Geology topics

Thomas Monecke

Publications and source records attributed to Thomas Monecke.

15 recordsLinked to original sources

USGS2026

Microtextural characteristics of adularia in banded quartz veins from the Midas low-sulfidation epithermal deposit, Nevada

High-grade ores at the Miocene Midas low-sulfidation epithermal deposit in northern Nevada are confined to crustiform quartz veins containing abundant adularia. Micro-X-ray fluorescence elemental mapping reveals that adularia is a common gangue mineral occurring in colloform bands, bands showing bladed textures, and bands with dendritic terminations. The adularia aggregates have delicate shapes and are comprised of stacked, submillimeter crystals hosted by fine-grained quartz. The textural evidence suggests that the adularia aggregates originally formed within a gel-like, noncrystalline silica matrix, which subsequently transformed into quartz. This indicates that the adularia did not precipitate in open space along the vein walls. Correlative microscopy, involving scanning electron microscopy-based automated mineralogy and optical petrography, demonstrates that bands containing abundant adularia are not the primary host to ore minerals. The ore minerals occur in different bands within the crustiform veins, implying that adularia and ore mineral precipitation did not always occur simultaneously. It is hypothesized here that fluid flow at Midas involved intermittent short-lived events of fluid flashing, causing rapid solute supersaturation in the liquid. During each flashing event, different amounts of vapor were produced along a given vein. Compositional differences between adjacent bands in the crustiform quartz veins may, therefore, be linked to variations in the amount of vapor formed during each flash event.

Nevada↗

USGS2025

Utilizing downhole datasets for modelling the aeromagnetic signature of the Iron Creek Co-Cu deposit in the Idaho Cobalt Belt

The Idaho Cobalt Belt in east-central Idaho is host to some of the largest domestic Co resources, including the informal Iron Creek deposit. The two main ore zones of this deposit, the Iron Creek and the Ruby, are hosted in greenschist-grade interbedded argillite/siltstone and quartz-rich units of the Mesoproterozoic Apple Creek Formation of Lemhi Group. The primary ore mineral is cobalt-bearing pyrite, which occurs with pyrrhotite, chalcopyrite, and magnetite. This study integrates mineralogical and core-scale geophysical measurements with regional-scale aeromagnetic surveys. The high magnetite content within the Ruby zone produces elevated magnetic susceptibility, but the relatively limited spatial footprint of the ore zone would produce a small-scale anomaly that may be overlooked in regional surveys. The low magnetite content in the Iron Creek zone results in low magnetic susceptibility, creating a relatively low amplitude geophysical response. By characterizing the magnetic properties and mineralogy of these ore zones, this study enhances the interpretation of aeromagnetic data, enabling the identification of small or faint anomalies as potential Co targets. These findings improve can improve exploration strategies, both within the Idaho Cobalt Belt as well as for similar deposit types globally.

Idaho↗

USGS2025

Low-sulfidation epithermal deposits of the central Basin and Range Province, USA

The Basin and Range Province is host to many important low-sulfidation epithermal deposits. Within this broad zone of extension, epithermal deposits are hosted by specific areas of Miocene and younger bimodal volcanism. In northern Nevada, rifting and related volcanic activity occurred in response to thermal bulging during the development of the Yellowstone hotspot. The Colorado River Extensional Corridor in southern Nevada, adjoining eastern California and northwestern Arizona, is a major zone of Miocene crustal extension formed during the transformation of the western margin of North America from a convergent to a transform plate boundary. The style of mineralization in the low-sulfidation epithermal deposits in these areas is strongly controlled by the nature of the volcanic successions. Highgrade, low-tonnage deposits are commonly found in flow-dominated volcanic successions where ore deposition occurred through short-lived periods of fluid flashing. In contrast, low-grade, large-tonnage deposits are more commonly located in clastic-dominated successions where fluid infiltration of the permeable hosts and cooling predominated.

Arizona, California, Nevada, Oregon↗

USGS2024

Microtextural evidence for the recrystallization of opal-A to quartz in epithermal veins: A case study from the McLaughlin deposit, California

High-grade ores in low-sulfidation epithermal deposits commonly consist of banded veins containing quartz as the most abundant gangue mineral. Previous studies suggested that at least some of the quartz has formed as a product of recrystallization from a noncrystalline silica precursor. Detailed petrographic studies confirm that high-grade veins from the <2.2 Ma McLaughlin deposit in California were originally entirely composed of opal-A G . The noncrystalline silica is isotropic in crossed-polarized light and consists of compacted microspheres that are up to several micrometers in size. In many bands of the high-grade veins, the thermodynamically unstable opal-A G has matured to quartz. The recrystallization of the noncrystalline silica resulted in the development of quartz and ore textures that mask the original conditions of vein formation. Incipient recrystallization of the opal-A G caused the formation of lepispheres consisting of opal-CT as well as the development of concentrically banded silica spheres. Continued maturation led to the growth of elongated quartz crystals or complexly shaped quartz aggregates in the cores of the concentrically banded silica spheres. Amalgamation of quartz crystals caused the development of mosaic quartz or flamboyant quartz. Ripening produced large prismatic quartz crystals, which are characterized by zones of feathery appearance. Fluid inclusions within the quartz formed through recrystallization are typically highly irregular in shape and show inconsistent liquid-to-vapor volumetric proportions, but assemblages with consistent ratios are also present. Bands rich in fluid inclusions occurring along former recrystallization fronts in prismatic quartz crystals resemble growth zones in zonal quartz. Recrystallization of the silica matrix resulted in grain coarsening of the ore minerals and encapsulation of ore minerals by quartz. In areas where recrystallization of the noncrystalline silica proceeded to completeness, the quartz textures could be easily misinterpreted to indicate that quartz growth occurred in open space with the ore minerals infilling vug spaces. Correct recognition of recrystallization textures has significant implications for paragenetic investigations on vein material from epithermal deposits as well as the design of fluid inclusion studies.

California↗

USGS2023

Evidence for a high-level porphyritic intrusion below the Sunnyside epithermal vein deposit, Colorado

High-temperature quartz veins were identified in drill core at ~600 m below the Sunnyside epithermal base and pre-cious metal deposit in southwestern Colorado. The veins consist of early anhedral quartz that shows a bluish ca-thodoluminescence emission and hosts heterogenous silicate melt inclusions. The early quartz is overgrown by a later generation of quartz that exhibits euhedral termina-tions with oscillatory growth zones showing a bright pink to purple cathodoluminescence emission. Both types of quartz are crosscut by ubiquitous planes of vapor-rich inclusions and some hypersaline liquid inclusions. In addi-tion, secondary planes of intermediate-density inclusions occur. The petrographic characteristics of the two quartz types are similar to those in ‘A’ and ‘B’ veins encountered in shallow- and intermediate-depth porphyry deposits. The relationships at Sunnyside imply that these high-temperature veins formed from magmatic-hydrothermal fluids derived from an intrusion located not far below the lowest level of drilling. Sunnyside appears to be a rare example of an epithermal deposit that is directly connected to a high-level porphyritic intrusion.

Colorado, New Mexico↗

USGS2022

Formation of orogenic gold deposits by progressive movement of a fault-fracture mesh through the upper crustal brittle-ductile transition zone

Orogenic gold deposits are comprised of complex quartz vein arrays that form as a result of fluid flow along transcrustal fault zones in active orogenic belts. Mineral precipitation in these deposits occurs under variable pressure conditions, but a mechanism explaining how the pressure regimes evolve through time has not previously been proposed. Here we show that extensional quartz veins at the Garrcon deposit in the Abitibi greenstone belt of Canada preserve petrographic characteristics suggesting that the three recognized paragenetic stages formed within different pressure regimes. The first stage involved the growth of interlocking quartz grains competing for space in fractures held open by hydrothermal fluids at supralithostatic pressures. Subsequent fluid flow at fluctuating pressure conditions caused recrystallization of the vein quartz and the precipitation of sulfide minerals through wall-rock sulfidation, with some of the sulfide minerals containing microscopic gold. These pressure fluctuations between supralithostatic to near-hydrostatic conditions resulted in the post-entrapment modification of the fluid inclusion inventory of the quartz. Late fluid flow occurred at near-hydrostatic conditions and resulted in the formation of fluid inclusions that have not been affected by post-entrapment modification as pressure conditions never returned to supralithostatic conditions. This late fluid flow is interpreted to have formed the texturally late, coarse native gold that occurs along quartz grain boundaries and in open spaces. The systematic evolution of the pressure regimes in orogenic gold deposits such as Garrcon can be explained by relative movement of fault-fracture meshes across the base of the upper crustal brittle-ductile transition zone. We conclude that early vein quartz in orogenic deposits is precipitated at near-lithostatic conditions whereas the paragenetically late gold is introduced at distinctly lower pressure.

Scientific Reports↗

USGS2022

Late Jurassic-Early Cretaceous orogenic gold mineralization in the Klamath Mountains, California: Constraints from 40Ar/39Ar dating of hydrothermal muscovite

The Klamath Mountains gold province is the second most important historical producer in California, having produced more than 7 Moz of gold from both lode and placer sources. Hydrothermal muscovite grains from gold-bearing veins provide the first 40 Ar/ 39 Ar age constraints indicative of a protracted period of mineralization in the Klamath Mountains. The data indicate that the window for orogenic gold mineralization in the Klamath Mountains was from ∼160–140 Ma, coinciding in age with the oldest orogenic gold deposits in the Sierra Nevada foothills province to the south, although mineralization continued in the latter along the Mother Lode belt for another 20–30 million years. Despite the broad age overlap between hydrothermal activity and magmatism, the former relates to a regional thermal event, and there is no genetic link between the two. Instead, a correlation exists between the timing of lode gold formation and discrete tectonic events, changes in stress regime, and fault movement. The maximum age corresponds to a major plate reorganization in the Pacific basin and initial gold mineralization continuing into the Sierra Nevada foothills gold province to the south. The minimum age corresponds to the westerly lateral offset of the Klamath Mountains from the Sierra Nevada and the then-active arc, marking the termination of both magmatism and gold-producing hydrothermal activity in the Klamath Mountains. Lode gold mineralization in the Klamath Mountains is compatible with a crustal source of metals and fluids that were released during metamorphic devolatilization and focused via hydrothermal fluid flow along regional faults.

California↗

USGS2021

Paragenesis of an orogenic gold deposit: New insights on mineralizing processes at the Grass Valley District, California

The Grass Valley orogenic gold district in the Sierra Nevada foothills province, central California, is the largest historical gold producer of the North American Cordillera. Gold mineralization is associated with shallowly dipping north-south veins hosted by the 160 Ma Grass Valley granodiorite to the southwest of the Grass Valley fault and steeply dipping east-west veins in accreted oceanic rocks to the northeast of this major fault. Quartz veins from both vein types show well-preserved primary textural relationships. Using a combination of petrographic and microanalytical techniques, the paragenetic sequence of minerals within the veins and the compositions of ore minerals were determined to constrain the mechanisms of quartz vein formation and gold deposition. The veins are composed of early quartz that formed through cooling of hydrothermal fluids derived from a geopressured reservoir at depth. The early quartz shows growth zoning in optical cathodoluminescence and contains abundant growth bands of primary inclusions. The primary inclusion assemblages and myriads of crosscutting secondary fluid inclusions have been affected by postentrapment modification, suggesting that early quartz formation was postdated by pronounced pressure fluctuations. These pressure fluctuations, presumably involving changes from lithostatic to hydrostatic conditions, may be related to fault failure of the host structure as predicted by the fault-valve model. Fluid flow associated with pressure cycling took place along microfractures and grain boundaries resulting in extensive recrystallization of the early quartz. Deposition of pyrite, arsenopyrite, and first-generation gold from these hydrothermal fluids causing recrystallization of the early quartz occurred as a result of wall-rock sulfidation. The gold forms invisible gold in the compositionally zoned pyrite or micron-sized inclusions within pyrite growth zones. The latest growth zones in euhedral quartz crystals that formed in association with this stage of the paragenesis contain very rare primary fluid inclusions that have not been affected by postentrapment modification. The hydrothermal system transitioned entirely to hydrostatic conditions immediately after formation of the latest quartz, explaining the preservation of the primary fluid inclusions. The formation of minor quartz in open spaces was followed by the deposition of second-generation native gold and telluride minerals that are commonly associated with base metal sulfides. Ore formation at this stage of the paragenesis is attributed to the rapid decompression of hydrothermal fluids escaping from the geopressured part of the crust into the overlying hydrostatic realm. There is no fluid inclusion evidence that this pressure drop resulted in fluid immiscibility of the hydrothermal fluids. Fluid inclusion evidence suggests that the north-south veins formed at a paleodepth of ~8 km, whereas the east-west veins appear to have formed at ~10 to 11 km below surface, confirming previous inferences that the NE-dipping Grass Valley reverse fault accommodated a large displacement. The findings of the study at Grass Valley have significant implications for the model for orogenic gold deposits, as the reconstruction of the paragenetic relationships provides evidence for the occurrence of two discrete events of gold introduction that occurred at different conditions during the evolution of the hydrothermal system.

California↗

USGS2019

Formation mechanisms of quartz veins in orogenic gold deposits: Insights from Grass Valley, California, USA

The orogenic gold veins of Grass Valley, California, USA, compose the historically richest lode gold district in the North American Cordillera. Petrographically, the veins exhibit a range of primary textural relationships allowing the reconstruction of the paragenetic sequence of mineral formation. Two generations of quartz are distinguished by optical cathodoluminescence microscopy and fluid inclusion petrography. Early quartz formed under conditions of variable pressures, ranging from supralithostatic to sublithostatic, whereas late quartz formed entirely at hydrostatic conditions. Subsequent sulfide formation occurred from fluids increasingly buffered by the host rocks. Late growth zones in pyrite are enriched with elements derived from the local host rock and contain abundant gold inclusions. Gold was never observed encapsulated by the quartz, rather it resides along grain boundaries and fractures in the quartz. The results of this study underscore the importance of pressure fluctuations during quartz vein formation but suggest that the bulk of the gold in these orogenic systems was introduced late in the paragenesis when fluid flow occurred at hydrostatic conditions.

California↗

USGS2018

Structural evolution of a gold-bearing transtensional zone within the Archean Porcupine-Destor deformation zone, southern Abitibi greenstone belt, eastern Ontario, Canada

The Garrison camp comprises four structurally distinct orogenic gold deposits that formed in different host lithologies during progressive deformation. Detailed field mapping, drill core logging, and geochronological constraints suggest that the 2678 ± 2 Ma Garrison granitic stock played a fundamental rheological role in the location of the four deposits. Initial local shear movement occurred along the southwestern margin of the stock leading to the development of the NW-trending sinistral NE-side-up Buffonta shear zone, which hosts the Buffonta deposit. Subsequently, a transtensional zone formed between the NE-trending sinistral Porcupine-Destor and Munro deformation zones, which host the 903 and Jonpol deposits, respectively. Finally, a local change in shortening orientation from NE to NNW caused westerly directed extension resulting in the formation of the younger gold-bearing veins composing the Garrcon deposit. In situ U-Pb laser ablation-inductively coupled plasma-mass spectrometry performed on monazite grains formed within the shear fabric of the Munro deformation zone indicates that transtension occurred at 2657 ± 15 Ma. Therefore, at least three of the four deposits formed subsequent to crystallization of the Garrison granitic stock. The reported U-Pb dates represent the first direct age constraints on the movement along a gold-bearing deformation zone in the southern Abitibi greenstone belt.

Ontario↗

USGS2018

Quartz solubility in the H2O-NaCl system: A framework for understanding vein formation in porphyry copper deposits

Porphyry copper deposits consist of low-grade stockwork and disseminated sulfide zones that contain characteristic vein generations formed during the evolution of the magmatic-hydrothermal systems. The present contribution proposes an interpretive framework for the formation of porphyry veins that is based on quartz solubility calculations in the H 2 O-NaCl system at temperatures of 100° to 1,000°C and pressures of 1 to 2,000 bar. The model predicts that high-temperature (≳500°C) quartz in A veins of deep (≳4 km) porphyry deposits forms as a result of the cooling of ascending intermediate-density fluids at lithostatic conditions. In deposits of intermediate depths (~1.5–4 km), A vein quartz is mostly formed through cooling of ascending hydrothermal fluids under closed-system conditions or quasi-isobaric cooling under open-system conditions within the two-phase field of the H 2 O-NaCl system. In shallow (≲1.5 km) porphyry deposits, rapid decompression can also result in quartz precipitation, forming so-called banded veins. The high-temperature quartz in A veins is associated with potassic alteration. During continued cooling of the magmatic-hydrothermal system, quartz is formed at intermediate temperatures (≳375°–500°C). This quartz overprints earlier A veins and forms B veins. The fluid inclusion inventory of this quartz generation suggests formation at fluctuating pressure conditions, marking the lithostatic to hydrostatic transition, and the change of wall-rock behavior from ductile to brittle conditions. The quartz is precipitated because of cooling and decompression of the magmatic-hydrothermal fluids under K-feldspar-stable conditions. Textural evidence from many porphyry veins suggests that hypogene sulfide minerals present in A and B veins postdate the quartz, as contacts between quartz and sulfide minerals commonly show dissolution textures. Hypogene sulfide minerals in C veins form at conditions of retrograde quartz solubility, explaining why these veins contain little to no quartz. The quartz solubility calculations suggest that C vein formation occurs at temperatures of ~375° to 450°C from low-salinity, single-phase fluids escaping from the lithostatic to the hydrostatic environment. At the upper end of this temperature range, C veins are biotite stable. However, these veins are associated with chlorite, chlorite-K-feldspar, or chlorite-sericite alteration in most deposits. Late quartz is formed during continued cooling of the hydrothermal fluids at ≲375°C within the single-phase field of the H 2 O-NaCl system as quartz solubility under these conditions decreases with temperature. This process is responsible for the formation of quartz in D veins and later base metal-bearing E veins, which are associated with phyllic, advanced argillic, or argillic alteration.

Economic Geology↗

USGS2017

Mineral potential mapping in an accreted island-arc setting using aeromagnetic data: An example from southwest Alaska

The distribution of volcanogenic massive sulfide (VMS), porphyry-epithermal, Alaska-type ultramafic-mafic complexes, intrusion-related Au, and granitoid Sn-W ore deposits in southwest Alaska supports current metallogenic models linking the formation of these deposit types to the emplacement of different suites of igneous rocks during the evolution of this convergent plate margin. Regional-scale aeromagnetic data provide a continuous set of observations over the deposits and show contrasting patterns over the igneous rock suites hosting the various deposit types. Combined with surface geologic data and regional metallogenic constraints, aeromagnetic data—filtered to enhance the anomalous magnetic field and map magnetic domains—were used to produce a mineral potential map across this accreted island-arc setting. The reduced-to-pole, upward continuation, and total horizontal gradient transform maps show anomalies that could represent porphyry-epithermal deposits within the intraoceanic- and continental-arc terranes. The tilt derivative transform highlights lineaments within the back arc that may represent zones with potential for VMS deposits. The truncations of tilt derivative lineaments outline a major magnetic domain boundary between the back-arc and craton margin, which is prospective for granitoid Sn-W deposits. Annular tilt derivative highs outline granitoids that could be associated with intrusion-related Au deposits within the craton margin. Shallow, magnetite-rich Alaska-type ultramafic-mafic complexes are mapped by their short-wavelength, high-amplitude anomalies. Successful mineral potential mapping across southwestern Alaska as performed in the present study suggests that filtered aeromagnetic data can be effectively used in mineral exploration in convergent continental margin settings.

Alaska↗

USGS2015

Application of U-Th-Pb phosphate geochronology to young orogenic gold deposits: New age constraints on the formation of the Grass Valley gold district, Sierra Foothills province, California

The Grass Valley orogenic gold district in the Sierra Nevada foothills province, central California, the largest historic gold producer of the North American Cordillera, comprises both steeply dipping east-west (E-W) veins located along lithologic contacts in accreted ca. 300 and 200 Ma oceanic rocks and shallowly dipping north-south (N-S) veins hosted by the Grass Valley granodiorite; the latter have yielded about 70 percent of the 13 million ounces of historic lode gold production in the district. The oceanic host rocks were accreted to the western margin of North America between 200 and 170 Ma, metamorphosed to greenschist and amphibolite facies, and uplifted between 175 and 160 Ma. Large-scale magmatism in the Sierra Nevada occurred between 170-140 Ma and 120-80 Ma, with the Grass Valley granodiorite being emplaced during the older episode of magmatism. Uranium-lead isotopic dating of hydrothermal xenotime yielded the first absolute age of 162±5 Ma for the economically more significant N-S veins. The vein-hosted xenotime, as well as associated monazite, are unequivocally of hydrothermal origin as indicated by textural and chemical characteristics, including grain shape, lack of truncated growth banding, lack of a Eu anomaly, and low U and Th concentrations. Furthermore, the crack-seal texture of the veins, with abundant wallrock slivers, suggests their formation as a result of episodic fluid flow possibly related to reoccurring seismic events, rather than a period of fluid exsolution from an evolving magma. The N-S veins are temporally distinct from a younger 153-151 Ma gold event that was previously reported for the E-W veins. Overlapping U-Pb zircon (159.9±2.2 Ma) and 40Ar/39Ar biotite and hornblende (159.7±0.6 to 161.9±1.4 Ma) ages and geothermobarometric calculations indicate that the Grass Valley granodiorite was emplaced at ca. 160 Ma at elevated temperatures (~800°C) within approximately 3 km of the paleosurface and rapidly cooled to the ambient temperature of the surrounding country rocks (<300°C). The age of the granodiorite is indistinguishable from that of the N-S veins, as recorded by the U-Pb age of xenotime in those veins. Consequently, the N-S veins must have formed between 162 and 157 Ma, the maximum permissive age of magma emplacement and the youngest permissive xenotime U-Pb age, respectively, during an E- to ENE-directed compressional regime. The geochemistry of the Grass Valley granodiorite is consistent with it being the product of arc magmatism. It served as a receptive host for mineralization, but it is has no direct genetic relationship to gold mineralization. Initial uplift of the intrusive mass correlates with the initial voluminous fluid flow event and vein formation at depths of no greater than 3 km. The E-W gold-bearing veins hosted within greenschist-facies country rocks adjacent to the intrusion formed during a second hydrothermal event 5-10 million years later than the magmatism and were contemporaneous with a shift to a transtensional deformation denoted by sinistral strike-slip faulting.

California↗

USGS2014

Three-dimensional distribution of igneous rocks near the Pebble porphyry Cu-Au-Mo deposit in southwestern Alaska: constraints from regional-scale aeromagnetic data

Aeromagnetic data helped us to understand the 3D distribution of plutonic rocks near the Pebble porphyry copper deposit in southwestern Alaska, USA. Magnetic susceptibility measurements showed that rocks in the Pebble district are more magnetic than rocks of comparable compositions in the Pike Creek–Stuyahok Hills volcano-plutonic complex. The reduced-to-pole transformation of the aeromagnetic data demonstrated that the older rocks in the Pebble district produce strong magnetic anomaly highs. The tilt derivative transformation highlighted northeast-trending lineaments attributed to Tertiary volcanic rocks. Multiscale edge detection delineated near-surface magnetic sources that are mostly outward dipping and coalesce at depth in the Pebble district. The total horizontal gradient of the 10-km upward-continued magnetic data showed an oval, deep magnetic contact along which porphyry deposits occur. Forward and inverse magnetic modeling showed that the magnetic rocks in the Pebble district extend to depths greater than 9 km. Magnetic inversion was constrained by a near-surface, 3D geologic model that is attributed with measured magnetic susceptibilities from various rock types in the region. The inversion results indicated that several near-surface magnetic sources with moderate susceptibilities converge with depth into magnetic bodies with higher susceptibilities. This deep magnetic source appeared to rise toward the surface in several areas. An isosurface value of 0.02 SI was used to depict the magnetic contact between outcropping granodiorite and nonmagnetic sedimentary host rocks. The contact was shown to be outward dipping. At depths around 5 km, nearly the entire model exceeded the isosurface value indicating the limits of nonmagnetic host material. The inversion results showed the presence of a relatively deep, northeast-trending magnetic low that parallels lineaments mapped by the tilt derivative. This deep low represents a strand of the Lake Clark fault.

Alaska↗

USGS2013

Geological analysis of aeromagnetic data from southwestern Alaska: Implications for exploration in the area of the Pebble porphyry Cu-Au-Mo deposit

Aeromagnetic data are used to better understand the geology and mineral resources near the Late Cretaceous Pebble porphyry Cu-Au-Mo deposit in southwestern Alaska. The reduced-to-pole (RTP) transformation of regional-scale aeromagnetic data shows that the Pebble deposit is within a cluster of magnetic anomaly highs. Similar to Pebble, the Iliamna, Kijik, and Neacola porphyry copper occurrences are in magnetic highs that trend northeast along the crustal-scale Lake Clark fault. A high-amplitude, short- to moderate-wavelength anomaly is centered over the Kemuk occurrence, an Alaska-type ultramafic complex. Similar anomalies are found west and north of Kemuk. A moderate-amplitude, moderate-wavelength magnetic low surrounded by a moderate-amplitude, short-wavelength magnetic high is associated with the gold-bearing Shotgun intrusive complex. The RTP transformation of the district-scale aeromagnetic data acquired over Pebble permits differentiation of a variety of Jurassic to Tertiary magmatic rock suites. Jurassic-Cretaceous basalt and gabbro units and Late Cretaceous biotite pyroxenite and granodiorite rocks produce magnetic highs. Tertiary basalt units also produce magnetic highs, but appear to be volumetrically minor. Eocene monzonite units have associated magnetic lows. The RTP data do not suggest a magnetite-rich hydrothermal system at the Pebble deposit. The 10-km upward continuation transformation of the regional-scale data shows a linear northeast trend of magnetic anomaly highs. These anomalies are spatially correlated with Late Cretaceous igneous rocks and in the Pebble district are centered over the granodiorite rocks genetically related to porphyry copper systems. The spacing of these anomalies is similar to patterns shown by the numerous porphyry copper deposits in northern Chile. These anomalies are interpreted to reflect a Late Cretaceous magmatic arc that is favorable for additional discoveries of Late Cretaceous porphyry copper systems in southwestern Alaska.

Alaska↗