Generation of the Giant Porphyry Cu-Au Deposit by Repeated Recharge of Mafic Magmas at Pulang in Eastern Tibet
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Abstract The giant Pulang porphyry Cu-Au district (446.8 Mt at 0.52% Cu and 0.18 g/t Au) is located in the Yidun arc, eastern Tibet. The district is hosted in an intrusive complex comprising, in order of emplacement, premineralization fine-grained quartz diorite and coarse-grained quartz diorite, intermineralization quartz monzonite, and late-mineralization diorite porphyry, which were all emplaced at ca. 216 ± 2 Ma. Mafic magmatic enclaves are found in both the coarse-grained quartz diorite and quartz monzonite. The well-preserved primary mineral crystals in such a systematic magma series (including contemporaneous relatively mafic intrusions) with well-defined timing provide an excellent opportunity to investigate upper crustal magma reservoir processes, particularly to test the role of mafic magma recharge in porphyry Cu formation. Two groups of amphibole crystals, with different aluminum contents, are observed in these four rocks. Low-Al amphibole crystals (Аl2О3 = 6.2–7.6 wt %) with crystallization temperatures of ~780°C mainly occur in the coarse-grained quartz diorite and quartz monzonite, whereas high-Al amphibole crystals (Al2O3 = 8.0–13.3 wt %) with crystallization temperatures of ~900°C mainly occur in the fine-grained quartz diorite and diorite porphyry. These characteristics, together with detailed petrographic observations and mineral chemistry studies, indicate that the coarse-grained quartz diorite and quartz monzonite probably formed by crystal fractionation in the same felsic magma reservoir, whereas the fine-grained quartz diorite and diorite porphyry formed from relatively mafic magmas sourced from different magma reservoirs. The occurrence of mafic magmatic enclaves, disequilibrium phenocryst textures, and cumulate clots indicates that the coarse-grained quartz diorite and quartz monzonite evolved in an open crustal magma storage system through a combination of crystal fractionation and repeated mafic magma recharge. Mixing with incoming batches of hotter mafic magma is indicated by the appearance of abundant microtextures, such as reverse zoning (Na andesine core with Ca-rich andesine or labradorite rim overgrowth), sharp zoning (Ca-rich andesine or labradorite core with abrupt rimward anorthite decrease) and patchy core (Ca-rich andesine or labradorite and Na andesine patches) textured plagioclase, zoned amphibole, high-Al amphibole clots, skeletal biotite, and quartz ocelli (mantled quartz xenocrysts). Using available partitioning models for apatite crystals from the coarse-grained quartz diorite, quartz monzonite, and diorite porphyry, we estimated absolute magmatic S contents to be 20–100, 25–130, and >650 ppm, respectively. Estimates of absolute magmatic Cl contents for these three rocks are 1,000 ± 600, 1,800 ± 1,100, and 1,300 ± 1,000 ppm, respectively. The slight increase in both magmatic S and Cl contents from the premineralization coarse-grained quartz diorite magma to intermineralization quartz monzonite magma was probably due to repeated recharge of the relatively mafic diorite porphyry magma with higher S but similar Cl contents. Mass balance constraints on Cu, S, and Cl were used to estimate the minimum volume of magma required to form the Pulang porphyry Cu-Au deposit. Magma volume calculated using Cu mass balance constraints implies that a minimum of 21–36 km3 (median of 27 km3) of magma was required to provide the total of 2.3 Mt of Cu at Pulang. This magma volume can explain the Cl endowment of the deposit but is unlikely to supply the sulfur required. Recharge of 5–11 km3 of diorite porphyry magma to the felsic magma reservoir is adequate to account for the additional 6.5–15 Mt of S required at Pulang. Repeated diorite porphyry magma recharge may have supplied significant amounts of S and some Cl and rejuvenated the porphyry system, thus aiding formation of the large, long-lived magma reservoir that produced the porphyry Cu-Au deposit at Pulang.Keywords:
Diorite
Quartz monzonite
Amphibole
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Felsic
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Abstract The giant Pulang porphyry Cu-Au district (446.8 Mt at 0.52% Cu and 0.18 g/t Au) is located in the Yidun arc, eastern Tibet. The district is hosted in an intrusive complex comprising, in order of emplacement, premineralization fine-grained quartz diorite and coarse-grained quartz diorite, intermineralization quartz monzonite, and late-mineralization diorite porphyry, which were all emplaced at ca. 216 ± 2 Ma. Mafic magmatic enclaves are found in both the coarse-grained quartz diorite and quartz monzonite. The well-preserved primary mineral crystals in such a systematic magma series (including contemporaneous relatively mafic intrusions) with well-defined timing provide an excellent opportunity to investigate upper crustal magma reservoir processes, particularly to test the role of mafic magma recharge in porphyry Cu formation. Two groups of amphibole crystals, with different aluminum contents, are observed in these four rocks. Low-Al amphibole crystals (Аl2О3 = 6.2–7.6 wt %) with crystallization temperatures of ~780°C mainly occur in the coarse-grained quartz diorite and quartz monzonite, whereas high-Al amphibole crystals (Al2O3 = 8.0–13.3 wt %) with crystallization temperatures of ~900°C mainly occur in the fine-grained quartz diorite and diorite porphyry. These characteristics, together with detailed petrographic observations and mineral chemistry studies, indicate that the coarse-grained quartz diorite and quartz monzonite probably formed by crystal fractionation in the same felsic magma reservoir, whereas the fine-grained quartz diorite and diorite porphyry formed from relatively mafic magmas sourced from different magma reservoirs. The occurrence of mafic magmatic enclaves, disequilibrium phenocryst textures, and cumulate clots indicates that the coarse-grained quartz diorite and quartz monzonite evolved in an open crustal magma storage system through a combination of crystal fractionation and repeated mafic magma recharge. Mixing with incoming batches of hotter mafic magma is indicated by the appearance of abundant microtextures, such as reverse zoning (Na andesine core with Ca-rich andesine or labradorite rim overgrowth), sharp zoning (Ca-rich andesine or labradorite core with abrupt rimward anorthite decrease) and patchy core (Ca-rich andesine or labradorite and Na andesine patches) textured plagioclase, zoned amphibole, high-Al amphibole clots, skeletal biotite, and quartz ocelli (mantled quartz xenocrysts). Using available partitioning models for apatite crystals from the coarse-grained quartz diorite, quartz monzonite, and diorite porphyry, we estimated absolute magmatic S contents to be 20–100, 25–130, and >650 ppm, respectively. Estimates of absolute magmatic Cl contents for these three rocks are 1,000 ± 600, 1,800 ± 1,100, and 1,300 ± 1,000 ppm, respectively. The slight increase in both magmatic S and Cl contents from the premineralization coarse-grained quartz diorite magma to intermineralization quartz monzonite magma was probably due to repeated recharge of the relatively mafic diorite porphyry magma with higher S but similar Cl contents. Mass balance constraints on Cu, S, and Cl were used to estimate the minimum volume of magma required to form the Pulang porphyry Cu-Au deposit. Magma volume calculated using Cu mass balance constraints implies that a minimum of 21–36 km3 (median of 27 km3) of magma was required to provide the total of 2.3 Mt of Cu at Pulang. This magma volume can explain the Cl endowment of the deposit but is unlikely to supply the sulfur required. Recharge of 5–11 km3 of diorite porphyry magma to the felsic magma reservoir is adequate to account for the additional 6.5–15 Mt of S required at Pulang. Repeated diorite porphyry magma recharge may have supplied significant amounts of S and some Cl and rejuvenated the porphyry system, thus aiding formation of the large, long-lived magma reservoir that produced the porphyry Cu-Au deposit at Pulang.
Diorite
Quartz monzonite
Amphibole
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Felsic
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Fractional crystallization (geology)
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1977)
Chemical data on amphibole phenocrysts in andesitic to rhyolitic rocks have been compiled mainly from the literature. It is inferred that most amphibole phenocrysts have been oxidized more or less by the loss of hydrogen in connection with volcanic activities from the fact that the volcanic amphiboles are generally higher in Fe3+/ΣFe ratio and lower in H2O+ content than their plutonic equivalents. Bimodal distribution of Fe3+/ΣFe ratio in the phenocrysts suggests that change of “usual amphibole” into “oxyhornblende” is very rapid. Kaersutite, usually considered to be confined to alkaline rocks, seems to occur in some calc-alkaline volcanics. Assuming that the original Fe3+/ΣFe ratio was 0.15, the numbers and ratios of cations in primary amphiboles are frequently in the following ranges on the basis of O=23 (average in parenthesis): Si=6.2-7.0 (6.5), AlIV=1.0-1.8 (1.5), AlVI=0.0-0.5 (0.3), Ti=0.1-0.3 (0.2), ΣFe=1.4-2.2 (1.7), Mn=0.01-0.06 (0.04), Mg=2.6-3.4 (2.9), Ca=1.6-1.9 (1.8), Na=0.3-0.8 (0.5), K=0.0-0.2 (0.1), Na+ K=0.4-0.9 (0.6), mg=0.5-0.8 (0.6) and K/Na=0.0-0.4 (0.2).
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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.
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Abstract Amphiboles in dykes from the dominantly silica-undersaturated Monteregian series range from pargasitic megacrysts and xenocrysts to kaersutitic, pargasitic and hastingsitic phenocrysts, groundmass prisms and reaction rims. Amphiboles in dykes and plutons from the silica-oversaturated White Mountain Magma Series range from kaersutite, through hornblende, hastingsite and edenite, to sodic-calcic and sodic varieties. This contrast between the amphiboles from the two series is probably a reflection of differing melt silica activity and is a useful petrologic discriminant. In most cases, pargasitic amphibole megacrysts from Monteregian monchiquites reflect the Mg numbers of their host rocks and are considered cognate. The megacrysts are lower in Ti and higher in Mg and Al than their phenocryst mantles and rims. This is probably a result of higher pressures of formation.
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Alkali basalt
Hornblende
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