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Types and origin of quartz and quartz-hosted fluid inclusions in mineralised porphyries

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Vasyukova, OV (2011) Types and origin of quartz and quartz-hosted fluid inclusions in mineralised porphyries. PhD thesis, University of Tasmania.

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Abstract

The formation of porphyry deposits is related to the intrusion of intermediate
to felsic magmas, its cooling and crystallisation, followed by exsolution of a metalrich
magmatic vapour phase (MVP), its accumulation under the intrusion cupola and
escape into country rocks. A ubiquitous phenomenon of the deposit-related intrusions
is a porphyritic texture that is typically defined by feldspar and quartz crystals. It is
believed that porphyry textures can be formed as a result of temperature quenching
during intrusion of hot magma into cold country rocks, or pressure quenching
after fluid escape. In spite of extensive research on porphyry deposits there is still
uncertainty about when and where quartz eyes crystallise. Do they crystallise in deep
magma chambers and travel with magma to a place of intrusion? Do quartz eyes
crystallise after magma emplacement? How is crystallisation of quartz eyes related
to mineralisation in time and space? Is it coeval with exsolution of mineralising fluid
and its accumulation under the intrusion carapace? If they are coeval then study of
quartz eyes may be highly beneficial for our knowledge of porphyry systems. The term
‘quartz eyes’ is used in preference to other terms (e.g. phenocrysts, porphyroblasts or
clasts) to emphasise that their origin is yet to be established.
Quartz eyes from seven porphyry deposits were studied. These are
Antapaccay (Peru), Batu Hijau (Indonesia), Climax (USA), Panguna (PNG), Far
Southeast porphyry (Philippines), Rio Blanco (Chile) and Omsukchan (Russia). The
research was focused on the internal textures of the quartz eyes and inclusions
they contain. To study quartz textures SEM-based cathodoluminescence (CL) and
hyperspectral CL mapping were applied. Both techniques were combined with Electron
Probe Microanalysis (EPMA). For inclusion study optical and electron microscopy,
LA-ICP-MS, Raman spectroscopy and microthermometry were applied.
Quartz eyes commonly showed strongly contrasted CL patterns, in many
grains core and rim zones were revealed. CL study showed significant diversity of
internal patterns of quartz eyes within a single thin section; grains with CL-dark cores
and CL-bright rims were sometimes next to grains with reverse zoning or grains
showing irregular or no zonation. Clusters of quartz grains were often found; diversity
of CL patterns within grains of a single cluster was found typical. Quartz eyes with
a distinct rim-and-core pattern often demonstrated enrichment of cores in Al, Li
and OH-, whereas rims were Ti-rich and contained variable Al. CL-dark cores often
showed sector zoning.Fluid inclusion studies showed that inclusion assemblages in quartz eyes are
unique; feldspar phenocrysts in the same thin sections contained only melt inclusions
whereas adjacent quartz eyes were sponge-textured due to the abundance of fluid
inclusions in them. Fluid inclusions in quartz eyes were found distributed along healed
fractures, and often had halos of secondary quartz around them, indicating that they
were most likely partially decrepitated. Such fractures were distributed only within
quartz eyes and did not extend into the matrix. Inclusions displayed extreme diversity
in composition that could be caused by a combination of processes upon cooling, such
as precipitation of daughter crystals, necking down, leakage of the fluid components,
etc. implying that inclusions underwent post-entrapment modification and cannot be
used as characteristic fluid during conditions of crystallisation of quartz eyes.
According to LA-ICP-MS analyses fluid inclusions in the quartz eyes and
veins contained Al, Na, K, Fe, Mn, Cl, S, W, Pb, Cu, Zn, Ca, Ag, Sn, Bi and Rb above
their detection limits. The element ratios varied significantly within adjacent inclusions.
Liquid-rich inclusions are usually chlorine-rich, and sometimes are B-rich. Vapour-rich
inclusions are often Al- and S-bearing with higher Cu/Zn, Cu/Rb and Cu/Sr ratios than
those in liquid-rich and multiphase inclusions. Multiphase inclusions are enriched in
metals (Cu, Zn, Rb, Sr, Ag, Pb, Mo and W) and showed very low Al concentrations.
Microthermometry experiments demonstrated atypical behaviour of the fluid
upon heating and cooling: some phases swirl (shrink in some parts while growing
and coalescing in others without general changes in a size) upon heating, showing
that those phases are more like immiscible liquids than solids. Upon cooling multiple
episodes of fluid immiscibility were observed. Every immiscibility event led to separation
of a phase, which appeared to be initially liquid-like and later could gradually reshape
to form crystals. Fluid inclusion study showed that behaviour of such complex fluid
is significantly different from that of model (NaCl) fluid and thus, using the NaClequivalent
fluid properties to reconstruct conditions of formation of porphyry deposits
can lead to erroneous results.
Distribution of metal-bearing phases (MBP) and trace metals in quartz eyes
showed that the sulphides are secondary, that their distribution is confined by quartz
eyes and that they are closely related to secondary CL-dark quartz. MBP in quartz
veins are interstitial between quartz grains and are also related to late CL-dark quartz.
MBP also form separate globules, which are associated with non-metal phases such
as apatite, biotite and fluorite. Non-metal phases are also associated with MBP within
quartz eyes and veins.Although it is traditionally assumed that quartz eyes from porphyries are
phenocrysts, comparison with quartz phenocrysts from lava samples showed
significant difference. Quartz phenocrysts from lavas are often fragments of crystals,
they show low contrast CL pattern (often oscillatory zoning only) with no healed
fractures and they usually contain melt inclusions of rhyolitic compositions but no
fluid inclusions. The difference in shapes, internal textures and inclusion assemblages
can indicate different origin. Porphyry systems are related to intermediate intrusions.
In such systems quartz will crystallise as the last magmatic phase when necessary
silica saturation occurs (for crystallisation). Thus, quartz eyes should crystallise after
magma emplacement during late magmatic stages. Preserved quartz clusters in
the studied porphyry samples is consistent with late crystallisation. Abundance of
oscillatory zonation together with preserved clusters can also indicate crystallisation
in stagnant magma. Abundance of fluid inclusions in quartz eyes and their distribution
indicate that they crystallised in an extremely fluid-rich environment. The absence of
similar fluid inclusion assemblages in adjacent crystals (other than quartz) implies that
quartz eye crystallisation was related to a unique event.
A new model for crystallisation of quartz eyes was proposed in this study, which
accounts for the marked differences between quartz phenocrysts and quartz eyes.
As a result of fractional crystallisation residual melt became enriched in silica, alkali,
volatile and metal components. Cooling and/or further crystallisation induced liquidliquid
immiscibility within the melt, dividing it into peraluminous water-poor melt and
peralkaline water- and silica-rich melt (heavy fluid). As a result of silica oversaturation
during further cooling, heavy fluid underwent immiscibility with formation of silica-rich
globules (>90wt% SiO2+Na(K, Li, Al)2O, H2O and metals); from these globules quartz
eyes crystallised. Crystallisation of quartz eyes could take place inwards when firstly
Ti-rich rims (high temperature) formed and then high Al, Li and OH- cores with sector
zoning crystallised. During the quartz crystallisation the other components (Na (K, Li,
Al)2O, H2O and metals) were expelled from the crystal lattice and formed abundant
fluid inclusions in quartz. Further cooling and fracturing of the pluton caused massive
(partial) decrepitation of fluid inclusions. The released fluid healed multiple fractures.
This scenario of formation of quartz eyes is in a good agreement with obtained CL
and fluid inclusion data.
The residual phase (left after separation of silica-rich globules) was extremely
mobile and easily migrated along fractures. The alkali-, volatile- and metal-rich phase
was chemically aggressive, and probably very unstable under cooling conditions. It
precipitated the remaining silica as veinlets, and then metal-bearing and alkali-bearing phases in interstitial spaces between quartz grains or as late veinlets. This residual
fluid may have colloidal nature; as soon as the fluid became oversaturated relative to
any of its components, multiple centres of crystallisation formed, turning it into colloid
(solid phase suspended in liquid phase). If the excess of this component was removed
from the system (precipitated) the fluid could convert back into solution.
This mechanism of evolution of the residual melts and eventual formation
of mineralising fluids from heavy fluid through a series of immiscibility events has
significant advantages in terms of efficiency of metal extraction over the transition
melt/aqueous fluid through the fluid exsolution. High efficiency of metal extraction
allows formation of an economically significant deposit from a rather small porphyry
stocks without necessity to invoke magma reservoirs of batholithic size. This scenario
of formation of mineralised fluid is consistent with obtained CL data as well as with the
observed fluid behaviour upon cooling. Suggested mechanism of the evolution of the
fluid is also in a good agreement with co-deposition of quartz and sulphides, so typical
for porphyry-type deposits.

Item Type: Thesis (PhD)
Keywords: porphyry-style mineralisation, quartz eyes, cathodoluminescence, fluid and melt inclusions
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Date Deposited: 14 Dec 2011 00:07
Last Modified: 11 Mar 2016 05:53
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