The Permian Watershed tungsten deposit (northeast Queensland, Australia): fluid inclusion and stable isotope constraints

The Watershed scheelite deposit is located in an extinct fore-arc basin in the Mossman Orogen of North Queensland. This fore-arc region comprises multiply deformed, Ordovician–Silurian metasedimentary rocks of the Hodgkinson Formation, and it was intruded by Carboniferous–Permian granites of the Kennedy Igneous Association. At Watershed, the Hodgkinson Formation includes strongly deformed skarn-altered conglomerate, psammite and slate units, which record four deformation events that evolved from ductile (D_1–3) to brittle–ductile (D₄). Early, D_1–2 scheelite mineralisation in Carboniferous monzonite and skarn-altered conglomerate formed during regional prograde metamorphism, which reached upper greenschist to lower amphibolite facies conditions. Permian, D₄ scheelite mineralisation was deposited in transtensional, shear-related veins, vein haloes and skarn-altered conglomerate during retrograde, lower greenschist facies metamorphism. During D₄, four stages of retrograde alteration (retrograde stages 1–4) affected the rocks. Fluid inclusion assemblages in retrograde stage 2, vein scheelite and quartz are characterised by a low-salinity H₂O–NaCl ± CH₄ fluid (<i>X</i>CH₄ &lt;0.01, 1.4–8.0 wt% NaCl_eq). The fluid inclusions show evidence for fluid mixing between a low (∼0 wt% NaCl_eq) and a medium (&lt;8 wt% NaCl_eq) saline fluid. Scheelite mineralisation <i>P–T</i> conditions were determined at ∼300 °C and 1–1.8 kbar (<i>i.e.</i> a depth of 3.7–6.7 km), indicative of a high geothermal gradient (35–75 °C/km), which was likely caused by the heat from the Permian granites. The presence of pyrrhotite and arsenopyrite in D₄ veins (retrograde stage 4), plus graphite and methane in the fluid inclusions in scheelite, indicates reduced mineralisation conditions. Oxygen isotope compositions (δ¹⁸O_VSMOW) of retrograde stage 2 scheelite (+3.8 to +7.3‰), plagioclase (+7.0 to +11.8‰) and quartz (+12.6 to +15.5‰) indicate a fluid temperature of 306 ± 56 °C with δ¹⁸O_VSMOW values between +4.7 and +8.3‰. Retrograde stage 3 muscovite δD_VSMOW (−73.4 to −62.7‰) and δ¹⁸O_VSMOW (+11.5 to +13.2‰) values were used to calculate the O–H isotopic compositions of the fluids in equilibrium with the minerals at various possible temperatures (250–300 °C). The results are consistent with a metamorphic origin for the mineralising fluid. Sulfur isotope compositions (δ³⁴S_CDT between −2.5 and +2.8‰) for vein-hosted, retrograde stage 4 sulfides indicate that sulfur could have come from seawater or seawater sulfate, which is consistent with the local geology, even though this range overlaps with magmatic sulfur isotope compositions. Metamorphic fluids probably originated from devolatilisation reactions in the Hodgkinson Formation during prograde metamorphism. Permian intrusions acted as heat source enhancing metamorphic fluid flow and metal transport. The main economic D₄ scheelite mineralisation in veins occur at temperatures of 306 ± 56 °C and depths between 3.7 and 6.7 km.D₄ scheelite mineralisation precipitated predominantly from a low- to medium-salinity metamorphic fluid, and with both H₂O–NaCl and H₂O–NaCl–CH₄ compositions.The presence of methane (CH₄) plus graphite in some fluid inclusions and the sulfide mineral phases indicates reduced mineralisation conditions. D₄ scheelite mineralisation precipitated predominantly from a low- to medium-salinity metamorphic fluid, and with both H₂O–NaCl and H₂O–NaCl–CH₄ compositions. The presence of methane (CH₄) plus graphite in some fluid inclusions and the sulfide mineral phases indicates reduced mineralisation conditions.

分水岭白钨矿（Watershed scheelite deposit）矿床坐落于昆士兰州北部莫斯曼造山带（Mossman Orogen）的消亡弧前盆地（extinct fore-arc basin）内。该弧前区域发育多期变形的奥陶纪-志留纪霍奇金森组（Hodgkinson Formation）变沉积岩，且被石炭纪-二叠纪肯尼迪火成岩组合（Kennedy Igneous Association）花岗岩侵入。在分水岭矿区，霍奇金森组包含强烈变形的夕卡岩蚀变（skarn-altered）砾岩、砂质岩（psammite）与板岩单元，其记录了从韧性变形（D₁–₃）向脆韧性变形（D₄）演化的四期变形事件。早阶段D₁–₂期白钨矿化形成于石炭纪二长岩（monzonite）与夕卡岩蚀变砾岩中，对应区域进变质作用（prograde metamorphism），变质条件达绿片岩相（greenschist facies）上部至角闪岩相（amphibolite facies）下部。二叠纪D₄期白钨矿化则形成于走滑拉张（transtensional）剪切相关脉体（shear-related veins）、脉晕（vein haloes）及夕卡岩蚀变砾岩中，对应退变质作用（retrograde metamorphism）下的低绿片岩相条件。在D₄变形期，四期退变质蚀变（退变质阶段1–4）改造了区内围岩。退变质阶段2、脉体白钨矿与石英中的流体包裹体（fluid inclusion）组合以低盐度（low-salinity）H₂O–NaCl ± CH₄流体为特征，其中甲烷摩尔分数XCH₄<0.01，氯化钠当量重量百分比为1.4–8.0 wt% NaCl_eq。该流体包裹体显示出低盐（约0 wt% NaCl_eq）与中盐（<8 wt% NaCl_eq）流体混合的证据。白钨矿化的压力-温度（P–T）条件经测定约为300 °C，压力1–1.8 千巴（kbar，即埋藏深度3.7–6.7 km），指示高地温梯度（35–75 °C/km），该梯度大概率由二叠纪花岗岩提供的热源所致。D₄期脉体（退变质阶段4）中产出磁黄铁矿（pyrrhotite）与毒砂（arsenopyrite），加之白钨矿流体包裹体中的石墨（graphite）与甲烷，表明矿化环境为还原环境。退变质阶段2白钨矿的δ¹⁸O_VSMOW值为+3.8至+7.3‰，斜长石（plagioclase）为+7.0至+11.8‰，石英为+12.6至+15.5‰，三者的氧同位素（oxygen isotope）组成显示，成矿流体温度为306 ± 56 °C，δ¹⁸O_VSMOW值介于+4.7至+8.3‰之间。通过退变质阶段3白云母（muscovite）的δD_VSMOW（−73.4至−62.7‰）与δ¹⁸O_VSMOW（+11.5至+13.2‰）值，可计算出250–300 °C不同温度下与该矿物平衡的流体的氧-氢同位素组成，结果与成矿流体的变质起源一致。脉体中赋存的退变质阶段4硫化物的硫同位素（sulfur isotope）组成（δ³⁴S_CDT介于−2.5至+2.8‰）表明，硫可能源自海水或海水硫酸盐（seawater sulfate），这与区域地质背景相符，尽管该范围与岩浆硫（magmatic sulfur）同位素组成存在重叠。变质流体可能源自霍奇金森组在进变质作用过程中的脱挥发分反应（devolatilisation reactions）。二叠纪侵入体作为热源，促进了变质流体活动与金属搬运（metal transport）。区内主要具有经济价值的D₄期脉状白钨矿化形成于306 ± 56 °C的温度条件下，埋藏深度介于3.7–6.7 km之间。D₄期白钨矿化主要源自低盐度至中盐度的变质流体，流体体系兼具H₂O–NaCl与H₂O–NaCl–CH₄两种类型。部分流体包裹体中产出甲烷（CH₄）与石墨，加之硫化物矿物相，均指示还原的矿化环境。D₄期白钨矿化主要源自低盐度至中盐度的变质流体，流体体系兼具H₂O–NaCl与H₂O–NaCl–CH₄两种类型。部分流体包裹体中产出甲烷（CH₄）与石墨，加之硫化物矿物相，均指示还原的矿化环境。