Porphyry Copper-molybdenum Deposit Research Articles

Magmatic and Hydrothermal Chronology of the Giant Rio Blanco Porphyry Copper Deposit, Central Chile: Implications of an Integrated U-Pb and 40Ar/39Ar Database

The history of hypabyssal intrusion and hydrothermal activity in the northeastern and central parts of the be-hemothian (sensu Clark, 1993) Rio Blanco-Los Bronces porphyry copper-molybdenum deposit is clarified on the basis of integrated U-Pb and 40Ar/39Ar geochronology. Isotope dilution thermal ion mass spectrometry (ID-TIMS) U-Pb dates for zircon separates and ID-TIMS and sensitive high resolution ion microprobe (SHRIMP) dates for single zircon grains in pre-, syn- and late-mineralization volcanic and intrusive host rocks in the Rio Blanco, Don Luis, and Sur-Sur mining sectors provide a temporal framework for interpretation of incremental-heating and spot-fusion 40Ar/39Ar dates for, respectively, magmatic biotite and hydrothermal biotite, muscovite, and orthoclase. The ore deposit is hosted in part by 16.77 ± 0.25 to 17.20 ± 0.05 (2σ) Ma andesitic volcanic strata of the Farellones Formation, but the major host rocks are units of the San Francisco batholith, including the 11.96 ± 0.40 Ma Rio Blanco granodiorite (mine terminology), the 8.40 ± 0.23 Ma Cascada granodiorite, and the 8.16 ± 0.45 Ma diorite. Hypabyssal dacitic intrusions (late porphyries) emplaced into the batholith yield 206Pb/238U ID-TIMS dates ranging from 6.32 ± 0.09 Ma (quartz monzonite porphyry), through 5.84 ± 0.03 Ma (feldspar porphyry) to 5.23 ± 0.07 Ma (Don Luis porphyry). The late-mineralization Rio Blanco dacite plug yields a SHRIMP zircon age of 4.92 ± 0.09 Ma. The 40Ar/39Ar plateau ages for phenocrystic biotites in quartz monzonite porphyry, feldspar porphyry, and Don Luis porphyry, as well as the preore diorite, range only from 5.12 ± 0.07 to 4.57 ± 0.06 Ma. All are significantly younger than the corresponding zircons and exhibit no correlation with intrusive sequence. The 40Ar/39Ar ages for hydrothermal biotite and orthoclase veins within the San Francisco batholith units fall in a narrow interval from 5.32 ± 0.27 to 4.59 ± 0.11 Ma. Hydrothermal sericites (muscovite), one associated with chalcopyrite, yielded spot-fusion ages of 4.40 ± 0.15 Ma (Rio Blanco granodiorite hosted) and 4.37 ± 0.06 Ma (Don Luis porphyry hosted). Comparison with the ID-TIMS and SHRIMP zircon ages indicates that most of the 40Ar/39Ar ages, even 95 percent plateaus, do not record initial magmatic cooling or hydrothermal alteration-mineralization events, evidence for quasipervasive reheating to at least 300°C by successive intrusions. Published Re-Os ages for two molybdenite samples range from 5.4 to 6.3 Ma and overlap extensively with the zircon U-Pb ages for the late porphyries. They imply that Cu-Mo mineralization overlapped temporally with the emplacement of, at least, quartz monzonite porphyry and feldspar porphyry units of the late porphyry suite and was, therefore, contemporaneous with the rise of dacitic melts to subvolcanic levels. Hydrothermal activity is inferred to have continued until 4.37 ± 0.06 Ma, following intrusion of the Don Luis porphyry and the early stages of emplacement of the Rio Blanco dacite plug complex. Hypogene Cu-Mo mineralization therefore probably persisted for 2 m.y. The geochronologic data do not resolve whether ore formation was continuous or episodic, but the observed crosscutting relationships between intensely altered and mineralized country rocks and less altered and mineralized late porphyry bodies support a model in which the ascent of metal-rich brines from an unexposed zone of the parental magma chamber was periodically stimulated by magma perturbation and hypabyssal intrusion.

Regional Geochemistry of Tertiary Igneous Rocks in Central Chile: Implications for the Geodynamic Environment of Giant Porphyry Copper and Epithermal Gold Mineralization

The giant porphyry copper-molybdenum deposits of central Chile formed within a thick sequence of Cretaceous to Pliocene volcanic rocks. The predominantly calc-alkaline basaltic andesites of the Cretaceous Las Chilcas Formation are characterized by La/Smn ratios of 1.8 to 2.5 and Sm/Ybn of 1.8 to 2.8. The upper Oligocene to lower Miocene Abanico Formation (variously defined as the Los Pelambres, Abanico, or Coya Machali Formations) ranges from basalt to rhyolite in composition and exhibits a broad southward transition from calc-alkaline to tholeiitic. All samples from this formation are characterized by LREE enrichment and moderately, or locally strongly, fractionated HREE (La/Smn = 1.3-4.1; Sm/Ybn = 1.5-5.8). The basaltic andesites and andesites of the middle Miocene Salamanca Formation have REE chemistry similar to that of the Upper Cretaceous strata (La/Smn = 1.5-2.7; Sm/Ybn = 1.6-2.7). The overlying middle Miocene Farellones Formation ranges from tholeiitic to calc-alkaline and from basalt to andesitic and has a similar LREE enrichment and fractionated HREE (La/Smn = 1.7-2.5; Sm/Ybn = 1.7-3.3). The Pliocene La Copa Rhyolite Complex, however, is strongly LREE enriched and HREE depleted (La/Smn = 3.8-3.9; Sm/Ybn = 4.2-4.7). Overall, the trace element geochemistry of the Cretaceous to middle Miocene volcanic rocks is characterized by enriched LREE and negative Nb anomalies, consistent with an arc setting, with only minor differences in the abundance of most elements. Crustal thickening during the Miocene in central Chile has been suggested to have been responsible for a transition from an amphibole- to garnet-dominated residual mineralogy resulting in the release of fluids that enabled the formation of giant copper porphyry deposits. However, the gradual increase in La/Yb through the early, middle, and late Miocene reported in earlier studies and interpreted to be a response to crustal thickening is not observed in the regional data. Instead, a rapid change in the geochemical signature between the end of the eruption of the Farellones Formation and the eruption of the high La/Yb La Copa Rhyolite Complex implies a more abrupt change in the tectonic environment. Isotopic data broadly support this, although lower epsilon Nd values in the Farellones Formation imply a greater role for crustal contamination in younger suites. In the absence of gradual crustal thickening, it is suggested that the subduction of the Juan Fernandez Ridge may have been the key geodynamic process responsible for the genesis of the three middle Miocene to lower Pliocene giant porphyry copper deposits in central Chile, possibly by promoting crustal-scale faulting and even acting as a source of metals.

Multifractal magnetic susceptibility distribution models of hydrothermally altered rocks in the Needle Creek Igneous Center of the Absaroka Mountains, Wyoming

Abstract. Magnetic susceptibility was measured for 700 samples of drill core from thirteen drill holes in the porphyry copper-molybdenum deposit of the Stinkingwater mining district in the Absaroka Mountains, Wyoming. The magnetic susceptibility measurements, chemical analyses, and alteration class provided a database for study of magnetic susceptibility in these altered rocks. The distribution of the magnetic susceptibilities for all samples is multi-modal, with overlapping peaked distributions for samples in the propylitic and phyllic alteration class, a tail of higher susceptibilities for potassic alteration, and an approximately uniform distribution over a narrow range at the highest susceptibilities for unaltered rocks. Samples from all alteration and mineralization classes show susceptibilities across a wide range of values. Samples with secondary (supergene) alteration due to oxidation or enrichment show lower susceptibilities than primary (hypogene) alteration rock. Observed magnetic susceptibility variations and the monolithological character of the host rock suggest that the variations are due to varying degrees of alteration of blocks of rock between fractures that conducted hydrothermal fluids. Alteration of rock from the fractures inward progressively reduces the bulk magnetic susceptibility of the rock. The model introduced in this paper consists of a simulation of the fracture pattern and a simulation of the alteration of the rock between fractures. A multifractal model generated from multiplicative cascades with unequal ratios produces distributions statistically similar to the observed distributions. The reduction in susceptibility in the altered rocks was modelled as a diffusion process operating on the fracture distribution support. The average magnetic susceptibility was then computed for each block. For the purpose of comparing the model results with observation, the simulated magnetic susceptibilities were then averaged over the same interval as the measured data. Comparisons of the model and data from drillholes show good but not perfect agreement.

Compositions of magmatic hydrothermal fluids determined by LA-ICP-MS of fluid inclusions from the porphyry copper–molybdenum deposit at Butte, MT

We analyzed 85 fluid inclusions from seven samples from the porphyry Cu–Mo deposit in Butte, MT, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The Butte deposit formed at unusually great depth relative to most porphyry deposits, and fluid inclusions in deep veins trapped a low-salinity, CO 2-bearing, magmatically derived, supercritical fluid as a single aqueous phase. This fluid is interpreted to be the parent fluid that cooled, decompressed, unmixed, and reacted with wall rock to form the gigantic porphyry Cu deposit at Butte. Few previous analyses of such fluids exist. Low-salinity, aqueous fluids from the earliest veins at Butte are trapped in deep veins with biotite-rich alteration envelopes (EDM veins). These veins, and the Butte quartz monzonite surrounding them, host much of the Butte porphyry Cu mineralization. Twenty fluid inclusions in one EDM quartz vein are dominated by Na, K, Fe (from ∼0.1 to ∼1 wt.%) and contain up to 1.3 wt.% Cu. These inclusions contain only small amounts (tens of ppm) of Pb, Zn, and Mn, and typically contain Li, B, Ca, As, Mo, Ag, Sn, Sb, Ba, and W in less than detectable quantities. The abundance of Cu in early fluids indicates that a low-salinity, Cu-rich, aqueous ore fluid can be directly produced by aqueous fluid separation from a granitic magma. Similar inclusions (eight) in an early deep quartz–molybdenite vein with a K-feldspar selvage have similar compositions but contain significantly less Cu than most inclusions in the biotite-altered vein. Analyzed inclusions in both veins contain less than detectable concentrations of Mo even though one is molybdenite-bearing. Low-salinity, CO 2-bearing aqueous fluids are also trapped in pyrite–quartz veins with sericitic selvages. These veins cut both of the above vein types and contain inclusions that were trapped at lower pressure and temperature. Thirty-nine inclusions in two such veins have compositions similar to early fluids, but are enriched by up to a factor of 10 in Mn, Pb, and Zn relative to early fluids, and are slightly depleted in Fe. Many of these inclusions contain as much or more Cu than early fluids, although little chalcopyrite is found in or around pyrite–quartz veins. Eighteen halite-bearing inclusions from three veins from both chalcopyrite-bearing and barren veins with both K-silicate and sericitic selvages were analyzed as well. Halite-saturated inclusions are dominated by Na, K, Fe, and in some inclusions Ca. Whereas these inclusions are significantly enriched in Ca, Mn, Fe, Zn, and Pb, fluids in all three veins contain significantly less Cu than early, high temperature, low-salinity inclusions. Analyses of all inclusion types show that whereas bulk-salinity of the hydrothermal fluid must be largely controlled by the magma, fluid–rock interactions have a significant role in controlling fluid compositions and metal ratios. Cu concentrations range over an order of magnitude, more than any other element, in all four samples containing low-salinity inclusions. We infer that variations are the result of fluid trapping after different amounts of fluid–rock reaction and chalcopyrite precipitation. Enrichment, relative to early fluids, of Mn, Pb, and Zn in fluids related to sericitic alteration is also likely the result of fluid–rock reaction, whereby these elements are released from biotite and feldspars as they alter to sericite. In halite-bearing inclusions, concentrations of Sr, Ca, Pb, and Ba are elevated in inclusions from the pyrite–quartz vein with sericitic alteration relative to halite-bearing inclusions from unaltered and potassically altered samples. Such enrichment is likely caused by the breakdown of plagioclase and K-feldspar in the alteration envelope, releasing Sr, Ca, Pb, and Ba.

40Ar/39Ar and Re-Os Geochronology of Porphyry Copper-Molybdenum Deposits and Related Copper-Silver Veins in the Collahuasi District, Northern Chile

40Ar/39Ar and Re-Os Geochronology of Porphyry Copper-Molybdenum Deposits and Related Copper-Silver Veins in the Collahuasi District, Northern Chile

40Ar/39Ar and Re-Os Geochronology of Porphyry Copper-Molybdenum Deposits and Related Copper-Silver Veins in the Collahuasi District, Northern Chile

The Rosario and Ujina porphyry Cu-Mo deposits, together with porphyry copper mineralization at the nearby Quebrada Blanca deposit, constitute the third-largest concentration of copper mineralization associated with the Domeyko fault system in northern Chile. At Rosario, fault-hosted Cu-Ag-rich massive sulfide veins are associated with pyrophyllite-alunite-quartz altered rocks. The Rosario massive sulfide veins cut biotitealbite-magnetite, K-feldspar and illite-chlorite altered rocks associated with porphyry-style copper-molybdenum mineralization. Similar massive sulfide veins occur in the La Grande area, 1 to 2 km south of Rosario. Copper ore at Ujina is associated with a K-feldspar-biotite-altered quartz monzonite intrusion. The K-feldspar-biotite assemblage has been overprinted by white mica-chlorite alteration. New geochronologic data presented in this paper constrain the ages of hydrothermal activity in the Rosario and Ujina deposits. The 40Ar/39Ar biotite, illite, and alunite dates (at 2 sigma error), and a Re-Os molybdenite date (at 0.5% error) are reported for porphyritic rocks and hydrothermal alteration from the Rosario and Ujina mineralized centers. A weighted mean 40Ar/39Ar plateau age of 34.4 plus or minus 0.3 Ma is obtained for igneous biotite in a monzonite porphyry that hosts copper mineralization at the Rosario deposit. Illite and hypogene alunite from separate overprinting alteration events yielded 40Ar/39Ar ages of 34.5 plus or minus 0.5 Ma (plateau age) and 32.6 plus or minus 0.3 Ma (plateau age), respectively. An Re-Os age of 33.3 plus or minus 0.2 Ma for molybdenite at Rosario is slightly younger than the 40Ar/39Ar age of illite, but older than the alunite. A weighted mean plateau age of 32.7 plus or minus 1.6 Ma for hypogene alunite from the La Grande Cu-Ag-(Au) vein south of Rosario is indistinguishable from the age of Rosario alunite. At Ujina, the weighted mean 40Ar/39Ar plateau age of igneous biotite for a monzonite intrusion that hosts copper mineralization is 35.2 plus or minus 0.3 Ma. The monzonite is intruded by postmineralization porphyry dikes of similar composition, from which igneous biotite yielded a 40Ar/39Ar age of 34.7 plus or minus 0.3 Ma. Igneous biotite in the Rosario Porphyry cooled through its closure temperature during and after the formation of illite, implying that porphyry-style ore and alteration minerals in the Rosario deposit had formed by 34.3 Ma. The age of alunite at Rosario and La Grande indicate that a second discrete episode of hydrothermal activity was superimposed, 1.8 plus or minus 0.4 m.y. later, onto the earlier-formed porphyry Cu system. Hydrothermal activity at Ujina is constrained by the 40Ar/39Ar ages of igneous biotite in the premineralization and postmineralization intrusions and occurred during a minimum interval of 0.5 plus or minus 0.4 m.y. The biotite granite at La Profunda, 1.5 km east of Ujina, has a igneous biotite age of 81.2 plus or minus 2.9 Ma, indicating that this intrusion is unrelated to the mineralized Eocene-Oligocene porphyry intrusions at Ujina, Rosario, and Quebrada Blanca. The 40Ar/39Ar data for igneous biotite in the Rosario and Ujina porphyries are cooling ages after multiple hydrothermal events. In most cases, these are minimum hydrothermal ages (i.e., ages of the last high-temperature hydrothermal event to have affected the samples), not magmatic ages. However, the biotite age for the Inca Porphyry is possibly only slightly younger than the intrusion age because biotite has not been affected by later hydrothermal alteration.

Lead sources in Mesozoic and Cenozoic Andean ore deposits, north-central Chile (30–34°S)

Mesozoic and Cenozoic ore deposits in the Chilean Andes between La Serena (~30°S) and Santiago (~34°S) include polymetallic vein, low- and high-sulfidation epithermal vein, skarn, porphyry copper-molybdenum and porphyry copper-gold. These deposits are associated with volcanic and plutonic complexes emplaced in eastward-migrating longitudinal arcs which formed during subduction along the continental margin of South America since the Middle Jurassic. Stratabound, but epigenetic, volcanic rock- and sedimentary rock-hosted manto deposits contain additional copper resources. Lead isotopic compositions in ore minerals from 29 deposits vary with age and geographic location, and hence with basement and host rocks. Lead in most ore deposits is derived from temporally related igneous rocks, except for the manto deposits whose lead is derived from host volcanic and sedimentary rock sequences. Lead in the ore deposits is dominated by two crustal sources. Low 207Pb/204Pb characterizes one source whereas high 207Pb/204Pb characterizes the second source. Lead isotopic compositions of Jurassic and Miocene ore minerals (206Pb/204Pb>18.50; 207Pb/204Pb>15.61) lie along the average crustal growth curve. By contrast, most Cretaceous deposits have ore minerals with lower 206Pb/204Pb (<18.39) and 207Pb/204Pb (<15.58) than Jurassic ore minerals. The shift in lead isotopic composition to lower lead isotopic values precludes derivation of lead from a source of similar composition to those in the Jurassic or Tertiary deposits. For Cretaceous deposits, polymetallic and low-sulfidation epithermal veins and a skarn have lower 206Pb/204Pb than a porphyry copper-gold system and peripheral gold veins at Andacollo (18.43–18.50). Late Cretaceous veins from the Bellavista deposit have the lowest 206Pb/204Pb (18.33) of all deposits. Ore minerals in Miocene and Pliocene porphyry copper-molybdenum deposits have higher 206Pb/204Pb (18.58–18.67) than Cretaceous deposits, consistent with their age being younger. The Miocene and Pliocene ore minerals also have higher 207Pb/204Pb (15.58–15.66) than Cretaceous ore minerals, thereby requiring an additional input from the high-207Pb/204Pb source into the younger deposits. Miocene auriferous deposits in the north have similar 206Pb/204Pb values as the Miocene and Pliocene porphyry copper-molybdenum deposits in the south, but they are distinguished by higher and variable 207Pb/204Pb (15.61–15.66) and 208Pb/204Pb (38.54–39.01), which are arrayed along steep mixing trends. These ore minerals have the largest input of high-207Pb/204Pb material in the deposits studied. By contrast, lead in the epigenetic manto deposits appears to be derived from the host volcanic or sedimentary rock-dominated sequences, and locally exhibits large-scale isotopic heterogeneity within a deposit. Overall, the lead isotopic compositions of ore minerals mimic the values and variations established in age-equivalent rock sequences. The low-207Pb/204Pb material in the deposits is derived from Cretaceous igneous rocks or their sources as they evolved with time; low 207Pb/204Pb characterizes these rocks. By contrast, high-207Pb/204Pb material is likely derived from Carboniferous to Triassic igneous rocks or their sources, as this lead isotopic characteristic dominates these rocks.

Two ages of porphyry intrusion resolved for the super-giant Chuquicamata copper deposit of northern Chile by ELA-ICP-MS and SHRIMP

Zircon U-Pb ages measured in situ by excimer laser ablation–inductively coupled plasma–mass spectrometry (ELA-ICP-MS) and verified by sensitive high-resolution ion microprobe (SHRIMP) on ore-bearing felsic porphyries from the Chuquicamata porphyry copper-molybdenum deposit, northern Chile, identify two discrete igneous events. The volumetrically dominant East porphyry has an age of 34.6 ± 0.2 Ma, whereas the Bench and West porphyries yield ages of 33.3 ± 0.3 Ma and 33.5 ± 0.2 Ma, respectively. The age of the East porphyry is indistinguishable from a Re-Os age for early molybdenite mineralization (35 Ma) and the oldest reported 40 Ar- 39 Ar ages for hydrothermal alteration, confirming a genetic link with mineralization. Previous geological studies and 40 Ar- 39 Ar and Re-Os geochronology identify two main hydrothermal events: high-temperature potassic alteration with chalcopyrite at 33.4 ± 0.3 Ma followed by lower temperature quartz- sericite alteration with pyrite at 31.1 ± 0.3 Ma. The ages of the West and Bench porphyries match the ages for potassic alteration. Younger quartz-sericite alteration may reflect an additional fourth intrusion concealed at depth. The anomalously large size of Chuquicamata appears to be due to a protracted igneous history resulting in the superposition of at least two temporally distinct magmatic-hydrothermal systems.

Laser probe 40 Ar/ 39 Ar ages of early- and late-stage alteration assemblages, Rosario porphyry copper-molybdenum deposit, Collahuasi District, I Region, Chile

Sensibly identical laser probe 40 Ar/ 39 Ar dates of 32.9 + or - 0.5 (2Sigma ) and 32.9 + or - 0.6 Ma are determined for, respectively, early-stage hydrothermal biotite and late-stage hydrothermal muscovite from the giant Rosario porphyry copper deposit in the Collahuasi district of northern Chile. The age determinations were carried out on ca. 1 cm 2 , 1- to 0.5-mm-thick wafers of altered rock or on smaller chips broken therefrom, obviating the requirement for mineral separation and permitting direct petrographic control of the analytical procedure. The age data, although not unambiguous, imply that this extremely large hydrothermal center was emplaced over a short interval. Hydrothermal activity in this central Andean transect migrated eastward over a period of ca. 3 to 4 m.y., from Copaquire, through Quebrada Blanca to Rosario, the intensity of mineralization increasing with time. Rosario, Chuquicamata, and La Escondida, the largest of the West Fissure (Domeyko fault system) array, were apparently emplaced coevally, in the terminal stage of the late Eocene-early Oligocene metallogenic episode.

The Myszków Porphyry Copper-Molybdenum Deposit, Poland

The porphyry copper-molybdenum deposit at Myszkow, south-central Poland, lies in the Cracow-Silesian orogenic belt, in the vicinity of a Paleozoic boundary between two tectonic plates. The deposit ...

Some secondary ore formation features of the Sar-Cheshmeh porphyry copper-molybdenum deposit, Kerman, Iran

New secondary mineral features and the control of secondary molybdenum enrichment have now been recognized at the Sar-Cheshmeh copper deposit. In part of the deposit, copper is locally concentrated within a subvolcanic unit — the so-called Late Fine Porphyry in amounts that seem too great to have come from the primary phase of the same unit. Clay minerals are believed to have influenced the deposition of copper by through-flowing oxidized zone solutions within this otherwise subeconomic unit. These solutions have caused argillic alteration and copper concentration within the plagioclase phenocrysts. Fractures are filled with chrysocolla, but little chrysocolla was observed within the altered plagioclase phenocrysts. The scattered layers of sedimentary “unroofing breccia” — horizontal layers of Quaternary age accumulated within the depressions at the top of the deposit — contain minor amounts of native copper. The breccia consists of angular to subangular clasts of local derivation cemented by a matrix of hematite, limonite, geothite, and small rock fragments. Native copper, the only copper mineral associated with the breccia, mainly adhers to the cavity walls in the cementing materials. The amount of molybdenum within the leached capping of the Sar-Cheshmeh deposit is generally controlled by the amount of pyrite. Abundant pyrite has caused the retention of molybdenum within the leached capping of the pyritic halo as a result of the more acidic solution during weathering, in contrast to the leached zone of the pyrite-poor stock, where molybdenum is partially mobile.

Hypogene alteration at the Berg porphyry copper-molybdenum property, north-central British Columbia

The Berg porphyry copper-molybdenum deposit is in the Tahtsa mountain ranges, approximately 84 km southwest of Houston, British Columbia. It is localized in and adjacent to one of several mineralized ca. 50-m.y.-old quartz monzonite intrusions in the area. Two potential orebodies with estimated reserves of 308 million metric tons of 0.348 percent Cu and 0.052 percent MoS 2 (with a 0.25% copper equivalent cutoff) are centered in a highly fractured zone. Hydrothermal alteration zones are related spatially to a central quartz monzonite stock and extend up to 1,000 m from the contact. Hypogene alteration types are divided into facies, zones, and subzones based on mineralogic associations. Potential orebodies occur in potassically altered, hornfelsic volcanic rocks close to the stock. Potassium enrichment of the hornfels is demonstrated using major element ternary and binary plots that compare rocks from the mineralized zone with fresh, albeit distant, regional equivalents. Hypogene mineralization is characterized primarily by several generations of veins; disseminated sulfides are important only in the central part of the stock and in the quartz diorite to the northeast, where fracture densities are low. The earliest veins, designated as types 1 and 2, contain much of the copper and molybdenum mineralization. Associated alteration envelopes are either potassic or nonexistent, implying equilibrium with the potassically altered wall rocks. Later veins defined as types 3 and 4 are characterized by low sulfide contents and phyllic or propylitic mineral assemblages.--Modified journal abstract.

Metallogeny and tectonics of porphyry copper–molybdenum deposits in British Columbia

Metallogenic studies in British Columbia are based on data for over 1200 deposits that include metals and ore minerals recorded, age and lithology of sedimentary and volcanic host rocks, and the nature of any related granitoid intrusions. Analysis of these data defines a bimodal source of metals in porphyry deposits. Cu and Mo are derived from the primary granitoid melt, and the Cu/Mo ratio reflects the composition of that melt. Pb, Zn, Ag, and Au occur in veins peripheral to porphyry intrusions, and the metal content of these veins is controlled by the lithology of their wall rocks.Petrogenetic constraints imply that granitoids are predominantly anatectic crustal melts; thus we use the distribution of porphyries with varying metal ratios to define a map of the lower crust, which then constrains a tectonic model. Two major volcanic–plutonic arcs (Coastal: Jurassic – early Tertiary; Takla: Triassic) were generated above two subduction zones. Anatexis of the associated underplates produced Cu porphyries. The Cu–Mo porphyries of the Intermontane Belt were produced by anatexis of mixed volcanic, volcaniclastic, and clastic sedimentary rocks deposited on a fragment of oceanic crust trapped between the two magmatic arcs in the Jurassic.

The geological context of molybdenum occurrences in the Southern Norrbotten Region, Northern Sweden

A molybdenum province of Middle Proterozoic age is described from the Southern Norrbotten region (Northern Sweden). This province occurs at the margin of an inferred Karelian continent and is limited to the south by an important ESE-WNW trending palaeogeographic divide between a marine, possibly oceanic domain, and a continental domain. The molybdenum mineralizations occur as: (a) porphyry copper-molybdenum deposits found in calc-alkaline plutonic rocks which belong to an early orogenic event and, speculatively, are related to subduction movements; (b) Mo-mineralizations associated with aplitic granites, granite cupolas, pegmatites and brecciated, strongly altered acid volcanics. These deposits are generally associated with acid magmatic rocks of a more alkaline nature. They probably belong to a late tectono-magmatic event and seem to be related to the development of rift systems.

Characteristics of igneous intrusions and various wall rocks in some Precambrian porphyry copper-molybdenum deposits in Pohjanmaa, Finland

Several minor porphyry-type deposits occur at the margin of the granitoid complex of central Finland. They are associated with Proterozoic felsic to intermediary porphyries that intruded a previously deformed and metamorphosed metaturbidite-volcanic rock environment. Three examples from Pohjanmaa, central Finland, are given.The Cu-Au mineralization of Kopsa is located in an elongate, hypabyssal stock of tonalite porphyry, which measures 600 X 2,000 m, and is controlled by the intersection of northeast- and northwest-striking faults. The deposit shows a concentric zonal arrangement of base metal contents. The host rock has been affected by potassium metasomatism and by propylitic alteration, and pyrite-pyrrhotite zone occurs in the surrounding metaturbidites.The Mo mineralization of Rautio was emplaced at the center of a multiphase batholith, 15 X 25 km in size. The batholith exhibits a concentric arrangement of plutonic rocks; these are, successively from the margin to the center, diorite, monzodiorite, quartz monzodiorite, and granodiorite, the last being the host rock of the mineralization.The Mo occurrence of Vinnolinneva is located at the edge of a large continuous plutonic area. The host rock is a tonalite porphyry cutting a large quartz diorite to diorite body, 3x6 km in extent. The mineralization is of stockwork type with a dense network of quartz veins.

Acoustic core logging in blast-damaged rock

Thill, R.E. and D' Andrea, D. V., 1975. Acoustic core logging in blast-damaged rock. Eng. Geol., 10: 13–36. The Bureau of Mines, in cooperation with the Duval Corp., conducted a blast-fragmentation experiment to determine the feasibility of preparing a porphyry copper-molybdenum deposit for in-situ leaching. The blast was designed with ten 9-inch-diameter blastholes to depths of 110 feet in an equilateral triangle configuration; spacings between blastholes were 15, 20, and 25 ft. One of the major problems in the experiment was in assessing blast damage. Acoustic core-logging equipment and methods were devised and used as one approach in solving this problem. Ultrasonic pulse travel-times were determined in four diametral directions at 2-ft intervals of depth to a final depth of 120 ft in three preblast and six postblast drill cores at the Duval test site. The acoustic logging program provided compressional wave travel-time at 0°, 45°, 90°, and 135° around the core circumference, maximum travel-time difference, mean compressional-wave velocity, and an anisotropy factor. Other acoustic parameters introduced in the analyses were stiffness modulus, seismic quality designation (SQD), and a compensated velocity to account for portions of the core that were nonrecoverable or too highly fractured to permit diametral travel-time measurements. The acoustic parameters all indicated the deterioration in structural quality from the preblast condition, in which the rock already was badly fractured and weathered, to the more highly fractured postblast condition. Because of the highly fragmented, poor structural condition of the rock after blasting, the rock was indicated to be suitable for in-situ leaching, at least at the 20- and 15-ft blasthole spacings, and even in some zones in the rock at the 25-ft blasthole spacing.

The zonal distribution of selected elements above the Kalamazoo porphyry copper deposit, San Manuel district, Pinal County, Arizona

Abstract There may be many as-yet-undiscovered porphyry copper deposits that exist as blind deposits deep within exposed rock bodies. The Kalamazoo porphyry copper-molybdenum deposit is a blind deposit present at depths up to at least 1,000 m (about 3,200 ft) that contains zoning features common to many of the known porphyry copper deposits found in western North and South America. As the preliminary phase in a geochemical study of the Kalamazoo deposit, whole-rock samples of core and cuttings from two drill holes have been analyzed for 60 different elements. Each hole represents a different major rock unit and each has penetrated completely through all the existing alteration zones and the ore zone. Plots of concentration vs. depth for 17 selected elements show distinct high- or low-concentration zones that are spatially related to the ore zone. For most of the ore-related elements no significant correlation with the two lithologies is apparent. The spatial distribution and abundance of elements such as Co, Cu, S, Se, Mn, Tl, Rb, Zn, B, and Li may be useful in determining the direction for exploration to proceed to locate a blind deposit. Trace element studies should be valuable in evaluating areas containing extensive outcrops of rocks with disseminated pyrite. Elemental zoning should be at least as useful as alteration-mineralization zoning for evaluating rock bodies thought to contain blind deposits similar to the Kalamazoo deposit.

Alternative Interpretations for the Casino Complex and Klotassin Batholith in the Yukon Crystalline Terrane

A small part of the Klotassin batholith, that intrudes the Yukon Metamorphic Complex, has been studied in detail near the Casino porphyry copper–molybdenum deposit, Yukon Territory. Eleven potassium–argon model ages, including concordant biotite and hornblende ages, indicate a mid-Cretaceous (100 Ma) age for this batholith. All ages are indistinguishable statistically. This age, therefore, is interpreted as the age of emplacement of the Klotassin batholith. Older dates can be expected from rocks of the Yukon Metamorphic Complex, the Carmacks batholith, and Aishihik batholith.The Casino complex, host to the Casino deposit, is dated at latest-Cretaceous (70 Ma), based on potassium–argon model ages from two samples of biotite. The complex is clearly intrusive into and younger than the Klotassin batholith. Previously, this unit was interpreted as Early Tertiary. Possibly the several so called Early Tertiary volcanic and hypabyssal rocks in Yukon are not strictly contemporaneous.

Mineral Resources of Ecuador-Development and Prospects: ABSTRACT

Mineral deposits in Ecuador are many but only one metalliferous mine, the Portovelo gold mine, now is operating. It was worked in colonial times but on a systematic basis only since 1904. Gold was the principal product together with silver, copper, lead, and zinc. At present, copper is more important but the ore reserves are virtually exhausted. The Macuchi mine was worked for copper, gold, and silver in the 1940s and prospects containing various combinations of copper, silver, lead, and zinc were discovered at La Plata, Sigchos, Molleturo, and Pilshum. In 1965 the United Nations in cooperation with the newly established Servicio Nacional de Geologia y Mineria began a seven-year mineral resources survey. The principal discovery was the Chaucha porphyry copper-molybdenum deposit where 55 million tons of 0.7-percent copper were proved. Other porphyry copper-molybdenum deposits were discovered at Los Linderos, Rio Playas, and Fierro Urco. At Fierro Urco 50 million tons of ore with low-gold values was indicated. Vein-type mineralization at Angas (copper, lead, zinc, silver, and gold), San Bartolome (silver and lead), and Uritohuaser (zinc, lead, and silver) also were discovered. Anomalies in tin and tungsten minerals were found near Saraguro. Most surveys have been in the southern Andes where exposed Tertiary and older rocks are intruded by granitic bodies. The flanks of the Andes have not been explored thoroughly because of difficult access and dense forest. Much of the north-central Andes has a cover of Quaternary volcanic rocks mantling the older rocks, but until there are methods to probe through this thick mantle for possible mineralization the potential is unknown. The prospects however must be good because the Ecuadorian Sierra is part of the great mineralized mountain belt extending through the western Americas. End_of_Article - Last_Page 1451------------