Big Data received from sources such as social media, in-stream monitoring systems, networks, and markets is often mined for discovering patterns, detecting anomalies, and making decisions or predictions. In distributed learning and real-time processing of Big Data, ensemble-based systems in which a fusion center (FC) is used to combine the local decisions of several classifiers, have shown to be superior to single expert systems. However, optimal design of the FC requires knowledge of the accuracy of the individual classifiers which, in many cases, is not available. Moreover, in many applications supervised training of the FC is not feasible since the true labels of the data set are not available. In this paper, we propose an unsupervised joint estimation-detection scheme to estimate the accuracies of the local classifiers as functions of data context and to fuse the local decisions of the classifiers. Numerical results show the dramatic improvement of the proposed method as compared with the state of the art approaches.