White mica and tourmaline are the dominant hydrothermal alteration minerals at the world-class Panasqueira W–Sn–Cu deposit in Portugal. Thus, understanding the controls on their chemical composition helps to constrain ore formation processes at this deposit and determine their usefulness as pathfinder minerals for mineralization in general. We combine whole-rock geochemistry of altered and unaltered metasedimentary host rocks with in situ LA-ICP-MS measurements of tourmaline and white mica from the alteration halo. Principal component analysis (PCA) is used to better identify geochemical patterns and trends of hydrothermal alteration in the datasets. The hydrothermally altered metasediments are enriched in As, Sn, Cs, Li, W, F, Cu, Rb, Zn, Tl, and Pb relative to unaltered samples. In situ mineral analyses show that most of these elements preferentially partition into white mica over tourmaline (Li, Rb, Cs, Tl, W, and Sn), whereas Zn is enriched in tourmaline. White mica has distinct compositions in different settings within the deposit (greisen, vein selvages, wall rock alteration zone, late fault zone), indicating a compositional evolution with time. In contrast, tourmaline from different settings overlaps in composition, which is ascribed to a stronger dependence on host rock composition and also to the effects of chemical zoning and microinclusions affecting the LA-ICP-MS analyses. Hence, in this deposit, white mica is the better recorder of the fluid composition. The calculated trace-element contents of the Panasqueira mineralizing fluid based on the mica data and estimates of mica-fluid partition coefficients are in good agreement with previous fluid-inclusion analyses. A compilation of mica and tourmaline trace-element compositions from Panasqueira and other W–Sn deposits shows that white mica has good potential as a pathfinder mineral, with characteristically high Li, Cs, Rb, Sn, and W contents. The trace-element contents of hydrothermal tourmaline are more variable. Nevertheless, the compiled data suggest that high Sn and Li contents are distinctive for tourmaline from W–Sn deposits.

Reconstructing the thermal evolution of hydrothermal ore deposits mainly relies on their fluid inclusion record, which is limited by favorable trapping conditions, calling for alternative temperature constraints. Muscovite and tourmaline coexist in many hydrothermal ore deposits and in the granites or pegmatites related with them. Whereas in situ analyses of boron isotopes in tourmaline are widely applied to constrain fluid sources and evolution, muscovite has seldom been used in this way, and the potential for isotope exchange thermometry with this mineral pair is unexplored. The different boron coordination in muscovite and tourmaline causes a significant, temperature-dependent isotopic fractionation between them, which has been determined experimentally. We used this relationship to study mineralization conditions and fluid evolution at the Panasqueira W-Sn-Cu deposit in Portugal, where the source and evolution of the mineralizing fluids are still debated. The difference in ¹¹B/¹⁰B ratios of coexisting muscovite and tourmaline, expressed as Δ¹¹B_{mica-tourmaline} = δ¹¹B_{mica –}δ¹¹B_tourmaline, yields median temperatures for vein selvages from 400° to 460°C within a total range of 350° to 600°C, which agrees with published Ti-in-quartz temperatures. Mineral pairs from a late fault zone yield a lower median temperature of about 250°C (range 220°–320°C), which fits with published homogenization temperatures of quartz-hosted fluid inclusions from the veins. Taking these temperatures into account, the calculated fluid composition of the early and late muscovite generations is about δ¹¹B_fluid = −6 ± 2‰, which indicates that the recurrent fluid pulses had a uniform composition and a magmatic-hydrothermal origin.

Faults and fractures can be permeable pathways for focused fluid flow in structurally controlled ore‐forming hydrothermal systems. However, quantifying their role in fluid flow on the scale of several kilometers with numerical models typically requires high‐resolution meshes. This study introduces a modified numerical representation of m‐scale fault zones using lower‐dimensional elements (here, one‐dimensional [1D] elements in a 2D domain) to resolve structurally controlled fluid flow with coarser mesh resolutions and apply the method to magmatic‐hydrothermal ore‐forming systems. We modeled horizontal and vertical structure‐controlled magmatic‐hydrothermal deposits to understand the role of permeability and structure connectivity on ore deposition. The simulation results of vertically extended porphyry copper systems show that ore deposition can occur along permeable vertical structures where ascending, overpressured magmatic fluids are cooled by downflowing ambient fluids. Structure permeability and fault location control the distribution of ore grades. In highly permeable structures, the mineralization can span up to 3 km vertically, resulting in heat‐pipe mechanisms that promote the ascent of a magmatic vapor phase to an overlying structurally controlled epithermal system. Simulations for the formation of subhorizontal vein‐type deposits suggest that the major control on fluid flow and metal deposition along horizontal structures is the absence of vertical structures above the injection location but their presence at greater distances. Using a dynamic permeability model mimicking crack‐seal mechanisms within the structures leads to a pulsating behavior of fracture‐controlled hydrothermal systems and prevents the inflow of ambient fluids under overpressured conditions.