Related Intrusive Rocks Research Articles

From the mantle source to the crustal sink: magmatic differentiation and sulfide saturation of the Paleoproterozoic komatiites of the Central Lapland Greenstone Belt, Finland

Paleoproterozoic (2.05 Ga) komatiites are widespread in the Central Lapland Greenstone Belt (CLGB), northern Finland. Close association with sulfur (S)-rich country rocks and spatiotemporal connection with the Cu-Ni(-PGE) deposits of Kevitsa and Sakatti make these komatiites interesting targets for sulfide deposit exploration. We provide whole-rock geochemical data from Sattasvaara komatiites and combine it with literature data to form a geochemical database for the CLGB komatiites. We construct a model for the komatiites from adiabatic melting of the mantle source to fractional crystallization at crustal conditions. Using MELTS, we calculate three parental melts (MgO = 20.6–25.7 wt%) in equilibrium with Fo92, Fo93, and Fo94 olivine for the CLGB komatiites. Based on REEBOX PRO simulations, these parental melts can form from a single mantle source by different pressures and degrees of melting when the potential temperature is 1575–1700 °C. We calculate ranges of S contents for the parental melts based on the different mantle melting conditions and degrees of melting. We use Magma Chamber Simulator to fractionally crystallize the parental melt at crustal conditions. These simulations reproduce the major element oxide, Ni, Cu, and S contents from our komatiite database. Simulated Ni contents in olivine are compatible with literature data from Kevitsa and Sakatti, hence providing a baseline to identify Ni-depleted olivine in CLGB komatiites and related intrusive rocks. We show that fractional crystallization of the komatiitic parental melt can form either Ni-rich or Cu-rich sulfide melt, depending on the initial Ni and S content of the parental melt.

Geoscience Poster G16: Potential field interpretation and modelling in the deep-water Otway Basin

Poster G16 The NW–SE trending Otway Basin is a well-established hydrocarbon province with onshore and shallow-water offshore discoveries. However, the deep-water offshore is underexplored due to the sparsity of data in the past. New 2D seismic, gravity and magnetic data, acquired during the 2020 Otway Basin Seismic Program, were combined with legacy seismic data to improve the understanding of this part of the basin. The new data have been processed and integrated with existing open-file data to derive Moho and lithosphere–asthenosphere boundary surfaces. Additional wavelength filtering of the newly acquired magnetic and gravity data, using constraints from seismic data, provides valuable information on the geometry and the spatial extent of igneous rocks in the deep-water basin. While extrusive igneous rocks and related intrusive rocks in the Otway Basin can be inferred from seismic reflection data, the new magnetic and gravity data provide additional constraints that help to map their distribution across the basin. To access the poster click the link on the right. To read the full paper click here

The first identification of early Paleoproterozoic (2.46–2.38 Ga) supracrustal rocks in the Daqingshan area, northwestern North China Craton: Geology, geochemistry and SHRIMP U-Pb dating

Early Paleoproterozoic supracrustal rocks are relatively rare worldwide and only a few have been reported in the North China Craton (NCC). Daqingshan is a typical area of the Khondalite Belt, northwestern NCC, where early Precambrian supracrustal and intrusive rocks are widely distributed, although the precise age of all units remains unconfirmed. The Lower Wulashan Group is mainly composed of acidic to mafic volcanic and volcano-sedimentary rocks that have undergone amphibolite to granulite facies metamorphism; it was previously considered to be of Mesoarchean age. This study utilized integrated field and petrographic work, whole-rock geochemistry, and Nd whole-rock and zircon U-Pb-Hf isotopic analyses on the Lower Wulashan Group and related intrusive rocks. SHRIMP U-Pb dating of eight samples from the supracrustal rocks, interpreted as metavolcanics, yielded magmatic zircon ages of 2455–2382 Ma, thus for the first time identifying early Paleoproterozoic supracrustal rocks in the Daqingshan area. Thus it is of great importance for understanding the Precambrian evolution of the NCC as well as the Earth because this is within the period when magmatism was globally weak. Six samples from a variety of intrusive rocks have zircon U-Pb ages of 2436–2362 Ma, thus further confirming the depositional ages of the supracrustal rocks. These rocks include meta-alkali basalt, and are high in total rare earth element contents and have magmatic zircon εHf(t) values of −3.00 to +6.62 and crustal model ages of 3.12–2.52 Ga (38 analyses), and whole-rock εNd(t) of +1.0 to +2.6 with crustal model ages of 2.74–2.60 Ga (5 analyses). It is considered that the early Paleoproterozoic supracrustal rocks of the Lower Wulashan Group and related intrusive rocks formed in a continental environment, but more work is required to determine the tectonic regime (extension or subduction).

Potential field interpretation and modelling in the deep-water Otway Basin

The NW–SE trending Otway Basin is a well-established hydrocarbon province with onshore and shallow-water offshore discoveries. However, the deep-water offshore is underexplored due to the sparsity of data in the past. New 2D seismic, gravity and magnetic data, acquired during the 2020 Otway Basin Seismic Program, were combined with legacy seismic data to improve the understanding of this part of the basin. The new data have been processed and integrated with existing open-file data to derive Moho and lithosphere–asthenosphere boundary surfaces. Additional wavelength filtering of the newly acquired magnetic and gravity data, using constraints from seismic data, provides valuable information on the geometry and the spatial extent of igneous rocks in the deep-water basin. While extrusive igneous rocks and related intrusive rocks in the Otway Basin can be inferred from seismic reflection data, the new magnetic and gravity data provide additional constraints that help to map their distribution across the basin.

Mineral chemistry of magnetite from magnetite-apatite mineralization and their host rocks: examples from Kiruna, Sweden, and El Laco, Chile

Interpretation of the mineralizing environment of magnetite-apatite deposits remains controversial with theories that include a hydrothermal or magmatic origin or a combination of those two processes. To address this controversy, we have analyzed the trace element content of magnetite from precisely known geographic locations and geologic environments from the Precambrian magnetite-apatite ore and host rocks in Kiruna, Sweden, and the Pliocene-Holocene El Laco volcano in the Atacama desert of Chile. Magnetite samples from Kiruna have low trace element concentrations with little chemical variation between the ore, host, and related intrusive rocks. Magnetite from andesite at El Laco, and dacite from the nearby Lascar volcano, has high trace element concentrations typical of magmatic magnetite. El Laco ore magnetite have low trace element concentrations and displays growth zoning in incompatible elements (Si, Ca, and Ce), compatible elements (Mg, Al, and Mn), large-ion lithophile element (Sr), and high field strength element (Y, Nb, and Th). The El Laco ore magnetite are similar in composition to magnetite that has been previously interpreted to have crystallized from hydrothermal fluids; however, there is a significant difference in the internal zoning patterns. At El Laco, each zoned element is either enriched or depleted in the same layers, suggesting the magnetite crystallized from a volatile-rich, iron-oxide melt. In general, the compositions of magnetite from these two deposits plot in very wide fields that are not restricted to the proposed fields in published discriminant diagrams. This suggests that the use of these diagrams and genetic models based on them should be used with caution.

The Empire Cu-Zn Mine, Idaho: Exploration Implications of Unusual Skarn Features Related to High Fluorine Activity

The Empire Cu-Zn skarn deposit is unusual because of the proximal position of Zn mineralization, abundance of endoskarn, and the extremely vermicular texture of quartz phenocrysts in the related intrusive rocks. Cu-Zn skarn occurs at the contact between Upper Mississippian White Knob limestone and the granite porphyry phase of the Mackay Stock which consists, from early to late, of quartz monzodiorite, granophyre, granite porphyry, porphyritic granite, and many dikes. The late phases have high F and also extremely vermicular quartz phenocrysts. Endoskarn is more abundant than exoskarn. The earliest alteration of the intrusive rocks consists of disseminated diopsidic pyroxene (Di64Hd36 to Di88Hd12), actinolite, and titanite. This assemblage was cut by early scapolite (Me18 to Me35, mostly Me18-26) and/or green pyroxene (Di14Hd80 to Di20Hd77) veinlets, with or without wollastonite halos. These early veins were then cut by main-stage endoskarn veins that typically have a garnet + minor pyroxene inner zone, a wollastonite and/or pyroxene ± Ca-rich plagioclase (An56 to An89) envelope, and a halo containing disseminated, fine-grained alteration minerals of the same assemblage as the envelope. Some veins contain only the envelope assemblage and are interpreted to represent the alteration front. The inner zone locally contains vesuvianite. Where many veins intersect, endoskarn is massive. Pyroxene is zoned around fluid conduits; the distal pyroxene is Fe rich (hedenbergitic) whereas the proximal pyroxene is Fe poor (diopsidic). The garnet changes in the opposite way, being Fe poor-Al rich (grossularitic) in locations distal to the fluid conduits, and Fe rich (andraditic) in proximal locations. In contrast, in the exoskarn, all pyroxene is diopsidic and garnet is andraditic. Weak, retrograde alteration composed of quartz + calcite + chlorite with minor fluorite, talc, and epidote overprinted both endoskarn and exoskarn. Magnetite precipitated after garnet-pyroxene in both endoskarn and exoskarn. Sphalerite precipitated together with chalcopyrite in proximal locations and is associated with retrograde alteration. Other ore minerals include minor molybdenite, bornite, pyrite, galena, arsenopyrite, native Au, as well as supergene minerals such as chrysocolla, malachite, azurite, native Cu, and limonite. Fluid xenoliths from pyroxene in early endoskarn veinlets homogenize at >600°C. Massive endoskarn and exoskarn replacing limestone inclusions in granite porphyry formed at 500° to >700°C, whereas the highest temperature inclusions, >700°C, occur in narrow garnet + minor pyroxene veins. Fluid inclusions in exoskarn replacing wall rock have homogenization temperatures of 500° to 650°C. Retrograde alteration and Cu-Zn mineralization occurred at 250° to 300°C. Fluid inclusions in prograde minerals contain daughter minerals, whereas fluid inclusions in retrograde minerals do not, indicating a decrease in salinity with time. Late-stage fluids have low eutectic temperatures, indicating the possible presence of KCl, NaCl, FeCl2, CaCl2, MgCl2, K2CO3, and/or Na2CO3. Formation of the unusually abundant endoskarn, the proximal position of Zn mineralization, and the extremely vermicular texture of quartz phenocrysts are all believed to have been promoted by the high F content of the magmatic fluid. These features may serve as exploration indicators of associated high F mineralization such as buried porphyry Mo deposits.

Evidence for simultaneous contraction and extension at different crustal levels during the Caledonian orogeny in NE Greenland

Leucogranites are common in the East Greenland Caledonides. Most granites cut across Caledonian contractional structures and have been interpreted as decompression melts related to late to post-orogenic collapse. To constrain the timing of the collisional event responsible for crustal thickening, zircon, monazite and xenotime were extracted from syn-contractional magmatic rocks from André Land, NE Greenland (73°N) and analysed using the U–Pb isotope dilution thermal ionization mass spectrometry method. The resulting ages range between 429 and 425 Ma. An age of 424 Ma on monazites from a kyanite–garnet–mica gneiss (granite melt source rock) is interpreted as dating late-stage metamorphism and migmatization in the area. A large syn-extensional granite pluton yielded an age of 429 ± 1 Ma. These ages, combined with field observations, demonstrate that subhorizontal upper crustal extension was contemporaneous with mid- to lower crustal subhorizontal contraction, and that the upper crustal strain field is not necessarily representative of the coeval strain fields in the middle and lower crust. The presence of 429–425 Ma migmatites and related intrusive rocks structurally above low-grade metasediments demonstrates that the late and post-tectonic leucogranites are not late orogenic but related to a separate early Caledonian tectonomagmatic phase predating Early Devonian nappe translation.

Ar40-Ar39 dating of biotite and sanidine in tuffaceous sediments and related intrusive rocks: Implications for the Early Miocene evolution of the Gördes and Selendi basins, W Turkey

The precise ages of the sedimentary successions within two prominent NE-SW-trending basins, the Gördes Basin and the Selendi Basin, are critical to an understanding of the Neogene evolutionary setting of western Turkey and associated calc-alkaline magmatism. Early radiometric dating was not always sufficiently precise to resolve alternative interpretations. During this study, high-precision Ar40-Ar39 radiometric ages were determined on single crystals of biotite and sanidine from silicic tuffs and associated intrusive rocks. Finegrained tuffaceous sediments near the top of the sedimentary succession in the Selendi Basin gave ages of 18.89 ± 0.58 Ma to 16.42 ± 0.09 Ma. Coarser-grained tuffaceous sediments near the top of the equivalent sedimentary succession in the Gördes Basin to the NW yielded ages of 18.78 ± 0.3 Ma to 17.04 ± 0.35 Ma. Associated intrusive rocks were dated at 20.86 ± 0.08 Ma to 17.62 ± 0.07 Ma. An andesitic body on the northern margin of the Gediz (Alaşehir) Graben further south gave ages of 16.08 ± 10.91 to 14.65 ± 0.06 Ma. Combined with published radiometric age data, these new results confirm an Early Miocene age for the clastic sedimentary fills of the Gördes and Selendi basins. The results from the Gediz Graben are consistent with its formation in Early Miocene time, earlier than the Late Miocene or Plio-Quaternary ages suggested in some interpretations.

Sm-Nd dating of scheelite and tourmaline; implications for the genesis of Archean gold deposits, Val d'Or, Canada

Sm-Nd analyses of 19 scheelite samples from auriferous quartz-carbonate-tourmaline-scheelite-gold veins from three mines in the Val d'Or gold camp in the Abitibi greenstone belt in western Quebec (the Sigma, Pascalis-North, and Siscoe Extension mines) yield a linear array on an isochron diagram with a mean square weighted deviation (MSWD) of 3.6 corresponding to an age of 2596 + or - 33 Ma and epsilon (sub Nd 1 ) = 2.5. Eight scheelite samples from the Pascalis-North mine alone define an excellent linear array with an MSWD = 0.31, an age of 2593 + or - 18 Ma, and epsilon (sub Nd 1 ) = 2.7. These ages are interpreted to represent the age of quartz-carbonate-tourmaline-scheelite vein-hosted gold mineralization in the three mines sampled. Sm-Nd analyses of tourmaline suggest that this mineral may also be useful for Sm-Nd dating of hydrothermal mineralization. The Sm-Nd scheelite age of ca. 2600 Ma is in good agreement with U-Pb ages obtained from mineralization-related rutile and titanite in the Val d'Or area. These results indicate that the quartz-carbonate-tourmaline-scheelite vein-hosted gold mineralization was emplaced approximately 70 m.y. later than the youngest observed premineralization magmatic activity and approximately 60 m.y. later than peak regional metamorphism in the Val d'Or area. These ages do not support hypotheses for deposit formation which involve fluids derived from spatially related intrusive rocks or metamorphism, although they do not preclude an earlier stage of introduction of gold into the Val d'Or area.The scheelite Sm-Nd age and depleted mantlelike epsilon (sub Nd 1 ) , and the mineralogy and alteration of the quartz-carbonate-tourmaline veins, are consistent with the derivation of the mineralizing fluids from a relatively homogeneous external source, and introduction into the upper crust as part of a single, large hydrothermal system. The formation of the Val d'Or gold deposits also appears to be broadly synchronous with the growth of zircon in high-grade metamorphic rocks of the lower crust underlying the greenstone belt material, suggesting that these may be related processes.

Speculations on the origin of the North American Midcontinent rift

The Midcontinent rift is an example of lithospheric extension and flood basalt volcanism induced when a new mantle plume arrived near the base of the lithosphere. Very large volumes of basaltic magma were generated and partly erupted before substantial lithospheric extension began. Volcanism continued, along with extension and deep rift subsidence, for the ensuing 15 m.y. Much of the basaltic magma, including some of the earliest flows, was formed by partial melting of isotopically primitive asthenosphere contained in the plume head. The intense but relatively short duration of rifting and magmatism is a result of the dissipation of thermal and mechanical energy in the plume head. As the plume head spread beneath the lithosphere, it stretched the overlying lithosphere radially away from the Lake Superior region, the triple junction of the rift system, and partially melted to form the great volume of basalt and related intrusive rocks of the region. The plume arrived beneath a continent that was under compression as a result of the ongoing Grenville orogeny that affected a large region east of the rift. That compression prevented full continental separation and eventually returned the region to compressional tectonics as the energy of the plume head waned.

Relationship of Sonoran tungsten mineralization to the metallogenic evolution of Mexico

The tungsten province of northwestern Mexico is the southernmost of the Cordilleran tungsten provinces of North America and the westernmost of a parallel series of Mexican metal provinces which contain Cu, Au-Ag, Pb-Zn-Ag, F, Sn, Mn, U, Fe, and other metals. On its western side, the tungsten province primarily contains skarns associated with Paleozoic limestones. The eastern side primarily contains pegmatites and quartz veins in Laramide plutons. The eastern margin of the tungsten province overlaps the porphyry copper province where a series of breccia pipes contain copper and lesser tungsten.Isotopic dating, using K-Ar, 40 Ar/ 39 Ar, and Rb-Sr methods, indicates that tungsten mineralization is Laramide with ages ranging from 47 to 65 m.y. Hornblende and biotite K-Ar and 40 Ar/ 39 Ar ages are discordant by an average of 11 m.y., and biotite and muscovite associated with mineralization record ages averaging 14 m.y. younger than related intrusive rocks. These large discordances are inferred to be the result of slow cooling in a relatively deep batholithic environment.Sr isotope initial ratios for intrusive rocks associated with tungsten deposits indicate that mineralization occurred independently of basement rock type. Deposits within terranes underlain by cratonic basement are associated with intrusive rocks having higher initial ratios than deposits within terranes underlain by accreted oceanic basement. Initial ratios range from 0.70901 at San Antonio (eratonic) to 0.70570 at San Alberto (accreted).The age of the tungsten province is similar to that of the porphyry copper province and is approximately 20 m.y. older than Ag-Au and Pb-Zn-Ag mineralization in the Sierra Madre Occidental and the central plateau. Tungsten mineralization is approximately 10 m.y. older than rhyolite-hosted U mineralization and approximately 25 m.y. older than rhyolite-hosted F, Sn, and Fe mineralization. The northwestern Mexico tungsten province is slightly younger than other Cordilleran tungsten provinces which are pre-Cenozoic in age.

Phase Relations of Peraluminous Granitic Rocks and Their Petrogenetic Implications

Peraluminous granitic rocks are magmatic rocks that contain quartz, po­ tassic feldspar and/or sodic plagioclase, and one or more aluminum-rich minerals; thus these rocks contain more aluminum than could be accom­ modated by feldspar in a CIPW normative calculation and show normative corundum. Peraluminous granites are defined by their chemical nature, but their recognition is often based on petrography; this discrepancy thus may lead to inconsistency. Although granitic rocks are com­ monly true granite (Streckeisen 1973), they range from tonalite to granite. Of special interest are strongly peraluminous (as defi ned below) granite, granodiorite, or, in rare instances, trondhjemite; these rocks contain alumi­ nous minerals (almandinic garnet, cordierite, tourmaline, muscovite) that are thought to be stable in silicate magmas only if these magmas are themselves peraluminous. The origin of granite and related intrusive rocks is of much current interest. Many of these form shallow-level intrusions, and they can reach enormous sizes; examples are the batholiths in the Lachlan Fold Belt in southeastern Australia (White & Chappell 1983), the Cornub­ ian batholith in England (Exley & Stone 1966), the South Mountain batholith in Nova Scotia (McKenzie & Clarke 1975), the Manaslu batho­ lith in the Himalayas (LeFort 1981), much of the Idaho batholith

Geophysical expression of the batholith beneath Questa Caldera, New Mexico

Gravity gradients delineate uncharacteristically straight, north‐south trending graben faults in the Rio Grande rift west of Questa caldera, and gravity and audiomagnetotelluric (AMT) data show north‐south trending low‐density and high‐resistivity zones from the caldera southward. A gravity inversion technique was used to isolate the gravity anomaly of the caldera and related intrusive rocks from the complicated Bouguer gravity field. The residual gravity anomaly together with AMT data reported in a companion paper by C. L. Long seem to delineate the subcaldera batholith. Geophysical models indicate this to be a north‐south trending dikelike or beamlike body about 25 km long, 5 km wide, and >4 km thick, thickness estimate being limited by the penetration depth of the AMT soundings. The north‐south trend is not consistent with the regional pattern of early Miocene southwest directed extension and instead may reflect passive control by preexisting north‐south strike‐slip faults.

Chemistry and origin of the Eocene Metchosin Volcanics, Vancouver Island, British Columbia

The Metchosin Volcanics of southern Vancouver Island consist of submarine pillow lavas, tuffs, and breccias, overlain by subaerial flows of mainly tholeiitic composition. They overlie basaltic dyke complexes and gabbro and are interpreted as Eocene (and older?) oceanic floor and oceanic island tholeiites. They are separated by the fundamental Leech River Fault from the Paleozoic and Mesozoic rocks underlying Vancouver Island. Major and trace element chemical analyses of the volcanics and a few related intrusive rocks are compared with those of recent to late Tertiary tholeiites from known oceanic settings and with the coeval Crescent Volcanics of the Olympic Peninsula. Possible settings are: (1) oceanic ridge (e.g., mid-Atlantic, Juan de Fuca); (2) oceanic ridge-island (e.g., Iceland); (3) oceanic intra-plate island (e.g., Hawaii); and (4) oceanic island arc (e.g., Tonga–Kermadec). Alkali–silica and AFM diagrams clearly define the subalkaline and tholeiitic character but do not allow distinction of these four oceanic settings. Major element factor analysis yields doubtful results but could perhaps be improved by increased data base. Several orthogonal plots of abundances of Ti, Fe, Mg, Zr, and Cr seem to permit distinction of the four types of tholeiites.On geological and chemical bases Metchosin and Crescent formations correspond most closely to the Icelandic ridge-island setting; the Hawaiian intra-plate island setting is less probable in view of several chemical distinctions. The ocean floor setting, though chemically similar in several plots, is precluded at least for the upper, nonmarine parts of the formations. Lastly, an island arc setting is not indicated by either general lithology or chemistry.

Critical remarks on the genesis of ore as applied to future mineral exploration

Challenges the position taken by Anton Gray in a lecture on the future of mineral exploration, in which the importance of igneous rocks as guides to ore deposits is minimized, and critically discusses syngenetic theories of origin recently advocated for the copper deposits of Northern Rhodesia. Evidence in support of a magmatic source for the ore-bearing solutions of the copper belt is summarized, and the significance of the association between ore deposits and calc-alkaline and related intrusive rocks is reaffirmed.

Geology of the Clay Deposits in Parts of Washington and Idaho

AbstractThe clay deposits of eastern Washington and northern Idaho are along the eastern edge of the Columbia River Plateau physiographic province. Three types of clay occur in the area: (a) residual clay derived from the Columbia River basalt of Tertiary age; (b) residual clay derived from the Cretaceous granodiorite and related intrusive rocks of the Idaho batholith; and (c) transported clay. The two types of residual clay were formed during a protracted lull in the outpouring of the Columbia River basalt, when the relief of land surface was low to moderate and the climate was warm and humid. The topography and climate during this interval were probably ideal for leaching and oxidation. The transported clay is chiefly a product of the weathered granodiorite and related intrusive rocks. Most of it accumulated in basins formed by damming of streams by basalt flows.Kaolinite and halloysite are the principal clay minerals found in the area. The residual clay derived from basalt is chiefly composed of halloysite. Most of this clay is colored blue, gray or grayish black by finely disseminated ilmenite and locally it is stained blown by limonite; most of it has a relict basaltic texture.The residual clay derived from granodiorite and related intrusive rocks contains both kaolinite and halloysite, and the transported clay is predominantly kaolinite. Both of these clays are white and contain abundant quartz grains and mica flakes. The residual clay has a relict granitic texture, and the transported clay has a well-developed bedding and is interbedded with elastic material.