INTRODUCTION
Applied mineralogy is pivotal in exploring and processing porphyry copper deposits. It aids in characterizing host rocks, identifying minerals, assessing quantities, and understanding textural relationships between minerals and surrounding rock. Skilled petrographers and collaboration with field geologists are crucial for comprehensive insights. In mineral processing, it helps identify minerals, assess quantities, determine grain sizes, evaluate minor and trace elements in key ore minerals, and identify clay minerals within the ore.
Porphyry copper systems span around 2,450 meters vertically. At the base, there are narrow late magmatic quartz veinlets with traces of minerals like molybdenite. About 2,100 to 1,500 meters down, distinct signs of the system emerge with blue corundum and changes in quartz composition. Lower, around 900 to 1,500 meters, classic alteration zoning forms with wider quartz veins and increased copper content. In the upper 900 meters, rock textures transform, dominated by sericite, creating the quartz-sericite alteration zone.
Vein density varies significantly within porphyry copper deposits, ranging from <9 per meter at the periphery to 63-90 per meter near the center, showing complexities like fracture filling and replacement.
Primary ore minerals in porphyry copper deposits, found in irregular grains within quartz veins and wallrock veinlets, display varying zonal distributions. Bornite-rich zones are common in orebody centers but absent in some deposits. Mineralization typically spans bornite-chalcopyrite, chalcopyrite-bornite-pyrite, and chalcopyrite-pyrite sequences, with the latter dominating deposits. Chalcopyrite often accompanies molybdenum, enargite, and gold, albeit with low gold content in molybdenite-rich deposits. Depositional sequences observed in different sites vary, such as magnetite-pyrite-chalcopyrite. Late-stage occurrences of various minerals are found in many deposits.
Silicification, a significant trait, is dominated by quartz in stockworks, veins, and country rock. It's primarily found in fracture-controlled veins, occasionally as overgrowths on primary quartz, or locally replacing the country rock due to hydrothermal or silicate mineral alteration.
Hydrothermal alteration in these deposits forms distinct but often overlapping zones: potassic, phyllic, argillic, and propylitic. Potassic alteration generates biotite or K-feldspar zones, typically at the orebody center, sometimes surrounded by a chlorite-carbonate zone in specific mines. The phyllic zone contains sericite and quartz and aligns with the main ore mineral zone, while the argillic zone consists of clay minerals. Propylitic zones, mainly at the deposit edges, often contain epidote. Unique mineral zones like magnetite at the periphery of certain deposits highlight the alteration's diverse nature.
Potassic alteration, marked by hydrothermal biotite or K-feldspar alteration in the central ore deposit, is evident in various deposits like Valley Copper in Canada and Malanjkhand in India. Biotite replacement of primary mafic minerals characterizes the alteration, transitioning from brown to green towards the orebody's center. Both K-feldspar and biotite alterations are significant in the El Teniente mine in Chile.
Phyllic alteration, seen as quartz and sericite enveloping quartz and mineralized veins, is widespread within ore zones, as observed in the Lomex deposit in Canada, where it extends into the argillic zone. It commonly involves sericite associated with small amounts of kaolinite, montmorillonite, and occasionally calcite and epidote.
Argillic alteration refers to transforming feldspars and sometimes mafic minerals to an assemblage of sericite, kaolinite, and minor chlorite. This alteration extends beyond minable grade isopleths, showing varying intensity, as seen in different deposits like Valley Copper and Highmont in Canada.
Propylitic alteration usually follows the argillic alteration, with epidote being a characteristic mineral. It involves alterations of feldspars to sericite, carbonate, and clay minerals, while mafic minerals transform into chlorite, carbonates, sericite, and epidote. Different deposits show varying degrees of propylitic facies assemblages, sometimes overlapping with argillic zones, as observed in Highland Valley and Lomex deposits in British Columbia.
Other Minerals, including but not limited to gypsum, anhydrite, and zeolites like laumontite, stilbite, chabazite, and heulandite, are present as veinlets, disseminated grains, and intergrown with calcite and sulfide minerals. Tourmaline, commonly found near breccia bodies, occurs as crystalline aggregates, replacing fractured rock, and disseminated grains in the wallrock, typically associated with minerals like quartz, hematite, epidote, calcite, and various sulfides.
Maturity in porphyry copper systems, determined by factors like rock type, alteration, and mineralization processes, varies from tonalite to quartz monzonite. Mature deposits showcase extensive quartz-sericite-pyrite zones, with the best copper values often at the boundary between phyllic and potassic alteration zones. In contrast, immature systems lack increased copper grades toward the potassic alteration zone. Immature intrusions retain deuteric water in specific minerals, while mature intrusions display deuteric veinlets with sericite selvages. Additionally, immature systems, notably associated with tonalite intrusions, often contain higher gold and silver contents and higher bornite: chalcopyrite ratios than mature equivalents.
The sulfur content, reaching up to 8% in porphyry copper deposits, plays a vital role. Higher sulfur contents correlate with greater maturity, occur at higher crustal levels, and coincide with broader quartz-sericite alteration aureoles. Sulfur contributes to altering the crystal structure of mafic silicate minerals, releasing elements like K, Si, Al, and H2O, which facilitate the formation of sericite and influence the development of hydrothermal alteration zones.
Supergene Mineralization characterizes the uppermost part of most porphyry copper deposits, comprising an oxidized cap and an underlying blanket zone. The cap typically contains diverse supergene copper minerals and might be enriched in gold. However, in certain deposits, the cap lacks copper minerals as elements from the copper sulfides were washed down and precipitated at the top of the blanket zone. Below the water table, the blanket zone hosts supergene copper minerals, mainly Cu sulfides. The lower boundary of the supergene blanket transitions into the underlying hypogene Cu sulfide mineralization.
Mineralogy of Cap:
At the top of certain ore bodies, extensive oxidation of ore minerals resulted in the formation of supergene minerals on-site. The supergene copper minerals in the cap encompass Cu oxides (like tenorite, cuprite, delafossite, and copper-bearing goethite), Cu carbonates (primarily malachite, less so azurite), Cu sulfates, Cu chlorides, Cu silicates, and native copper.
For instance, cuprite at the Malanjkhand deposit in India occurs in several-centimeter-wide veins associated with native copper, goethite, and delafossite, surrounded by massive goethite. More abundant than azurite, Malachite appears as intensely oxidized material and encrustations on cuprite and goethite. The presence of malachite often obscures primary ore features, with associated plagioclase altered to clay minerals, leaving only quartz and residual minerals like brochantite preserved.
Additionally, native copper is found in various forms within the oxidized zone, constituting a significant percentage of copper minerals at locations like the Afton deposit, presenting as dendrites, films, granules, and masses up to 5 mm in size.
Mineralogy of blanket zone:
Secondary copper and iron sulfides dominate in the blanket zone of porphyry copper deposits, with variable oxidation levels. Main copper minerals include chalcocite, covellite, bornite, and remnants of unaltered chalcopyrite. Chalcocite is particularly abundant, seen in disseminations and veins up to 25 mm wide in deposits like Afton. During alteration, primary ore minerals transform into pyrrhotite, marcasite, iron oxide phases, and ferrimolybdite.
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