CASSERITE U-Pb GEOCHRONOLOGY OF THE SANTA BÁRBARA TIN DISTRICT, RONDÔNIA TIN PROVINCE, BRAZIL

The Mesoproterozoic Rondônia Tin Province of the Amazonian craton records a protracted history of about 600 m.y. of successive rare-metal granite intrusions and hosts the youngest known event of tin-granite emplacement of the craton—a rare-metal granite suite known as the Younger Granites of Rondônia intrusive suite. The ~1 Ga suite is currently interpreted as intracratonic magmatism resulting from a Grenvillian-age orogeny during the assembly of Rodinia. The Santa Bárbara massif is a tin-granite system of the Younger Granites of Rondônia intrusive suite that hosts Sn-Nb-Ta-W–bearing endogreisen and stockwork, as well as important placer deposits. The Santa Bárbara mine produces about 800 to 1,000 t Sn/year from placers and weathered greisen and represents about 20% of the tin mine output of the Rondônia Tin Province. Here, we report laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) cassiterite U-Pb ages of 989 ± 3 and 987 ± 6 Ma for the Santa Bárbara greisen and the cassiterite-quartz vein system, respectively. Alluvial cassiterite from placer mining has a U-Pb age of 995 ± 4 Ma, which is, within uncertainty, indistinguishable from those of primary cassiterite. These ages agree well with the previously published zircon and monazite U-Pb ages for the Santa Bárbara granite (978 ± 13 and 989 ± 13 Ma), which indicate a coeval relationship between hydrothermal tin mineralization and granite magmatism. The previously suggested 20- to 30-m.y. time span between granite magmatism and hydrothermal tin mineralization, which was based on mica K-Ar and Ar-Ar age data, is likely due to younger thermal disturbance of the isotopic systems.

The Revsegg and Kvitenut Allochthons, Scandinavian Caledonides: origins and evolutions in the Caledonian Wilson cycle

The Caledonian Orogen preserves the record of a complete Wilson cycle from rifting to continent-continent collision and orogenic collapse. The Revsegg Allochthon, the uppermost tectonostratigraphic unit of the Hardangervidda-Ryfylke Nappe Complex of the southern Scandinavian Caledonides, is an understudied example illustrating key temporal and tectonic stages in a Wilson cycle. It overlies 1600-1500 Ma gneisses of the Kvitenut Allochthon that were deformed, metamorphosed and juxtaposed onto the Dyrskard Allochthon at 1000 Ma. The Revsegg Allochthon consists of leucosome-bearing mica-schists with meta-sandstone, and amphibolite and granitoids lenses. The timing of sedimentation of metasedimentary rocks is constrained to the period 780 - 495 Ma, but its association with a 495 Ma bimodal mafic and felsic intrusive suite suggests concurrent sedimentation in a Cambrian extensional setting. The Revsegg Allochthon underwent peak metamorphism at 480-470 Ma, followed by several metamorphic stages from 460 to 440 Ma, probably at an active margin outboard of Baltica, as postulated for the eclogite-bearing Jæren Nappe to the southeast. The Revsegg Allochthon was thrust onto the Kvitenut-Dyrskard duplex from 437 Ma to 434 Ma during an early Scandian phase also recognized in the Seve Nappe. Syn-deformational pegmatites, emplaced at 428 Ma represent the final stage in the nappe stack development. Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/cc/caledonian-wilson-cycle

Exploring the emplacement of Cretaceous arc granites and dextral shear zones in northwestern Nevada, USA, using the StraboSpot data system

ABSTRACT This field trip traverses the Sahwave and Nightingale Ranges in central Nevada, USA, and northward to Gerlach, Nevada, to the Granite, northern Fox, and Selenite Ranges. Plutonic bodies in this area include the ca. 93–89 Ma Sahwave nested intrusive suite of the Sahwave and Nightingale Ranges, the ca. 106 Ma Power Line intrusive complex of the Nightingale Range, the ca. 96 Ma plutons in the Selenite Range, and the ca. 105–102 Ma plutons of the Granite and Fox Ranges. Collectively these plutons occupy nearly 1000 km2 of bedrock exposure. Plutons of the Sahwave, Nightingale, and Selenite Ranges intrude autochthonous rocks east of the western Nevada shear zone, while plutons of the Granite and Fox Ranges intrude displaced terranes west of the western Nevada shear zone. Integrated structural, geochemical, and geochronological studies are used to better understand magmatic and deformation processes during the Early Cretaceous, correlations with Cretaceous plutons in adjacent areas of Idaho and California, and regional implications. Field-trip stops in the Sahwave and Nightingale Ranges will focus on: (1) microstructure and orientation of magmatic and solid-state fabrics of the incrementally emplaced granodiorites-granites of the Sahwave intrusive suite; and (2) newly identified dextral shear zones hosted within intrusions of both the Sahwave and Nightingale Ranges. The Sahwave intrusive suite exhibits moderate to weak magnetic fabrics determined using anisotropy of magnetic susceptibility, with magnetic foliations that strike NW-NE and magnetic lineations that plunge moderately to steeply. Microstructural analysis indicates that these fabrics formed during magmatic flow. The older Power Line intrusive complex in the Nightingale Range is cross-cut by the Sahwave suite and contains a N-S–trending solid-state foliation that reflects ductile dextral shearing. Field-trip stops in the plutons of the Gerlach region will focus on composition, texture, and emplacement ages, and key differences with the younger Sahwave suite, including lack of evidence for zoning and solid-state fabrics. The field trip will utilize StraboSpot, a digital data system for field-based geology that allows participants to investigate the relevant data projects in the study areas.

Lithostratigraphy of the Vuurdood Subsuite, an early mafic-ultramafic phase of the Palaeoproterozoic Vioolsdrif Intrusive Suite, Richtersveld Magmatic Arc, Northern Cape Province

Mafic-ultramafic plutonic intrusions form an early phase in the emplacement of the predominantly granitic Vioolsdrif Suite, which together with its extrusive carapace, the Orange River Group, form the Richtersveld Magmatic Arc, a Palaeoproterozoic crustal segment formed between 1910 and 1865 Ma. Their lithologic character and distinctive dark weathering features in the mountain desert landscape of the Richtersveld, neighbouring regions of the Northern Cape Province and southern Namibia, make them a separate mappable unit in what is a predominantly granitic terrain. The name of the subsuite is taken from a spectacular twin peak massif near Goodhouse (Vuurdoodberg), while the type locality is one of the best preserved central-type intrusive bodies at Swartkop, situated 2 km off the N7 highway about 20 km south of the border town of Vioolsdrif, where the rock types present include gabbro, metagabbro, quartz-metagabbro, peridotite and troctolite.

CONTROLS ON THE METAL ENDOWMENT OF PORPHYRY Mo DEPOSITS: INSIGHTS FROM THE LUMING PORPHYRY Mo DEPOSIT, NORTHEASTERN CHINA

Processes controlling the metal endowment of arc-related porphyry Mo deposits are not well understood. Located in northeastern China, the arc-related Luming porphyry Mo deposit has a proven reserve of 0.75 Mt Mo at an average grade of 0.092 wt % and is characterized by multiple pulses of alteration and mineralization. These features make this deposit an ideal location to investigate the role of multiple pulses of magmatism and fluid release in the evolution and formation of an arc-related porphyry Mo deposit. Molybdenum mineralization at Luming is typically observed as a series of molybdenite-bearing veins hosted within a composite intrusive complex, referred to as the Luming Intrusive Suite. Crosscutting relationships between intrusive units and off-set veins indicate that the Luming Intrusive Suite is composed of five major, successive granitic intrusions: the premineralization plutonic biotite monzogranite and monzogranite units, and the synmineralization stock- and dike-like porphyritic monzogranite, granite porphyry, and syenogranite units. Each synmineralization unit is associated with similar vein sequences that comply with the general form of early EB-type biotite veins, through A-type quartz ± biotite and B-type quartz-molybdenite veins, to late D-type quartz-molybdenite ± pyrite ± chalcopyrite, molybdenite, quartz-pyrite ± calcite, and calcite ± clays veins. The intensity and volume of alteration and mineralization within a given synmineralization unit decrease from early- through inter- to late-mineralization units. Although minor Mo mineralization is associated with potassic alteration along B-type veins, the majority of the ore is associated with D-type quartz-molybdenite-pyrite and molybdenite veins rimmed by sericite-chlorite-pyrite alteration, which are primarily hosted in the two premineralization units. A combination of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb and hydrothermal biotite 40Ar/39Ar studies, together with available isotope dilution-inductively coupled plasma-mass spectrometry (ID-ICP-MS) molybdenite Re-Os data, has resulted in a substantial reappraisal of the timing of magmatism and its association with molybdenite mineralization at Luming. The volumetrically dominant premineralization intrusive units have indistinguishable zircon U-Pb weighted mean 206Pb/238U ages ranging from 187.5 ± 2.8 to 186.5 ± 3.6 Ma (2σ), whereas the synmineralization units yield weighted mean 206Pb/238U ages from 178.6 ± 2.2 to 175.6 ± 3.0 Ma (2σ). The zircon U-Pb weighted mean 206Pb/238U ages of the synmineralization units are indistinguishable from the mean molybdenite Re-Os model (178.1 ± 2.7; 2σ) and hydrothermal biotite 40Ar/39Ar plateau (174.7 ± 1.1 Ma; 2σ) ages within uncertainty, confirming a genetic link with mineralization. Melt inclusion data show that the synmineralization intrusions were Mo poor, with Mo concentrations <4 ppm. The data presented here suggest that molybdenite mineralization at Luming was most likely accomplished through three discrete magmatic-hydrothermal events during assembly of the Mo-poor synmineralization intrusive complex. The giant Luming deposit appears to be related to multiple pulses of magmatic-hydrothermal activities, resulting in the superposition of temporally distinct mineralization events. Our results suggest that pulsed release of ore-forming magmas and fluids, which are channeled along focusing structures like small porphyry fingers within a focused area, from a large magma chamber at depth may play a major role in the formation of large to giant porphyry Mo deposits of both the arc-related and Climax types. This conclusion is in line with field observations of a number of large to giant porphyry Mo deposits, which commonly show reversals in magmatic-hydrothermal evolutionary trend and are associated with multiple pulses of small stocks and dikes that are separate in time and space.

Telluride Mineralogy of the Deer Horn Au-Ag-Te-(Bi-Pb-W) Deposit, British Columbia: Implications for the Generation of Tellurides

ABSTRACT The Deer Horn deposit, located 150 km south of Smithers in west-central British Columbia, is an Eocene polymetallic system enriched in Au-Ag-Te with lesser amounts of Bi-Pb-W; the Au and Ag are hosted in Te-bearing minerals and Ag-rich gold (Au-Ag alloy). A quartz-sulfide vein system containing the main zones of Au-Ag-Te mineralization and attendant sericite alteration occurs in the hanging wall of a local, spatially related thrust fault and is genetically related to the nearby Eocene Nanika granodiorite intrusive suite. Tellurium-bearing minerals commonly form isolated euhedral to subhedral grains or composite grains (up to 525 μm in size) of Ag-, Bi-, Pb-, and Au-rich tellurium-bearing minerals (e.g., hessite, tellurobismuthite, volynskite, altaite, and petzite). Panchromatic cathodoluminescence imaging revealed four generations of quartz. Within remnant cores of quartz I, local oscillatory zoning occurs in quartz II. Fine-grained veinlets of quartz III and IV crosscut quartz I and II, showing evidence of at least two deformation events; late-forming veinlets of calcite crosscut all generations of quartz. The tellurides and Ag-rich gold occur in stage III quartz. Three types of fluid inclusions were observed in stage III and IV quartz: (1) aqueous liquid and vapor inclusions (L-V); (2) aqueous carbonic inclusions (L-L-V); and (3) carbonic inclusions (vapor-rich). Primary fluid inclusions related to the telluride mineralization within quartz III were tested with microthermometry, along with a few primary inclusions from quartz IV. Homogenization temperatures are 130.0–240.5 °C for L-V inclusions and 268.0–336.4 °C for L-L-V inclusions. Aqueous carbonic inclusions had solid CO2 melting temperatures from –62.1 to –56.8 °C, indicating the presence of ≈1 to 30 mol.% dissolved methane in these inclusions. The Deer Horn Au-Ag-Te-(Bi-Pb-W) deposit is a reduced intrusion-related gold system characterized by sheeted veins, metal zoning, low salinity aqueous-carbonic fluids, and a genetic relationship to an Eocene granodiorite. Values of δ34S of pyrite vary from –1.6 to 1.6 per mil and are compatible with a magmatic source of sulfur.