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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 metal-rich 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 NaCl-equivalent 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 liquid-liquid 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% SiO\(_2\)+Na(K, Li, Al)\(_2\)O, H\(_2\)O 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)\(_2\)O, H\(_2\)O 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
Authors/Creators:	Vasyukova, OV
Keywords:	porphyry-style mineralisation, quartz eyes, cathodoluminescence, fluid and melt inclusions
Copyright Information:	Copyright © 2011 the author
Item Statistics:	View statistics for this item

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