrite, hematite
silicic Represents the addition of silica to the rock, resulting
in replacement or, more commonly, the fill to vugs
created during intense leaching. Silicification is
common in high-sulphidation systems at porphyry to
epithermal depths. It is sometimes confused with
intense quartz stockwork veining at the top of some
porphyry deposits.
quartz, kaolinite/dickite,
alunite, diaspore, pyrophyllite,
rutile, zunyite, alumino
phosphate-sulphates, native
sulphur, pyrite, hematite
advanced argillic –
acid sulphate
Forms widespread zones in the upper parts of some
porphyry systems (lithocap); also as more restricted
alteration haloes around high-sulphidation
epithermal deposits.
kaolinite/dickite,
montmorillonite, illite-
smectite, quartz, pyrite
argillic,
intermediate
argillic
May be present as a zone of alteration between
advanced argillic and propylitic alteration,
particularly in the high-sulphidation epithermal
setting.
calcite, chlorite, epidote,
albite, sericite, clay, pyrite
propylitic May occur as an outer regionally extensive alteration
zone in systems at moderate depths (>500m).
Low-sulphidation Epithermal – Geothermal
quartz, chalcedony, opal, pyrite, hematite
silicic Pervasive replacement of the rock by silica minerals.
Occurs in some epithermal and geothermal systems
as wallrock alteration around fractures and veins or
within permeable zones, usually at relatively shallow
levels. Also forms blanket-like zones of replacement
at the water table below steam-heated advanced
argillic alteration. Stratiform replacement
silicification may be mistaken for sinter.
orthoclase (?adularia”),
quartz, sericite-illite, pyrite
adularia” Varies from wallrock alteration around veins,
fractures and permeable zones to selective
replacement of plagioclase in alteration envelopes.
Common at shallow to intermediate depths in
epithermal or geothermal systems; may be associated
with boiling. Pervasive replacement by “adularia” is
difficult to distinguish from silicification.
-sericite (muscovite), illite-
smectite, montmorillonite,
kaolinite, quartz, calcite,
sericitic, argillic Occurs as wallrock alteration around veins and
replacement zones in permeable lithologies. May
exhibit progression from sericite to mixed layer clays
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dolomite, pyrite with increasing distance from mineralized (upflow)
zones. Blanket-like carbonate-bearing alteration
zones in the upper part of some
geothermal/epithermal systems may reflect the
condensation of gases (CO2) from deeper boiling
zones. Carbonate may also be important in some
deeper base metal-rich systems.
kaolinite, alunite, cristobalite (opal, chalcedony), native
sulphur, jarosite, pyrite
advanced argillic –
acid-sulphate
Forms extensive areas of alteration above the water
(paleowater) table related to the condensation and
oxidation of gases (H2S). Associated with mud pools,
fumaroles and deposits of native sulphur.
quartz, calcite silica-carbonate Replacement of ultramafic rocks in the shallow parts
(low temperature) or geothermal systems.
calcite, epidote, wairakite, chlorite, albite, illite-smectite,
montmorillonite, pyrite
propylitic, zeolitic
alteration
Regionally extensive alteration around epithermal
and geothermal systems. Mineralogical changes from
zeolite-rich to propylitic assemblages reflect
increasing depth and temperature. The concentration
of CO2 also influences the stability of zeolites and
the relative importance of calcite versus epidote.
Mesothermal
calcite, ankerite, dolomite,
quartz, muscovite (Cr-/V-rich),
chlorite, pyrite, pyrrhotite
carbonate Wallrock alteration in and around veins or shear
zones, and extensive replacement of ultramafic to
mafic rocks. Carbonate-rich alteration may be
regionally extensive and is not always related to
mineralization.
chlorite, muscovite, quartz,
actinolite, pyrite, pyrrhotite
chloritic Wallrock alteration in and around veins and shear
zones, particularly in mafic volcanic and
volcaniclastic sedimentary rocks.
biotite, chlorite, quartz, pyrite,
pyrrhotite
biotitic Wallrock alteration in and around veins or shear
zones, particularly in sedimentary rocks.
Sediment-hosted gold
quartz, pyrite, hematite jasperoid Complete replacement of limestone, and
occasionally other rock types, by fine-grained quartz;
often associated with brecciation. Jasperoids can
form as regionally extensive zones, as small bodies
related to sediment-hosted Au deposits (‘Carlin-
type’), and as the upper or outer alteration zones
associated with intrusion-related skarn/sulphide
replacement bodies. Depth of formation is probably
moderate (>2km – ‘mesothermal’), although
shallower zones of jasperoid may form; fluids may
be metamorphogenic (classic mesothermal), connate,
or magmatic.
Volcanogenic massive sulphide
sericite, quartz, pyrite, chlorite,
andalusite, chloritoid
sericitic Pervasive replacement of rocks in the footwall below
massive sulphide lenses; concentrated in stockwork
feeder zones but may be laterally extensive both
deeper in the footwall and extending into the hanging
wall in some deposits. Most common in intermediate
to felsic volcanic rocks but may also replace the
more mafic units in lower temperature systems.
Andalusite and chloritoid occur in metamorphosed
alteration zones.
chlorite, quartz, sericite, pyrite, chloritic Pervasive replacement of rocks in the footwall below
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cordierite, biotite massive sulphide deposits. Fe-rich chlorite occurs in
the core of stockwork feeder zones in mafic footwall
sequences (e.g. Archaean deposits) whereas Mg-rich
chlorite has a more erratic distribution, generally
around the periphery or upper parts of stockwork
zones. Cordierite ± biotite is common in
metamorphosed Mg-Fe-rich alteration zones.
quartz, pyrite, sericite, K-
feldspar
silicic Pervasive replacement of rocks in the footwall below
massive sulphide deposits; particularly common in
permeable siliceous ash-rich beds, where the
silicified rock may be mistaken for cherts (chemical
sediments). Also occurs as wallrock alteration in
some quartz vein stockwork zones.
dolomite, siderite, ankerite,
calcite, quartz, sericite, chlorite,
pyrite
carbonate Usually occurs as disseminated alteration in footwall
sequences, commonly over extensive lateral and
stratigraphic intervals. The composition of
carbonates may change with distance from ore zones.
Sediment-hosted massive sulphide
quartz, muscovite, siderite,
dolomite, garnet, celsian,
pyrrhotite, pyrite, barite
silicic Pervasive replacement of footwall strata below
massive sulphide; also of baritic facies of the
sulphide body and less commonly in the hanging
wall. Well developed within calcareous strata but
more cryptic in siliciclastic strata. Can be mistaken
for siliceous shale or chert. Occurs as “garnet
quartzite” in high grade metamorphic rocks.
tourmaline, muscovite, quartz,
pyrrhotite
tourmalinite Pervasive replacement of footwall strata below
massive sulphide. Associated with abundant
disseminated sulphide in shallow footwall. Limited
to feldspathic strata.
ankerite, siderite, calcite,
quartz, muscovite, pyrrhotite
carbonate Disseminated carbonate, often as euhedral crystals,
in the shallow footwall below massive sulphide and
baritic facies. Disseminated ankerite/siderite can be
very extensive within calcareous strata along major
structures.
sericite, chlorite, quartz,
pyrrhotite, pyrite, albite
sericitic Pervasive replacement of strata in broad halo around
massive sulphide deposit; forms discordant bodies in
structurally controlled vent areas. Occurs as
potassium feldspar in high-grade metamorphic rocks.
Best developed in feldspathic strata.
albite, chlorite, muscovite, biotite
albitic Pervasive replacement of strata, and massive
sulphide; more typically along structures and around
mafic intrusions. Limited to feldspathic strata.