Water inrush disasters under high geothermal conditions pose major threats to deep underground engineering. Grouting is an essential technique for water hazard treatment. However, traditional cement grouts show poor injectability in microfractures and obvious performance deterioration at high temperatures. To address this gap, a low-carbon grouting material was prepared by incorporating mechanically activated gold tailings (MGT) and polycarboxylate superplasticizer (PCE) into superfine Portland cement (SPC). A self-designed testing system was developed to evaluate grout injectability in microfractures. The rheological properties, injectability, fluidity, bleeding rate, setting time, hydration heat evolution, and mechanical strength of the grouts were systematically tested at 20 °C, 50 °C, and 80 °C. Results show that MGT increased the grout viscosity and yield stress, whereas PCE extended percolation time, reduced the rigidification rate and effectively regulated the structural build-up of grouts under high-temperature conditions. The gold-tailings based superfine cement (GSC) grout blended with 0.6–1.2 wt% PCE exhibited excellent injectability in 0.3–1 mm rough, angled, and intersecting fractures across all test temperatures. Negative linear correlations between the injectability index and rheological parameters of grouts were established, providing quantitative references for engineering applications. Performance evaluation revealed that 0.6 wt% PCE yielded the optimal comprehensive performance for GSC at 20 °C and 50 °C, while 1.2 wt% PCE was the preferred dosage at 80 °C. Benefiting from the synergistic effect of MGT and PCE, this eco-friendly grouting material achieves a balanced high-temperature performance, presenting a promising technical solution for water plugging in deep underground engineering.