ALBERT

Description: Regional metamorphic rocks in the Pakistan Himalaya include both UHP coesite eclogite-facies and MP/T kyanite-sillimanite-grade Barrovian metamorphic rocks. Age data show that peak metamorphism of both was c. 47 Ma. 40Ar-39Ar hornblende cooling ages date post-peak metamorphic cooling of both through 500 {degrees}C by 40 Ma, some 20 Ma earlier than for metamorphic rocks in the central and eastern Himalaya. Typically these ages have been explained by obduction of the Kohistan arc onto the Indian plate at about 50 Ma and India-Asia collision. We suggest instead that the earlier metamorphic and cooling ages of the Pakistani Barrovian metamorphic sequence could be partially explained by Late Cretaceous to Early Paleocene crustal thickening linked to obduction of an ophiolite thrust sheet onto the leading edge of the Indian plate, similar to the Spontang Ophiolite in Ladakh. Heating following on from this Paleocene crustal thickening explains peak Barrovian metamorphism within 5-10 Ma of subsequent obduction of Kohistan. Remnants of the ophiolite sheet, and underlying Tethyan sediments, are preserved in NW India and in western Pakistan but not in northern Pakistan. Tectonic erosion removed all cover sequences (including the ophiolites) from the Indian plate basement.

Description: The world-class Loulo mining district (15.5 Moz resource), in the Birimian terrane of western Mali, contains a range of mineralogically diverse styles of orogenic gold mineralization. The district is distinguished by As-rich orebodies as at Yalea, as well as tourmaline-bearing, Fe-rich, orebodies as at Gara. New fluid inclusion and stable isotope data presented here constrain the nature of the hydrothermal fluids responsible for these different types of mineralization, and point towards the role of multifluid sources (metamorphic and magmatic) in the formation of orogenic gold deposits. Microthermometric and laser Raman studies from Yalea Main and two other similar orebodies (Loulo-3 and Baboto) reveal the dominance of coexisting CO 2 -N 2 ±CH 4 (type 1) and H 2 O-NaCl (type 2) fluid inclusions. These inclusions show evidence of fluid unmixing from reduced (quartz-faylite-magnetite, QFM, buffered), low-salinity (≤10 wt % NaCl equiv), CO 2 -rich-H 2 O-NaCl-N 2 ± CH 4 primary ore fluids. The combination of microthermometric data and geothermometry based on ore and alteration assemblages indicate trapping temperatures and pressures of 270° to 340°C at 1.4 to 1.8 kbar. The P-T-X state of the Yalea-style mineralizing fluids, along with the 18 O fluid compositions of 8.8 to 10.7, is consistent with the derivation of auriferous fluids during greenschist facies regional metamorphism of the host terrane. Similar fluid compositions were previously reported elsewhere in the Birimian crust of West Africa and in other orogenic gold districts worldwide. The precipitation of gold from the H 2 S-rich metamorphic fluid is primarily linked to phase separation of the ore fluid, which is controlled by P-T fluctuations and/or fluid-rock interaction with carbonaceous host sedimentary rocks (confirmed by low 13 C values of –21.7 to –15.8). Fluid inclusion investigations from Gara and a similar style orebody (Yalea North) indicate the presence of coexisting CO 2 ± N 2 ± CH 4 (type 1) and mixed-salinity (5–21 wt % NaCl equiv) CO 2 -rich-H 2 O-NaCl inclusions (type 3). Inclusion assemblages also contain common oxidized (hematite-magnetite, HM, buffered), high temperature (〉400°C), hypersaline (~35–50 wt % total dissolved solids), metalliferous (Na+Fe+Ca+Cu+Ni+W+ Pb+Zn), multiphase H 2 O-rich-CO 2 -NaCl-FeCl 2 inclusions (type 4). This inclusion type has not been previously reported in other Birimian terranes. The composition of the brines, along with carbon isotope data ( 13 C of –14.4 to –4.5), suggests a magmatic input to the Gara-style hydrothermal system. The coexistence in the fluid inclusion assemblages of the magmatic brines with Yalea-style, CO 2 -rich, metamorphic fluids and the positive correlation between salinity and homogenization temperatures suggest mineralization was locally controlled by fluid mixing. The interaction of these two chemically contrasting fluids explains the distinctive petrographic characteristics of the Gara-style orebodies. This includes the growth of widespread multi-stage Fe 3+ -rich tourmaline (B-rich granite source) and sodic alteration, and ore assemblages consisting of abundant nickeloan pyrite, (REE)-phosphates, Ni ± Co ± Pb ± Zn minor/trace sulfides, and scheelite. Gold deposition in the Gara-style hydrothermal system is related to physical and chemical changes of the two fluids during mixing (e.g., decreases in f O2 and T in the brines and retrograde boiling of the CO 2 component in the metamorphic fluids, a "salting out effect").

Description: Loulo is a world-class orogenic gold mining district in the Birimian terrane of western Mali. Orebodies are located along second or higher order shears associated with the Senegal-Mali shear zone, with gold mineralization largely linked to a transtensional event. The mine camp is divided into two distinct styles of gold deposit on the basis of differing geologic characteristics. One group is typified by the Gara deposit, whereas the other by the Yalea deposit. Both deposit styles are hosted by similar rock types (calcareous graywackes and calciticdolomitic marbles). Gara-style orebodies occur as sulfide disseminations or ankerite-rich shear vein stockworks, hosted in folded tourmalinized sandstones and breccias mainly within 2 km of the Senegal-Mali shear zone. These deposits are characterized by intense multi-stage albitization and tourmalinization (pre-, syn- and postmineralization). Gold lodes are Fe-rich (dominated by nickeloan pyrite), contain Cu-Ni ± Co minor and trace sulfides (e.g., chalcopyrite, gersdorffite, pentlandite, cobaltite, millerite), and show consistently high levels of P-REE-W–bearing phases (apatite, monazite, xenotime, and scheelite). Base metal concentrations show a marked increase in marble host rocks, with the formation of nickeloan pyrite-cobaltite-clausthalite ores. In contrast, Yalea-style deposits are associated with quartz ± ankerite vein lodes and disseminated sulfide stringer zones. Mineralization occurs along highly altered (tourmaline-absent), brittle-ductile, shears up to 8 km away from the Senegal-Mali shear zone. Wall-rock alteration is characterized by addition of K 2 O, CaO, CO 2 , H 2 O, and SiO 2 , with mineral assemblages consisting of chlorite-sericite-carbonate-quartz ± albite. Ore paragenesis is enriched in As, mainly as multistage growth of arsenopyrite and arsenian pyrite. Base metal sulfides, scheelite, and (REE)-phosphates are extremely rare. The diversity in the ore paragenesis is controlled by a dynamic hydrothermal system that sourced fluids and metals from different reservoirs within the region. The As-rich Yalea-style deposits have characteristics typical of Birimian gold mineralization in Ghana, and auriferous fluids are likely derived from the dewatering of sedimentary rocks during regional metamorphism. On the other hand, the polymetallic, Fe-B–rich, Gara-style orebodies show atypical features for Birimian gold mineralization. Instead, field relations and the mineralogy and geochemistry of the Gara-style lodes indicate a strong hydrothermal influence from surrounding intrusive stocks, with possible links between gold mineralization and iron oxide skarn development in the region. The data collected at Loulo highlight the diverse nature of orogenic gold deposits, especially in West Africa. This style of ore deposit can form from a variety of fluid sources, both metamorphic and magmatic.

Description: The Massawa gold project is situated on the Senegalese side of the highly prospective/productive Palaeo-Proterozoic (Birimian) Kédougou–Kéniéba inlier, which hosts several world-class orogenic gold deposits/districts in western Mali (e.g. Loulo and Sadiola). The Massawa ore body has a strike length of at least 4 km and a current resource of 3.61 Moz at a grade of 2.8 g t –1 . The ore body is structurally controlled and located within a package of low-grade regionally metamorphosed volcaniclastic sediments (agglomerates, tuffs and ash-tuffs), quartz–feldspar and lithic wackes, carbonaceous shales, hydrothermal breccias, and gabbro and porphyry sills. These rocks have undergone pervasive silica alteration followed by a sericite–ankerite–chlorite alteration event related to mineralization. Two major styles of mineralization are recognized at Massawa from field and laboratory studies. The first stage of sulphide–Au mineralization is associated with disseminated arsenopyrite–pyrite, which follows shear zones in the sedimentary and volcano-sedimentary host rocks. The second stage consists of quartz–stibnite±tetrahedrite veining distinguished by coarse visible gold and represents a late stage overprint on the primary mineralization. The two stages of gold mineralization are separated by a phase of quartz–molybdenite veining. A distinctive base metal trace assemblage is linked to stibnite formation including multiple Sb phases such as chalcostibite, zinkenite, roshchinite, aurostibite, jamesonite and robinsonite. Secondary ion mass spectroscopy-based gold deportment data indicate that up to 90% of stage 1 gold is held as a solid solution within either arsenopyrite or arsenian pyrite. Stable isotope data yield 34 S sulphide values of between 0 and 4.1 and 18 O H2O values of 5.5–10.9 for all stages of mineralization, suggesting a magmatic fluid influence. This is consistent with field data that suggest that mineralization is synchronous with emplacement of a sequence of concordant felsic sheets. That mineralization occurred at shallow (〈6 km) depths is suggested both by the presence of stibnite and by fluid inclusion studies. Low-temperature (homogenization temperatures between 150 °C and 230 °C) H 2 O–NaCl fluids (〈6 wt% NaCl equiv.) and coeval CO 2 –CH 4 inclusions, observed in both phases of mineralization, indicate trapping conditions of 220–315 °C at 1–1.65 kbar. A combination of phase petrology, fluid inclusion and stable isotope data suggests deposition of gold from low-salinity, magmatic fluids, most probably released from felsic rocks similar to those emplaced into the Massawa sequence during mineralization.

Description: 〈p〉The Himalaya resulted from collision of the Indian plate with Asia and is well known as the highest, youngest, and one of the best studied continental collision orogenic belts. It is frequently used as the type example of a continental collision orogenic belt in studies of older Phanerozoic orogenic belts. The beauty of the Himalaya is that, on a broad scale they form a relatively simple orogenic belt. The major structural divisions, the Indus-(Yarlung Tsangpo) suture zone, the Tethyan Himalaya sedimentary units, Greater Himalaya Sequence (GHS) metamorphic rocks, the Lesser Himalaya (LH) fold and thrust belt, and the Sub-Himalaya Siwalik molasse basin are present along the entire 2000 km length of the Himalaya (Figs. 1, 2). Likewise, the major structures, the Indus – Yarlung Tsangpo suture with north-vergent backthrusts, the South Tibetan Detachment (STD) low-angle normal fault, locally called the Zanskar Shear zone (ZSZ) in the west, the Main Central Thrust (MCT) zone, and the Main Boundary Thrust (MBT) are all mapped along the entire length of the mountain belt between the western (Nanga Parbat) and eastern (Namche Barwa) syntaxes. Klippen of low-grade or unmetamorphosed sedimentary rocks lie above the GHS high-grade rocks in places (e.g. Chamba klippe in India; Lingshi klippe in Butan), and far-travelled klippen of GHS rocks occur in places south of the main MCT and GHS rocks (e.g. Darjeeling klippe).〈/p〉

Description: 〈p〉The Pakistan part of the Himalaya has major differences in tectonic evolution compared with the main Himalayan range to the east of the Nanga Parbat syntaxis. There is no equivalent of the Tethyan Himalaya sedimentary sequence south of the Indus-Tsangpo suture zone, no equivalent of the Main Central Thrust, and no Miocene metamorphism and leucogranite emplacement. The Kohistan Arc was thrust southward onto the leading edge of continental India. All rocks exposed to the south of the arc in the footwall of the Main Mantle Thrust preserve metamorphic histories. However, these do not all record Cenozoic metamorphism. Basement rocks record Palaeo-Proterozoic metamorphism with no Cenozoic heating; Neo-Proterozoic through Cambrian sediments record Ordovician ages for peak kyanite and sillimanite grade metamorphism, although Ar–Ar data indicate a Cenozoic thermal imprint which did not reset the peak metamorphic assemblages. The only rocks that clearly record Cenozoic metamorphism are Upper Palaeozoic through Mesozoic cover sediments. Thermobarometric data suggest burial of these rocks along a clockwise 〈i〉P–T〈/i〉 path to pressure–temperature conditions of ~10–11 kbar and ~700 °C. Resolving this enigma is challenging but implies downward heating into the Indian plate, coupled with later development of unconformity parallel shear zones that detach Upper Palaeozoic-Cenozoic cover rocks from Neoproterozoic to Palaeozoic basement rocks and also detach those rocks from the Paleoproterozoic basement.〈/p〉 〈p〉Supplementary material at 〈a href="https://doi.org/10.6084/m9.figshare.c.4496246"〉https://doi.org/10.6084/m9.figshare.c.4496246〈/a〉〈/p〉