Fine-grained Porphyritic Granite Research Articles

Magmatic–Hydrothermal Evolution at the Barren Wushan Pluton, Southeast China: Insight into Controls on Mineralization Potential

Abstract A-type granites typically exhibit enrichment and mineralization of critical metals such as molybdenum and tin, essential for emerging technologies. However, the key factors influencing their mineralization potential remain elusive. The scarcity of studies on barren systems impedes the understanding of this question. Here, a detailed melt and fluid inclusion study was conducted on the barren Wushan pluton to reconstruct its magmatic evolution and magmatic–hydrothermal transition and explore the factors controlling the metallogenic potential of Mo and Sn in A-type granites. The Wushan pluton displays apparent lithological zoning consisting of two major phases, i.e., medium-grained seriate to porphyritic alkali feldspar granite and fine-grained porphyritic granite. Miarolitic cavities are widely developed in each lithofacies. The silicate melt inclusions from two granitic phases are rhyolitic, with moderate F contents (0.06–0.53 wt %) and depleted H2O contents (2.0–3.5 wt %). Melt inclusions show a wide range of incompatible element contents, such as Cs (9–1977 μg/g) and Rb (268–2601 μg/g), suggesting that Wushan has undergone a high degree of magma evolution. Mo behaves incompatibly in the magmatic evolution, and its content is enriched with the increasing degree of fractional crystallization, but remains constant after the Cs content exceeds 50 μg/g. Rayleigh fractionation model suggests that a large amount of Mo is extracted from fluid exsolution, which restrains Mo from further enrichment. In contrast, Sn behaves as a mildly incompatible element during the entire magmatic evolution history. The contents of Sn increase slowly compared to the trend of Mo, and the maximum contents reach ~30 μg/g in the highly evolved melts. The separation and crystallization of Sn-bearing minerals such as biotite, magnetite, and titanite inhibit the enrichment of Sn. Intermediate-density (ID-type) fluid inclusions hosted in the miarolitic quartz, representing the initial fluid exsolving from magma, display high Mo but low Sn concentrations. Constrained from two assemblages of coexisting ID-type fluid and melt inclusions, the fluid/melt partition coefficients of metals are obtained, with DMo, fluid/melt at 16–19, while DSn, fluid/melt is only about 1. The comparison between Mo-mineralized and barren intrusions worldwide shows that the metal contents in melts and fluids are not fundamentally different. The mineralized intrusions are characterized by the lower melt viscosity and the development of apophyses, both of which facilitate the extraction of metals and fluids from large magma chambers, followed by their concentration into a small rock volume. Consequently, it appears that physical and structural conditions rather than chemical compositions play a crucial role in the Mo mineralization process. Enrichment of Sn in melts is necessary but not decisive for Sn mineralization, whereas Sn enrichment in the initial exsolving fluid determines the Sn mineralization potential of a given granitic system. Compared to Sn enrichment in source melting and fractional crystallization which commonly enhance final Sn fertility in the highly evolved melts, the efficiency of Sn partitioning between melt and fluid plays a fundamental role in converting melt fertility into Sn-enriched fluids and thereby high mineralization potential of the magmatic–hydrothermal system. Our findings suggest a prospect for Mo exploration in the coastal A-type granite belt in South China, while the potential for Sn mineralization is expected to be limited.

Mineralization, alteration, structure, and Re–Os age of the Lanjiagou porphyry Mo deposit, North China Craton

Lanjiagou is a porphyry Mo deposit in terms of its alteration zonation and mineralization associated with granitic intrusions and predominance of quartz vein-hosted molybdenum mineralization. It is the largest Mo deposit in North China Craton (404,000 t). There is an intimate spatial/temporal association between all stages of mineralization and Early Jurassic granitic intrusions at Lanjiagou. Most of the molybdenum was emplaced during the principal hydrothermal (PH) stage (184.6 ± 1.3 – 185.6 ± 1.4 Ma), contemporaneously with intrusion of fine-grained porphyritic granite (188.9 ± 1.2 Ma) into a granite batholith (193 ± 3 Ma). The PH mineralization stage is mainly hosted by a quartz-dominated stockwork associated with phyllic alteration in the fine-grained porphyritic granite. This stage was followed by the late hydrothermal (LH) activity. Thick Mo-rich quartz veins were emplaced during the LH stage and cut the porphyry ore bodies. A ring breccia zone formed during the last hydrothermal stage and apparently cuts both the porphyry and the quartz vein ore bodies. The main hydrothermal vein stages have predominantly concentric and radial vein orientations centred on the ring breccia zone. Most of the concentric veins have shallow dips, whereas the radial veins are subvertical. The LH veins have predominantly NEE and NW orientations in the deposit and are moderately inclined. We surmise that the veining was controlled by the local stress regime generated by the intrusion of a large, deep pluton that we interpreted to be the source of the granites, the breccia zone, and the molybdenum mineralization. Resurgence within the magma chamber reactivated the steep concentric structures in a reverse sense, and accumulation of magmatic and/or fluid pressure resulted in explosive brecciation, producing the ring breccia zone. A predominantly late set of NW-trending, post-ore felsic dikes, associated with the regional structures, are a consequence of far-field stresses exceeding local stresses in the deposit.

Mafic enclaves densely concentrated in the upper part of a vertically zoned felsic magma chamber: The Kurobegawa granitic pluton, Hida Mountain Range, central Japan

The Kurobegawa granitic pluton is a Pliocene batholith exposed over 700–2900 m elevation in the northern Hida Mountain Range of central Japan. The pluton is vertically zoned from the lower granite (a medium-grained equigranular to porphyritic granite, 70–74 wt% SiO 2 ) to the upper granite (a fine-grained porphyritic granite, 72–77 wt% SiO 2 ). Field relationships of the pluton and its textural, mineralogical, and geochemical features indicate that the pluton represents a well-exposed felsic magma chamber that was zoned as a result of fractional crystallization. Densely concentrated mafic microgranular enclaves (MMEs, 0.1–2 m major axis) of basaltic to dacitic composition (54–68 wt% SiO 2 ) occur throughout the pluton except most of the lowermost part. The MMEs generally have sharp, fine-grained, chilled margins and show no in situ mixing with the host granite except for in the northern area where the two magmas locally mixed and mingled to form hybrid rocks. Field relationships reveal that mafic magma intruded into the chamber from the bottom and ascended as a restricted feeder dike through the lower crystal mush. The mafic magma eventually fragmented to form MMEs, and the MMEs rose upward, resulting in their dense distribution throughout the pluton. Mafic silicate phases of the MMEs are amphibole and biotite without anhydrous mafic silicates, and the association magnetite-titanite-quartz occurs extensively, indicating that the mafic magma was originally hydrous and relatively oxidized, characteristic of subduction-related backarc basalt. We infer that the MMEs underwent vapor exsolution by second boiling on cooling during their ascent, reducing their bulk density relative to that of the host felsic magma. Relatively H 2 O-rich MMEs, indicated by the larger modal content of biotite compared to that of amphibole, are abundant in the shallower level of the Kurobegawa pluton. To achieve the MMEs9 dense distribution in the upper granite, we interpret that their flotation was sustained because of bubbles trapped in the segregated, interstitial melt by the MMEs9 chilled crustal margins. Although no evidence for settling, such as load-cast, silicic pipe, and tightly packed structures, is observed, the bubbles may have eventually been lost, with the result that the MMEs became denser and eventually settled onto the then-current top of the crystal mush.

Geochemical characteristics of cryolite-tin-bearing granites from the Pitinga Mine, northwestern Brazil — A review

Mineralization in granites bearing cryolite, tin and rare elements (F, Zr, Nb, Ta, Y, REE, Rb) in the Pitinga region is related to the Madeira and Agua Boa Granites. These alkaline granitic bodies have Proterozoic Rb/Sr ages of 1689 Ma, and are interpreted to be cogenetic due to strong similarities in their petrological, geochemical and isotopic characteristics. The Madeira Granite is composed of an early rapakivi facies (RPK) intruded by a biotite granite (BTG) with an apogranitic (APG) facies at the core. The RPK is mildly metaluminous, the BTG is metaluminous to peralkaline and the APG ranged from peraluminous to peralkaline, due to different stages of albitization, microclinization, or greisenization. The Agua Boa Granite also has an early rapakivi facies that is intruded by a fine-grained porphyritic granite (FRG) and by a late biotite granite. All facies are peraluminous. The apogranite, which is closely associated with primary mineralization of cassiterite, cryolite, malacon, xenotime, REE, Nb, Ta, Y and Rb, can be subdivided into a marginal subfacies and a core subfacies. The marginal subfacies is strongly oxidized, with red staining and with fluorite and cassiterite as accessory phases. The core subfacies consists grey, peralkaline rocks with seams enriched in riebeckite and cryolite that are associated with strongly albitized bands with peraluminous affinity. Evidence linking the APG to metasomatic/late magmatic processes includes the occurrence of high albite with 0% An, high contents of F, Zr, Nb, Ta, Y, Li, Ga, Na 2O, Rb and HREE, low contents of MgO, TiO 2, P 2O 5, LREE, Ba and Sr, and very low normalized La/Yb.

MORPHOLOGICAL STUDIES OF ZIRCONS FROM THE IBARAGI GRANITIC COMPLEX, OSAKA, JAPAN

The Ibaragi granitic complex comprises four rock types, that is, (I) medium-grained quartz diorite, (II) medium-grained adamellite, (III) fine-grained porphyritic granite and (IV) fine-grained adamellite. These granitic rocks intruded into Paleozoic formations successively from type I to IV in the late Cretaceous age. Zircon crystals from these granitic complex have been studied on their crystal habit, length along the c-axis and breadth along the a1 or a2 axis. A crystal habit which has two prisms and one pyramid increases towards the later intrusion. A growth trend of zircon populations is indicated by the “a” value, that is, ratio of standard deviation of length by that of breadth. The “a” value decreases towards the later intrusion. The metamict zircon is most abundant in the fine-grained adamellite that is the latest intrusive rocks. Parameters such as crystal habit or “a” value vary continuously with the characters of the host rocks.

Geochemical characters of muscovite from the Pan African Mandi Granite, and its emplacement and evolution

DOI = 10.3126/hjs.v5i7.1291 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.98