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Ingrid M. Kjarsgaard M.Beth McClenaghan Bruce A. Kjarsgaard Larry M. Heaman 《Lithos》2004,77(1-4):705-731
Sixteen kimberlite boulders were collected from three sites on the Munro and Misema River Eskers in the Kirkland Lake kimberlite field and one site on the Sharp Lake esker in the Lake Timiskaming kimberlite field. The boulders were processed for heavy-mineral concentrates from which grains of Mg-ilmenite, chromite, garnet, clinopyroxene and olivine were picked, counted and analyzed by electron microprobe. Based on relative abundances and composition of these mineral phases, the boulders could be assigned to six mineralogically different groups, five for the Kirkland Lake area and one for the Lake Timiskaming area. Their indicator mineral composition and abundances are compared to existing data for known kimberlites in both the Kirkland Lake and Lake Timiskaming areas. Six boulders from the Munro Esker form a compositionally homogeneous group (I) in which the Mg-ilmenite population is very similar to that of the A1 kimberlite, located 7–12 km N (up-ice), directly adjacent to the Munro esker in the Kirkland Lake kimberlite field. U–Pb perovskite ages of three of the group I boulders overlap with that of the A1 kimberlite. Three other boulders recovered from the same localities in the Munro Esker also show some broad similarities in Mg-ilmenite composition and age to the A1 kimberlite. However, they are sufficiently different in mineral abundances and composition from each other and from the A1 kimberlite to assign them to different groups (II–IV). Their sources could be different phases of the same kimberlite or—more likely—three different, hitherto unknown kimberlites up-ice of the sample localities along the Munro Esker in the Kirkland Lake kimberlite field. A single boulder from the Misema River esker, Kirkland Lake, has mineral compositions that do not match any of the known kimberlites from the Kirkland Lake field. This suggests another unknown kimberlite exists in the area up-ice of the Larder Lake pit along the Misema River esker. Six boulders from the Sharp Lake esker, within the Lake Timiskaming field, form a homogeneous group with distinct mineral compositions unmatched by any of the known kimberlites in the Lake Timiskaming field. U–Pb perovskite age determinations on two of these boulders support this notion. These boulders are likely derived from an unknown kimberlite source up-ice from the Seed kimberlite, 4 km NW of the Sharp Lake pit, since indicator minerals with identical compositions to those of the Sharp Lake boulders have been found in till samples collected down-ice from Seed. Based on abundance and composition of indicator minerals, most importantly Mg-ilmenite, and supported by U–Pb age dating of perovskite, we conclude that the sources of 10 of the 16 boulders must be several hitherto unknown kimberlite bodies in the Kirkland Lake and Lake Timiskaming kimberlite fields. 相似文献
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L. P. Black J. L. Everard M. P. McClenaghan C. R. Calver A. M. Fioretti 《Australian Journal of Earth Sciences》2013,60(7):933-968
Devonian–Carboniferous granites are widespread in Tasmania. In eastern Tasmania, Devonian granites intrude Ordovician–Early Devonian quartz-rich turbidites of the Mathinna Supergroup. The earliest (～400 Ma) I-type granodiorites may be arc-related. Following the Tabberabberan Orogeny (～389 Ma), more felsic and, finally, strongly fractionated I- and S-type granites were emplaced until ～373 Ma. In contrast, western Tasmania granites intrude a more diverse terrane of predominantly marine shelf successions, with depositional ages as old as Late Mesoproterozoic. They are mostly felsic and fractionated I- and S-types emplaced from ～374–351 Ma, possibly in response to post-collisional crustal extension following juxtaposition of the eastern and western Tasmanian terranes. Granites from the two terranes are readily distinguishable by the age spectra of their inherited zircon, which are noticeably similar to those of the detrital zircon from sedimentary successions in their respective terranes. Furthermore, within each terrane, both I and S-types yield similar inheritance patterns. This suggests a pivotal role for the sedimentary successions in the petrogenesis of both types. Western Tasmanian granites are also enriched in ～1600 Ma zircon, which is essentially unrepresented in the exposed supracrustal succession. Subtle differences between the inheritance and detrital age spectra in eastern Tasmania probably relate to unrepresentative sampling of the supracrustal rocks. Nd, Sr and Pb isotopic characteristics of the granites are consistent with their derivation by mixing of magmas derived from the mantle, possibly the lower crust, and from supracrustal rocks. Systematic isotopic trends in some eastern Tasmanian I-types, particularly in the Scottsdale Batholith, correlate well with major and trace element geochemistry and age. The isotopes are inconsistent with simple restite unmixing or crystal fractionation in a closed magma chamber, and indicate progressive contamination by the Mathinna Supergroup, or similar rocks. The isotopic characteristics of late, strongly fractionated granites, although sometimes obscured by hydrothermal alteration, are also consistent with concurrent assimilation-fractional crystallisation processes. Together with the close association of some strongly fractionated I- and S-types, this suggests that such granites were generated directly in the lower crust, and were not derived from unfractionated parental granite magmas. 相似文献
3.
L. P. Black M. P. McClenaghan R. J. Korsch J. L. Everard C. Foudoulis 《Australian Journal of Earth Sciences》2013,60(6):807-829
A series of new Sensitive High-Resolution Ion MicroProbe (SHRIMP) U – Pb ages is presented for Palaeozoic (mainly Devonian and Carboniferous) granites from Tasmania. In virtually all instances the new ages are significantly older than previously determined Rb – Sr and K – Ar ages, even though the level of emplacement had been thought to be too shallow to allow loss of radiogenic daughter products. In two extreme cases, granite bodies at South West Cape and Elliott Bay that had previously yielded Carboniferous Rb – Sr and Early Devonian K – Ar ages, respectively, are now both shown to be Late Cambrian. In northeast Tasmania, granitic activity in the Blue Tier Batholith lasted for about 22 million years, with I-type magmas being followed by S-types only toward the end of that time. The exclusively I-type granites of the Scottsdale Batholith formed about 10 million years after the initiation of igneous activity in the Blue Tier Batholith, and were emplaced over a comparatively short time interval (4 – 5 million years). The new data confirm a previously held view, based on Rb – Sr analysis, that the economically important Lottah Granite crystallised roughly 9 million years later than the nearby Poimena Granite and, therefore, could not have been derived by magmatic fractionation of the latter. A regional deformation equated with the Tabberabberan Orogeny has been dated at about 390 Ma in northeastern Tasmania, based on the presence or absence of a northwest-trending foliation in the different granite bodies. The oldest granites occur in the northeast of Tasmania, with an irregular progression of ages to the west coast. A trend of this type could have arisen in an arc-free or arc-related environment. If the latter applies, either flat subduction or processes associated with the amalgamation of eastern and western basement terranes might be the controlling mechanism. Eastern Tasmania experienced a trend from mafic I-type to progressively more felsic, largely S-type igneous activity, but the trend for western Tasmania is not as obvious. The trend for eastern Tasmania is an exception to the general rule for the Lachlan Orogen, possibly signifying that the mid-crust was relatively cool when the first I-type granites were generated. Crustal thickening during the Tabberabberan Orogeny may have been a prerequisite for the generation of later, more felsic, S- and I-types. 相似文献
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This paper summarizes advances since 1987 in the application of glacial sediment sampling to mineral exploration (drift prospecting) in areas affected by continental or alpine glaciation. In these exploration programs, clastic glacial sediments are tested by geochemical or mineralogical methods to detect dispersal trains of mineral deposit indicators that have been glacially transported from source by mechanical processes. In glaciated terrain the key sampling medium, till, is produced by abrasion, crushing and blending of rock debris and recycled sediment followed by down-ice dispersal ranging from a few metres to many kilometres. As a consequence of the mid-1980s boom in gold exploration, the majority of case studies and regional till geochemical surveys published in the past decade deal with this commodity. Approximately 30% of Canada and virtually all of Fennoscandia have been covered by regional till geochemical surveys that aid mineral exploration and provide baseline data for environmental, agricultural, and landuse planning. The most profound event in drift prospecting in the last decade, however, has been the early-1990s explosion in diamond exploration which has dramatically increased the profile of glacial geology and glacial sediment sampling and stimulated changes in sampling and analytical methods. 相似文献
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Rmi Coltat Yannick Branquet Pierre Gautier Hector Campos Rodriguez Marc Poujol Ewan Pelleter Sean McClenaghan Gianreto Manatschal Philippe Boulvais 《地学学报》2019,31(6):549-561
Mid‐Ocean Ridges host various types of hydrothermal systems including high‐T black‐smokers found in ultramafic rocks exhumed along slow spreading ridges. These systems are mostly described in two dimensions as their exposure on the present‐day seafloor lacks the vertical dimension. One way to understand these systems at depth is to study their fossilized equivalents preserved on‐land. Such observation can be done in the Platta nappe, Switzerland, where a Jurassic‐aged mineralized system is exposed in 3D. Serpentinites host a Cu‐Fe‐Ni‐Co‐Zn‐rich mineralization made of sulphides, magnetite and Fe‐Ca‐silicates either replacing serpentinites or within stockwork. Fe‐Ca‐silicates, abundant at the deepest levels, vanish in the mineralization close to the palaeo‐detachment. Fluids were channelized along mafic dykes and sills acting as preferential drains. Warm carbonation (~130°C) is the latest hydrothermal record. We propose that this system is an analog to the root zone of present‐day serpentinite‐hosted hydrothermal systems such as those found along the Mid‐Atlantic Ridge. 相似文献
6.
Sean H. McClenaghan David R. Lentz Jillian Martin Wilfredo G. Diegor 《Mineralium Deposita》2009,44(5):523-557
The 329-Mt Brunswick No. 12 volcanogenic massive sulfide deposit (total resource of 163 Mt at 10.4% Zn, 4.2% Pb, 0.34% Cu,
and 115 g/t Ag) is hosted within a Middle Ordovician bimodal volcanic and sedimentary sequence. Massive sulfides are for the
most part syngenetic, and the bulk of the sulfide ore occurs as a Zn–Pb-rich banded sulfide facies that forms an intimate
relationship with a laterally extensive Algoma-type iron formation and defines the Brunswick Horizon. Zone refining of stratiform
sulfides is considered to have resulted in the development of a large replacement-style Cu-rich basal sulfide facies, which
is generally confined between the banded sulfide facies and an underlying stringer sulfide zone. Complex polyphase deformation
and associated lower- to upper-greenschist facies regional metamorphism is responsible for the present geometry of the deposit.
Textural modification has resulted in a general increase in grain size through the development of pyrite and arsenopyrite
porphyroblasts, which tend to overprint primary mineral assemblages. Despite the heterogeneous ductile deformation, primary
features have locally been preserved, such as fine-grained colloform pyrite and base and precious metal zonation within the
Main Zone. Base metal and trace element abundances in massive sulfides from the Brunswick No. 12 deposit indicate two distinct
geochemical associations. The basal sulfide facies, characterized by a proximal high-temperature hydrothermal signature (Cu–Co–Bi–Se),
contains generally low Au contents averaging 0.39 ppm (n = 34). Conversely, Au is enriched in the banded sulfide facies, averaging 1.1 ppm Au (n = 21), and is associated with an exhalative suite of elements (Zn–Pb–As–Sb–Ag–Sn). Finely laminated sulfide lenses hosted
by iron formation at the north end of the Main Zone are further enriched in Au, averaging 1.7 ppm (n = 41) and ranging up to 8.2 ppm. Laser ablation inductively coupled plasma-mass spectrometry (ICP-MS) analyses of pyrite
(n = 97) from the north end of the Main Zone average 2.6 ppm Au and range from the detection limit (0.015 ppm) to 21 ppm. Overall,
these analyses reveal a distinct Au–Sb–As–Ag–Hg–Mn association within pyrite grains. Gold is strongly enriched in large pseudo-primary
masses of pyrite that exhibit relict banding and fine-grained cores; smaller euhedral pyrite porphyroblasts, and euhedral
rims of metamorphic origin surrounding the pyrite masses, contain much less Au, Sb, Ag, As, and Sn. Arsenopyrite, occurring
chiefly as late porphyroblasts, contains less Au, averaging 1.0 ppm and ranging from the detection limit (0.027 ppm) to 6.9 ppm.
Depth profiles for single-spot laser ablation ICP-MS analyses of pyrite and arsenopyrite display uniform values of Au and
an absence of discrete microscopic inclusions of Au-bearing minerals, which is consistent with chemically bonded Au in the
sulfide structure. The pervasive correlation of Au with Sn in the Zn–Pb-rich banded sulfide facies suggests similar hydrothermal
behavior during the waxing stages of deposition on the seafloor. Under high temperature (>350oC) and moderate- to low-pH conditions,
Au and Sn in hydrothermal fluids would be transported as chlorocomplexes. An abrupt decrease in temperature and aH2S, accompanied by an increase in fO2 and pH during mixing with seawater, would lead to the simultaneous destabilization of both Au and Sn chlorocomplexes. The
enrichment of Au in fine-grained laminated sulfides on the periphery of the deposit, accompanied by sporadic occurrences of
barite and Fe-poor sphalerite, supports lower hydrothermal fluid temperatures analogous to white smoker activity on the flanks
of a large volcanogenic massive sulfide system. In lower temperature (<350oC) and mildly acidic hydrothermal fluids, Au would
be transported by thiocomplexes, which exhibit multifunctional (retrograde–prograde) solubility and a capacity to mobilize
Au to the outer parts of the sulfide mound. The sluggish nature of this low-temperature venting together with larger variations
in ambient fO2 could lead to a sharp enrichment of Au towards the stratigraphic hanging wall of massive sulfide deposits.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
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