Resource Geology Flashcards
Deposits provide more than 50% of the worlds copper from over 100 producing mines.
Porphyry copper
Closest relative of porphyry copper deposits
Porphyry molybdenum deposit
Sulphide minerals in the porphyry copper system are associated with _____________.
hydrothermal alteration of the hostrock intrusion and wallrocks
Stockworks of quartz veinlets, quartz veins, closely spaced fractures and breccias contain ________
pyrite and chalcopyrite
________, ________, and _______ occur in large zones of economically bulk mineable mineralization.
Molybdenite, bornite, and magnetite
T or F. Most porphyry copper deposits are commonly related to subduction-related magmatism.
T
The age of most porphyry copper deposits are of _________ to _________ age
Mesozoic and Cenozoic
Porphyry copper deposits have a commonly depositional environment of _______ and ________.
oceanic volcanic island and continent-margin arcs
The typical form of porphyry copper deposit are commonly ______ or _______.
cylindrical or stock-like
Always present. Characterized by secondary K-spar, biotite and/or chlorite replacing primary K-spar, plagioclase and mafics. Minor sericite may be present.
Potassic Zone
Not always present. Characterized by vein quartz, sericite and pyrite and minor chlorite, illite and rutile replacing K-spar and biotite.
Phyllic zone
Not always present. Characterized by the clay minerals kaolinite and montmorillonite with minor disseminated pyrite. Plagioclase is strongly altered, K-spar unaffected and biotite chloritized.
Argillic zone
Always present. Characterized by chlorite, calcite and minor epidote. Mafic minerals highly altered and plagioclase less so.
Porpylitic zone
Coincident with the potassic alteration zone. Generally, several hundred meters in diameter. Relatively LOW SULFIDE CONTENT, but molybdenum is higher than anywhere else in the deposit. Pyrite 2-5% and pyrite/chalcopyrite ratio about 3:1. Mineralization is disseminated rather than stockwork.
Inner zone
Lies roughly at the POTASSIC-PHYLLIC BOUNDARY. Pyrite 5-10% and pyrite/chalcopyrite ratio about 2.5:1. Main ore mineral chalcopyrite occurring as stockwork veinlets. Other ore minerals include bornite, enargite and chalcocite.
Ore zone
Includes much of the phyllic and argillic (if present) zones. Pyrite quite high (10-15%) and pyrite/chalcopyrite ratio about 15:1. Mineralization both as veins and disseminations. Many additional exotic sulfide phases begin to show up.
Pyrite Zone
Coincides with the propylitic zone. Pyrite minor, and copper mineralization rare. Sphalerite and galena common, but usually sub-ore grade. Mineralization approaches true veins.
Outer zone
Often major ore carriers in the porphyry system. Have very high grades (2-5% Cu) and can occur both in the porphyry or the country rock. May be formed by hydrothermal activity, gravitational collapse or later explosive volcanism
Breccia zone
Predominant sulfide mineral; in some deposits the Fe oxide minerals magnetite, and rarely hematite, are abundant.
Pyrite
Chalcopyrite; molybdenite, lesser bornite and rare (primary) chalcocite.
Ore minerals
Tetrahedrite/tennantite, enargite and minor gold, electrum and arsenopyrite. In many deposits late veins commonly contain galena and sphalerite in a gangue of quartz, calcite and barite.
Subordinate minerals
Both internal between intrusive phases and external with wall rocks; cupolas and the uppermost, bifucating parts of stocks, dikes swarms.
Igneous contacts
Mainly early formed intrusive and hydrothermal type.
Breccias
Give rise to ore-grade vein stockworks, notably where there are coincident or intersecting multiple mineralized fracture sets.
Zones of intensely developed fracturing
Carry chalcocite, covellite and other Cu*2S minerals.
Secondary (supergene) zone
Marked by ferruginous ‘capping’ with supergene clay minerals, limonite (geothite, hematite, and jarosite) and residual quartz.
Oxidized and leached zones
T or F. Porpylitic alteration extends upward into the volcano.
T
T or F. Porphyry deposits are not associated with small, high level stocks ans subaerial calc-alkaline volcanism.
F. It is associated
There are two types of Porphyry Copper Deposit and which one is hosted by quartz monzonites to grandiorites?
Lowell and Guilbert type
There are two types of Porphyry Copper Deposit and which one is hosted by quartz diorite and diorite?
Diorite type
Type of granite that is a product of crustal anatexis.
S-type
Type of granite that represents the final stage differentiation product of a partial melt generated during plate subduction.
I-type
What are the following features of calc-alkaline intrusion and volcanic model?
- Caps of advances argillic alteration
- Diatreme breccia associations
- Intra-mineral intrusion
- Vertically elongated porphyry system
- SCC (sericite, clay, chlorite)
Sequence of events associated with emplacement and cooling of a melt at shallow (<2 km depth) crustal levels, as an apophysis to a larger magma source, and the exsolution of magmatic fluids and metals from the upper levels of the porphyry stock.
Prograde
This stage involves the subsequent cooling of the intrusion, and the porphyry stock and its host rocks become an environment of metal deposition.
Retrograde
Initial emplacement of melts at shallow crustal levels and the associated cooling and crystallization is accompanied by the formation of zoned alteration assemblages formed in response to the transfer of heat from the melt into the host lithologies.
Stage 1: Heat transfer and zoned alteration
Dominated by biotite in calc-alkaline magmatic arcs. In areas of crustal rifting involving more silicic intrusions, the potassic alteration is dominated by K-feldspar.
Potassic alteration
Distal to the central potassic core, reflecting progressively cooler conditions. Secondary amphibole (actinolite) in calc-alkaline environment locally occur with biotite in the outer alteration zone and with epidote in the inner zone.
Propylitic alteration
Some intrusions, the deep propylitic alteration assemblage is dominated by albite, formed during late stage crystallization of the melt.
Albitization
Cooling and crystallization of the melt results in the fracturing, especially around the carapace of the intrusion, accompanied by the exsolution of magmatic volatiles. The build up of gas pressure within the cooling magma may initiate fracturing in the brittle host rocks
STAGE II: Exsolution of magmatic fluids
The most common mineral deposited from calc-alkaline intrusions
Quartz
The most abundant gangue mineral in the stockwork and sheeted veins
Quartz
The pre-mineralization gangue minerals can be deposited within the fracture network and result in the formation of a _________.
stockwork
T or F. This high density of quartz veins implies that a significant quantity of volatiles were released from the melt.
T
T or F. Fluid inclusion studies indicate that the majority of the stockwork and sheeted quartz veins were deposited from a hot (>300-500ºC), hypersaline (>25-30 wt% NaCl equiv.), two-phase (boiling) brine.
T
T or F. Fluids which formed the stockwork quartz veins are significantly enriched in quartz.
F. Metals
Cu-Au mineralization in many porphyry Cu systems post-date Stage I potassic/propylitic alteration, quartz vein formation and advanced argillic alteration, attributed to a progressive change from an environment dominated by magmatic fluids to one dominated by cooler and more dilute meteoric waters.
STAGE III: Late stage cooling and mineralization
Biotite and K-feldspars form as early Stage II minerals, in thin veinlets w/ quartz, magnetite &/or pyrite and are associated with variable abundances of chalcopyrite.
Potassic alteration assemblages
Calc-silicate minerals (actinolite, epidote and zeolites occur in fractures and in open spaces replacing wall rock minerals (mainly mafic phases) and post-date the biotite and k-fp.
Propylitic alteration assemblages
Most Cu-Au mineralization in the SW Pacific is intimately related w/ late chlorite &/or sericite or illitic clay deposition and wall rock alteration. Chlorite dominates at depth and is early, whereas sericite dominates at shallower levels and is late. This zonation is indicative of a progressive decrease in fluid pH at shallower levels.
Phyllic alteration assemblages
Pervasive quartz-ser-pyrite alteration w/c displays an increasing abundance of chlorite at depth, and post-dates Cu-Au mineralization overprints other alteration assemblages and veins at shallow levels and along the margins.
STAGE IV: Post-mineral phyllic, argillic and advanced argillic overprint
Derived from a fluid dominated by meteoric water.
Sericite
Alteration and deposition in open spaces and fractures are inferred to have formed as late hydrothermal alteration minerals
Smectite &/or kaolinite ± siderite
