In the Lojane area (North Macedonia) ores of Sb (stibnite), As (realgar), and Cr (chromite) were mined and processed in a metallurgical plant until 1979. Over one million tons of flotation tailings containing As, Sb, and other hazardous substances are located in an open dump site for flotation waste created by the mine. The tailings site is completely unprotected, and its orange color reflects a very high concentration of arsenic (fine-grained realgar superficially altered to pararealgar). In order to better understand the weathering behavior of these tailings, which is necessary to evaluate the environmental risks (mainly from the mobilization of As-Sb-Cr), solid waste material was sampled and studied from the chemical and mineralogical point of view. The material was characterized by inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma-optical emission spectrometry (ICP-OES), X-ray diffraction analysis (both single crystal and powder), scanning electron microscopy (SEM) with energy-dispersive microanalysis (EDX), Raman spectroscopy, and transmission electron microscopy (TEM) with selected area electron diffraction (SAED), energy-dispersive X-ray analysis (EDX), and electron energy loss spectrometry (EELS). The studied tailings material is comprised mostly of well-crystallized realgar, gypsum, and quartz, and minor amounts of stibnite, pararealgar, chromite, and sulfur. Very minor pyrite is found within quartz aggregates. The most abundant secondary phase, which forms thin coatings around realgar and stibnite grains, is an As-Sb-Fe-Ca-(Ni)-oxide/hydroxide in which the As:Sb ratio varies from ca. 2:1 to 1:2.2 and Fe contents are variable. Antimony-dominant variants of this oxide also form larger homogeneous grains up to 500 μm in size, characterized by broad dehydration cracks suggesting original formation as a gel. Both As-rich and -poor variants were identified as members of the roméite group. EELS showed that all the Fe is ferric. Further secondary phases originated from the weathering of realgar, stibnite, and other primary phases are As-bearing sulfur, scorodite (often slightly Sb-bearing, locally common), arsenolite, “limonite”, pickeringite (Ni- and Fe-bearing), alunogen, and annabergite. The weathering of primary sulfides in the flotation tailings at Lojane proceeded under mostly oxidizing, acidic, and temporarily wet conditions. Highly acidic conditions on the surface of the tailings dump imply dissolution of arsenolite and scorodite, thus causing contamination of the environment and high mobility of arsenic.