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.

分水岭白钨矿矿床（Watershed scheelite deposit）产于北昆士兰州莫斯曼造山带（Mossman Orogen）的一个消亡弧前盆地（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，霍奇金森组包括强烈变形的夕卡岩（skarn）蚀变砾岩、砂屑岩（psammite）和板岩单元，其记录了从韧性（ductile）变形（D₁–₃）到脆韧性（brittle–ductile）变形（D₄）的四期构造变形事件。
早期D₁–₂期白钨矿矿化形成于石炭纪二长岩（monzonite）和夕卡岩蚀变砾岩中，对应区域进变质作用（prograde metamorphism），变质程度达绿片岩相上部至角闪岩相下部。
二叠纪D₄期白钨矿矿化则形成于退变质（retrograde metamorphism）、低绿片岩相条件下的走滑伸展剪切相关脉体、脉体晕圈及夕卡岩蚀变砾岩中。
在D₄期变形过程中，四期退变质蚀变作用（退变质阶段1-4）改造了围岩。退变质阶段2的流体包裹体（fluid inclusion）组合、脉体白钨矿和石英以低盐度H₂O–NaCl ± CH₄流体为特征（X_CH₄ <0.01，1.4–8.0 wt% NaCl_eq）。该流体包裹体证据显示，低盐度（~0 wt% NaCl_eq）与中等盐度（<8 wt% NaCl_eq）的流体发生了混合。
白钨矿矿化的温压条件（P–T conditions）测定为~300 °C、1–1.8 kbar（即深度3.7–6.7 km），指示高地温梯度（geothermal gradient，35–75 °C/km），该梯度可能源自二叠纪花岗岩提供的热流。
D₄期脉体（退变质阶段4）中发育磁黄铁矿（pyrrhotite）与毒砂（arsenopyrite），且白钨矿内流体包裹体中含有石墨与甲烷，表明矿化环境为还原环境。
退变质阶段2白钨矿（δ¹⁸O_VSMOW为+3.8‰~+7.3‰）、斜长石（δ¹⁸O_VSMOW为+7.0‰~+11.8‰）与石英（δ¹⁸O_VSMOW为+12.6‰~+15.5‰）的氧同位素组成显示，流体温度为306 ± 56 °C，δ¹⁸O_VSMOW值介于+4.7‰~+8.3‰之间。
通过退变质阶段3白云母（muscovite）的δD_VSMOW（−73.4‰~−62.7‰）与δ¹⁸O_VSMOW（+11.5‰~+13.2‰）数值，可计算出在不同可能温度（250–300 °C）下与矿物平衡的流体的O-H同位素组成，结果指示成矿流体为变质成因。
脉体中D₄期退变质阶段4硫化物的硫同位素组成（δ³⁴S_CDT为−2.5‰~+2.8‰）表明，硫可能源自海水或海水硫酸盐，尽管该范围与岩浆硫同位素组成重叠，但与区域地质背景相符。
变质流体可能起源于霍奇金森组在进变质作用过程中的脱挥发分反应（devolatilisation reaction），二叠纪侵入体作为热源促进了变质流体活动与金属运移。
具有经济价值的主要D₄期脉状白钨矿矿化形成于306 ± 56 °C的温度区间与3.7~6.7 km的深度范围内。D₄期白钨矿矿化主要源自低盐度至中等盐度的变质流体，流体体系包括H₂O–NaCl与H₂O–NaCl–CH₄两种类型。部分流体包裹体中发育甲烷（CH₄）与石墨，结合硫化物矿物相特征，进一步证实了还原的矿化环境。
D₄期白钨矿矿化主要源自低盐度至中等盐度的变质流体，且兼具H₂O–NaCl与H₂O–NaCl–CH₄两种流体组成。部分流体包裹体中含有甲烷与石墨，结合硫化物矿物相，指示还原的矿化环境。